J. MATER. CHEM., 1991, 1(3), 387-391 Nature of Dangling-bond Sites in Native Plasma-polymerized Films of Unsaturated Hydrocarbons, and Electron Paramagnetic Resonance Kinetics on Heat Treatment of the Films Masayuki Kuzuya,*" Masanao Ishikawa," Akihiro Noguchi," Hideki Ito," Kaoru Kamiyab and Tohru Kawaguchib a Laboratory of Pharmaceutical Physical Chemistry, Gifu Pharmaceutical University, 56-I, Mitahora-Higashi, Gifu 502, Japan Tomei Sangyo Co. Ltd. Noritake-Sinmachi, Nishiku, Nagoya 467, Japan We describe the electron paramagnetic resonance (EPR) study of dangling-bond sites (DBS) of plasma- polymerized films prepared from three unsaturated hydrocarbons, phenylacetylene, styrene and hex-3-yne. It has been shown that all the EPR spectra of DBS in such native films (before exposure to air) are much more intense than those after exposure to air.The DBS are quite stable at room temperature under anaerobic conditions, but their number begins to decrease towards a limiting value on heat treatment. However, when exposed to air the DBS decay rather rapidly; this decay tends to level off , indicating that there exist two different types of DBS, reactive and non-reactive. We discuss the difference in the nature of dissipation of DBS between oxidative and non-oxidative conditions. We also present the EPR spectral interpretations of the DBS and the reactions of several plasma-polymerized films previously reported. Keywords: Plasma polymerization; Dangling-bond site; Electron paramagnetic resonance spectroscopy It is a well known fact that glow-discharge plasmolyses of nearly all organic vapours produce unique plasma-poly- merized films on the surface of glass reaction vessels, although the rate of plasma polymer deposition depends on the nature of the organic compounds used.This process is referred to as plasma-state polymerization.ls2 One of the special features of the plasma-state polymerization is the fact that the resulting polymers contain large amounts of stable trapped free radicals at room temperature, all of which show a broad single line in their EPR ~pectra.~-'~ For instance, the EPR spectrum of styrene plasma-polymerized films (PPST) is shown in Fig. 1 as a representative example.' Note the isotropic broad single line with a peak-to-peak width (AHms,)(msl =maximum slope) of cu.1.7-1.8 mT. The pioneering studies of Yasuda indicated more than a decade ago that there is a definite correlation between the radical concentration and the chemical structure of organic compounds used for plasmolysis.2 Plasma-polymerized films of organic vapours that contain triple bonds and aromatic rings, including hetero-aromatic rings, produce the highest level of trapped free radicals, whereas saturated hydrocarbons produce the lowest number of trapped free radicals. These trends roughly parallel those for the rate of plasma-state polymerizations. In view of the fact that plasma-polymer deposition occurs directly on the surface of solid materials from the vapour Fig.1 EPR spectrum of trapped free radicals of styrene plasma- polymerized film' phase through 'atomic polymerization'2 involving a variety of plasma-fragmented species, it can be considered that the higher level of trapped free radicals observed in the plasma- polymerized films from unsaturated organic compounds could result from the involvement of the unsaturated n-bond of the monomer; the regenerated radicals thus formed are immobil- ized in the polymer matrices as the plasma-polymer deposition proceeds. 'Trapped free radicals' in the plasma-pol ymerized films can be considered not to be discrete organic free-radical species, but to be immobilized dangling-bond sites (DBS). These incorporate a variety of chemical structures including conju- gated and non-conjugated radical centres, and are of no structural significance." Therefore, the EPR spectra show only outlines of overlapping multicomponent radical centres, which are not represented by either Lorentzian or Gaussian curves.This is consistent with the assignment of broad single- line spectra observed in plasma-irradiated polymers to compo- nent Most of the spectra of plasma-polymerized films previously reported, however, have been recorded using films (or powders scraped from the glass surface) that had been exposed to air by withdrawal from the plasma reactor, even for those measurements described as in uucuo. When the films were exposed to air, some of the DBS in the surface layer could have reacted rapidly with oxygen to give the corresponding peroxy radicals followed by the termination reactions.We believe that detailed EPR study on the DBS of native plasma-polymerized films without exposure to air can provide more definitive information concerning the nature of the DBS as well as the characteristics of the resulting films. Thus, we have undertaken EPR spectral measurement of DBS in native plasma-polymerized films prepared from unsaturated hydro- carbons such as phenylacetylene (PA), styrene (ST) and hex-3-yne (3H). Furthermore, we have also conducted EPR kinetics of DBS on the heating of such films to gain further insight into the physicochemical properties of the DBS. We report that the spectral intensities of DBS thus observed were much greater than those observed after exposure to air, and J.MATER. CHEM., 1991, VOL. 1 vacuum line T er inlet sealed -0-Fig. 2 Schematic representation for plasma polymerization and EPR spectral measurement of the plasma-polymerized films that they can be assigned to two different types of DBS: reactive sites and non-reactive sites. Thus, we discuss the difference in the nature of DBS decay under oxidative and non-oxidative conditions. We also present the interpretations of EPR spectra of DBS reported in several previous papers relevant to the present work. Experirnental Materials Organic compounds, phenylacetylene (PA), styrene (ST), and hex-3-yne (3H) used for plasma-polymerization are all com- mercially available and were used without further purification.Method of Plasma Polymerization A neutral glass capillary (1 mm i.d., 65 mm long) was placed in a specially designed ampoule with a side branch (30mm i.d., 100 mm long) connected to a capillary tube (2 mm i.d.) at the upmost part of the ampoule. The ampoule was connec- ted to a vacuum line and degassed (0.001 Torrt). The plasma polymerization was carried out using inductively coupled plasma in the region encircled by the radiofrequency discharge coil at 13.56 MHz. The supplied power was 40 W over a period of 10 min for PA, and 1 h for ST and 3H for EPR study, with a monomer flow rate of 3.5 x cm3 min-' for PA and ST and 2.5 x cm3 min-' for 3H. After the plasma irradiation was discontinued, the ampoule reactor was kept in uucuo for 30 min to remove the remaining low-molecular- weight materials.The EPR spectral measurements of plasma-polymerized films formed on the glass capillary were performed by turning the ampoule upside down (Fig. 2). The EPR spectral intensity was determined by double integration. EPR spectra were recorded with a JES-RE1X spectrometer (JEOL) with X-band and 100 kHz field modu- lation, and extra care was taken to ensure that the observed spectra were not saturated by keeping the microwave power level below 0.01 mW. IR Spectral Measurement the IR spectra were measured on a JASCO A-102 spec- trometer. Results EPR Spectra Fig. 3 shows the EPR spectra of immobilized DBS observed with three plasma-polymerized films, PPPA, PPST and PP3H, formed from PA, ST and 3H, respectively, before and after exposure to air.It can be seen that the spectral features are more or less the same and they were unchanged at any stage of plasma polymerization. The spectra are characterized by an isotropic broad single line with AHmslof 1.76 mT for PPPA, 1.90 mT for PPST and 1.89 mT for PP3H. Note that the broad single-line spectrum of PPST is also essentially identical with that of the radicals produced in plasma-irradiated polystyrene (PST), with longer plasma dur- ation.16 A notable feature in Fig. 3(u) is that the spectral intensity of PPPA is markedly larger than for the other two films, PPST and PP3H.t This trend parallels the report of Yasuda.2 Thus, a large number of the DBS in PPPA result t The spectral intensity of PP3H is still much larger than that of other compounds such as methyl methacrylate (MMA), methyl is0 butyrate (MIB) and hexa-1,Sdiene.A B C (b) (b)A-7 (& Fig. 3 EPR spectra of dangling-bond sites of (A) plasma-polymerized Plasma-polymerized films for IR spectral measurements were films of phenylacetylene (PPPA), (B) styrene (PPST) and (C) hex-3-yneprepared on KBr discs in a manner similar to the above, and (PP3H): (a) before exposure to air, (b) after exposure to air. Values of AH,,,/mT: (A) (a) 1.76; (b) 1.47; (B) (a) 1.90, (b) 1.33; (C) (a) 1.89, t 1 Torr x 133.322 Pa. (b) 1.13 J. MATER. CHEM., 1991, VOL. 1 from the presence of both effective structural features for DBS formation: an aromatic ring and a triple bond in PA.The spectral intensities observed after exposure to air are appreciably reduced relative to those prior to exposure as shown in Fig. 3(b). This indicates that a considerable number of DBS react with oxygen and are terminated to stable diamagnetic molecules at room temperature (uide infru). It is known that even a brief plasma-exposure of various kinds of glass substrate produces intense paramagnetic centres, i.e. 'glass radicals', evidenced by the EPR spectroscopic meas~rements,~~~*~~~~~although the EPR spectral features vary depending on the nature of glass substrate. Yasuda and Hsu have shown, however, that glass radicals have not been formed in the plasma polymerizations of unsaturated organic vap~urs.~It was also confirmed by separate experiments that the EPR spectra of the films formed under the present flow plasma conditions shown in Fig.3 were not contaminated with those of the glass radicals. EPR Kinetics Fig. 4 shows the progressive changes in EPR spectral intensity (determined by double integration) of the DBS in PPPA, PPST and PP3H in the course of plasma polymerization. It is clear in all cases that the spectral intensity increases linearly as the reaction proceeds, but the rate varies with the com- pounds used for plasmolysis. The rate of plasma-polymeriz- ation was also evaluated by monitoring the growth of the characteristic peak of the IR spectra in each film formed on KBr discs with various plasma duration.Thus, as shown in Fig. 5, the rate of DBS formation was found to be well correlated to the rate of plasma-polymerized film formation (linear relationship). Fig. 6 shows the EPR spectral changes of DBS in PPPA, PPST and PP3H on standing at room temperature under aerobic conditions. It is seen that the spectral intensity decreases quite rapidly with time under aerobic conditions toward a limiting value. After a few hours the spectral intensity tends to level off and persists essentially unchanged for a long period of time at room temperature. Note that the decay of the spectral intensity under aerobic conditions is accompanied by a decrease in AH,,, of EPR spectra in all cases (from cu. 1.8-1.9 mT to cu. 1.1-1.5 mT) (uide infru).These results demonstrate that the DBS are quite stable at room tempera- ture so long as the films are kept under anaerobic conditions in all cases, indicating that all the DBS are immobilized and 0 20 40 60 plasma duratiodmin Fig.4 Progressive changes in EPR spectral intensity of the DBS in the course of plasma-polymerization: @, PPPA; 0,PPST; @, PP3H P plasma duratiodmin Fig. 5 Progressive changes in characteristic band absorbance (2920- 2930 an-') of IR spectra in the course of plasma-polymerization:0, PPPA; 0,PPST; @, PP3H 1.o 0.8 .-5 0.6 v) ac .-0.4 0.2 0 1 2 3 standing time/h Fig.6 Progressive changes of relative EPR spectral intensity of the DBS on standing in air at room temperature: @, PPPA 0,PPST; 0, PP3H do not readily undergo termination at room temperature.However, the fact that dissipation of the DBS in air tends to level off indicates the presence of unreactive DBS. Thus, it is considered that immobilized DBS can be divided into two different DBS, reactive and non-reactive sites. It is apparent that the former lie in the surface layer, where oxygen can diffuse readily to react with the DBS, producing the corre- sponding peroxy radicals followed by well known complex chain-termination reactions. The latter sites would appear to lie in the bulk where oxygen can not penetrate readily. Thus, most of the DBS previously reported by EPR spectra were probably only the non-reactive DBS located in the bulk of the plasma-polymerized films.Furthermore, as will become more apparent later, the surface layer of such films is less cross-linked than the bulk, this being equivalent to the pres- ence of a larger quantity of lower-alkyl carbons. The bulk layer is of a higher cross-linked network containing a larger quantity of higher-alkyl carbons. Since oxygen was observed to react only with those DBS in the surface layer, it can be reasonably assumed that the remaining DBS consist of more higher-alkyl carbon-centred radicals, which possess fewer hydrogens capable of coupling with free-radical electrons. We believe this is the essential reason for the decrease in AHm1 when films have been left to stand in air. Heat Treatment With a view to improving the dielectric properties of styrene plasma-pol ymerized films, several authors have conducted annealing experiments under a variety of condition^.^*'^^" A detailed kinetic study, however, has not been undertaken.Since we believe that the kinetic study of the native DBS on heat treatment under anaerobic conditions could provide important information of intrinsic properties of DBS, includ-ing the neighbouring structure and the degree of cross-linking of such films, we have conducted the kinetic study in more detail. Fig. 7 shows the progressive changes in the spectral intensity of three films, when heated at various temperatures under anaerobic conditions. It is seen that, although such DBS persisted unchanged in intensity at room temperature, heat treatment caused a decay of the spectral intensities.The nature of the decay depends on the type of film and the temperature, but the intensity tends to level off gradually in all cases. It was also found that all the decay curves at earlier stages could be described by second-order kinetics, indicating that the dissipation of DBS follows a diffusion-controlled bimolecular reaction. Comparison of the three parts of Fig. 7 disclosed several interesting features. The DBS of PPPA appear to be the most stable, so the progressive changes in intensity at 125 "C were nearly the same as those at lOO"C, and both spectral intensities remained at the highest level. This indicates that, of the three films, PPPA is the most rigid in the polymer matrix, probably owing to the presence of both a triple bond and an aromatic ring.On the other hand, the DBS of PPST is much less stable so that the DBS is hardly detectable at 125 "C. This indicates that PPST has the least cross-linked network in the matrix structure. Results of spin-trapping reactions of styrene plasma-polymerized ultrathin films sup- ports this.21 Although the spectral intensity gradually decreased, the AH,,, of the spectra persisted unchanged during heat treat- ment in all cases,t which is in sharp contrast to the case of progressive changes of the spectra under aerobic conditions at room temperature. These results indicate that the surface layer should be less cross-linked than the bulk and may be mobile enough to undergo the termination reaction at higher temperatures even in the absence of oxygen.This view is also consistent with the results of spectral decay under aerobic conditions at room temperature, as described above. Note that heat treatment at a higher temperature than 125 "C accelerated the rate of dissipation of DBS, and after most of 7 This fact differs from that reported previo~sly.~*'' This discrep- ancy may stem from the fact that our PPST had never been exposed to air. 1.o 0.5 -2 0 0 0 J. MATER. CHEM., 1991, VOL. 1 the radicals had dissipated a new EPR signal with much smaller AH,,, appeared at temperatures higher than 200 "C, as has been reported previously (see below). Discussion As stated in the introduction the EPR spectra of stable free radicals in plasma-polymerized films have been.observed by a number of author^.^-'^ In connection with our present work, we wish to discuss the EPR spectral interpretation of the structure of radicals involved in plasma-polymerized films in several previous reports. In the EPR study on the annealing of PPST, several authors have observed that the heating of PPST in air at a higher temperature than ca. 250 "C produced a new EPR signal with a much smaller AH,,, value (ca.0.5-0.8 mT) after the original DBS had nearly disappeared, and the spectral intensity con- tinued to increase as the temperature was raised f~rther.~*'~*" The relatively sharp single-line spectrum with small AHms, thus observed is indicative of the presence of far fewer kinds of component spectra.In the above reports it was postulated that the radical initially observed was due to carbon-centred radicals, while at higher temperatures a new radical was formed as a result of reaction with oxygen in the system. However, there was no further structural characterization. The comparison of elemental analyses before and after heat treatment indicates that hydrogens have been eliminated oxidatively to form a more cross-linked network. In fact, such films have been shown to carbonize on heat treatment resulting in the formation of dark-coloured films. Such a small AH,,, is a spectral feature that is strikingly similar to those of DBS in amorphous carbon films obtained by arc evapor- ation of pure graphite rods22 and diamond thin films formed by microwave plasma chemical vapour depo~ition.~~ More-over, the calculated g value of the new single-line spectrum (g=2.002 for PPST) is consistent with carbon-centred, and not oxygen-centred, radicals.Based on these facts, it seems logical to consider that the dominant formation of tertiary carbon-centred DBS is responsible for a new isotropic single- line spectrum, which possesses essentially no hydrogen capable of coupling with the free-radical electron. Likewise, the observed EPR spectra with a small value of AH,,, (0.6-0.8 mT) in plasma-polymerized films of siloxane compounds7 could be explained similarly in terms of an absence of hydro- gens capable of coupling with the silicon-centred free-radical electron.One should remember that this view is totally consistent with the foregoing interpretation of the tendency for AH,,, to decrease when the films are exposed to air. Several authors have reported that the EPR spectra of plasma-polymerized perfluorinated compounds show excep- 1.o 0.5 1 2 0 1 2 0 1 2 heating time/h heating time/h heating time/h Fig. 7 Progressive changes in relative EPR spectral intensities of DBS on heat treatment in uucuo for (a) PPPA, (b)PPST and (c)PP3H: 0, 70; 0,100; @, 125°C J. MATER. CHEM., 1991, VOL. 1 tionally large values of AHms,of ca.4.0mT, which is much broader than those of non-fluorinated organic polymers (1.5- 2.0 mT).6,7,12,13 Millard et al. have explained that this kind of feature probably results from a combination of inhomo- geneous broadening and exchange narrowing.6 We believe, however, that all these broad single-line spectra are assignable to the same type of DBS as those in other plasma-polymerized films without invoking any special property, and the large AHmslof the spectra can be interpreted by the fact that the hyperfine splitting of fluorine atoms is more than three times larger than that of hydrogen atoms (e.g.ca. 7.5 mT for a- fluorinez4us. ca. 2.0 mT for a-hydrogen coupling). Among previous work dealing with the DBS of plasma- polymerized films, we also wish to comment on the result reported by Venugopalan and co-w~rkers.~ The authors have observed that heat treatment of xylene plasma-polymerized film at 100 "C in air gave a new peak, a doublet separated by 12.5 mT.This new doublet has speculatively been assigned to hydroxybenzyl-type radicals resulting from the oxidation of xylene. As described above, however, the hyperfine splitting of a-hydrogen is normally ca. 2.0 mT and such a large splitting of 12.5 mT for a-hydrogens is unrealistic in any interatomic hydrogen coupling in any organic compound. We have already undertaken a number of plasma-irradiation studies on various kinds of inorganic materials including glass substrates such as soft glasses, neutral glasses, Pyrex and quart^.^'*^' We have similarly observed the doublet separated by 12.0-12.5 mT on quartz by Ar plasma irradiation." Thus, the feature observed by Venugopalan is undoubtedly assignable to the paramag- netic centres generated by plasma irradiation in the quartz substrate, which we believe became observable in the spectrum owing to a significant reduction in intensity of the other peaks by heat treatment.In summary, the results reported here combined with the reinterpretations of the previously reported EPR spectra demonstrate that only the DBS in the surface layer of plasma-polymerized films dissipate oxidatively in air at room tempera- ture, and the broad single-line spectrum is caused by aniso- tropic hyperfine splitting with the superposition of various kinds of radical centres, i.e. DBS. Heat treatment of the films caused further dissipation of the radicals, either oxidatively or non-oxidatively, with oxidative heating (under aerobic conditions) being more prone to dissipation than non-oxidat- ive heating (under anaerobic conditions). Further heat treat- ment at higher temperatures is distinct from the reaction of the original DBS in plasma-polymerized films, since, appar- ently, it incorporates the carbonization of the film.Based on these considerations, the value of AHmslof EPR spectra may 39 1 be taken as an indication of the structure of DBS and the rigidity of plasma-polymerized film matrices. References 1 M. Hudis, Techniques and Applications of Plasma Chemistry, ed. J. R. Hollahan and A. T. Bell, Wiley, New York, 1974. 2 H. Yasuda, Macromolecular Reviews, Wiley, New York, 198I, vol.16. 3 F. J. Vastola and J. P. Wightman, J. Appl. Chem., 1964, 14, 69. 4 J. P. Wightman and N. J. Johnston, Adv. Chem. Ser., 1969, 80, 322. 5 S. Morita, T. Mizutani and M. Ieda, Jpn. J. Appl. Phys., 1971, 10, 1275. 6 M. M. Millard, J. J. Windle and A. E. Pavlath, J.Appl. Polym. Sci., 1973, 17, 2501. 7 H. Yasuda and T. Hsu, J. Polym. Sci., Polym. Chem. Ed., 1977, 15, 81. 8 T. W. Scott, K. Chu and M. Venugopalan, J. Polym. 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Crist, M. Bumgarner, T. Hsu and H. Yasuda, J. Macromol. Sci. Chem., 1976, 10, 451. 20 M. Kuzuya, A. Noguchi, S. Ito, R. Itatani and A. Hatta, to be published. 21 M. Kuzuya, S. Nakai and A. Ito, Chem. Lett., 1987, 1083. 22 S. Orzeszko, W. Bala, K. Fabisiak and F. Rozploch, Phys. Status Solidi A, 1984, 81, 579. 23 I. Watanabe and K. Sugata, Jpn. J. Appl. Phys., 1988,27, 1808. 24 R. J. Rontz and W. Gordy, J. Chem. Phys., 1962,37, 1357. 25 M. Kuzuya, T. Kawaguchi, Y. Yanagihara and T. Okuda, Nippon Kaguku Kuishi, 1985, 1007. Paper 0/05121G; Received 14th November, 1990