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Microwave Plasma Reactions of Imidazole on Poly(dimethylsiloxane) Elastomer Surfaces: A Spectroscopic Study Heung Kim and Marek W. Urban* Department of Polymers and Coatings, North Dakota State University, Fargo, North Dakota 58105 Received December 8, 1994. In Final Form: April 3, 1995@ Although there are various forms of energy available for conducting surface and interfacial reactions, microwave energy generated plasmas appears to be an effective source for reacting monomeric molecules to elastomeric surfaces. In this study, closed and open flow microwave plasma reactors were developed and utilized to react imidazole molecules to cross-linked polydimethylsiloxanes (PDMS) surfaces. Using closed reactor, imidazolemolecules are chemicallybonded on the PDMS surface through hydrogen abstraction of the N-H bonds of imidazole. Their orientation, as determined by polarized ATR FT-IR spectroscopy, appears to be preferentially parallel to the PDMS surface. The amount of imidazole reacted to the PDMS surface increases at discharge pressures not exceeding 53.3 Pa, and discharge times not exceeding 20 s. Extended discharge times, however, are destructive to chemically attached imidazole molecules to PDMS. In the open flow reactor, imidazole molecules are reacted to the PDMS surface by ring opening of the imidazole entity to form C s N surface species. Both reactivity and the extent of the ring opening reactions can be controlled by the plasma reaction parameters, discharge pressure, and time. The presence of silica microdomains on PDMS surface inhibits imidazole reactions to PDMS.
Introduction Surface reactions utilizing plasmas are a n attractive means for conducting chemical reactions, in particular, when localized and short pulses of energy are required. For that reason, utilization of microwave plasma leading to modifications of polymer surfaces and interfaces is quite appea1ing.l In this context, one of the merits of plasma reactions is the ability of plasmas to alter surfaces without altering bulk polymer properties.2 This approach opens numerous opportunities for modifying not only surface properties, but also creating reactive sites for further surface and interfacial reaction^.^ In contrast to traditional chemical reactions, one of the drawbacks of the reactions conducted in the plasma gas phase is the complexity of reaction mechanism^.^^^ Multimolecular or multiatomic, often simultaneous and spontaneous collisions, and extremely fast reaction rates6make predictions and control of the chemical processes occurring in plasma environments challenging. Therefore, considerable difficulties may be encountered when detailed analysis of the surface species created by plasma reactions is attempted.' utilized microwave energy to generate Recently, plasmas which allowed us to create new functional groups on cross-linked poly(dimethylsi1oxane) (PDMS) elastomers. Analysis of the surface functional groups on PDMS and their quantitative analysis resulting from
* To whom all corrrespondence should be addressed.
* Abstract published in Advance ACS Abstracts, June 1, 1995.
(1)Stewart, M.; DiDomenico, E.; Urban, M. W. U S . Patent 5,364,662. (2) Yasuda, H. Plasma Polymerization; Academic Press: Orlando, FL,1985. (3)Yasuda, H.; Shamara,A. K. J . Polym. Sci., Polym. Phys. Ed. 1981, 19, 1285. (4)Kobayashi, H.;Shen, M.; Bell, A. T. J. Mucromol. Sci., Chem. 1974,At?, 1354. (5)Yasuda, H.; Lamaze, C. E. J . Appl. Polym. Sci. 1973,17, 1533. (6) Westwood, A. R. Eur. Polym. J . 1971, 7,363. (7) Griesser, H.J.;Chatelier, R. C. J.Appl. Polym. Sci., Appl. Polym. Symp. 1990,46, 361. (8) Gaboury, S. R.; Urban, M. W. Polym. Commun. 1991,32(13),390. (9)Gaboury, S. R.;Urban, M. W. Polymer 1992,33(23),5085. (10)Gaboury, S.R.;Urban, M. W. Langmuir 1993,9, 3225. (11)Gaboury, S.R.;Urban, M. W. Langmuir 1994,10,2289.
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microwave plasma reactions of solid monomers were conducted using attenuated total reflectance (ATR) FTIR spectroscopy. I n these studies, we established that the monomers containing C=C bonds, such as acrylamide and maleic anhydride, react with the PDMS surface through C=C double bond opening, and maintain their original structure when reaction times do not exceed 10 s. However, extended reaction times lead to limited monomer supply to the reaction sites due to conversion of monomers to polyacrylamide or maleic acid. Therefore, a cleavage of the previously reacted surface amide or anhydride species occurs. Using ATR FT-IR spectroscopy, quantitative analysis of newly created surface species was performed. l1 In this study, our efforts will concentrate on surface reactions of imidazole on PDMS elastomer surfaces using microwave plasma energy. Similarly to the previous studies, formation of surface reacted imidazole molecules on PDMS surface, effects ofmicrowave plasma parameters on surface reactivity, orientation of the surface species, and how closed or open flow reactors may affect reaction mechanisms will be investigated.
Experimental Section Substrate Preparation. Poly(dimethylsi1oxane)(PDMS) was prepared from a linear, vinyl terminated dimethylvinylmethylsiloxane polymer (M,= 28 000, Huh American Inc.).
Reactions between vinyl groups forming cross-linked PDMS networks were initiated by addition of 0.5% (w/w) tert-butyl perbenzoate (Aldrich Chemical)to PDMS. PDMS oligomer and initiator were first premixed for 24 h to ensure complete dissolution of initiator in PDMS. Films of cross-linked PDMS were prepared by pressure molding the oligomer-initiator solution for 15 min at 149 "C, and post-cross-linking for an
additional 4 h at 210 "C. Cross-linked PDMS films containing Si02 were prepared by addition of 5% (w/w) of Aerosil 200 (Degussa Corp.) SiOz. Oligomer-initiator solution prepared in such a way was combined with silica and mixed in a rolling ball mill for additional 24 h. Cross-linking was accomplished by pressure molding a specimen under approximately 330 psi for 15 min at 149 "C using Carver Lab. Press, Model C, and postcross-linkingfor an additional 4 h at 210 "C. Potential surface contaminants and residual low molecular weight species were
0 1995 American Chemical Society
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2072 Langmuir, Vol. 11, No. 6, 1995 Closed Plasma Reactor
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removed by stirring PDMS films in methylene chloride for 5 h. Residual methylene chloride was removed from the PDMS network by vacuum desiccating each specimen for 24 h at room temperature. Surface Reactions. Plasma reactions were conducted using closed and open flow reactors which are schematically depicted in Figure 1. In the open flow reactor, reactions were conducted in a continuous flow of gas under a specific pressure. Crosslinked PDMS substrate, with approximate dimensions of 50 x 25 x 2 mm, and approximately 50 mg of solid imidazole were placed into a reactor. The reactor was evacuated to 1.3 Pa, followed by purging it with Ar gas to the desired pressures, until a steady-state pressure ofAr gas flow was reached. At this point, microwaveradiation of approximately 600W of power with output frequency of 2.45 GHz using microwave source KMC Model KMO24G to induce plasma reactions was turned on. For experiments conducted in a closed reactor, the same procedure was utilized except, after imidazole and PDMS were placed into the reactor, the chamber was evacuated to approximately 1.3Pa and brought back to atmospheric pressure by introducing Ar gas. The reactor was evacuated again to the desired pressures, followed by microwaveexposure to induce plasmas. In both cases,gas plasma reactions on PDMS surface were carried out using imidazole (Aldrich Chemical) which at 1.3 Pa exhibits a partial vapor Pa.12 Under these conditions,the pressure pressure of 2.6 x in the reaction chamber increases continuously duringmicrowave plasma discharge. However,under the same pressure conditions, the pressure in the reaction chamber remains constant, and no sorption of imidazole into the PDMS network was detected without microwave plasma discharge. Spectroscopic Measurements. ATR FT-IR spectra were collected on a Digilab FTS-14B equipped with a liquid nitrogen cooled MCT detector. A resolution of 4 cm-I and a mirror speed of 0.3 cm s-l were used. The ATR cell was aligned at a 45"angle of incidence using a 45" angle parallelogram KRS-5 crystal. In an effort t o determine orientation ofthe surface species,90"(TE) and 0' (TM)polarizedinfrared light was used. TE is a transverse vector of the incidence light polarized at 90" with respect to the sample surface, whereas TM is a transverse magnetic vector polarized at 0" with respect to the sample surface. Other experimental details concerning the setup are published e l s e ~ h e r e . ~ Each - ~ ~ Jspectrum ~ represents 300 coaded scans ratioed against a reference spectrum obtained from 300 coaded (12) De Wit, H. G.;VanMiltenburg, J. C. J . Chem. Thermodyn. 1983, 15(7), 651. (13)Thorstenson, T. A.; Tebelius, L. K.; Urban, M. W. J . A p p l .Polym. Sci. 1993,49, 103.
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Figure 2. ATR FT-IR spectra in the 2300-1300 cm-I region of imidazole reacted to PDMS surface under various pressure conditions using a closed reactor: A, unreacted PDMS; B, imidazole reacted to PDMS at 106.7PdlO s;C, imidazole reacted to PDMS at 53.3 PdlO s; D, imidazole reacted to PDMS at 26.6 PdlO s.
scans of an empty ATR cell. All ATR spectra were corrected for spectral distortions using Q-ATR ~0ftware.l~
Results and Discussion Figure 2 illustrates ATR FT-IR spectra in the C=N and C=C stretching regions for imidazole reacted to PDMS surface under various initial discharge pressures. In this case, experiments were conducted in a closed reactor. For reference purposes, trace A of Figure 2 illustrates the spectrum of unreacted PDMS surface. The spectra of imidazole reacted to PDMS surface exhibit the appearance of the bands a t 1603 cm-' and 1559 cm-l which are attributed to the C=C and C=N stretching modes of imidazole. It appears that when the reaction is conducted a t 106.7 Pa (trace B), the C=C stretching band a t 1603 cm-l and the C=N stretching band a t 1559 cm-l are detected. The bands become stronger when initial discharge pressures are dropped to 53.3 Pa (trace C) and 26.6 Pa (trace D). In addition, when pressures below 106.7 Pa are used, the C-H deformation region of imidazole reacted to PDMS surface exhibits a new band a t 1393 cm-l attributed to the C-H deformation modes of the -CH=CH- groups. Although these observations suggest that imidazole reacts with the PDMS surface, if this is indeed the case, the N-H stretching bands should allow us to confirm these findings. Figure 3 illustrates ATR FT-IR spectra in the N-H and C-H stretching regions of the same specimens and exhibit the appearance of two new bands at 3161 cm-l and 3121 cm-l attributed to antisymmetric and symmetric stretching modes of the -CH=CH- entities on the surface reacted imidazole. Again, when initial discharge pressures are decreased from 106.7 Pa (trace B) to 26.6 Pa (trace D), the intensity ofthe C-H stretching bands of the -CH=CH- groups also increases. These observations indicate that imidazole indeed reacts at the PDMS surface, most likely by the hydrogen abstraction of the N-H bonds, but without C=C cleavage and subsequent ring opening. A possible mechanism of imidazole reaction at the PDMS surface is shown in Figure 4,path A. In this figure, "box"represents PDMS substrate. (14)Huang,J. B.; Urban, M. W. Appl. Spectrosc. 1992,46(11),1666.
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Figure 3. ATR FT-IR spectra in the 3400-2800 cm-l region of imidazole reacted to PDMS surface under various pressure conditions using a closed reactor: A, unreacted PDMS; B, imidazole reacted to PDMS at 106.7PdlO s;C, imidazole reacted to PDMS at 53.3 PdlO s; D, imidazole reacted to PDMS at 26.6 PdlO 5. Pat? A
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Figure 4. Schematic representation of imidazole reacted to PDMS surface in microwave plasma reactions: path A, imidazole reacted under closed reactor conditions; path B, imidazole reacted under open reactor conditions.
Although a t this point we do not know the origin of the reaction sites on PDMS available for surface reactions with imidazole, this issue is currently under investigation.15 It should be also realized that similar spectroscopic observations could be made if imidazole molecules were physically deposited on the surface ofthe PDMS elastomer. In a n effort to resolve this concern, similarly to the previous studies,1° all microwave plasma reacted PDMS samples were boiled in water for 20 min., followed by a removal and drying specimens under vacuum for 24 h. After such treatments, which are considered to be sufficient to remove all physisorbed molecules, ATR FT-IR spectra were recorded again. In all cases presented in this study, the spectra before and after surface reactions were identical, indicating that imidazole is chemically attached and not physisorbed a t PDMS surface. Figure 5 illustrates ATR FT-IR spectra in the C=C and C=N stretching region for imidazole reacted at the PDMS surface under closed reactor conditions with a change of discharge times. Again, for reference purposes, trace A illustrates the unreacted PDMS spectrum. As the discharge times increase from 5 s (trace B) to 10 s (trace c), (15)Kim, H.; Urban, M. W. In preparation.
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Figure 5. ATR FT-IR spectra in the 2300-1300 cm-l region of imidazole reacted to PDMS surface under various discharge time conditions using a closed reactor: A, unreacted PDMS; B, imidazole reacted to PDMS at 26.6 Pd5 s; C, imidazole reacted to PDMS at 26.6 PdlO s; D, imidazole reacted to PDMS at 26.6 Pd20 s; E, imidazole reacted t o PDMS at 26.6 Pd30 s.
intensities of the C=C and C=N stretching bands a t 1603 cm-’ and 1551cm-’increase. However, extended reaction times over 20 s result in a decrease of the band intensities. This is illustrated in traces C and D, which represent ATR FT-IR spectra for the specimens with 20 and 30 s discharge time. These relatively long discharge times do not allow residual monomer to evaporate, most likely due to a n increase of the inner pressure a t longer discharge times in a closed reactor. Therefore, the monomer cannot be further supplied to a gas phase, thus preventing a continuation of surface reactions. Silica in a form of powder particles is commonly used as a PDMS/elastomer reinforcing agent. Its presence, however, many cause a significant effect on surface reactions. Indeed, our earlier s t u d i e ~indicated ~ ~ ~ that the presence of silica may affect surface reactions. For example, PDMS without Si02 exposed to microwave plasmas in the presence of Ar atmosphere leads to the formation of Si-H surface functionalities. However, the same reactions conducted on a PDMS containing Si02 inhibits the formation of Si-H functionalities. These findings stimulated us to pursue similar experiments when imidazole is microwave plasma reacted to PDMS. Figure 6 illustrates ATR FT-IR spectra of imidazole reacted to PDMS containing 5% (w/w) of silica a t various initial discharge pressures. As the initial discharge pressures are diminished from 106.7 Pa (trace B)to 26.6 Pa (trace D), intensities of the C-C and C=N stretching bands at 1601 cm-l and 1551 cm-l increase. However, the intensity changes are not as pronounced as compared to imidazole reacted to the PDMS surface without silica. A comparison of the results for initial discharge pressures of 26.6 Pa is shown in Figure 7, and indicates that no change of imidazole reactions for the discharge times ranging from 5 to 30 s are detected. This is shown in Figure 7,traces B and C, respectively. For comparison purposes, trace D of Figure 7 illustrates a spectrum of the PDMS specimen without silica exposed to 26.6 P a for 30 s discharge time. Intensities of the C=C and C=N stretching bands at 1598 cm-l and 1551cm-l of imidazole reacted to PDMS with silica are diminished. In a n effort to verify t h a t indeed the presence of silica is detrimental to the formation of PDMS-imidazole
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Figure 6. ATR FT-IR spectra in the 2300-1300 cm-' region ofimidazole reacted to PDMS surface containing 5%(w/w)silica under various pressure conditions using a closed reactor: A, unreacted PDMS; B, imidazole reacted to PDMS at 106.7 P d 10 s; C, imidazole reacted to PDMS at 53.3 PdlO s;D, imidazole reacted t o PDMS at 26.6 PdlO s.
Figure 8. ATR FT-IR spectra in the 1900-900 cm-l region of imidazole reacted to silica film surface using a closed reactor: A, unreacted silica film; B, imidazole reacted to silica film at 26.6 PdlO s; C, imidazole reacted to PDMS containing 5%(w/ w) silica at 26.6 PdlO s.
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Figure 7. ATR FT-IR spectra in the 2300-1300 cm-l region of imidazole reacted to PDMS surface containing 5% (w/w)silica under various discharge time conditions using a closed reactor: A, unreacted PDMS; B, imidazole reacted to PDMS at 26.6 P d 5 s; C, imidazole reacted t o PDMS at 26.6 Pd30 s; D, imidazole reacted to PDMS without silica at 26.6 Pd30 s.
linkages, polycrystalline silica films were microwave plasma treated. Trace A of Figure 8 illustrates the ATR FT-IR spectrum of unreacted silica film. Trace B illustrates the spectrum of the same silica film, but exposed to 26.6 Pa for 10 s discharge time with imidazole present in the reaction chamber. For comparison purposes, trace C illustrates ATR FT-IR spectrum of imidazole reacted to PDMS containing 5%(w/w)of silica exposed to 26.6 Pa for 10 s discharge time. Analysis of the spectra shown in Figure 8 indicates that no C=C and C=N stretching modes due to imidazole are detected on silica film. Although it is beyond the scope of these studies to determine a mechanism of silica inhibition, based on these findings, it is believed that lower surface reactivity of imidazole on PDMS containing silica is attributed to the formation of
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Figure 9. Polarized ATR FT-IR spectra in the 2300-1300 cm-l region of imidazole reacted to PDMS surface under various pressure conditions using a closed reactor: A, TM polarization of imidazole reacted to PDMS at 106.7 PdlO s; B, TM polarization of imidazole reacted to PDMS at 53.3 PdlO s; C, TM polarization of imidazole reacted to PDMS at 26.6 PdlO s; D, TE polarization of imidazole reacted to PDMS at 106.7 PdlO s; E, TE polarization of imidazole reacted to PDMS at 53.3 PdlO s; F, TE polarization of imidazole reacted to PDMS at 26.6 PdlO s.
microdomains containing silica on the PDMS surface, which inhibits surface reactions. It is well known that one of the advantages of using polarized ATR FT-IR spectroscopy is the ability to determine orientation of the surface species.13 If one considers symmetry and reactivity of five- and sixmembered rings, these entities have the tendency to take a certain surface orientation. This stimulated us to further investigate orientation of imidazole reacted to PDMS surface. Figure 9 illustrates ATR FT-IR spectra in the C=C and C=N stretching region for imidazole reacted to PDMS surfaceat 106.7,53.3,and26.6Painaclosedreactor using transverse magnetic (TM) (traces A, B, and C) and
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Figure 10. Polarized ATR FT-IR spectra in the 3400-2800 cm-l region of imidazole reacted to PDMS surfaceunder various pressure conditions using a closed reactor: A, TM polarization of imidazole reacted to PDMS at 106.7 PdlO s; B, TM polarization of imidazole reacted to PDMS at 53.3 PdlO s; C, TM polarization of imidazole reacted t o PDMS at 26.6 PdlO s; D, TE polarization of imidazole reacted to PDMS at 106.7 PdlO s; E, TE polarization of imidazole reacted to PDMS at 53.3 PdlO s; F, TE polarization of imidazole reacted to PDMS at 26.6 PdlO s.
transverse electric (TE)(traces D, E, and F) polarizations. Definitions of TM and TE polarizations along with the experimental setup are published elsewhere.16 As shown in Figure 9, the intensities of the C=C and C-N stretching bands a t 1606 and 1599 cm-l are significantly lower for the TM polarization (traces A, B, and C). This observation indicates t h a t the newly formed species are preferentially parallel to the surface. This assessment is supported by the results shown in Figure 10 for antisymmetric and symmetric C-H stretching bands at 3160 and 3116 cm-l recorded in the TE polarization, which are virtually absent when the spectra are recorded using TM polarization. Again, this observation supports the fact that imidazole rings are preferentially oriented parallel to the PDMS surface. One of the issues t h a t is rarely addressed when gas plasma reactions are conducted is the effect of initial discharge pressures in the plasma reactor and their changes during plasma reactions. This issue is important because chemical structures produced by microwave plasma reactions may vary due to inner pressure changes in the plasma reactor. While the velocity of a gas molecule and ionization of gases in plasma states is inversely proportional to the discharge pressures during plasma reactions, chemical structures formed on the surface will vary with the inner pressure changes. In addition, the plasma state is controlled by a so-called energy factor, which is the amount of microwave energy divided by velocity of a gas molecule. Therefore, a n increase of the discharge pressures during plasma reactions results in a significant decrease in velocity of gas phase molecules, thus lowering their kinetic energy, and consequently making a lower energy state. So far, we discussed microwave plasma reactions conducted under closed reactor conditions. Before we focus on the open microwave plasma reactions, it should be (16)Urban, M. W. Vibrational Spectroscopy of Molecules and Macromolecules on Surfaces;John Wiley: New York,1993and references therein.
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Figure 11. ATR FT-IR spectra in the 2300-1300 cm-l region of imidazole reacted to PDMS surface under various pressure conditions using an open flow reactor: A, unreacted PDMS; B, imidazolereactedtoPDMSat 106.7PdlO s; C, imidazole reacted to PDMS at 53.3PdlO s; D, imidazole reacted to PDMS at 26.6 PdlO s.
realized that the primary difference between the closed and open flow reactors is the change of discharge pressures during plasma reactions. Under closed reactor conditions, discharge pressures increase rapidly with increase of discharge times during plasma reactions, resulting in a lower energy state. In contrast, when a n open flow reactor is used, discharge pressures maintain steady state initial pressures during plasma reactions, which result in a higher energy state. This higher energy plasma state under open flow reactor conditions is due to lower steady state pressures during microwave plasma reactions, and results in the ring opening of imidazole molecules shown in Figure 4, path B. Let us now focus on the surface reactions conducted under open flow reactor conditions. Figure 11illustrates ATR FT-IR spectra in the C-C and C=N stretching regions for imidazole reacted to PDMS surface with a change of the initial discharge pressures in the open flow reactor. The spectrum of imidazole reacted to the PDMS surface at 106.7 Pa (trace B) exhibits a new band at 1658 cm-l, which is attributed to the CHz=CH- stretching modes resulting from the ring opening of imidazole molecules. Compared to the spectra obtained on the specimens reacted in a closed reactor (Figures 2 and 3), the C-N stretching bands a t 1559 cm-l are not detected. However, the spectrum of imidazole reacted to the PDMS surface a t 53.3 P a (trace C) exhibits a new band a t 2183 cm-', which is attributed to the CEN groups. These observations indicate that the presence of these species results from the ring opening reaction of the imidazole ring, which appears to occur only when open flow reactor conditions are employed. The higher energy state of plasma in the open flow reactor is most likely responsible for the ring opening reaction, which is caused by significantly lower steady state pressures during the plasma reactions. A mechanism responsible for the C e N formation is proposed in Figure 4, path B. As the initial discharge pressures decrease from 53.3 P a to 26.6 Pa, the intensity of the CEN stretching band a t 2183 cm-l increases. Figure 12 illustrates ATR FT-IR spectra in the C=C and C=N stretchingregions for imidazole reacted to the PDMS surface with a change of the discharge times in the open flow reactor. As discharge times increase,
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Figure 12. ATR FT-IR spectra in the 2300-1300 cm-' region of imidazole reacted to PDMS surface under various discharge time conditions using an open flow reactor: A, unreacted PDMS; B imidazole reacted to PDMS at 26.6 Pd5 s; C, imidazole reacted to PDMS at 26.6 PdlO s; D, imidazole reacted to PDMS at 26.6 Pd20 s; E, imidazole reacted t o PDMS at 26.6 Pd30 s. intensities of the CHZ=CH- and C=C stretching bands a t 1655 cm-l and 1596 cm-l, respectively, increase, when the plasma reactions do not exceed 20 s. However, extended discharge times over 20 s result in lower intensities, indicating again that the species that were created during microwave plasma exposure are being cleaved from the surface. To summarize the results of surface reactions conducted in the open flow condition, it appears that the C=N stretching band a t 2180 cm-l is detected when the discharge times are between 10 (trace C) and 20 s (trace D). Above 20 s discharge time, the 2180 cm-l band is not detected (trace E). This behavior results from a lack of supply of a monomer and removal of the imidazole molecules reacted to the PDMS surface by plasma etching.
Surface analysis of imidazole monomers reacted to PDMS surface using ATR FT-IR spectroscopy reveals that imidazole molecules are reacted to the PDMS surface through a hydrogen abstraction of the N-H bonds. This reaction can be conducted in a closed reactor using microwave plasma environments. Both imidazole ring structure and PDMS cross-linked network are maintained. Reactivity of imidazole reacted to the PDMS surface increases a t lower discharge pressures, and its amount increases up to approximately 20 s. Discharge times exceeding 20 s result in a decrease ofthe imidazole content reacted to the PDMS surface. This behavior is attributed to a lack of ability of imidazole to evaporate to a gas phase and subsequent removal of already reacted groups. For the PDMS network containing 5% (w/w) of silica, the presence of silica microdomains results in a small imidazole content reacted to PDMS surface. This behavior is attributed to lower surface reactivity of silica microdomains. Finally, orientation of imidazole reacted to the PDMS surface using TE and TM polarized ATR FT-IR spectroscopy reveals that the imidazole rings are preferentially oriented parallel to PDMS surface. Analysis of microwave plasma reactions conducted under open flow reactor conditions reveals different structures formed on the PDMS surface. Imidazole reacts to a PDMS surface by ring opening, resulting in the formation of C=N surface groups. The reactivity of imidazole to form C=N surface species increases a t lower discharge pressures, and the amount of the C=N groups reacted to PDMS surface also increases with discharge times not exceeding 20 s. Discharge times exceeding 20 s are destructive to the C=N group stability, most likely due to a localized thermal energy input from a microwave source.
Acknowledgment. The authors are thankful to the members of the NSF Coatings Research Center for supporting this work. LA9409721
