PHYSICAL AND CHEMICAL PROPERTIES OF ATMOSPHERIC PRESSURE PLASMA POLYMER FILMS O. Goossens, D. Vangeneugden, S. Paulussen and E. Dekempeneer VITO Flemish Institute for Technological Research, Boeretang 200, B-2400 Mol, Belgium Abstract: The aim of this work was to perform a detailed study of the basic properties of plasma polymers produced from ethylene, ethylene oxide and propylene. Films were deposited in an atmospheric pressure dielectric barrier discharge (DBD). Physical properties were characterized by field emission scanning electron microscopy (FESEM) and contact angle measurements. On the other hand, chemical properties were characterized by means of Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR) and electron spin resonance (ESR) techniques. 1. INTRODUCTION When introducing molecular gases into a plasma, chemically active species are formed such as molecules in excited states, radicals and ions. These species can react with each other, neutral molecules or with the surface of a substrate. Depending on the nature of the molecules and the process conditions, this may result in the deposition of a thin film. Films resulting from organic precursors are generally known as plasma polymers. The reaction chain leading to a plasma polymer film is not comparable to common polymerization reactions. For this reason, the properties of plasma polymer films can significantly differ from their classic chemical counterparts. Although widely investigated for many years, plasma polymers are still not well understood with respect to their physical and chemical properties. In particular, this is true for the more recently developed deposition technologies based on atmospheric pressure plasmas like dielectric barrier discharges (DBDs) [1, 2, 3]. When hydrocarbons are introduced in a DBD at atmospheric pressure, plasma polymerization occurs which results in the deposition of polymer-like material on the surface of the electrodes. This paper reports on a more detailed investigation of plasma polymer coatings obtained from ethylene, propylene and ethylene oxide in discharges sustained in He. The aim of this work is to gain more insight in the morphology and chemical structure of these materials. Currently, coatings obtained in Ar and N 2 discharges are also being investigated. 2. EXPERIMENTAL SETUP Plasma polymerization is performed in the system shown in figure 1. The discharge is obtained between two disk-shaped electrodes with a diameter of 150 mm, both covered with an Al 2 O 3 plate of 2 mm thickness. To ensure stable plasma operation, the gap width is typically limited to a few millimeters. The bottom electrode is connected to a variable frequency AC power source. The frequency can be varied between 1 and 30 khz with a maximum output voltage of 20 kv. In this setup the top electrode is grounded. In order to obtain a well controlled atmosphere the configuration is mounted in a vacuum chamber. This vacuum chamber is evacuated before each run and subsequently filled with a specific process gas mixture. Electronic address: Olivier.Goossens@vito.be
Stainless steel electrode Pumping system 1:1000 Gas inlet Power source ~ Spectrometer R Al v r ~ i 2 O 3 Oscilloscope FIGURE 1. Experimental setup. By means of mass flow controllers about 5 % of reactive gas (0.5 l/min) was added to the inert carrier gas flow (20 l/min). The deposition time was set at 10 min for deposition experiments with ethylene and propylene. In case of ethylene oxide a deposition time of 30 min was used. In order to obtain samples for the different analysis techniques, various substrates were placed between the electrodes such as glass slides, pieces of single and double polished wafers and polished stainless steel plates. The optical emission spectra (OES) of the DBD are sampled through an optical fiber placed at a distance of a few centimetres from the discharge. The optical fiber is connected to a spectrometer, which has a focal length of 320 mm (Jobin Yvon, HR 320). Plasma polymer coatings used for FESEM analysis (Jeol, JSM-6340F), FTIR analysis (Thermo Nicolet, Avatar 360 E.S.P.) and contact angle measurements (Dataphysics, OCA 15 plus) were deposited on -substrates placed on the bottom electrode. The plasma polymers used for NMR and ESR measurements were recovered from deposits on polished glass substrates. The plasma polymer deposition time varied from 10 to 30 min. 3. RESULTS AND DISCUSSION Some qualitative observations are readily made. Plasma polymerization of ethylene and propylene in He results in opaque and sticky materials, this in contrast to coatings obtained from ethylene oxide which are clear and non-sticking. In figure 2 FESEM pictures are shown that were taken from cross sections of coatings deposited on single polished. As such the thickness of coatings and hence the deposition rates could be evaluated. The film growth rate appears to be much higher when ethylene or propylene is used as precursor (about 100 nm/min) than when ethylene oxide is used (about 20 nm/min). Films deposited by the addition of mentioned monomers to He discharges appear to have a very smooth surface. This in contrast to coatings obtained from the same monomers in Ar discharges [3]. Under both discharge conditions coatings obtained appear homogeneous and without pinholes or structural defects. From FTIR analysis some information can be gathered concerning the presence of functional groups in the coatings [4]. It is however quite difficult to obtain adequate information about the hydrocarbon backbone. As can be readily seen from the infrared spectra in figure 3, the ester group (C=O stretching around 1700 cm -1 ) and the hydroxyl group (O-H stretching around 3400 cm -1 ) are more pronounced in the plasma polymer obtained from ethylene oxide compared to that from ethylene. The presence of the latter groups in the ethylene and propylene plasma polymer originates from exposure to air after deposition. However, it has to be remarked that absorption of water vapor present in the atmosphere can also contribute to the observed O-H signal. In case of the ethylene oxide plasma polymer, oxygen already present in the monomer gives rise to more oxygen containing functionalities such as esters, ketons, ethers and alcohols. (The C-O stretching of ethers is situated around 1120 cm -1 ). Furthermore
Coating surface Coating surface Coating surface 100 nm 1 µm 100 nm FIGURE 2. SEM cross sections of coatings obtained by plasma polymerization of respectively (from left to right) ethylene, propylene and ethylene oxide. The substrate material was and the deposition time was 10 minutes in case of ethylene and propylene and 30 minutes in case of ethylene oxide. all spectra contain C-H stretching (2955 cm -1 ) and C-H deformation (1456 cm -1 ) bands from the obtained hydrocarbon network. X-ray photoelectron analysis has been carried out to determine the relative amount of oxygen and carbon at the surface of the plasma polymer films. The first results correlate well with the results obtained by FTIR spectroscopy. A higher oxygen content is observed in the polymer deposited from ethylene oxide. The question whether surface properties of the plasma polymers are representative for the bulk of the material is currently being addressed. GD-OES (glow discharge optical emission spectrometry) seems to be more appropriate for this purpose than XPS. Nuclear magnetic resonance (NMR), and in particular 13 C-NMR, has proven to be a powerful technique to perform detailed investigations of the hydrocarbon backbone of polymers [5, 6]. It also seemed an interesting technique to gain some more insight in the chemical structure of the obtained plasma polymers. In principle, one should be able to evaluate for instance the retention of the chemical structure of the monomers in the plasma polymers and the degree of crosslinking. Therefore, 13 C- NMR measurements were performed on plasma polymers from ethylene and propylene recovered from depositions on large glass plates. However, the recorded 13 C-NMR spectra only showed very weak signals that were strongly broadened and hence not very informative. This could be due to the presence of strong paramagnetic sites, such as remaining stable radicals. In contrast to the 13 C-NMR measurements, 1 H-NMR analysis of the plasma polymers resulted in more useful spectra (figure 4). Although the line broadening in these spectra is also considerably higher than usual for polymers, some qualitative information can be extracted. First of all, the spectra of plasma polymers from ethylene and propylene are very similar. However, the CH 3 -signal (at about 0.8 ppm) seems to be disproportionately large compared to the CH 2 - and CH-groups (at about 1.2 to 1.4 ppm). As can be expected, the CH 3 signal is relatively higher for plasma polymers from propylene compared to those obtained from ethylene. Furthermore, the spectra indicate the presence of remaining C=C double bonds (signals around 4.5 to 6 ppm) and some oxygen containing unsaturated 150 140 130 120 110 100 90 80 70 60 50 40 3500 3000 2500 2000 1500 Wavenumber [cm -1 ] 1000 500 FIGURE 3. FTIR spectra of plasma polymers from ethylene (left) and ethylene oxide (right) obtained in He discharges. 45 40 35 30 25 3500 3000 2500 2000 1500 Wavenumber [cm -1 ] 1000 500
FIGURE 4. 1 H-NMR spectra of plasma polymers obtained from ethylene (left) and propylene (right) in He discharges. functionalities like OCHO or C=CHO (around 8 ppm). The broad signal from about 1.7 to 2.2 ppm is attributed to CH x -groups next to double bonds. To verify the hypothesis of remaining stable radicals in plasma polymers from ethylene, propylene and ethylene oxide, ESR measurements were performed. The ESR spectra presented in figure 5 confirm the presence of remaining radicals in plasma polymers obtained from ethylene and propylene. However, ESR measurements on plasma polymers obtained from ethylene oxide, didn t reveal any detectable signal of remaining radicals. This indicates that the monomer structure has an important impact on the formation of stable radicals. In case of plasma polymers from ethylene and propylene, the presence of remaining double bounds (as present in the monomers) is expected to have a stabilizing effect on some radical sites. By delocalization across several atoms radicals become less reactive and hence more stable. In order to substantiate this theory additional ESR experiments are being performed including the use of several ESR spectral bands (X-, Q- and W-band) and measurements under different conditions (e.g. temperature). Furthermore, plasma polymers obtained from propane and butane will be investigated. In order to follow up the coating deposition in situ, a link between the plasma polymerization reactions and optical emission spectroscopy measurements would be very useful. It seems however quite difficult to link the physical and chemical structure of the obtained coatings to the optical emission spectra. Gathering information on plasma composition and reaction mechanisms seems rather difficult to target in this way [7]. For this purpose modeling seems to be a very powerful tool. 6000 6000 2000 0 3350 3370 3390 3410 3430 3450-2000 - 2000 0 12050 12070 12090 12110 12130 12150 12170 12190-2000 - -6000-6000 B (10-4 T) -8000 B (10-4 T) FIGURE 5. Examples of ESR measurements on plasma polymers from ethylene (left, X-band spectrum) and propylene (right, Q-band spectrum) obtained in He discharges.
4. CONCLUSIONS Coatings were obtained by plasma polymerization of ethylene, propylene and ethylene oxide and analyzed with a broad spectrum of analysis techniques. The results presented were focused on coatings obtained in a He discharges. The research done gives some more inside in the chemical structure of the obtained coatings. Nevertheless, it remains difficult to correlate the results of analysis on plasma polymers to plasma chemical conditions or to OES spectra taken during the deposition process. The presented results suggest that no polymerization in the conventional sense of the word occurs. Plasma polymerization results in a network structure. In case ethylene or propylene is used as monomers, some double bounds remain present in the plasma polymers obtained. These double bonds can have a stabilizing effect on the radicals which makes them long living. The presence of such radicals was first indicated by difficulties to perform ( 13 C-)NMR measurements and later confirmed on the basis of ESR-analysis. A more elaborate investigation of the influence of the chemical structure of the monomer on the presence of stable radicals in the plasma polymer is presently ongoing. ACKNOWLEDGMENTS The authors would like to acknowledge Prof. E. Goovaerts from the University of Antwerp (UIA) and Prof. F. Callens from Ghent University (RUG) for the ESR measurements. The authors would also like to thank Prof P. Adriaensens from the Limburg University Centre (LUC) for the NMR measurements. REFERENCES [1] Prat R., Koh Y. J., Babukutty Y., Kogoma M., Okazaki S., Kodama M.,, 41, 7355-60 (2000) [2] Salge J., Surf. Coat. Technol., 80, 1-7 (1996) [3] Goossens O., Dekempeneer E., Vangeneugden D., Van de Leest R., Leys C., Surf. Coat. Technol., 142-144, 474-81 (2001) [4] Ren W., Colloid. Polym. Sci., 270, 747-52 (1992) [5] Randall J. C., Ruff C. J., Kelchtermans M., Recl. Trav. Chim. Pais-Bas, 110, 543-52 (1991) [6] Prasad J. V., J. Polym. Sci.: Part A: Polym. Chem., 30, 2033-36 (1992) [7] Gherardi N., Martin S., Massines F., J. Phys. D-Appl. Phys., 33, L104-8 (2000)