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Langmuir 1996, 12, 3282-3288
Microwave Plasma Reactions of Imidazole on Poly(dimethylsiloxane) Elastomer Surfaces: Attenuated Total Reflectance Fourier Transform Infrared Spectroscopic and Atomic Force Microscopic Studies Heung Kim and Marek W. Urban* Department of Polymers and Coatings, North Dakota State University, Fargo, North Dakota 58105
Feifei Lin and Dale J. Meier Michigan Molecular Institute, Midland, Michigan 48640 Received December 5, 1995. In Final Form: April 9, 1996X While in the recent studies we utilized microwave plasma energy to react imidazole molecules to crosslinked poly(dimethylsiloxane) (PDMS) surfaces, the issue of surface morphological changes resulting from microwave plasma reactions of imidazole on PDMS surfaces remained open. In this study, we will combine attenuated total reflectance Fourier transform infrared analysis and atomic force microscope measurement to establish inhibition mechanism of silica and morphological changes resulting from both closed and open flow reactor conditions. These studies show that silica containing PDMS is composed of PDMS-rich and silica-rich domains. The aggregations of silica particles in silica-rich domains is responsible for the inhibition of imidazole reactions on PDMS surfaces. Under a closed reactor condition, a multilayer of imidazole rings is formed on PDMS surfaces. On the other hand, in an open flow reactor condition, imidazole ring is opened and grafted to form -(CHdCH)n- species on PDMS surfaces.
Introduction Surface reactions of gaseous monomers on various substrates using microwave plasma energy are an attractive means to modify surface properties. Its attractiveness comes from the fact that microwave plasma reactions are fast, clean, and do not alter bulk properties of a substrate.1-3 Although one could argue, and perhaps rightfully so, that one of the drawbacks of the reactions conducted in the plasma gas phase is the complexity of reaction mechanisms, the advantages are overwhelming. For that reason, significant efforts have been made to analyze reaction mechanisms conducted by microwave energy. Recently, we4 utilized attenuated total reflectance Fourier transform (ATR FT-IR) spectroscopy to analyze the imidazole reactions on poly(dimethylsiloxane) (PDMS) surfaces using closed and open flow reactor conditions. Using a closed reactor, we managed to chemically bond imidazole rings to the PDMS surfaces through hydrogen abstraction of the N-H bonds. On the other hand, using an open flow reactor, imidazole ring opening occurred, resulting in the formation of the -CtN species. In both cases, the presence of silica filler inhibited imidazole reactions on the PDMS surface. Although ATR FT-IR spectroscopy has been an invaluable tool in detecting newly formed species resulting from the microwave plasma surface reactions of imidazole to PDMS surfaces, surface morphology after microwave plasma exposure, and therefore accessibility for further reactions, has not been addressed. For that reason, and to identify morphology changes resulting from the microwave plasma reactions, we have combined ATR FT-IR analysis and atomic force microscope (AFM) measure* To whom all correspondence should be addressed. X Abstract published in Advance ACS Abstracts, June 1, 1996. (1) Stewart, M.; DiDomenico, E. Urban, M. W. US Patent 5,364,662. (2) Yasuda, H. Plasma Polymerization; Academic: Orland, FL, 1985. (3) Yasuda, H.; Shamara, A. K. J. Polym. Phys. Ed. 1981, 19, 1285. (4) Kim, H.; Urban, M. W. Langmuir 1995, 11, 2071.
S0743-7463(95)01512-5 CCC: $12.00
ments.5 Because AFM has the ability to perform imaging of solid surfaces,6,7 morphological changes of the PDMS surfaces resulting from microwave plasma reactions of imidazole can be detected and analyzed. In this study, our efforts are focused on the inhibition mechanisms of silica in imidazole/PDMS reactions and on the morphological changes of PDMS surfaces resulting from the imidazole reactions in both closed and open flow reactor conditions. Experimental Section Substrate Preparation. Poly(dimethylsiloxane) (PDMS) was prepared from a linear, vinyl-terminated dimethylvinylmethylsiloxane polymer (Mn ) 28 000; Huls American Inc.). Reactions between vinyl groups to form a cross-linked PDMS network were initiated by addition of 0.5% (w/w) tert-butyl perbenzoate (Aldrich Chemical) to PDMS. The 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 postcuring for an additional 4 h at 210 °C. Cross-linked PDMS films containing SiO2 were prepared by the addition of 5% (w/w) of Aerosil 200 (Degussa Corp.) SiO2. The oligomer-initiator solution prepared in this 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 a Carver Lab. Press, Model C, and post-cross-linking for an additional 4 h at 210 °C. Potential surface contaminants and residual low molecular weight species were removed by stirring PDMS films in methylene chloride for 5 h. The 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 elsewhere.4 In the open flow reactor, reactions were conducted in a continuous flow of gas under a specific pressure. The cross(5) Sarid, D. Scanning Force Microscope; Oxford University Press: New York, 1991. (6) Magonov, S. N. J. Appl. Polym. Sci., Appl. Polym. Symp. 1992, 51, 3. (7) Albrecht, T. R.; Kuan, S. W. J. Appl. Phys. 1988, 64 (3), 1178.
© 1996 American Chemical Society
Mechanism of Silica in Imidazole/PDMS Reactions linked PDMS substrate, with approximate dimensions of 50 × 25 × 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 of Ar gas flow was reached. At this point, microwave radiation of approximately 600 W of power with an output frequency of 2.45 GHz using a KMC Model KMO-24G microwave source 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 1.3 Pa, and brought back to atmospheric pressure by introducing Ar gas. The reactor was evacuated again to the desired pressures, followed by plasma treatment. In both cases, gas plasma reactions on PDMS surface were carried out using imidazole (Aldrich Chemical), which at 1.3 Pa exhibits partial vapor pressure of 2.6 × 10-6 Pa. This is the actual vapor pressure before plasma state is generated. However, when plasma discharge occurs, the pressure in the reaction chamber increases continuously during microwave plasma discharge. Without microwave plasma discharge and under the same pressure conditions, the pressure in the reaction chamber remains constant, and no sorption of imidazole into the PDMS network was detected. Spectroscopic Measurements. ATR FT-IR spectra were collected on a Digilab FTS-14B spectrometer equipped with a liquid nitrogen cooled MCT detector. A resolution of 4 cm-1 and a mirror speed of 0.3 cm s-1 were used. The ATR cell was aligned at a 45° angle of incidence using a 45° angle parallelogram KRS-5 crystal. In an effort to determine the orientation of the surface species, 90° (TE) and 0° (TM) polarized infrared light was used. All ATR spectra were corrected for spectral distortions using Q-ATR software, which account for surface coverage as well as optical property changes resulting from surface nonhomogeity.8 Atomic Force Microscopy. A TopoMetrix TMX 2000 small stage atomic force microscope (AFM) was used to image the sample surfaces. All images were obtained on PDMS samples glued to an AFM sample holder and scanned under ambient conditions. The samples were imaged in a contact mode, using a force of 10-9 N, except for the soft PDMS containing no SiO2. In this case, a “tapping mode” was utilized.
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Figure 1. ATR FT-IR spectra in the 2300-1300 cm-1 region of imidazole reacted to the PDMS surface under various discharge time conditions using a closed reactor: A, unreacted PDMS; B, imidazole reacted to PDMS at 26.6 Pa/5 s; C, imidazole reacted to PDMS at 26.6 Pa/10 s; D, imidazole reacted to PDMS at 26.6 Pa/20 s; E, imidazole reacted to PDMS at 26.6 Pa/30 s.
Results and Discussions As our previous studies4 indicated, ATR FT-IR spectra of imidazole reacted onto the PDMS surface exhibit the appearance of the bands at 1598 and 1551 cm-1, which are attributed to the CdC and CdN stretching modes of imidazole. The bands resulting from the imidazole reactions to the PDMS surface changed when the initial discharge pressure and time were altered. Figure 1 illustrates ATR FT-IR spectra in the CdC and C)N stretching region for imidazole reacted to the PDMS surface at various discharge times under closed reactor conditions. For reference purposes, trace A of Figure 1 illustrates the spectrum of the unreacted PDMS surface. As the discharge times increase from 5 s (trace B) to 10 s (trace C), intensities of the CdC and CdN stretching bands at 1603 and 1551 cm-1 increase. However, discharge times exceeding 20 s (traces D and E) result in a decrease of the band intensities, which is attributed to an increase of the inner pressure at longer discharge times in a closed reactor. It should be noted that the plasma surface reactions are competing with the microwave surface degradation of PDMS. After 20 s of discharge time the surface degradation of newly formed imidazole layers dominates the process, which is illustrated by the diminishing intensities of the CdC and CdN stretching normal vibrations. Figure 2 illustrates ATR FT-IR spectra recorded using transverse magnetic (TM) and transverse (8) Urban, M. W. Attenuated Total Reflectance Spectroscopy of PolymerssTheory and Practice; American Chemical Society: Washington, DC, 1996.
Figure 2. Polarized ATR FT-IR spectra in the 2300-1300 cm-1 region of imidazole reacted to PDMS surface under various pressure conditions using a closed reactor: A, TE polarization of imidazole reacted to PDMS at 106.7 Pa/10 s; B, TE polarization of imidazole reacted to PDMS at 53.3 Pa/10 s; C, TE polarization of imidazole reacted to PDMS at 26.6 Pa/10 s; D, TM polarization of imidazole reacted to PDMS at 106.7 Pa/ 10 s; E, TM polarization of imidazole reacted to PDMS at 53.3 Pa/10 s; F, TM polarization of imidazole reacted to PDMS at 26.6 Pa/10 s.
electric (TE) polarizations in the CdC and CdN stretching regions for imidazole molecules reacted to PDMS surface at various discharge pressures in a closed reactor. Intensities of the CdC and CdN stretching bands at 1606 and 1557 cm-1 in TE polarization increase as the discharge
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Figure 3. ATR FT-IR spectra in the 2300-1300 cm-1 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 Pa/5 s; C, imidazole reacted to PDMS at 26.6 Pa/30 s; D, imidazole reacted to PDMS without silica at 26.6 Pa/30 s.
pressures become lower (traces A, B, and C), and a significant drop of intensities is observed when the spectra are recorded using polarization (traces D, E, and F). This observation indicates that newly reacted imidazole rings to the PDMS surface are preferentially oriented parallel. Finely-divided silica is commonly used as a PDMS/ elastomer reinforcing agent. However, as our earlier studies4,9 indicated, its presence may have a significant effect on the surface reactions. Figure 3 illustrates ATR FT-IR spectra in the CdC and CdN stretching regions for imidazole reacted on the PDMS surfaces. The cross-linked PDMS polymer network contains 5% (w/w) of silica with a particle size of 0.03 µm. However, in the reactions of the PDMS containing silica under 26.6 Pa pressures, no change of imidazole content reacted on the PDMS surfaces is detected for the discharge times ranging from 5 (trace B) to 30 s (trace C). For comparison purposes, trace D of Figure 3 illustrates a spectrum of the PDMS surface without silica which was exposed at 26.6 Pa for 30 s discharge times. A comparison of these data indicates that the intensities of the CdC and CdN stretching bands at 1598 and 1551 cm-1 of imidazole reacted to the PDMS containing silica are diminished. Because the presence of silica may significantly alter the extent of microwave plasma reactions, an effort was made to verify that the presence of silica is detrimental to the formation of PDMS-imidazole reactions by studying imidazole reactions on polycrystalline silica films. Trace A of Figure 4 illustrates the ATR FT-IR spectrum of polycrystalline silica film, and trace B shows the spectrum of the same silica film, but exposed to a microwave plasma of imidazole under 26.6 Pa for 10 s discharge time. For comparison purposes, trace C illustrates the ATR FT-IR spectrum of imidazole reacted on the PDMS containing 5% (w/w) of silica after exposure to the microwave plasma of imidazole under 26.6 Pa for 10 s of discharge time. As seen, no CdC and CdN stretching modes due to imidazole reactions are detected on the polycrystalline silica, thus confirming that SiO2 does not react with imidazole. (9) Gaboury, S. R.; Urban, M. W. Polymer 1992, 33 (23), 5085.
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Figure 4. ATR FT-IR spectra in the 1900-900 cm-1 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 Pa/10 s; C, imidazole reacted to PDMS containing 5% (w/ w) silica at 26.6 Pa/10 s.
In an effort to determine the surface morphological changes on PDMS resulting from the presence of silica upon unsuccessful imidazole microwave plasma reactions, AFM can be used. As a first step, let us examine the PDMS surface which does not contain silica. Figure 5 illustrates an AFM image of the PDMS surface and indicates that the only detected morphological features result from the formation of the film. In this case, the PDMS surface exhibits a maximum height of 1082 nm. On the other hand, the AFM image of the silica containing PDMS illustrated in Figure 6 exhibits a much smoother surface but contains many cracks with a minimum-tomaximum peak height of about 3500 nm. Since this sample is reinforced with silica, the film rigidity is increased, and its flow is restricted. Therefore, even though there is a significant fraction of an elastomeric component, surface cracks may form because the film is unable to relax during its formation. Although it is rather speculative, aggregation of silica may occur because PDMS is known to form aggregates. If this is the case, some regions may be PDMS-rich regions and other regions may be silica-rich. When regions are non-homogeneous, the more rigid SiO2 aggregates will not allow the elastic component to relax the film, and cracking will occur, such as shown in Figure 6. Although at this point the mechanism of the crack formation is not quite clear, it is believed that it involves a non-homogeneous dispersion of the silica particles, thus giving a significant difference between the highest and lowest surface elevations, i.e., 3546.34 nm. On combination of this information with the spectroscopic data discussed earlier, the presence of silica islands on PDMS diminishes the extent of imidazole reactions. In our study,10 imidazole radicals resulting from the hydrogen abstraction of the N-H bonds react with the SiCH2• radicals through hydrogen abstraction of SiCH3 groups on PDMS, to form Si-CH2-imidazole entities. Silica particles not only have no reactive sites for reactions with imidazole but also inhibit the SiCH2• radical formation on PDMS. Therefore, the presence of silica particles act as an anticatalyst for the imidazole-PDMS microwave plasma reaction. (10) Kim, H.; Urban, M. W. In preparation.
Mechanism of Silica in Imidazole/PDMS Reactions
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Figure 5. Surface views of PDMS imaged using an atomic force microscope.
Figure 6. Surface views of PDMS with 5% (w/w) silica imaged using an atomic force microscope.
Keeping in mind that the above data were obtained on a closed system, let us now focus on the open flow reactor
surface reactions. Figure 7 illustrates ATR FT-IR spectra in the CdC and CdN stretching regions for imidazole
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Figure 7. ATR FT-IR spectra in the 2300-1300 cm-1 region of imidazole reacted to PDMS surface under various pressure conditions using an open flow reactor: A, unreacted PDMS; B, imidazole reacted to PDMS at 26.6 Pa/5 s; C, imidazole reacted to PDMS at 26.6 Pa/10 s; D, imidazole reacted to PDMS at 26.6 Pa/20 s; E, imidazole reacted to PDMS at 26.6 Pa/30 s.
reacted on a PDMS surface containing no silica under 26.5 Pa pressure as a function of the discharge times in the open flow reactor. Again, for reference purposes, trace A illustrates the unreacted PDMS spectrum. The spectrum of imidazole reacted to the PDMS surface for 5 s discharge time (trace B) exhibits a new band at 1658 cm-1,
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which is attributed to the -CHdCH- stretching modes resulting from the ring opening of imidazole molecules. Compared to the spectra obtained on specimens reacted in a closed reactor (Figure 1), the CdN stretching bands at 1559 cm-1 are not detected. On the other hand, the spectrum of imidazole reacted to PDMS surface for 10 s discharge times (trace C) exhibits a new band at 2183 cm-1, which is attributed to the CtN groups. These species, however, which result from imidazole ring opening, show no preferential orientation on the PDMS surface, as the TE and TM polarized spectra exhibit no differences. The presence of these bands indicates that the ringopening reactions of the imidazole ring appear to occur when the open flow reactor conditions are employed. The higher energy state of the plasma gas in the open flow reactor is most likely responsible for the ring opening, which is caused by the significantly lower steady state pressures during the plasma reactions.2,4 In order to identify morphological surface changes resulting from the open flow reactor conditions on PDMS, AFM images were obtained. Figure 8 illustrates AFM images of imidazole reacted to PDMS surfaces, under 26.6 Pa for 10 s discharge time using closed reactor conditions, and Figure 9 shows the AFM images recorded from imidazole reacted to PDMS under the same conditions using the open flow reactor. In both cases, the PDMS elastomer did not contain silica. It appears that the difference between the highest and lowest surface elevations detected from the AFM images of imidazole reacted to PDMS surface in closed reactor conditions is 54.79 nm from the z-axis scale. Again, no significant morphological changes are detected when compared with the AFM images of unreacted PDMS (Figure 5). On the basis of the ATR FT-IR data illustrated in Figure 1, imidazole rings are chemically attached to the PDMS surface through
Figure 8. AFM image of the surface of the imidazole reacted on PDMS under 26.6 Pa for 10 s discharge time in closed reactor conditions.
Mechanism of Silica in Imidazole/PDMS Reactions
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Figure 9. AFM image of the surface of the imidazole reacted on PDMS under 26.6 Pa for 10 s discharge time in open flow reactor conditions.
hydrogen abstraction of the N-H bonds. In the plasma reactions of gaseous imidazole, there are various possibilities which may result in monolayer and/or multilayer formation. On combination of the results of ATR FT-IR analysis (Figure 1) and the AFM images (Figure 8) for the imidazole reacted to PDMS under closed reactor conditions, it seems that imidazole rings are uniformly formed on the PDMS surface through hydrogen abstraction of the N-H groups on imidazole. This was demonstrated by the presence of the 1603 and 1551 cm-1 bands due to imidazole (Figure 1) and no significant morphological deviations in the AFM images compared to unreacted PDMS surfaces (Figure 5). As the discharge pressure decreases, band intensities of the CdC and CdN stretching bands at 1606 and 1599 cm-1 in Figure 2 increase relatively faster, compared to the intensities of the C-H deformation bands of -CdC-H. On the basis of these results obtained in a closed reactor, imidazole rings are formed on a PDMS surface through hydrogen abstraction of N-H bonds, followed by hydrogen abstraction of the -CdC-H groups on imidazole rings, to form multilayers of imidazole rings on PDMS. On the other hand, the AFM images of the imidazole reacted to the PDMS surface under open flow reactor conditions (Figure 9) exhibit elevations reaching 3766.78 nm, and the presence of irregular blocks is detected. Following the results of ATR FT-IR spectra illustrated in Figure 7, it appears that, when the open flow reactor conditions are employed, two types of reactive groups are produced: •CtN and CH2dCH•. They result from the imidazole ring opening reactions in the presence of an Ar microwave plasma. The CH2dCH• species are grafted to the PDMS surface by hydrogen abstraction, to form the -(CHdCH)n- linkages on the surface. On the other hand, the •CtN species act as a terminal group in the CH2dCH• grafting process. This is verified by the
Figure 10. A schematic representation of PDMS surface structure resulting from imidazole reaction in the presence of Ar microwave plasma: A, closed chamber; B, open flow chamber.
presence of the CdC stretching modes at 1601 cm-1 (Figure 7). Therefore, morphological changes of imidazole reacted to PDMS under an open flow reactor conditions result from the grafting of imidazole entities reacting on the PDMS surface through the CH2dCH- entities. A schematic representation of the PDMS surface structures generated using closed and open flow reactor conditions is shown in Figure 10. Conclusions Analysis of ATR FT-IR data of imidazole reacted to PDMS surfaces indicates that hydrogen abstraction from
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the N-H groups of imidazole occurs under closed reactor conditions. On the other hand, imidazole rings open up to form the CH2dCH• and •CtN reactive entities on the PDMS surfaces when an open flow reactor conditions are employed. The presence of silica on the PDMS surfaces inhibits the imidazole-PDMS surface reactions. On the basis of the analysis of the AFM images of imidazole reacted to PDMS surfaces, it appears that the silica containing PDMS forms PDMS-rich and silica-rich domains. In the PDMS-rich domains, silica particles are well dispersed, but the aggregation in the silica-rich domain results in crack formation. The presence of the silica-rich regions inhibits PDMS reactions with imidazole. AFM images of the imidazole-reacted PDMS surfaces under closed reactor conditions reveal no significant morphological changes and the maximum surface roughness is found to be 54.79 nm. On the other hand, using open flow reactor conditions, the surface roughness is much larger and results from the occurrence of irregular blocks
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formed on the PDMS surfaces. On the basis of these findings, under closed reactor conditions, a multilayer of imidazole rings are reacted on the PDMS surfaces through hydrogen abstraction of the N-H bonds of imidazole and followed by hydrogen abstraction of -CHdCH- of the imidazole molecules. Under open flow reactor conditions, imidazole entities of CH2dCH• resulting from imidazole ring opening are grafted to form -(CHdCH)n- linkages on PDMS surfaces and •CtN species acting as terminal groups in the grafting process. Acknowledgment. The authors are thankful to the National Science Foundation Industry/University Cooperative Research Center in Coatings at North Dakota State University, Michigan Molecular Institute, and Eastern Michigan University for financial support. LA9515128
