INFRARED-VISIBLE SUM FREQUENCY GENERGATION STUDIES OF WATER AT THE POLYMER/SAPPHIRE INTERFACE. A Thesis. Presented to

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1 INFRARED-VISIBLE SUM FREQUENCY GENERGATION STUDIES OF WATER AT THE POLYMER/SAPPHIRE INTERFACE A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements of the Degree Master of Science Jing Zhou August, 2013

2 INFRARED-VISIBLE SUM FREQUENCY GENERGATION STUDIES OF WATER AT THE POLYMER/SAPPHIRE INTERFACE Jing Zhou Thesis Approved: Accepted: Advisor Dr. Ali Dhinojwala Dean of the College Dr. Stephen Z. D. Cheng Faculty Reader Dr. Mesfin Tsige Dean of the Graduate School Dr. George R. Newkome Department Chair Dr. Coleen Pugh Date ii

3 ABSTRACT Water between two solid surfaces plays an important role in interfacial adhesion, catalysis, corrosion, microelectronic industry and biomaterials. (1, 2) However, research on molecular-level structure of confined water between two solid surfaces and the disruption of interactions between these two surfaces caused by water are lacking. Infrared-visible sum frequency generation spectroscopy (SFG) is used to directly probe confined water between polyurethane (PU) and the sapphire substrate after exposing the polyurethane films to liquid water and water vapor. In liquid water condition, the observation of SFG peaks associated with H 2 O bands ( cm -1 ) and D 2 O bands ( cm -1 ) indicate water molecules ingress to the buried interface and exist in the form of hydrogen-bonded water network. The water layer disrupts interactions between polyurethane and hydroxyl groups on the substrate. When PU films were exposed to water vapor, the SFG signal corresponding to PU hydrocarbon groups significantly increase, while the SFG signal of sapphire hydroxyl groups decrease, which indicates water molecules reach the interface. However, no hydrogen-bonded water network was observed, instead, H 2 O peak at 3555 cm -1 and D 2 O peaks ( cm -1 ) show up which can be assigned to low-coordination water. An alternate assignment could be the hydroxyl groups hydrogen bonded with carboxyl groups of PU. Water molecules cannot form a uniform monolayer at the interface and as a consequence cannot completely disrupt the PU-sapphire bonds. These results provide two distinct states of water at the polymer-solid interface, which iii

4 could influence the interfacial bonding state differently and have important implications in understanding interfacial adhesion, coatings and corrosion. iv

5 ACKNOWLEDGEMENTS I would like to thank my advisor, Dr. Ali Dhinojwala, for his encouragement, support and guidance to my research. He gave me the chance to fulfill my dream and enjoy the scientific exploration. I appreciate all the group members for discussing and sharing their knowledge with me. We had a great time working together in the past three years. In particular, I want to thank the whole SFG group for assisting me in SFG experiments. I am very grateful to my parents for their support. With everything they made available to me, I could pursue my dream without any burden. I also appreciate all of my friends for sharing happiness and pain with me. They gave me good memories in Akron. Finally, I would like to express my immense gratitude to my boyfriend Fei Lin for his love and care. He never stopped supporting me even in the most difficult time. This achievement would not have been possible without him. v

6 TABLE OF CONTENTS Page LIST OF TABLES...viii LIST OF FIGURES...ix CHAPTER I. INTRODUCTION...1 II. BACKGROUND Water Confined Between Two Solid Surfaces Infrared-Visible Sum Frequency Generation Spectroscopy Basic Principles SFG Study of Polymers SFG Study of Water...24 III. EXPERIMENTAL Sample Preparation Ellipsometry Measurements Quartz Crystal Microbalance (QCM) Measurements Sum Frequency Generation Spectroscopy Apparatus and Measurement Total Internal Reflection (TIR) Geometry Combined with SFG Measurement...40 vi

7 3.6 Sample Cell and the Humidity Controlling Set-Up...41 IV. RESULTS AND DISCUSSION Calculating Contributions of SFG Signal from Two Interfaces as a Function of the Film Thickness The Thickness of Polymer Films by Ellipsometry Water Uptake of Polymer Films by QCM PU-Air Interface and PU-Sapphire Interface of Samples in the Ambient Condition by SFG PU-Water Interface and PU-Sapphire Interface after Exposure of Samples to Liquid Water by SFG Sapphire-Water Vapor (or Dry N 2 Gas) Interfaces by SFG PU-Sapphire Interface after Exposure of Samples to Water Vapor by SFG The Effect of Hydrophobicity for Polymer Coating on Water Transport.62 V. CONCLUSION...65 BIBLIOGRAPHY...67 APPENDIX...77 vii

8 LIST OF TABLES Table Page QCM experimental data and calculated data SFG fitting parameters with PPP polarization deduced from Figure for the experiments of probing the PU-air interface and the PU-sapphire interface SFG fitting parameters with SSP polarization deduced from Figure for the experiments of probing the PU-air interface and the PU-sapphire interface SFG fitting parameters with PPP polarization deduced from Figure for the experiments of probing the PU-H 2 O interface and the PU-D 2 O interface SFG fitting parameters with PPP polarization deduced from Figure for the liquid H 2 O and D 2 O experiments of probing the PU-sapphire interface SFG fitting parameters with PPP polarization deduced from Figure for the water vapor experiment of probing the PU-sapphire interface at various RH (%) of H 2 O vapor SFG fitting parameters with PPP polarization deduced from Figure for the water vapor experiment of probing the PU-sapphire interface at various RH (%) of D 2 O vapor Relevant peak assignments for H 2 O, D 2 O and PU next to various interfaces. 61 viii

9 LIST OF FIGURES Figure Page Force between two mica surfaces in mm KCl solutions as a function of separation distance. Below 2 nm, the force is oscillating with a periodicity of the diameter of water molecules (2.5 ). Above 2 nm, the force is repulsive. Inset: theoretical prediction of force for this system based on a non-continuum molecular theory. (29) Whole-animal shear (frictional) adhesion forces from tokay geckos tested on four surfaces in dry or wet contact. Each gecko (n = 6) was tested three times on each surface (glass, PMMA, OTS-SAM coated glass, and PTFE), and the highest of the three tests was used for data analysis. Surfaces were tested either without water (dry) or fully submerged in water (wet). (30) (a) The model for the impedance of a polymer coated metal and (b) theoretical impedance spectra for a degraded polymer coating. (31) Solid points represent the moisture uptake into thin PnOMA films coated on quartz crystals. corresponds to the adsorption on the blank crystal. The dashed lines are linear fits to the data. (17) The X-ray reflectivity profiles for polyacrylate layer on Al 2 O 3 with different curing conditions in vacuum (gray curve) and exposed to saturated H 2 O vapor at 60 (black curve). The reflectivity curves are offset for clarity. (18) The Neutron-scattering length density profile from the fitting of the Neutron reflectivity profiles for samples (polyacrylate layer on Al 2 O 3 ) in vacuum (solid line) and exposed to saturated water vapor (dash line). (18) A model illustrating the contact region formed between the PDMS lens and the sapphire surface in the presence of confined water. A tentative physical model of the molecular structure in these two types of contact area s is also shown in the panels on the right. The relative sizes of these two regions cannot be quantified from measurements. (32)...13 ix

10 Schematic of the laser beams for SFG spectroscopy at an interface. (35) Definition of Euler angles relating lab axes to molecular axes The SFG spectra of the vapor/water interface, IR spectra of bulk water and bulk ice, and molecular structure of hexagonal ice. Upper-left: The SFG spectrum of the vapor/water interface shows three OH bands at 3200, 3400 and 3700 cm -1. Upper-right: side view of the bulk ice near the (0001) surface. Bottom-left: The IR spectrum of bulk ice shows a dominant peak at 3150 cm -1. Bottom-right: The IR spectrum of bulk water is dominated by a broad peak at ~3400 cm -1. (60) The SFG spectra of the vapor/water interface taken with (a) SSP, (b) PPP, and (c) SPS polarization combinations. (61) The Im spectrum of HOD at the vapor/water interface probed by phasesensitive sum frequency generation spectroscopy, and schematic of the first two molecular layers at the vapor/water interface occupied by DDA, DAA and DDAA water molecules. (70) The SFG spectra of water (D 2 O) film on mica as a function of the relative humidity (RH) at room temperature (296K) with SSP polarization, and scanning polarization force microscopy images, showing 2-dimensional islands of water (bright patches) produced by a brief contact of the atomic force microscope tip near the center of the image, that induces capillary condensation around the contact point. At relative humidity values below 70%, the clusters show a contrast of Å. The contrast decreases down to the noise level at high humidity (80%). (71) Possible hydrogen-bonding configuration of water molecules on hydrophilic silica surface: (a) protonated (SiOH) surface sites, low ph; (b) deprotonated (SiO - ) surface sites, high ph; (c) structure of water/silica interface at low ph. Red and gray spheres represent O and H atoms of water molecules; large graygreen, pink, and white spheres represent Si, O, and H atoms of SiOH groups at silica surface. Dotted lines indicate hydrogen bonds. (72) The SFG spectra the water/fused silica interface as a function of ph. Polarization combination is SSP. A spectrum of the ice/fused silica interface is shown for comparison (filled squares). The spectra are offset vertically by 2 units for clarity. (73)...32 x

11 The SFG spectra of the water/hydrophobic interfaces: (a) the water / octadecyltrichlorosilane (OTS) covered silica interface; (b) the water / vapor interface; (c) the water/hexane interface. (80) The chemical structure of polyurethane that is used in our study The schematic diagram of SFG apparatus The schematic of combining TIR geometry with SFG measurements Diagram of the sample geometry for SFG measurements. Liquid water and water vapor can be sealed well in the gap between the sapphire prism and the stainless steel sample cell Diagram of the humidity-controlling set-up. The water container is made of glass and other parts are made by stainless steel Predictions of interferences of the SFG signal for two interfaces as a function of the thickness of polymer films by a three-layered structural model. Red dashed line is the ratio of signals from PU-air interface over PU-sapphire interface at incident angle 2 degrees. Blue dashed line is the ratio of signals from PU-water interface over PU-sapphire interface at incident angle 2 degrees. Pink solid line is the ratio of signals from PU-sapphire interface over PU-air interface at incident angle 42 degrees. Black solid line is the ratio of signals from PU-sapphire interface over PU-air interface at incident angle 16 degrees Diagram of the thickness of PU films as a function of the solution concentrations varying from 0.3% to 1%. Each data point is the average result of three different samples The QCM spectra of the bare sensor and the sensor spin coated with the polyurethane film (300nm by ellipsometry) at 3 rd, 5 th, 7 th and 9 th overtones The SFG spectra in PPP polarization for the PU-air interface (a) and the PUsapphire interface (b). The solid lines are fit to a Lorentzian equation The SFG spectra in SSP polarization for the PU-air interface (a) and the PUsapphire interface (b). The solid lines are fit to a Lorentzian equation xi

12 The SFG spectra in PPP polarization for the PU-water interfaces: (a) the PU- H 2 O interface and (b) the PU-D 2 O interface. The solid lines are fit to a Lorentzian equation The SFG spectra in PPP polarization for the PU-sapphire interface upon exposure to liquid water. Both H 2 O (a) and D 2 O (b) reaches the PU-sapphire interface. The solid line in (a) is a guide to the eye. The solid line in (b) is fit to a Lorentzian equation The SFG spectrum in PPP polarization for the D 2 O/sapphire interface. The solid line is used to guide eyes The SFG spectra of the Sapphire-water vapor (or dry N 2 gas) interface in PPP polarization after the humidity-controlling set-up was integrated to the sample cell. The solid lines are used to guide eyes The SFG spectra of the PU-sapphire interface that are collected at various RH of H 2 O vapor in PPP polarization. These spectra are collected at 0%, 23%, 48% and 82% RH, respectively. The solid lines are fitted to a Lorentzian equation The SFG spectra of the PU-sapphire interface that were collected at various RH of D 2 O vapor. These spectra are collected at 0%, 23%, 53% and 80% RH, respectively. The solid lines are fitted by Lorentzian equation The contact angle images of the water droplet on (a) the polyurethane-coated sample and (b) the fluorinated layer-coated sample The SFG spectra of the PU-sapphire interface (of plasma treated samples) that were collected at various RH of D 2 O vapor. These spectra are collected at 0%, 23%, 49% and 69% RH, respectively. The solid lines are guides to the eyes A model illustrating the PU-sapphire interface in the presence of water molecules under liquid water and water vapor conditions, respectively A1. The IR spectrum of PU in the ambitious condition xii

13 CHAPTER I INTRODUCTION The incorporation of water between two solid surfaces plays an important role in many areas such as interfacial adhesion, corrosion, catalysis and lubrication. (1, 3-7) Interfaces that consist of various organic polymers combining with inorganic substrates are of technological consequence in understanding systems like protective coatings, adhesive joints, and polymer/inorganic composites. (8, 9) When such systems are exposed to liquid water or exposed to high humidity, water can ingress into the polymer-substrate interfaces by diffusing through intact polymer films or along the interfaces. (10, 11) As a result, water replaces the initial bonds between polymer and substrate and even causes delamination and mechanical failure, which is detrimental to the durability of polymer/substrate systems. (12) Various non-destructive measurements have been developed recently for probing water transport underneath polymer films such as electrochemical impedance spectroscopy (EIS), Fourier transform infrared multiple internal reflection (FTIR- MIR) spectroscopy, quartz crystal microbalance (QCM), and X-ray and neutron reflectometry (XR and NR). Wormwell and Brasher have utilized EIS to measure electrical properties of organic coatings after exposure to aqueous environments since early 1950s. (13) The analysis of EIS data is based on building proper electric-circuit models and thus depends on the validity of models. FTIR-MIR spectroscopy was developed by Nguyen et al. that enabled in situ and quantitative detection of water in vicinity of the substrate. (14, 15) In this technique, the probing depth is 1.75 and 1

14 thus detectable water molecules include water layer at the polymer-substrate interface and water absorbed in the polymer film within the probing depth. Even though FTIR- MIR is not interface specific, it provides information on water transport pathways and water-susceptibility of the polymer-substrate interface helping to understand the mechanisms of water-induced adhesion loss of polymer-coated systems. QCM, XR and NR were also used to study the absorption of water underneath polymer films. Because of better resolution of these techniques than EIS and IR, ultra thin films from tens to hundreds nanometers were used in these studies. QCM were widely used to measure water uptake in a variety of polymer-substrate systems as it s sensitive to even several nanometers thick water absorption. (16, 17) More recently, Vogt et al. (18, 19) applied NR and XR to study water transport kinetics in multilayered systems upon exposure to water vapor. The reflectivity data described the moisture permeation process and swelling of individual polymeric layers in detail. NR measurements with D 2 O provided information of the water distribution within each layer and water concentration accumulated at the interface. All these techniques used to monitor water transport in polymer-coated systems are of practical importance for predicting barrier properties of polymer coatings and understanding the impact of water on interfacial properties such as adhesion and electrochemistry. Previous studies showed that water accumulation at the polymer-substrate interface depends on substrate surface chemistry which influences the interaction with polymer coatings and water. Nguyen et al. pointed out that plasma treated substrate with enhanced interactions with PMMA lead to a decreased amount of water at the interface. (15) O Brien et al. found that hydrocarbon treatments which made the substrate more hydrophobic and hence weaker interactions with water suppressed the water accumulation at the interface. (19) An investigation was conducted by Karul et 2

15 al (17) to clarify the effect of polymer chain mobility on water accumulation content. They found that rubbery polymers are able to suppress the accumulation of moisture at interface with the explanation that the low-tg polymers (PnBMA and PnOMA) promote relatively unrestricted molecular movement towards a thermodynamically stable conformation such that the hydrophobic alkyl chains are closer to the interface. Although a variety of studies for quantifying the interfacial water have been conducted, the structure of this complex water interface is not understood. Understanding molecular structure of the polymer-substrate interface in the presence of water can help to clarify the mechanism of water-induced interfacial properties. To address this question, we have studied the confined water between the polymer filmsapphire (Al 2 O 3 ) interfaces using infrared-visible sum frequency generation spectroscopy (SFG) in total internal reflection (TIR) geometry. (20) SFG is a nonlinear optical spectroscopic technique that is highly surface-specific because second order nonlinear optical processes are forbidden in centrosymmetric media under electric-dipole approximation. (21) This makes it possible to probe various surfaces and interfaces where inversion symmetry is broken. (22-24) Briefly, SFG technique involves two incident beams, one visible beam and a tunable IR beam, overlapping temporally and spatially on the sample and generating a sum frequency signal (ω 1 + ω 2 ) at the interface. The SFG signals are enhanced when the scanning IR frequency overlaps with the molecular vibrational modes of molecules at the interface that are infrared and Raman active. The intensities and positions of peaks can provide chemical and orientation information of molecules. (25) Combining with total internal reflection geometry, SFG spectroscopy permits us to study the presence of water between the polymer-sapphire interface. By combining SFG with TIR geometry, it enables us to selectively probe the individual interface in a multilayered 3

16 system and the SFG output is enhanced times when the incidence angles are near the critical angle for TIR. (26, 27) Taking advantage of this approach, we studied the structure of the polyurethane (PU)-sapphire interface upon exposure to liquid water and water vapor with controllable relative humidity (RH). This technique can also be applied to a variety of polymer-substrate systems for monitoring the ingress of water to the coating-protected interface mimicking the natural environments such as rain and humid air. In this dissertation, we chose polyurethane as a model polymer for our study because polyurethane is widely used in the automotive refinish and large vehicle coating areas, and in metal, wood, plastic, glass, and textile coating. (28) We have addressed three main questions in this research. First, can water ingress to the polymer-sapphire interface? Second, what is the structure of water in this confined geometry? Third, what are the effects of water at the interface? Some basic concepts of water between two solid substrates are discussed in Chapter 2. It also provides a brief insight on our current understanding of this subject. In Section 2.2, we will provide an introduction to the principles of non-linear optics and the theoretical framework for SFG. At the end of Chapter 2, typical studies of water at various interfaces are presented to highlight the major advantages of this spectroscopic technique. Chapter 3 discusses the sample preparation and experimental techniques in this study. It also provides details of the development of the sample cell and the humidity-controllable set-up. Chapter 4 provides the results and discussion. To understand the ingress of water to the polymer-sapphire interface, we exposed samples in both liquid water and water vapor. H 2 O and D 2 O were used in the study to confirm our finding. At the end 4

17 of Chapter 4, the effect of hydrophobicity of the organic layer on preventing water penetration was presented. Finally, in Chapter 5 we conclude with a summary of the major results of this research. 5

18 CHAPTER II BACKGROUND 2.1 Water Confined Between Two Solid Surfaces In nature, we can see confined water in many areas such as gecko pads- wet solid substrate interfaces and cellular membrane interfaces. In industry, confined water layer involving in areas like coating-metal interfaces and tire-road interfaces is everywhere around us. Confined water in those systems plays very important roles in adhesion, corrosion, lubrication and friction. It s not easy to measure the properties of water confined between two solid substances because of this indirect experimental geometry. Israelachvili and coworkers used surface force apparatus to study the force between two smooth solid surfaces in the presence of water. At long range, a repulsion force exists between the two solids and it increases exponentially with decreasing separation distance, which is well described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. Measurements at short range reveal oscillations in the force with a period ~0.25nm which corresponds to the size of a water molecule. When the separation is approaching 0, the force is expected to be attractive. (29) Figure shows the measured force as a function of separation distance and it suggests that forces between surfaces are related to water structure, especially for the first layer of water in direct contact with surfaces. 6

19 Figure Force between two mica surfaces in mm KCl solutions as a function of separation distance. Below 2 nm, the force is oscillating with a periodicity of the diameter of water molecules (2.5 ). Above 2 nm, the force is repulsive. Inset: theoretical prediction of force for this system based on a non-continuum molecular theory. (29) Mimicking gecko adhesion on wet natural surfaces, Alyssa and coworkers tested the adhesion of geckos on submerged substrates with various surface wettabilities. (30) They found that (in Figure 2.1-2) geckos significantly lose their adhesion on a wet hydrophilic surface compared with a dry hydrophilic surface. They didn t see adhesion difference between dry and wet contact of an intermediated hydrophobic surface. However, geckos clung significantly better to a wet hydrophobic substrate than the dry substrate. Their experimental and calculated results suggested that gecko adhesion is highly dependent on surface wettability and 7

20 the presence of water or air between the toe pad and the contact surface. This study provides us direct evidence of water s impacts on varying interfacial properties in natural environment, and highlights the importance of studying water confined between two surfaces. Figure Whole-animal shear (frictional) adhesion forces from tokay geckos tested on four surfaces in dry or wet contact. Each gecko (n = 6) was tested three times on each surface (glass, PMMA, OTS-SAM coated glass, and PTFE), and the highest of the three tests was used for data analysis. Surfaces were tested either without water (dry) or fully submerged in water (wet). (30) As we all know, water penetrating protective coatings and reaching the coating-metal interface will potentially initiate corrosion. EIS was widely used to evaluate corrosion protection of metals and alloys by polymer coatings. Those studies include evaluation water uptake of coatings, degradation of coatings with exposure time, disbanding of coatings, determination of the active area at which corrosion occurs, and estimation of corrosion rates at the metal /coating interface. (31) As shown in Figure (a), the analysis of impedance data (in Figure (b)) of 8

21 polymer-coated metal exposed to corrosive media is based on the simple model where,,, and correspond to the uncompensated resistance between reference electrode and test electrode, the capacitance of the polymer coating, the pore resistance resulting from the formation of ionic conducting paths across the coating, and the polarization resistance of the area at the metal-coating interface at which corrosion occurs and the corresponding capacitance. However, the reliability of analyzed data depends on the validity of the model that was used for analyzing. Figure (a) The model for the impedance of a polymer coated metal and (b) theoretical impedance spectra for a degraded polymer coating. (31) 9

22 Recently, the detection and quantification of water at the polymer-solid interface can be achieved by QCM, NR and XR techniques. The mechanism of quantifying water by QCM is based on relating resonance frequency of piezoelectric sensors to the change in mass experienced by sensors because of molecular absorption. However, the mass change measured by QCM includes mass changes caused by absorption in the bulk film and molecules at the polymer-solid interface. The absolute quantification for the interface can be achieved by extrapolating of the uptake to zero thickness as shown in Figure (17) Figure Solid points represent the moisture uptake into thin PnOMA films coated on quartz crystals. corresponds to the adsorption on the blank crystal. The dashed lines are linear fits to the data. (17) The reflectivity-based metrology provides details about the moisture permeation process through multilayer stacks. Polymer films swelling upon exposure to water can be detected by XR as shown in Figure (18) Neutron reflectivity measurements with deuterated water are used to provide information of the water distribution within each layer. The amount of D 2 O at the buried interface can be 10

23 quantified by the change in the scattering length density after moisture exposure. The scattering length density profiles corresponding to the fit of the NR data are shown in Figure (18) Figure The X-ray reflectivity profiles for polyacrylate layer on Al 2 O 3 with different curing conditions in vacuum (gray curve) and exposed to saturated H 2 O vapor at 60 (black curve). The reflectivity curves are offset for clarity. (18) Understanding the structure of water helps clarify the mechanism how water influences the contact interfaces. Kumar and coworkers deformed a elastomeric PDMS lens against a flat solid surface to study confined water between them by infrared-visible sum frequency generation spectroscopy. (32) They correlated molecular structure of the contact region with friction of the system. They found that even though the sliding of PDMS lens on the solid substrate in the presence of water generated lower friction than dry sliding, the friction coefficients of wet sliding were much higher than those expected for a contact spot with a uniform layer of water (lubricated sliding). The origin of the higher friction coefficient was attributed to the 11

24 Figure The Neutron-scattering length density profile from the fitting of the Neutron reflectivity profiles for samples (polyacrylate layer on Al 2 O 3 ) in vacuum (solid line) and exposed to saturated water vapor (dash line). (18) patchy contact spot with regions where the elastomer was in direct contact with the solid surface in literature. (7) This hypothesis was proved in this study by observing specific vibrational bands that illustrated molecular structures in the contact area (as shown in Figure 2.1-7). However, the direct measurements of the structure of water confined between two solid surfaces are still lacking, especially for the water layer in atomic scale. Taking advantage of SFG spectroscopy, we are able to investigate confined water between two solid surfaces (polymer film and sapphire prism) in our study. Studying water in this system is very important for mimicking coating-metal interfaces in the presence of water. For example, coatings on aluminum are often exposed to humidity and rain and the study of these coatings to these environmental conditions is 12

25 important in preventing corrosion. The early detection of water at interfaces will help in designing polymers for corrosion protection. Figure A model illustrating the contact region formed between the PDMS lens and the sapphire surface in the presence of confined water. A tentative physical model of the molecular structure in these two types of contact area s is also shown in the panels on the right. The relative sizes of these two regions cannot be quantified from measurements. (32) 2.2 Infrared-Visible Sum Frequency Generation Spectroscopy In recent years, surfaces and interfaces science attract intensive attentions because of their important roles in many areas, such as interfacial adhesion, catalysis, electrochemistry and corrosion. However, there are still limitations of available 13

26 surfaces and interfaces probe techniques. Many electron probe techniques required ultrahigh vacuum (UHV) conditions e.g. Low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS), which limited their applications to dry sample surfaces. (33) Microscopic probes such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) can be applied to study samples with liquid on top, but suffer from molecular movements at liquid interfaces. (21) Some other techniques that have been used to study surfaces and interfaces e.g. X-ray spectroscopy and Neutron reflectivity are not highly surface-specific. On the other hand, second-order nonlinear optical spectroscopy, including second harmonic generation (SHG) and sum frequency generation (SFG), shows its advantages in surfaces and interfaces studies. It s highly surface-specific because nonlinear optical processes are forbidden in centrosymmetric media under electric-dipole approximation and only permitted at surfaces and interfaces where inversion symmetry is broken. (21) Basic Principles In 1961, the discovery of second-harmonic generation by Franken et al. opened the world of nonlinear optics. The phenomena are called nonlinear optics when a material responses to an applied optical field in a nonlinear manner. To describe more precisely what optical nonlinearity is, we can consider how the dipole moment per unit volume, or polarization of a material depends upon the strength of the applied optical field. In the case of linear optics, the induced polarization shows linear relationship with the electric field strength (34) 14

27 P(t) = χ (1) E(t) (2.1) where is the linear susceptibility. In nonlinear optics, the polarization can be expressed by a series of orders of the electric field strength. (2.2) The quantities and are the second-order and the third-order nonlinear optical susceptibilities, respectively. The second-order nonlinear optical interactions can only occur in noncentrosymmetric media. On the other hand, the third-order nonlinear optical interactions can occur both in noncentrosymmetric and centrosymmetric media. A laser beam whose electric field strength is represented as E(t) = Ee iωt + c.c. (2.3) is incident on a medium with a nonzero second-order susceptibility. The medium can generate polarization as P (2) (t) = 2χ (2) EE * + (χ (2) E 2 e 2ωt + c.c.) (2.4) where is the charge of an electron, is the frequency and is the time. This is the basic principle of SHG. Under proper experimental conditions, the incident beam with 15

28 frequency can be effectively converted to radiation with second harmonic frequency. For sum frequency generation, the incident laser beams consist of two distinct frequencies. The electric fields of incidence can be represented as E(t) = 1 2 (E 1 e iω 1t + E 2 e iω 2t + c.c.) (2.5) The generated nonlinear polarization is (2.6) = χ (2) [E 2 1 e 2iω 1t + E 2 2 e 2iω 2t + 2E 1 E 2 e i(ω 1 +ω 2 )t + 2E 1 E * 2 e i(ω 1 +ω 2 )t + c.c.] +2χ (2) [E 1 E * 1 + E 2 E * 2 ] The summation of nonlinear polarization can be expressed by different components that describe processes such as SHG, SFG, different-frequency generation (DFG) and optical rectification (OR) as shown below. (2.7) The geometry for SFG is shown in Figure Generally, two pulsed laser beams, including a visible beam in and a tunable infrared beam in, overlap 16

29 spatially and temporally on a sample and the reflected SFG light at the sum of the two incident frequencies enhanced when is detected. (35) The intensity of SFG light is resonantly overlaps with the resonant frequency of a molecular vibrational mode that is both infrared and Raman active. (20) Scanning the frequency of the IR laser therefore yields a SFG spectrum. The position and the magnitude of those resonance peaks provide chemical and orientation information of the molecules at a surface or an interface. Figure Schematic of the laser beams for SFG spectroscopy at an interface. (35) The intensity of SFG light depends on the second-order susceptibility which is a polar tensor of rank three and has 27 elements represented as. We change the sign of under the inversion operation: χ (2) (2) ijk = χ i j k. For centrosymmetric crystals which are invariant under the inversion operation: χ (2) (2) ijk = χ i j k, should be zero. As a consequence, SFG is forbidden in centrosymmetric media under electric-dipole approximation and only permitted at surfaces and interfaces where inversion symmetry is broken. 17

30 When we are probing the surface or the interface, we can choose either S or P polarization for incident beams and receive the output SFG by selecting a polarization for the detector. (Here, P polarization refers to light whose electric field is in the incident plane, while S polarization refers to light whose electric field is parallel to the surface.) Different combinations of laser polarization offer corresponding Cartesian components of. For a surface that is azimuthally isotropic, there are only four independent non-zero components of :,, and. In the case of S-polarized visible and S-polarized infrared beams, only is non-zero and contributes to P-polarized light. In the case of S-polarized visible and P- polarized IR beams, only is non-zero and gives rise to the S-polarized light. In the case of both P-polarized visible and IR lasers, all four independent elements of contribute to the output intensity. In most of current SFG measurements, SSP, PPP, SPS polarization combinations are chosen (e.g. SSP is given in the following sequence: S-polarized SFG output, S-polarized visible input and P-polarized IR input.) The relationship between the effective susceptibility with the combination of polarizations is shown below taking Fresnel effects into account. (36) (2) χ eff,ssp = L yy (ω 3 )L yy (ω 1 )L zz (ω 2 )sinθ 2 χ yyz (2.8) (2) χ eff,sps = L yy (ω 3 )L zz (ω 1 )L yy (ω 2 )sinθ 2 χ yzy (2.9) Here, and are the effective second-order nonlinear optical susceptibility measured in the experiments using SSP and SPS polarization combinations. is the frequency of SFG light equal to. (i = x, y, or z) are the transmission 18

31 Fresnel factors, and and are angles between the surface normal and the input visible beam, and the input IR beam, respectively. Take SSP as an example, can be fitted by Lorentzian equation as shown below, (2) χ eff,ssp A = χ (2) q NR + F sur (2.10) q ω 2 ω q + iγ q F sur = L yy (ω 3 )L yy (ω 1 )L zz (ω 2 )sinθ 2 (2.11) where is the nonresonant background. is the complex Fresnel coefficient., and are the strength, angular frequency and damping constant of the th resonant vibrational mode, respectively. The intensity of SFG signal is proportional to and the electric field of incident beams as shown below. I SFG = 8π 3 ω 2 3 sec 2 θ 3 c 3 n(ω 3 )n(ω 1 )n(ω 2 ) χ (2) 2 eff I Vis I IR (2.12) is the angel between the surface normal and SFG beam. is the refractive index of the medium at wavenumber. and are the intensity of visible beam and infrared beam, respectively. As we know SFG provides chemical and orientation information of molecules, we need describe how to relate the SFG signal with molecular characters at a surface 19

32 or an interface. The overall polarization of a system is the sum of the polarizations induced on individual molecules as shown in the equation below, (37) (2.13) where is the total number of molecules in the system, is the average polarization of individual molecules for the entire system. The hyperpolarizability of a molecule should be mentioned here, and the relationships between and and are shown below. p mol = β xyz : E 1 E 2 (2.14) χ (2) = β xyz = N β xyz (2.15) al lmolecules is the hyperpolarizability of a molecule defined in terms of the laboratory coordinate system. It can be converted into the hyperpolarizability defined in terms of the molecular coordinate system through a series of rotations using Euler angles. The relationship between the molecular axes (a, b, c) to the lav axes (x, y, z) can be seen in Figure The coordinate transformations are performed through a series of rotation, including rotating about the z-axis by an angle, rotating about the x-axis by an angle and rotating again about the z-axis by an angle. In many cases, the 20

33 Figure Definition of Euler angles relating lab axes to molecular axes. symmetry of the molecule will cause many of the molecular hyperpolarizability elements to become zero. Take a C 3V symmetric moiety of molecules (such as methyl groups) at a surface as an example, considering a azimuthally isotropic surface, the non-vanishing elements of can be presented by the hyperpolarizability elements in this way: (38) For the symmetric stretching mode of the methyl group, (2) χ yyz,s π 1 = 2 Nβ [cosθ(1+ r) ccc,s cos3 θ(1 r)] f (θ) sinθdθ (2.16) 0 (2) χ yzy,s π 1 = 2 Nβ [cosθ ccc,s cos3 θ](1 r) f (θ) sinθdθ (2.17) 0 For the asymmetric stretching mode of the methyl group, 21

34 (2) χ yyz,s π = 1 2 Nβ [cosθ caa,as cos3 θ] f (θ) sinθdθ (2.18) 0 (2) χ yzy,as π 1 = 2 Nβ caa,as cos3 θ f (θ) sinθdθ (2.19) 0 where is the angle between the surface normal and the principal axis of the methyl group which can be used to describe the molecular orientation. is the distribution function of. The Gaussian function is commonly used for the distribution function as the equation below. (39) (2.20) Here is the mean orientation angle, is a normalization constant, and is the root-mean-square width. By combining Equation (2.18), (2.19) and (2.20), we can relate the orientation angle with the ratio between and as shown in the equation below. R = χ yyz,as χ yzy,as F ssp F sps = cosθ cos 3 θ cos 3 θ (2.21) Here, and are Fresnel coefficients. The ratio can be experimentally obtained from SFG spectra conducted using SSP and SPS polarizations. By comparing the experimentally measured value with the calculated value as a function of, we can estimate the orientation angle of molecules at an interface. 22

35 2.2.2 SFG Study of Polymers As SFG has submonolayer sensitivity and surface specificity, and provides chemical and orientation information of the molecules at a surface or an interface, it has been extensively applied to investigate polymer surface composition and structure which are related to the properties and performance of polymer materials. Chen and coworkers study the surface structure of various polymer surfaces such as poly (methacrylate)s, poly (methyl methacrylate)/polystyrene blend, polyethylene oxide- polypropylene oxide copolymers etc. (40-42) Performing SFG experiments under different polarization enable a quantitative analysis of molecular orientation, in particular relating the intensities of SFG spectra to orientation information of methyl and methylene groups. Extensive research relates the microscopic structure with macroscopic properties such as adhesion, wettability, antifouling and antimicrobial abilities. (43-47) SFG was applied to monitor surface structure changes at a molecular level under various surface treatments, such as UV irradiation and plasma treatment, wet treatment, rubbing and sheardeposition. (48-53) Among these studies, extensive attentions were attracted by restructuring of the surface in contact with water. (20, 54-56) In the study of poly (ethylene glycol) surfaces (54), simultaneous spectral investigations in the C H ( cm -1 ) and O H ( cm -1 ) stretch regions showed that interactions with water induce dramatic conformational changes of the polymer backbone. As SFG spectroscopy is an interfacial specific technique, it widely applied to study the buried solid-solid interfaces. (20, 55, 56) Gautam et al. used a total internal reflection geometry to study both the sapphire polystyrene and air polystyrene 23

36 interfaces probed by SFG spectroscopy. They collected spectra related to these two different interfaces by using two different angles of incidence of the input beams, where only one interfacial (air polymer or sapphire polymer) contribution is amplified. They found the phenyl rings of polystyrene have significantly different orientations at these two interfaces. The molecular orientation at the buried solid polymer interface could also be correlated to adhesion in the case of polystyrene in contact with the modified glass substrate. Kurian and coworkers have used SFG spectroscopy to determine the interaction energies of various polymers in contact with the sapphire substrate which govern the wetting, adhesion, friction, chemical reactions and many other material and biological phenomena at interfaces. (57) Other SFG investigations of buried interfaces including polymer polymer and silane polymer interfaces were conducted by Chen and coworkers. (58, 59) In this latter case, they succeeded in monitoring the evolution of the microscopic structure of the interface during the diffusion of silanes SFG Study of Water The importance of water interfaces has long been well recognized because they re ubiquitous and play critical roles in many natural and technological processes. However, studies of these interfaces at the molecular level are somehow limited because of a few available techniques. Infrared-visible sum frequency generation spectroscopy which is interfacial specific and has submonolayer sensitivity has been proven to be the most powerful and versatile method to obtain surface vibrational spectra of liquid interfaces that yield information about liquid interfacial structure. 24

37 Vapor-Water Interfaces The first set of vibrational spectra for the vapor/water interface was obtained by Du and coworkers in (60) As displayed in Figure 6, the SSP spectrum of the vapor/water interface was compared with the IR spectra of bulk ice and liquid water. A sharp peak at 3700 cm -1 is assigned to the stretching mode of the dangling OH bonds pointing towards the vapor side as shown in Figure (61) The 3700 cm -1 peak is characteristic of the surface instead of bulk. The two broad peaks at 3400 cm -1 Figure The SFG spectra of the vapor/water interface, IR spectra of bulk water and bulk ice, and molecular structure of hexagonal ice. Upper-left: The SFG spectrum of the vapor/water interface shows three OH bands at 3200, 3400 and 3700 cm -1. Upper-right: side view of the bulk ice near the (0001) surface. Bottom-left: The IR spectrum of bulk ice shows a dominant peak at 3150 cm -1. Bottom-right: The IR spectrum of bulk water is dominated by a broad peak at ~3400 cm -1. (61) 25

38 and 3200 cm -1 resemble peaks for the stretching modes of the bonded OH in bulk water and bulk ice, respectively. We call them as liquidlike and icelike peaks, corresponding to tetrahedrally hydrogen-bonded and three-coordination hydrogenbonded water networks. Wei and coworkers also obtained SFG spectra for the vapor/water interface with three polarization combinations, SSP, PPP and SPS. (62) All the peaks in the PPP and SPS spectra shown in Figure are significantly weaker than those in SSP because the OH bonds of water at the interface are tilted close to the surface normal. As a consequence, its vibrational modes are more easily excited by S-polarized, rather than P-polarized, IR input. In the PPP and SPS spectra, the resonant feature at cm -1 can be assigned mainly to the bonded OH stretching mode of surface water molecules with one bonded OH and one dangling OH (low-coordination water). Infrared-visible sum frequency generation spectroscopy has been used to study various types of water interfaces. (63-68) However, detailed interpretation of the spectra often differs and causes a great deal of confusion. The difficulty usually arises because of ambiguity in analyzing the spectra and because of lack of sufficiently accurate theoretical calculation to compare with experiment. Bonn et al. reported that the interfacial water resonance in the hydrogen-bonded region ( cm -1 ) originates from intramolecular coupling of vibrational modes, rather than from the existence of distinct water substructures. (69) He supported his arguments based on the spectra of uncoupled HOD peaks, which reveal a smooth transition from two peaks to one peak in the hydrogen-bonded region as D 2 O is converted into HOD. Thus, they concluded that two prominent peaks observed at water interfaces are assigned to the symmetric stretching mode split by the Fermi resonance with the overtone of the bending mode. 26

39 Figure The SFG spectra of the vapor/water interface taken with (a) SSP, (b) PPP, and (c) SPS polarization combinations. (62) In response to Bonn s argument, Shen and coworkers developed a phasesensitive SFG technique, allowing direct measurements of both the amplitude and the phase of the SF response and helping in analysis and interpretation of the spectra of water interfaces. (70, 71) In their study of the vapor/water interface, they found the bonded-oh (or bonded-od) spectra of pure H 2 O (or pure D 2 O) exhibit a double-peak feature, while this feature convert into a single broad band with sufficient isotopic dilution. The spectra are consistent with those provided by other groups. However, the measured Im spectra of HOD at the vapor/water interface as displayed in Figure showed one positive resonance band around 3300 cm -1 and one negative 27

40 reaonance band around 3450 cm -1, which efficiently refuted Bonn s argument that the two peaks originates from intramolecular coupling of vibrational modes. They proposed a model of the first two molecular layers of the vapor/water interface that dominantly contribute to the SFG spectra, consisting of DAA, DDA, and DDAA molecules in a distorted hydrogen-bonding network. Here D and A denote single proton donor and single proton acceptor through which water molecules bond to neighbors. As shown in Figure 2.2-5, the sharp positive peak at 3690 cm 1 is attributed to the dangling OH stretching mode of DAA water molecules protruding at the surface. The positive band at 3300 cm -1 and the negative band at 3450 cm -1 appear at the same positions as the IR absorption bands of HOD in bulk ice and liquid water. Thus, they are assigned to icelike DDAA water molecules with dipole moment pointed towards the vapor side and liquidlike (loosely donor-bonded to molecules) DDAA, DAA and DDA water molecules with dipole moment pointed towards the bulk water. This work here shows that the spectra of Im can provide a much clearer picture of the vapor/water interfacial structure even though the overall broad spectra is complex due to continuous variation of hydrogen-bonding geometry and strength. Figure The Im spectrum of HOD at the vapor/water interface probed by phase-sensitive sum frequency generation spectroscopy, and schematic of the first two molecular layers at the vapor/water interface occupied by DDA, DAA and DDAA water molecules. (71) 28

41 Water-Hydrophilic Solid Interfaces Hydrophilic solid surfaces are often hygroscopic. The absorbed water on solid surfaces is critical in wetting, weathering and biological phenomenon. Miranda and coworkers studied the structure of a water film formed on mica as a function of relative humidity (RH) of water vapor by using SFG spectroscopy and scanning polarization force microscopy (SPFM). (72) From the SFG spectra as RH (%) increases shown in Figure 2.2-6, partial water coverage about 20% RH doesn t form an ordered water structure. At 90% RH, a full monolayer coverage is achieved, forming an ordered ice-like structure with no dangling OD groups. This result agrees with the prediction of molecular dynamics simulations. (73) They applied SPFM to image the water adsorption on mica and they found that the polygonal shaped islands that are generated by tip contact with the surface can be observed from 20% RH up to 70% RH. As RH is above 80%, the surface is nearly fully covered by an ice-like water film, and it becomes more difficult to observe an isolated island. Many studies were conducted for probing the water-hydrophilic solid interfaces such as silica-water, sapphire-water and CaF 2 -water interfaces. The isoelectric points of these solid surfaces determine whether the surfaces are neutral, positive charged or negative charged, which directly influences water structure forming on these surfaces. Take silica for an example, the surface remains neutral when the ph of bulk water is lower than 2, becomes increasingly deprotonated as ph increases, and is completely deprotonated and saturated with negative charges at ph=10. (74) Water molecules can form hydrogen-bonded networks on both neutral (SiOH) and negative charged (SiO - ) surfaces. As shown in Figure 2.2-7, when the surface is neutral, two water molecules can bind with H to O and one with O to H on 29

42 SiOH, while three molecules can bind with H to O of SiO - when the surface is deprotonated. The surface field at high ph should help orient the water molecules with hydrogen bonding to the surface and establish more ordered hydrogen-bonded networks. Figure The SFG spectra of water (D 2 O) film on mica as a function of the relative humidity (RH) at room temperature (296K) with SSP polarization, and scanning polarization force microscopy images, showing 2-dimensional islands of water (bright patches) produced by a brief contact of the atomic force microscope tip near the center of the image, that induces capillary condensation around the contact point. At relative humidity values below 70%, the clusters show a contrast of Å. The contrast decreases down to the noise level at high humidity (80%). (72) 30

43 Figure Possible hydrogen-bonding configuration of water molecules on hydrophilic silica surface: (a) protonated (SiOH) surface sites, low ph; (b) deprotonated (SiO - ) surface sites, high ph; (c) structure of water/silica interface at low ph. Red and gray spheres represent O and H atoms of water molecules; large graygreen, pink, and white spheres represent Si, O, and H atoms of SiOH groups at silica surface. Dotted lines indicate hydrogen bonds. (74) This is now confirmed by SFG spectroscopy. Shen and coworkers present in Figure a set of SSP spectra of silica/water interfaces at different bulk ph. (75) The observation of the liquidlike and icelike peaks indicates that the interfacial water molecules form a partially ordered hydrogen-bonded network. Both peaks increase with ph, but the icelike peak grows more significantly at high ph, indicating a betterordered water network at high ph. Yeganeh and coworkers probed the alumina/water interface by SFG spectroscopy and reported the isoelectric point at around ph=8. (76) From many other researches of alumina/water interfaces, the isoelectric points range from 3 to 9.5 that 31

44 relate to different crystallographic plane, experimental techniques, and surface treatments. (77-80) Adsorption of ions and charged polymers on the bare solid surface will significantly alter the point of zero charge and influences the hydrogen-bonded water structure as a consequence. (81) Figure The SFG spectra the water/fused silica interface as a function of ph. Polarization combination is SSP. A spectrum of the ice/fused silica interface is shown for comparison (filled squares). The spectra are offset vertically by 2 units for clarity. (75) 32

45 To sum up, there is a coexistence of icelike and liquidlike water structures on hydrophilic solid surfaces. Differing from the vapor/water interface, no dangling OH (or OD) band shows up at the water/hydrophilic solid interface. The hydrogen-bonded structures can be affected by modification of the surface through deprotonation, ion adsorption, or molecular adsorption at the surface which determine how the interfacial water molecules bond to the solid surface. Surface charges, and hence surface field, can induce better ordering of interfacial water molecules perhaps even up to a few monolayers Water-Hydrophobic Solid Interfaces Water molecules should poorly wet a hydrophobic surface due to stronger interactions among themselves compared with interactions between water and the substrate. As air is an ideal hydrophobic surface, we would expect the vibrational spectrum of the water/hydrophobic substrate interface to be similar to that of the vapor/water interface. Particularly, the peak for the dangling OH at 3700 cm -1 should show up since they won t bond to the substrate. Figure The SFG spectra of the water/hydrophobic interfaces: (a) the water/octadecyltrichlorosilane (OTS) covered silica interface; (b) the water/vapor interface; (c) the water/hexane interface. (82) 33

46 Du et al. first presented the SFG spectra of a water/octadecyltrichlorosilane (OTS)-covered silica interface as shown in Figure (82) The dangling OH band at 3680 cm -1 is apparent indicating that there is no hydrogen bond between water and OTS. Instead, the weak van der Waals interaction between the free OH and the CH 3 terminal group of OTS causes the red shift by 20 cm -1. Additionally, the SFG spectrum of a water/hexane interface looks quite similar to that of the vapor/water interface. Hus and Dhinojwala (23) have probed the contact interface between oil sapphire interfaces in aqueous media using SFG spectroscopy. A transition from an attractive contact to repulsive contact was observed above the isoelectric point (IEP) of the sapphire substrate. Below the IEP of the sapphire substrate, the hexadecane drops stick to the sapphire surface and Hus surprisedly observed a thin layer of water in the adhesive contact region between hexadecane drop and the sapphire substrate. The presence of this water layer in the adhesive contact region can be explained due to weaker repulsive double layer and the attractive van der Waals interactions. 34

47 CHAPTER III EXPERIMENTAL 3.1 Sample Preparation The sapphire prisms were purchased from Miller Optics with the c-axis parallel to the prism face. The prisms were cleaned by following procedure: (1) sonicate in toluene for more than 2 hours using a Branson model 610 sonicator; (2) blow with nitrogen gas; (3) expose to an air plasma treatment for 2-3 minutes using a Harrick PDC-32G scientific plasma cleaner which is connected with a Varian 3201 direct drive rotary vane pump. Polyurethane (PU) films were obtained by spin coating 0.3% - 1% solution in chloroform on one face of the sapphire prism (spin coating conditions: the rotating speed is 2000 RPM and the rotating time is 1 minute), and dried in a vacuum oven at room temperature for 12 h. Polyurethane (M w =100 kg/mol) was purchased from Sigma-Aldrich and was used as received. It consists of alternating dicyclohexylmethane-4,4 -diisocyanate and polytetramethylene oxide (PTMO) in the chemical structure of the polymer as shown in Figure Spin coating was conducted by using a Specialty Coating Systems model P6700 spin coater. Ultrapure distilled H 2 O (18 MΩ cm) used in our study was obtained from a Millipore filtration system. D 2 O was purchased from Cambridge Isotopes (D: 99.9%). 35

48 Figure The chemical structure of polyurethane that is used in our study. 3.2 Ellipsometry Measurements Film thickness (Polymer films were spin coated on clean silicon wafers.) was obtained by spectroscopic ellipsometry measurements using a phase-modulated ellipsometer purchased from MRL. The thickness of PU films was measured by a phase-modulated ellipsometer purchased from MRL. Three angles of incident beam, and were used to obtain spectra with a range from 3000 to wavenumbers. Data analysis was performed by creating a three-layer model (Si + SiO 2 + PU) and fitting the data. During fitting, the thickness of Si layer can be assumed as 1 mm and the thickness of SiO 2 layer can be obtained by measuring bare silicon wafer using ellipsometry. 3.3 Quartz Crystal Microbalance (QCM) Measurements Q-sense E4 operator from Biolin Scientific AB was used to study the water uptake of polymer thin film. SiO 2 coated crystal sensor X301 (5 MHz resonant frequency) was chosen as a substrate for spin coating. The sensor crystals were spin coated with 300 nm PU films and placed into the QCM flow cell. H 2 O was introduced into the flow cell (Liquid water is contacting with polymer films.) at a rate of ml/min -1 at 25 for certain hours until a stable baseline was obtained. D 2 O was 36

49 introduced into the flow cell and the change in frequency was recorded. The shift due to the change in solvent occurred in <30 s. After certain minutes, H 2 O was again introduced to the cell, and the frequency returned to the initial baseline. The differences in density of H 2 O and D 2 O caused changes in the resonant frequency of sensor crystals, and permitted calculation of the mass density of absorbed water by polymer films. (16) The calculations can be performed based on two equations listed below. (3.1) ( C=0.177 mg.s/m 2 ) (3.2) Where and are the frequency differences for bare sensor crystals and crystals coated with polymer films (n represents n th overtone) caused by solvent exchange. Using Equation 2 and Equation 3, we can know frequency change cause by the absorbed water and water uptake value. 3.4 Sum Frequency Generation Spectroscopy Apparatus and Measurement The SFG measurement involves a visible pulse at 798 nm overlapping 37

50 spatially and temporally with a tunable infrared pulse at 3-10 µm. The two pulses have a 1 ps width and 1 khz repetition rate. As shown in Figure 3.4-1, the SFG apparatus has two laser sources, a Ti: Sapphire laser and a Nd: YLF laser. Millenia and Tsunami combine together to generate the first laser. Millenia produces a solidstate green laser whose output (at 532 nm with the power of 5W) is directed into Tsunami. In Tsunami, a Ti: Sapphire laser (mode locked) at 800 nm wavelength and 1 Watt power with an 82 MHz repetition rate and a pulse width of 150 femtosecond is generated and used to drive the amplifier. The laser is stretched to 1 picosencond and it forms the seed before entering into the amplifier. The second laser produced by Empower is a pulsed, green, 527 nm beam with 1 khz repetition rate and 30 Watt power. It is also pumped into the amplifier (Spitfire). In Spitfire, the seed and the pump pass through a Ti: Sapphire laser rod and the input pulse from Tsunami is amplified more than 6 orders of magnitude in energy. The amplified pulse is then compressed and directed into optical parametric amplifier (OPA) with the width of ~1 ps and ~ 1 Watt power. In OPA, the input beam is split into two limbs, including the majority of energy (~ 96%) is reflected and used for pumping the OPA and the rest (~ 4%) is transmitted and used to produce a white light continuum which provides the seed. The major portion of the amplified beam is split into two pump beams, the first path (15%) and the second path (85%). The seed pulse and the first pumping path overlap in a beta barium borate (BBO) crystal and signal and idler are generated. The amplified idler beam passes through the crystal and is reflected back by a mirror WLR 3 (diffraction grating), and then overlaps with the second pumping path in BBO crystal for final amplification. These beams are sent to an AgGaS2 crystal (different frequency mixing (DFM)) to generate an IR pulse and a visible beam output. The IR 38

51 beam wavelength can be tuned by rotating the BBO crystal which tunes the wavelengths of the signal and idler beams. By rotating the diffraction grating and DFM crystal, we can maximize the output intensity. The IR and visible output beams from OPA are adjusted to overlap spatially and temporally on the sample surface, and the produced SFG beam is directed into a photomultiplier tube (PMT) and the SFG signal is detected by a Stanford Research Systems SR400 gated photon counter. Simultaneously, a Stanford Research Systems SR850 DSP lock-in amplifier detects IR intensity. Then the SFG intensity (number of photon counts) is normalized with the IR intensity and plotted as a function of the IR wavenumber as shown in a SFG spectrum. Figure The schematic diagram of SFG apparatus. 39

52 3.5 Total Internal Reflection (TIR) Geometry Combined with SFG Measurement We combined TIR with SFG measurement and chose sapphire prisms as a model substrate in our study because SFG signal can be enhanced by 1-2 order of magnitude near critical angles in this geometry (The critical angles were calculated using Snell s law). As shown in Figure 3.5-1, a sapphire prism is coated with the polymer film in contact with water or air (or other medium). The refractive index of water, air, polyurethane and sapphire used in the calculation are 1.33, 1, 1.51 and 1.76, respectively. By performing experiments at proper incident angles, we can control angles between refracted beams and the prism face that is coated with polymer. Total internal reflection takes place when those angles are near critical angles, which allow us to selectively probe the sapphire-polymer interface and the polymer-air (or water) interface. In our study, we used incident angles (with respect Figure The schematic of combining TIR geometry with SFG measurements. 40

53 to the surface normal of the sapphire prism face) of 42, 16, 2 and 2 degrees to probe PU-air, PU-H 2 O (and PU-D 2 O), PU-sapphire and PU-vapor interfaces, respectively. 3.6 Sample Cell and the Humidity Controlling Set-Up We designed a SFG cell that can be used to add water or water vapor in contact with the polymer layer as Figure shown. Water between polymer and the sample cell was sealed well to prevent any loss of water during experiments. The stainless steel cell was cleaned by the same procedure of cleaning sapphire prisms before using. Figure Diagram of the sample geometry for SFG measurements. Liquid water and water vapor can be sealed well in the gap between the sapphire prism and the stainless steel sample cell. A humidity-controlling set-up was also built to control the real-time RH and this setup was integrated with the SFG setup to continuously flow water vapor on the top of polymer layer. Figure shows the humidity meter (ithx-d3 from Omega) to monitor humidity values during the experiments. The controlled humidity was obtained by adjusting the flow rate of N 2 gas bubbling through water and the flow rate 41

54 of dry N 2 gas mixing with water vapor to lower the humidity. The sample was fixed to the cell exposing to water vapor with certain RH and equilibrated for 15 minutes before we collected the SFG spectra. Figure Diagram of the humidity-controlling set-up. The water container is made of glass and other parts are made by stainless steel. 42

55 CHAPTER IV RESULTS AND DISCUSSION 4.1 Calculating Contributions of SFG Signal from Two Interfaces as a Function of the Film Thickness Before we test whether water molecules can penetrate polymer films and reach the polymer-sapphire interface, we need to understand the contributions of SFG signal from two interfaces that are involved in our measurements. For dry samples, we have polyurethane-air and polyurethane-sapphire interfaces that can contribute to the SFG signal; while we have polyurethane-water and polyurethane-sapphire interfaces for samples exposed to liquid water. A mathematical model for the three-layered system developed by Guifeng et al. (83) was used to demonstrate the dependence of interference effects of signals from two interfaces on the PU film thickness as a function of incident angles. We have focused our analysis for incident angles of, and because these were the conditions we used to probe PU-sapphire, PUwater and PU-air (or PU-vapor) interfaces, respectively. Figure presents the ratio of the signals from two interfaces as a function of film thickness (smaller ratio indicates weaker interference effects). As shown in Figure 4.1-1, the interference of the upper interface to the signal of the bottom interface decreases as the thickness of the PU film increases. The interference of the bottom interface to the signal of the upper interface fluctuates as varying the thickness 43

56 of the PU film. Ratios for all the four curves are smaller than 0.1 when the film thickness ranges from 260 nm to 300 nm. We can conclude that when we use PU films with thickness locate in this range, interferences of signals between two interfaces will be reasonably minimized. Figure Predictions of interferences of the SFG signal for two interfaces as a function of the thickness of polymer films by a three-layered structural model. Red dashed line is the ratio of signals from PU-air interface over PU-sapphire interface at incident angle 2 degrees. Blue dashed line is the ratio of signals from PU-water interface over PU-sapphire interface at incident angle 2 degrees. Pink solid line is the ratio of signals from PU-sapphire interface over PU-air interface at incident angle 42 degrees. Black solid line is the ratio of signals from PU-sapphire interface over PU-air interface at incident angle 16 degrees. In this model, we assume that the refractive index of all materials in our study is a real quantity, and absorption in the IR wavelength is neglected. When studying the possible four-layered system (A water film exists between the sapphire and the PU film, and the thick water layer sits on the top of the PU film.), we assume that the three-layered model can work well because the water film at the bottom interface is 44

57 too thin to influence the optical results. 4.2 The Thickness of Polymer Films by Ellipsometry Figure shows the thickness of PU films (They are spin coated on silicon wafers) measured by ellipsometer as a function of PU solution concentration. The thickness increases as increasing the solution concentration. A 292 ± 4 nm thick PU films were obtained using 1 wt. % solution and this film thickness was used for the SFG measurements. Figure Diagram of the thickness of PU films as a function of the solution concentrations varying from 0.3% to 1%. Each data point is the average result of three different samples. 4.3 Water Uptake of Polymer Films by QCM As shown in the Figure 4.3-1, we can clearly see the change in frequency because of the solvent exchange (H 2 O D 2 O H 2 O). The eight lines correspond to 45

58 bare and coated sensors under 3 rd, 5 th, 7 th and 9 th overtones. We collect the frequency change for bare sensors (n represents n th overtone) and sensors coated with PU films, and calculate the contribution of water and water uptake value. The data for different sensors are shown in Table According to, we can calculate that the weight percentage of water in the PU film and between the sensor and the PU film is 0.3% - 1.7% (wt%). Figure The QCM spectra of the bare sensor and the sensor spin coated with the polyurethane film (300nm by ellipsometry) at 3 rd, 5 th, 7 th and 9 th overtones. We also use microbalance to measure the water uptake of PU. We soak five PU beads in H 2 O for 5 days, and then take the beads out of H 2 O and dry the beads surface. We measure the weight of PU beads before and after soaking in H 2 O, which are 54.8 mg and 55.7 mg. The water uptake of PU is 1.6%. This method can only give approximate water uptake value of PU. By comparing the results of these two 46

59 techniques, we can confirm that PU absorbs 1-2 wt.% water. As we ve already known that the polymer films can absorb water, the interfacial specific technique SFG will be used to detect whether water reaches the polymer-substrate interface. TABLE QCM experimental data and calculated data Sensor 1 Sensor 2 Sensor 3 Sensor 4 n= n= n= (s -1 ) n= (s -1 ) (mg/m 2 ) n= n= n= n= n= n= n= n= PU-Air Interface and PU-Sapphire Interface of Samples in the Ambient Condition by SFG Figure shows the SFG spectra in PPP polarization for the PU-air interface (a) and the PU-sapphire interface (b) respectively. The fitting parameters are shown in Table From the PU-air interface, we see three characteristic peaks at 2800, 2850, and 2920 cm -1. The three peaks shown in IR spectra provided in appendix 47

60 and can be assigned to α-ch 2 (s) stretching, normal CH 2 (s) stretching and normal CH 2 (as) stretching modes, respectively. (51, 84-86) The spectrum of the PU-sapphire interface reveals three peaks at 2850, 2940 and 3650 cm -1. Assignment of the band at 2940 cm -1 is less definite. Even though it was attributed to the Fermi resonance of the Figure The SFG spectra in PPP polarization for the PU-air interface (a) and the PU-sapphire interface (b). The solid lines are fit to a Lorentzian equation. 48

61 CH 2 (s) stretch with the overtone of the methylene deformation in literature, this assignment is not reasonable in our study because we didn t observe comparable intensity of CH 2 (s) stretch at the PU-sapphire interface. Thus, the 2940 cm -1 peak is tentatively assigned to an asymmetric CH 2 vibration of soft segment (PTMO) moiety. (54, 87) The 2940 cm -1 peak is significantly different from those for the PU-air interface, indicating the interaction between PU and sapphire. We attributed the 3650 cm -1 peak to the surface OH of sapphire in contact with PU. We can see, based on the magnitude of the acid-base interactions, the surface OH peak shifts to lower wavenumber (The 3720 cm -1 peak is assigned to free surface OH peak on the sapphire surface.), which confirm the interaction between PU and sapphire. (75) TABLE SFG fitting parameters with PPP polarization deduced from Figure for the experiments of probing the PU-air interface and the PU-sapphire interface. PU-air interface PU-sapphire interface TABLE SFG fitting parameters with SSP polarization deduced from Figure for the experiments of probing the PU-air interface and the PU-sapphire interface. PU-air interface PU-sapphire interface

62 We also took spectra of the PU-air and the PU-sapphire interfaces by SSP polarization as show in Figure Similar peaks were observed in spectra of both PPP and SSP polarizations. The fitting results are summarized in Table Figure The SFG spectra in SSP polarization for the PU-air interface (a) and the PU-sapphire interface (b). The solid lines are fit to a Lorentzian equation. 50

63 4.5 PU-Water Interface and PU-Sapphire Interface after Exposure of Samples to Liquid Water by SFG Figure (a) shows the SFG spectrum of PU-H 2 O interface using PPP polarization. The fitting parameters are summarized in Table We observed two H 2 O peaks at 3100 and 3350 cm -1 which are assigned to strongly tetrahedrally coordinated (ice-like) and lower coordination (liquid-like) hydrogen-bond stretch, respectively. (57) The peak at 2940 cm -1 comes from PU-sapphire that is different from hydrocarbon peaks at PU-air interface, indicating the reorientation of polymer chains because of the interaction between PU and H 2 O. The Figure (b) shows the SFG spectrum of PU-D 2 O interface. The observation of peaks at 2410, 2500 and 2940 cm -1 are consistent with those in Figure (a), corresponding to ice-like, liquid-like D 2 O bands and CH 2 (as) stretch of PTMO moiety. The ice-like water bands in Figure indicate an ordered layer of water is formed on PU surface. As suggested by other SFG studies of water/hydrophilic solid interfaces (such as silica and polyethylene oxide (PEO)), (62, 88, 89) the hydrogen bonds between water and silanol groups (SiOH) (or PEO headgroup) contribute to the ordering of water. We TABLE SFG fitting parameters with PPP polarization deduced from Figure for the experiments of probing the PU-H 2 O interface and the PU-D 2 O interface. PU-H 2 O interface PU-D 2 O interface

64 can conclude that there are hydrogen bonds forming between polar groups (including ether, ester and amine groups) of PU chain and water, which causes the coexistence of ice-like and liquid-like water structures at the PU-water interfaces. Figure The SFG spectra in PPP polarization for the PU-water interfaces: (a) the PU-H 2 O interface and (b) the PU-D 2 O interface. The solid lines are fit to a Lorentzian equation. 52

65 The Figure (a) shows the spectrum of the PU-sapphire interface after liquid H 2 O was filled between the cell and the sample using PPP polarization. In this figure, we observed two H 2 O bands at ~3150 and ~3400 cm -1, and a PU peak at 2940 SFG Intensity (a. u.) (a) Wavenumber (cm -1 ) Figure The SFG spectra in PPP polarization for the PU-sapphire interface upon exposure to liquid water. Both H 2 O (a) and D 2 O (b) reaches the PU-sapphire interface. The solid line in (a) is a guide to the eye. The solid line in (b) is fit to a Lorentzian equation. 53

66 cm -1, which indicates the ingress of water to the PU-sapphire interface. In addition, we observed a peak at ~3700 cm -1 that is associated with water and surface OH groups. The fitting paremeters are provided in Table The interpretation and assignment of this peak is less definite because of overlapping of vibrational frequencies for water and surface OH. Our system is also very complicated, including water in contact with PU, water in contact with sapphire and possible sapphire in contact with PU. Thus, the opposite directions of dipole moment would make our interpretation more difficult. TABLE SFG fitting parameters with PPP polarization deduced from Figure for the liquid H 2 O and D 2 O experiments of probing the PU-sapphire interface. PU-sapphire interface PU-sapphire interface We replaced H 2 O with D 2 O and measured the SFG spectra of the PU-sapphire interface in the presence of D 2 O, which makes our interpretation easier because the D 2 O peaks are between cm -1. Figure (b) shows the spectrum of PU/D 2 O/sapphire interface. The peaks at 2400 and 2490 cm -1 are assigned to ice-like and liquid-like D 2 O peaks, which indicate the presence of a water layer confined between PU and sapphire. The peak at 3620 cm -1 is assigned to sapphire OH peak in contact with D 2 O. A previous study of the hexadecane/d 2 O/sapphire interface by Ping 54

67 et al. suggested this peak corresponded to the surface OH band. (23) We also observed this peak at the D 2 O-sapphire interface as shown in Figure 4.5-3, which confirms the assignment. The sapphire OH peak in contact with polyurethane is expected to be near 3650 cm -1. The red shift to 3620 cm -1 indicates that the sapphire OH is now in direct contact with D 2 O, and as the water layer confined between polyurethane film and sapphire substrate disrupts their bonds as a consequence. Figure The SFG spectrum in PPP polarization for the D 2 O/sapphire interface. The solid line is used to guide eyes. 4.6 Sapphire-Water Vapor (or Dry N 2 Gas) Interfaces by SFG In our water vapor experiments, the humidity-controlling set-up was integrated with SFG set-up. Even though we used a careful cleaning procedure to clean our samples, potential contamination that came from the humidity-controlling set-up was a big issue as SFG is very sensitive to organic contaminations. Any organic contamination that absorbed on our samples can be detected by SFG spectroscopy and 55

68 can cause additional peaks in the SFG spectrum. Thus, it will bring us confusion and difficulty for interpreting the real SFG spectrum. At the beginning of building the humidity-controlling set-up, all the tubes we used were polymeric tubes such polyethylene (PE), polyvinyl chloride (PVC) and silicone rubber. Because of evaporative additives were added in these tubes, they produced serious contaminations during experiments. Thus, we replaced all the polymeric tubes with stainless steel tubes and checked again whether the contamination issue is solved. To make sure the whole experimental set up is clean enough, we collected spectra of the sapphire-water vapor interface before we studied the PU-sapphire interface after the humidity-controlling set-up was integrated to the sample cell as show in Figure The cleaned blank sapphire prism was first exposed to dry N 2 gas (0% RH), then exposed to water vapor with 72% RH, and finally exposed to dry N 2 gas again for 30 minutes and 60 minutes. We didn t see hydrocarbon peaks in the region cm -1 after N 2 gas passed through the humidity chamber and blew the sapphire surface, indicating that there is no organic contamination caused by the humidity-controlling set-up. The sharp peak at 3730 cm -1 in the red curve is assigned to the free surface OH of sapphire which confirms that there is no absorbate on sapphire. This sapphire OH peak became broader and red shifted to lower wavenumber after the sample is exposed to water vapor because water molecules absorbed to sapphire surface and interacted with surface OH groups. After the sample was exposed to dry N 2 gas again, the 3730 cm -1 peak partially recovered, which means a non-full water coverage on sapphire surface. The peak intensity increased with increasing the period of time for exposing the sample in the dry N 2 because of 56

69 Figure The SFG spectra of the Sapphire-water vapor (or dry N 2 gas) interface in PPP polarization after the humidity-controlling set-up was integrated to the sample cell. The solid lines are used to guide eyes. less water coverage. The recovery of free OH peak also indicates that sapphire surface is covered by water instead of organic moieties. In the whole experiment, we didn t found organic contamination issue, and thus the humidity-controlling set-up is clean enough and can be applied for our water vapor experiments. 4.7 PU-Sapphire Interface after Exposure of Samples to Water Vapor by SFG For comparing with the spectra of the PU/water/sapphire interfaces, we also collected the SFG spectra of the PU-sapphire interface after samples are exposed to water vapor. Figure shows the spectra of the PU-sapphire interface that were collected when we exposed the sample to H 2 O vapor and gradually increased the RH. 57

70 Figure The SFG spectra of the PU-sapphire interface that are collected at various RH of H 2 O vapor in PPP polarization. These spectra are collected at 0%, 23%, 48% and 82% RH, respectively. The solid lines are fitted to a Lorentzian equation. The fitting parameters are shown in Table The intensity of the 2940 cm -1 peak assigned for PU peak increases as RH increases, while the intensity of the sapphire OH peak at 3650 cm -1 deceases when RH reaches 82%. All these changes indicate H 2 O molecules reach the buried interface and interact with PU and the sapphire substrate. The intensity of the region from 3100 cm -1 to 3400 cm -1 increases slightly as we increased RH. Different from the results of liquid H 2 O experiments, we did not observe any obvious features for ice-like and liquid-like water in the humidity experiments, which indicates that water molecules at the interface don t form a 58

71 uniform monolayer to disrupt PU-sapphire bonds. This interpretation is also supported by the existence of 3650 cm -1 peak in the humidity experiments in which water at the interface partially disrupt the interaction between polymer and substrate. We observed a peak at ~3550 cm -1. For interpreting this peak correctly, we did humidity experiments by using D 2 O vapor. TABLE SFG fitting parameters with PPP polarization deduced from Figure for the water vapor experiment of probing the PU-sapphire interface at various RH (%) of H 2 O vapor. 0% 23% 48% 82% Figure shows the SFG spectra from D 2 O vapor experiment. Table provides the fitting parameters and Table summarizes the peak assignments for H 2 O, D 2 O and PU next to various interfaces in the thesis. The observation of intensity changes of PU hydrocarbon peak and sapphire OH peak are consistent with those shown in H 2 O vapor experiments. In addition, we also observed signal of D 2 O at ~2630 cm -1 and ~2700 cm -1 which are at higher wavenumbers than those for icelike and liquidlike D 2 O peaks. This is again consistent with the results for the H 2 O vapor that the water molecules do not form a highly coordinated hydrogen-bonded water layer. 59

72 Figure The SFG spectra of the PU-sapphire interface that were collected at various RH of D 2 O vapor. These spectra are collected at 0%, 23%, 53% and 80% RH, respectively. The solid lines are fitted by Lorentzian equation. Wei al et. obtained SFG spectra for the vapor/water interface and suggested that the resonant feature at cm -1 could be assigned mainly to the bonded OH stretching mode of water molecules with one bonded OH and one dangling OH (low-coordination water). (62) Another study of sorption of water into a poly (2- methoxyethyl acrylate) film by Morita al et. pointed out that two bands at 3628 and 3558 cm -1 can be assigned to the antisymmetric and symmetric OH stretching modes of water with the C = O H O type of hydrogen bond. (90) The three peaks observed at ~3550, ~2630 and ~2700 cm -1 in our spectra may be assigned to lowcoordination water or water with the C = O H (D) O type of hydrogen bond. 60

73 TABLE SFG fitting parameters with PPP polarization deduced from Figure for the water vapor experiment of probing the PU-sapphire interface at various RH (%) of D 2 O vapor. 0% 23% 53% 80% TABLE Relevant peak assignments for H 2 O, D 2 O and PU next to various interfaces. Origin Peak position (cm -1 ) Surface OH in contact with PU 3650 Surface OH in contact with D 2 O 3620 Liquid-like H 2 O network ~3400 Ice-like H 2 O network ~3150 PTMO segments CH 2 symmetric 2940 CH 2 symmetric 2850 CH 2 asymmetric 2920 PU CH 2 -O symmetric 2795 Liquid-like D 2 O network ~2500 Ice-like D 2 O network ~2400 Low-coordination H 2 O 3550 Low-coordination D 2 O 2630 and

74 4.8 The Effect of Hydrophobicity for Polymer Coating on Water Transport The liquid repellency (lyophobicity) is of very importance for organic coatings. Extensive studies were conducted for controlling wetting properties of a surface, which is important in specialty coatings like corrosion resistant, anti-fog, and anti-ice coatings, and in engineering devices for a variety of technological applications such as micro-fluidics, self-cleaning surfaces, bio-mimetic surfaces. (91-95) Surface wettability control can be achieved by changing surface chemistry and surface roughness. (96) Fluorinated surfaces are widely used to enhance water repellency because of low surface energy. However, bulk fluorinated materials are generally expensive and difficult to process. (97) Instead, surface treatments such as plasma chemical vapor deposition (PCVD) have been applied to coat fluorinated layer on the surface. In our lab, PCVD technique was well built and used to create superhydrophobic surfaces. Taking this advantage, we coat a fluorinated layer on the top of PU film by PCVD and detect whether it can prevent water transport through the organic coating. Contact angles (CA) are used to prove the success of PCVD and SFG is used to measure the existence of water at the polymer-sapphire interface. Figure The contact angle images of the water droplet on (a) the polyurethanecoated sample and (b) the fluorinated layer-coated sample. 62

75 As shown in Figure 4.8-1, the contact angle for the polyurethane film spin coated on the silicon wafer is 86 (obtain the same angle when testing different places on two samples). Then PU-coated samples were coated with a fluorinated layer on the top by PCVD technique. The contact angle of fluorinated samples increases to 110 (also obtain the same angle by testing different places on samples). It indicates that the fluorinated layer was successfully deposited on PU films. The controlling sample (the blank silicon wafer coated with a fluorinated layer) also has similar contact angle with the three-layer sample. Figure The SFG spectra of the PU-sapphire interface (of plasma treated samples) that were collected at various RH of D 2 O vapor. These spectra are collected at 0%, 23%, 49% and 69% RH, respectively. The solid lines are guides to the eyes. 63

76 The ellipsometry was applied to measure the thickness of the fluorinated layer. We obtained the thickness for this layer by measuring the thickness of polymer film before and after plasma treatment and subtracting the former thickness from the latter one. The result was nm. As we have developed an experimental protocol to study water at the polymer-sapphire interface using sum frequency generation (SFG) spectroscopy, we also collected the SFG spectra of the PU-sapphire interface after samples (plasma treated) were exposed to D 2 O vapor. As shown in Figure 4.8-2, the intensity of the peak at 2940 cm -1 assigned for PU peak increases as RH increases, while the intensity of the sapphire OH peak at 3650 cm -1 deceases when RH is high. We also observed signal of D 2 O at cm -1 region that are at higher wavenumbers than those for icelike and liquidlike D 2 O peaks. All these changes indicate the hydrophobic fluorinated layer can t prevent the ingress of D 2 O molecules to the buried interface. Water molecules do not form a highly coordinated hydrogen-bonded water layer at the PU-sapphire interface. 64

77 CHAPTER V CONCLUSION In conclusion, we have studied water at polymer-solid interfaces using surface-sensitive sum frequency generation spectroscopy. We have used this setup to study penetration of water through a model polyurethane film after exposure to liquid water and water vapor. In the case of liquid water experiments, water reaches the PUsapphire interface, forming a highly coordinated hydrogen-bonded layer and disrupting the bond between polyurethane and sapphire. The water layer is ultra thin because of the observation of the hydrocarbon peak from PU. In the case of water vapor, we observed water molecules penetrating to the PU-sapphire interface. However, water molecules are unable to form a fully covered layer and they partially Figure 5-1. A model illustrating the PU-sapphire interface in the presence of water molecules under liquid water and water vapor conditions, respectively. 65

78 disrupt the PU-sapphire bonds. A simple model illustrating the results is shown in Figure 5-1. The direct observations of water at the buried interface in liquid water and humidity experiments have important impacts on adhesion of polymer coatings to solid surfaces, and also provide useful information for designing coating and preventing corrosion. 66

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89 APPENDIX The IR Spectrum of Polyurethane: Figure A1 shows the IR spectrum of PU obtained in the ambient condition. Three peaks at 2795, 2850 and 2920 cm -1 were observed from the spectrum which also presented in the SFG spectrum of the PU-air interface. Figure A1. The IR spectrum of PU in the ambient condition. 77