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Langmuir 2003, 19, 2110-2114
Atmospheric Pressure Glow Discharge Deposition of Polysiloxane and SiOx Films L. J. Ward, W. C. E. Schofield, and J. P. S. Badyal* Department of Chemistry, Science Laboratories, Durham University, Durham DH1 3LE, England, U.K.
A. J. Goodwin Dow Corning Ltd., Barry CF63 2YL, Wales, U.K.
P. J. Merlin Dow Corning SA, Parc Industriel, 7180 Seneffe, Belgium Received May 7, 2002. In Final Form: December 13, 2002 Hydrophobic polysiloxane-like thin films have been deposited onto polyethylene by introducing octamethylcyclotetrasiloxane and tetramethylcyclotetrasiloxane precursors through an ultrasonic atomizer into an atmospheric pressure glow discharge. Enrichment of the gas feed with oxygen gives rise to the formation of hydrophilic SiOx coatings which exhibit an improvement in gas barrier.
Introduction Silicon-containing films find many technological applications: these include the packaging,1-4 biomedical,5,6 automotive,7,8 and microelectronics9-11 industries. Chemical vapor deposition (CVD)12-15 and the pyrolytic degradation16 of silicon-containing precursors are often used in this context. However, the inherently high temperatures (typically over 400 °C) make these techniques unsuitable for treating temperature-sensitive materials, such as polymers and semiconductors17 (e.g., GaAs, InP, and a-Si: H). One common approach employed to overcome this limitation has been to use low pressure glow discharge methods such as plasma enhanced CVD (PECVD)18,19 and remote plasma enhanced CVD (RPECVD).20,21 The prerequisite vacuum necessary for these processes makes * To whom correspondence should be addressed. (1) Inagaki, N.; Tasaka, S.; Makino, M. J. Appl. Polym. Sci. 1997, 64, 1031. (2) Frisk, P.; Jaccoud, B.; Roulin, A.; Johansson, H. U.S. Patent 5,721,027, Feb 24, 1998. (3) Frisk, P. U.S. Patent 5,916,685, June 29, 1999. (4) Fukugami, N.; Nishino, H.; Yanaka, M.; Tomiyama, K.; Tsukahora, Y. J. Vac. Sci. Technol., A 1999, 17, 1840. (5) Hettlich, H.-J.; Otterback, F.; Mittermoyer, C.; Kaufmann, R.; Klee, D. Biomaterials 1991, 12, 521. (6) Fox, C. L.; Modak, S. M.; Sampath, L. A. U.S. Patent 5,616,338, April 1, 1997. (7) Gordon, N. U.S. Patent 5,098,745, March 24, 1992. (8) Inokuchi, Y.; Kuwata, S. U.S. Patent 5,760,109, June 2, 1998. (9) Engle, G. M. U.S. Patent 4,223,048, Sept 16, 1980. (10) Dory, T. S. U.S. Patent 4,877,651, Oct 31, 1989. (11) Batey, J.; Tierney, E. U.S. Patent 5,068,124, Nov 26, 1991. (12) Gill, W. N.; Ganguli, S. J. Vac. Sci. Technol., B 1997, 15, 948. (13) Fujino, K.; Nishimoto, Y.; Tokumasu, N.; Maeda, K. Jpn. J. Appl. Phys. 1994, 33, 2019. (14) Ikeda, K.; Maeda, M. Jpn. J. Appl. Phys. 1996, 35, 1573. (15) Fujino, K.; Nishimoto, Y.; Tokumasu, N.; Maeda, N. J. Electrochem. Soc. 1990, 137, 2883. (16) Seyferth, D. Adv. Chem. Ser. 1990, 224, 565. (17) Bergonzo, P.; Kogelschatz, U.; Boyd, I. W. Appl. Surf. Sci. 1993, 69, 393. (18) Nguyen, S.; Dobuzinsky, D.; Harmon, D.; Gleason, R.; Fridman, S. J. Electrochem. Soc. 1990, 7, 2209. (19) Rats, D.; Hajek, V.; Martinu, L. Thin Solid Films 1999, 340, 33. (20) Regnier, C.; Tristant, P.; Demaison, J. Surf. Coat. Technol. 1996, 80, 18.
them expensive and limited to batch treatments. UV irradiation22 and UV/ozone23 treatment are viable alternatives capable of operating at atmospheric pressure and low temperature; however, film growth typically tends to be slow. In this article, an atmospheric pressure glow discharge (APGD) fitted with an ultrasonic atomizer is used to deposit polysiloxane and silicon oxide coatings at high growth rates. APGD plasmas typically use a helium diluent and a high frequency (>1 kHz) power supply to generate a homogeneous glow discharge at atmospheric pressure via a Penning ionization mechanism.24-27 These plasmas are highly uniform in nature compared to other types of nonequilibrium atmospheric plasma (such as coronas or dielectric barrier discharges, where the reactive species tend to be concentrated into narrow filaments and therefore can cause localized surface damage and degradation28,29). Octamethylcyclotetrasiloxane (OMCTS) and tetramethylcyclotetrasiloxane (TMCTS) have been chosen as precursors in the present study, since they consist of cyclic Si-O structures, which can potentially ring open to provide polysiloxane networks; see structures 1 and 2. The physicochemical nature of the deposited films has been examined by X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (ATR-FTIR), contact-angle, solid (21) Wrobel, A. M.; Walkiewicz-Pietrzykowska, A.; Wickramanayaka, S.; Hatanaka, Y. J. Electrochem. Soc. 1998, 145, 2866. (22) Niwano, M.; Kinashi, K.; Kazuhiko, S.; Miyamoto, N. J. Electrochem. Soc. 1994, 141, 1556. (23) Muisener, R. J.; Koberstein, J. T. Abstracts of Papers, 214th National Meeting of the American Chemical Society, 1997; American Chemical Society: Washington, DC, 1997; p 56. (24) Kanazawa, S.; Kogoma, M.; Moriwaki, T.; Okazaki, S. J. Phys. D: Appl. Phys. 1988, 21, 838. (25) Okazaki, S.; Kogoma, M. Proc. Jpn. Symp. Plasma Chem 1989, 2, 95. (26) Kanazawa, S.; Kogoma, M.; Okazaki, S.; Moriwaki, T. Nucl. Instrum. Methods Phys. Res. 1989, B37/38, 842. (27) Yokoyama, T.; Kogoma, M.; Kanazawa, S.; Moriwaki, T.; Okazaki, S. J. Phys. D: Appl. Phys. 1990, 23, 374. (28) Greenwood, O. D.; Boyd, R. D.; Hopkins, J.; Badyal, J. P. S. J. Adhes. Sci. Technol. 1995, 9, 311. (29) Massines, F.; Gouda, G. J. Phys. D.: Appl. Phys. 1998, 31, 3411.
10.1021/la0204287 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/14/2003
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Chart 1
Figure 1. Schematic of the APGD deposition reactor.
state nuclear magnetic resonance (NMR) spectroscopy, and gas permeation measurements. Experimental Section The home-built atmospheric pressure glow discharge (APGD) deposition reactor constituted a high voltage 15 kHz ac power supply applied across two aluminum electrodes spaced 12 mm apart, with the lower live electrode shielded by a glass dielectric plate; see Figure 1. An ultrasonic nozzle (Sono-tek, 8700-120) was embedded flush within the upper, earthed electrode and powered by a broad-band ultrasonic generator (Sono-tek, 06-05108). This employs a piezoelectric transducer to convert liquid feed into an atomized spray30 (in principle permitting the rapid vaporization of even nonvolatile precursors). Octamethylcyclotetrasiloxane (OMCTS; Fluka >99%) and tetramethylcyclotetrasiloxane (TMCTS; Fluorochem 95% minimum) were introduced into the ultrasonic nozzle using a syringe pump (Harvard Apparatus) via a nonlubricated ball valve. Plasma deposition entailed placing a piece of polyethylene film substrate (ultrasonically washed for 30 s in a 1:1 mixture of propan-2-ol (BDH, AnalaR grade) and cyclohexane (BDH, AnalaR grade)) on top of the glass dielectric plate and evacuating the chamber with a liquid nitrogen trapped rotary pump. The process gas was then admitted at a flow rate of 1900 sccm and a pressure of 1020 mbar. Two compositions were used: helium (BOC, 99.996%) and a 99% helium/1% oxygen (BOC, 99.998%) mixture. After 10 min of purging, the monomer reservoir syringe pump was switched on at a flow of 0.12 mL/h. When the liquid precursor OMCTS or TMCTS reached the ultrasonic nozzle, the ultrasonic generator was switched on (2.5 W) to initiate atomization. Subsequently, the APGD plasma was ignited by applying 1.5 kV across the electrodes. Following 10 min of deposition, any labile material was pumped off by placing the samples under vacuum for 20 min. XPS analysis was performed using a Kratos ES300 electron spectrometer equipped with an unmonochromated Mg KR X-ray source and a concentric hemispherical analyzer operating in the FRR mode (22:1). Photoelectrons were detected at a takeoff angle (30) Ultrasonic-nozzle Product Information, Sono-tek, U.S.A.
of 30° from the substrate normal. Instrumentally determined sensitivity factors were taken as C(1s):O(1s):Si(2p) ) 1.00:0.57: 0.72. Sample charging was taken into consideration by referencing all spectra relative to the C(1s) hydrocarbon peak at 285.0 eV.31,32 The oxidation state of silicon centers was estimated using Gaussian peak shapes to fit the Si(2p) envelope. The Si-O peak position was set at 102.0 eV, Si2O3 at 103.0 eV, and SiO2 at 104.O eV.33,34 The total amount of siliceous material (SiOx>1) was then calculated by summing the amount of silicon and oxygen in the Si2O3 and SiO2 environments. ATR-FTIR spectra of the deposited films were recorded using 100 scans at 4 cm-1 resolution on a Mattson Polaris spectrometer fitted with an attenuated total reflectance (ATR) accessory (Graseby Specac Golden Gate). A spectrophotometer (Aquila Instruments, nkd-6000) was used to determine film thickness. Transmittance-reflection curves over the 350-1000 nm range were fitted to a Drude-Lorentz model using a modified Levenburg-Marquardt procedure.35 Repeatability and sample homogeneity were assessed by recording multiple readings from several different samples. Contact angle values were obtained with a video capture apparatus (AST Products VCA2500XE) using sessile 2 µL droplets of deionized water. Solid state NMR spectroscopic analysis was performed upon coatings scraped from the glass dielectric and packed into a 6 mm rotor. 29Si NMR spectra were acquired at 79.4 MHz with a gated decoupling sequence on a JEOL Lambda 400 MHz spectrometer, using magic angle spinning (MAS) at 5 kHz and a pulse delay time of 35 s. Under these conditions, analysis is considered to be semiquantitative. Octakistrimethylsilyloxysilsesquioxane (Dow Corning) was used as an external reference. Gas permeation measurements were acquired using a mass spectrometric device. This comprised inserting the coated polyethylene substrate between two drilled-out stainless steel flanges and a viton gasket. This assembly was attached to a ultrahigh vacuum (UHV) chamber via a gate valve (base pressure of 5 × 10-10 Torr) with the coated side of the polymer film exposed to an oxygen (BOC, 99.998%) pressure of 1000 Torr. A UHV ion gauge (Vacuum Generators, VIG 24) and a quadrupole mass spectrometer (Vacuum Generators SX200) interfaced to a PC computer were used to monitor the permeant pressure drop across (31) Beamson, G.; Briggs, D. High resolution XPS of Organic Polymers: The Scienta ESCA300 Database, 1st ed.; John Wiley & Sons: Chichester, 1992. (32) Clark, D. T.; Shuttleworth, D. J. Polym. Sci. Chem. Ed. 1980, 18, 27. (33) Alfonsetti, R.; Lozzi, L.; Passacantando, M.; Picozzi, P.; Santucci, S. Appl. Surf. Sci. 1993, 70/71, 222. (34) Alexander, M. R.; Short, R. D.; Jones, F. R.; Michaeli, W.; Blomfield, C. J. Appl. Surf. Sci. 1999, 137, 179. (35) Technical Information, Aquila Instruments, Cambridge, UK.
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Table 1. Summary of XPS, Contact Angle, and Deposition Rate Measurements XPS sample
%C
%O
% Si
% SiOx>1
OMCTS, theoretical monomer OMCTS, 100% He OMCTS, 99% He, 1% O2 TMCTS, theoretical monomer TMCTS, 100% He TMCTS, 99% He, 1% O2
50 43.3 ( 2.5 25.5 ( 5.2 33.3 32.5 ( 0.3 9.2 ( 0.2
25 29.3 ( 0.8 48.5 ( 4.5 33.3 39.1 ( 1.0 61.4 ( 2.2
25 25.8 ( 0.6 26.0 ( 0.7 33.3 28.4 ( 0.6 29.5 ( 2.0
0 35.7 ( 6.5 74.4 ( 2.8 0 65.4 ( 1.3 81.5 ( 5.9
a
contact angle (deg)
deposition rate (nm min-1)
107.8 ( 9.2a 56.4 ( 9.8
28 ( 4 29 ( 4
102.3 ( 2.8 wets
82 ( 11 244 ( 38
Clean polyethylene has a contact angle of 105.8 ( 1.5°.
Figure 2. Si(2p) XPS spectra of octamethylcyclotetrasiloxane films deposited in (a) helium APGD and (b) helium/oxygen APGD. the substrate. The quadrupole mass spectrometer’s response per unit pressure was independently calculated by introducing oxygen directly into the chamber via a leak valve and recording the mass spectrum at a predetermined pressure of 4 × 10-7 Torr (taking into account ion-gauge sensitivity factors36). This was then used to calculate mean equilibrium permeant partial pressure (MEPPP) of oxygen.37 Finally, the barrier improvement factor (BIF) for each sample was determined by referencing with respect to the MEPPP value measured for the untreated polyethylene substrate.
Results (a) Octamethylcyclotetrasiloxane Deposition. APGD deposition of OMCTS in the presence of helium carrier gas yielded hydrophobic coatings with surface elemental abundances resembling those of the monomer; see Table 1. Mixing oxygen with the helium feed gas produced a hydrophilic (low contact angle), oxidized surface containing a reduced amount of carbon. A corresponding increase in the Si(2p) binding energy intimated the formation of a more inorganic, SiOx>1-rich environment;38-41 see Figure 2. Spectrophotometric thickness measurements indicated that the films deposited in pure He and the He/O2 mixture were 279 ( 36 and 286 ( 38 nm thick, respectively; see Table 1. ATR-FTIR has a penetration depth of approximately 0.1-10 µm;40,42 it can therefore provide insight into the (36) Pressure Measurement Technical Information, Vacuum Generators. (37) Crank, J.; Park, G. S. In Diffusion in Polymers; Crank, J., Park, G. S., Eds.; Academic Press: London, 1968; Chapter 1. (38) Owen, M. J.; Smith, P. J. J. Adhes. Sci. Technol. 1994, 8, 1063. (39) Murakami, T.; Kuroda, S.-I.; Osawa, Z. J. Colloid Interface Sci. 1998, 202, 37. (40) Hillborg, H.; Gedde, U. W. Polymer 1998, 39, 1991. (41) Morra, M.; Occhiello, E.; Marola, R.; Garbassi, F.; Humphrey, P.; Johnson, D. J. Colloid Interface Sci. 1990, 137, 11.
Figure 3. ATR-FTIR spectra of (a) polyethylene substrate, (b) octamethylcyclotetrasiloxane liquid, (c) helium APGD deposited octamethylcyclotetrasiloxane, and (d) helium/oxygen APGD deposited octamethylcyclotetrasiloxane.
bulk chemical structure of the coating (whereas XPS is only sensitive to the outermost 2-5 nm). Films deposited onto polyethylene substrates using an OMCTS/He APGD displayed many IR spectral features similar to those seen for the liquid OMCTS precursor; see Figure 3. The main differences were an enhancement of the peak at ∼850 cm-1 and the emergence of a shoulder on the band at 11001000 cm-1 (Si-O-Si stretch18,23,43). The precise nature of the polymerized product is difficult to determine since absorbances due to Si-CH2-Si and Si-O-C coincide with Si-O-Si (1020-1090 cm-1)44,45 and Si-O, Si-C, Si-H, and Si-Me3 features all overlap in the 880-800 cm-1 region.21,44,52 The absence of polyethylene bands at 1450, 2850, and 2950 cm-1 indicated that the deposited films were sufficiently thick to preclude infrared sampling of the underlying substrate. OMCTS coatings deposited in the presence of a helium/ oxygen mixture exhibited several new infrared spectral features; see Figure 3. The Si-O-Si stretch (1100-1000 cm-1) broadened and shifted toward higher wavenumber; (42) Gaboury, S. R.; Urban, M. W. Analysis of Gas Plasma Modified Poly(dimethylsiloxane) Elastomer Surfaces. Attenuated-Total-Reflectance-Fourier Transform Infrared Spectroscopy. In Structure-Property Relations in Polymers; Urban, M. W., Craver, Clara, D., Eds.; American Chemical Society: Washington, DC, 1993; Vol. 236, p 778. (43) Fujii, T.; Hiramatsu, M.; Nawata, M. Thin Solid Films 1999, 343-344, 457. (44) Bogart, K. H. A.; Ramirez, S. K.; Gonzales, L. A.; Bogart, G. R. J. Vac. Sci. Technol., A 1998, 16, 3175. (45) Kim, M. T. Thin Solid Films 1999, 347, 99. (46) Zajickova, L.; Janca, J.; Perina, V. Thin Solid Films 1999, 338, 49. (47) Veprek-Heijman, M. G. J.; Boutard, D. J. Electrochem. Soc. 1991, 138, 2042. (48) Okimura, K.; Maeda, N. J. Vac. Sci. Technol., A 1998, 16, 3157. (49) Williams, E. A. Annu. Rep. NMR Spectrosc. 1983, 15, 235. (50) Engelhardt, G.; Jancke, H.; Lippmaa, E.; Samoson, A. J. Organomet. Chem. 1981, 210 (3), 295. (51) Greenwood, O. D. Ph.D. Thesis, University of Durham, UK, 1997. (52) Agres, L.; Segui, Y.; Delsol, R.; Raynaud, P. J. Appl. Polym. Sci. 1996, 61, 2015.
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Figure 4. 29Si solid state NMR spectrum of helium APGD deposited octamethylcyclotetrasiloxane. Table 2. Oxygen Permeation Measurements deposition conditions
barrier improvement factor
polyethylene substrate OMCTS, 100% He OMCTS, 99% He, 1% O2 TMCTS, 100% He TMCTS, 99% He, 1% O2
1.0 (by definition) 0.9 ( 0.2 6.8 ( 3.5 0.9 ( 0.1 4.5 ( 1.6
this can be taken as being representative of quartz-like material.23 In contrast, the Si-Me band at 1260 cm-1 became attenuated, intimating the oxidative removal of methyl groups.13,23 Bands at ∼910 and 3500-3000 cm-1 also emerged, indicative of Si-OH and SiOH-H2O/ absorbed H2O, respectively.18,20,46,47 The absence of the C-H stretch at 2950 cm-1 48 (a band seen for both liquid OMCTS and the helium deposited polymer) provided evidence to support the preferential loss of organic material. 29 Si solid state NMR analysis of the material deposited in helium showed predominantly (Me2SiO)n at a chemical shift of approximately -20 ppm, indicative of a polysiloxane polymer and any entrapped OMCTS monomer; see Figure 4. The signal at -67 ppm is due to MeSiO3/2 signifying the presence of some cross-linking or branching. The small signals at -8.9 and -58.5 ppm were attributed to HOMe2SiO1/2 and MeSi(OH)O2/2, respectively.49,50 Oxygen gas permeability measurements of the coating obtained from the OMCTS/helium APGD were comparable to the untreated polyethylene substrate; see Table 2. In contrast, OMCTS films deposited in a helium/oxygen mixture gave rise to a marked improvement in the gas barrier characteristics of the substrate. (b) Tetramethylcyclotetrasiloxane Deposition. XPS analysis showed that the hydrophobic TMCTS films deposited in helium contained more oxygen than predicted from the monomer structure; see Table 1. The Si(2p) binding energy was also higher than expected, exhibiting a value (∼102.8 eV) akin to a cross-linked structure such as a silsesquioxane51 (Si(2p) binding energy ∼103 eV); see Figure 5. Addition of oxygen to the plasma resulted in carbon removal and a corresponding rise in oxygen concentration. This, taken into consideration with an increase in the Si(2p) binding energy, intimated the formation of oxidized coatings which are more SiOx>1 rich than those deposited from OMCTS and He/O2. The deposition rates were also much greater for the TMCTS precursor, especially in the presence of He/O2, where several micron-thick films were grown. ATR-IR spectra of the films deposited from the TMCTS/ He APGD displayed features similar to those of the liquid monomer; see Figure 6. The main differences were a broadening of the Si-O-Si stretch (1100-1000 cm-1) and
Figure 5. Si(2p) XPS spectra of tetramethylcyclotetrasiloxane films deposited in (a) helium APGD and (b) helium/oxygen APGD.
Figure 6. ATR-FTIR spectra of (a) polyethylene substrate, (b) tetramethylcyclotetrasiloxane liquid, (c) helium APGD deposited tetramethylcyclotetrasiloxane, and (d) helium/oxygen APGD deposited tetramethylcyclotetrasiloxane.
a diminishment of the Si-H band at ∼2200 cm-1.44,52,53 These features can be attributed to Si-O-Si cross-linking with a corresponding partial loss of Si-H functionality. Addition of oxygen to the APGD plasma appeared to augment these phenomena; films deposited in a He/O2 mixture possessed a larger, broader Si-O-Si stretch, a weaker Si-Me band (1260 cm-1), and an almost extinct Si-H peak. A broad band at 3500-3000 cm-1 also emerged, indicative of SiOH-H2O/absorbed H2O, the presence of which explains the enhanced hydrophilicity of this system. 29 Si solid state NMR analysis of the material deposited in helium showed predominantly MeHSiO2/2 at -30 to -40 ppm, a functionality associated with poly(methylhydrogensiloxane) polymer and some entrapped TMCTS; see Figure 7. Bands observed at -60 to -70 ppm and -80 to -90 ppm were indicative of MeSiO3/2 and HSiO3/2, respectively.49,50 Their intensities demonstrated that a significant degree of cross-linking or branching had taken place. TMCTS coatings deposited from helium APGD did not significantly alter the oxygen permeability characteristics of the polyethylene substrate; see Table 2. In contrast, TMCTS films deposited from a helium/oxygen mixture (53) Lin, C.-F.; Tseng, W.-T.; Feng, M. S. J. Appl. Phys. 2000, 87, 2808.
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produced a significant improvement in the gas barrier characteristics of the substrate.
able to the Si-H bonds present in the precursor being particularly labile (as shown by ATR-IR and 29Si NMR), leading to the formation of Si-O-Si linkages. This is based on the weakness of the Si-H bond relative to the Si-O bond (average bond dissociation enthalpies: Si-H ) 318 kJ mol-1 and Si-O ) 466 kJ mol-1).57 The greater volatility of TMCTS compared to OMCTS may also be a contributing factor for the greater deposition rate. The introduction of oxygen into both the OMCTS and TMCTS helium APGD mixtures leads to significant levels of oxygen incorporation at the expense of carbon and hydrogen removal to yield inorganic SiOx cross-linked structures. The accompanying attenuation in oxygen permeation is most likely to stem from a rise in film density52 (i.e., drop in free volume) and hindrance in segmental mobility associated with an oxidized siliceous phase. A similar fall in oxygen transmission rate has previously been observed for SiOx films deposited by CVD and low pressure PECVD.52
Discussion
Conclusions
Siloxane coatings deposited from OMCTS in a helium APGD display many XPS and IR spectral features associated with the OMCTS monomer. The observed differences are attributable to the opening and polymerization of the siloxane ring. The high organic content measured and fairly low level of cross-linking (shown by the low Si(2p) binding energy characteristic of linear siloxanes) accounts for the negligible improvement in gas barrier.52 However, gas absorption by such plasma polymerized OMCTS films may find application in selective gas sensors.54,55 Also, surface hydrophobicity was noted to be comparable to that obtained for conventional low pressure plasma deposited water-repellent polysiloxane films.56 In comparison, the higher deposition rate and enhanced cross-linking observed for TMCTS films could be attribut-
Introduction of substituted cyclotetrasiloxane precursors into a helium atmospheric pressure glow discharge using an ultrasonic atomizer gives rise to the fast deposition of hydrophobic polysiloxane films onto polyethylene. In the presence of a helium/oxygen gas mixture, SiOx>1-rich coatings are produced which display a significant improvement in hydrophilicity and gas barrier. This novel combination of a liquid atomizer combined with an atmospheric pressure plasma effectively circumvents a lot of the drawbacks associated with speed and efficiency for conventional vacuum plasma methods.
Figure 7. 29Si solid state NMR spectrum of helium APGD deposited tetramethylcyclotetrasiloxane.
(54) Janca, J.; Sodomka, L. Surf. Coat. Technol. 1998, 98, 851. (55) Guo, S.; Rochotzki, R.; Lundstrom, I.; Arwin, H. Sens. Actuators, B 1997, 44, 243. (56) Mahlberg, R.; Niemi, H. E.-M.; Denes, F.; Rowell, R. M. Int. J. Adhes. Adhesives 1998, 18, 283.
Acknowledgment. L.J.W. thanks EPSRC and Dow Corning for financial support. The authors thank Simeon Bones for his assistance in acquiring solid state NMR spectra, and Dan Futter for useful discussions throughout this work. LA0204287 (57) Atkins, P. W. Physical Chemistry, 5th ed.; Oxford University Press: Oxford, UK, 1994; p C7.
