UNIVERSITY OF CINCINNATI

Transcription

1 UNIVERSITY OF CINCINNATI Date: I,, hereby submit this work as part of the requirements for the degree of: in: It is entitled: This work and its defense approved by: Chair:

2 FLUORINATION OF SILICONE RUBBER BY PLASMA POLYMERIZATION A dissertation submitted to the Division of Graduate Studies and Research of the University of Cincinnati in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemical and Materials Engineering of the College of Engineering 2004 by Jennifer Chase Fielding B. S. University of Florida May 1998 M. S. University of Cincinnati December 2000 Committee Chair: Dr. F. James Boerio

3 ABSTRACT Plasma polymerized fluorocarbon (PPFC) films were deposited onto various silicone rubber substrates, including O-rings, to decrease oil uptake. Depositions were performed using a radio frequency (rf)-powered plasma reactor and various fluorocarbon monomers, such as C 2 F 6, C 2 F 5 H, C 3 F 6, and 1H,1H,2H-perfluoro-1-dodecene. PPFC films which were most promising for inhibiting oil uptake were deposited with 1H,1H,2H-perfluoro-1-dodecene, and were composed predominantly of perfluoromethylene (CF 2 ) species. These films displayed low critical surface energies (as low as 2.7 mj/m 2 ), and high contact angles with oil (84 ), which were correlated with the amount of CF 2 species present in the film. For the films with the highest degree of CF 2 (up to 67 %), CF 2 chains may have been oriented slightly perpendicular to the substrate and terminated by CF 3 species. Adhesion of the PPFC films directly to silicone rubber was found to be poor. However, when a plasma polymerized hydrocarbon interlayer was deposited on the silicone rubber prior to the fluorocarbon films, adhesion was excellent. O-rings coated with multilayer fluorocarbon films showed 2.6 % oil uptake after soaking in oil for 100 hrs at 100 C. Due to variability in data, and the low quality of the industrial grade silicone rubber, the oil uptake mechanism was determined to be from oil flowing through flaws in the film due to defects within the substrate, not from generalized diffusion through the film. This mechanism was confirmed using higher quality silicone rubber, which showed little or no oil diffusion. Therefore, this film may perform well as an oil-repelling barrier when deposited on a high quality silicone rubber.

4

5 ACKNOWLEDGEMENTS There are many people I would like to thank for their encouragement and for their contribution to the success of this research, either directly or indirectly. Specifically, I would like to thank my husband, Randy Fielding, for his love, patience and understanding as I pursue this degree. I would also like to thank my parents, Arthur and Lucille Chase, for their invaluable guidance and support. I would like to thank Dr. F. James Boerio for his excellent advising over the course of this research. He has encouraged creative thinking and independence, and has continued to reach beyond my expectations of an advisor. He is a very honorable person and a trusted friend. Many members of Dr. Boerio s research group also deserve recognition for their assistance and companionship. In particular, Dr. Giles Dillingham, Dr. Bhaskar Gopalanarayanan, Dr. Robert Turner, and Dr. Craig Bertelsen for their mentoring in the initial months of my graduate work. I would also like to acknowledge Dr. Robert Hudgins and Dr. Andrew Steckl for the training on the use of the AFM, and for the time that I was allowed to use it for my sample analysis. Finally, this project would not have been successful without the financial support of Caterpillar, Inc. David Penning, Teresa Cushing, Darren Gedes, Matthew Hess, and Alan Dickey were helpful in guiding the progress of the research.

6 TABLE OF CONTENTS LIST OF TABLES. 5 LIST OF FIGURES I. OBJECTIVES II. INTRODUCTION A. Problem statement swelling of silicone rubber B. Fluoroelastomers. 39 C. Fluoropolymers D. Fluorocarbon films E. Plasma deposition F. Reactor types for plasma polymerized fluorocarbon (PPFC) films 45 G. Fluorocarbon plasma chemistry H. PPFC films deposited with low molecular mass monomers 47 I. Multilayer films. 50 J. Hydrocarbon films K. Perfluoromethylene (CF 2 ) films using low molecular mass monomers.. 51 L. Perfluoromethylene (CF 2 ) films using high molecular mass monomers. 52 M. CF 2 -dominated films prepared using techniques other than plasma. 53 N. Contact angle measurements and surface energy O. Methods to find surface tension of a liquid P. Characterization techniques Fourier transform infrared (FTIR) spectroscopy X-ray photoelectron spectroscopy (XPS) Table of Contents 1

7 3. Ellipsometry Microscopy III. EXPERIMENT A. Substrate Preparation Polyethylene (PE) Ferrotype plate Industrial grade silicone rubber plaques Industrial grade silicone rubber O-rings Food grade silicone rubber plaques Medical grade silicone rubber plaques 73 B. Substrate characterization Polyethylene film Ferrotype plate Silicone rubber swatches Industrial grade silicone rubber O-rings.. 75 C. Characterization of the diesel engine oil D. Plasma film deposition Description of reactor Deposition of PPFC films using low molecular mass monomers Depositions of LCF films Depositions of multilayer (FC/HC) films Deposition conditions for multilayer (HC/FC) films on O-rings E. Film characterization Table of Contents 2

8 1. Ellipsometry Fourier transform infrared (FTIR) spectroscopy X-ray photoelectron spectroscopy Contact angle measurements and surface energy Microscopy Adhesion testing Oil uptake on silicone rubber plaques O-ring oil uptake testing using a soak test 91 IV. RESULTS AND DISCUSSION. 92 A. Substrate Characterization Characterization of PE substrate Characterization of ferrotype plate Characterization of silicone rubbers B. Characterization of the Caterpillar diesel engine oil (DEO) Infrared spectroscopy Surface energy C. Short monomer depositions Plasma glow Deposition rate Refractive index Film chemistry and structure Contact angle and surface energy Table of Contents 3

9 6. Adhesion Testing AFM of films produced with small molecular mass monomers E. Multilayer films F. Long chain fluorocarbon (LCF) films Deposition rate Morphology Structure Contact angles and surface energies of LCF films Discussion of PPFC structure and surface energy Adhesion of the LCF films Oil Uptake of the LCF/multilayer films O-ring coating using the PPFC films. 137 V. CONCLUSIONS VI. FUTURE WORK. 145 VII. TABLES VIII. FIGURES IX. BIBLIOGRAPHY Table of Contents 4

10 LIST OF TABLES TABLES DESCRIPTION PAGES TABLE I. TABLE II. TABLE III. TABLE IV. TABLE V. TABLE VI. Bond energies for bonds relevant to the study of fluorocarbon 16, 17 plasmas and films. Binding energies and assignments for peaks used in curvefitting the XPS C(1s) region for many PP films. Fluorocarbon films were dominated by C-F bonding and 8, 71, 88 hydrocarbon films were dominated by C-O bonding. Experimental reactor parameters used for deposition of plasma polymerized fluorocarbon films with low molecular mass fluorocarbon monomers, such as C 2 F 6, C 2 F 5 H, and C 3 F 6. Deposition parameters of the multilayer film M1 which consisted of a 7-layer film of varying fluorination, deposited by varying the ratio of C 2 F 5 H/C 2 H 2. As each layer was deposited, the degree of fluorination increased. Deposition parameters of the multilayer film M2 which consisted of a 3-layer film of varying fluorination, deposited by varying the ratio of C 2 F 6 /C 2 H 2. Deposition parameters of the multilayer film M3 which consisted of a 4-layer film of varying fluorination, deposited by varying the ratio of C 3 F 6 /C 2 H 2, with a long chain (LCF) film deposited at 160 W of rf power as the final layer. p. 146 p. 146 p. 147 p. 147 p. 148 p. 148 List of Tables 5

11 TABLE VII. TABLE VIII. TABLE IX. TABLE X. TABLE XI. Deposition parameters of the multilayer film M4 which consisted of a 3-layer film with varying fluorination, deposited with a hydrocarbon layer, a fluorocarbon layer, then with a long chain fluorocarbon (LCF) film deposited with 2 W of rf power as the final layer. Deposition parameters of the multilayer film M5 which consisted of a 3-layer film with varying fluorination, deposited with a hydrocarbon layer, a fluorocarbon layer, then with a long chain fluorocarbon (LCF) film deposited at 160 W of rf power as the final layer. ATR-IR band assignments for the industrial grade silicone rubber, used as a substrate for many PP film depositions. 66 The IR spectrum is shown in Fig. 20. Mechanical property data for the three types of silicone rubber used to measure oil uptake. Numbers shown are an average of three samples. Stress-strain curves are shown in Fig. 21. Transmission IR absorbance band assignments for spectra shown in Fig. 31 of the diesel engine oil used in the oil uptake experiments, before and after heating for 72 hrs at 150 C. 66 p. 149 p.149 p.150 p. 150 p. 151 List of Tables 6

12 TABLE XII. ATR-IR absorbance band assignments for plasma p. 151 TABLE XIII. TABLE XIV. polymerized fluorocarbon films deposited on PE using C 3 F 6 /Ar = 10/90, with deposition time of 5 min, total flow rate of 20 sccm, and varying rf power from 10 to 140 W. Spectra are shown in Fig. 37. RAIR absorbance band assignments for films deposited using C 2 F 5 H/C 2 H 2 with varying % C 2 H 2 from 30 to 90 %. Other deposition parameters were 20 sccm total flow rate, 50 W of rf power, and 5 min deposition time. RAIR spectra for these films are shown in Figs RAIR absorbance band assignments for the deposition of a hydrocarbon film on ferrotype plate using C 2 H 2 at a flow rate p. 152 p. 152 of 20 sccm for 5 min at 50 W of rf power. 66 A large degree of TABLE XV. oxidation was shown by the peaks at 3454 and 1725 cm -1. RAIR spectra are shown in Fig. 53. Oil uptake results for three O-rings coated with the multilayer film M1 and tested using the soak test for 24 hrs at room temperature. The average oil uptake for these coated O-rings was 2.7 %, compared to the uncoated control of 7.0 %. p. 153 List of Tables 7

13 TABLE XVI. O-ring oil uptake results for three O-rings coated with the multilayer film M1 and tested using the soak test, for 24 hrs at 80 C. The average oil uptake was 11.5 %, which was approximately equal to the estimate for the control value at this temperature. p. 153 TABLE XVII. RAIR absorbance bands assignments for the long chain p. 154 fluorocarbon (LCF) monomer LCF plasma polymerized films, as shown in Figs TABLE XVIII. CF 3 /CF 2 ratio of the areas found from peak-fitting the XPS p. 154 TABLE XIX. C(1s) spectra for the long chain fluorocarbon (LCF) films deposited with various RF powers (2 and 160 W). XPS analysis was done at take-off angles of 10 and 80, and it was shown that a change in the % CF 3 on the surface was present for the film deposited with 2 W of rf power. Atomic concentration results found using XPS for the tape side of the three tape peel tests on different areas of the multilayer film M5 deposited on industrial grade silicone rubber swatches. Deposition parameters are shown in (Table VIII). Results show excellent adhesion, as indicated by very low values of the % F on the tape side of the peel surfaces, with an average % F of 1.9 %. p. 155 List of Tables 8

14 TABLE XX. TABLE XXI. Atomic concentration results found using XPS for the tape side of the three tape peel tests on different areas of the long chain fluorocarbon (LCF) film layer described in the M5 film, deposited individually on industrial grade silicone rubber swatches. Deposition parameters are shown in (Table VIII). Results show very poor adhesion, indicated by a high % F on the tape side of the peel surface, with an average % F of 54.3 %. This experiment showed the necessity of the multilayer film with the hydrocarbon interlayer for adhesion. O-ring oil uptake results for six O-rings coated with the multilayer film M3 and tested using the soak test for 100 hrs at various temperatures. These O-rings were cleaned using Kimwipes and compressed N 2. Oil uptake was varied, and no longer dependent on temperature. Therefore, the oil uptake was not by diffusion, since it did not correlate with temperature. Oil uptake by flow through defects was predicted to be the mechanism of oil uptake. p. 155 p. 156 List of Tables 9

15 TABLE XXII. Oil uptake results for eight O-rings coated with the multilayer film M5 and tested using the soak test for 24 hrs at 100 C. The O-rings were cleaned using the microscope cleaning method, where flaws on the surface of the O-ring were removed at 16 X magnification, and flash lines were smoothed. The average oil uptake was only 0.9 %, well below the uncoated value of 7.2 %. p. 156 TABLE XXIII. Oil uptake results for eight O-rings coated with the multilayer p. 157 film M5 and tested using the soak test for 100 hrs at 100 C. The O-rings were cleaned using the microscope cleaning method at 16 X magnification, where flaws on the surface were removed and the flash lines were smoothed. The average oil uptake was 5.5 %, higher than that of the project objectives (< 2 %), but less than the control value of 13.0 %. TABLE XXIV. Repeat experiment of that shown above in XXII. Average oil p. 158 uptake was still 5.5 %, and the oil uptake between O-rings showed similar variability. List of Tables 10

16 LIST OF FIGURES FIGURES DESCRIPTION PAGES FIG. 1. FIG. 2. FIG. 3. Diagram showing the mechanism of HF elimination in a FC plasma due to the addition of H 2, or another source of H, to the feed of a saturated FC monomer. The result is an increase in the relative concentration of CF x species in the plasma environment, which shifts the plasma mechanism towards film deposition. Diagram showing the behavior of a droplet of water on two different surfaces: one with high surface energy and one with low surface energy. The surface energies of the liquid-vapor (γ lv ), liquid-solid (γ ls ), and solid-vapor (γ sv ) interfaces, and the contact angle (θ) are labeled, and may be related by the Young-Dupre equation [see Eq. (1)]. Energy level diagram showing the photoelectron and Auger electron emission process which occurs during an x-ray photoelectron spectroscopy (XPS) experiment: (a) initial state, showing incoming x-ray photon with energy hν approaching filled atomic orbitals, (b) photoemission process, with the release of a core electron to the vacuum (c) Auger emission process with the relaxation of the atom due to a L 2,3 electron falling to a lower energy level, K, with simultaneous emission of an electron from the L 2,3 orbital. Image adapted from Watts. 68 p. 158 p. 158 p. 159 List of Figures 11

17 FIG. 4. FIG. 5. Diagram of the photoemission of an electron, via bombardment with an X-ray photon. Analysis depth d 1 depends on the take off angle φ 1 and the mean free path λ of the electrons. Changes in take-off angle from tilting the sample stage result in changes in the sampling depth of the XPS experiment. In (a), the sample stage was horizontal so that φ In (b), the sample stage was tilted, so that φ 2 is less than φ 1, resulting in a shallower sampling depth (d 2 less than d 1 ). Diagram showing the reflections and transmissions of light directed onto a reflective surface with a film present, with thickness d. Two interfaces are shown, the first being the air-film interface, the second being the film-substrate interface. The angle of the light will change according to Snell s Law [see Eq. (12)] when the light reaches a material with a different complex refractive index N ~. Image adapted from Tompkins. 74 p. 160 p. 160 FIG. 6. Schematic of an atomic force microscope (AFM) in p. 161 TappingMode showing the cantilever and tip oscillation in the vertical (z) direction while the tip is rastered across the sample in the x and y directions. AFM is often used to study the morphology and roughness of plasma polymerized films. Image adapted from Digital Instruments Nanoscope IIIa Instruction Manual. 75 List of Figures 12

18 FIG. 7. FIG. 8. FIG. 9. FIG. 10. O-ring surface refinishing and cleaning process, shown at 16 X magnification using an optical microscope: (a) Before cleaning, arrows show imbedded particles and surface debris (b) Imbedded particle cut out with a razor blade, as shown by arrow (c) Surface and flash lines were smoothed with polishing papers of 400, 800, and 1200 grit. Loose rubber particles remained on the surface, as indicated by the arrows. (d) Wiped with an ethanol-soaked Kimwipe and then blown with compressed N 2 to remove any loose rubber particles. Digital image of rf-powered, parallel-plate reactor chamber, showing placement of the powered and grounded electrodes separated by a 3.5 cm gap. The monomer and carrier gas inlet is shown above the powered electrode. All substrates were placed in the center of the reactor on the grounded electrode. Structures of low molecular mass fluorocarbon monomers used for film deposition: (a) hexafluoroethane (C 2 F 6 ), (b) pentafluoroethane (C 2 F 5 H), and (c) hexafluoropropylene (C 3 F 6 ). Chemical structures of the long chain fluorocarbon (LCF) monomers used for plasma polymerization: (a) 1H,1H,2Hperfluoro-1-dodecene and (b) 1H,1H,2H-perfluoro-1-decene. These monomers were unique since their molecular mass was higher than monomers typically used for plasma polymerization. p. 162 p. 163 p. 163 p. 164 List of Figures 13

19 FIG. 11. FIG. 12. FIG. 13. HTML program written to calculate the polar (γ P s ) and dispersive (γ D s ) components of a solid's surface energy using the Kaelble method, which may calculate the surface energy using the contact angle of two liquids on the surface. Diagram showing tape peel test method for adhesion of the PP films to the substrates. Pressure sensitive double-sided tape was applied to the coated rubber with finger pressure, then peeled back. The presence of fluorine on the tape surface as shown by XPS analysis would indicate poor adhesion of a PPFC film. Digital image of coated swatch of industrial grade rubber, under 5.9 % tensile strain, using hose clamps. The specimen is shown inside a Phillips XL30 ESEM chamber, and the sample was analyzed to view film cracking under this amount of strain. p. 165 p. 165 p. 166 FIG. 14. FIG. 15. FIG. 16. XPS survey spectrum of the PE substrate, used for many film depositions. The atomic concentrations were 97.6 % C, 1.8 % O, and 0.6 % Si. XPS C(1s) region of the spectrum of the PE substrate, used for many film depositions, showing a single peak at ev due to C-C, C-H. ATR spectrum of the PE substrate, where all absorbance bands corresponded to known bands for CH 2 stretching (2915, 2847 cm - 1 ), deformation (1466 cm -1 ), and rocking (721 cm -1 ). p. 166 p. 167 p. 167 List of Figures 14

20 FIG. 17. FIG. 18. FIG. 19. XPS survey scan of ferrotype plate (a) before and (b) after the cleaning process. Before the cleaning process the atomic concentration of C was 61.1 %, and after the cleaning process the atomic concentration of C dropped to 25.0 %, which indicated a removal of much of the carbonaceous contaminant from the protective covering. XPS survey spectrum of the industrial grade silicone rubber substrate. Atomic concentration analysis revealed 45.1 % C, 27.2 % O, 25.7 % Si, and 2.0 % F. XPS C(1s) spectrum of the industrial grade silicone rubber p. 168 p. 169 p. 169 FIG. 20. FIG. 21. FIG. 22. substrate, showing a single, symmetric peak due to the Si-CH 3 environment. ATR-IR of industrial grade silicone rubber plaques, used as substrates for PP film deposition. Absorbance band assignments are given in Table IX. Stress-strain curves for various silicone rubbers. a) industrial grade with modulus of 4.26 MPa, b) food grade silicone rubber with modulus of 0.52 MPa, and c) medical grade with modulus of 0.93 MPa. Averages for stress and strain at failure, Young s modulus, and fracture toughness are shown in Table X. TGA experiments on medical, food, and industrial-grade silicone rubber used for testing oil uptake. p. 170 p. 171 p. 172 List of Figures 15

21 FIG. 23. FIG. 24. FIG. 25. DSC runs of medical, food, and industrial-grade silicone rubber used for testing oil uptake. Optical microscopy at 32 X of the industrial grade silicone rubber swatches showing imbedded debris, and foreign particles. SEM images of the industrial grade silicone rubber O-ring: (a) Surface flaw present on O-ring surface. (b) Cross section of an O- p. 173 p. 174 p. 175 ring where a flaw was noticed. It is shown to extend approximately 800 µm below the O-ring surface. (c) Fibrous rubber particles connected to and extending from the rubber surface. (d) Cross section, showing the flash line of the O-ring. (e) Cross section of O-ring, showing dimensions of flash line. FIG. 26. AFM image of untreated industrial grade silicone rubber on a 50 p. 176 FIG. 27. FIG.28. µm scale. The arrow shown indicates the peak-to-trough distance and is equal to 5.4 µm. The average roughness was 849 nm. Dyed oil droplet test showing the oil uptake of the uncoated industrial grade silicone rubber at 100 C and various times: (a) initial application of oil droplets, (b) 1 hr, (c) 2 hrs, (d) 12 hrs, (e) 24 hrs. Dyed oil droplet test showing the oil uptake of the uncoated food grade silicone rubber where (a) is the swatch after droplet placement and (b) is the swatch after 100 hrs at 100 ºC, showing diffusion of oil into the rubber. p. 177 p. 177 List of Figures 16

22 FIG. 29. FIG. 30. FIG. 31. Dyed oil droplet test showing the oil uptake of the uncoated medical grade silicone rubber where (a) is the swatch after droplet placement and (b) is the swatch after 100 hrs at 100 ºC, showing diffusion of oil into the rubber. Oil uptake of uncoated O-rings, as a function of immersion time at 24 hrs ( ) and 100 hrs ( ), and as a function of temperature. These values were used as controls, for comparison to the % oil uptake of coated O-rings. Transmission IR of DEO (a) after heating at 150 C for 72 hrs and (b) before heating. Band assignments are given in Table XI and reveal changes to the oil due to increased CH 3 and C=O species. An inset of the region between 1950 cm -1 and 1100 cm -1 is shown in Fig 32. p. 178 p. 178 p. 179 FIG. 32. Expansion of the region from 1950 to 1100 cm -1 of the p. 180 transmission IR shown in Fig. 31 of the diesel engine oil before and after heating to 150 C for 72 hrs. The dashed line is the spectrum of the oil before heating and the solid line is the spectrum after heating. An increase in the bands at 1706 and 1230 cm -1 were apparent, both due to oxidation, and a splitting of the bands around 1377 cm -1 was also shown, due to increased CH 3 species. List of Figures 17

23 FIG. 33. FIG. 34. FIG. 35. FIG. 36. Surface tension of diesel engine oil before ( ) and after ( ) heating in air for 72 hrs at 150 C. There was a decrease in the surface tension as temperature increased. The previously heated oil also showed a slightly lower surface tension for all temperatures. Effect of power on deposition rate of PPFC films deposited using two different monomer systems: C 2 F 6 /C 2 H 2 = 90/10 ( ) and C 3 F 6 /Ar = 10/90 ( ), with rf power of 50 W, total flow rate of 20 sccm for C 2 F 6 /C 2 H 2 and 50 sccm for C 3 F 6 /Ar and deposition time of 5 min. Effect of concentration of C 2 H 2 in the feed on deposition rate for monomers C 2 F 5 H ( ) and C 2 F 6 ( ), with rf power of 50 W, total flow rate of 20 sccm, and deposition time of 5 min. Dashed lines indicate the deposition rate assuming no interaction between the C 2 H 2 and the fluorocarbon monomer. Effect of the concentration of C 2 H 2 in the feed gas on the refractive index of the resulting PPFC films, deposited using C 2 F 5 H ( ) and C 2 F 6 ( ). Films were deposited using 50 W rf power, 20 sccm total flow rate and 5 minutes deposition time. p. 181 p. 182 p. 183 p. 184 FIG. 37. ATR-IR spectra of PPFC films deposited using C 3 F 6 /Ar = 10/90, p. 185 deposition time was 5 min, total flow rate was 20 sccm, with varying rf powers. Peak assignments are shown in Table XII. List of Figures 18

24 FIG. 38. RAIR of PPFC film on ferrotype plate, with C 2 H 2 /C 2 F 5 H = 30/70, p. 186 total flow of 20 sccm, rf power of 50 W, and deposition time of 5 min, revealing a sharp band at 1237 cm -1 due to C-F stretching. Other band assignments for spectra shown in Figs are shown in Table XIII. FIG. 39. RAIR of PPFC film on ferrotype plate, with C 2 H 2 /C 2 F 5 H = 50/50, p. 187 total flow of 20 sccm, rf power of 50 W, and deposition time of 5 min. Band assignments are shown in Table XII. FIG. 40. RAIR of PPFC film on ferrotype plate, with C 2 H 2 /C 2 F 5 H = 70/30, p. 188 total flow of 20 sccm, rf power of 50 W, and deposition time of 5 min. FIG. 41. RAIR of PPFC film on ferrotype plate, with C 2 H 2 /C 2 F 5 H = 90/10, p. 189 FIG. 42. total flow of 20 sccm, 50 W of rf power, and deposition time of 5 min. RAIR of PPFC film on ferroplate substrates with varying degrees of C 2 H 2 in the feed gas of C 2 F 5 H. with a total flow rate of 20 sccm, 50 W of rf power, and deposition time of 5 min. Spectra are enlarged from Figs , but centered on the region of the main fluorocarbon peak. p. 190 List of Figures 19

25 FIG. 43. XPS C(1s) spectra of PPFC films deposited using C 2 F 5 H and varying amounts of C 2 H 2 in the feed gas as follows: (a) 30 %, (b) 50 %, and (c) 70 %. The films were deposited with 50 W rf power, 20 sccm total flow rate, and deposition time of 5 min. Changes to the degree of fluorination are shown by the changes in the % of peaks due to CF 3 and CF 2 at and ev. p. 191 FIG. 44. Films deposited using C 2 F 6 monomer, as a function of C 2 H 2 p. 192 FIG. 45. concentration in the feed, with a total flow rate of 20 sccm, rf power of 50 W, process pressure of 40 mtorr, and deposition time of 5 min. Correlation between the F/C ratio found using XPS in the films and the C 2 H 2 in the feed gas, for C 2 F 6 ( ) and C 2 F 5 H ( ), where the rf power was 50 W, total flow rate was 20 sccm, and deposition time was 5 min. p. 193 FIG. 46. XPS survey spectrum of a film deposited on PE using Ar and C 3 F 6 p. 194 FIG. 47. in a ratio of 90/10, with rf power of 20 W and a total flow rate of 50 sccm, process pressure of 500 mtorr. Atomic concentration analysis revealed 63.8 % F and 36.2 % C, with a F/C ratio of XPS C(1s) region of the spectrum of the film deposited on PE using C 3 F 6 /Ar in a ratio of 10/90, with rf power of 20 W, total flow of 50 sccm, process pressure of 500 mtorr. Curve-fitting revealed the following main peaks: 27.5 % CF 3, 26.5 % CF 2, and 21.2 % C-CF. p. 194 List of Figures 20

26 FIG. 48. XPS C(1s) region of the spectrum of the film deposited on PE using C 3 F 6 /Ar in a ratio of 10/90, with rf power of 140 W and total flow rate of 50 sccm. Atomic concentration analysis revealed 64 % F, 33.5 % C and 1.4 % O and 1.1 % N. Curve-fitting of the C(1s) revealed 21.2 % CF 3 and 29.8 % CF 2. p. 195 FIG. 49. XPS C(1s) spectrum of a film deposited on PE using C 2 F 6 /C 2 H 2 = p. 196 FIG. 50. FIG /10, rf power of 50 W and deposition time of 5 min. Atomic concentration analysis revealed 63.9 % F, 34.8 % C, and 1.3 % O. Curve-fitting revealed 25.8 % CF 3, 0.9 % CF 3 -CF 2, 31.4 % CF 2, 11.6 % CF-CF x, 9.2 % C-F, and 17.0 % C-CF. XPS survey spectrum of a film deposited on PE with a similar film structure deposited with C 2 F 6 /C 2 H 2 = 80/20, with 100 W of rf power and deposition time of 40 min. Atomic concentration analysis revealed 61.0 % F, 36.5 % C, 1.5 % O and 1.0 % N. Curve-fitting of the C(1s) region revealed 24.8 % CF 3, 1.7 % CF 3 - CF 2, 24.8 % CF 2, 13.6 % CF-CF x, 14.1 % C-F, and 21.0 % C-CF. XPS C(1s) spectrum of the film deposited on PE using only C 2 F 5 H at 20 sccm with 50 W rf power, process pressure of 40 mtorr, for a deposition time of 5 min. The atomic concentration analysis revealed 60.1 % F, 37.5 % C, and 2.4 % O, for a F/C of Curve-fitting of the C(1s) revealed 20.0 % CF 3, 4.2 % CF 3 -CF 2, 22.3 % CF 2, 16.7 % CF-CF x, 15.2 % C-F, 20.0 % C-CF, and 1.6 % C-C. p. 197 p.198 List of Figures 21

27 FIG. 52. Schematic showing the relationship between low molecular weight fluorocarbon monomers, the resulting plasma species and approximate film structure. Low molecular weight monomers result in a highly crosslinked and branched polymeric film. p. 199 FIG. 53. RAIR spectrum of plasma polymerized C 2 H 2 at 20 sccm with 50 p. 200 W rf power, for 5 min, showing peaks due to hydrocarbon (2971, 2935, 1456, 1383, and 627 cm -1 ) and oxidized (3454 and 1725 cm - 1 ) species. Specific absorbance band assignments are given in Table XIV. FIG. 54. XPS survey spectrum of a hydrocarbon film deposited with 20 p. 201 FIG. 55. sccm C 2 H 2, process pressure of 50 mtorr, 50 W of rf power, flow rate of 20 sccm, and deposition time of 5 min. Atomic concentration analysis revealed 78.8 % C, 20.4 % O, 0.8 % F. XPS C(1s) spectrum of a plasma polymerized hydrocarbon film, deposited using 20 sccm C 2 H 2 and 50 W rf power, revealing peaks at 286.1, 287.4, and ev due to oxidation. The film was deposited with 50 mtorr, 50 W of rf power, and deposition time of 5 min. p. 202 FIG. 56. RAIR spectrum of PPFC film using C 3 F 6 /C 2 H 2 = 50/50, with 50 p. 203 W rf power, total flow rate of 20 sccm, and deposition time of 5 min. The band at 1727 cm -1 was due to C=O stretching, and the band at 1232 cm -1 was due to C-F stretching. List of Figures 22

28 FIG. 57. XPS survey spectrum of a film, deposited using C 3 F 6 /C 2 H 2 = p /50, and 50 W of rf power and deposition time of 5 min. Atomic concentration analysis revealed 46.8 % C, 49.2 % F, and 4.0 % O. FIG. 58. XPS C(1s) spectrum of a film deposited using C 3 F 6 /C 2 H 2 = 50/50, p. 205 FIG. 59. FIG W of rf power, and deposition time of 5 min, showing a highly crosslinked film with a high concentration of C-CF. Effect of the concentration of C 2 H 2 in the feed gas for the monomers C 2 F 6 ( ) and C 2 F 5 H ( ) on the contact angle of water on the resulting film, deposited on PE using 50 W rf power, total flow rate of 20 sccm, and deposition time of 5 min. The uncoated contact angle of water on PE was 90, represented by the bolded line. Effect of C 2 H 2 in the feed of C 2 F 6 ( ) and C 2 F 5 H ( ) on the surface energy of the films deposited with 50 W of rf power, total flow rate of 20 sccm, and deposition time of 5 min. Surface energy was calculated using the Kaelble method. The surface energy of uncoated PE was 33.1 mj/m 2, and is indicated by the bolded line. p. 206 p. 207 List of Figures 23

29 FIG. 61. FIG. 62. FIG. 63. XPS survey spectrum of the pressure sensitive adhesive doublesided tape used for the adhesion testing of the PPFC films on silicone rubber. Atomic concentration analysis revealed 81.6 % C and 18.4 % O. Presence of F(1s) at 686 ev in the XPS survey spectrum of the tape side of a tape peel test of a PPFC films would be indicative of film failure. XPS C(1s) of the pressure sensitive adhesive double-sided tape used for adhesion testing of the PPFC films on silicone rubber. Curve-fitting of the C(1s) region revealed peaks at ev due to C-O and at ev due to C=O bonding. After a tape peel test on a PPFC film, the appearance of peaks at or ev would indicate the presence of CF 3 and CF 2 bonded species, indicating failure of adhesion. XPS survey spectrum of the tape side of a peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, with 20 W of rf power and a total flow rate of 50 sccm. Atomic concentration analysis revealed 46.5 % F, 47.3 % C, and 6.2 % O. The presence of oxygen in the spectrum indicated a patchy removal of the PPFC film. p. 208 p. 209 p. 210 List of Figures 24

30 FIG. 64. FIG. 65. FIG. 66. FIG. 67. XPS C(1s) region of the tape side of a peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, with rf power of 20 W and total flow of 50 sccm. Curve-fitting analysis revealed 13.6 % CF 3, 13.1 % CF 2, 10.8 % CF-CF x, 11.4 % C-F, 13.6 % C-CF, and 37.5 % C-C. The XPS C(1s) of the film before the tape peel test is shown in Fig. 47. XPS survey spectrum of the substrate side of the tape peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, with rf power of 20 W, and a total flow rate of 50 sccm. This sample was tape tested 5X. Atomic concentration analysis revealed 50.3 % F, 47.2 % C, 2.5 % O, and 0.1 % N. XPS C(1s) region of the substrate side of the tape peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, 20 W of rf power, and a total flow rate of 50 sccm. This sample was tape tested 5X. Curve-fitting revealed 15.2 % CF 3, 8.6 % CF 2, 3.6 % CF-CF x, 8.6 % C-F, 14.1 % C-CF, and 50.0 % C-C. XPS survey spectrum of the substrate side of the tape peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, 20 W of rf power, and a total flow rate of 50 sccm. The sample was tape tested 20 X. Atomic concentration analysis revealed 47.6 % F, 50.1 % C, and 2.3 % O. p. 211 p. 212 p. 213 p. 214 List of Figures 25

31 FIG. 68. FIG. 69. FIG. 70. FIG. 71. XPS C(1s) region of the spectrum of the substrate side of the tape peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, 20 W of rf power, and a total flow rate of 50 sccm. The sample was tape tested 20 X. Curve-fitting revealed 13.4 % CF 3, 5.9 % CF 2, 3.3 % CF-CF x, 6.9 % C-F, 14.7 % C-CF, and 56.8 % C-C. F/C ratio vs. the number of tape peels onto the film side of the PE, with the film deposited using C 3 F 6 /C 2 H = 9/10, with rf power of 20 W and deposition time of 5 min, showing that interfacial failure is not an issue for even a film with a large degree of fluorination. Cohesive failure occurred within this film. XPS C(1s) of the tape side of the peel test for the PPFC film deposited using C 3 F 6 /Ar in a ratio of 10/90, with 140 W of rf power, and deposition time of 15 minutes, showing partial film failure. The C(1s) of the original film revealed 21 % CF 3 and 30 % CF 2 %, and is shown in Fig 48. AFM images (topography and phase) in TappingMode of PP hydrocarbon film on industrial grade silicone rubber, deposited using 50 W of rf power, 20 sccm C 2 H 2 and 5 minutes deposition time. The average roughness determined by the AFM software was nm and the film thickness found using ellipsometry was 73.6 nm. p. 215 p. 216 p. 217 p. 218 List of Figures 26

32 FIG. 72. FIG. 73. FIG. 74. FIG. 75. XPS survey spectrum of the tape side of the peel test for adhesion of films deposited on the industrial grade silicone rubber using (a) a single layer deposited with 20 sccm C 2 F 5 H, 50 W of rf power, and deposition time of 5 min and (b) a multilayer film M1 (deposition parameters described in Table IV) a 7-layer film deposited with varying amounts of C 2 F 5 H/C 2 H 2. The single layer film showed film failure where the tape side of the peel test revealed 28 % F, and the multilayer film showed 3 % F, indicating adhesion of the multilayer film M1 to the silicone rubber. XPS survey spectrum of the tape side of a peel test on a multilayer film (M2, a 3-layer film deposited with varying amounts of C 2 F 6 /C 2 H 2 ), resulting in 6.3 % F on the tape side of the peel surface. XPS C(1s) spectrum of the above tape peel test on a multilayer film (M2, a 3-layer film deposited with varying amounts of C 2 F 6 /C 2 H 2 ), showing that the predominant species due to the delaminated film were CF 3 species. Oil uptake experiments with the medical grade rubber coated with p. 219 p. 220 p. 220 p. 221 the multilayer film M1. Neither the uncoated nor the coated sample buckled up from the glass due to oil uptake, but the samples tested at 80 C showed differences where the coated sample buckled much less than the uncoated rubber. Arrows show slight buckling of the coated film tested at 80 C. List of Figures 27

33 FIG. 76. FIG. 77. Dyed oil droplet test showing oil uptake at 100 C industrial grade silicone rubber coated with the multilayer film M1 at various times: (a) initial application of oil droplets, before heating, and after heating for (b) 1 hr, (c) 12 hrs, (d) 24 hrs. AFM topography and phase image on a 10 x 10 µm scale of a multilayer film deposited on silicone rubber (M1). Surface roughness was found using the AFM software to be 233 nm. p. 222 p. 223 FIG. 78. FIG. 79. Deposition rate of the films deposited using the long chain fluorocarbon (LCF) monomer ( ) and the C 2 F 6 /C 2 H 2 = 90/10 system ( ) as a function of rf power. The LCF monomer was deposited at 0.14 g/min for 15 min. The C 2 F 6 /C 2 H 2 monomers were deposited at 20 sccm for 5 min. Deposition rate decreased as a function of rf power for both systems, but the LCF monomer plasma did not reach an etching condition, as the C 2 F 6 /C 2 H 2 did. AFM phase images of PPFC film deposited on a ferrotype plate substrate using the LCF monomer and 2 W of rf power on two different scales: a) 2 x 2 µm, with R a = 6.6 nm and b) 10 x 10 µm, with R a = 8.5 nm. p. 223 p. 224 FIG. 80. AFM phase images of PPFC film deposited on a ferrotype plate substrate using the LCF monomer and 160 W of rf power on two different scales: a) 2 x 2 µm, with R a = 7.1 nm and b) 10 x 10 µm, with R a = 12.4 nm. p. 225 List of Figures 28

34 FIG. 81. RAIR of LCF monomer smeared on ferrotype plate. Bands at 1282, 1252, 1230, and 1149 cm -1 were the most prominent and were assigned to CF 2 symmetric and asymmetric stretching p. 226 modes. A notable band was also present at 1423 cm -1 due to CH 2 deformation and at cm -1 due to CH and CH 2 wagging. All band assignments are given in Table XVII. FIG. 82. RAIR spectra of the film on ferrotype plate, deposited using (a) 2 p. 227 FIG. 83. FIG. 84. W rf power, and (b) using 160 W rf power. Broadening of the bands at cm -1 showed an increase in the variety of C- F species incorporated into the film. No peaks due to the vinyl portion of the LCF monomer appeared in the spectra of the films. All band assignments are given in Table XVI. XPS C(1s) spectra of the films deposited using the LCF monomer as a function of rf power: (a) 2 W, (b) 20 W (c) 40 W, and (d) 160 W. Increased fragmentation of the monomer was shown with greater rf power by increased CF 3 and CF species relative to the peak due to CF 2. XPS C(1s) spectra of PPFC films deposited using the LCF monomer at 2 W rf power, for various take-off angles: (a) 10, and (b) 80. A greater concentration of CF 3 was present on the surface, indicating structural orientation of the CF 2 chains, perpendicular to the substrate, and terminated by CF 3 species. p. 228 p. 229 List of Figures 29

35 FIG. 85. XPS C(1s) spectra of the PPFC films deposited using the LCF monomer and 160 W rf power for various take-off angles: (a) 10, and (b) 80. Little changes between the concentration of CF 3 and CF 2 species were shown. p. 229 FIG. 86. Schematic showing the relationships between a long chain p. 230 fluorocarbon monomer, gentle plasma conditions, and the resulting approximate film structure. Using mild plasma FIG. 87. FIG. 88. conditions, PTFE-like films may be deposited with a high concentration of CF 2 and light crosslinking. Contact angle measurements for hexane, hexadecane, toluene, cyclohexane, and DEO for the LCF films deposited at various rf powers. The highest contact angle of diesel engine oil was 84 for the LCF film deposited at 2 W of rf power. All liquids shown in this graph wetted the PE substrate before film deposition (θ = 0), except for DEO (θ = 17). Surface energy found by the Kaelble method, using hexadecane and glycerol as test liquids, for the LCF films deposited as a function of rf power. p. 231 p. 232 List of Figures 30

36 FIG. 89. Zisman plot showing the critical surface energy (γ c ) for the LCF films deposited at 2 W of rf power ( ) and 160 W of rf power ( ), as found by extrapolating the surface energy of four hydrocarbon liquids to the cos(θ) of their contact angle on the film surfaces. The film at deposited at 2 W of rf power showed a γ c of 2.7 mj/m 2, and the film deposited at 160 W rf power showed a γ c of 17.6 mj/m 2. p. 233 FIG. 90. Relationship between structure and surface energy as a plot CF 3 p. 234 and CF 2 as found by XPS with the surface energy from depositions with C 2 F 5 H/C 2 H 2, indicating importance of CF 2 species. For these film depositions, the rf power was 50 W, total FIG. 91. flow rate was 20 sccm, and deposition time was 5 min. CF 2 species appeared to be strongly correlated with decreased surface energy, more so than CF 3 species. XPS C(1s) region of the tape side of the peel surface of two long chain fluorocarbon (LCF) films deposited at rf powers of (a) 2 W, and (b) 160 W. The tape peel of the film deposited at 2 W of rf power revealed 26 % F, indicating poor adhesion. Also the peak at ev showed that the majority of the film species were p. 235 removed as CF 3. The tape peel of the film deposited with 160 W of rf power revealed 0.5 % F and showed excellent adhesion. List of Figures 31

37 FIG. 92. FIG. 93. FIG. 94. XPS C(1s) region of the substrate side of the tape peel test for adhesion of the long chain fluorocarbon (LCF) film deposited on PE with 2 W of rf power. The spectrum shown here was virtually identical to the original spectrum of the film, as shown in Fig. 83(a), indicating cohesive failure within the PPFC film. Comparing adhesion of the LCF films to PE, deposited with varying rf power, as estimated by the % F on the tape side of the peel surface after a tape peel test, where the films with a low % F on the tape surface adhered well. The trend showed increased film cohesion with increased crosslinking, due to the higher rf power. XPS survey spectrum of the tape side of the peel test of the multilayer film M5 deposited on industrial grade silicone rubber. This tape peel test revealed only 3.9 % F on the tape side of the peel surface. Atomic concentration values for three tape peel tests are shown in Table XIX. The average of the three tape peel tests on the film was 1.9 % F, showing excellent adhesion of the film to the silicone rubber. p. 236 p. 237 p. 238 List of Figures 32

38 FIG. 95. FIG. 96. FIG. 97. XPS survey spectrum of the tape side of the peel test of only the LCF top layer of M5, deposited directly on the industrial grade silicone rubber. This tape peel test revealed 58.6 % F on the tape side of the peel surface. Atomic concentration values for three tape peel tests are shown in Table XX. The average of the three tape peel tests on the sample was 54.3 % F, showing very poor adhesion without the multilayer film between the rubber and the fluorocarbon film. Dyed oil droplet test showing the extent of oil uptake on (a) the multilayer film M5 deposited on silicone rubber using the conditions outlined in Table VIII, compared to (b) the uncoated control sample. The samples were tested for 24 hrs at room temperature. No oil uptake shown for the coated sample. Dyed oil droplet test showing the extent of oil uptake on (a) the multilayer film M5 deposited on silicone rubber using the conditions outlined in Table VIII, compared to (b) the control sample. The samples were tested for 24 hrs at 100 C. There was slight oil uptake on the coated sample, pointed out by the arrow. p. 239 p. 240 p. 240 List of Figures 33

39 FIG. 98. FIG. 99. FIG Dyed oil droplet test showing the extent of oil uptake on (a) the multilayer film M5 deposited on silicone rubber using the conditions outlined in Table VIII, compared to (b) the control sample. The samples were tested for 24 hrs at 150 C. Oil uptake was extensive, though localized underneath the droplet region, as compared to the uncoated control sample. Dyed oil droplet test on a multilayer film M5 deposited on silicon rubber using the conditions outlined in Table M5 (a) After oil droplet placement, before test, (b) after 100 hrs at 100 C. Clean areas were targeted for placement of the oil droplets by looking under microscope and applying oil to regions where there were no visible flaws under 16 X magnification. An experiment similar to that shown previously in Fig. 99 was conducted. The multilayer film M5 was deposited on each swatch of industrial grade silicone rubber. Regions (a) without flaws and (c) with flaws were targeted with the dyed oil droplets. The flipside of each rubber swatch after 100 hrs at 100 C are shown in (b) and (d). The rubber swatch with droplets of oil on flawed regions showed more oil uptake than the swatch without. p. 241 p. 241 p. 242 List of Figures 34

40 FIG Dyed oil droplet test for oil uptake on the multilayer/lcf film M5 deposited on medical grade and food grade rubber, showing specimens (a) before and (b) after 100 hrs at 100 ºC. No evidence of oil uptake was shown, indicating the success of the M5 film deposited on high quality rubber in a static environment. p. 243 FIG Stretched oil uptake test on industrial grade silicone rubber. p. 244 FIG Silicone rubber swatches coated with multilayer film M5 were stretched to 5.9 % strain (simulating 31 % compression in use environment). Droplets were placed on the coated rubber and observed after (a) 3 hours and (b) 100 hours at 100 ºC. SEM image from the M5 coated industrial grade silicone rubber under 5.9 % tensile strain (simulating 31 % compression in the use environment). Images show cracking in the direction perpendicular to the imposed strain, indicating that this film was not applicable to environments with this amount of static strain. p List of Figures 35

41 I. OBJECTIVES The objectives of this research were twofold, with both commercial and scientific goals. The immediate, commercial objective of this research was to explore the usefulness of plasma polymerized fluorocarbon (PPFC) films for the prevention of the diffusion of oil into silicone rubber. O-rings made of silicone rubber are currently used as seals in the engines of heavy machinery due to their outstanding resistance to high temperatures and flexibility. The silicone rubber seals are currently used in an oily and hot environment. As a function of time and temperature, oil diffuses into the silicone rubber. When an O-ring swells with oil, it fails to meet the tolerance level specified for the part, and a leak can occur. Although silicone rubber seals are relatively inexpensive, the costs of replacement in terms of machinery downtime and labor costs are notable. The objective was to reduce the oil uptake to less than 2 % by weight of the O-ring, which would reduce the swelling to a range level which is acceptable and would not cause a leakage and need for replacement. Fluorosilicone rubber O-rings were another option for this part, but were much more expensive. The approach taken in this research was to deposit a film with a surface energy less than that of the engine oil. In this condition, oil could not wet the surface and would not diffuse into the rubber. However, this film must also be adhesive to the silicone rubber. The goal which was set by Caterpillar, the industrial sponsor for the project was for the coated O-rings to perform with less than 2 % oil uptake by weight, for 24 hrs at 100 C. This goal was then heightened to an oil uptake of less than 2 % for 100 hrs at 100 C. The knowledge gained through this research may also be applied to seal technology at lower use temperatures. Objectives 36

42 The Introduction section of this thesis extensively details numerous other commercial applications of imparting a low surface energy film on a surface. Therefore, the applicability of this research extends beyond this narrowed use for the heavy machinery industry. Broader and more academic-based objectives of this research included an investigation into the production of plasma polymerized fluorocarbon films in general. Specifically, effects of various monomers, both low molecular mass and high molecular mass were studied. Effects of various reactor parameters, such as the rf power and pressure were also studied. Another objective was to determine specifically what structure of plasma polymerized fluorocarbon films results in the lowest surface energy and the greatest cohesion. The issue of adhesion of a low surface energy film to many materials is not a trivial one. An investigation into how this may be accomplished solely using plasma techniques was another goal of this project. Objectives 37

43 II. INTRODUCTION A. Problem statement swelling of silicone rubber Polydimethylsiloxane (PDMS) or silicone rubber is frequently the material of choice for seals, such as O-rings, in the engines of heavy construction equipment. Silicone rubber is advantageous for a number of reasons such as the material can withstand high temperatures (up to 200 C), is resistant to oxidation, and has a relatively low cost. Silicone rubbers are typically mixed with crosslinking agents and fillers to produce a lightly crosslinked polymer. One drawback of this type of rubber is that the crosslinking causes the silicone rubber to swell when exposed to various solvents and oils. Since the silicone rubber O-rings used in the engines are in an oily environment, swelling occurs, and causes the part to leak. Therefore, the O-rings need to be replaced frequently when leaks occur. While this does not result in extensive material cost, the loss from machinery downtime and labor is notable. To examine oil uptake in silicone rubber materials, first the mechanisms by which a polymer swells from a penetrating liquid need to be examined. Diffusion of a liquid into a polymer will take place if the following occurs. Initially, the liquid must adsorb onto the surface of the polymer. In this application, since the O-ring is in an oily environment, the presence of the oil cannot be changed. Secondly, the oil must wet the surface of the polymer, meaning that the surface energy of the rubber must be greater than the surface energy of the oil. Wetting of the rubber by the oil may be prevented if the surface energy of the rubber is less than the surface energy of the oil. The surface energy of the oil cannot be changed for this application. Thirdly, once the oil passes the silicone rubber surface, the diffusion rate of the oil is dependent on the temperature and Introduction 38

44 the physical and chemical structure of the polymer, namely the molecular weight and the crosslink density. Other factors of diffusion into rubbers include the crosslink density, chain flexibility (determining free volume), and the molecular weight. For rubbers, the effect of fillers will alter the diffusivity, where increased filler content provides a more tortuous path for the solvent, reducing the diffusion rate 1. For example, Lawandy and Wassef concluded that the diffusion rate of motor oil into polychloroprene rubber decreased with the addition of carbon black 2. Many authors have studied and produced models for the diffusion of organics into rubber. However, studying the uptake of oil into rubber was not the aim of this project. Also, for this project, the bulk properties of the silicone rubber could not be altered, as this would affect the material s cost. Therefore, the focus of this project was to prevent wetting by lowering the surface energy of the silicone rubber below that of the oil. A similar method for decreasing the diffusion of volatile organic compounds has been shown by Mishima and Nakagawa by plasma grafting fluoroalkyl methacrylates to PDMS 3. B. Fluoroelastomers Currently, many fluoroelastomers are used as materials for seals, instead of silicone rubber. Because of the chemistry and structure of the fluoroelastomers, these materials impart high thermal and chemical resistance 4. Due to their low surface energy, fluorosilicone elastomers are one class of fluoroelastomers which are frequently used to seal parts in an oily environment 5, 6. These materials have a backbone of Si-O bonds, but have perfluoro- groups attached to the Si atom, which imparts oil-repelling properties to the material 4. O-rings and seals manufactured from this rubber are resistant to high temperatures (up to 200 C), and swelling by solvents, fuels, and oils. One drawback of Introduction 39

45 fluorosilicones is that they show limited abrasion resistance and are only used for static seals for this reason. Another common fluoroelastomer is Viton, manufactured by DuPont Dow Elastomers 6. This product is typically used for O-rings functioning in a dry chemical processing environment. The greatest drawback to these versatile polymers is the high cost, as they are far more expensive than conventional silicone rubber seals 6. Although, a fluorosilicone O-ring would perform well for this application, the focus of this research was to explore the performance of a fluorocarbon coating on silicone rubber. C. Fluoropolymers Fluoropolymers possess many desirable qualities including hydrophobicity, 7, 8 oleophobicity (oil-repellency), 4, 8, 9 low coefficient of friction, 7, 10 8, 11 low surface energy, electrical resistivity, 10 high melting point, 4, 11, 12 biocompatibility, 13, 14 inhibition of biofouling, 15 5, 7, 11, 12 and resistance to chemical and oxidative degradation. The unique characteristics of the fluorine atom give fluoropolymers outstanding properties. The bond energy of the C-F bond is remarkably high (488 kj/mol), as compared to the C-C bond (348 kj/mol). 8 Other bond energies relevant to this research are listed in Table I. 12, 16, 17 The uniqueness of the C-F bond is apparent not only by its bond strength, but also by the distribution of electrons. 12 The F atom is the most electronegative atom, imparting a highly dispersive intermolecular force to the C-F bond. 12 5, 10 Discovered accidentally in 1938 by R. J. Plunkett at DuPont, poly(tetrafluoroethylene) (PTFE) may be polymerized by a free-radical process from tetrafluoroethylene. PTFE is a linear polymer with a repeat unit of perfluoromethylene, CF 2. PTFE exhibits high molecular weights and some degree of crystallinity, due to the linearity of the polymer. PTFE is currently produced at 1 6 MPa and C with Introduction 40

46 a water-soluble radical initiator. 4 The polymerization is carried out by suspension polymerization with a dispersing agent and vigorous mixing. PTFE is extremely resistant to corrosion 7 and has excellent barrier properties against organic solvents. 18 There is no known solvent which will dissolve, or even swell, PTFE (below 300 C). 7 Because of these properties, fluoropolymers are often used as gaskets, vessel liners, and coatings for cookware and fabrics. D. Fluorocarbon films Applications for fluorocarbon polymers are diverse and widespread due to their unique properties, as outlined in the previous section. Since many of these properties are surface phenomena (coefficient of friction, biocompatibility, surface energy), a fluoropolymer coating could impart these surface qualities to other polymers, while retaining bulk properties. 19 Fluoropolymers, in general, are more expensive to synthesize in a bulk form than other polymers. 11 Therefore, a less expensive polymer with a fluorocarbon coating is a cost-effective alternative for many applications. 11 This was the driving force of this project to impart a fluoropolymer coating on an inexpensive silicone rubber O-ring, which could combine the desirable bulk properties of the silicone rubber with the desirable surface properties of a fluoropolymer coating. Currently, fluorocarbon coatings are used as linings of fuel tanks in automobiles. 18 Fluorocarbon films also have one of the lowest dielectric constants of a 4, 8, 20 dense material ( ), which makes them useful in microprocessors. Fluorocarbon films have shown promise for low coefficient of friction coatings on various biomaterials, such as the lead wires for pacemakers 13 and artificial heart valves. 14 Introduction 41

47 Fluorocarbon films have been shown to greatly decrease platelet adhesion to polyethylene terephthalate (PET), in another biomedical application. 21 Many of these coatings show promise, but have not yet been commercialized due to processing challenges. There are many obstacles to producing fluoropolymer coatings. Since the melt viscosity is very high, PTFE coatings cannot be produced by conventional melt processing techniques. 4 Films are usually produced from pressing and sintering PTFE powder or solvent spin coating. 22 However, the sintering powder technique is complicated, may leave pinholes, and is difficult to process in a reproducible manner. 23 Solvent spin coating may result in films with less thickness control, and it is desirable to limit solvent use, if possible. Some surface fluorination of polymeric surfaces may be accomplished using fluorine gas, but this method is undesirable due to the toxicity of F 2 gas. 4 Therefore, producing films using plasma deposition is a viable alternative to conventional fluorination techniques. E. Plasma deposition Due to the difficulties cited in the last section on fluoropolymer film production, alternative methods of film deposition have been investigated. Plasma deposition of fluorocarbon films has been studied extensively in the past 20 years, as there are many advantages to this type of deposition method when compared to conventional techniques. Advantages of using plasma include processing at low temperatures and fluorination without the use of harmful solvents or F 2 gas. 5 Plasma deposition also avoids the use of solvents, and lowers the environmental impact of methods such as spin coating. Plasma deposition also gives better control of film thickness than spin coating. Introduction 42

48 A plasma is also known as a glow discharge, and is generated by applying an electric field to a volume of gas typically at low pressure. 8 This causes excitation of electrons, ions and neutral species within the plasma, in much the same way as chemical vapor deposition does using high temperatures. Instead of the high temperature providing the energy, the electric field provides the energy to ionize the gas. Plasmas can be used for different types of surface functionalization, namely film deposition, surface treatment, or etching of the substrate. The focus during the course of this research was on the film deposition which is a physical growth of a new material on the substrate from the fragmentation of a gaseous precursor in the plasma. This process is also often referred to as plasma polymerization. However, an actual, clearly-defined polymerization mechanism, as cited in conventional polymer science texts, frequently does not occur in the plasma environment. Typically, when chemical bonds in a precursor are dissociated by the plasma, the recombination of these species to produce a film is random and undefined. The structure of a plasma polymer is also unlike those of conventional polymers since there are few chains with a repeat unit. Therefore, plasma polymers typically form a 3-D, crosslinked structure. 24 In rare cases, a free-radical, conventional polymerization method may produce a more linear plasma polymer. However, this is unlikely and requires precise reactor conditions and unique monomers. Throughout the literature, the terms deposition and polymerization are often used interchangeably, though by definition, are different. The starting material is also termed both a precursor and a monomer, though the use of monomer implies a true polymerization mechanism. In this thesis, the terms deposition and polymerization and precursor and monomer are used interchangeably as well. Introduction 43

49 The second type of plasma processing is plasma treatment and refers to the change in surface chemistry or the grafting of functional groups onto a surface, without the production of an actual film. 14 The third type of plasma processing is etching, where the impact of ionized species causes the substrate to be physically removed. 14 The surface may become rough and/or pitted due to differential etching if the material is heterogeneous. The etched material may also recombine with other species in the plasma phase and be redeposited. FC plasmas are frequently used to etch dielectrics for the semiconductor industry. 25 A wide variety of plasma reactors have been designed and their types vary in terms of reactor geometry, reactor volume, and frequency of the glow discharge 26 Common frequencies used to excite a plasma are in the radio frequency (rf) and microwave range. 14 Plasma reactors typically operate at low pressure with the use of a vacuum pump, and gases may be flowed through the reactor to carry out the desired process. Specialty plasma reactors have been designed such as continuous plasma reactors for processing webs of polymeric material 26 and fluidized bed reactors for coating high density polyethylene powders with fluorocarbon species. 27 Numerous reactor variables can affect the film deposition process. The main parameters include the composition and flow rates of the monomer(s) and carrier gas(es), the reactor geometry, the frequency and amount of power, the base pressure and processing pressure of the reactor chamber, and the deposition time. In addition, variables such as the substrate temperature, the pretreatment of the reactor chamber, and the substrate placement within the reactor may also have an effect on the properties of the film. 28 In general, the higher the energy flux, the more crosslinked and dense the film. Introduction 44

50 However, if the energy flux is low, the structure of the monomer is more likely to be retained. 28 When comparing plasma polymerized fluorocarbon (PPFC) films from various sources in literature, difficulties arise since so many reactor parameters can affect the properties of the films. Therefore, it is often difficult to compare PPFC films deposited with the same monomers since identical reactor parameters are rarely encountered in literature. F. Reactor types for plasma polymerized fluorocarbon (PPFC) films The majority of reactors used in literature for the deposition of fluorocarbon films are parallel-plate, capacitively-coupled rf reactors. Very few microwave-powered reactors are used. In one case, a PPFC film was produced in a microwave reactor from the copolymerization of CF 4 and H Upon varying the CF 4 /H 2 ratio, many types of films with various structures were produced in a similar manner as the studies using rf 8, 29 reactors. Therefore, the widespread use of rf-powered reactors for the production of FC films appears to be due simply to the popularity and simplicity of rf-powered reactors. G. Fluorocarbon plasma chemistry An excellent review of the literature on plasma polymerization has been written by d Agostino, where a chapter is devoted entirely to the chemistry of fluorocarbon plasmas and the effect on the properties of FC films. 8 A typical monomer for a PPFC film may be a fluorocarbon gas, such as hexafluoropropylene (C 3 F 6 ). Upon excitation of a FC monomer in a plasma, two main active species are present: CF x radicals (where x = 1-3) and F radicals. The CF x radicals (where x = 1-2) are considered to be the species that result in the initial formation and growth of a PPFC film. Therefore, CF x radicals are referred to as the building blocks of the film, and are sometimes refered to as true Introduction 45

51 monomers. 8 F radicals tend to bombard the surface heavily and promote etching, or fluorination, of the substrate. F radicals may also recombine with the CF x radicals in the gas phase and lead to saturated FC species (such as CF 4 ) which may not contribute to the growth of a film. 8 depositing species. 8 CF 3 + species have also been shown to be an etching, rather than a Therefore, in most FC plasmas, etching and deposition occur simultaneously and are competing mechanisms. Coburn and Winters have shown that there is a boundary between the conditions at which a FC plasma will etch or deposit, and this was shown to be roughly approximated by the F/C atomic ratio of the precursor. 30 Typically, a monomer with a F/C ratio greater than 3 will tend to etch the substrate, rather than deposit a PPFC film. 30 Monomers with a F/C ratio less than three, will be more likely to deposit a film. This approximation was shown for a plasma reactor operating with an RF power of 100 W, a moderately powerful plasma power. 30 Varations in rf power may alter this boundary, as higher powers will tend to shift the process towards etching. A more accurate indication of depositing or etching ability of the precursor may be the actual ratio of active species in the plasma, such as the [F]/[CF x ] value. 8 The concentration of F and CF x radicals can be altered based on the monomer type, monomer flow rate, the applied power, the pressure and the reactor geometry. 8 The F and CF x species may be measured within the plasma by spectroscopic techniques, such as time-offlight mass spectroscopy (TOFMS) and optical emission spectroscopy (OES). Typically when plasma polymerizing a film using a low molecular weight monomer, such as C 2 F 6 or C 3 F 6, the precursor is fragmented into many different species, such as CF, CF 2, CF 3, and F radicals. 8 These species may recombine to produce a film, and/or they may etch the surface. The resulting film from this type of deposition may Introduction 46

52 contain a structure that is very different from the starting monomer. However, when a monomer with a higher molecular weight is used while controlling the chemistry by using low or pulsed power, the film that results may be very similar in structure to that of the original monomer. Studying these types of films is important since the greatest drawback to the potential viability of PPFC films is the lack of control over chemistry of the plasma. Due to the lack of structural control, PPFC films currently have limited use in commercial applications. 19 H. PPFC films deposited with low molecular mass monomers There are many types of monomers currently used to deposit fluorocarbon films and these may be classified in a few different categories. Conventionally, plasma polymerization of fluorocarbon films has been accomplished using small molecular mass monomers, such as hexafluoroethane (C 2 F 6 ), 8, 31 8, 32 hexafluoroproplyene (C 3 F 6 ), trifluoromethane (CHF 3 ), 18, 33, 34 tetrafluoroethane (C 2 H 2 F 4 ), 35 pentafluoroethane (C 2 F 5 H), 20 hexafluoroacetone (C 3 F 6 O), 36 and octafluorobutylene (C 4 F 8 ). 37 Fluorocarbon plasmas have been produced using saturated fluorocarbons, such as tetrafluoromethane, 38, CF 39 8, 39, 40 4 and C 2 F 6. Since these monomers have a F/C ratio greater than or equal to three, plasmas ignited with these monomers typically etch, rather than deposit, and technically should not be referred to as monomers when considered alone. 30 This etching mechanism is due to a high concentration of excited F radicals heavily bombarding the sample. Although these saturated FC monomers will etch a surface, they can also fluorinate a surface. This mechanism is not one of film deposition, but simply a grafting of F to the surface. Introduction 47

53 Conveniently, the plasmas formed from saturated FC monomers may be forced 8, 40 towards a depositing condition by the addition of H to the feed gas. Masouka et al. and d Agostino et al. discovered a sharp increase in deposition rate with an increase of H 2 8, 40 in the feed of C 2 F 6. H readily bonds to F and the HF molecule is eliminated from the plasma region by the vacuum environment and pumped out of the reactor. A schematic of this mechanism in a typical FC plasma environment is shown in Fig. 1. Due to its high bond strength (564 kj/mol), the H-F molecule is very stable, much more so than other chemical bonds in a FC plasma, as shown in Table 1. 5 The mechanism of HF elimination 8, 20, 30, 33, 41 is well-established in the literature by various research groups. For example, Agraharam et al. found using mass spectroscopy analysis of the residual gas from the plasma that excess F was scavenged as CF 4 and HF from a mixture of C 2 F 5 H and Ar. 20 d'agostino has also shown the effect of F scavenging using optical emission spectroscopy. 8 Senkevich et al. not only showed an increase in deposition rate, but also an increase in the refractive index with the addition of H 2 to a feed of CF 3 H. 33 This indicated that with higher feed concentrations of H 2, the film was more highly crosslinked and contained less fluorination. 33 Conversely, the addition of a saturated monomer (C 3 F 8 ) to the feed of an unsaturated monomer (C 3 F 6 ) will turn the plasma towards an etching mode, resulting in a lowered deposition rate. 42 As another source of hydrogen, a hydrocarbon gas may be added to the feed along with the fluorocarbon monomer. Wang et al. have investigated the effect of adding CH 4 to the feed of saturated fluorocarbon monomers, which resulted in a rise in deposition rate. 39 Inagaki et al. have studied the effect of the addition of ethane (C 2 H 6 ), ethylene (C 2 H 4 ), and acetylene (C 2 H 2 ) to CF 4 plasmas. 43 Also, Golub et al. have investigated the Introduction 48

54 effect of copolymerization of C 2 H 4 and tetrafluoroethylene (C 2 F 4 ). 44 These researchers have shown that the addition of a hydrocarbon to a saturated fluorocarbon also led to an increase in deposition rate of the PPFC film, due to a relative increase in CF x species, 8, 41 compared to F radicals, which resulted to film formation. Researchers have also examined fluoroalkyl monomers, with H and F atoms present in the same molecule, such 33, 35, 36, 45 as CHF 3, C 2 H 2 F 4, and C 2 F 5 H. These monomers have been shown to polymerize readily as well, due to the presence of H in the molecule. Unsaturated monomers composed of only fluorine and carbon have also been used for deposition. Unlike saturated fluorocarbons, these monomers tend to deposit quite readily due to the presence of the highly reactive double bond. Samukawa and Mukai have shown that in fluorocarbon monomers, the C=C bond is five times more likely to dissociate than the C-C bond and this has been found to occur even with higher molecular mass monomers. 32 With the C 3 F 6 monomer, the C=C bond ruptures preferentially, leaving a CF 2 group, which is useful for film growth. Much research has been done on the polymerization of unsaturated fluorocarbon monomers, such as hexafluoropropylene (C 3 F 6 ) 16, 39 and tetrafluoroethylene C 2 F Aside from the propensity for deposition, there are environmental advantages to using unsaturated fluorocarbon monomers for film production. During plasma processing, some monomer vapor always exits the exhaust vent from the pump in an unreacted state. This can happen either while gases are flowing in the reactor chamber before the plasma is ignited, or upon venting of the chamber. Saturated FC monomers, such as CF 4 and C 2 F 6, have been shown to have very high global warming potentials (GWP) ( ) and lifetimes of approximately years in the Introduction 49

55 atmosphere. 46 Many semiconductor companies are currently searching for alternative etchants other than C 2 F 6 for microprocessor fabrication because of this environmental concern. In contrast, unsaturated monomers, such as C 3 F 6 or C 5 F 8 have much lower GWPs (~100) and they survive in the atmosphere at most around 10 yrs. 46 I. Multilayer films PPFC films have shown unsatisfactory adhesion to many substrates, including polymers such as silicone rubber. The issue of adhesion was a great concern over the course of this research. Chemical bonding does not occur at the interface of silicone rubber and fluorocarbon films. PTFE coatings are currently most notably used in the cookware industry for anti-stick surfaces. 4 This coating is forced to adhere to the metal by roughening the surface and applying a primer. Then, the PTFE powder is applied and heated, and the process is repeated. The PTFE layers are mechanically interlocked with the primer, which adheres well to the metal. Therefore, this is a physical interlocking mechanism of adhesion, and a commonly used method to achieve adhesion of a polymer to a substrate. There are other methods to induce adhesion of the PPFC film to a surface. Srividya et al. deposited a PPFC film onto a polysilicon layer on stainless steel using a multilayer hydrocarbon and fluorocarbon film for purposes of adhesion. 47 However, no direct adhesion testing was shown by Srividya et al. 47 Multilayer films have been deposited for many purposes, along with improving adhesion. 48 A low surface energy (19 mj/m 2 ), multilayer coating was applied to a metal using C 3 F 6 O and C 2 H 2 with increasing amounts of hexafluoropropylene oxide (C 3 F 6 O) in each subsequent layer. 45 A multilayer PPFC film was deposited using CF 3 H and C 2 H 4 Introduction 50

56 for reducing toluene permeation through PE. 29 Again, the degree of fluorination was increased and the surface energy was lowered with each subsequent layer. 29 In a study by Walker et al., an electron cyclotron resonance (ECR) reactor was used to decrease the permeation of toluene by a factor of 100 with the application of a single fluorocarbon layer. Remarkably, when a multilayer coating was applied, the permeability decreased by X. 45 J. Hydrocarbon films Plasma polymerized hydrocarbon films (PPHCs) have been deposited previously by many researchers. These films were investigated in this work as the first layer of the multilayer film to increase adhesion of the PPFC film to the silicone rubber substrate. Retzko et al. have studied the deposition rates of C 2 H 2, C 2 H 4 and butadiene (C 4 H 6 ). 49 One finding by Retzko et al. was that the degree of unsaturation present in the HC monomer directly influenced the deposition rate. They showed that alkynes possessed a higher deposition rate than alkenes, and alkenes displayed a higher deposition rate than alkanes. 49 C 2 H 2 has one of the highest deposition rates of HC monomers, and was shown to be readily polymerizable using plasma, due to the highly dissociative triple bond. Tsai and Boerio have also examined PP acetylene films thoroughly, for use in improving rubber-to-metal bonding. 50 K. Perfluoromethylene (CF 2 ) films using low molecular mass monomers PTFE-like films, where the structure was predominantly composed of perfluoromethylene (CF 2 ) species, have been produced using low molecular mass monomers. Specifically, these films have been deposited using C 3 F 6 O in a pulsed plasma 13, 36 or by remote plasma, where the substrates are positioned downstream from the glow region. 36 Pulsed plasma methods have been explored as a method to increase Introduction 51

57 CF 2 content in the PPFC film, and have resulted in films with up to 80 % CF 2 species. 36 Mackie et al. also obtained films dominated by CF 2 using pulsed C 2 F 6 /H 2 plasmas, with long off times. 51 Pulsed plasma films deposited using perfluorocyclohexane showed a predominance of CF 2 species as the plasma off time increased. 52 Yet another researcher, Limb et al., also produced a film containing 65 % CF 2 using a pulsed plasma of C 3 F 6 O. 13 Other films deposited using CF 4 and H 2 in an ECR microwave plasma have shown up to 85 % CF 2 and resulted in surface energies reported to be as low as 4.2 mj/m Grainger et al. have also performed some interesting studies with the remote deposition of C 2 F 4 films. 19 When using remote deposition, films could be produced with a structure predominantly of CF 2 (up to 90%). 19 Interestingly, they also showed using angle-resolved XPS and near-edge x-ray absorption fine structure (NEXAFS) that the CF 2 groups were organized in chains perpendicular to the substrate, terminated by CF In contrast, films deposited directly in the glow discharge region showed a more crosslinked, less CF 2 -dominated structure, with no orientation. L. Perfluoromethylene (CF 2 )-dominated films using high molecular mass monomers Plasma polymerized fluorocarbon (PPFC) films with a structure predominantly composed of perfluoromethylene (CF 2 ) groups have also been produced using higher molecular mass momomers. 53,54 These films were deposited using monomers such as 53, 54 1H,1H,2H-perfluoro-1-dodecene, and 1H,1H,2H,2H-heptadecafluorodecyl acrylate. In this document, these types of monomers are referred to as long chain fluorocarbon (LCF) monomers, and the films which result from their polymerization, are termed LCF films. These films have shown to be highly successful at repelling hydrocarbon liquids, such as pentane and hexadecane, an indication of oleophobic behavior. 53 One possibility Introduction 52

58 of the oleophobic behavior may be the presence of CF 2 groups in a chain perpendicular to the substrate and terminated by a CF 3 species. 53 Banks et al. claimed that a terminal CF 3 group on a surface was not sufficient for low surface energy and oleophobicity, but rather the CF 2 chain beneath it was necessary for vertical alignment of the CF 3 group towards the surface. 5 Orientation of the CF 2 chains on the surface was cited as the reason for the oleophobicity of these films. 53 M. CF 2 -dominated films prepared using techniques other than plasma Long chain fluorocarbon (LCF) films have been produced on surfaces using techniques other than plasma polymerization and studied for their extremely low surface energy. For example, films deposited using LCF monomers have also been deposited using chemical vapor deposition (CVD) for adhesion control in microelectronic. 55 In another case, a silane-coupling agent with a LCF chain of ten carbon atoms long was used to produce a plaque-controlling surface, for the inhibition of bioadhesion. 56 Fluoroalkylsilanes were also studied by Hozumi et al., where the contact angle of water on these thin films was Interestingly, the contact angle of water increased as fluoroalkylsilanes with longer perfluoroalkyl groups were used. Also, in another study, fluoroalkyl disulfides with long fluoroalkyl chains assembled on gold substrates gave higher water contact angles than shorter chains. 58 Finally, in an experiment by Zisman, the lowest surface energy he reported was 6 mj/m 2, found for a monolayer of CF 3 (CF 2 ) 10 CO 2 H on platinum. 59 On this surface, CF 3 groups would have been aligned vertically towards the surface. Therefore, there are many examples in literature where LCF films show promise for producing extremely low surface energy coatings, due to structural alignment of the films. Introduction 53

59 N. Contact angle measurements and surface energy Low surface energy surfaces are useful for many applications, including hydrophobicity, oleophobicity, and inhibiting adhesion. 55 Biofouling may also be prevented. Barnicles are unable to attach to ship hulls with surface energies less than 12 mj/m Wetting behavior and surface energy are properties that are characterized frequently for PPFC films, since the films are typically hydrophobic and can exhibit low surface energies. In this research, the lowering of the surface energy of the silicone rubber by the deposition of a PPFC film was crucial. Therefore, the concept of surface energy is central to this research and needs to be thoroughly addressed. Surface energy is the free energy of a solid per unit area and is typically expressed in mj/m 2. Some other units which are equivalent, although used less frequently in the 60, 61 literature, are mn/m or dynes/cm. It is important to note here that surface energy is, as its name implies, a surface property and not necessarily a bulk property. 12 Low surface energies are a consequence of weak intermolecular forces in materials. A commonly used method to analyze surface energies is through measurement of contact angles of liquids on surfaces. 12 The contact angle is the angle between the edge of a liquid droplet and the surface measured in the direction toward the center of the droplet. The contact angle measurement is a simple, fast and direct indicator of the strength of the interaction between the liquid and the solid surface. If the contact angle is 60, 61 zero, then the liquid is referred to as wetting the surface. A droplet of water (with a high surface tension and strong polar forces) will rest on a PTFE surface (with mostly dispersive forces) with a high contact angle. This is because the intermolecular interactions between the two substances are weak, and energy would need to be applied Introduction 54

60 to the system to maximize the interface. 12 A large interfacial tension is present resulting in the minimization of contact area between the liquid and the solid two substances, resulting in a high contact angle of the dropled. 12 This concept of wettability can also be thought of as a solubility issue of like materials. The term like materials implies the concept of similar intermolecular forces. Unlike materials seek to minimize the free energy in the interfacial region, which is accomplished by minimizing the contact area. 12 Figure 2 shows a schematic of a surface with high surface energy and a surface with low surface energy, with two droplets of water. The water droplet has a high contact angle (θ) when resting on the low surface energy material and a low contact angle when resting on the high surface energy material. The Young-Dupré equation, which describes the relationship between the surface energy of the solid, liquid, and vapor interfaces with the contact angle of the liquid 60, 61 droplet, is as follows: ( θ ) ( γ γ ) sv sl cos = (1) γ lv In this expression, γ sv, γ sl, and γ lv are energies at the solid-vapor, solid-liquid, and liquidvapor interfaces, and θ is the contact angle that the droplet makes with the surface (Fig. 2). The contact angle may be reported in literature as advancing (swelling droplet), receding (shrinking droplet), or sessile (stationary droplet). Contact angles are typically analyzed using a contact angle goniometer, in which the liquid droplet rests on the sample surface, where it is illuminated and magnified through an eyepiece. The contact angle is measured by manually rotating a protractor inside the eyepiece. This measurement is usually done visually, by reading the angle on the protractor. However, Introduction 55

61 some goniometers have the capability to determine θ digitally using a video contact angle measurement device, which may be more precise and automated. 62 The surface energy has been described as being composed of two types of physical attractive forces, polar and dispersive. 61 Polar forces arise from electric dipoles and the effect of these dipoles on polarizable molecules. 61 Dispersive forces arise from internal electron movements, independent of dipole moments, and are generally weaker than the polar forces. 61 The polar (γ P s ) and dispersive (γ D s ) components of the surface energy may be related to the contact angle through an expression developed by Owens and Wendt: 61 ( ) ( ) ( ) ( ) D 1/ 2 s lv D 1/ 2 lv P 1/ 2 s P 2 γ γ 2 γ γ lv 1 + cosθ = + (2) γ γ lv 1/ 2 This equation is also known as the Kaelble equation from the work by Kaelble and Uy, and is also sometimes also referred to as the geometric mean equation. 61 Calculating the polar and dispersive components of the surface energy of a material is relatively simple when using contact angle data of two liquids with known surface energies. From Eq. (2), a set of two equations (one for each liquid) can easily be solved for the two unknowns, γ D s and γ P s. The total surface energy for the material is then simply the sum of these values, as shown below in Eq. (3): 61 γ = γ + γ (3) s D s P s For example, the surface energy of bulk PTFE is 19.1 mj/m 2, where γ s D = 18.6 mj/m 2 and γ s P = 0.5 mj/m 2. 5 For PPFC films, the surface energy depends greatly on the film chemistry and structure, which depends on reactor parameters and the monomer. For most PPFC films, surface energy typically ranges from 9.7 to 15.5 mj/m 2 29, 43. Introduction 56

62 Another method to compare surface energies of materials is using the Zisman critical surface energy method. 59 The contact angles of liquids with the test surface are measured using liquids with known surface energies. Then a plot is generated of cos(θ) vs. γ lv, and the γ lv value where cos(θ) = 1 gives the critical surface energy of the substrate. The value is known as the "critical" surface energy because any liquid with a lower γ lv will wet the surface. Liquids with higher surface energies will not wet the surface. Zisman showed that this method was valid and produced a linear relationship for a variety of liquids, but the value which results is purely dispersive in nature. 59 Zisman s prediction of surface energy is not valid when strong hydrogen bonding or acid-base interactions occur. This method is a useful method to compare surface energies of many polymers and PPFC films, since these materials contain predominantly dispersive forces. Therefore, when nonpolar surfaces are studied, the Zisman method gives good agreement with theory. 59 The critical surface energy is an empirical value, and does not have a physical meaning. However, it may be used to compare wettability of surfaces with each other. 63 Roughness of a surface can also alter the contact angle, resulting in the calculation of surface energies for surfaces with different degrees of roughness. 60 This was determined by Wenzel, Cassie, Shuttleworth and Bailey, and was shown by the following equation: 53 ( θ ) ( cos ) cos measured = R θ true (4) Equation (4) indicates that if the absolute contact angle (θ true ) on a perfectly smooth surface is greater than 90, then a surface with a higher degree of roughness R will result in a higher measured contact angle (θ measured ). Conversely, if the θ true on a smooth surface Introduction 57

63 is less than 90, then a rougher surface will cause the θ measured to decrease. 60 If the surface is so rough that air is trapped between the solid and the liquid, then more complex theories need to be considered. 60 This concept of the degree of roughness altering θ has been applied to adhesion mechanics. An adhesive on a surface with high surface energy will wet the surface more when roughened, and result in better adhesion. The opposite effect has been shown with certain embossed polymers produced specifically for water repellency. This effect has also been shown to exist in nature and is known as the Lotuseffect, named after the flower which exhibits the phenomenon. 64 The leaves of the Lotus plant are highly hydrophobic, but this is not entirely due to the surface chemistry of the leaf, but due to a combination of the surface chemistry and the microroughness present which inflates the contact angle. 64 Even the feathers of a bird have notches that result in a heightened water contact angle for repellency. 60 Contact angles of water around 150 are typical for ridged feathers, whereas without the ridges, the contact angle of water is only around Sato et al. have also shown the effect of increased surface roughness resulting in inflated water contact angles in studies of PPFC films deposited using vinylidene fluoride. 65 The increased roughness was due to powder formation in the PPFC which inflated the contact angle of water. 65 O. Methods to find surface tension of a liquid In this work, in order to determine the critical surface energy of the silicone rubber for prevention of oil uptake, an analysis must be done of the surface tension of the oil. Most oils display a change in surface energy with temperature, where the surface energy decreases with increasing temperature. 60 This occurs due to a decrease in density, Introduction 58

64 or molar volume of the liquid, with temperature, due to an increased kinetic energy from molecular motion. The most common and simple method to measure the surface tension of a liquid is using a Du Noüy tensiometer. 60 Using the Du Noüy method, a horizontal loop of wire is submerged in the liquid, and the force necessary to displace it from the liquid/air interface is measured. This analytical method is a type of surface tension measurement known as a detachment method, of which the Wilhelmy slide and drop weight are also members of this group. Equation (5) has been used to calculate the surface tension of a liquid using a Du Noüy tensiometer. 60 γ ideal ( W W ) total i ring =, (5) 4πD where W total and W ring are the total weight and the weight of the ring, respectively. D i is the inner diameter of the ring. This results in a calculation of γ ideal, the ideal surface tension. Then, a correction factor, f, is necessary to calculate the real surface tension, γ real, as shown below in Eq. (6). 60 γ real = f ( γ ideal ) (6) According to Adamson, the theory behind the need for a correction factor is quite complicated, but has been worked out by a few authors, showing the error between theory and experiment to be about 0.25 %. 60 P. Characterization techniques 1. Fourier transform infrared (FTIR) spectroscopy PPFC films are typically characterized using surface analysis techniques, as are many plasma polymerized films. Fourier transform infrared (FTIR) spectroscopy is a Introduction 59

65 useful characterization technique for many plasma polymers. When a film is exposed to IR radiation of many wavelengths, it is absorbed by the sample at discrete wavelengths due to the vibrational and rotational modes of the chemical bonds present in the film. 66 Several different techniques may be used to characterize films by FTIR. Films may be deposited on polished metals and analyzed by reflection-absorption IR (RAIR). Since this is a reflection experiment, the spectrum is typically subtracted by a background of the reflective substrate prior to film deposition. This results in a spectrum with peaks only due to the film, without interference from peaks due to the substrate. Also through the use of RAIR, some understanding about the orientation of chemical groups may be obtained. The absorbance intensity of a band in RAIR is proportional to the square of the dot product of the transition dipole moment vector, M r, and the electric field vector, E r 66, 67. r r I E M (7) In RAIR, the electric field vector is positioned perpendicular to the film or substrate surface. Therefore, if the transition dipole moment vector from the vibrational mode detected by IR was also perpendicular to the substrate surface, it would appear at a heightened intensity than if it were parallel or angled to the surface. Films may be deposited on soft materials (such as polymers) and analyzed by attenuated total reflectance (ATR) IR. 66 In this analysis mode, a crystal prism with a high refractive index physically contacts the surface of the material. Absorption bands are detected from those corresponding vibrational modes within the surface of the material contacting the prism. The penetration depth of the IR and the analysis depth of the Introduction 60

66 surface depend on the refractive index of the prism and the material by the following equation: d p = 2πn 2 sin n θ s n c λ c (8) where d p is the depth of penetration, θ is the incident angle of the IR radiation, and λ is the wavelength of the radiation in the prism. n c and n s are the indices of refraction of the crystal and the material, respectively. When studying thin films using ATR, bands typically appear due to both the film and the substrate. The intensity of the bands due to the film and those due to the substrate are directly dependent on the film thickness and the refractive index of the prism (related to d p ). Yet another method to study thin films via IR is to deposit the film on a KBr disc and acquire the spectrum via transmission IR. A pure spectrum of the PPFC film may be obtained since KBr contains no absorption bands in the IR region. The film may also be scraped off of a substrate and mixed with KBr of a fine grain to make a pellet for analysis. 66 Transmission IR may also be useful for the investigation of liquids such as the diesel engine oil (DEO). The IR of DEO may be gathered by mixing the liquid with KBr and performing transmission IR analysis, or by placing a droplet of the liquid on a KBr pellet. 2. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) is a surface characterization technique that has also been shown to be slightly more useful than ATR or RAIR spectroscopy for the analysis of the structure of PPFC films. The photoemission process which occurs during an XPS experiment is represented by an energy-level diagram of atomic orbitals Introduction 61

67 (see Fig. 3). The black circles represent electrons, and the unfilled circles represent vacancies in an atomic orbital. Figure 3(a) shows the initial, filled state of the atom, with an x-ray photon approaching. The energy of a photon can be expressed as hν where h is Planck s constant (6.62 x J s) and ν is the frequency of the radiation. If hν is greater than the binding energy of the electron to the atom, then the electron will be ejected from the orbital. Fig. 3(b) shows the release of a core electron (K shell) to the vacuum. As shown in Eq. (9) by Einstein in 1900, hν represents the energy of the x-ray beam, BE is binding energy of the electron to the nucleus of the atom, and KE is the 68, 69, 70 kinetic energy of the detected electron when it is ejected and reaches the detector: BE = hν KE (9) In a typical XPS experiment, hν is known, and the KE of the detected electrons is measured. Therefore, the BE is easily found using Eq. (9). Photoelectron peaks on the binding energy scale are discrete for each atom and correspond to electrons from unique atomic orbitals. Therefore, the binding energy peaks in an XPS spectrum are specific for each atom. After photoemission, which results in an electron vacancy within the atom, another process occurs known as the Auger electron emission. An electron from a higher energy level falls to a lower energy level to fill the vacancy from the ejected photoelectron. 68 Immediately after this occurrence, due to the energy gain of the atom, another electron must be lost, and is ejected. Fig. 3(c) shows the Auger electron emission process with the relaxation of the atom by a L 2,3 electron falling to the core energy level, K, with simultaneous emission of an electron from the L 2,3 orbtal. 68 Therefore, this Auger electron would be referred to as a KLL electron since the original vacancy was from the Introduction 62

68 K shell, an electron fell from an L shell to fill the K, and an electron from the L shell was ejected. Auger electrons are detected in an XPS experiment as well as the photoelectrons, since only kinetic energy of electrons is measured. Since the Auger emission is a secondary process, Auger electrons have a fixed KE, which is independent of hν. Therefore, on a typical XPS binding energy scale, peaks due to Auger electrons will shift if the x-ray photon energy is changed due to Eq. (9). XPS is a more surface sensitive technique than ATR, resulting in the analysis of the uppermost 10 nm of the material. 69 Since the mean free path of electrons in solids is small, the only electrons that escape to the surface and are detected arise from up to 10 nm within the surface. Since contact angle and surface energy are the dependent almost entirely on the chemistry of the first few atomic layers, XPS is an ideal technique to use for correlating structure and surface energy effects. Secondly, a more detailed description of the type of C-F bonding may be gained by using XPS rather than IR. Nearly every element can be detected by XPS, with the exception of H and He, since they lack an inner atomic shell. For polymeric analysis, one of the limitations of XPS is the lack of detection of the presence of H or H-bonding. However, using IR spectroscopy in conjunction with XPS allows for a greater understanding of the structure of PPFC films, since IR is sensitive to bonds involving H. Therefore, researchers that use IR in conjunction with XPS to study PPFC films are at an advantage to fully understanding the structure of these films. 33 Also, using quantitative analysis of the areas under the core peaks, the atomic concentration of the surface may be determined. The binding energy of a core electron is also very sensitive to the chemical state. For example, if highly oxidizing or electronegative atoms are bonded to carbon, this will Introduction 63

69 result in peaks in the spectrum due to C, but will appear slightly shifted in binding energy. Analysis of chemical shifts is also extremely useful since they, like the core binding energy peaks, are discrete, known, and vary from 0.1 ev up to 10 ev. Since F is such an electronegative element, large chemical shifts in the binding energy of the C(1s) region of the XPS spectra are present when F is bonded to C. This results in many well-defined peaks due to both primary F atoms and secondary F atoms 8, 71, 72 (those in the β-position). Binding energies and their assignments for peaks in the 8, 71, 72 C(1s) spectrum which are shifted due to fluorination are shown in Table II. The underlined atom in the table is the atom from which the electron is originating. Note that the peak at ev in Table II is labeled as C-C or C-H, since the electrons resulting from these bonding situations are identical. Peaks with higher binding energies than the C-C peak have more fluorine atoms adjacent to them or in the β-position. The β- substituted fluorinated carbon peak at ev (C-CF) is an indication of the degree of crosslinking of a PPFC film, especially when the C-C peak is not present. 33 Symmetric peaks placed at these binding energies with varying intensity may be used to represent the composite curve from the C(1s) orbital using a process known as curve-fitting. This results in quantitative information on the types of bonding present. If there is significant oxygen incorporation into the films, peaks may also appear at 286.1, 287.4, and ev in the C(1s) spectra. These peaks are due to C-OR and/or C-OH, C=O, and O-C=O and/or HO-C=O, respectively, also shown in Table II. 71 Therefore, by combining atomic concentration data with curve-fitting data, XPS analysis may reveal the concentrations of certain functional groups on the surface. Introduction 64

70 XPS may also be conducted with the x-ray beam positioned at various take-off angles (φ) from the sample to achieve depth profiling of the surface. 68 Spectra obtained at lower take-off angles show the composition of the material closer to the surface and spectra obtained at higher take-off angles show chemistry slightly deeper into the material. As the stage which holds the sample is tilted, the take-off angle may be varied, as shown in Fig. 4. Fig. 4(a) shows a take-off angle φ 1 of approximately 45, with an analysis depth of d 1. λ represents the mean free path for the electron to be emitted from the surface. As the sample stage is tilted [see Fig. 4(b)], the take-off angle decreased φ 2 < 45, resulting in a shallower analysis depth d 2. XPS must be performed in an ultra high vacuum (UHV) environment for several reasons. At a pressure greater than 1 x 10-7 torr, a monolayer of adsorbed gases will accumulate on almost any surface within one min. If XPS was performed at higher pressures than 1 x 10-7 torr, these adsorbed gases would be detected along with the solid surface. Another reason for the necessity of the UHV condition is that if an appreciable amount of gas molecules were present in the analysis chamber, the electrons from the sample would be scattered on the gas molecules before reaching the detector. Also, in the chamber, the majority of the electrons detected would originate from the gas molecules and not the sample itself. During XPS analysis of insulating samples, a positive charge will build up on the surface, due to the loss of electrons. The effect of surface charging manifests itself in the XPS spectra as a shift of the entire spectrum to higher binding energies. The electrons emitted have some degree of attraction back to the surface due to the positive charge, resulting in a slightly decreased kinetic energy of the electrons. When converted to the Introduction 65

71 binding energy scale via Eq. (9), this results in an inflated binding energy. To compensate for this, the x-axis is usually corrected for charging by shifting the C-C, C-H peak down to ev. 68 Another method to prevent charging is to use an electron flood gun on the sample surface, which neutralizes the positive charge accumulation. 3. Ellipsometry Film thickness and optical constants may be determined using variable angle spectroscopic ellipsometry (VASE) analysis of a film on a metallic substrate. Ellipsometry is a quick and nondestructive technique frequently used to determine the thickness and refractive index of plasma polymerized films. An ellipsometer measures the change in polarization of linearly polarized light to elliptically polarized light as it is reflected off of a reflective surface. 73 The ellipsometer measures the values of psi (ψ) and delta ( ), which are related to the Fresnel coefficients, R ~ p and R ~ s, for elliptically polarized light by the following equation: 73 ~ R ~ R p s = tan ψ i ( ) e, (10) where p is the direction perpendicular to the propagation of the light wave and parallel to the surface, and s is the direction perpendicular to the propagation of the light wave and in the plane of incidence. The Fresnel coefficients are given by the following: 73 ~ ~ ~ N cosφ N cosφ R p ~ ~ N cosφ + N cosφ =, (11) 1 2 ~ ~ ~ N cosφ N R s ~ ~ N cosφ + N cosφ =, (12) cosφ 2 Introduction 66

72 ~ ~ where N 1 and N2 are the complex indices of refraction of two substances, through which light is being reflected and transmitted. R ~ p and R ~ s are the ratios of the amplitude of the reflected wave to the amplitude of the incident wave for an interface, which were derived from Snell s law, shown in Eq. (13). ~ ~ N = φ (13) 1 sinφ1 N 2 sin 2 ~ As light passes from one medium with complex refractive index N1 at an angle φ 1, the direction of light will change by another angle φ 2, according to Eq. (13). Figure 5 shows the simultaneous transmission and reflection of light as it is passed through and/or reflected off of a surface with a film of thickness d. The multiple interfaces, reflections and transmissions of light are shown. The complex index of refraction is equal to the following: 74 ~ N = n ik (14) In Eq. (14), n is the index of refraction and k is the extinction coefficient. After the light is reflected off of the substrate, ψ and will change as there is a shift in the phases of both s and p. To obtain useful data from the values of ψ and, a model must be created which describes the optical behavior of the substrates and the deposited film(s). This leads to the determination of not only film thickness, but also refractive index. The refractive index of the film gives information which may shed light on other properties, such as dielectric constant and porosity. To determine film thickness, a model of the ψ and for the film must be chosen, along with prior knowledge of the n and k of the substrate, as a function of the wavelength of light. Introduction 67

73 Determination of film thickness using ellipsometry is valid for films which are close in thickness to the wavelength of light used. VASE is useful to measure the film thickness and N ~ for films greater than 5 nm or less then 1 µm. 73 However, film or substrate conditions such as excessive roughness, nonuniformity, or presence of powder or dust particles will lead to interference of the propagation of the light and inaccurate measurements. Film thickness is important for the determination of deposition rate, and for determination of whether the FC plasma is depositing a film or etching the surface. Deposition rates typical for many PPFC films are in the range of nm/min and have been shown to depend greatly on the monomer type and the reactor conditions Microscopy Scanning electron microscopy (SEM) is typically used to show the morphology of the plasma polymerized films and other materials, as this may affect the wettability of liquids on the surface. Atomic force microscopy (AFM) is another popular characterization technique to evaluate morphology, but can also be used to quantify surface roughness. The two types of AFM are contact mode and TappingMode. In conctact mode, the tip is dragged across the surface and the topography is mapped out. Using TappingMode, a cantilever with a tip on the end oscillates vertically at a certain frequency and traces out the topography of the surface, while a feedback loop to the electronics maintains a constant oscillation amplitude. 75 A Si tip of radius of approximately 10 nm is attached to a cantilever. As the cantilever is oscillated at its resonance frequency, a laser is shown on the back of it and reflected to a split photodiode detector. The information on the tip deflection is used to generate a map of topography and/or other properties of interest, such as modulus of the surface. Introduction 68

74 TappingMode AFM is typically used on polymeric surfaces rather than contact mode, since contact mode AFM may deform the polymers and lead to an erroneous image. A schematic of TappingMode is shown in Fig. 6, which was adapted from the Digital Instruments manual on the Nanoscope IIIa AFM. TappingMode also allows for higher resolution (1 nm 5 nm) and uses lower tip force, thereby reducing damage to polymeric samples. One disadvantage of TappingMode AFM is that this method takes slightly longer to collect an image. Two different images are typically gathered simultaneously during an AFM image scan. The first is topography mode, where only the topography is shown in varying shades of color, where the lighter shades are higher and the darker shades are lower. These images may be displayed in 3-D for easier visualization. The second mode is phase mode, where the phase lag of the oscillation is measured. Changes in this value may be due to a variety of material properties, including friction, modulus, composition, and adhesion. These changes will also be shown as a variation in color of the image. In terms of modulus, stiffer regions are shown as darker in color. Introduction 69

75 III. EXPERIMENT A. Substrate Preparation 1. Polyethylene (PE) As discussed in the previous section of this manuscript, fluorocarbon films have been found to adhere poorly to silicone rubber. However, PPFC films do adhere well to hydrocarbon substrates. Therefore, polyethylene was chosen as a substrate for initial depositions of PPFC films and for testing of the cohesion of the films when deposited on a substrate with known adhesion. Polyethylene (PE) sheets were obtained from a proprietary source. No cleaning was done to these substrates prior to deposition. 2. Ferrotype plate Ferrotype plates (chrome-plated steel) were also used as substrates for every experiment in which a film was deposited on a polymer. These were supplied by Doran Enterprises (Milwaukee, WI). These were used for analysis of film thickness and refractive index, since analysis using VASE requires that the film be deposited on a reflective substrate. Analysis of the film thickness on a polymeric substrate would have been impossible to carry out using VASE since polymers are not reflective. Therefore, the ferrotype plate substrate was used as a witness coupon. The assumption was made that the same film in terms of thickness, refractive index and chemistry was deposited on both substrates. The ferrotype plate substrate was placed directly adjacent to the polymeric substrates on the grounded electrode in the reactor chamber. Also, by depositing the film on the ferrotype plate, reflection-absorption infrared spectroscopy (RAIR) could be performed, which is useful since it the IR shows no interference from peaks due to the substrate. Experiment 70

76 The ferrotype plates were 0.65 mm thick and had a protective polymeric covering on the polished side. The covering was easily peeled off with tweezers, and the reflective side of the plate was rinsed with acetone (Pharmco, 99.9 %) to remove any residue from the polymeric covering. The ferrotype plate was then flame-cleaned using a Bunson burner. XPS analysis of the ferrotype plate was carried out after the polymeric covering was removed, and before and after the cleaning process. A rinsed and flame-cleaned ferrotype plate was used as the background for the RAIR experiments, so that only absorption bands due to the plasma deposited films would be analyzed. 3. Industrial grade silicone rubber plaques Industrial grade silicone rubber plaques were manufactured by Wynn s Precision, Inc. (Lebanon, TN), the same supplier as the O-rings. The silicone rubber swatches were rinsed with ethanol and wiped with a Kimwipe to remove any loose debris. The swatches were then placed in a stainless steel tray, and covered with Al foil until needed for the experiments. This procedure reduced the probability that any dust would settle on the substrates from the laboratory. The plaques were yellow in color and were supplied in dimensions of 15 cm x 15 cm x 2 cm. These swatches were cut in half for each experiment to make plaques in dimensions of 15 cm x 7.5 cm x 2 cm. 4. Industrial grade silicone rubber O-rings O-rings composed of silicone rubber were also obtained from a Wynn s Precision, Inc. The same grade of silicone rubber was used for the O-rings as was used to manufacture the silicone rubber plaques described previously. The dimensions of the O- rings were 6.3 cm ID, 6.8 cm OD, and 2.5 mm thick. Most O-rings tested were simply wiped with ethanol and a Kimwipe and the surfaces were blown dry with nitrogen. The Experiment 71

77 O-rings were also placed in the stainless steel tray and covered with Al foil until needed. Along the ID and OD of the O-rings, a flash line was present, presumably from excess rubber squeezing out of the mold during manufacturing. The O-rings also had extensive imbedded debris throughout the part and on the surface. Throughout this research, it was desirable to coat the rubber surface without any imbedded debris. However, this was virtually impossible, as most all contained imbedded debris to some degree on the surface. Therefore, a surface refinishing process was developed. Figure 7(a) shows the surface of a typical O-ring at 16 X magnification before any cleaning process, where arrows indicate flaws in the surface. While illuminating the O-rings with an optical microscope at 16 X magnification, the following steps were taken, with images shown in Fig. 7(b) 7(d): Step 1: Imbedded debris was plucked out using fine-tipped tweezers. Step 2: Pores left from the removed debris were cut smoother with a razor blade [Fig. 7(b)]. Step 3: Flash lines on the ID and OD of the O-rings were smoothed using polishing paper of 400, 800, and 1200 grit, in that sequence [Fig. 7(c)]. Step 4: The surface was wiped with an ethanol-soaked Kimwipe to remove loose rubber from the previous step. Step 5: The surface was blown with compressed N 2 to ensure removal of any loose particles [Fig. 7(d)]. The surface which was produced [see Fig. 7(d)] was much smoother and contained no defects, as shown at this magnification. 5. Food grade silicone rubber plaques To examine the effect of the imbedded particles in the surface of the industrial grade rubber, a food grade rubber was hand cast in the laboratory. The rubber was a 2- part room temperature vulcanizing (RTV) silicone rubber with a Pt catalyst (Factor II, Inc., Lakeside, AZ). The rubber was mixed with the catalyst in a 10 : 1 mix ratio by Experiment 72

78 weight, and poured into a shallow stainless steel tray. Compressed air was blown over the surface to remove any bubbles trapped in the rubber. The rubber was cured at room temperature for 24 hrs and then in an oven for 1 hour at 100 C, as recommended by the supplier. Oil-repelling properties of films deposited on this substrate were compared to those deposited on the industrial grade silicone rubber to more clearly determine the effect of the surface flaws in the industrial grade food rubber. 6. Medical grade silicone rubber plaques For a few experiments, plaques of medical grade silicone rubber were used from an unknown supplier. No surface cleaning was necessary prior to film deposition as the rubber was kept uncontaminated by being sandwiched between two polymeric coverings. B. Substrate characterization 1. Polyethylene film ATR and XPS spectra were obtained for the PE film, used as a base, polymeric substrate for many depositions. These characterization methods are described thoroughly in a later section. Contact angle measurements and surface energy calculations were also done for the untreated PE substrate for comparison to the coated samples. Oil uptake properties were not measured for this substrate, nor was any further characterization necessary. 2. Ferrotype plate XPS analysis of the ferrotype plate was performed after removal of the polymeric covering, but before the plates were rinsed with acetone and flamed over a Bunson burner. The XPS survey spectrum obtained for the cleaned sample after the rinsing and the flaming and was compared to the XPS survey spectrum obtained for the uncleaned Experiment 73

79 ferrotype plate sample. Changes to the amount of carbon present on the surface upon cleaning were noted. 3. Silicone rubber swatches Chemical analysis of the industrial grade silicone rubber was done using XPS and ATR. An analysis of the debris imbedded in the surface of the rubber was done using SEM and optical microscopy. The surface morphology of the rubber was also studied by SEM. The specifics of these characterization methods are discussed later in this section. Contact angle measurements, surface energy calculations, and oil uptake experiments were also done for the uncoated silicone rubber for comparison to the coated samples. The mechanical properties of the silicone rubbers were measured by tensile testing using an Instron 4465 load frame. The specimen dimensions were 1 cm wide, 2 mm thick, with a 4 mm gauge length. Crosshead speed was 10 cm/min and three specimens were tested of each rubber. Young s modulus was calculated for the three silicone rubbers by dividing the stress over the strain in the initial, linear portion of the stress vs. strain curve. The thermal properties of the silicone rubbers were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The DSC experiments were carried out using a TA instruments 2010 DSC was from 150 to 200 C at 10 C/min. The samples were contained in Pt pans. The data was collected using the Instrument Control Software, Thermal Advantage. The TGA experiments were carried out with a temperature ramp at 20 C/min from room temperature to 900 C. The same mechanical, thermal and oil uptake tests were performed on the food grade and medical grade silicone rubber swatches as the industrial grade rubber swatches. Experiment 74

80 4. Industrial grade silicone rubber O-rings Optical microscopy of the O-rings was performed to observe the imbedded debis in the surface of the O-rings. SEM images of the O-rings were also obtained, specifically of the O-ring surfaces, the flash lines and the cross section. C. Characterization of the diesel engine oil Diesel engine oil (DEO) with a multiweight viscosity grade of 15W-40 was used as the testing oil for all contact angle measurements and oil uptake experiments. Multiweight oil typically is used in applications with widely varying use temperatures. Multiweight oil typically contains polymer additives (7 8 %, by weight), which uncoil at higher temperatures to slightly counterbalance the drop in viscosity from the oil at high temperatures. The chemistry of the DEO was analyzed using transmission IR by placing a droplet of the oil on a KBr pellet. The transmission IR of the DEO was also analyzed after heating for 72 hrs at 150 C to observe changes to the chemistry of the DEO. The surface tension of the DEO was measured at various temperatures using a Du Noüy interfacial tensiometer. This was a precision direct reading tensiometer manufactured by Central Scientific Co. (Chicago, USA). Care was taken to flame clean the Pt ring before each use to remove any contaminants which could affect the measurement. The Pt ring was rinsed with acetone and flame-cleaned with a Bunson burner. A bath of oil on a hot plate was heated to 180 C as quickly as possible, while stirring with a magnetic stirrer. Then, the heat was removed while the stirring continued. Measurements of surface tension were taken repeatedly as the oil cooled to room temperature. The surface tension was calculated using Eqs. (5) and (6) and was plotted as a function of temperature. The same experiment was performed using oil which had Experiment 75

81 previously been heated for 72 hrs 150 C. Therefore, the surface tension of the temperature-aged oil could be compared to that of fresh oil. D. Plasma film deposition 1. Description of reactor Films were deposited using a parallel-plate, capacitively-coupled, radio frequency (rf)-powered reactor (Advanced Plasma Systems, Inc., St. Petersburg, FL) operating at a frequency of MHz. A Manitou Systems rf generator (Model PB-3) with a maximum rf power of 300 W was used to produce the glow discharge. The reactor volume was 38 L and the powered (top) electrode was separated from the grounded (bottom) electrode by 3.5 cm, as shown in Fig. 8. The plasma glow region was concentrated between the parallel plates, resulting in an active plasma volume of 4.0 L. MKS mass flow meters (Model 1259C) were used to regulate the flow of the gases into the reactor, and each had a maximum flow capacity of 50 sccm. These mass flow controllers were calibrated to flow N 2. Therefore, correction factors were necessary to adjust the flow for other gases. As shown in Fig. 8, the monomer entered the reactor chamber above the powered electrode through a showerhead configuration with many tiny holes facing downward into the chamber. The deposition time was varied from a few seconds to 40 min and the rf power was varied from 2 to 250 W. The reflected power was typically less than 3 W, but was as low as 0 for the depositions at 2 and 5 W of RF power. A 651C MKS pressure controller was used to set the pressure, which regulated a 253B MKS throttle valve placed between the vacuum pump and the reactor chamber. The chamber pressure was measured using a 626A MKS Baratron Experiment 76

82 capacitance manometer. Prior to each experiment, the leak rate of the reactor was checked to be less than 0.3 mtorr/sec. All substrates were placed in the center of the reactor chamber on the grounded electrode. Since the PE substrate has such low mass, it was adhered to the electrode using double-sided tape, so that it would not move upon venting of the reactor. The ferrotype plate and silicone rubber substrates were of greater mass than the PE substrate, so they did not need to be anchored using double-sided tape. Prior to each film deposition, the reactor chamber was etched with an O 2 (Matheson Gas, ultra-high purity) plasma using 200 W of RF power for approximately 30 minutes. This was sufficient to remove any film deposited on the chamber walls and electrodes during previous experiments. Schaepkens et al. also showed that an O 2 plasma effectively cleaned an rf plasma reactor contaminated with a fluorocarbon film. 76 The species were removed from the walls of the vessel as COF 2, as shown by infrared laser absorption spectroscopy Deposition of PPFC films using low molecular mass monomers PPFC films were deposited using various monomer systems: (1) hexafluoroethane (C 2 F 6 ) (Matheson Tri-Gas, semiconductor grade), and acetylene (C 2 H 2 ) (Matheson Tri- Gas, 99.6 %), (2) pentafluoroethane (C 2 F 5 H) (Lancaster, 99 + %) and C 2 H 2, and (3) hexafluoropropene (C 3 F 6 ) (Aldrich, 99 + %) and Argon (Ar) (Matheson Tri-Gas, ultrahigh purity). The structures of these monomers are shown in Fig. 9. All experimental variables are outlined in Table III for depositions with small molecular mass FC monomers. For the C 2 F 6 /C 2 H 2 and the C 2 F 5 H/C 2 H 2 systems, the concentration of the C 2 H 2 was varied from 0 to 100 % of the total flow rate. For the C 2 F 6 /C 2 H 2 and the Experiment 77

83 C 3 F 6 /Ar systems, the rf power was varied from 10 to 140 W. For the C 2 F 6 /C 2 H 2 and the C 2 F 5 H/C 2 H 2 experiments, the process pressure was kept fairly low (20 60 mtorr). However, for the C 3 F 6 /Ar experiments, the process pressure was higher (500 mtorr). The effect of these reactor variables on parameters such as deposition rate, refractive index, chemical structure, contact angle of DEO and surface energy was determined. 3. Depositions of LCF films Two long chain fluorocarbon (LCF) monomers: 1H, 1H, 2H, perfluoro-1- dodecene (Lancaster Synthesis, 97 %) and 1H, 1H, 2H, perfluoro-1-decene (Lancaster, 99 + %) were used for depositions. These monomers consisted of a fully fluorinated carbon chain with a vinyl end group (see Fig. 10). Frequently in this research, both monomers are referred to as the LCF monomer because of their similarity in structure and deposition parameters. The colorless, liquid monomer was placed in a flask that was attached directly to the inlet of the reactor, bypassing the mass flow controllers. This was necessary because the vapor emitted from the liquid in the flask was very small and the mass flow controllers were not sensitive enough to measure the flow. The vapors from the liquid produced a flow rate of 0.14 g/min, as measured by monitoring the weight of the flask before and after film deposition. The temperature of the flask was not controlled, and remained at room temperature. The monomer was allowed to flow for 5 minutes before ignition of the plasma and no carrier gases were used with the LCF monomer depositions. The base pressure of the reactor was 20 mtorr and the process pressure was 50 mtorr for the experiments with 2 to 40 W rf power. Due to difficulties in sustaining a Experiment 78

84 plasma at higher powers with only 50 mtorr of pressure, the pressure was increased to 150 mtorr for the experiments with 80 and 160 W rf power. 4. Depositions of multilayer (FC/HC) films Multilayer films were deposited on the industrial grade, food grade, and medical grade silicone rubber plaques, and on the PE substrate. Multilayer films with hydrocarbon (HC) and FC layers were developed to study adhesion of these films to the silicone rubber substrate. A number of multilayer films were developed using various monomers, reactor parameters, and numbers of layers. The flow rates of the various monomers and carrier gases could not be altered while the plasma was being generated. Therefore, between each film layer, the RF power was halted and the gas flow ratios changed. A multilayer film was developed with seven layers and consisted of varying the concentrations of C 2 F 5 H and C 2 H 2 in the total flow to the reactor. Table IV shows the deposition parameters for this film. The parameters for the deposition were rf power of 50 W, process pressure of 40 mtorr, total flow rate of 20 sccm, and 5 min deposition time per layer. For clarity, the multilayer films discussed in this document were labeled with a code. Films deposited with these parameters were labeled M1 films. Other multilayer films were deposited with fewer layers than M1. The parameters for a multilayer film M2 deposited using varying concentrations of C 2 F 6 /C 2 H 2 with 3 layers are shown in Table V. Here, the layer with C 2 F 6 /C 2 H 2 = 90/10 was the final layer. Also, multilayer films with a LCF film as the final layer were deposited. These were labeled as M3, M4, and M5 and the corresponding reactor parameters are listed in Experiment 79

85 Tables VI VIII. Here, the number of layers was altered, along with the rf power used to deposit the final layer. 5. Deposition conditions for multilayer (HC/FC) films on O-rings These experiments were carried out using a number of different techniques, the most successful of which are reported here. O-rings were coated by placing the O-ring flat on the grounded electrode, in the same position as the silicone rubber swatches. Since plasma is a line-of-sight coating process, when coating a 3D object the object must be rotated so that each side is exposed to the plasma for an equal amount of time to ensure uniformity of the deposition. When depositing a multilayer film, a layer of the film was deposited, then the O-ring was flipped over so that the other side would be coated, and the process was repeated for each layer. The following process produced a 3-layer film on an O-ring using the M5 multilayer coating parameters shown in Table VIII. When the reactor was brought up to atmospheric pressure to open the door to the chamber and flip the O-rings, the O-rings were removed and the reactor was cleaned (wiped out and etched with O 2 as described previously). While the reactor was being cleaned, the O-rings were held in the stainless steel tray covered with Al foil. Deposition steps for coating O-rings with the multilayer film (M5) using the LCF monomer were as follows: Step 1: O-rings were placed on the grounded electrode, and the reactor was pumped down to base pressure (20 mtorr). Step 2: 20 sccm C 2 H 2 were flowed for 2 min at 150 mtorr. Step 3 (Layer 1): Deposited C 2 H 2 for 5 min at 50 W rf power. Step 4: Reactor was brought up to air, O-rings placed in holding tray, flipped, and covered. Step 5 (Cleaning): Reactor was cleaned with 40 sccm O 2 for 30 min at 200 W rf power. Step 6: O-rings were placed on the grounded electrode, and the reactor was pumped down to base pressure (20 mtorr). Experiment 80

86 Step 7: 20 sccm C 2 H 2 were flowed for 2 min at 150 mtorr. Step 8 (Layer 1 and Layer 2): Deposited C 2 H 2 for 5 min at 150 mtorr, and C 2 H 2 /C 3 F 6 = 50/50 for 5 min at 50 W rf power. Step 9: Reactor was brought up to air, O-rings placed in holding tray, flipped, and covered. Step 10 (Cleaning): Reactor was cleaned with 40 sccm O 2 for 30 min at 200 W rf power. Step 11: O-rings were placed on the grounded electrode, and the reactor was pumped down to base pressure (20 mtorr). Step 12: 20 sccm C 2 H 2 /C 3 F 6 = 50/50 were flowed for 2 min at 150 mtorr. Step 13 (Layer 2 and Layer 3): Deposited C 2 H 2 /C 3 F 6 = 50/50 at 50 W for 5 min, then LCF monomer at 160 W rf power for 5 min. Step 14: Reactor was brought up to air, O-rings placed in holding tray, flipped, and covered. Step 15 (Cleaning): Reactor was cleaned with 40 sccm O 2 for 30 min at 200 W rf power. Step 16: O-rings were placed on the grounded electrode, and the reactor was pumped down. Step 17 (Layer 3): Deposited LCF monomer at 160 W rf power for 5 min. E. Film characterization 1. Ellipsometry Films deposited on ferrotype plates were analyzed using a variable angle spectroscopic ellipsometer (VASE) (J. A. Woollam Co., Inc., Lincoln, NE). The data were collected through angles of incidence of 60 to 75 in increments of 5 and wavelengths of 300 to 1000 nm in increments of 10 nm. A Xe lamp was used within the ellipsometer as a source. For these wavelengths of light, films above 5 nm and below 1 µm could be measured with accuracy. 73 The ellipsometer was calibrated by finding the n and k of a SiO 2 wafer, and calibrating this to the known values in the VASE software. The wafer was blown with compressed N 2 to remove any dust prior to running the calibration sample. Within the ellipsometer software, a Lorentz model [see Eq. (15)] was used to determine the refractive indices as a function of wavelength for the ferrotype Experiment 81

87 plate substrate. This is a model commonly used to find the real and imaginary parts of the refractive index for a metal. 73 ~ Ak ε ( hν ) = ε1 + iε 2 = ε E ib hν k k ( hν ) k (15) For the k th oscillator, A k is the amplitude, E k is the center energy, B k is the broadening of each oscillator. hν is the photon energy in ev. The values of n and k were found for the ferrotype plate as a function of wavelength using this model. Then to find thickness, n and k of the film, another model was used. A common model used for polymeric, dielectric, and semiconducting films is the Cauchy-Urbach model, as follows: 73 Bn Cn n ( λ) = An + +, (16) 2 4 λ λ ( λ) 1 1 β λ γ k = αe (17) The fitting parameters, A n, B n, and C n, the coefficient amplitude α, the exponent factor β, and the band edge γ were regressed to fit the model and the software measured the quality of the fit with a mean squared error (MSE) value. 73 PPFC films have been fitted previously using this model by other researchers. 22 For a Cauchy fitted material, n should decrease with wavelength. Each of these fit parameters were fitted and values for n and k were found. Mean-squared error (MSE) values less than 10 for the fitting of the models were considered to be sufficient. 2. Fourier transform infrared (FTIR) spectroscopy a. General description. FTIR spectra were obtained using a Nicolet Magna-IR 760. Typically, 256 scans were averaged to produce spectra with resolution of 4 cm -1. In Experiment 82

88 some cases, the number of scans was increased to improve the signal to noise ratio, which may be small for very thin films. Also, in some cases increasing the number of scans allowed easier distinction between peaks which were close together. b. Transmission IR. Prior to deposition of films using the LCF monomers, the structure of the monomer itself was confirmed by transmission IR. Since the monomer was a liquid, a KBr pellet was pressed from KBr powder, and then a droplet of the monomer was placed on the KBr pellet. The DEO was also analyzed using this method. The same KBr pellet, before the liquid was placed on it, was used for the background, to correct for any absorbed water vapor within the KBr pellet. c. Reflection-absorption IR (RAIR). For films deposited on the ferrotype plate substrates, the spectrometer was used in conjunction with a Spectratech FTS-85 grazing angle reflection accessory. A blank, cleaned ferrotype plate was used for the background. The LCF monomer was also analyzed using reflection mode by smearing a droplet on a cleaned ferrotype plate. This method allowed for direct comparison of IR peaks for the films deposited on ferrotype plates. d. Attenuated total reflectance (ATR) IR. ATR was used with two different crystals of varying refractive index. As discussed previously, the refractive index of the prism will affect the penetration depth of IR radiation into the analysis surface. The two prisms used in this work were composed of Ge and ZnSe, with refractive indices of 4.0 and 2.4, respectively cm -1 is a common peak found in the IR of PPFC films, which converts to a λ of um. Using this value for λ and an angle of 85 in Eq. (7), the penetration depths were calculated for each crystal. The value for n s which was used in the equation was 1.39 (that of PTFE). 77 d p of 0.34 µm was determined when using the Experiment 83

89 Ge prism, and the d p for the ZnSe prism was 0.64 µm. Hence, for thinner films, peaks due to the substrate could appear more intense especially when using the ZnSe crystal. Therefore, the ZnSe crystal was used when either a bulk material characterization was desired or for analysis of a thick film on a polymer. The Ge crystal was only used when attempting to obtain an IR of a very thin film. 3. X-ray photoelectron spectroscopy Further chemical analysis of the films was performed using a Perkin-Elmer model 5300 x-ray photoelectron spectrometer (XPS) with either Mg K α x-rays at an energy of ev or Al K α x-rays at an energy of ev. High-resolution scans were obtained at a pass energy of ev and used to determine the atomic concentrations of C, F, O, and any other elements present. To determine orientation effects, some films were analyzed at varying take-off angles, such as at 10 and 80 º. If the take-off angle is not specified, it can be assumed to be the standard angle of 45 º. Spectra obtained at lower take-off angles showed the chemical composition more at the surface and higher take-off angles show chemistry slightly deeper into the material. Take-off angle, θ, is related to sampling depth as shown in Eq. (18), 78 d = λ sin( ) (18) 1 3 θ1 where d 1 is the sampling depth and λ is the inelastic mean free path. The inelastic mean free path for organic compounds can be approximated by the following equation: 79 ( KE ) ( KE ) avg avg λ = (19) where KE avg is the average kinetic energy of photoelectrons reaching the detector. Many PPFC films were composed of approximately 65 % F and 35 % C, by atomic Experiment 84

90 concentration analysis. However, the electrons detected are a function of the atomic species by volume, so converting to a volume fraction using the values of the covalent radii of C and F, resulted in 60 % F and 40 % C. KE avg may be determined by Eq. (20): KE = V KE V KE 1 (20) avg F ( s ) C C( s ) F 1 + In Eq. (20), V F and V C are the volumetric concentrations of fluorine and carbon, and KE F(1s) and KE C(1s) are the kinetic energies of the F(1s) and C(1s) photoelectron peaks. For Mg K α radiation, the F(1s) and C(1s) peaks correspond to kinetic energies of and ev, resulting in a value of KE avg of ev. Using this result along with Eqs. (18) and (19), the sampling depths at take-off angles of 10 and 80 were found to be 1.5 and 8.7 nm, respectively. The C(1s) spectra of the films were fitted using 90 % Gaussian peaks with a full-width at half maximum of 1.9 ev. X-ray beam exposure caused sample degradation in some samples, which was evident by a decrease in the peak due to CF 2 and an increase in peaks due to other C-F bonded species. This effect was shown after approximately 20 min of x-ray beam exposure time. Degradation of fluorocarbon materials such as PTFE during x-ray 72, 80 photoelectron analysis has been shown previously. In order to avoid misleading results from beam damage, the C(1s) spectra were collected from a previously unanalyzed sample within 10 minutes of x-ray beam exposure time. The atomic concentrations of the various elements found were determined by the software using the following equation: 68 I x S x C x =, (21) Ii S i Experiment 85

91 where I x is the area under the curve for a certain element X, and S x is the sensitivity factor for that element which was put into the software and was specific for the spectrometer used. Curve-fitting was performed according to the values shown in Table II due to C-O and C-F bonding in each film. Nanse et al. has done extensive investigation into the exact binding energies of the peaks due to C-F bonding, and many of these were used for reference. 72 In addition, other XPS resources by Beamson and Briggs were also used as references. 71 Curve-fitting was carried out by inputting peaks at these known binding energies, which were adjusted at most ± 0.2 ev. The peaks were fitted to % Gaussian peaks. The theoretical line shape of photoelectron peaks is Lorentzian. However, contributions from the X-ray beam and instrument change this shape to mostly Gaussian. 4. Contact angle measurements and surface energy The contact angles (θ) of distilled water, glycerol (Aldrich, 99.5 %), diiodomethane (Aldrich, 99 + %), toluene (Fisher Sci., 99.8 %), hexane (Aldrich, 99 + %), hexadecane (Aldrich, 99 + %), and cyclohexane (Fisher Sci., 99.0 %) on the films were measured using the sessile droplet technique and a contact angle goniometer. The data presented was an average of 10 readings on 5 droplets, for both sides of each droplet. Films which display oleophobicity should show large contact angles of hydrocarbon liquid droplets, such as hexadecane. Hexadecane is typically used as a probe for oleophobicity, as it is a long chain hydrocarbon liquid similar to oil. 12 Surface energies of the films were compared using two techniques. The first was the method outlined by Kaelble described thoroughly in the Introduction section. 61 Experiment 86

92 A set of two equations was developed for two test liquids and solved for γ D s and γ P s. As shown in Fig 11, a HTML program was written to run in Microsoft Internet Explorer. This program used the Kaelble equation and led to rapid and convenient calculation of surface energies using any two liquids used for surface energy analysis. The total surface energy of the solid γ s, γ D s, and γ P s were all determined by entering θ and choosing the corresponding test liquid from the select box. The surface energies of the test liquids were all written into the program, so that only the test liquid needed to be selected for each calculation. For convenience, γ D lv and γ P lv did not need to be referenced each time when changing test liquids. The test liquids chosen often were glycerol (γ D lv = 34.0 mj/m 2, γ P lv = 30.0 mj/m 2 ), and hexadecane (γ D lv = 27.6 mj/m 2, γ P lv = 0.0 mj/m 2 ). These liquids were used since they displayed high contact angles on the surfaces studied, making the measurement of the contact angle easier. Also, if one of the test liquids wets the surface, there is no contact angle, and surface energy calculation using the Kaelble approach would not be possible. The second method used for comparison of surface energy of the films was the Zisman critical surface energy method. 71 Four liquids were used to generate the plot of cos(θ) vs. γ lv for the critical surface energy analysis: hexane, hexadecane, toluene, and cyclohexane. The surface energies of these liquids were mostly dispersive and were as D follows: hexane, γ lv = 18.4 mj/m 2 ; cyclohexane, γ D lv = 25.5 mj/m 2 ; hexadecane, γ D lv = 27.6 mj/m 2, and toluene, γ D lv = 26.1 mj/m 2 (γ P lv = 2.3 mj/m 2 ). 5. Microscopy Atomic force microscopy (AFM) images of the industrial grade silicone rubber and PPHC and PPFC films on PE and the silicone rubber were taken using a Digital Experiment 87

93 Instruments Nanoscope IIIa AFM in TappingMode. Images were gathered by scanning at 0.5 Hz and gathering data at a resolution of 256 pixels/line. In TappingMode, the tip lightly touches the surface of the sample, as the cantilever and tip are oscillated. Images were captured on a scanned area of 2 x 2 µm, 10 x 10 µm, 50 x 50 µm, and 100 x 100 µm in both topography and phase mode. The scan area chosen depended on the extent of surface roughness and the scale of the features on each surface. The surface morphology was more clearly shown in phase mode, but topography mode images were used for determination of the degree of roughness. The root-mean-squared (RMS) roughness was calculated within the Digital Instruments software using the following formula: RMS roughness = N ( Z i Z avg ) i= 1 N 2 (22) where Z i is the current Z value, Z avg is the average Z value and N is the number of points analyzed in the area. A peak-to-trough distance, or a difference between the highest and lowest points in the vertical (z) direction, was also determined in many of the images. 75 A Hitachi S-4000 Field Emission SEM was used for analysis of the silicone rubber O-rings and the silicone rubber industrial grade plaques. The acceleration voltage was 15 kv. Samples were coated with Au using a Denton vacuum sputter coater for 1 min prior to SEM analysis to prevent sample charging. 6. Adhesion testing Adhesion of the films to silicone rubber and PE was qualitatively evaluated using a tape peel test followed by visual and XPS analysis of the failure surfaces, as shown in Fig. 12. A pressure sensitive tape was applied to the PP film side of the sample with Experiment 88

94 finger pressure and peeled back slowly in as close to a 180º angle as possible. The surfaces were examined visually for any evidence of film transfer and failure of adhesion. Typically, only the tape side of the peel test was examined by XPS, but at times the substrate side was analyzed as well. For PPFC films, an indication of adhesion was taken to be the absence of peaks in the region between and ev on the tape side of the peel surface. Peaks in this range were due to CF 3 and CF 2 and were not present on the tape surface prior to peel testing. Also, the atomic % F present on the tape side of the peel surface in the atomic concentration analysis was used as another qualitative measurement of adhesion. 7. Oil uptake on silicone rubber plaques For the oil uptake experiments on films deposited on the industrial grade and food grade silicone rubber plaques, an oil-soluble dye (#7050, Robert Koch Industries) was added to the diesel engine oil (DEO) at 1 % by weight. The dye color was deep blue, so that the infiltration of the oil into the rubber could be visually evaluated. Without the addition of the dye to the DEO, it was difficult to see the diffusion of the oil into the rubber since the oil was a golden yellow color, and the rubber was bright yellow. Dyed oil has been used in food science research to monitor vegetable oil uptake in potato slices during deep frying. 81 The contact angle of the dyed oil on various substrates did not change with the addition of 1 % of the dye, so it was concluded that the addition of the dye did not alter the surface tension of the oil. Therefore, since no change in contact angle was shown, the presence of the dye at 1 % should not alter the oil uptake properties of the oil. Digital images of the plaques with droplets were taken before and after each experiment and at various times during the experiment. Experiment 89

95 Another experiment was performed in which the silicone rubber was in a simulated compression environment. According to Caterpillar, Inc., these O-rings will undergo approximately 31 % compression. Using finite element analysis, this was translated into 5.9 % tensile strain on a silicone rubber plaque. Therefore, to simulate the performance of the film and substrate under compression, this was done using a tensile strain experiment. Plaques of dimensions of 10 mm x 120 mm were coated. These were marked at a length of 100 mm, with 10 mm of extra material on each end. The rubber was then stretched and clamped with hose clamps mm, to impose 5.9 % strain. Oil uptake droplet tests were carried out on this coated rubber with the imposed strain. Also, a smaller sample (able to fit in an ESEM) was prepared with the same amount of strain imposed using the same type of clamps. The film surface was examined using a Phillips XL 30 ESEM at 30 kv and 500 X magnification to determine if the film was delaminating or cracking upon imposition of this amount of strain. Figure 13 shows the rubber stretched using the hose clamps inside the ESEM chamber. The oil uptake into the medical grade silicone rubber was tested using a different technique. Coated and uncoated samples of the medical grade silicone rubber were placed with the film side facing up in watch glasses, placed concave upward. The medical grade silicone rubber was very thin, so it could rest on the watch glass smoothly and follow its contour. Upon any swelling from the oil, the rubber would buckle up away from the glass. 2 g of DEO were added to each watch glass with the coated and uncoated medical grade silicone rubber samples. The glasses were allowed to rest at room temperature and at 80 C for 24 hrs. Oil uptake was visually assessed by the degree of Experiment 90

96 buckling of the silicone rubber, and images of the samples were taken using a digital camera. 8. O-ring oil uptake testing using a soak test The indigo dye did not need to be added to the oil for the O-ring testing since the degree of oil uptake was quantified as the % increase in weight of the O-ring. Each O- ring was weighed before and after immersion in a glass Petri dish filled with oil set in an oven at temperatures between 50 and 150 C for times spanning 24 to 100 hrs. The oil was preheated in the oven for 5 min to reach the testing temperature before the O-rings were submerged. Fresh oil which had not been heated previously was used for each experiment. The O-rings were also tested at room temperature. Glass petri dishes with dimensions of 10 cm in diameter and 1.5 cm in height were used to hold the O-rings individually. After soaking in the oil, the O-rings were removed from the petri dishes of oil carefully with tweezers, and blotted with a KimWipe to remove any oil remaining on the surface. A small amount of weight gain (0.2 %) may be attributed to oil which could not be removed from the surface. Care was taken to not squeeze the O-rings while blotting them. The % oil uptake by weight was calculated as the following: Wa Wb % Uptake = *100 (23) W b where W a was the weight of the O-ring after the immersion in the oil, and W b was the weight of the O-ring before immersion. Experiment 91

97 IV. RESULTS AND DISCUSSION A. Substrate Characterization 1. Characterization of PE substrate Figure 14 shows the XPS survey spectrum of the PE substrate used for many film depositions. The atomic concentration was found to be mostly C (97.6 %), as expected. However, trace amounts of O (1.8 %) and Si (0.6 %) were also found, and these elements may be attributed to additives in the PE blend. Figure 15 shows the C(1s) region of the XPS spectrum, which revealed a single, symmetric peak due to C-C, and C-H bonding at ev. The ATR-IR spectrum for PE is shown in Fig. 16. Bands due to CH 2 asymmetric and symmetric stretching appeared at 2915 cm -1 and 2847 cm -1. Other bands due to CH 2 deformation and in-phase rocking appeared at 1466 cm -1 and 721 cm -1. The contact angle of water on PE was 90, which was similar to that found by other researchers. 61 Diesel engine oil droplets on PE resulted in an immediate contact angle of 16.8, which continued to spread over the course of few minutes and resulted in wetting of the surface. The surface energy (γ s ) calculated for uncoated PE was 33.1 mj/m 2 (γ s D = 33.0, γ s P = 0.1). These values were very close to those cited by other researchers (γ s = 32.4, γ s D = 31.3 and γ s P = 1.1 mj/m 2 ). 61 Variation in surface energy of PE may be due to variation in the roughness of the PE, which would affect the contact angle of the test liquids slightly. Also, any additives in commercial PE will alter the surface energy. Since, Si and O were detected by XPS, this indicated that an additive was present which may have slightly increased the surface energy. Results and Discussion 92

98 2. Characterization of ferrotype plate Figure. 17(a) shows the survey spectrum of the ferrotype plate after the polymeric covering was removed and before the cleaning process. Atomic concentration revealed 61.8 % C, 34.1 % O, and 4.1 % Cr. After the ferrotype plate was cleaned with acetone and flamed over a Bunson burner, the XPS survey spectrum was gathered again [see Fig. 17(b)]. The atomic concentration was 54.0 % O, 24.8 % Cr, 21.0 % C and 0.2 % N. This indicated that much of the carbonaceous contamination from the polymeric covering was removed by this cleaning process. 3. Characterization of silicone rubbers a. Chemistry/structure. Figure 18 shows the XPS survey spectrum of the industrial grade silicone rubber substrate, used for many film depositions. Atomic concentration analysis revealed 45.1 % C, 27.2 % O, 25.7 % Si and 2.0 % F. Here, the presence of F was due to a contaminant from the XPS chamber itself. These values are roughly equal to that for silicone rubber (50 % C, 25 % Si, and 25 % O). Recall that XPS cannot detect the presence of H. The C(1s) region of the spectrum is shown in Fig. 19 and revealed a symmetric peak, similar to that shown in PE. The single, symmetric peak was present since all the C atoms in silicone rubber have an identical chemical environment, CH 3 Si. Figure 20 shows the ATR-IR spectrum of the industrial grade silicone rubber. The main absorbance band at 1003 cm -1 and was due to asymmetric Si-O-Si stretching. The peak at 1258 cm -1 was due to Si-CH 3 symmetric deformation. The peak at 783 cm -1 was due to Si(CH 3 ) 2 bending. Peaks due to CH 3 asymmetric and symmetric stretching Results and Discussion 93

99 were found at 2963 cm -1 and 2906 cm -1. Other bands are listed in Table IX with their corresponding assignments. 66 b. Mechanical Testing. Tensile testing on the various silicone rubbers indicated that the rubbers were quite different. Figure 21 shows the stress vs. strain curves for three samples of each rubber. Table X shows the stress and strain at failure, modulus, and fracture toughness of each silicone rubber. The industrial grade rubber displayed both the highest stress at failure and the largest modulus. The industrial grade and the food grade silicone rubber both failed at approximately 365 % strain. The medical grade rubber survived to a higher % strain 625 %, and displayed the greatest fracture toughness at 14.8 MPa. c. Thermal Properties. The TGA data for the three rubbers tested is shown in Fig. 22. All rubbers began to degrade slightly at 400 ºC. After testing, the industrial and medical grade rubber still contained some weight, indicating the presence of fillers in these rubbers which survived to 900 ºC. Very little mass was left of the food grade rubber after testing indicating no fillers present in the rubber. The possibility exists that during the oil uptake tests, oil may have removed a portion of lower molecular weight polymer or filler material. However, this effect would be very small, and may not be significant. Also, if this effect is present and constant for all the industrial grade rubber O-rings, then the comparison of each film on the same substrate would not be adversely affected. DSC analysis revealed little changes in thermal properties between the rubbers (Fig. 23). Tg was shown at approximately -120 ºC and crystallization at approximately - 50 ºC. Based on the thermal and mechanical analysis, the industrial grade and medical grade rubbers were tougher than the food grade rubber. Results and Discussion 94

100 d. Morphology. Optical microscopy images are shown in Fig. 24 of the industrial grade silicone rubber surface at 32 X magnification. The images showed a substantial amount of fibers, imbedded debris, and surface impurities. There were very few regions on the rubber which did not reveal any contaminants. SEM images were also obtained of the industrial grade silicone rubber surface of the O-rings (Fig. 25). In Fig. 25(a), a depressed flaw is shown on the surface of the industrial grade silicone rubber O-ring, approximately 200 µm in length. This defect was probably due to the rubber not properly filling the mold during processing. Figure 25(b) shows a cross section of an O-ring, which was cut with a razor blade where a surface flaw was noticed. This imbedded fibrous debris extended approximately 800 µm below the O-ring surface. These defects could provide an efficient mechanism of oil uptake by capillary action into the O-ring. Another surface flaw that was noticed is shown in Fig. 25(c), and consists of fibrous strands of silicone rubber extending outwards from the main rubber surface. These 3-D strands were approximately 200 µm in length, and would present a problem for plasma treatment, since it is a line-of-sight process. An industrial grade silicone rubber O-ring was cut using a razor blade so that the flash line on the OD and ID could be viewed easily by SEM. Figure 25(d) shows the cross-section of an O-ring and Fig. 25(e) shows that this flash line extends up to approximately 80 µm from the O-ring surface. Difficulties inhibiting oil uptake on these substrates may arise from this relatively large, rough flash line. Improving the process parameters used for the manufacturing of the O-rings could reduce the width of the flash line. Results and Discussion 95

101 AFM images showed that the rubber itself was quite rough, which made imaging difficult at times. Loose rubber and/or debris on the surface often caused the AFM tip to smear the debris, resulting in streaking in the image. Figure 26 shows an AFM image of the industrial grade silicone rubber on a 50 x 50 µm scale. The image shows a depressed feature typical of the surface of this substrate. The peak-to-trough value for this image was 5.4 µm and the roughness was 849 nm, indicating a very rough and convoluted surface. e. Wettability. The uncoated industrial grade silicone rubber resulted in a contact angle of 48.3 with the DEO. Total surface energy was calculated to be 25.7 mj/m 2 D (γ S = 25.2 mj/m 2, γ P s = 0.5 mj/m 2 ). This value may be compared to that found in literature of 21.8 mj/m The discrepancy between the surface energy calculated for the industrial grade silicone rubber and that found for pure PDMS in literature may be due to the contribution of fillers and additives resulting in an inflation of surface energy. In addition, there is a relationship between molecular weight and surface energy. 82 As molecular weight increases, so does surface energy. This is basically an effect of the end groups. f. Oil Uptake. Uncoated industrial grade silicone rubber swatches were tested as controls for the dyed oil droplet test, as shown in Fig. 27, as a function of time. Digital images were gathered after the initial application of the dyed oil droplets, after 1 hr, 2 hrs, 12 hrs, and 24 hrs. The temperature was held at 100 C for this set of experiments. Initial uptake of the oil into the rubber was clearly shown by the blue haze surrounding each droplet. After 24 hrs, there was very little oil remaining on the surface of the swatches, as the majority of the oil diffused into the rubber. Results and Discussion 96

102 Uncoated food grade and medical grade swatches were also tested as controls for the dyed oil droplet test, and are shown in Figs. 28 and 29. The food grade rubber and the industrial grade rubber seemed to absorb more oil than the medical grade rubber. This may be due to the medical grade rubber being thinner than the others. Diffusion of oil was greater for the food grade rubber than the medical grade rubber, which may be related to its mechanical properties of enhanced toughness. The immersion test for oil uptake was performed on the uncoated O-rings, to obtain control values for oil uptake as a function of time and temperature. Six O-rings were tested for 24 hrs. Two were tested at each temperature of 23, 50 and 100 C. Results of the % oil uptake are shown in Fig. 30. The % oil uptake ranged from 7.0 % for those tested at 24 hrs at room temperature up to 13.0 % for those tested at 100 hrs at 100 C. Upon visual and manual examination of these O-rings, they appeared swollen and easily deformed. The % oil uptake was very similar for the O-rings tested for 24 hrs and 100 hrs, indicating that the majority of oil uptake occurred within in the first 24 hrs after which the rubber was essentially saturated. This result was found to be consistent also for the experiments at 50 and 100 C. From these results it appeared that temperature was a more important factor for rate of oil uptake than the duration of the immersion test. B. Characterization of the Caterpillar diesel engine oil (DEO) 1. Infrared spectroscopy It was suspected that there may be changes to the chemistry of the diesel engine oil with elevated temperatures. Therefore, IR spectra of the oil were gathered before and after heating for a period of time. Figure 31 shows the transmission IR spectrum of the Results and Discussion 97

103 diesel engine oil (DEO) before and after heating for 72 hrs at 150 C, with band assignments in Table XI. There were a few notable changes to the DEO after heating which appeared when comparing the IR spectra. First, the ratio of CH 3 to CH 2 species increased after heating the oil. This was shown in two regions of the spectra. The band at 2954 cm -1 was due to CH 3 stretching, and the band 2924 cm -1 was due to CH 2 stretching. The ratio of the intensities of these two peaks changed upon heating of the oil, where less CH 2 species and more CH 3 species occurred in the heat treated DEO. A change in the CH 3 species was also shown at lower wave numbers, around the region of 1377 cm -1, due to CH 3 in-phase deformation. Figure 32 shows an expanded view of this region. Before heating, the peak at 1377 cm -1 was a single peak, with a small shoulder present at lower wave numbers. However, after heating of the oil, the IR revealed a split in the peak at 1377 cm -1. According to Colthup et al., this change was due to steric repulsion of CH 3 groups which are adjacent to each other. 66 When there are two or three CH 3 groups on a saturated carbon, the close proximity of the H atoms alters the resulting frequency of the vibrational mode. When there are two CH 3 groups this band will split into two bands at 1388 and 1366 cm -1, which are shown in Fig. 32. This change in structure indicated that shorter molecular chains of oil, or more branched oil molecules, were present after the heating, possibly indicating a degradation of the oil. Other changes occurred to the structure of the oil as well. The band at 970 cm -1 (see Fig. 32) in the spectrum of the oil before heating was due to a trans C-H wagging in CH=CH groups. Interestingly, this peak disappeared after heating. Oxidation of oil normally occurs when atmospheric oxygen attacks weak points in the hydrocarbon chain, such as a C=C bond. Peaks due to these oxidized species were found in the oil after Results and Discussion 98

104 heating at 1708 cm -1 due to C=O stretching and at 1230 cm -1 due to C-O-O stretching. Therefore, the disappearance of the band due to CH=CH and the appearance of the bands due to C=O and C-O-O indicated oxidation of the oil at continuous elevated temperature in the presence of oxygen. Polymer additives (7 8 %) are typically added to multiweight oil to prevent them from thinning excessively with increases in temperature. As the temperature is increased, the polymers uncoil and help to counteract the decreasing viscosity of the oil due to the increased temperature. When these polymers uncoil at higher temperatures, they may display greater susceptibility to both oxidation, and chain degradation. 2. Surface energy The surface tension of the DEO was also measured as a function of temperature. This was important to determine because in the theory of wetting explained earlier, in order to prevent oil uptake the surface energy of the oil should not fall below the surface energy of the rubber or film. If this occurs, then the oil will wet the surface, and oil uptake can occur. Also, since oil uptake testing using the dyed oil droplet test and the O- ring soak test was performed at elevated temperatures, it was useful to gather information on the surface tension of the oil at these temperatures. As shown in Fig. 32, the surface tension of the oil as a function of temperature decreased substantially. The trends were fitted with a linear regression with an R 2 greater than The surface tension of oil at room temperature was 25.3 mj/m 2. Most PPFC films had a lower surface energy than this value, so oil uptake would not be expected to occur at room temperature. When the temperature of the oil increased to 100 C, the surface tension of the oil dropped to 19.5 mj/m 2, which was close to some of the surface energies of the PPFC films. At 150 C, Results and Discussion 99

105 the surface tension of the oil reached 16.1 mj/m 2. This was lower than the surface energy of some of the PPFC films deposited during the course of this research. As shown later in this document, the % oil uptake of many coated samples increased remarkably with temperature, especially for those tested above 100 C. The decrease in the surface tension of the oil with increased temperature may be the cause. For the DEO which had been previously heated and showed oxidation by IR analysis, the surface tension was slightly lower, for all tested temperatures, as shown in Fig. 33. Therefore, this justified the consistent use of fresh (unheated) oil for the O-ring soak tests. C. Short monomer depositions 1. Plasma glow The plasma glow is an important effect of the plasma chemistry, and was monitored for each deposition. FC plasmas typically emitted a blue/purple glow. Acetylene plasmas typically emitted a white/light grey glow. The presence of a pinkish/orange glow was due to nitrogen and was indicative of a leak in the reactor or gas lines. Depositions where the glow was pink or orange were leak checked and repeated. Plasmas which ran a combination of hydrocarbon monomers such as acetylene and fluorocarbon monomers gave a light blue/grey glow, depending on the ratio of the monomers. The O 2 plasmas used to clean the reactor after deposition of PPFC films were light blue/purple initially due to FC species being etched off of the chamber walls. After some time, the glow changed to light grey, indicating a pure O 2 plasma. Results and Discussion 100

106 2. Deposition rate Deposition rate was calculated by taking the film thickness value found using ellipsometry and dividing by deposition time. The deposition rates for two of the monomer systems were compared, as a function of rf power: C 2 F 6 /C 2 H 2 and C 3 F 6 /Ar [see Fig. 34]. For the C 2 F 6 /C 2 H 2 system, the deposition rate increased up to 20 W of rf power. The sharp increase of deposition rate with a small amount of the HC monomer has been shown previously, 8, 39 and is due to the H-F elimination mechanism within the plasma. Then, it continued a downward trend until the plasma was mostly etching at approximately 100 W of rf power. At this point, the amount of H in the plasma was just not sufficient to counteract the etching F radicals. As rf power increased, more C-F bonds were broken in the C 2 F 6 molecules, releasing an excess of F, and turning the plasma mechanism towards etching. This is comparable since with greater H present, the maximum will shift. Wang et al. also showed similar trends of deposition rate of C 2 F 6 with varying amounts of CH The C 3 F 6 /Ar system did not reach a point of etching, as the C 2 F 6 /C 2 H 2 system did. With unsaturated monomers, the C=C bond was predicted to break preferentially, leaving a highly reactive radical for deposition. As rf power increased, more bonds would have broken, but the trend was still towards deposition, rather than etching. C 3 F 6 also led to a higher maximum deposition rate although a higher flow rate was used for the experiment. Samukawa and Mukai have also found that C 3 F 6 generated a much higher density of CF 2 radicals than C 2 F 6, as shown by infrared diode laser absorption spectroscopy. 32 Therefore, C 3 F 6 plasmas lead to a high concentration of CF 2 radicals in the plasma, which Results and Discussion 101

107 promotes the growth of the film, without the need to break C-C or C-F bonds as is necessary when polymerizing with C 2 F 6. Deposition rate was also found to be affected by the ratio of C 2 H 2 to the feed gas for the deposition of films using C 2 F 6 and C 2 F 5 H. Figure 29 shows the trends of deposition rate with increased C 2 H 2 concentration for these monomers, and was shown to reach a maximum for each. Other researchers have also reported a maximum in deposition rate as a function of the concentration of a hydrocarbon species in a FC 8, 44 plasma. The concave-downward shape of the curves found for the deposition rate of C 2 F 5 H and C 2 F 6 as a function of % C 2 H 2 were also very similar to those found by Golub for C 2 F 4 and C 2 H 4 copolymerization. 44 In research by Mackie et al., deposition rate also reached a maximum when a feed of C 2 F 6 /H 2 = 50/50 was used. 51 The presence of a concave-downward curve in Fig. 35 indicated a positive interaction (copolymerization) between the hydrocarbon and fluorocarbon monomers. The dashed lines in Fig. 35 indicate the predicted deposition rate if there were no interactions occurring between the FC and HC monomers. The deposition rate curve for C 2 F 5 H was slightly shifted to the right of the C 2 F 6 curve, due to the presence of the extra H atom in C 2 F 5 H. Also shown in Fig. 35, C 2 F 6 was merely an etching/fluorinating plasma without the addition of C 2 H 2. The use of a hydrocarbon as a source of H to alter the plasma chemistry towards deposition has been shown by other researchers, and follows with the 8, 41, 44 theories of H-F elimination outlined in the Introduction section of this paper. Mackie et al. also showed the absence of film deposition for plasmas composed entirely of C 2 F However, also shown in Fig. 29, a film was produced with a feed of only C 2 F 5 H since the H in the monomer may be used to eliminate excess F and inhibit etching. Results and Discussion 102

108 In this set of experiments, this condition showed the lowest deposition rate (aside from the etching conditions of pure C 2 F 6 ). This was consistent since the pure contained the lowest amount of H in the feed gas. Scavenging of excess F radicals in the polymerization of C 2 F 5 H has also been shown by Agraharam et al. where the F radicals were primarily scavenged as HF and CF Refractive index Figure 36 shows the variation in refractive index n, found using ellipsometry, on the PPFC films deposited with C 2 F 6 and C 2 F 5 H as a function of % C 2 H 2 in the feed gas. As the concentration of C 2 H 2 increased in the feed gas, n increased. This is desirable since n may be an accurate approximation of the density of the films. The film deposited purely with a feed of C 2 F 5 H was found to have the lowest refractive index (1.33). This is very similar to the refractive index of PTFE (1.35). 83 The film deposited entirely with C 2 H 2 was found to have the highest refractive index (1.89). Refractive indices found for films deposited using similar monomers by other researchers were varied. For a film deposited with CHF 3, n was 1.42, and for a film deposited with CF 3 CH 2 F n was Films deposited with a higher presence of H in the feed tended to have larger refractive 33, 83 indices. 4. Film chemistry and structure The ATR-IR spectra of the films deposited on PE using C 3 F 6 /Ar are shown in Fig. 37. Bands due to the PE substrate were shown, since for thin films analyzed using ATR, the IR radiation penetrates the film and detects vibrations due to the substrate and the film. The peaks identified due to the substrate were those at 2915, 2847, 1466 and 721 cm -1, from comparison with those found in Fig. 16. The main bands due to the film Results and Discussion 103

109 were a large absorbance band at 1225 cm -1 and a much weaker peak at 984 cm -1. The main band at 1225 cm -1 was due to a variety of CF 2, CF 3, and C-F stretching vibrations. According to d Agostino, absorbance bands due to CF 3 appear close to cm -1, while those due to CF 2 arise closer to cm The weak band at 984 cm -1 was assigned to CF 3. 8 Band assignments are shown in Table XII. The intensity of the bands due to the film increased as the film thickness increased, which is evident by comparing the deposition rate graph (see Fig. 34) to the spectra in Fig. 37. Another example of how the PPFC film structure changed with reactor parameters was shown by the RAIR spectra of films deposited on ferrotype plates using C 2 F 6 with varying amounts of C 2 H 2 (see Figs ). Since the spectra were taken in RAIR mode, all C-H peaks were shown to be due to the structure of the film. For these experiments, the rf power was held at 50 W, the total flow rate was held at 20 mtorr, and the deposition time was 5 min. Figure 38 shows a typical RAIR spectrum of a PPFC film with a large degree of fluorination, as shown by the relatively narrow peak at 1237 cm -1, due to C-F stretching. Table XIII shows all the band assignments for the spectra shown in Figs As the % C 2 H 2 increased, the main band at 1237 cm -1 shifted to lower wave numbers, indicating a decrease in the amount of fluorination. An interesting observation was the lack of peaks due to C-H stretching (shown in the region of cm -1 ) even for up to 70 % C 2 H 2 in the feed in Fig. 40. Many other researchers have also shown this phenomenon, which is presumably due to the C-H bonds inherently weaker than the C-F bonds, and the H-F elimination mechanism described previously. Winder and Gleason observed no C-H incorporation into a film deposited using a pulsed rf reactor and the CHF 3 monomer. 34 In Fig. 41, the film contained the main FC peak, but Results and Discussion 104

110 showed absorbance bands at 1734 cm -1 due to C=O stretching and 2968 cm -1 due to CH 3 stretching. As shown in Fig. 42, on a more narrowed scale around lower wave numbers, the changes to the IR spectra were predominantly to the main band for FC films, centered at approximately 1225 cm -1. As the degree of fluorination increased in the total flow rate, the band became narrower. As more C 2 H 2 was added to the feed gas, the peak became broader, which indicated a more randomly structured film. Golub et al. also have shown a broader main IR peak when copolymerizing C 2 F 4 with 50 % C 2 H 4, as compared to polymerizing with only C 2 F Also, a general observation that leads to some knowledge of structure is a broadening and a shift of the large band in the region of cm - 1 toward higher frequencies with increasing fluorination. The maximum of this absorbance band shifted from 1201 to 1235 cm -1, as the flow ratio of the fluorinated species increased. The slight shift of this peak towards higher wave numbers was due to the presence of higher fluorinated species, and this has also been shown by a number of 33, 40, 44 authors. The appearance of the distinct band at 980 cm -1 due to CF 3 appeared when the % C 2 H 2 was 30 % of lower. For all the films analyzed using XPS, it is important to note that all the films were much thicker than 100 Å, as found using ellipsometry. Therefore, when evaluating the structure of these films using XPS, all the peaks which appeared were due to the structure of the film and not to the substrate, since the escape depth of the photoelectrons is no greater than approximately 100 Å. Although ATR and XPS probe different depths of these films, both complimented each other to give an overall structure of the PPFC films. Results and Discussion 105

111 XPS analysis was done of the films deposited with varying C 2 H 2 in a feed of C 2 F 6. The C(1s) regions of the XPS spectra are shown in Fig. 43 for films deposited with C 2 F 6 and 30, 50, and 70 % C 2 H 2 in the feed. In Fig. 43(a), the C(1s) region of the XPS spectrum for the film deposited using a feed of C 2 F 6 and 30 % C 2 H 2 is shown. The film was highly fluorinated, as shown by the large peaks at and ev, due to CF 3 and CF 2 species. As the concentration of C 2 H 2 in the feed gas increased [see Figs. 43(b) and 38(c)], the peaks due to highly fluorinated species (CF 3 and CF 2 ) decreased, and peaks due to a less-fluorinated and more crosslinked film became dominant, such as the peak due to C-CF. This peak at ev (C-CF) is representative of a branch point in the polymer, and also may represent the degree of crosslinking in the PPFC film. Since the IR spectrum for these same films deposited with up to 70 % C 2 H 2 showed no peaks due to C-H bonding, this confirmed that the peak at ev in the XPS spectrum was solely due to C-C bonding. The values for the curve-fits for this series of experiments are plotted in Fig. 44. Both the CF 3 and CF 2 groups decreased with increased C 2 H 2 in the feed, as the C-CF peak rapidly increased. Therefore, the extent of crosslinking increased with increased concentration of the hydrocarbon monomer. The F/C ratio found from the XPS atomic concentration analysis was plotted as a function of the concentration of C 2 H 2 in the feeds of the monomers C 2 F 5 H and C 2 F 6, and shown in Fig. 45. Both monomers were shown to vary linearly with the feed of C 2 H 2, with R 2 values greater than or equal to The F/C ratio for the film deposited with C 2 F 5 H was 2.1, and indicated the presence of a lightly crosslinked film. Highly crosslinked films were roughly categorized as those with a F/C ratio less than 1.0, shown as those deposited with greater than or equal to 50 % C 2 H 2. The C 2 F 6 monomer resulted Results and Discussion 106

112 in films which were consistently more fluorinated than C 2 F 5 H. This was expected, since C 2 F 6 contains one more fluorine atom in its structure than C 2 F 5 H. The XPS and IR results correlated well since as C 2 H 2 was increased, the fluorination decreased, and the extent of crosslinking increased. As shown by this set of experiments, the degree of fluorination and the degree of crosslinking were altered by adjusting the feed ratio of the FC monomer and the C 2 H 2, which produced films with widely varying chemistry and structure. Figure 46 showed the XPS survey spectrum of the film deposited using C 3 F 6 /Ar in a ratio of 10/90 for a deposition time of 5 min., rf power of 20 W, total flow rate of 50 sccm, and process pressure of 500 mtorr. The survey spectrum showed peaks at 689 ev due to electrons from the F(1s) orbital and a broader peak at ev due to electrons from the C(1s) orbital. No oxygen was present in this film, as a peak would have appeared near 535 ev due to electrons from the O(1s) orbital. The atomic concentration analysis of the spectrum revealed that the film was composed of 63.8 % F and 36.2 % C, giving a F/C ratio of Looking closer at the C(1s) region in Fig. 47, the curve can be resolved into many peaks using curve-fitting of those assigned to C-F chemical shifts, as shown in Table II. The film was found to be composed of a variety of C-F bonded species, the majority of which were CF 3 (27.5 %), CF 2 (26.5 %), and C-CF (21.2 %). Therefore, this film was highly branched, with many end-groups, as shown by the high concentration of CF 3, but also highly crosslinked, as shown by the notable presence of the C-CF peak. Wang and Chen also found similar results with films deposited using C 3 F 6, where they concluded that the concentration of CF 3 incorporated into the film would always be larger than that of the CF 2 concentration, when conditions Results and Discussion 107

113 of low power and high pressure are used, as in this experiment. 39 This also correlated well with the IR (see Fig. 37) where the peak at 984 cm -1 appeared due to CF 3. Figure 48 shows the C(1s) region of a film with a similar structure to that shown in Fig. 47. This film was deposited using the same conditions (C 3 F 6 /Ar = 10/90, with a total flow rate of 50 sccm and 5 min. deposition time). However, the film was deposited with a higher rf power (140 W). The atomic concentration of this film was found to be 57.4 % F, 39.5 % C, 1.9 % O, and 1.2 % N. This film was deposited at a relatively high process pressure, which may have contributed to the small amount of oxygen and nitrogen in the film. Also, the increase in rf power may have removed more oxygenated species from the reactor walls and incorporated them into the film. The structure of the film revealed lower fluorination than the previous film deposited with 20 W of rf power (Fig. 47), as the F/C ratio for this film was only Also, the concentration of CF 3 (21.2 %) was lower than the concentration of CF 2 (29.8 %). A change in the F/C ratio was also noticed by Wang and Chen for films with the same monomer, where films deposited at 10 W of rf power resulted in a F/C ratio of 1.75, and films deposited at 120 W of rf power resulted in a F/C of only Fig. 49 shows the XPS C(1s) spectrum of a PPFC film deposited using the C 2 F 6 /C 2 H 2 = 90/10 system at an rf power of 50 W, and deposition time of 5 min. The atomic concentration analysis revealed that the film was composed of 63.9 % F, 34.8 % C, and 1.3 % O, with a F/C ratio of Curve-fitting of the C(1s) revealed 25.8 % CF 3, 0.9 % CF 3 -CF 2, 31.4 % CF 2, 11.6 % CF-CF x, 9.2 % C-F, and 17.0 % C-CF. Again, for this film the amount of CF 2 was slightly greater than the amount of CF 3. In the next spectrum shown, the film was produced with slightly more C 2 H 2 in the feed (20 %) with Results and Discussion 108

114 100 W of rf power and a deposition time of 40 min. Figure 50 shows the C(1s) region of the XPS spectrum for this film deposited on PE, where the atomic concentration analysis revealed 61.0 % F, 36.5 % C, 1.5 % O, and 1.0 % N, with a F/C ratio of Curvefitting of the C(1s) revealed 24.8 % CF 3, 1.7 % CF 3 -CF 2, 24.8 % CF 2, 13.6 % CF-CF x, 14.1 % C-F, and 21.0 % C-CF. Here the amount of CF 3 and CF 2 was approximately equal. Therefore, as the rf power was increased and the amount of C 2 H 2 was increased, the variability in the type of FC species increased. For another experiment, C 2 F 5 H was used solely as the monomer to make a PPFC film without flowing any C 2 H 2. The C(1s) region of the XPS spectrum is shown in Fig. 51. This film was deposited using a flow of 20 sccm, 50 W of rf power, process pressure of 40 mtorr, and a deposition time of 5 min. The atomic concentration analysis revealed 60.1 % F, 37.5 % C, and 2.4 % O, resulting in a F/C of Curve-fitting of the C(1s) revealed 20.0 % CF 3, 4.2 % CF 3 -CF 2, 22.3 % CF 2, 16.7 % CF-CF x, 15.2 % C-F, 20.0 % C-CF, and 1.6 % C-C. Films produced using monomers similar in structure to C 2 F 5 H, such as CF 3 H and C 2 F 4 H 2 were studied by Doucoure et al. 83 The presence of the increased amount of H was found to induce crosslinking reactions resulting in decreased concentration of F in the films. 83 As shown by Doucoure et al., depositions with CHF 3 resulted in 48 % F, 49 % C, and 3 % O, while depositions with C 2 F 4 H 2 resulted in 37 % F, 52 % C, and 9 % O. 83 Strobel et al. also showed a very similar C(1s) spectrum for a film deposited using CF 3 H. 83 An approximation of film structure obtained using small molecular mass monomers, such as C 2 F 5 H, C 2 F 6 /C 2 H 2, and C 3 F 6 are shown in Figure 52. This structure, as approximated by XPS and IR spectra, may be used for visual comparison to other film structures produces using the LCF monomer. Results and Discussion 109

115 Figure 53 shows a RAIR spectrum of a C 2 H 2 film deposited on ferrotype plate using only C 2 H 2 at 20 sccm, a process pressure of 50 mtorr, and rf power of 50 W. The spectrum revealed bands due to CH 3 and CH 2 components, at 2971, 2935, 1456, and 1383 cm -1. A strong band at 1725 cm -1 was assigned to C=O stretching and at 3454 cm -1 due to O-H stretching, which indicated that the film underwent a large degree of oxidation. Other specific band assignments are listed in Table XIV. As shown in Fig. 54, the XPS survey spectrum of the HC film deposited with only C 2 H 2 revealed peaks due to electrons from the O(1s) orbital at 535 ev and electrons from the C(1s) orbital at ev. Also a small peak was present at 686 ev due to electrons from the F(1s) orbital. The presence of F in the film was probably due to the walls of the reactor chamber from previous fluorocarbon depositions, or may have been due to a small amount of contamination discovered in the XPS chamber itself. The atomic concentration was found to be the following: C = 78.8 %, O = 20.4 %, and F = 0.8 %. Even though the reactor cleaning process was rigorous, the walls may still have contained some fluorocarbon species. These may have been removed from the walls during the plasma, and then were incorporated into the film. The extent of this would depend on the cleanliness of the reactor chamber. The C(1s) region of the XPS spectrum of the HC film deposited with C 2 H 2 is shown in Fig. 55. Since the atomic concentration of F was so low, bands due to C-F bonding were omitted from the curve-fit, and only bands due to C-O bonding were labeled and curve-fitted according to the values in Table II. The predominant functional group in the curve-fit corresponded to C-OH or C-OC. Based on the presence of the OH Results and Discussion 110

116 group in the IR spectrum, the predominant C-O bonding peak in the C(1s) was probably due to C-OH. Another common issue with C 2 H 2 films is the propensity for them to incorporate oxygen in the films. The oxygen concentration found in this film was rather high (20.4 %). The oxidation of C 2 H 2 plasma films has been studied previously by Retzko et al., using in-situ XPS analysis. A PPHC film produced using C 2 H 2 showed approximately 2 % of O by XPS analysis, when analyzed in-situ (without exposure to the atmosphere). However, upon exposure to air for only 5 min, 7.5 % O was shown in the film. After exposure of the film to the atmosphere for one week, 16.0 % O was shown. 49 Therefore, the propensity for these films to oxidize was great, and the oxidation rates were similar for the butadiene and the ethylene films as well. 49 Exposure to air allows for the diffusion of molecular oxygen into the bulk, which may recombine with C * radical sites. 49 This HC film was not expected to perform well as an oleophobic coating due to the presence of oxidation, giving a high surface energy. However, the presence of oxygenated species within PPHC films produced using C 2 H 2 has shown to increase the adhesive properties of these films to many substrates. Therefore, this film may be useful in the multilayer films described later in this document for adhesion purposes. The RAIR spectrum of a PPFC film deposited with C 3 F 6 /C 2 H 2 = 50/50, with 50 W of rf power is shown in Fig. 56. The spectrum consisted of an intense peak due to a variety of C-F stretching modes at 1232 cm -1. Also, oxidation was shown due to the C=O stretching peak at 1727 cm -1. Figure 57 shows the XPS survey spectrum of this film which revealed 49.2 % F, 48.8 % C, and 4.0 % O. Again, since the % O was so much smaller than the % F, the peaks in the C(1s) were only fit to those due to C-F bonding. Results and Discussion 111

117 Looking more closely at the C(1s) region in Fig. 58, there was a predominance of the C- CF band, revealing a highly crosslinked film. For this film, the band in the IR due to C=O was quite strong (see Fig. 53), although the % O according to the XPS analysis was small (4.0 %). Therefore, much of the oxidation was present deeper in the film. 5. Contact angle and surface energy The contact angle was found to vary widely depending on the film structure and chemistry. The contact angle of water on the C 2 F 6 and C 2 F 5 H films as a function of C 2 H 2 in the feed was examined. As shown in Fig. 59, the contact angle of water varied greatly with the % C 2 H 2 in the feed. As C 2 H 2 was added to the feed gas, the contact angle of water on the resulting film decreased. The major changes shown in film structure (plotted in Figs. 44 and 45) affected the water contact angle and also the surface energy, as calculated using the liquid contact angle measurements of di-iodomethane and glycerol with the Kaelble method. The total surface energies of the films were plotted as a function of the % C 2 H 2 in the feed in Fig. 60. As less % C 2 H 2 was added to the feed gas, the hydrophobicity increased and the surface energy decreased. The highest water contact angle measured for a film in this set of experiments was 111, and was obtained on the film deposited using C 2 F 5 H. In research by Mackie et al., the contact angle of water of the C 2 F 6 /H 2 films also steadily increased as the % C 2 F 6 in the feed gas increased. 51 A higher contact angle (118 ) resulted from the PE which was exposed to the C 2 F 6 plasma. Recall that this plasma did not deposit a film, as shown by ellipsometry on the ferrotype plate (see Fig. 34). However, it was likely that the plasma fluorinated the PE surface, resulting in an increase in the water contact angle from 90 to 118. Surface Results and Discussion 112

118 fluorination of polymeric substrates has been shown previously by Strobel et al. In that study, CF 4, another saturated fluorocarbon, fluorinated the surface of polypropylene resulting in a contact angle with water of Both the pure C 2 F 5 H and the C 2 F 6 plasmas resulted in surfaces with energies of approximately 14.1 mj/m 2. The PP film deposited with only C 2 H 2 resulted in a surface energy of 32.5 mj/m 2. Using C 2 F 6 and C 2 H 2, Srividya et al. also showed a correlation with increased C 2 F 6 in the feed and a higher F content, as shown by XPS, along with a decrease in surface energy from 52 mj/m 2 (100 % C 2 H 2 ) to 20 mj/m 2 (100 % C 2 F 6 ). 47 The calculated surface energy values followed a less linear trend than the water contact angle data, with greater scatter in the data. 6. Adhesion Testing Prior to adhesion testing by the tape peel test, the XPS of the tape surface itself was evaluated. As shown in Fig. 61, the XPS survey spectrum of the tape is composed only of C and O. The atomic concentration analysis revealed 81.6 % C and 18.4 % O. The lack of F within the tape indicates that if F is present on the tape after a tape peel test, it must be due to the PPFC film itself. The extent of the F present on the tape may be used as a qualitative measure of the failure of adhesion of the PPFC film to the substrate. Looking closer at the C(1s), in Fig. 62, it was clear that there was bonding of C-O and C=O present in the tape surface. These peaks appeared at and ev, which overlap somewhat with peaks due to some fluorinated species, such as C-F and C-CF. However, when performing a tape peel test, and analyzing the tape side of the peel surface by XPS, peaks in the C(1s) region at and ev due to CF 3 and CF 2 would indicate poor adhesion of the PPFC film. Results and Discussion 113

119 The film deposited with C 3 F 6 /Ar = 10/90, with a total flow of 50 sccm, rf power of 20 W, and 15 min deposition time was tested for adhesion to PE, and was shown to perform quite poorly. The original XPS spectrum of the film is shown in Fig. 47. After one tape peel test, the PPFC film could be visually shown on the tape side of the peel surface. As shown in Fig. 63, the XPS survey spectrum of the tape side of the peel test revealed a substantial peak due to F at 689 ev. The atomic concentration analysis revealed 46.5 % F, 47.3 % C and 6.2 % O. The presence of some O on the tape side of the peel surface indicated parts of the tape were still being analyzed by XPS, and that the film failure was either a patchy removal or removed less than 50 Å of material. The C(1s) region of the tape side of the peel test, shown in Fig. 64, was curve-fitted, again neglecting peaks due to carbon bonded with oxygen, for clarity. Analysis of the C(1s) revealed 13.6 % CF 3, 13.1 % CF 2, 10.8 % CF-CF x, 11.4 % C-F, 13.6 % C-CF, and 37.5 % C-C. The same region on this PPFC film sample was then tape tested repeatedly up to 20 X using fresh pieces of tape each time. Figure 65 shows the substrate side of the tape peel test, after being tape peeled 5 X, revealing that still there was still a large presence of F on the surface. The C(1s) region of the 5 th tape peel to the substrate still showed substantial presence of a thin FC film (Fig. 66). After tape peeling for 20 X, the XPS survey spectrum of the substrate side of the peel test is shown in Fig. 67, still showing a large F peak. Atomic concentration analysis revealed 47.6 % F, 50.1 % C, and 2.3 % O. Figure 68 shows the XPS C(1s) region of the substrate side of the peel test, after 20 tape peels, showing that there were still peaks due to fluorocarbon bonding present. Up to 28.5 % of carbons were bonded to one or more fluorine atoms. Therefore, if all of the area detected from the peak at C-C was due to the PE substrate and a uniform film layer Results and Discussion 114

120 was still present, then it could be assumed that the remaining film was approximately 30 Å. However, a patchy failure also may have occurred. Figure 69 shows a plot of the F/C ratio of the substrate side of the tape peel test, as a function of the number of time the peel test was performed. This shows that the adhesion of this highly fluorinated PPFC film to PE is very good. Interfacial failure did not occur even after 20 tape peel tests on PE. This strongly indicated that the adhesion of PPFC films to PE was adequate, but the cohesion of this PPFC film needed to be improved, even if patchy failure occurred. A similar film as the last one discussed was deposited with C 3 F 6 /Ar = 10/90 and 50 sccm of a total flow rate, but deposited with 140 W of rf power and was shown previously (see Fig. 48). The structure of this film was slightly different and showed a higher concentration of CF 2 (30 %) species than CF 3 (21 %). An adhesion test was conducted on this film deposited on PE, and XPS C(1s) of the tape side of the peel test is shown in Fig. 70. The % F revealed by atomic concentration analysis revealed only 17.3 % F on the tape side of the peel test. This was a substantial improvement from the tape peel test of the film deposited with 20 W of rf power, shown in Fig. 64, which revealed 50.3 % F on the tape side of the peel surface. The improvement of adhesion of this film was thought to be due to the lowering of the CF 3 /CF 2 ratio in the original XPS of the film. Performance of films with this structure indicated that CF 2 was important to the structure of the PPFC films for improving film cohesion. A film with a high concentration of CF 3 groups, while may impart low surface energy, also imparts many end groups and discontinuities into the polymer, giving less cohesion and mechanical integrity. Results and Discussion 115

121 7. AFM of films produced with small molecular mass monomers AFM images were obtained for the PP films deposited using C 2 H 2 and an example of these images, on a 10 x 10 µm scale, in both topography and phase mode, is shown in Fig. 71. This film was deposited on the industrial grade silicone rubber using 50 W of rf power, 20 sccm C 2 H 2, and 5 min deposition time. Within the AFM software, the roughness was calculated to be 480 nm, and the thickness of this film found using ellipsometry of the same film on ferrotype plate revealed a thickness of 73.6 nm. Therefore, most of the roughness was from the silicone rubber substrate. The morphology of the C 2 H 2 film was typical of many PP films shown since it was composed of spherical particles clustered together. In general, most plasma polymerized films show a globular, cauliflower-type morphology, which arises from gas-phase dominated plasma 85, 86 polymerization. Rapid polymerization may occur in the gas phase and produce solid 46, 85, 86 nano- and microparticles, which then aggregate on the surface of the substrate. Further polymerization on the substrate and growing film fills in the area between the globules. Since C 2 H 2 is a rapidly dissociating and depositing monomer, it was expected that gas phase polymerization would occur, resulting in formation of spherical globules within the growing film. The average size of the globules was approximately 0.25 µm. This particle size was similar to that found by Silverstein and Chen, where spherical particles formed in an acetylene plasma with a diameter ranging from 0.2 to 0.5 µm. 16 E. Multilayer Films First, the contact angle and surface energy of the multilayer films were tested. For multilayer film M1 (Table IV), the contact angle for DEO was 56.1 and the contact angle for H 2 O was After a tape peel test was performed on this film, similar Results and Discussion 116

122 contact angles remained: 54.6 for DEO and for H 2 O. Since the contact angle is a measure of the surface energy, and surface energy is a measurement of the top few monolayers of material, then the layers below the final layer should not have an effect on the surface energy, unless their presence alters roughness greatly. The contact angle values obtained here for M1 were very similar to those obtained for the final film, deposited directly on silicone rubber. The value for H 2 O contact angle was shown to be for the C 2 F 5 H single film as shown previously in Fig. 59. Therefore, the multiple layers under the C 2 F 5 H layer did not alter the wettability of the surface. A number of multilayer films were tested for adhesion to the industrial grade silicone rubber. M1 was found to display adhesion to the silicone rubber, as shown in Fig. 72(b), where the XPS survey spectra of the tape side of the peel test was compared for M1 to a single layer film [see Fig. 72(a)] on silicone rubber deposited only with C 2 F 5 H, the final layer of M1. Therefore, the surface properties of the two films were the same. The only difference was the interlayers in the multilayer film M1. The tape peel test for M1 revealed that only 3 % F was present on the tape side of the peel test, while the tape peel test for the single layer of C 2 F 5 H [see Fig. 72(a)] revealed 28 % F on the tape surface. Therefore, it appeared that the multilayer film, M1, was successful in imparting adhesion of the PPFC film to the silicone rubber. Another multilayer film was deposited with three layers, M2, using varying concentrations of C 2 F 6 /C 2 H 2, the parameters of which are listed in Table V. XPS survey (Fig. 73) of the tape side of the peel test for this film revealed only 6.3 % F on the tape surface, indicating fairly good adhesion. The C(1s) spectrum of the tape side of the peel test of M2 (see Fig. 74) revealed the majority of fluorocarbon species which appeared on Results and Discussion 117

123 the tape were CF 3. The M2 multilayer film gave similar contact angles with water and DEO as the other multilayer films: H 2 O = 106 and DEO = 59. M1 was deposited on medical grade silicone rubber and was tested for oil uptake using the watch glass method. Figure 75 shows the four samples which were tested: M1 at room temperature, M1 at 80 C, uncoated at room temperature, and uncoated at 80 C. The results show substantial buckling of the uncoated silicone rubber upon swelling in the experiment at 80 C. The silicone rubber sample coated with M1 showed significantly less buckling. However, there was still some buckling present, as shown by the arrows in Fig. 75, which indicated that there was some degree of swelling due to the oil. However, the deposited film did provide a substantial degree of protection to the rubber. There was no buckling the rubber away from the glass at room temperature for both the coated and the uncoated films, indicating again that swelling is strongly dependent on temperature, as was shown in Fig. 30 for the O-ring oil swelling test. Oil uptake testing was performed using the dyed oil droplet method with the M1 multilayer film deposited on the industrial grade silicone rubber plaques. These were tested for 24 hrs. at 100 C. Figure 76 shows the digital images of the plaques during the course of this experiment. Initially, the contact angles of the oil droplets on the surface were large, as shown in Fig. 76(a). After just 1 hour at 100 C, the oil began to spread slightly, as shown in Fig. 76(b), and oil uptake was shown to occur slightly around a few droplets as shown by the black arrows. After 12 hrs, there appeared to be extensive oil uptake of the rubber, shown by the dark diffused pattern surrounding the droplets. Interestingly, the rate of oil uptake for the film was not uniform for each droplet [see Fig. 76(c)]. However, after 24 hrs of testing, all the regions of rubber around each oil droplet Results and Discussion 118

124 showed a dark patch, indicating that oil diffused into the surrounding rubber. Therefore, the multilayer film M1 failed to inhibit oil uptake at 100 C for oil droplet exposure times up to 24 hrs. O-ring oil uptake tests were also done on the multilayer films M1 deposited on the O-rings. Six O-rings were coated with the multilayer film M1 and soaked in DEO for 24 hours, three O-rings at room temperature and three O-rings at 80 C. Table XV shows the values for oil uptake for the three O-rings tested at room temperature for 24 hrs. The average weight increase for the O-rings at room temperature was 2.7 %, well below the control value for oil uptake of uncoated O-rings at 7.0 %. Table XVI shows the values for oil uptake for the three O-rings tested at 80 C. The average oil uptake for these samples was 11.5 %. The average weight increase for the O-rings at 80 C, was approximately what was expected for a control O-ring % uptake at 80 C, based on extrapolating between the 50 C and the 100 C points in Fig. 24. Therefore, at 80 C, the multilayer film M1 did not show good barrier properties against the oil. Failure of the coatings on the O-ring may be either through solubility and diffusion mechanisms, or through a flow through defects mechanism. 26 For the experiments just described, it appeared that the coating was undergoing a solubility and diffusion mechanism of oil uptake since the data points are all relatively close for each temperature, and they were all higher for increased temperatures. However, the oil uptake data for the dyed-oil droplet tests seemed to follow a flow through defects mode, and then a diffusion mode, since the rates of oil uptake vary at the 12 hr point. Figure 77 shows an AFM image in topography and phase mode showing the multilayer film M1 on the industrial grade silicone rubber on a 10 x 10 µm scale. The Results and Discussion 119

125 roughness was found to be equal to 233 nm, and the image showed that the film displayed very similar globular morphology as described previously for the PP hydrocarbon film deposited using only C 2 H 2 (Fig. 71). F. Long chain fluorocarbon (LCF) films 1. Deposition rate Deposition rate as a function of rf power was determined for the films deposited using the LCF monomer. These values, along with those for another fluorocarbon system studied previously, C 2 F 6 /C 2 H 2, are shown in Fig. 78. Using the LCF monomer, with only 2 W of rf power, the film deposition rate was 3X greater than that of the C 2 F 6 /C 2 H 2 system at its highest deposition rate. Interestingly, both these monomers exhibited two maxima in the deposition rate as a function of rf power. Typically, most fluorocarbon monomers, such as C 3 F 6 and perfluoro-1,3-dimethylcyclohexane showed a single 85, 87 maximum in a plot of deposition rate vs. rf power. For the films deposited with the LCF monomer, the highest deposition rate occurred at the lowest rf power value. A possible explanation is that the deposition rate was initially high due to the rapid dissociation of the C=C bond leaving a large, CF 2 -dominated radical for deposition. This explanation is consistent with the work by Coulson et al. who also showed high deposition rates when using low rf powers, such as 1 W of average, pulsed rf power, and an inductively coupled rf reactor. 53 In this work, deposition rate decreased with rf power due to the increased etching ability of F radicals in the plasma from dissociation of C-F bonds. As rf power increased, smaller fragments of the original monomer were available due to further bond dissociation. With very high powers (160 W), more F radicals were available to contribute to etching mechanisms, and smaller fragments of the original Results and Discussion 120

126 monomer were incorporated into the film. Deposition rate decreased at very high rf powers in films deposited with both the LCF monomer and C 2 F 6 /C 2 H 2 due to the presence of etching mechanisms from a high concentration of F radicals present in the plasma at higher powers. Coulson et al. also showed a maximum in deposition rate at low rf powers (1W) using an inductively coupled rf reactor and a similar monomer. 53 Film thicknesses on ferrotype plate substrates ranged from 450 nm (rf power = 2 W) to 90 nm (rf power = 160 W), for deposition times of 15 min. An attempt was made to deposit a film with LCF monomer using only 1 W of rf power. However, this resulted in a sticky, viscous film on the inner surfaces of the reactor. This sticky film was assumed to be oligomeric, from monomer which had not fully polymerized in the plasma of this low power. Another experiment was done where ferrotype plate and PE substrates were placed in the reactor, and the monomer flowed for 10 minutes with no rf power (no plasma). The DEO contact angle on the PE was still 0, indicating that no monomer was adsorbed to the substrate, and none polymerized without the rf power present. Also, XPS analysis was done on the PE sample and no F was detected in the survey spectrum. Therefore, a deposition of a solid film requires that at least 2 W rf power be used in the plasma for this reactor. The average refractive index for these films was 1.393, which was very similar to that reported for PTFE (1.38), 75 and for other plasma polymerized fluorocarbon films (1.38) Morphology AFM images for the films deposited using 2 W of rf power revealed an unusual morphology from that typically shown for PPFC films, as shown in Fig. 79. In general, most plasma polymerized films show a globular, cauliflower-type morphology, which Results and Discussion 121

127 arises from gas-phase dominated plasma polymerization, which was discussed previously for the lower molecular mass monomers. 85 For the film deposited using 2 W of rf power, the images show a convoluted morphology without globules, as shown on a 10 x 10 µm and 2 x 2 µm area. Average roughness for the 2 x 2 µm and 10 x 10 µm areas were 6.6 and 8.5 nm, respectively. The AFM image shown in Fig. 79 displayed morphology very similar to that shown by Coulson et al. for a film deposited using the same LCF monomer and pulsed power, with a time-averaged equivalent power of only 0.07 W. 53 The globular morphology described earlier was present for the film deposited at 160 W of rf power, and is shown on a 10 x 10 µm and 2 x 2 µm area in Fig. 80. For the film deposited with 160 W of rf power, the average size of the globules was approximately 0.2 µm. Roughness values for the images were 7.1 nm and 12.4 nm for the 2 x 2 µm and 10 x 10 µm areas, respectively. Therefore, slightly rougher films were deposited at higher rf powers. This globular morphology for the films deposited with high rf power indicated the presence of gas-phase polymerization due to rapid monomer fragmentation. 85 However, the cauliflower-type morphology was not as pronounced as that seen for the PPHC films, or the PPFC films with low molecular weight monomers. The lack of globules in the AFM of the films deposited at 2 W of rf power may indicate a surface polymerization mechanism, rather then a gas-phase polymerization. The difference in morphology with rf power is likely to be due to the differences in the extent of fragmentation of the monomer. Another factor that may affect the extent of gas-phase polymerization is the process pressure. Recall that the film produced with 2 W of rf power was deposited at a pressure of 50 mtorr, and the film produced with 160 W of rf power was deposited at 150 mtorr. As pressure increases, the extent of gas-phase Results and Discussion 122

128 polymerization would also increase, so this may also have contributed somewhat to the differences in morphology Structure The IR spectrum of the LCF monomer applied to a ferrotype plate is shown in Fig. 81, with corresponding band assignments shown in Table XVI. The bands at 1655 and 1423 cm -1 53, 66 correspond to C=C stretching and CH=CH 2 in-plane deformation. Other bands due to C-H in the vinyl component of the monomer were present at 1065, 1018, 966, 888, 779 and 710 cm -1, due to C-H and CH 2 wagging. Upon plasma deposition of the monomer, all bands due to the C=C and C-H groups were absent in the IR spectra of the films, even when deposited with only 2 W rf power [see Fig. 82(a)]. Therefore, even for low rf powers, the C=C and C-H bonds were dissociated, and these species were not incorporated into the film structure. The CF 2 portion of the monomer remained roughly intact, as shown by the sharp absorbance at 1273 cm -1 due to CF 2 stretching. Since the spectra were gathered in RAIR mode, some understanding about the orientation of the chemical species in the film may be gained. Comparing the spectra in Fig 81 and in Fig 82, not only are the C-H and C=C species absent in the polymerized films, but there are also other changes to the structure of the C-F species. Band assignments are also given in Table XVII. A few notable changes are the blending of the two peaks at 1282 and 1252 cm -1 in the monomer into one peak centered at 1273 cm -1. Another noteworthy change is the reduction in the peak due to C-C-C bending of the CF 2 chain at 1200 cm -1. Also, the peak due to CF 2 asymmetric stretching at 1230 cm -1 was reduced, along with the peak at 1149 cm -1 due to CF 2 symmetric stretching. These three Results and Discussion 123

129 peaks all were lower in intensity upon polymerization, and have dipole moments in the direction perpendicular to the chain axis. 67 This indicated that the film may be slightly aligned with CF 2 chains perpendicular to the substrate. Figure 82b shows the IR spectrum of the film deposited at 160 W of rf power. The bands due to the vinyl portion of the monomer were again absent, and the spectrum was representative of a purely fluorocarbon film. These results correlate well with the results by Samukawa and Mukai who showed that the C=C bond will be broken preferentially in a plasma when using any unsaturated FC monomer, regardless of the molecular mass. 32 Coulson et al. have also shown this result of preferential C=C bond dissociation with both pulsed and continuous plasmas. 53 With higher rf power (160 W), the main band at 1269 cm -1 was broader than that shown in the spectrum for the film deposited at 2 W. This indicated that at higher powers the film contained less CF 2 species and a wider variety of C-F bonded species. A randomly structured FC film resulting in broadening of this band has been shown previously by Limb et al. 13 The RAIR spectrum also shows an increase in the peak at 1200 cm -1 due to C-C-C bending and an increase due to CF 2 symmetric stretching at 1153 cm -1. Since this chemical bend is polarized perpendicular to the chain axis, this indicates that there was less orientation in the film deposited at 160 W of rf power. 67 XPS analysis revealed that the films were composed predominantly of F and C, with a trace amount of O (< 1%). Since the presence of oxygen in a PPFC film has been shown to increase surface energy, 57 a goal of this research was to reduce the amount of oxygen incorporated into the films. The LCF films deposited showed a much lower amount of oxygen as compared to those films deposited with the lower molecular mass Results and Discussion 124

130 monomers. Also, although the process pressure (50 mtorr) was not that different than the base pressure of the reactor (20 mtorr) very little residual oxygen in the chamber was incorporated into the film. The XPS C(1s) spectra were fitted with the peaks shown in Table II, and given the 8, 68, 88 corresponding assignments. Since the atomic concentration of oxygen in the films was less than 1%, possible contributions to the C(1s) spectra at and ev, due to C-O and C=O, were not considered in the curve fit since they overlap with more prominent C-F peaks. 68 The peak at ev was a major contribution to the curve-fit, and was attributed to CF 2 -CF 3, the secondary effect of having an additional F in the β- position from CF The presence of this peak in the curve-fit of the LCF films was one of the notable differences between these curve-fits and those for the low molecular weight monomers. In the curve fit, the area of this peak was constrained to be less than or equal to the area of the CF 3 peak, since it would be impossible for more electrons to be detected from the secondary CF 3 environment than CF 3 itself. However, the CF 3 peak may be greater than the CF 2 CF 3 peak. In fact, the extent of the CF 2 CF 3 peak indicated the connectivity of the CF 3 species to the plasma polymer. The placement of all CF 3 species at the end of a CF 2 chain or adjacent to a less fluorinated species was significant to understanding the structure of these films. Figure 83(a) shows the XPS C(1s) spectra for the films deposited at various rf powers. The film deposited at 2 W of rf power was predominantly composed of CF 2 (67 %). As the rf power was increased to 160 W [see Fig. 83(d)] the concentration of CF 2 in the film was decreased, and the film contained a higher concentration of other FC species, such as CF 3 CF 2 and CF-CF 2, CF-CF. The C(1s) spectra of the films at Results and Discussion 125

131 intermediate RF powers {20 W [see Fig. 83(b)] and 40 W [see Fig. 83(c)]} revealed structures intermediate of those at 2 W and 160 W. For the 2, 20 and 40 W, the area of the secondary peak, CF 2 CF 3 was equal to the area of the CF 3 peak. This indicated that all of the CF 3 species were most likely located at the end of a CF 2 chain, and little or no CF 3 species were formed during the plasma process, but were all present due to the monomer. This results in an increased likelihood that the CF 3 species are exposed to the surface of the film. For the C(1s) in Fig. 83(d), for the film deposited with 160 W rf power, the end of the CF 2 chain was slightly disrupted, with less CF 3 species on the ends of the CF 2 chains. Therefore, it was less likely that these CF 3 species were exposed, or aligned, toward the film surface. These films were further examined by studying the XPS C(1s) as a function of take-off angle in order to depth-profile the surface. Recall that at low take-off angles, more of the surface was probed, and at high take-off angles, more of the bulk was examined. The XPS C(1s) spectra for the LCF film deposited using 2 W of rf power are shown in Fig. 84. For the 10 take-off angle, the film showed a high concentration of CF 2 species (67.2 %), which was calculated from the sum of the areas of the peaks at ev (CF 2 ) and ev (CF 2 -CF 3 ). Therefore, the large concentration of CF 2 indicated extensive retention of the perfluoromethylene monomer structure. The film also showed a slightly higher concentration of CF 3 closer to the surface than deeper in the film. This noteworthy result may indicate some degree of molecular orientation with chains of CF 2 tending toward the direction normal to the substrate, terminating with CF 3 species on the film surface. The spectra in Fig. 84 displayed a C(1s) spectrum which was very similar to that shown by Limb et al. for a film deposited in a pulsed reactor using Results and Discussion 126

132 C 3 F 6 O as the monomer. 13 The XPS C(1s) spectrum of the films produced by Limb et al. showed 65 % CF 2, 13 which was very similar for films analyzed in this research using the LCF monomer. As shown in Fig. 85, a remarkable decrease in the amount of CF 2 species (43.3 %) for the film deposited with 160 W of rf power, was shown, as compared to the film deposited with 2 W of rf power. With higher rf powers, the monomer became more fragmented in the plasma, resulting in a more varied structure than that for the films deposited with lower rf power. There were very slight differences between the spectra for each take-off angle for the film deposited at 160 W rf power, especially in the % CF 3. As shown in Table XVIII, the ratio of CF 3 to CF 2 as a function of take-off angle varied slightly for the films deposited at 2 W rf power. The ratio changed from 0.14 for the 80 take-off angle to 0.20 for the 10 take-off angle, indicating the presence of a change in the degree of CF 3 within the first 1.5 nm of film. However, this ratio remained relatively constant for the film deposited at 160 W rf power, where for the 80 take-off angle the ratio was 0.27 and for the 10 take-off angle the ratio was The change in the extent of CF 3 with very low take-off angle for the film deposited with only 2 W of rf power has only been seen in films deposited at low powers (< 5 W), and has been repeated. The XPS and IR results correlated well for the experiments with films deposited using the LCF monomer, as both showed increased fragmentation of the monomer and increased crosslinking, as shown by an increase in the peak due to C-CF bonding in the C(1s) region of the XPS spectra, with films deposited with higher rf powers. At low rf power, a more 2-D, CF 2 -dominated film was produced. Films deposited at rf powers Results and Discussion 127

133 between 2 W and 160 W revealed intermediate structures. Shown in Fig. 86 is a rough approximation of the LCF film deposited with 2 W rf power, as predicted from the XPS and the IR data. The sketch shows a film with some degree of orientation of CF 3 groups towards the surface, and light crosslinking throughout, with a high concentration of CF 2 species. 4. Contact angles and surface energies of LCF films Recall that all droplets of hydrocarbon liquids applied to the uncoated PE wetted the surface, while water produced a contact angle of 90. The water contact angles on the plasma coated PE using the LCF monomer at varying rf power ranged from 103 to 106 º. The higher values were obtained for the film deposited at 2 W of rf power. Hence, changes in the contact angles of water on these films as a function of rf power were not substantial, since the contact angle data for water varied only slightly. Therefore, water was shown to be an insufficient probe for oleophobic properties. A much more reliable indicator was hexadecane. This is somewhat intuitive since the surface energy of hexadecane is entirely dispersive, and the DEO should also be a highly dispersive liquid. On the other hand, H 2 O is a partially polar liquid. The changes in the contact angles of the hydrocarbon liquids, with varying rf power (CF 2 retention) are shown in Fig. 87. The error bars show the range of data for 10 measurements. Standard deviations in contact angle measurements were 3 4 and these were typical of those reported by other 38, 88 authors. All of the films increased the contact angles of the hydrocarbon liquids with the surface, since all hydrocarbon liquids wetted the PE surface prior to film deposition. The highest contact angle for hexadecane (73.8 ) was for the film deposited using only 2 W Results and Discussion 128

134 of rf power, as was the highest contact angle of DEO seen in this research (84 ). The other hydrocarbon liquids followed the same trend of higher contact angles with the films deposited at lower rf power, with greater CF 2 retention. Total surface energies, calculated using the Kaelble method, are shown in Fig. 88. For the PPFC films deposited with the LCF monomer, the surface energy decreased for films that were deposited with lower rf powers. A purely dispersive surface energy of 11.2 mj/m 2 was calculated for the film deposited with 2 W rf power. For the film deposited using 160 W of rf power, a surface energy of 19.5 mj/m 2 (γ D s = 18.0 mj/m 2 P, γ s = 1.5 mj/m 2 ) was determined. This value was fairly close to that of bulk PTFE (18.5 mj/m 2 ). 61 The Zisman plot which shows the critical surface energy can be seen in Fig. 89 for the films deposited with 2 and 160 W of rf power. γ c for the film deposited using 2 W of rf power was found to be 2.7 mj/m 2, with an R 2 of 0.99 for the linear extrapolation to the cos(θ) = 1 line. γ c for the film deposited using 160 W of rf power was found to be 17.6 mj/m 2 (R 2 = 0.98). The critical surface energy for the film deposited at 2 W of rf power was similar to that reported by Coulson et al. for a film deposited using the same monomer and pulsed power (1.5 mj/m 2 ). 53 The chemical structure of the films directly affected the surface energy, where the retention of CF 2 species resulted in high contact angles and low surface energies. This is interesting since some researchers conclude that changes in film structure and surface energy are due mainly to variation in the amount of CF 3 species. 43 However, these studies are typically of films with a highly varied structure, not dominated by CF 2. In these films, an increase in the amount of CF 3 species will lower surface energy, as Results and Discussion 129

135 usually the amount of CF 2 is relatively constant. 43 In some studies, lowered surface energy could be correlated in general with an increase in the degree of fluorination. 38 However, in this study, changes to the degree of fluorination with varying rf power were minor. For films deposited at 160 W and 2 W of rf power, the degree of fluorination only varied from 64 % to 66 % F, respectively. Therefore, the large changes in surface energy shown in Figs. 88 and 89 were not due to the extent of fluorination or an increase in CF 3, but rather the extent of CF 2 retention. The films deposited with very low rf powers also showed effects of orientation, and this has been shown to have a substantial impact on 53, 59 lowering surface energy. 5. Discussion of PPFC structure and surface energy During the course of this research, the relationship between the structure of PPFC films and the resulting surface energy was studied, as there are competing theories on this relationship in current literature. Some authors claim that maximizing CF 3 in a film will result in low surface energy. 19 Others claim that it is not simply the amount of CF 3, but rather the orientation which is important. The orientation of CF 3 on the surface may be perfected by an alignment of CF 2 groups terminating with CF 3 species. In the experiments with the LCF monomer, the surface energy clearly decreased as a function of perfluoromethylene (CF 2 ) retention, as shown when comparing the surface energy results in Figs. 88 and 89 to the structure of the films as shown in the XPS C(1s) spectra in Fig. 82. This relationship between CF 2 for lowering surface energy was examined for the films deposited using the lower molecular mass monomers as well. The relationship between the surface energy and the % CF 3, % CF 2 and % CF 3 + CF 2 (as found using Results and Discussion 130

136 XPS) was investigated, and plotted in Fig. 90 for the experiments performed with C 2 F 5 H/C 2 H 2 at varying % C 2 H 2. There was a trend with each of these species and their ability to decrease surface energy. However, the trend for decreased surface energy with CF 2 was the strongest correlation with an R 2 of CF 3 concentration has also been cited previously as the factor in decreasing surface energy, simply due to its correlation with increased fluorination and decreased surface energy. However, in one case, this was for a film deposited with HFA and C 2 H The correlation made in this case was with an increase in CF 3 content and also an increase in the overall F content. 29 Other researchers claim that it is not specifically the CF 3 group, but just an increase in fluorination, or extent of nonpolar groups. Inagaki et al. found that only CF 3, not CF 2, lowered surface energy, using films deposited with hexafluoroacetone. 43 This effect has also been shown by other researchers who have used monomers such as 2,3,5- trimethyl-3-hexene, which is a branched, unsaturated fluorocarbon with five CF 3 groups. 39 Siggurdson and Shishoo, however, have shown that increasing amounts of both CF 2 and CF 3 contributed to the lowering of surface energy. 38 In this study, retention of CF 2 content was prominent and shown to decrease surface energy, as CF 2 chains are also nonpolar. Many authors claim that the result of low surface energy is both the chemistry of the surface coupled with the organization of the CF 3 towards the surface. 19 Because the density of CF 3 groups on the surface is necessary to achieve the lowest surface energies, the % CF 3 found in a film is not the only factor to reduce the solid s surface energy. More importantly is the alignment of chains terminated by CF 3 to reduce surface Results and Discussion 131

137 energy. 19 In work by Zisman and coworkers it was shown that a perfluorinated dodecanoic acid adsorbed as a monolayer yielded a critical surface tension of 6 mj/m This showed the importance of the immediate surface in repelling liquids, and specifically the importance of the CF 3 group. 89 However, these monolayers are not useful as a coating because they are readily attacked when exposed to water. 63 Moreover, the CF 2 species is as important, if not more so than the CF 3, since this controls the orientation of the terminal CF 3 group outward towards the surface. A film containing a very high degree of CF 3 would represent a polymer with little cohesion, due to the large degree of end groups. Also, the CF 3 species in a polymer with a high crosslink density and a large degree of CF 3 are all orientated randomly in many different directions, thereby providing only a small amount of CF 3 on the film s surface. 6. Adhesion of the LCF films Results of the tape peel tests for the films deposited with 2 and 160 W of rf power are shown in Fig 91. The tape side of the peel surface for the film deposited with 2 W of rf power showed poor film adhesion by the appearance of a peak at ev in the C(1s) XPS spectra due to CF 3. However, the film deposited using 160 W of rf power showed excellent adhesion, as no peaks were present in the XPS C(1s) spectrum due to fluorocarbon bonding on the tape side of the peel surface. Looking closer at the atomic concentration analysis for the film deposited using 2 W of rf power showed the presence of 26 % F on the tape side of the peel surface. Poor adhesion for films deposited with low rf power was attributed to the fairly linear structure of the polymer. Since the structure of the films was predominantly CF 2 (67.2 %), there was not a high degree of crosslinking. Failure occurred cohesively within the film, as Results and Discussion 132

138 shown by the XPS C(1s) of the substrate side of the failure surface (Fig. 92). The peak shape was virtually identical to that of the film surface before the peel test [see Fig. 83(a)] indicating cohesive failure within the film. The tape side of the peel test for the film deposited at 160 W of rf power showed only a trace amount of F (0.5 %). The lack of an appreciable amount of F on the tape side of the peel test indicated adhesion of the film to PE and also cohesion within the PP film. This increase in film adhesion with a change in only rf power can be explained by the increased crosslinking present in the film, shown in the XPS C(1s) spectrum by the peaks due to C-F and C-CF [see Fig. 83(d)]. Crosslinking within the film was important since this gave the film a 3-D structure, which was essential for cohesion. Interestingly, the quality of adhesion followed a clear trend as a function of rf power, due to these changes in structure as shown in Fig. 93. This result was not an effect of differences in wettability of the films, as the tape adhered well to all samples. Rather, cohesive failure occurred in films deposited with low rf power due to the low degree of crosslinking. A small amount of crosslinking is very beneficial, since it inhibits the thermal motion of the polymeric chains. Therefore, transport of any hot liquid through the chains would be inhibited by the crosslinks because of the reduction in free volume. A final adhesion test was performed using a multilayer film on industrial grade silicone rubber, with the LCF film deposited with 160 W of rf power as the top layer. This multilayer film was labeled as M5, and the experimental parameters are outlined in Table VIII. Three tape peel tests were done on this film, with analysis of the tape side of the failure surface by XPS. These results were compared to the tape side of the failure surface of a peel test for the same LCF film deposited using the parameters M5, but Results and Discussion 133

139 without the interlayers. Therefore, the LCF film at 160 W of rf power was deposited directly on the silicone rubber. Upon performing the tape peel test on the M5 film, no film was visually shown as coming off on the tape. The XPS survey spectra of the tape side of the peel test of the multilayer film with the LCF top layer deposited with 160 W of rf power, M5, is shown in Fig. 94. This tape test for the multilayer film revealed 3.9 on the tape side of the peel surface. The other two results are shown in Table XIX, giving an average % F of 1.9. Therefore, this indicated that the multilayer film, M5, was adhesive to silicone rubber. When the tape peel test was performed only on the LCF film layer described in the M5 film parameters, but deposited individually on the silicone rubber, the film was clearly shown to not adhere directly to the silicone rubber. Fig. 95 shows the XPS survey spectrum of one of these tape surfaces. Atomic concentration values for three tape peel tests are shown in Table XIX, where the average of the three samples was 54.3 % F, indicating very poor adhesion without the multilayer film. The XPS C(1s) spectrum for the tape side of the peel test shown in Fig. 95 revealed the 58.6 % F on the tape side of the peel surface of the three samples. This result clearly showed the necessity of the hydrocarbon interlayers to promote adhesion of the highly fluorinated LCF film to the silicone rubber. 7. Oil Uptake of the LCF/multilayer films Oil uptake results for the LCF/multilayer films are perhaps the most promising thus shown. The multilayer film M5, with deposition parameters outlined in Table VIII was tested for oil uptake using the dyed oil droplet test. This film showed the most promise due to its adhesive properties to the silicone rubber, and its low surface energy Results and Discussion 134

140 and oleophobic behavior. Figs. 96, 97, and 98 show the dyed oil droplet tests for the coated silicone rubber, with the uncoated samples to the right of each for comparison. With the droplets still on the rubber, no oil uptake was observed for any of the samples at various temperatures. The DEO droplets were gently absorbed using a cotton swab to view the rubber underneath. The oil did not penetrate through the film for the room temperature test (Fig. 96), but did show some diffusion into the rubber for a few of the regions on the rubber for the 100 C test (Fig. 97), but was very promising for this. The oil droplet test for the 150 C test (Fig. 98) showed some penetration into the rubber, but it was very localized and did not show diffusion laterally into the area surrounding the oil droplet. There was still a large difference between the coated and the uncoated sample, where the oil diffused throughout the rubber plaque. One interesting observation is that the extent of oil uptake was much greater for the coated plaques tested at 150 C than 100 C. Recall that the surface energy of the oil drops to approximately 17 mj/m 2, as shown in Fig. 33, which is lower than the surface energy calculated for this film, 19.5 mj/m 2. However, at 100 C, the oil uptake is very slight, localized, and more likely attributed to flaws, rather than a diffusion mechanism. At 100 C, the surface tension of the oil is 20.5 mj/m 2, which is just slightly higher than the surface energy of the film (19.5 mj/m 2 ). On another piece of industrial grade silicone rubber, the multilayer film with the LCF film using 160 W of rf power was deposited (M5). To test if the film would have better oil barrier properties on areas of the rubber where there were no visible flaws swatches of the industrial grade rubber were cut and examined under the microscope at 16 X magnification. Dyed oil droplets were targeted on areas where no flaws were shown. Fig. 99(a) shows a sample with the M5 multilayer film after the placement of the Results and Discussion 135

141 droplets. Figure 99(b) shows the sample after 100 hrs at 100 ºC and no oil uptake was visible. This experiment showed evidence that with the deposition of this film, the diffusion of oil is likely to occur if there is a visible flaw on the surface and a flaw in the rubber. There is a high likelihood that these flaws are sand and dirt (silica). The significance of these surface flaws are notable when the coefficients of thermal expansion (CTE) are considered. The CTE values for the PTFE-like films and the silicone rubber are similar since they are both polymers. They are also both quite a bit larger than that of sand (silica). This type of CTE mismatch has been shown throughout literature to cause film cracking and delamination. This theory was further examined in another experiment which targeted the dyed oil droplets onto regions where visible flaws were shown on one sample, and onto regions were there were no flaws visible on another sample. Figure 99 shows the results from this experiment. Figure 100(a) shows the sample with droplets at flawless regions on the rubber and Figure 100(c) shows another sample with droplets at regions of flaws in the rubber. After testing for 100 hrs at 100 ºC, the samples were viewed. For these samples, it was more clear to view the oil uptake from the backside of the rubber. Figures 100(b) and 100(d) show the backside of each swatch. There was oil uptake present as shown by the blue hue in multiple places across the rubber. However, the swatch in which the droplets were placed in flawless rubber areas, there was no oil uptake. To further test the theory of oil uptake from the presence of flaws in the industrial grade rubber, the same film (M5) was deposited on swatches of food grade rubber which was mixed and cast in the laboratory. This rubber was covered during curing and stored in extremely clean conditions to minimize surface contamination. Medical grade rubber Results and Discussion 136

142 was also tested at the same time. Oil droplets were placed as shown in Fig After the 100 hr test at 100 C, there was no evidence of oil uptake on both the food grade and medical grade rubbers. This experiment was repeated with similar results. The M5 coating deposited on the industrial grade rubber was also tested in tension at 5.9 % strain, to observe the same type of effect as 31 % compression, which was the typical use environment of these O-rings. The dyed oil droplet test was carried out on the M5 film deposited onto the industrial grade silicone rubber, then strained to 5.9 % in tension. In Fig 102, after just 3 hrs at 100 ºC, the swatch showed signs of oil diffusion. After 100 hrs at 100 ºC, the stretched film/rubber allowed extensive diffusion of oil, as shown in Fig 102. Upon examination of the M5 film on industrial grade silicone rubber by ESEM at 500 X magnification, the film showed cracks in the direction perpendicular to the direction of strain (Fig. 103). This result strongly indicated that this film could not be applied to an environment where the film is strained to this degree. PPFC films have not been mechanically tested extensively in the literature. In one study by Butoi et al., a PPFC films was deposited on a copper wire, with ~ 80 % CF 2 species. 36 The film thickness ranged from 48 nm to 1920 nm. The film did not show delamination or cracking when the copper wire was bent in a loop of 0.5 cm diameter and observed via SEM. Therefore, there may be possibilities for enhanced mechanical behavior of the M5 film. The deposition of a thinner film may be possible to help counteract the strain imposed on the sample. 8. O-ring coating using the PPFC films O-rings were cleaned by simply wiping them with an ethanol-soaked Kimwipe, followed by drying with compressed N 2. They were then coated using the multilayer film Results and Discussion 137

143 M3, the experimental parameters of which are shown in Table VI. Six O-rings were coated and the oil uptake results are shown in Table XXI. Interestingly, the samples which showed the least amount of oil uptake were at the highest temperatures. Since more oleophobic samples were those tested at higher temperatures, the method of uptake must now be based on flaws in the film or the substrate. Since the O-rings were the industrial grade, the same as the industrial grade swatches, and show the same amount of flaws in SEM, it can be concluded that this is the most probable issue. The two 100 hr, 100 C O-ring samples showed 2.6 % oil uptake, by weight on average. An attempt was made to remove the surface debris imbedded in the industrial grade rubber O-rings by manual examination and excision of the flaws. This was done using an optical microscope at 16 X. This process, the microscopic cleaning process has been described thoroughly in the Experimental section of this document. Eight O- rings were cleaned in this manner, coated using the multilayer film M5 (Table VIII) then tested for oil uptake using by soaking in oil at 100 C for just 24 hrs as shown in Table XXII. The results for the O-rings were the lowest % oil uptake ever shown for a 24 hour test at this temperature, with an average of 0.9 %, with the highest % uptake of only 1.2 %. All the O-rings were firm to the touch and not mushy like an O-ring that has failed and swollen with oil. This test was then increased in time with a new batch of 8 O-rings up to the 100 hr at 100 C test point as shown in Table XXIII. These results were not as promising, as the oil uptake average was now 5.5 %. This is still a reduction of over 50 % that of the control value. However, it is over the goal of this project, which was less than 2 %. This exact experiment was repeated, with very similar results, as shown in Table XXIV, with an average of 5.5 % oil uptake for the O-rings. Results and Discussion 138

144 The difficulty with coating O-rings is that once there is a point of entry that the oil can follow diffuse through, then the limiting factor is just the diameter of this entry point, time and temperature. Eventually, the part will saturate with oil. Therefore, the coatings for the O-rings must be absolutely perfect, with no surface debris, pinholes, or cracking. Otherwise, these flaws must be small enough and the surface energy difference between the oil and the surface to be large enough that the oil does not seep into the flaw. The possibility is great that O-rings manufactured in a clean environment with food grade rubber, with the M5 coating would inhibit oil uptake successfully. Results and Discussion 139

145 V. CONCLUSIONS Plasma polymerized fluorocarbon (PPFC) films were deposited using a variety of monomers in an rf-powered plasma reactor. Conventional, low molecular mass monomers, such as C 2 F 6, C 2 F 5 H, and C 3 F 6 were studied as well as higher molecular mass monomers, such as 1H,1H,2H-perfluoro-1-dodecene. When C 2 H 2 was added to the feed of the low molecular mass monomers, a film was deposited and the deposition rate was higher than when the fluorocarbon monomers were used independently. The effect of H in the feed gas was shown to follow the theory of F scavenging to promote deposition, as shown in current literature. The refractive index of the films and the degree of crosslinking was also shown to increase with increased amounts of C 2 H 2. However, when a small amount of C 2 H 2 was added to the feed, films with increased fluorination and low surface energies were deposited, a desirable result for increasing oil repellency. C 3 F 6 and C 2 F 5 H could be used as monomers to deposit a film for all reactor conditions studied and did not reach an etching regime. For the C 3 F 6 monomer, this was due to the presence of unsaturation, and for the C 2 F 5 H monomer, this was due to the presence of H. In contrast, the saturated monomer, C 2 F 6, could not deposit a film when used independently. However, with the addition of C 2 H 2 to the feed of C 2 F 6, a film was deposited due to the F scavenging properties of the added H. PPFC films deposited with a high concentration of C 2 H 2 also showed oxidation, which increased surface energy. This was not advantageous for oleophobicity, but may have contributed to the films adhesive properties to silicone rubber. For the PPFC films deposited using low molecular mass monomers, increased fluorination led to higher contact angles of oil and lower Conclusions 140

146 surface energies. Nonpolar liquids such as hexadecane and toluene were found to predict oleophobicity much better than polar liquids, such as water. For the experiments with the LCF monomer, the C=C bond in the monomer was found to be dissociated by the plasma, resulting in films composed predominately of CF 2 species with no unsaturation. A variety of film structures were produced using this monomer by varying the RF power. Higher structural retention of the CF 2 chain from the precursor was obtained by reducing the RF power, thereby decreasing the fragmentation of the monomer. Some degree of orientation was shown for LCF films deposited with very low rf power (2 W), and showed a linear type of polymer composed almost predominantly of perfluoromethylene (CF 2 ) species. Films deposited with this monomer at 2 W of rf power displayed the highest contact angle of engine oil, 84, seen on any surface during the course of this research. This structure resulted in highly oleophobic films, with a critical surface energy of 2.7 mj/m 2. Increasing rf power produced films with a wider variety of fluorocarbon species, and resulted in a surface energy closer to that of bulk PTFE (γ c = 17.6 mj/m 2 ). Both CF 2 and CF 3 species were important to the proliferation of the polymer chain and the contribution to lower surface energy. While some researchers may cite CF 3 as the sole factor in low surface energy, CF 2 groups have also been found to be very significant, not only for continuity of the polymer chain, but also for orientation of the CF 3 to the surface. For the films deposited with the LCF monomer, those produced with a high concentration of CF 2 (~67 %) led to a non-cohesive film, due to too few crosslinks. The most adhesive film was a film with varied types of C-F bonding, but with the predominant type being CF 2 at approximately 46 % and crosslinking present. These Conclusions 141

147 films were deposited with the LCF monomer at higher rf powers ( W) and were shown to be both cohesive and oleophobic. Tape peel tests with XPS analysis allowed comparison of adhesive properties and correlation to structure. PPFC films adhered well to hydrocarbons, such as polyethylene (PE). Even after applying a tape peel test 20 X to a film deposited with C 3 F 6 and C 2 H 2 on PE, the presence of a thin fluorocarbon film remained on the PE surface. When PPFC films failed the tape peel test on a hydrocarbon surface, it was often due to cohesive failure. In films deposited with small molecular mass monomers, this was due to the high concentration of CF 3 end groups in the film. These films with a high concentration of CF 3 (~30 %) led to a non-cohesive film, due to a low continuity and high branching of the polymer. Multilayer films were developed which showed adhesion to both PE and to silicone rubber. These films consisted of a PP hydrocarbon first layer, and ended with a highly fluorinated layer. The hydrocarbon layer and the fluorocarbon layer in the multilayer film M5 were necessary for adhesion of the long chain fluorocarbon (LCF) film. An average of 1.9 % F was found on the tape surface for the tape test on the multilayer film M5, indicating excellent adhesion. An average of 54.3 % F was found on the tape surface of the tape test on just the top layer of M5, the LCF layer on silicone rubber, indicating poor adhesion. Therefore, the hydrocarbon film interlayers were necessary for adhesion of the fluorocarbon film to the silicone rubber. The multilayer film M5 deposited with hydrocarbon layers and the final layer with the LCF monomer showed excellent oleophobicity. Depositions of multilayer films with the LCF film as the top layer showed improved barrier properties when placed on Conclusions 142

148 regions of the silicone rubber swatches which did not show flaws under the optical microscope at 16 X magnification. Flaws were shown imbedded in the industrial grade rubber by optical microscopy and SEM. This led to the indication that the flaws in the rubber may be sources of oil uptake. This test was done for 100 hrs at 100 C, the test goals for the project. This test was reported on industrial grade rubber as well as food and medical grade rubber. None of these types of rubber showed oil uptake indicating that flaws in the industrial grade rubber were the cause of oil diffusion. This was confirmed by targeting areas with flaws, which did show oil diffusion. Therefore, the mechanism for oil uptake for the M5-coated industrial grade rubber was a flow-throughdefects mechanism and not a linear diffusion. Uncoated O-rings showed increased oil uptake with time and temperature, with the temperature being the more important variable to the % uptake. Two O-rings were coated and tested for oil uptake using the soak test for 100 hrs at 100 C and they resulted in 2.9 and 2.2 % oil uptake by weight. A surface refinishing process of the O-rings themselves did not decrease the likelihood of oil uptake, and showed similar variability in the oil uptake. Using the microscope cleaning process produced some O-rings with similar oil uptake, but resulted in variability of the % oil uptake, with an average oil uptake of 5.5 for 100 C and 100 hrs. This value was lower than the control value of 13 %, but higher than the value set for the goals of the project ( 2 %). The variations in values for oil uptake of the O-rings may also be attributed to the flow through defects issue. Based on these results, the films produced in this research showed promise for oilrepellent coatings up to 100 C and 100 hrs of testing. However, the substrate that the Conclusions 143

149 film is deposited on must be of high quality, and the flaws on the industrial grade rubber may have led to the variability in the oil uptake data seen for the oil uptake soaking experiments. Therefore, the oil uptake was not by diffusion, since it did not correlate with temperature or time. Due to the flaws in the industrial grade rubber, oil uptake by flow through defects was predicted to be the mechanism of oil uptake. Therefore, the films may show promise for rubbers of higher quality in static environments. Conclusions 144

150 VI. FUTURE WORK There are a few areas of future work that should be addressed for this project. For the films which show some evidence of alignment could be further characterized using near edge X-ray absorption fine structure (NEXAFS). X-ray diffraction could also be used to investigate if some degree of crystallinity was obtained for the films with long fluorocarbon chains, as is shown with PTFE. Flaws in the surface of the industrial grade silicone rubber were cited to be the most likely cause of film failure. The success of the films developed in this research deposited onto rubber of higher grades, deposited onto 3-D substrates could be investigated for the inhibition of solvents or oil into the silicone rubber. The films may also exhibit other useful qualities, such as inhibiting biofouling and lowering the coefficient of friction of silicone rubber. Future Work 145

151 VII. TABLES TABLE I. Bond energies for bonds relevant to the study of fluorocarbon plasmas and 16, 17 films. Bond Energy (kj/mol) H-F 568 C-F 488 C-H 413 C-C 348 TABLE II. Binding energies and assignments for peaks used in curve-fitting the XPS C(1s) region for many PP films. Fluorocarbon films were dominated by C-F bonding and 8, 71, 88 hydrocarbon films were dominated by C-O bonding. Binding Energy Assignment for Binding Energy Assignment for (±0.2 ev) FC Films (±0.2 ev) HC Films C-C, C-H C-C, C-H C-CF C-OR, C-OH C-CF C=O C-F CF-CF, CF-CF C=O-OR, C=O-OH CF CF 3 -CF CF 3 Tables 146

152 TABLE III. Experimental reactor parameters used for deposition of plasma polymerized fluorocarbon films with low molecular mass fluorocarbon monomers, such as C 2 F 6, C 2 F 5 H, and C 3 F 6. FC 2 nd Ratio of 1 st Total Flow Base Process RF Dep. Monomer gas gas to 2 nd gas rate Pressure Pressure Power Time (1 st gas) (sccm) (mtorr) (mtorr) (W) (min) C 2 F 6 C 2 H 2 0/ / C 2 F 5 H C 2 H 2 0/ / C 3 F 6 Ar 10/ TABLE IV. Deposition parameters of the multilayer film M1 which consisted of a 7- layer film of varying fluorination, deposited by varying the ratio of C 2 F 5 H/C 2 H 2. As each layer was deposited, the degree of fluorination increased. M1 7-layer C 2 F 5 H/C 2 H 2 Total Flow Process RF Deposition film layers flow ratio Rate Pressure Power Time (sccm) (mtorr) (W) (min) 1 0/ / / / / / / Tables 147

153 TABLE V. Deposition parameters of the multilayer film M2 which consisted of a 3- layer film of varying fluorination, deposited by varying the ratio of C 2 F 6 /C 2 H 2. M2 3-layer C 2 F 6 /C 2 H 2 Total Flow Process RF Dep. film layers flow ratio Rate Pressure Power Time (sccm) (mtorr) (W) (min) 1 0/ / / TABLE VI. Deposition parameters of the multilayer film M3 which consisted of a 4- layer film of varying fluorination, deposited by varying the ratio of C 3 F 6 /C 2 H 2, with a long chain (LCF) film deposited at 160 W of rf power as the final layer. M3 4-layer C 3 F 6 /C 2 H 2 LCF Total Flow Process RF Dep. film layers flow ratio monomer Rate Pressure Power Time (sccm) (mtorr) (W) (min) 1 0/ / / (a) (a) g/min Tables 148

154 TABLE VII. Deposition parameters of the multilayer film M4 which consisted of a 3- layer film with varying fluorination, deposited with a hydrocarbon layer, a fluorocarbon layer, then with a long chain fluorocarbon (LCF) film deposited with 2 W of rf power as the final layer. M4 3-layer C 3 F 6 /C 2 H 2 LCF Total Flow Process RF Dep. Film layers flow ratio monomer Rate Pressure Power Time (sccm) (mtorr) (W) (min) 1 0/ / (a) (a) g/min TABLE VIII. Deposition parameters of the multilayer film M5 which consisted of a 3- layer film with varying fluorination, deposited with a hydrocarbon layer, a fluorocarbon layer, then with a long chain fluorocarbon (LCF) film deposited at 160 W of rf power as the final layer. M5 3-layer C 3 F 6 /C 2 H 2 LCF Total Flow Process RF Dep. Film layers flow ratio monomer Rate Pressure Power Time (sccm) (mtorr) (W) (min) 1 0/ / (a) (a) g/min Tables 149

155 TABLE IX. ATR-IR band assignments for the industrial grade silicone rubber, used as a substrate for many PP film depositions. 66 The IR spectrum is shown in Fig. 20. Band Location (cm -1 ) Assignment 2963 CH 3 asymmetric stretching 2906 CH 3 symmetric stretching 1258 Si-CH 3 symmetric deformation 1003 Si-O-Si asymmetric deformation 865 Si(CH 3 ) 2 bending 783 Si(CH 3 ) 2 bending 696 CH 3 rocking TABLE X. Mechanical property data for the three types of silicone rubber used to measure oil uptake. Numbers shown are an average of three samples. Stress-strain curves are shown in Fig. 21. Industrial Food Medical Stress at Failure (MPa) Strain at Failure (%) Modulus (MPa) Fracture Toughness (MPa) Tables 150

156 TABLE XI. Transmission IR absorbance band assignments for spectra shown in Fig. 31 of the diesel engine oil used in the oil uptake experiments, before and after heating for 72 hrs at 150 C. 66 Band Location (cm -1 ) Assignment 2954 CH 3 asym. stretching 2924 CH 2 asym. stretching 2854 CH 2 symm. stretching 1706 C=O stretching 1464 CH 2 deformation + CH 3 asym. deformation 1366 C-H wagging in CH 3, with steric repulsion 1377 CH 3 symm. deformation 1388 C-H wagging in CH 3, with steric repulsion 1230 C-C-O stretching 722 CH 2 in-phase rocking TABLE XII. ATR-IR absorbance band assignments for plasma polymerized fluorocarbon films deposited on PE using C 3 F 6 /Ar = 10/90, with deposition time of 5 min, total flow rate of 20 sccm, and varying rf power from 10 to 140 W. Spectra are shown in Fig. 37. Band Location (cm -1 ) Assignment 2915 CH 2 asymmetric stretching in PE substrate 2847 CH 2 symmetric stretching in PE substrate 1466 CH 2 deformation in PE substrate 1225 CF 2 stretching in PPFC film 984 CF 3 in PPFC film 721 CH 2 in-phase rocking in PE substrate Tables 151

157 TABLE XIII. RAIR absorbance band assignments for films deposited using C 2 F 5 H/C 2 H 2 with varying % C 2 H 2 from 30 to 90 %. Other deposition parameters were 20 sccm total flow rate, 50 W of rf power, and 5 min deposition time. RAIR spectra for these films are shown in Figs Band Location (cm -1 ) Assignment 2968 CH 3 asymmetric stretching 1734 C=O stretching C-F and CF 2 stretching 738 CF 2, amorphous PTFE TABLE XIV. RAIR absorbance band assignments for the deposition of a hydrocarbon film on ferrotype plate using C 2 H 2 at a flow rate of 20 sccm for 5 min at 50 W of rf power. 66 A large degree of oxidation was shown by the peaks at 3454 and 1725 cm -1. RAIR spectra are shown in Fig. 53. Band Location (cm -1 ) Assignment 3454 O-H stretch (bonded) 2971 CH 2 asymmetric stretch in CH CH 3 out-of-phase stretch CH C=O stretch in R-CO-CH CH 2 deformation 1383 CH 3 deformation 627 CH 2 wagging Tables 152

158 TABLE XV. Oil uptake results for three O-rings coated with the multilayer film M1 and tested using the soak test for 24 hrs at room temperature. The average oil uptake for these coated O-rings was 2.7 %, compared to the uncoated control of 7.0 %. O-ring Weight Before Weight After Weight % Oil Uptake Number Immersion (g) Immersion (g) Gain (mg) by Weight TABLE XVI. O-ring oil uptake results for three O-rings coated with the multilayer film M1 and tested using the soak test, for 24 hrs at 80 C. The average oil uptake was 11.5 %, which was approximately equal to the estimate for the control value at this temperature. O-ring Weight Before Weight After Weight % Oil Uptake Number Immersion (g) Immersion (g) Gain (mg) by Weight Tables 153

159 TABLE XVII. RAIR absorbance bands assignments for the long chain fluorocarbon (LCF) monomer LCF plasma polymerized films, as shown in Figs Band Location (cm -1 ) Assignment 1655 C=C stretching 1423 CH 2 deformation 1368 C-F symmetric stretching in CF 3 (CF 2 ) n 1340 C-F asymmetric stretching in CF 3 (CF 2 ) n 1282 CF 2 stretching 1270 CF 2 stretching 1252 CF 2 asymmetric stretching 1230 CF 2 asymmetric stretching 1200 C-C-C bending in CF 2 chain 1150 CF 2 symmetric stretching 1065 CH or CH 2 wagging 1018 CH 2 wagging 966 CH wagging 888 CH 2 wagging 779 CH 2 wagging 740 CF 2 wagging, amorphous PTFE 710 CH wagging 662 CF 2 wagging and rocking 560 CF or CF 2 wagging 529 CF or CF 2 wagging TABLE XVIII. CF 3 /CF 2 ratio of the areas found from peak-fitting the XPS C(1s) spectra for the long chain fluorocarbon (LCF) films deposited with various RF powers (2 and 160 W). XPS analysis was done at take-off angles of 10 and 80, and it was shown that a change in the % CF 3 on the surface was present for the film deposited with 2 W of rf power. RF power Take-off Angle (W) Tables 154

160 TABLE XIX. Atomic concentration results found using XPS for the tape side of the three tape peel tests on different areas of the multilayer film M5 deposited on industrial grade silicone rubber swatches. Deposition parameters are shown in (Table VIII). Results show excellent adhesion, as indicated by very low values of the % F on the tape side of the peel surfaces, with an average % F of 1.9 %. Tape Peel Test % F % C % O Average TABLE XX. Atomic concentration results found using XPS for the tape side of the three tape peel tests on different areas of the long chain fluorocarbon (LCF) film layer described in the M5 film, deposited individually on industrial grade silicone rubber swatches. Deposition parameters are shown in (Table VIII). Results show very poor adhesion, indicated by a high % F on the tape side of the peel surface, with an average % F of 54.3 %. This experiment showed the necessity of the multilayer film with the hydrocarbon interlayer for adhesion. Tape Peel Test % F % C % O Average Tables 155

161 TABLE XXI. O-ring oil uptake results for six O-rings coated with the multilayer film M3 and tested using the soak test for 100 hrs at various temperatures. These O-rings were cleaned using Kimwipes and compressed N 2. Oil uptake was varied, and no longer dependent on temperature. Therefore, the oil uptake was not by diffusion, since it did not correlate with temperature. Oil uptake by flow through defects was predicted to be the mechanism of oil uptake. Temperature Weight Before Weight After Weight % Oil Uptake ( C) Immersion (g) Immersion (g) Gain (mg) by Weight TABLE XXII. Oil uptake results for eight O-rings coated with the multilayer film M5 and tested using the soak test for 24 hrs at 100 C. The O-rings were cleaned using the microscope cleaning method, where flaws on the surface of the O-ring were removed at 16 X magnification, and flash lines were smoothed. The average oil uptake was only 0.9 %, well below the uncoated value of 7.2 %. O-ring Weight Before Weight After Weight % Oil Uptake Number Immersion (g) Immersion (g) Gain (mg) by Weight Tables 156

162 TABLE XXIII. Oil uptake results for eight O-rings coated with the multilayer film M5 and tested using the soak test for 100 hrs at 100 C. The O-rings were cleaned using the microscope cleaning method at 16 X magnification, where flaws on the surface were removed and the flash lines were smoothed. The average oil uptake was 5.5 %, higher than that of the project objectives (< 2 %), but less than the control value of 13.0 %. O-ring Weight Before Weight After Weight % Oil Uptake Number Immersion (g) Immersion (g) Gain (mg) by Weight TABLE XXIV. Repeat experiment of that shown above in XXII. Average oil uptake was still 5.5 %, and the oil uptake between O-rings showed similar variability. O-ring Weight Before Weight After Weight % Oil Uptake Number Immersion (g) Immersion (g) Gain (mg) by Weight Tables 157

163 VIII. FIGURES CF H-F H CF 2 H-F F F CF 3 H CF 2 F H-F CF FIG. 1. Diagram showing the mechanism of HF elimination in a FC plasma due to the addition of H 2, or another source of H, to the feed of a saturated FC monomer. The result is an increase in the relative concentration of CF x species in the plasma environment, which shifts the plasma mechanism towards film deposition. γ lv γ lv θ γ sv γ sl γ sv γ sl Low Contact Angle High Surface Energy Ex. Water on Glass High Contact Angle Low Surface Energy Ex. Water on PTFE FIG. 2. Diagram showing the behavior of a droplet of water on two different surfaces: one with high surface energy and one with low surface energy. The surface energies of the liquid-vapor (γ lv ), liquid-solid (γ ls ), and solid-vapor (γ sv ) interfaces, and the contact angle (θ) are labeled, and may be related by the Young-Dupre equation [see Eq. (1)]. Figures 158

164 Vacuum (a) hν (b) Ejected K electron (c) Ejected KL 2,3 L 2,3 electron Valence band L 2,3 L 1 K FIG. 3. Energy level diagram showing the photoelectron and Auger electron emission process which occurs during an x-ray photoelectron spectroscopy (XPS) experiment: (a) initial state, showing incoming x-ray photon with energy hν approaching filled atomic orbitals, (b) photoemission process, with the release of a core electron to the vacuum (c) Auger emission process with the relaxation of the atom due to a L 2,3 electron falling to a lower energy level, K, with simultaneous emission of an electron from the L 2,3 orbital. Image adapted from Watts. 68 Figures 159

165 (a) hν e - (b) hν e - φ 1 φ 2 d 1 λ d 2 FIG. 4. Diagram of the photoemission of an electron, via bombardment with an X-ray photon. Analysis depth d 1 depends on the take off angle φ 1 and the mean free path λ of the electrons. Changes in take-off angle from tilting the sample stage result in changes in the sampling depth of the XPS experiment. In (a), the sample stage was horizontal so that φ In (b), the sample stage was tilted, so that φ 2 is less than φ 1, resulting in a shallower sampling depth (d 2 less than d 1 ). ~ Air N 1 φ 1 ~ Film N 2 φ 2 d ~ Substrate N 3 φ 3 FIG. 5. Diagram showing the reflections and transmissions of light directed onto a reflective surface with a film present, with thickness d. Two interfaces are shown, the first being the air-film interface, the second being the film-substrate interface. The angle of the light will change according to Snell s Law [see Eq. (12)] when the light reaches a material with a different complex refractive index N ~. Image adapted from Tompkins. 74 Figures 160

166 x, y Split z Photodiode Cantilever Tip FIG. 6. Schematic of an atomic force microscope (AFM) in TappingMode showing the cantilever and tip oscillation in the vertical (z) direction while the tip is rastered across the sample in the x and y directions. AFM is often used to study the morphology and roughness of plasma polymerized films. Image adapted from Digital Instruments Nanoscope IIIa Instruction Manual. 75 Figures 161

167 (a) (b) (c) (d) FIG. 7. O-ring surface refinishing and cleaning process, shown at 16 X magnification using an optical microscope: (a) Before cleaning, arrows show imbedded particles and surface debris (b) Imbedded particle cut out with a razor blade, as shown by arrow (c) Surface and flash lines were smoothed with polishing papers of 400, 800, and 1200 grit. Loose rubber particles remained on the surface, as indicated by the arrows. (d) Wiped with an ethanol-soaked Kimwipe and then blown with compressed N 2 to remove any loose rubber particles. Figures 162

168 Monomer and Carrier Gas Inlet Powered Electrode 3.5 cm Grounded Electrode FIG. 8. Digital image of rf-powered, parallel-plate reactor chamber, showing placement of the powered and grounded electrodes separated by a 3.5 cm gap. The monomer and carrier gas inlet is shown above the powered electrode. All substrates were placed in the center of the reactor on the grounded electrode. (a) F (b) F F (c) H F F F F F F F F F F F F F F FIG. 9. Structures of low molecular mass fluorocarbon monomers used for film deposition: (a) hexafluoroethane (C 2 F 6 ), (b) pentafluoroethane (C 2 F 5 H), and (c) hexafluoropropylene (C 3 F 6 ). Figures 163

169 (a) CF 3 CF 2 CF 2 CF 2 CF 2 CF 2 CF 2 (b) CF 2 CF 2 CF 2 CF 2 CF 3 CF 2 CF 2 CF 2 CF 2 C H 2 CF 2 CH H 2 C CF 2 CH FIG. 10. Chemical structures of the long chain fluorocarbon (LCF) monomers used for plasma polymerization: (a) 1H,1H,2H-perfluoro-1-dodecene and (b) 1H,1H,2Hperfluoro-1-decene. These monomers were unique since their molecular mass was higher than monomers typically used for plasma polymerization. Figures 164

170 Select the 1st test liquid that you used: Enter the average contact angle of the 1st liquid droplet on the sample: Select the 2nd test liquid that you used: Enter the average contact angle of the 2nd liquid droplet on the sample: Glycerol 100 Di-iodomethane 65 Reset The surface energy of your sample is: Polar 0.8 Dispersive 13.5 Total 14.3 mj/m 2 mj/m 2 mj/m 2 FIG. 11. HTML program written to calculate the polar (γ s P ) and dispersive (γ s D ) components of a solid's surface energy using the Kaelble method, which may calculate the surface energy using the contact angle of two liquids on the surface. Presence of F from film? Pressure Sensitive Tape Pressure sensitive tape PP Film(s) Substrate PP Film(s) Substrate FIG. 12. Diagram showing tape peel test method for adhesion of the PP films to the substrates. Pressure sensitive double-sided tape was applied to the coated rubber with finger pressure, then peeled back. The presence of fluorine on the tape surface as shown by XPS analysis would indicate poor adhesion of a PPFC film. Figures 165

171 FIG. 13. Digital image of coated swatch of industrial grade rubber, under 5.9 % tensile strain, using hose clamps. The specimen is shown inside a Phillips XL30 ESEM chamber, and the sample was analyzed to view film cracking under this amount of strain. FIG. 14. XPS survey spectrum of the PE substrate, used for many film depositions. The atomic concentrations were 97.6 % C, 1.8 % O, and 0.6 % Si. Figures 166

172 FIG. 15. XPS C(1s) region of the spectrum of the PE substrate, used for many film depositions, showing a single peak at ev due to C-C, C-H. Absorbance Wave numbers (cm -1 ) FIG. 16. ATR spectrum of the PE substrate, where all absorbance bands corresponded to known bands for CH 2 stretching (2915, 2847 cm -1 ), deformation (1466 cm -1 ), and rocking (721 cm -1 ). Figures 167

173 (a) (b) FIG. 17. XPS survey scan of ferrotype plate (a) before and (b) after the cleaning process. Before the cleaning process the atomic concentration of C was 61.1 %, and after the cleaning process the atomic concentration of C dropped to 25.0 %, which indicated a removal of much of the carbonaceous contaminant from the protective covering. Figures 168

174 C(KLL) O(1s) N(E)/E O(KLL) F(1s) C(1s) Si(2s) Si(2p) Binding Energy (ev) FIG. 18. XPS survey spectrum of the industrial grade silicone rubber substrate. Atomic concentration analysis revealed 45.1 % C, 27.2 % O, 25.7 % Si, and 2.0 % F N(E)/E Binding Energy (ev) FIG 19. XPS C(1s) spectrum of the industrial grade silicone rubber substrate, showing a single, symmetric peak due to the Si-CH 3 environment. Figures 169

175 FIG. 20. ATR-IR of industrial grade silicone rubber plaques, used as substrates for PP film deposition. Absorbance band assignments are given in Table IX. Figures 170

176 7 6 5 Stress (MPa) Strain (%) Stress (MPa) Strain (%) Stress (MPa) Strain (%) FIG. 21. Stress-strain curves for various silicone rubbers. a) industrial grade with modulus of 4.26 MPa, b) food grade silicone rubber with modulus of 0.52 MPa, and c) medical grade with modulus of 0.93 MPa. Averages for stress and strain at failure, Young s modulus, and fracture toughness are shown in Table X. Figures 171

177 FIG. 22. TGA experiments on medical, food, and industrial-grade silicone rubber used for testing oil uptake. Figures 172

178 FIG. 23. DSC runs of medical, food, and industrial-grade silicone rubber used for testing oil uptake. Figures 173

179 FIG. 24. Optical microscopy at 32 X of the industrial grade silicone rubber swatches showing imbedded debris, and foreign particles. Figures 174

180 100 µm 500 µm (a) (b) 200 µm 500 µm (c) (d) 100 µm (e) FIG. 25. SEM images of the industrial grade silicone rubber O-ring: (a) Surface flaw present on O-ring surface. (b) Cross section of an O-ring where a flaw was noticed. It is shown to extend approximately 800 µm below the O-ring surface. (c) Fibrous rubber particles connected to and extending from the rubber surface. (d) Cross section, showing the flash line of the O-ring. (e) Cross section of O-ring, showing dimensions of flash line. Figures 175

181 50 µm 25 FIG. 26. AFM image of untreated industrial grade silicone rubber on a 50 µm scale. The arrow shown indicates the peak-to-trough distance and is equal to 5.4 µm. The average roughness was 849 nm µm Figures 176

182 (a) (b) (c) (d) (e) FIG. 27. Dyed oil droplet test showing the oil uptake of the uncoated industrial grade silicone rubber at 100 C and various times: (a) initial application of oil droplets, (b) 1 hr, (c) 2 hrs, (d) 12 hrs, (e) 24 hrs. (a) (b) FIG 28. Dyed oil droplet test showing the oil uptake of the uncoated food grade silicone rubber where (a) is the swatch after droplet placement and (b) is the swatch after 100 hrs at 100 ºC, showing diffusion of oil into the rubber. Figures 177

183 (a) (b) FIG 29. Dyed oil droplet test showing the oil uptake of the uncoated medical grade silicone rubber where (a) is the swatch after droplet placement and (b) is the swatch after 100 hrs at 100 ºC, showing diffusion of oil into the rubber Oil uptake (%) Temperature ( C) FIG. 30. Oil uptake of uncoated O-rings, as a function of immersion time at 24 hrs ( ) and 100 hrs ( ), and as a function of temperature. These values were used as controls, for comparison to the % oil uptake of coated O-rings. Figures 178

184 FIG. 31. Transmission IR of DEO (a) after heating at 150 C for 72 hrs and (b) before heating. Band assignments are given in Table XI and reveal changes to the oil due to increased CH 3 and C=O species. An inset of the region between 1950 cm -1 and 1100 cm - 1 is shown in Fig 32. Figures 179

185 Absorbance After Heating 0.00 Before Heating Wavenumbers (cm -1 ) 1200 FIG. 32. Expansion of the region from 1950 to 1100 cm -1 of the transmission IR shown in Fig. 31 of the diesel engine oil before and after heating to 150 C for 72 hrs. The dashed line is the spectrum of the oil before heating and the solid line is the spectrum after heating. An increase in the bands at 1706 and 1230 cm -1 were apparent, both due to oxidation, and a splitting of the bands around 1377 cm -1 was also shown, due to increased CH 3 species. Figures 180

186 Surface tension (mj/m 2 ) Temperature ( C) FIG. 33. Surface tension of diesel engine oil before ( ) and after ( ) heating in air for 72 hrs at 150 C. There was a decrease in the surface tension as temperature increased. The previously heated oil also showed a slightly lower surface tension for all temperatures. Figures 181

187 Deposition rate (nm/min) rf Power (W) FIG. 34. Effect of power on deposition rate of PPFC films deposited using two different monomer systems: C 2 F 6 /C 2 H 2 = 90/10 ( ) and C 3 F 6 /Ar = 10/90 ( ), with rf power of 50 W, total flow rate of 20 sccm for C 2 F 6 /C 2 H 2 and 50 sccm for C 3 F 6 /Ar and deposition time of 5 min. Figures 182

188 20 Deposition rate (nm/min) % C 2 H 2 in Feed FIG. 35. Effect of concentration of C 2 H 2 in the feed on deposition rate for monomers C 2 F 5 H ( ) and C 2 F 6 ( ), with rf power of 50 W, total flow rate of 20 sccm, and deposition time of 5 min. Dashed lines indicate the deposition rate assuming no interaction between the C 2 H 2 and the fluorocarbon monomer. Figures 183

189 Refractive Index, n % C 2 H 2 in Feed FIG. 36. Effect of the concentration of C 2 H 2 in the feed gas on the refractive index of the resulting PPFC films, deposited using C 2 F 5 H ( ) and C 2 F 6 ( ). Films were deposited using 50 W rf power, 20 sccm total flow rate and 5 minutes deposition time. Figures 184

190 Absorbance Wave numbers (cm -1 ) FIG. 37. ATR-IR spectra of PPFC films deposited using C 3 F 6 /Ar = 10/90, deposition time was 5 min, total flow rate was 20 sccm, with varying rf powers. Peak assignments are shown in Table XII. Figures 185

191 FIG. 38. RAIR of PPFC film on ferrotype plate, with C 2 H 2 /C 2 F 5 H = 30/70, total flow of 20 sccm, rf power of 50 W, and deposition time of 5 min, revealing a sharp band at 1237 cm -1 due to C-F stretching. Other band assignments for spectra shown in Figs are shown in Table XIII. Figures 186

192 FIG. 39. RAIR of PPFC film on ferrotype plate, with C 2 H 2 /C 2 F 5 H = 50/50, total flow of 20 sccm, rf power of 50 W, and deposition time of 5 min. Band assignments are shown in Table XII. Figures 187

193 FIG. 40. RAIR of PPFC film on ferrotype plate, with C 2 H 2 /C 2 F 5 H = 70/30, total flow of 20 sccm, rf power of 50 W, and deposition time of 5 min. Figures 188

194 FIG. 41. RAIR of PPFC film on ferrotype plate, with C 2 H 2 /C 2 F 5 H = 90/10, total flow of 20 sccm, 50 W of rf power, and deposition time of 5 min. Figures 189

195 Absorbance 0.02 C 2 H 2 = 30 % C 2 H 2 = 50 % C 2 H 2 = 70 % Wavenumbers (cm -1 ) FIG. 42. RAIR of PPFC film on ferroplate substrates with varying degrees of C 2 H 2 in the feed gas of C 2 F 5 H. with a total flow rate of 20 sccm, 50 W of rf power, and deposition time of 5 min. Spectra are enlarged from Figs , but centered on the region of the main fluorocarbon peak. Figures 190

196 (a) (b) (c) FIG. 43. XPS C(1s) spectra of PPFC films deposited using C 2 F 5 H and varying amounts of C 2 H 2 in the feed gas as follows: (a) 30 %, (b) 50 %, and (c) 70 %. The films were deposited with 50 W rf power, 20 sccm total flow rate, and deposition time of 5 min. Changes to the degree of fluorination are shown by the changes in the % of peaks due to CF 3 and CF 2 at and ev. Figures 191

197 60 % of the C(1s) Curve-Fit CF 3 CF 2 CF-CF, CF-CF 2 CF C-CF C-C % C 2 H 2 FIG. 44. Films deposited using C 2 F 6 monomer, as a function of C 2 H 2 concentration in the feed, with a total flow rate of 20 sccm, rf power of 50 W, process pressure of 40 mtorr, and deposition time of 5 min. Figures 192

198 2.5 F/C Atomic Concentration Ratio R 2 = 0.97 R 2 = % C 2 H 2 FIG. 45. Correlation between the F/C ratio found using XPS in the films and the C 2 H 2 in the feed gas, for C 2 F 6 ( ) and C 2 F 5 H ( ), where the rf power was 50 W, total flow rate was 20 sccm, and deposition time was 5 min. Figures 193

199 FIG. 46. XPS survey spectrum of a film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, with rf power of 20 W and a total flow rate of 50 sccm, process pressure of 500 mtorr. Atomic concentration analysis revealed 63.8 % F and 36.2 % C, with a F/C ratio of FIG. 47. XPS C(1s) region of the spectrum of the film deposited on PE using C 3 F 6 /Ar in a ratio of 10/90, with rf power of 20 W, total flow of 50 sccm, process pressure of 500 mtorr. Curve-fitting revealed the following main peaks: 27.5 % CF 3, 26.5 % CF 2, and 21.2 % C-CF. Figures 194

200 FIG. 48. XPS C(1s) region of the spectrum of the film deposited on PE using C 3 F 6 /Ar in a ratio of 10/90, with rf power of 140 W and total flow rate of 50 sccm. Atomic concentration analysis revealed 64 % F, 33.5 % C and 1.4 % O and 1.1 % N. Curvefitting of the C(1s) revealed 21.2 % CF 3 and 29.8 % CF 2. Figures 195

201 FIG. 49. XPS C(1s) spectrum of a film deposited on PE using C 2 F 6 /C 2 H 2 = 90/10, rf power of 50 W and deposition time of 5 min. Atomic concentration analysis revealed 63.9 % F, 34.8 % C, and 1.3 % O. Curve-fitting revealed 25.8 % CF 3, 0.9 % CF 3 -CF 2, 31.4 % CF 2, 11.6 % CF-CF x, 9.2 % C-F, and 17.0 % C-CF. Figures 196

202 FIG. 50. XPS survey spectrum of a film deposited on PE with a similar film structure deposited with C 2 F 6 /C 2 H 2 = 80/20, with 100 W of rf power and deposition time of 40 min. Atomic concentration analysis revealed 61.0 % F, 36.5 % C, 1.5 % O and 1.0 % N. Curve-fitting of the C(1s) region revealed 24.8 % CF 3, 1.7 % CF 3 -CF 2, 24.8 % CF 2, 13.6 % CF-CF x, 14.1 % C-F, and 21.0 % C-CF. Figures 197

203 FIG. 51. XPS C(1s) spectrum of the film deposited on PE using only C 2 F 5 H at 20 sccm with 50 W rf power, process pressure of 40 mtorr, for a deposition time of 5 min. The atomic concentration analysis revealed 60.1 % F, 37.5 % C, and 2.4 % O, for a F/C of Curve-fitting of the C(1s) revealed 20.0 % CF 3, 4.2 % CF 3 -CF 2, 22.3 % CF 2, 16.7 % CF-CF x, 15.2 % C-F, 20.0 % C-CF, and 1.6 % C-C. Figures 198

204 Monomers F F F F F F C 3 F 6 hexafluoropropylene F F HC CH F F C 2 H 2 F F C acetylene 2 F 6 hexafluoroethane F H F F F F C 3 F 5 H pentafluoroethane CF H-F F CF 2 H Plasma CF 2 H-F F H-F F CF 3 CF H CF 3 C CF 3 C CF 2 Approximate Film Structure CF C C CF C CF 3 CF CF 3 C C CF 2 CF C C C CF CF 3 C CF 2 FIG. 52. Schematic showing the relationship between low molecular weight fluorocarbon monomers, the resulting plasma species and approximate film structure. Low molecular weight monomers result in a highly crosslinked and branched polymeric film. Figures 199

205 FIG. 53. RAIR spectrum of plasma polymerized C 2 H 2 at 20 sccm with 50 W rf power, for 5 min, showing peaks due to hydrocarbon (2971, 2935, 1456, 1383, and 627 cm -1 ) and oxidized (3454 and 1725 cm -1 ) species. Specific absorbance band assignments are given in Table XIV. Figures 200

206 FIG. 54. XPS survey spectrum of a hydrocarbon film deposited with 20 sccm C 2 H 2, process pressure of 50 mtorr, 50 W of rf power, flow rate of 20 sccm, and deposition time of 5 min. Atomic concentration analysis revealed 78.8 % C, 20.4 % O, 0.8 % F. Figures 201

207 C-C C-H C-OH C-OC C=O-OH C=O-OC C=O FIG. 55. XPS C(1s) spectrum of a plasma polymerized hydrocarbon film, deposited using 20 sccm C 2 H 2 and 50 W rf power, revealing peaks at 286.1, 287.4, and ev due to oxidation. The film was deposited with 50 mtorr, 50 W of rf power, and deposition time of 5 min. Figures 202

208 FIG. 56. RAIR spectrum of PPFC film using C 3 F 6 /C 2 H 2 = 50/50, with 50 W rf power, total flow rate of 20 sccm, and deposition time of 5 min. The band at 1727 cm -1 was due to C=O stretching, and the band at 1232 cm -1 was due to C-F stretching. Figures 203

209 FIG. 57. XPS survey spectrum of a film, deposited using C 3 F 6 /C 2 H 2 = 50/50, and 50 W of rf power and deposition time of 5 min. Atomic concentration analysis revealed 46.8 % C, 49.2 % F, and 4.0 % O. Figures 204

210 C-CF CF 3 CF 2 FIG. 58. XPS C(1s) spectrum of a film deposited using C 3 F 6 /C 2 H 2 = 50/50, 50 W of rf power, and deposition time of 5 min, showing a highly crosslinked film with a high concentration of C-CF. Figures 205

211 Contact angle of H2O % C 2 H 2 in Feed FIG. 59. Effect of the concentration of C 2 H 2 in the feed gas for the monomers C 2 F 6 ( ) and C 2 F 5 H ( ) on the contact angle of water on the resulting film, deposited on PE using 50 W rf power, total flow rate of 20 sccm, and deposition time of 5 min. The uncoated contact angle of water on PE was 90, represented by the bolded line. Figures 206

212 Surface Energy (mj/m 2 ) C 2 H 2 % in Feed FIG. 60. Effect of C 2 H 2 in the feed of C 2 F 6 ( ) and C 2 F 5 H ( ) on the surface energy of the films deposited with 50 W of rf power, total flow rate of 20 sccm, and deposition time of 5 min. Surface energy was calculated using the Kaelble method. The surface energy of uncoated PE was 33.1 mj/m 2, and is indicated by the bolded line. Figures 207

213 FIG. 61. XPS survey spectrum of the pressure sensitive adhesive double-sided tape used for the adhesion testing of the PPFC films on silicone rubber. Atomic concentration analysis revealed 81.6 % C and 18.4 % O. Presence of F(1s) at 686 ev in the XPS survey spectrum of the tape side of a tape peel test of a PPFC films would be indicative of film failure. Figures 208

214 FIG. 62. XPS C(1s) of the pressure sensitive adhesive double-sided tape used for adhesion testing of the PPFC films on silicone rubber. Curve-fitting of the C(1s) region revealed peaks at ev due to C-O and at ev due to C=O bonding. After a tape peel test on a PPFC film, the appearance of peaks at or ev would indicate the presence of CF 3 and CF 2 bonded species, indicating failure of adhesion. Figures 209

215 FIG. 63. XPS survey spectrum of the tape side of a peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, with 20 W of rf power and a total flow rate of 50 sccm. Atomic concentration analysis revealed 46.5 % F, 47.3 % C, and 6.2 % O. The presence of oxygen in the spectrum indicated a patchy removal of the PPFC film. Figures 210

216 FIG. 64. XPS C(1s) region of the tape side of a peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, with rf power of 20 W and total flow of 50 sccm. Curve-fitting analysis revealed 13.6 % CF 3, 13.1 % CF 2, 10.8 % CF- CF x, 11.4 % C-F, 13.6 % C-CF, and 37.5 % C-C. The XPS C(1s) of the film before the tape peel test is shown in Fig. 47. Figures 211

217 FIG. 65. XPS survey spectrum of the substrate side of the tape peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, with rf power of 20 W, and a total flow rate of 50 sccm. This sample was tape tested 5X. Atomic concentration analysis revealed 50.3 % F, 47.2 % C, 2.5 % O, and 0.1 % N. Figures 212

218 FIG. 66. XPS C(1s) region of the substrate side of the tape peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, 20 W of rf power, and a total flow rate of 50 sccm. This sample was tape tested 5X. Curve-fitting revealed 15.2 % CF 3, 8.6 % CF 2, 3.6 % CF-CF x, 8.6 % C-F, 14.1 % C-CF, and 50.0 % C-C. Figures 213

219 FIG. 67. XPS survey spectrum of the substrate side of the tape peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, 20 W of rf power, and a total flow rate of 50 sccm. The sample was tape tested 20 X. Atomic concentration analysis revealed 47.6 % F, 50.1 % C, and 2.3 % O. Figures 214

220 FIG. 68. XPS C(1s) region of the spectrum of the substrate side of the tape peel test for adhesion of the film deposited on PE using Ar and C 3 F 6 in a ratio of 90/10, 20 W of rf power, and a total flow rate of 50 sccm. The sample was tape tested 20 X. Curve-fitting revealed 13.4 % CF 3, 5.9 % CF 2, 3.3 % CF-CF x, 6.9 % C-F, 14.7 % C-CF, and 56.8 % C- C. Figures 215

221 F/C Ratio Number of Tape Peels FIG. 69. F/C ratio vs. the number of tape peels onto the film side of the PE, with the film deposited using C 3 F 6 /C 2 H = 9/10, with rf power of 20 W and deposition time of 5 min, showing that interfacial failure is not an issue for even a film with a large degree of fluorination. Cohesive failure occurred within this film. Figures 216

222 FIG. 70. XPS C(1s) of the tape side of the peel test for the PPFC film deposited using C 3 F 6 /Ar in a ratio of 10/90, with 140 W of rf power, and deposition time of 15 minutes, showing partial film failure. The C(1s) of the original film revealed 21 % CF 3 and 30 % CF 2 %, and is shown in Fig 48. Figures 217

223 FIG. 71. AFM images (topography and phase) in TappingMode of PP hydrocarbon film on industrial grade silicone rubber, deposited using 50 W of rf power, 20 sccm C 2 H 2 and 5 minutes deposition time. The average roughness determined by the AFM software was nm and the film thickness found using ellipsometry was 73.6 nm. Figures 218

224 (a) F(1s) C(KLL) F(KLL) O(1s) C(1s) F(2s) (b) C(1s) C(KLL) O(1s) F(KLL) F(1s) FIG. 72. XPS survey spectrum of the tape side of the peel test for adhesion of films deposited on the industrial grade silicone rubber using (a) a single layer deposited with 20 sccm C 2 F 5 H, 50 W of rf power, and deposition time of 5 min and (b) a multilayer film M1 (deposition parameters described in Table IV) a 7-layer film deposited with varying amounts of C 2 F 5 H/C 2 H 2. The single layer film showed film failure where the tape side of the peel test revealed 28 % F, and the multilayer film showed 3 % F, indicating adhesion of the multilayer film M1 to the silicone rubber. Figures 219

225 FIG. 73. XPS survey spectrum of the tape side of a peel test on a multilayer film (M2, a 3-layer film deposited with varying amounts of C 2 F 6 /C 2 H 2 ), resulting in 6.3 % F on the tape side of the peel surface. FIG. 74. XPS C(1s) spectrum of the above tape peel test on a multilayer film (M2, a 3- layer film deposited with varying amounts of C 2 F 6 /C 2 H 2 ), showing that the predominant species due to the delaminated film were CF 3 species. Figures 220

226 Untreated, Room Temperature Coated, Room Temperature Untreated, T = 80 C Coated, T = 80 C FIG. 75. Oil uptake experiments with the medical grade rubber coated with the multilayer film M1. Neither the uncoated nor the coated sample buckled up from the glass due to oil uptake, but the samples tested at 80 C showed differences where the coated sample buckled much less than the uncoated rubber. Arrows show slight buckling of the coated film tested at 80 C. Figures 221

227 (a) (b) (c) (d) FIG. 76. Dyed oil droplet test showing oil uptake at 100 C industrial grade silicone rubber coated with the multilayer film M1 at various times: (a) initial application of oil droplets, before heating, and after heating for (b) 1 hr, (c) 12 hrs, (d) 24 hrs. Figures 222

228 FIG. 77. AFM topography and phase image on a 10 x 10 µm scale of a multilayer film deposited on silicone rubber (M1). Surface roughness was found using the AFM software to be 233 nm. Deposition rate (nm/min) rf power FIG. 78. Deposition rate of the films deposited using the long chain fluorocarbon (LCF) monomer ( ) and the C 2 F 6 /C 2 H 2 = 90/10 system ( ) as a function of rf power. The LCF monomer was deposited at 0.14 g/min for 15 min. The C 2 F 6 /C 2 H 2 monomers were deposited at 20 sccm for 5 min. Deposition rate decreased as a function of rf power for both systems, but the LCF monomer plasma did not reach an etching condition, as the C 2 F 6 /C 2 H 2 did. Figures 223

229 a) b) FIG. 79. AFM phase images of PPFC film deposited on a ferrotype plate substrate using the LCF monomer and 2 W of rf power on two different scales: a) 2 x 2 µm, with R a = 6.6 nm and b) 10 x 10 µm, with R a = 8.5 nm.. Figures 224

230 a) b) FIG. 80. AFM phase images of PPFC film deposited on a ferrotype plate substrate using the LCF monomer and 160 W of rf power on two different scales: a) 2 x 2 µm, with R a = 7.1 nm and b) 10 x 10 µm, with R a = 12.4 nm. Figures 225

231 FIG. 81. RAIR of LCF monomer smeared on ferrotype plate. Bands at 1282, 1252, 1230, and 1149 cm -1 were the most prominent and were assigned to CF 2 symmetric and asymmetric stretching modes. A notable band was also present at 1423 cm -1 due to CH 2 deformation and at cm -1 due to CH and CH 2 wagging. All band assignments are given in Table XVII. Figures 226

232 (a) (b) FIG. 82. RAIR spectra of the film on ferrotype plate, deposited using (a) 2 W rf power, and (b) using 160 W rf power. Broadening of the bands at cm -1 showed an increase in the variety of C-F species incorporated into the film. No peaks due to the vinyl portion of the LCF monomer appeared in the spectra of the films. All band assignments are given in Table XVI. Figures 227

233 (a) (b) (c) (d) FIG. 83. XPS C(1s) spectra of the films deposited using the LCF monomer as a function of rf power: (a) 2 W, (b) 20 W (c) 40 W, and (d) 160 W. Increased fragmentation of the monomer was shown with greater rf power by increased CF 3 and CF species relative to the peak due to CF 2. Figures 228

234 (a) CF 3 = 13.5 (b) CF 2 = 67.2 CF 2 = 66.6 CF 3 = 9.0 FIG. 84. XPS C(1s) spectra of PPFC films deposited using the LCF monomer at 2 W rf power, for various take-off angles: (a) 10, and (b) 80. A greater concentration of CF 3 was present on the surface, indicating structural orientation of the CF 2 chains, perpendicular to the substrate, and terminated by CF 3 species. (a) CF 3 = 11.4 CF 2 = 43.3 (b) CF 3 = 12.4 CF 2 = 46.2 FIG. 85. XPS C(1s) spectra of the PPFC films deposited using the LCF monomer and 160 W rf power for various take-off angles: (a) 10, and (b) 80. Little changes between the concentration of CF 3 and CF 2 species were shown. Figures 229

235 H 2 C CF 2 CH Monomer CF 2 CF 2 CF 2 CF 2 CF 2 CF 2 CF 2 CF 2 CF 3 1H,1H,2H-perfluoro-1-dodecene Long chain fluorocarbon (LCF) F CF 3 CF 2 CF 2 Plasma, pulsed, low power or remote deposition CF 3 CF2 CF 2 CF2 CF 2 CF2 CF 2 CF2 CF 2 H H H-F CF2 CF 2 CF2 F CF 2 CF 3 CF 2 Approximate Film Structure CF 3 CF 2 CF 2 CF CF 2 CF 2 CF 2 CF 2 CF 3 CF 3 CF 2 CF 2 CF 2 CF 2 CF CF 2 CF 2 CF 2 CF 2 CF 2 CF 2 CF 2 CF CF FIG. 86. Schematic showing the relationships between a long chain fluorocarbon monomer, gentle plasma conditions, and the resulting approximate film structure. Using mild plasma conditions, PTFE-like films may be deposited with a high concentration of CF 2 and light crosslinking. Figures 230

236 Contact angle (θ) of liquid DEO Hexadecane Toluene Cyclohexane Hexane rf power (W) FIG. 87. Contact angle measurements for hexane, hexadecane, toluene, cyclohexane, and DEO for the LCF films deposited at various rf powers. The highest contact angle of diesel engine oil was 84 for the LCF film deposited at 2 W of rf power. All liquids shown in this graph wetted the PE substrate before film deposition (θ = 0), except for DEO (θ = 17). Figures 231

237 21 19 Total surface energy (mj/m 2 ) rf power (W) FIG. 88. Surface energy found by the Kaelble method, using hexadecane and glycerol as test liquids, for the LCF films deposited as a function of rf power. Figures 232

238 γ c = 17.6 mj/m 2 cos(θ) γ c = 2.7 mj/m Liquid Surface Energy FIG. 89. Zisman plot showing the critical surface energy (γ c ) for the LCF films deposited at 2 W of rf power ( ) and 160 W of rf power ( ), as found by extrapolating the surface energy of four hydrocarbon liquids to the cos(θ) of their contact angle on the film surfaces. The film at deposited at 2 W of rf power showed a γ c of 2.7 mj/m 2, and the film deposited at 160 W rf power showed a γ c of 17.6 mj/m 2. Figures 233

239 35.0 Surface energy (mj/m 2 ) CF 3 CF 2 CF 3 + CF 2 CF 2 + CF 3 : R 2 = 0.90 CF 3 : R 2 = 0.83 CF 2 : R 2 = % Fluorinated Species by XPS Analysis FIG. 90. Relationship between structure and surface energy as a plot CF 3 and CF 2 as found by XPS with the surface energy from depositions with C 2 F 5 H/C 2 H 2, indicating importance of CF 2 species. For these film depositions, the rf power was 50 W, total flow rate was 20 sccm, and deposition time was 5 min. CF 2 species appeared to be strongly correlated with decreased surface energy, more so than CF 3 species. Figures 234

240 (a) C-C, C-H CF 3 C=O C-O (b) C-C, C-H C=O C-O FIG. 91. XPS C(1s) region of the tape side of the peel surface of two long chain fluorocarbon (LCF) films deposited at rf powers of (a) 2 W, and (b) 160 W. The tape peel of the film deposited at 2 W of rf power revealed 26 % F, indicating poor adhesion. Also the peak at ev showed that the majority of the film species were removed as CF 3. The tape peel of the film deposited with 160 W of rf power revealed 0.5 % F and showed excellent adhesion. Figures 235

241 FIG. 92. XPS C(1s) region of the substrate side of the tape peel test for adhesion of the long chain fluorocarbon (LCF) film deposited on PE with 2 W of rf power. The spectrum shown here was virtually identical to the original spectrum of the film, as shown in Fig. 83(a), indicating cohesive failure within the PPFC film. Figures 236

242 % F on Tape Side of Peel Surface RF Power FIG. 93. Comparing adhesion of the LCF films to PE, deposited with varying rf power, as estimated by the % F on the tape side of the peel surface after a tape peel test, where the films with a low % F on the tape surface adhered well. The trend showed increased film cohesion with increased crosslinking, due to the higher rf power. Figures 237

243 FIG. 94. XPS survey spectrum of the tape side of the peel test of the multilayer film M5 deposited on industrial grade silicone rubber. This tape peel test revealed only 3.9 % F on the tape side of the peel surface. Atomic concentration values for three tape peel tests are shown in Table XIX. The average of the three tape peel tests on the film was 1.9 % F, showing excellent adhesion of the film to the silicone rubber. Figures 238

244 FIG. 95. XPS survey spectrum of the tape side of the peel test of only the LCF top layer of M5, deposited directly on the industrial grade silicone rubber. This tape peel test revealed 58.6 % F on the tape side of the peel surface. Atomic concentration values for three tape peel tests are shown in Table XX. The average of the three tape peel tests on the sample was 54.3 % F, showing very poor adhesion without the multilayer film between the rubber and the fluorocarbon film. Figures 239

245 a) b) FIG. 96. Dyed oil droplet test showing the extent of oil uptake on (a) the multilayer film M5 deposited on silicone rubber using the conditions outlined in Table VIII, compared to (b) the uncoated control sample. The samples were tested for 24 hrs at room temperature. No oil uptake shown for the coated sample. a) b) FIG. 97. Dyed oil droplet test showing the extent of oil uptake on (a) the multilayer film M5 deposited on silicone rubber using the conditions outlined in Table VIII, compared to (b) the control sample. The samples were tested for 24 hrs at 100 C. There was slight oil uptake on the coated sample, pointed out by the arrow. Figures 240

246 a) b) FIG. 98. Dyed oil droplet test showing the extent of oil uptake on (a) the multilayer film M5 deposited on silicone rubber using the conditions outlined in Table VIII, compared to (b) the control sample. The samples were tested for 24 hrs at 150 C. Oil uptake was extensive, though localized underneath the droplet region, as compared to the uncoated control sample. (a) (b) FIG. 99. Dyed oil droplet test on a multilayer film M5 deposited on silicon rubber using the conditions outlined in Table M5 (a) After oil droplet placement, before test, (b) after 100 hrs at 100 C. Clean areas were targeted for placement of the oil droplets by looking under microscope and applying oil to regions where there were no visible flaws under 16 X magnification. Figures 241

247 (a) (b) (c) (d) FIG An experiment similar to that shown previously in Fig. 99 was conducted. The multilayer film M5 was deposited on each swatch of industrial grade silicone rubber. Regions (a) without flaws and (c) with flaws were targeted with the dyed oil droplets. The flipside of each rubber swatch after 100 hrs at 100 C are shown in (b) and (d). The rubber swatch with droplets of oil on flawed regions showed more oil uptake than the swatch without. Figures 242

248 (a) Medical grade Food grade (b) Medical grade Food grade FIG Dyed oil droplet test for oil uptake on the multilayer/lcf film M5 deposited on medical grade and food grade rubber, showing specimens (a) before and (b) after 100 hrs at 100 ºC. No evidence of oil uptake was shown, indicating the success of the M5 film deposited on high quality rubber in a static environment. Figures 243

249 (a) (b) FIG Stretched oil uptake test on industrial grade silicone rubber. Silicone rubber swatches coated with multilayer film M5 were stretched to 5.9 % strain (simulating 31 % compression in use environment). Droplets were placed on the coated rubber and observed after (a) 3 hours and (b) 100 hours at 100 ºC. Figures 244

250 FIG SEM image from the M5 coated industrial grade silicone rubber under 5.9 % tensile strain (simulating 31 % compression in the use environment). Images show cracking in the direction perpendicular to the imposed strain, indicating that this film was not applicable to environments with this amount of static strain. Figures 245