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Surface Restructuring Behavior of Various Types of Poly(dimethylsiloxane) in Water Detected by SFG Chunyan Chen,† Jie Wang,‡ and Zhan Chen*,†,‡ Department of Macromolecular Science and Engineering and Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109 Received March 15, 2004. In Final Form: August 11, 2004
Surface structures of several different poly(dimethylsiloxane) (PDMS) materials, tetraethoxysilanecured hydroxy-terminated PDMS (TEOS-PDMS), platinum-cured vinyl-terminated PDMS (Pt-PDMS), platinum-cured vinyl-terminated poly(diphenylsiloxane)-co-poly(dimethylsiloxane) (PDPS-co-PDMS), and PDMS-co-polystyrene (PDMS-co-PS) copolymer in air and water have been investigated by sum frequency generation (SFG) vibrational spectroscopy. The SFG spectra collected from all PDMS surfaces in both air and water are dominated by methyl group stretches, indicating that all the surfaces are mainly covered by methyl groups. Other than surface-dominating methyl groups, some -Si-CH2-CH2- moieties on the Pt-PDMS surface have also been detected in air, which are present at cross-linking points. Information about the average orientation angle and angle distribution of the methyl groups on the PDMS surface has been evaluated. Surface restructuring of the methyl groups has been observed for all PDMS surfaces in water. Upon contacting water, the methyl groups on all PDMS surfaces tilt more toward the surface. The detailed restructuring behaviors of several PDMS surfaces in water and the effects of molecular weight on restructuring behaviors have been investigated. For comparison, in addition to air and water, surface structures of PDMS materials mentioned above in a nonpolar solvent, FC-75, have also been studied. By comparing the different response of phenyl groups to water on both PDPS-co-PDMS and PS-co-PDMS surfaces, we have demonstrated how the restructuring behaviors of surface phenyl groups are affected by the structural flexibility of the molecular chains where they are attached.
Introduction Poly(dimethylsiloxane) (PDMS), a good candidate for biofouling control coatings, has been extensively studied since the early 1970s.1-7 The physical nature of PDMS materials enables PDMS coatings to retard the onset of fouling and provide easy release when fouling does occur. It is believed that because PDMS has a very low surface energy, it can minimize chemical interactions of the surface with other adsorbents. The flexibility of the PDMS backbone causes minimization of the mechanical locking of the adsorbed organism, which can improve fouling release. There are several methods to create PDMS surfaces. The first two classes of such PDMS surfaces are alkoxysilane-cured and platinum-cured PDMS networks, both of which are based on cross-linking of functional group-terminated liquid PDMS. These two methods have been widely used in coating technology.8-15 However, in * To whom all correspondence should be addressed: e-mail
[email protected]; Fax 734-647-4865. † Department of Macromolecular Science and Engineering. ‡ Department of Chemistry. (1) Hogt, A. H.; Gregonis, D. E.; Andrade, J. D.; Kim, S. W.; Dankert, J.; Feijen, J. J. Colloid Interface Sci. 1985, 106, 289-298. (2) Holmstrom, C.; Kjelleberg, S. Biofouling 1994, 8, 147-160. (3) Adkins, J. D.; Mera, A. E.; Roeshort, M. A.; Pawlikowski, G. T.; Brady, R. F. Prog. Org. Coat. 1996, 29, 1-5. (4) Petronis, S.; Berntsson, K.; Gold, J.; Gatenholm, P. J. Biomater. Sci., Polym. Ed. 2000, 11, 1051-1072. (5) Morra, M. J. Biomater. Sci., Polym. Ed. 2000, 11, 547-569. (6) Berglin, M.; Larsson, A.; Jonsson, P. R.; Gatenholm, P. J. Adhes. Sci. Technol. 2001, 15, 1485-1502. (7) Callow, M. E.; Jennings, A. R.; Brennan, A. B.; Seegert, C. E.; Gibson, A.; Wilson, L.; Feinberg, A.; Baney, R.; Callow, J. A. Biofouling 2002, 18, 237-245. (8) Majumdar, S.; Bhaumik, D.; Sirkar, K. K. J. Membr. Sci. 2003, 214, 323-330. (9) Liu, W. M.; Zhou, F.; Yu, L. G.; Chen, M.; Li, B.; Zhao, G. H. J. Mater. Res. 2002, 17, 2357-2362.
some cases more strict requirements on durability or adhesion ability of polymer coating materials are needed, and other methods have been attempted to design such materials with both desired surface properties and enhanced bulk properties.6,16-20 For example, making polymer blends or copolymers is an easy and extensively used way to achieve both good surface and bulk properties for polymers. PDMS-based block or graft copolymers, such as PDMS-co-PMMA, PDMS-co-PS, and PDMS-co-nylon, are often used to create PDMS surfaces while maintaining other bulk characteristics.16,18,19 Antifouling or fouling release materials such as PDMS must be used in an aqueous environment. To understand the mechanism how PDMS coatings minimize the biofouling, it is crucial to elucidate in situ molecular surface structures of PDMS materials in water.3,6 Many surfacesensitive analytical techniques require high vacuum to operate; therefore, they cannot provide in situ surface (10) Zhang, S. L.; Tsou, A. H.; Li, J. C. M. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 1530-1537. (11) Badal, M. Y.; Wong, M.; Chiem, N.; Salimi-Moosavi, H.; Harrison, D. J. Chromatogr. A 2002, 947, 277-286. (12) Berglin, M.; Wynne, K. J.; Gatenholm, P. J Colloid Interface Sci. 2003, 257, 383-391. (13) Uilk, J. M.; Mera, A. E.; Fox, R. B.; Wynne, K. J. Macromolecules 2002, 36, 3689-3694. (14) Hawkridge, A. M.; Gardella, J. A.; Toselli, M. Macromolecules 2002, 35, 6533-6538. (15) Stein, J.; Truby, K.; Wood, C. D.; et al. Biofouling 2003, 19, 87-94. (16) Mera, A. E.; Goodwin, M.; Pike, J. K.; Wynne, K. J. Polymer 1999, 40, 419-427. (17) Spanos, C. G.; Ebbens, S. J.; Badyal, J. P. S.; Goodwin, A. J.; Merlin, P. J. Macromolecules 2003, 36, 368-372. (18) Hou, Y. X.; Tulevski, G. S.; Valint, P. L.; Gardella, J. A. Macromolecules 2002, 35, 5953-5962. (19) Ndoni, S.; Jannasch, P.; Larsen, N. B.; Almdal, K. Langmuir 1999, 15, 3859-3865. (20) Pike, J. K.; Ho, T.; Wynne, K. J. Chem. Mater. 1996, 8, 856-860.
10.1021/la049327u CCC: $27.50 © 2004 American Chemical Society Published on Web 10/14/2004
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Table 1. Physical Parameters of Liquid PDMS Materials and Cross-Linking Agents
vinyl-terminated PDMS
hydroxy-terminated PDMS cross-linking agents
V21 V31 V52 PDV1625 H25 HMS301 HPM502 TEOS
mol wt (MW)
viscosity (cst)
6000 28000 155000 9500 550 2000 4000 208
100 1000 165000 500 25 25-35 75-110
structures of PDMS in water. One of the most widely used methods to study surfaces of wet polymer materials is to monitor the water contact angle on such surfaces. Through dynamic contact angle (DCA) measurements, one can qualitatively evaluate the heterogeneity or structure reorganization of polymer surfaces, although no detailed molecular surface structures can be deduced.13,16 Freezedrying X-ray photoelectron spectroscopy (XPS) can detect the surface composition change of the polymer upon contacting water, but the sample handling tends to be complicated and no orientation information about surface chemical groups can be obtained.14 Recently, sum frequency generation (SFG) vibrational spectroscopy has been applied to study polymer surface restructuring behavior in water.20-23 Because of its submonolayer sensitivity, SFG can detect molecular structures such as functional group composition and orientation at the surface or interface.24-29 In addition, SFG can study in situ surface restructuring behaviors of polymer materials in water. In this paper, the molecular surface structures of several types of PDMS materials in air and in water will be investigated using SFG. These PDMS materials are tetraethoxysilane-cured hydroxy-terminated PDMS (TEOSPDMS), platinum-cured vinyl-terminated PDMS (PtPDMS), platinum-cured vinyl-terminated poly(diphenylsiloxane)-co-poly(dimethylsiloxane) (PDPS-co-PDMS), and PDMS-co-polystyrene (PDMS-co-PS) copolymer. The surface structures and detailed molecular group orientations of PDMS materials in both air and water will be elucidated. For comparison, such surfaces in a nonpolar solvent, FC75, will also be tested. Experimental Section Materials. Liquid vinyl-terminated PDMS with different molecular weights (V21, V31, V52) and cross-linker methylhydrosiloxane-dimethylsiloxane copolymer (HMS301), liquid vinylterminated PDPS-co-PDMS (PDV1625) and cross-linker methylhydrosiloxane-phenylmethylsiloxane copolymer (HPM502), and platinum (Pt) catalyst (SIP6831) were all purchased from Gelest Inc. Liquid hydroxy-terminated PDMS (H25), cross-linker tetraethyoxysilane (TEOS), and catalyst stannous 2-ethylhex(21) Zhang, D.; Ward, R. S.; Shen, Y. R.; Somorjai, G. A. J. Phys. Chem. B 1997, 101, 9060-9064. (22) Chen, C. Y.; Even, M. A.; Wang, J.; Chen, Z. Macromolecules 2002, 35, 9130-9135. (23) Wang, J.; Woodcock, S. E.; Buck, S. M.; Chen, C. Y.; Chen, Z. J. Am. Chem. Soc. 2001, 123, 9470-9471. (24) Ye, S.; Noda, H.; Morita, S.; Uosaki, K.; Osawa, M. Langmuir 2003, 19, 2238-2242. (25) Chen, C.; Wang, J.; Woodcock, S. E.; Chen, Z. Langmuir 2002, 18, 1302-1309. (26) Oh-E, M.; Lvovsky, A. I.; Wei, X.; Shen, Y. R. J. Chem. Phys. 2000, 113, 8827-8832. (27) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. Rev. B 1999, 59, 12632-12640. (28) Miranda, P. B.; Shen, Y. R. J. Phys. Chem. B 1999, 103, 32923307. (29) Miranda, P. B.; Pflumio, V.; Saijo, H.; Shen, Y. R. Chem. Phys. Lett. 1997, 264, 387-392.
mol % diphenylsiloxane
mol % methylhydrosiloxane
15-17 25-30 45-50
Scheme 1
anoate (tin) were purchased from Aldrich. PDMS-co-PS (Mw 3300co-28700) was purchased from Polymer Source Inc. The nonpolar solvent FC-75 was purchased from 3M. All chemicals were used as received, and the physical parameters for liquid PDMS materials are listed in Table 1. Fused silica (1 in. diameter, 1/ in. thickness) substrates were ordered from ESCO Products 8 Inc. The spin-coater was purchased from Specialty Coating Systems. Sample Preparation. The cross-linked PDMS films were prepared by spin-coating mixtures of PDMS liquid, cross-linking agent, and catalyst before being cured onto fused silica substrates at 5000 rpm for 30 s followed by a curing process. The film thickness was several micrometers. The PDMS-co-PS thin film was prepared by spin-coating 4 wt % solution of the polymer in toluene onto the fused silica at 2000 rpm for 30 s. All spin-cast samples were cured or dried in an oven at 80 °C for 12 h prior to measurements. The cross-linking processes for PDMS with different end groups are demonstrated in Scheme 1. For vinyl-terminated PDMS or PDPS-co-PDMS, the amount of cross-linker was chosen on the basis of equivalent ratios of Si-H to vinyl groups. The mixture of liquid PDMS and crosslinker was cured by the addition of 2-10 ppm Pt catalyst. For hydroxy-terminated PDMS, 40% extra cross-linker TEOS has been used to compensate for the evaporation of TEOS, and the mixture was also cured by the addition of 1% tin catalyst. SFG Setup. Details about the SFG theory have been reported previously.30-33 In a typical SFG setup, a pulsed visible laser beam (ωVis) and a pulsed tunable IR beam (ωIR) are overlapped spatially and temporally on a surface. The light emitted by the nonlinear process at the sum frequency, ωSF ) ωVis + ωIR, is detected by a photodetector. The intensity of the light at ωSF is proportional to the square of the sample’s second-order nonlinear
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susceptibility, which vanishes when a material has inversion symmetry. Therefore, bulk materials that possess inversion symmetry usually do not generate a sum frequency output, but surfaces that lack inversion symmetry do. Both theoretical calculations and experimental results show that SFG is submonolayer sensitive.30-47 As the IR beam is tuned over the vibrational resonance of surface/interface molecules, the effective surface nonlinear susceptibility χ(2) R can be enhanced. The plot of SFG signal vs the IR input frequency shows a polarized vibrational spectrum of the surface or interface. Our SFG system has been described before and will not be repeated here.22,34-36 In this research, SFG spectra with different polarization combinations including ssp (s-polarized SF output, s-polarized visible input, and p-polarized infrared input) and sps were collected with two input laser beams traveling through the fused silica substrate and overlapping on the polymer/air or polymer/ liquid interface. All SFG spectra were normalized by the intensities of the input IR and visible beams to compensate for the effects of intensity fluctuations and absorption. Our early research has demonstrated that SFG signals were dominated by the polymer/air or polymer/liquid interface, with almost no polymer/substrate or polymer bulk contributions to the spectra using this experimental geometry.22,34-36 Methods To Calculate the Orientation of Surface Functional Groups. Considering the nonresonant background χ(2) NR from the substrate, a tensor component of the measured nonlinear (2) (2) susceptibility χs , e.g., χs,ssp, can be written as (2) (2) (2) χs,ssp ) χR,ssp + χNR,ssp )
∑ω q
Aq,yyz IR - ωq + iΓq
(2) + χNR,yyz (1)
where Aq,yyz, ωIR, ωq, and Γq are strength of the qth vibrational mode, frequency of the infrared beam, resonance frequency, and damping constant of the qth vibrational mode, respectively.32,36-40 According to the relation between macroscopic second-order nonlinear susceptibility χ(2) s and microscopic hyperpolarizability R, the orientation of functional groups at the surface or interface can be deduced by analyzing SFG spectra obtained under different polarization combinations. As with the normal methyl group, the methyl group on PDMS chains can also be treated as having C3v symmetry. The orientation information on such methyl groups can be evaluated by measuring the value of |χyyz,as|/|χyzy,as| or |χyyz,as|/ |χyyz,s|.36,37,40 The relations between such ratios and the methyl (30) Chen, Z.; Gracias, D. H.; Somorjai, G. A. Appl. Phys. B: Laser Opt. 1999, 68, 549-557. (31) Shen, Y. R. Appl. Phys. A: Mater. Sci. Process. 1994, 59, 541543. (32) Hirose, C.; Akamatsu, N.; Domen, K. Appl. Spectrosc. 1992, 46, 1051-1072. (33) Shen, Y. R. The Principles of Nonlinear Optics; Wiley: New York, 1984. (34) Chen, C. Y.; Wang, J.; Even, M. A.; Chen, Z. Macromolecules 2002, 35, 8093-8097. (35) Wang, J.; Paszti, Z.; Even, M. A.; Chen, Z. J. Am. Chem. Soc. 2002, 124, 7016-7023. (36) Wang, J.; Chen, C. Y.; Buck, S. M.; Chen, Z. J. Phys. Chem. B 2001, 105, 12118-12125. (37) Gautam, K. S.; Dhinojwala, A. Macromolecules 2001, 34, 11371139. (38) Hirose, C.; Yamamoto, H.; Akamatsu, N.; Domen, K. J. Phys. Chem. 1993, 97, 10064-10069. (39) Hirose, C.; Akamatsu, N.; Domen, K. J. Chem. Phys. 1992, 96, 997-1004. (40) Kim, J.; Somorjai, G. A. J. Am. Chem. Soc. 2003, 125, 31503158. (41) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Phys. Rev. Lett. 2000, 85, 3854-3857. (42) Opdahl, A.; Somorjai, G. A. Langmuir 2002, 18, 9409-9412. (43) Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Langmuir 2000, 16, 4528-4532. (44) Chen, C. Y.; Even, M. A.; Chen, Z. Macromolecules 2003, 36, 4478-4484. (45) Chen, C. Y.; Loch, C. L.; Wang, J.; Chen, Z. J. Phys. Chem. B 2003, 107, 10440-10445. (46) Wang, J.; Even, M. A.; Chen, X. Y.; Schmaier, A. H.; Waite, J. H.; Chen, Z. J. Am. Chem. Soc., 2003, 125, 9914-9915. (47) Chen, C. Y.; Wang, J.; Loch, C. L.; Chen, Z. J. Am. Chem. Soc. 2004, 126, 1174-1179.
group orientation angle θ, the angle between the surface normal and the principal axis of the methyl group, are
| | | | | |
χyyz,as 〈cos θ〉 - 〈cos3 θ〉 ) χyzy,as 〈cos3 θ〉
|
χyyz,as 2Rcaa 〈cos θ〉 - 〈cos3 θ〉 ) χyyz,s Rccc 〈cos θ〉 (1 + r) - 〈cos3 θ〉 (1 - r)
(2)
|
(3)
where r ≈ Rcaa/Rcccfor methyl group vibrations and the values for r range from 1.6 to 4.2.27,37 The bracket 〈 〉 implies averages of tilting angles. For example
〈cosn θ〉 )
∫ cos π
0
n
θ f(θ) sin θ dθ
(4)
and
f(θ) ) C exp[-(θ - θ0)2/2σ2]
(5)
where f(θ) is a Gaussian function describing the angle distribution, C is the normalization constant, θ0 is related to the average orientation angle of the methyl group vs surface normal, and σ is the angle distribution parameter. The value of |χyyz,as|/|χyzy,as| or |χyyz,as|/|χyyz,s| can be obtained by fitting SFG spectra using eq 1. Therefore, it is possible to evaluate the orientation angle and angle distribution of the methyl group vs the surface normal based on the fitted results and an assumed value of r.
Results and Discussion 1. Surface Structures of PDMS in Air. First, we will discuss the surface structures of three PDMS materials prepared by different methods detected by SFG. As mentioned, these PDMS polymers are TEOS-PDMS, PtPDMS, and PDMS-co-PS block copolymer. The water contact angles of these PDMS surfaces measured using a KSV contact angle goniometer are 105°, 106°, and 107°, respectively, showing that the three PDMS surfaces have similar surface hydrophobicity. Here, the contact angle was determined by taking the average of several contact angle measurements in the first several minutes when a drop of water contacts the PDMS surface. The same standard was used for all three PDMS films; therefore, these contact angle values are reliable and comparable. In addition, these contact angle values are very close to the advancing contact angle measured by the DCA technique.13 SFG experiments have been performed on these PDMS surfaces to understand their detailed structures, which could not be obtained using contact angle measurements. Using different polarization combinations of the input and output laser beams including ssp and sps, we have collected SFG spectra from the three PDMS surfaces in air as shown in Figure 1. The SFG spectra of all three samples have many common characteristics and also have slight differences. All SFG spectra are dominated by SFG signals contributed by the methyl groups on the PDMS side chains. The ssp spectra are dominated by the symmetric C-H stretch of the methyl group at 2910 cm-1, and the sps spectra are dominated by the asymmetric stretch of the methyl group at 2965 cm-1. For Pt-PDMS, there are weak SFG signals at 2865 cm-1 (ssp) and 2940 cm-1 (sps). We assign them to the symmetric stretch of the methylene groups due to the presence of -Si-CH2CH2- moieties at cross-linking points on the surface and Fermi resonance. For the diblock PDMS-co-PS copolymer film, although the PDMS block only accounts for 11 wt % and can be viewed as the chain end, both contact angle measurement and SFG results confirm that it has very similar surface structures to TEOS-PDMS. The fact that
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Table 2. Vibrational Strength |Aq/Γq|2 of the Surface Dominating Chemical Groups for Three PDMS Materials TEOS-PDMS modes (q)
ωq
symmetric C-H stretch of -Si-CH2-CH2asymmetric C-H stretch of -Si-CH2-CH2symmetric C-H stretch of -Si-CH3 Fermi resonance asymmetric C-H stretch of -Si-CH3
2865 ( 5 2920 ( 5 2910 ( 5 2940 ( 5 2965 ( 5
ssp
sps
ssp
sps
9.00
0.04 0.16 15.80 0.04 2.56
1.56 4.00
27.04 6.25
Pt-PDMS
PDMS-co-PS ssp
sps
21.78 4.00
6.96
Figure 1. Measured and fitted SFG spectra (ssp and sps) collected from three PDMS surfaces in air (a) TEOS-PDMS, (b) Pt-PDMS, and (c) PDMS-co-PS.
the PDMS-co-PS surface is dominated by methyl groups from the PDMS block indicates that the PDMS block segregates to the surface. PDMS block has a lower surface energy than the PS block; therefore, it tends to segregate to the surface. In addition, PDMS exists at the chain end and the chain flexibility can facilitate the segregation process. In the spectra collected from the PDMS-co-PS surface, there are several peaks above 3000 cm-1 which are due to the aromatic C-H stretches from the phenyl group.34,41 We found that the ssp SFG spectrum at the region above 3000 cm-1 is dominated by the peak at 3055 cm-1, which is caused by the asymmetric stretch of the phenyl group.25,41-43 This demonstrates that the phenyl group tends to lie down at the PDMS-co-PS surface, unlike the phenyl group orientation at the pure PS surface41 but similar to the orientation of the phenyl group at the polymer blend surface,25 possibly due to interference with the methyl group on the surface. Using eq 1, we have fitted the spectra in Figure 1, and the fitted results for the dominating groups (methyl and methylene groups) are listed in Table 2. By analyzing the fitted results in Table 2, we should be able to obtain further structural information about these PDMS surfaces. For example, we can deduce orientation information about the surface dominating functional group. As described, for a typical methyl group with C3v symmetry, information about the average orientation angle (θ0) and angle distribution (σ) can be evaluated on
Figure 2. Calculated values of (a) |χyyz,as|/|χyzy,as| and (b) |χyyz,as|/ |χyyz,s| for the methyl groups at the PDMS/air or PDMS/water interface as a function of the tilting angle θ0 and angle distribution σ.
the basis of the value of |χyyz,as|/|χyzy,as| or |χyyz,as|/|χyyz,s|, which can be deduced by fitting SFG spectra using eq 1. Figure 2 shows the calculated |χyyz,as|/|χyzy,as| and |χyyz,as|/|χyyz,s| values as a function of θ0 for several different orientation angle distributions (σ ) 0°, 10°, 20°, 30°, 40°, 50°). According to Figure 2, a fixed value of |χyyz,as|/|χyzy,as| or |χyyz,as|/|χyyz,s| can correspond to various possible combinations of θ0 and σ. For example, if |χyyz,as|/|χyzy,as| is equal to 0.3 (Figure 2a), one such possible combination is θ0 ) 10° and σ ) 20°. Other combinations can be θ0 ) 26° and σ ) 10° or θ0 ) 32° and σ ) 0°. Similar analysis also applies to the curves displayed in Figure 2b, which shows a similar way to evaluate the functional group orientation and orientation distribution using another measured value of |χyyz,as|/|χyyz,s|. Figure 2a,b gives similar information; however, deduced results from these two figures can be compared to ensure more reliable information about the surface functional group orientation and orientation distribution, avoiding possible errors during experiments and data fitting. The measured values of |χyyz,as|/|χyzy,as| and |χyyz,as|/|χyyz,s| for three PDMS surfaces in air based on the detailed data fitting are presented in Table 4. The values of |χyyz,as|/|χyzy,as| are 0.83, 0.80, and 0.76. With the consideration of the experimental errors and errors
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Table 3. Vibrational Strength |Aq/Γq|2 of the Surface Chemical Groups for Three PDMS Materials at PDMS/Water and PDMS/FC-75 Interfaces TEOS-PDMS modes (q)
ωq
water
FC-75
symmetric C-H stretch of -Si-CH2-CH2asymmetric C-H stretch of -Si-CH2-CH2symmetric C-H stretch of -Si-CH3 Fermi resonance asymmetric C-H stretch of -Si-CH3
2865 ( 5 2920 ( 5 2910 ( 5 2940 ( 5 2965 ( 5
3.61
9.61
2.98
2.50
Pt-PDMS water 0.04 1.56 0.11 2.25
PDMS-co-PS
FC-75
water
FC-75
5.06
3.24
0.25
0.04
Table 4. Values of |χyyz,as|/|χyzy,as| and |χyyz,as|/|χyyz,s| Obtained from the PDMS/Air, PDMS/Water, and PDMS/FC-75 Interfaces for Three Different PDMS Materials materials
|χyyz,as|/|χyzy,as|a,c (air)
|χyyz,as|/|χyyz,s|b,c (air)
|χyyz,as|/|χyyz,s|b,d (water)
|χyyz,as|/|χyyz,s|b,d (FC-75)
TEOS-PDMS Pt-PDMS PDMS-co-PS
0.83 0.80 0.76
0.48 0.40 0.43
0.91 1.20
0.51
