photooxidation studies on branched polysilanes

Pergamon

Eur. Po/.vm. J. Vol. 33, No. I, pp. 81-88, 1997 Copyright 0 1996 Elsevier Science Ltd

PII: SOO14-3057(96)00107-3 Printed in Great Britain. All rights reserved

0014-3057/97 $17.00 + 0.00

PHOTOOXIDATION STUDIES ON BRANCHED POLYSILANES

PATRiCIA P. C. SARTORATTO,* CELSO U. DAVANZO and I. VALCRIA P. YOSHIDAt

University of Campinas, Institute of Chemistry 13083-970, Campinas, SP, Brazil

(Receiced 21 Augur 199.5; accepted in final form 15 Nooember 1995)

Abstract-The photooxidation of films of the branched [(SiEt),,(SiPhl).], (SiPh),, [(SiPh).,(SiMez)], [(Sic-hex),,,(SiPh,),,], and of the linear (SiPhMe). polymers, by UV irradiation (1 = 337 nm) was studied by IR and UV-vis spectroscopic measurements. The main chemical changes were the formation of siloxane and silanol groups, with a ratio that varied from 1.4 to 3.0, indicating partial degradation of the branched structures. At the saturation dose, the degree of oxidation of the polymer films was approximately the same (3541%), but the total amount of oxygen incorporation was greater for the more branched ones, due to the higher density of Si-Si bonds in those polymer films. Copyright 0 1996 Elsevier Science Ltd

INTRODUCTION

The high molecular weight polysilane (SiMe&, first synthesized in 1949, attracted little scientific interest due to its intractability and high insolubility [l]. Therefore, during the 1960s through the 1980s much of the research on silicon-silicon bonded compounds was concentrated on the preparation of oligomeric, cyclic, polycyclic and cage organosilanes [2-51, yielding a large variety of structures including the cubane and highly strained rings [6]. The synthesis of polycyclic and cage compounds, from the reductive coupling of mixtures of RzSiClz and RSiCl, or organochlorodisilanes, were very sensitive to the nature of the reductant employed (Na, Na/K or Li), stoichiometry of reagents, solvent, and reaction conditions [6]. In some cases only polymers were obtained, rather than the desired cyclic or cage compounds [7]. Photochemistry studies of oligomeric and cyclic polysilanes, as well as of polymeric disilanes, have been done, from which silyl radicals, silylenes and silenes were inferred as intermediates [8].

The new interest in polysilane research began with the synthesis of a number of soluble linear high molecular weight homo and copolymers, together with the development of a list of potential technological applications, most of them associated with their photochemistry and photophysics [9, lo]. This class of polymers exhibits strong near-UV absorption bands attributed to cr-o* transitions from delocalized bonding HOMO to antibonding LUMO [lo]. Their photochemical behavior upon exposure to UV light and oxygen, including bleaching and fragmentation of the main chain, and their resistance to oxygen plasma, allowed their development as

*Present address: University of Goi&, Institute of Chemistry Goilnia, GO, Brazil.

tTo whom all correspondence should be addressed.

81

positive photoresists for mid and deep UV mi- crolithography [ 111.

Recently, a series of soluble network silicon-based macromolecules (SIR),,, named polysilynes, and highly branched polysilanes [(SiR),“(Si&),] were prepared and characterized by Weidman and Bianconi from the coupling reaction of RSiCl, or RSiCI,/R2SiC12, using Na/K alloy and high intensity ultrasound [12-151. Although this method proved to be advantageous over the usual Wurtz type coupling of organochlorosilanes with sodium metal, the latter have also been used successfully by other authors [16-191.

Polysilynes and highly branched polysilanes also exhibit intense Si-Si o-o.* transitions, which appear as a broad UV-visible bandedge, rather than a discrete transition [12-191. Similar to polysilanes, polysilynes are optical recording materials and have been described as media for optical waveguide fabrication [20,21], and as negative photoresist for deep UV lithography [22,23]. The photooxidation of polyalkylsilyne thin films is accompanied by cross- linking reactions rather than degradation, resulting in their conversion to siloxane network solids [20].

In a recent work, changes in the optical properties of poly(phenylsilyne) thin films by photooxidation were studied [24]. Besides the decrease in the film refractive index, we also observed a significant increase in the film thickness, the extent of which was attributed to the extent of the photooxidation process. The formation of siloxane groups, and of a reasonable amount of stable silanol groups was noticed, the latter observation indicating the opening of the network structure.

Because most imaging processes use materials in thin film form, and are most often conducted in air, it is important to look more closely at the photooxidation process. Some detailed work has been done on linear polysilanes by Ban and Sukegawa who analyzed the siloxane content of

82 P. P. C. Sartoratto et nl.

irradiated films of poly(methylphenylsilane) and poly(n-propylmethylsilane) by IR spectroscopy [25].

In this paper a comparative study of the chemical changes induced by irradiation (j. = 337 nm) of the polyphenylsilyne (SiPh),m, of the highly branched copolymers [(SiPh),,,(SiMe&], [(SiEt),,,(SiPh&,], [(Sic- hex),,,(SiPh$], (c-hex = cycle-hexyl), and of the linear (SiPhMe),, thin films is presented. Infrared spectroscopy was used as a tool for both siloxane and silanol quantitative estimation. Gravimetric or volumetric methods for measuring 0: consumption during photooxidation cannot differentiate between production of SiOSi and SiOH groups. To circum- vent this situation an IR method was used, which was based on the assumption that integrated molar absorption coefficients (c) for the SiOSi and SiOH modes can be transferred from other compounds.

EXPERIMENTAL

Polymer syntheses

Poly(phenylsilyne) and the branched copolymers men- tioned above were synthesized by a Wurtz-type reaction of the appropriate organochlorosilanes and sodium dispersion in refluxing toluene, with stirring [26]. The scheme below summarizes the synthetic procedures.

.uRSiCh + Y’R’RSiCIl

Naitoluene

reflux/quenching with methanol 5 [(RSi)~,~(‘R2RSi),I]

(1) U/l = l.OjO.0

A = Phenyl

(2) X/Y = 0.75/0.25

The average molecular weight (An) of the polymers was determined by vapour pressure osmometry in toluene at 45 C with a Knauer instrument. Proton NMR spectra were recorded on a Brucker 300 spectrometer in CC14 solutions, using D20 in a capillary as an internal standard. UV-visible (UV-vis) spectra of polymers were measured at room temperature in THF (Uvasol) solutions. with a Perkin Elmer i.-3 instrument. Infrared spectra were recorded on a Perkin Elmer FT-IR 1600 spectrometer, in the 400&400cm ’ region as film on KBr windows.

R = Phenyl ‘R = ‘R = Methyl R = Ethyl ‘R = ‘R = Phenyl R = Cyclohexyl ‘R = :R = Phenyl

(3) X/J = 0.011.0

‘R = Methyl ?R = Phenyl Pol~wwr films: cl v CYpposure

Linear polymethylphenylsilane was obtained by a similar route, but the reaction temperature was 65’C and 15-crown-5 (5 mol% based on the sodium amount) and heptane (15% based on the toluene volume) were employed as co-catalysts [27]. All the reaction steps were carried out in an argon atmosphere and the system was protected from light. Phenyl, ethyl, cyclohexyl trichlorosilanes, and phenylmethyl, dimethyl and diphenyl dichlorosilanes (Hiils) were distilled in the presence of CaH?. All solvents used in this work were dried and distilled prior to use. Reaction time was typically 4 hr, after which the sodium metal residues

Solutions of the polymers were prepared by dissolving the solid polymers in THF (20% w/v) and filtering them through a 0.45 pm Millipore membrane. Thin films were deposited by spin-casting these solutions on to KBr (very well polished window) and quartz (Suprasil) substrates (4000 rpmi20 set). The thickness of the films was determined interferometrically either by an interferometric microscope or by recording the transmittance visible-near infrared spectra of the polymer films on quartz [28]. Film thickness was typically 0.7-1.2 pm.

Polymer films on KBr and on quartz substrates were irradiated in air with a I MW NZ pulsed laser (i. = 337 nm) operating at 20 Hz. The average irradiance per pulse was 1.5 mJ cm-‘. The laser beam was sized at the proper dimension so that the total area of both KBr and the quartz substrates would be irradiated and analyzed immediately after. Exposures were performed until the end of the photooxidation process, which was detected when no more changes on the UV-vis and IR absorbances were observed.

Table I. Polymer charactenstics and synthetic yields

Polymer Degree composition End of RSil’RlRSi group &fn polymn. Yield

P&mer m:n* -0MeiW a/mol N’ (Oh)

~fil$(SiPh~).,] [(!&‘h;:,(SiMe2),,]

0.86jO.14 l.O!O.O 0.1810.82 0.10~0.90 1930 1460 19 17 60 50 0.75jO.25 O.lS/O.SS 3330 34 80

((Sic-hex),(SiPh:),,] 0.7510.25 0.0530.95 1600 12 50 ISiPhMe~, O.O/l.O - 6920 58 40

nMolar fraction. bMolar fraction of methoxy ending groups in relation to the total

monomeric units. x,6: determined by integration of ‘H-NMR signals. Glculated from Mn values, polymer composition and end group

values.

‘W

Fig. 1. ‘H-NMR spectrum of [(Sic-hex),,,(SiPh?),,].

and NaCl were removed by filtration. Polymer chains were terminated by the addition of proper amounts of methanol to the filtrate, and by stirring for 12 hr under argon.

Purified highly branched homo and copolymers were obtained as amorphous yellow powders after various precipitations in Ihe THF!methanol solvent system, and were dried under vacuum, at 70 C for 6 hr. Linear poly(phenylmethylsilane) was obtained as a white powder after appropriate purification to eliminate cyclic compounds, a procedure that is described elsewhere [27].

PolJsmer films: IR measurements

Infrared spectra were obtained in a Perkin Elmer 1600 equipment, with nominal resolution of 4 cm-‘, using the strong apodization function by Norton and Beer [29] and co-adding 16 scans. The intensities of the appropriate bands were measured as an integrated area, using the calculator function area of the instrument, which was set up to find the area between two wavenumber limits above a baseline

Photooxidation studies on branched polysilanes 83

[(Sic-hex),(SiPhz),]

/!

I I I I I I I I

4000 3.500 3ooo 2500 2000 1500 loo0

Wavenumber (cm-‘)

Fig. 2. Infrared spectra of freshly synthesized homo and copolymers.

%a

through these limits. The reproducibility of this procedure was checked by allowing a variation of 5 cm-‘, within those wavenumber limits, and variations in the area values were

-..-..-.- [(SiEt),(SiPh,),J

---- (SiPh),

.......’ [(SiPh),(SiMez),]

-.----- [(Sic-hex),(SiPhz),

- (SiPhMe),

250 300 350 400 Wavelength (nm)

Fig. 3. Solution UV-vis spectra of homo and copolymers.

found not to be greater than 3-5%, which represented the experimental error.

The IR method for estimating the SiOSi and SiOH content produced by the photooxidation of the polymer films consisted of the following.

(i) Determination of an integrated molar absorption coefficient of &H (SiOH) oscillators (EOH). For this purpose, solutions of either PhrSi(OHh or (HOSiMerhGH4 in acetone with concentrations between 2.20 and 4.50 g L-’ and a liquid IR cell with 0.120 mm of optical path, were used. These compounds were synthesized and purified by methods described in Refs [30] and [31], respectively. The values of c for both compounds agreed within the experimental error. The average <OH value was 3.25 x 10’ cm2 mol-’

(ii) Determination of integrated molar absorption coefficient of Si&O-Si oscillators (CSiOSi). For this purpose, solutions of polydimethylsiloxane oils in either acetone or CHKh with concentrations between 2.5 and 6.5 g L-’ and the same liquid cell described above were employed. Two polydimethylsiloxane oils were used: PDMS-1 (Dow Corning, Mn = 3420 g mol-‘) and PDMS-2 (Aldrich, Rn = 47200 g mol-‘). Again, the values of c for both polymers agreed within experimental error. The average G,OS, value was 2.65 x IO4 cm2 mol-‘.

(iii) Determination of SiOSi and SiOH band areas produced by irradiation of polymer films. For this purpose band areas of irradiated films were subtracted from

84 P. P. C. Sartoratto et al.

-..-----.- [(SiEt),(SiPhz),] ----- (SiPh),

..- [(SiPh),(SiMe&] -.-.-.-. [(Sic-hex),(SiPh&] _ (SiPhMe),

300 350 4cm Wavelength (nm)

450

Fig. 4. UV-vis spectra of the polymer films before irradiation.

those of non-irradiated ones. Taking into account film thickness, the increase in SiOSi and SiOH contents were calculated.

The method was also checked gravimetrically. A polymer film was photooxidized and the increase in weight due to oxygen incorporation was measured using a Mettler microbalance. The spectroscopic method agreed with the gravimetric measurements within 5%.

RESULTS AND DlSCUSSlON

Characterization of’ the polymers

Table 1 shows the characteristics of the homo and copolymers, together with the synthetic yields. Polymers were obtained with relatively good yields (SO-SO%) and were all very soluble in aromatic solvents, chloroform and THF. No insoluble fraction was obtained in the syntheses of [(SiEt),,(SiPhz),,] and [(SiPh),,(SiMe&], although the equivalent homo- polymers (SiEt),, [13], (SiPhJ, [lo, 321 and (SiMe?),, [I] present limited solubility. The homopolymeriza- tion of c-hexSiC1, usually results in very low yields of (Sic-hex),n (typically 17%) [17], but the copolymeriza- tion of c-hexSiClz with Ph,SiCl, was very successful, yielding 50% of copolymers.

A representative ‘H-NMR spectrum of the copolymers is shown in Fig. 1 for the [(Sic- hex),,(SiPh$]. Besides the broad signals of the

Table 2. Absorption coefficients of the polymer solutions and concentration of Si-Si bonds in polymer films

Polvmer

css: cssst x IO-4 U” x IO”

CL cm-’ mol-‘) (urn-0 (mol urns’)

[fSiEt),(SiPh&] 1.9 58 31 (SiPh),, 1.0 24 24 [fSiPhk(SiMe&] 1.2 20 I7 [(Sic-hexb,,(SiPhz),,] 0.89 I5 I7 (SiPhMe), 6.7 49 7

‘Integrated absorption coefficients in the 337450 nm of the polymer

UV spectra.

-m- 350 nm -- 320 nm --c 290 nm

0 l’d 20 30 40 50 60 70

Dose (J/cm’)

Fig. 5. Decrease of the UV absorption coefficients of the [(SiEt),,,(SiPhl),,] film. in the course of photooxidation, at

different wavelengths.

cycle-hexyl protons at 1.7 and 1.2 ppm and of phenyl protons in the 6.3-8.0 ppm region, a weak broad signal at 3.5 ppm was apparent. This was attributed to the methoxy protons of the ending groups. Determination of m/n ratio in the copolymers was carried out by integration of the ‘H-NMR signals for the phenyl and alkyl groups. The molar fractions of RSi and RzSi units corresponded fairly well with those of the monomer feed composition. The ratio of methoxy groups in relation to the total units (-OMe/Si) was determined from the integration of the signal corresponding to methoxy protons and varied from 0.18/0.82 for (SiPh),,, to O.OSjO.95 for [(Sic-hex),,,(SiPh,),].

As a general trend, the incorporation of RISi units was accompanied by a decrease in the amount of methoxy ending groups. In addition, the polymers with SiPh units showed higher contents of methoxy groups than those with SiAlkyl units. This behavior can be a consequence of differences in the reactivity of silicon<hlorine bonds within the growing polymeric structures.

Molecular weights (n;in) of the poly(phenylsilyne) and the branched copolymers were relatively low (Table 1). The degree of polymerization N was calculated considering the copolymer composition (m/n) and the amount of methoxy groups. It varied from 12 for the [(Sic-hex),,,(SiPhl),,] to 34 for the [(SiPh),,,(SiMe&,]. The higher degree of polymerization of the latter, together with the higher percentage of methoxy ending groups, can be indicative that the SiMeQ monomer did not favor cyclization reactions in the reaction conditions.

The amorphous state of polysilynes makes their structure determination more difficult. ?Si-NMR spectrometry should be a convenient technique to get some insight into the structure of these homo and copolymers, but the lack of resolution of signals and


structural rigidity prevents this technique from being very useful. The branched polymers and copolymers can be visualized as extended structures of polycyclic and cage organosilanes, which are composed of condensed rings [ 12-191.

Figure 2 shows the infrared spectra of freshly synthesized homo and copolymers. Besides the bands associated with the organic side groups, and methoxy groups (v SiOC at 1090cm-I), some particular features can be observed. All polymers showed a weak broad band in the 1000-1100 cm-’ region, associated with SiOSi groups, indicating that a small degree of oxidation occurred during the synthesis or polymer isolation. The degree of oxidation of freshly synthesized branched polymers depended on the method employed for quenching the Si-Cl ending groups at the final stage of the polycondensation reaction. The best results were obtained when the reaction mixture was filtered under inert atmosphere before the addition of the quencher. The degree of oxidation was also diminished by the long quenching reaction time. Another important feature is that only a very weak band at the 2100 cm-’ region, associated with Si-H groups, was observed in the spectra of freshly synthesized material. The formation of Si-H bonds can be a consequence of the

presence of anionic intermediates or of highly strained rings which do not survive after the methanol quencher addition.

All branched polymers suffered from an ageing oxidation process, even when they were protected from ambient light. After approximately 2 months, they are considerably oxidized, losing their yellow color. When the polymers were isolated by evapor- ation of their THF solutions, bright yellow resinous-like solids were obtained.

Solution UV-vis spectra of homo and copolymers can be seen in Fig. 3. The profiles for the different branched copolymers were very similar to those of the poly(phenylsilyne) and were characterized by monotonic decays of the absorbances from the 200-400 nm region. In addition, the absorption coefficients for Si-Si bond (tSIS,) above 300 nm were also very similar. This suggested that the distributions of chromophores (segments or assemblies of silicon atoms) resembled each other [lo, 201. The linear homopolymer (SiPhMe). presented a discrete c--6* transition band at 335 nm, and a phenyl n-a* band at 270 nm. In the UV spectra of (SiPh),. and (SiPh),,(SiMe&, the higher cSIs, values of the absorptions in the 250-300 nm region also have contribution of the phenyl ring n-n* transitions.

. I - I . I ’ I ’ I ’ I ’

Wavenumber (cm-‘)

Fig. 6. Infrared spectra of [(SiEt),(SiPh&] film in the course of photooxidation.


[(Sic-hex),(SiPh&]

4000

Fig.

Film characterization

The UV-vis spectra films can be seen in

3500 3000 2500 2000 1500 IWO

Wavenumber (cm-‘)

7. Infrared spectra of polymer films, irradiated at the saturatron dose.

SO0

of non-irradiated polymer Fig. 4, where absorption

coefficients (a), which were the absorbances over film thickness, are plotted against wavelength (2). Although the profiles of the spectra of the different branched polymers resembled each other, the x values are quite different, which suggested variations in the density of chromophores within the polymer films. The specific nature of chromophores in branched polysilanes was not very well established, but their density in a polymer film is, obviously, proportional to the concentration of polymeric molecules and, furthermore, to the concentration of silicon-silicon bonds. The density of molecules in a polymer film is an intensive property which characterizes each polymeric system and can vary mainly according to structure, intermolecular forces, packing and molecular weight. In these polymers, the photooxidation process occurs by silicon-silicon bond scission, with the incorporation of oxygen, the amount of which varies according to the amount of broken Si-Si bonds per molecule, as well as to the concentration of molecules in the film.

Therefore, the molar concentration of SiXi bonds (C,,,) in each film was estimated by spectroscopic measurements, by dividing the absorption coefficients x of the polymer films by the absorption coefficient eSIL of the polymers solutions. It was more realistic to use integrated absorption coefficients so that any variation on the profile could be accounted for, considering from 337 nm, which was the wavelength used for chromophore excitation, to 450 nm.

As can be seen in Table 2, the concentration of Si-Si bonds differed significantly. The branched polymers films (SiPh),,, and [(SiEt),,,(SiPh$,], which were richer in the trifunctional SIR units, presented the largest concentrations of Si-Si bonds, while the linear polymer film (SiPhMe),, showed the lowest concentration. The values of C,, which were calculated by this spectroscopic measurement agreed within 5% with the gravimetric method.

Thus, the general trend was that the more branched the polymer, the larger the density of Si-Si bonds in the film. The larger density of silicon-silicon bonds influenced the amount of oxygen incorporation in the photooxidation process, as will be shown in the next section.


Table 3. Increase in the siloxane and silanol contents in irradiated polymer films at the saturation dose

CS,OS, CSIOH Degree of X IO” X 10’5 Ratio oxidation’

Polymer (mol WI-‘) fmol pm-‘) CS,OS,/CS,OH 1%)

[(SiEt),,(SiPh&] 8.8 4.0 2.2 41 (SiPh),, 6.5 3.1 2. I 40 [(SiPh),,(SiMe&,] 4.3 2.6 1.7 41 [(Sic-hex),(SiPh&] 4.3 1.6 2.7 35 (SiPhMe),, 1.4 0.5 3.0 27

Talculated by the ratio Go,, + Cs,OH/CS,S,.

-*- [(SiEt),(SiPh,),] -I- (SiPh),

-o- (SiPhMe), “T

0 ;5 3b 4; bh I

75

Dose (J/cm*)

Fig. 8. Increase in the concentration of siloxane groups in the photooxidation of polymer films.

Film photooxidation

General results. The UV irradiation of the branched polymer films resulted in a progressive bleaching of their absorbances which saturated with a dose of approximately 40 Jcm-‘. Figure 5 shows the decrease of the absorption coefficients with the radiation dose for the [(SiEt),,(SiPh&,] in three wavelengths: 350, 320 and 290 nm. Except for the initial a values, all branched polymers showed similar graph profiles, indicating similar sensitivities. The linear (SiPhMe), needed a lower dose (15 Jcm-‘) for saturation of the absorbances, which may be a consequence of its higher absorbtivity at the irradiation wavelength. Furthermore, radical recom- bination may be more efficient for the branched structures than for the linear ones, which may account for the lower sensitivitv of the branched polymers in the experimental coiditions.

Figure 6 shows the infrared spectra of [(SiEt),,,(SiPh,).] film in the course of photooxidation. The intensity and width of the asymmetric SiOSi stretching band in the 1000-1100 cm-’ region increased progressively. In addition, another major change occurred in the O-H stretching region (3750-3000 cm-‘), where a very broad band ap- peared, attributed to SiOH groups. Silanol groups were present in all irradiated films (Fig. 7), and may be an indication that the branched polymeric structures were degraded to some extent in the photochemical oxidation process. This was a quite different result from that which has been described for poly(alkylsilynes) [12-1.5, 20, 211 and is mainly due to the higher stability of silanols containing a phenyl group, to the presence of methoxy ending groups and to irradiation conditions.

Estimation of siloxane and silanol contents. The silanol acetone solutions, used as silanol standards and described in the experimental section, produced OH bands with maximum at 3450 cm-’ with full width at half height (fwhh) of -280 cm-‘. This feature did not differ significantly from those obtained for the irradiated films which presented OH bands at 3400 cm-’ with fwhh of - 300 cm-‘. Thus, it was a good approximation to use the average value of LoHI obtained from these solutions, to estimate the silanol contents in the irradiated films.

The two polysiloxane oils in either acetone or CH2CIz showed SiOSi bands at about 1056 cm-’ with fwhh of -125 cm-‘. This feature was also very similar to the SiOSi bands observed for the irradiated films, whose maximum occurred at - 1060 cm-’ with fwhh of - 140 cm-‘. The average csios, used to estimate the siloxane contents in the irradiated films is indicated in the experimental section.

Taking into account the tSIOSl and coH values, together with film thickness, the increase in the concentrations of siloxane (C,,,,,) and silanol (C,,,,) in the course of polymer film photooxidation were estimated and can be seen in Figs 8 and 9. Some observations can be made from these figures.

-+-- [(SiPh),(SiMe2),] --A- [(Sic-hex),(SiPh& --o- (SiPhMe).

(i) The oxygen incorporation showed a relation- ship with the radiation dose that did not fit a pure exponential rise, which may be indicative of a process that is also dependent on oxygen diffusion, as has already been suggested by other authors [22].

I I . I . , f

15 30 45 60

Dose (J/cm*)

Fig. 9. Increase in the concentration of silanol groups in the photooxidation of polymer films.

(ii) Silanol was generated at the same time as siloxane, suggesting that both were formed via photochemical pathways. However, it seems that as the saturation dose was approached, silanol formation was favored. This could be rationalized by


considering that silanol may be formed from &i-O-O. radicals by hydrogen abstraction from organic side groups [33-351. However, at the final stages of the photochemical process, a low quantity of silyl radicals (&i.) is formed, which may inhibit rapid siloxane formation, then favoring hydrogen abstraction.

(iii) The more branched polymers, (SiPh),,, and [(SiEt),,,(SiPh&,], incorporated a greater amount of oxygen than the less branched ones. This behavior may be understood firstly considering the fact that the more branched polymer films presented a higher density of Si-Si bonds, as already discussed. The percentage of broken Si-Si bonds in the films, which can be approximately understood as the degree of oxidation, may have also influenced the total amount of oxygen incorporation. To verify the degree of oxidation of the polymer films at the saturation dose, the sum of C,,,,, and CsloH was divided by &. The results are summarized in Table 3. The degree of oxidation for the different branched polymers was very similar, varying from 35% to 41%. These results suggested that the greater amount of oxygen incorporation in the higher branched polymer films is mainly due to higher Si-Si bonds densities in these films. The linear (SiPhMe), incorporated a much lower amount of oxygen which may be associated with both lower density and lower degree of oxidation, which was 27%.

The ratio of siloxane to silanol groups formed at the saturation dose (Table 3) varied from 1.4 to 3.0, suggesting that the network structures of the branched polymers were not maintained after photooxidation.

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photooxidation studies on branched polysilanes

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