Pyrolysis polysiloxanes

Another Py-GC/MS experiment was performed on polyacrylic /nfer-net-polysiloxane, a copolymer used as impact properties modifier. This is a polymer of butyl acrylate with low levels of allyl, methyl, and 3-(dimethoxymethylsilyl)propyl methacrylates interpenetrated with cyclic dimethylsiloxane. The copolymer has CAS 143106-82-5. The pyrolysis was done at 600 C in He similar to other experiments previously discussed. The pyrogram is shown in Figure 6.7.14 and peak identification is given in Table 6.7.10. 

The pyrolysate of polyacrylic-/nfer-nef-polysiloxane copolymer contains as main fragment molecules pyrolysis products similar to those of poly(butyl acrylate) and of poly(dimethylsiloxane (see Figure 6.7.8. and Section 16.1). The identification of fragments that would indicate sequences of other comonomers or any molecular connections between the two types of comonomer units was not possible. Other copolymers with acrylic acid as comonomer were studied using analytical pyrolysis. Among these are copolymers with special properties such as the copolymer with the formula shown below ... 

Pyrolysis of the formed polysilazane at 750° C generates several alkylsilanes such as dimethylsilane, trimethylsilane, ethyldimethylsilane, tetramethyidisiloxane, pentamethyidisiloxane, methylenebisdimethylsilane, hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane, etc. The presence of the cyclic compounds similar to those from poly(dimethylsiloxane) was an indication that some polysiloxane sequences may be present in the polymer. Thermal degradation studied between 350° C and 650° C showed the formation of some hydrogen, methane, ethane, and propene. 

Figure 4 X-ray diffraction (XRD) patterns of a polysilazane-polysiloxane hybrid polymer with respect to different pyrolysis temperatures.

Figure 4 shows weight loss curves for polycarbosilane, polysilazane, and polysiloxane, which are representative precursors for ceramics. The schematic mechanisms of each polymer pyrolysis are [32]... 

In some cases, cyclic and linear PDMS have been combined to form conetworks, and unsaturated cyclic side-chain fragments have been placed into polysiloxanes to make them thermoreactive. Interactions between ring polymers have also been analyzed and related to loops in chromatin. Also, some cyclic oligosiloxanes having polar end groups show liquid-crystal-line behavior, specifically smectic A and E phases. Finally, several PDMS cyclics have been exposed to vacuum pyrolysis and the products analyzed by matrix-isolation spectroscopy. The pyrolysis products obtained under a variety of conditions identified the radical reactions that were involved. 

Results presented in Fig. 20.20 show that polysiloxanes, obtained by ordinary sol-gel processing of TEOS, are also effective as nanometric size stabilizers. The role of the SiO covering in the stability of the SnO grains was discussed before. However, since the current interpretation of the phenomenon requires the absorption of hydrophilic species on the solid particles, the temperature treatment reduces the Si-OH population and favors the segregation of SnO this event parallels an important increase in the dimensions of metallic oxide crystallites. In contrast, pyrolysis of glucosidic moieties after treatment produces carboxylic groups able to coordinate metallic ions at the particle surface, and the related size stabilization, prolonged up to 600 °C, appears exclusive of starch. 

The degradation of linear siloxanes under conditions ranging from dry Michigan soils to onto- space continues to be a topic of great interest, both academically and industrially. The previous two reviews in this series have comprehensively covered the mechanisms and kinetics of thermolysis of linear polysiloxanes and little new work, on this aspect of siloxane degradation, appears to have been reported. The pyrolysis of resinous polysiloxane materials to give siUcon-containing ceramics is, however, an area of current interest and study. 

An SiC film formed by pyrolysis of polysiloxane gave IR preaks due to pSi-CHs vibrations in the range 790-810 cm". The FTIR spectra of silicon-carbon alloys synthesised by electron cyclotron resonance CVD include a band at 1180 cm", assigned as an SiC stretch, corresponding to an SiC bond order of 1.5. ° Raman spectra of Sii-yCy films formed by low-temperature CVD include a band at 607 cm", assigned to C atoms at Si substitutional sites. IR spectra of such films show a similar band, together with features at 475 and 810 cm 1312 methods were used to calculate the wavenumbers of substitu-... 

The methyl and phenyl groups of methyl phenyl polysiloxanes were determined as methane and benzene by alkaline pyrolysis gas chromatography [690]. Fig. 5.9 shows how this determination is performed. 

The presence of amorphous oxycarbide (Si-O-C) phase was also revealed in the Nicalon fibers at the interface between SiC core and surface SiOa layer (Pumpuck, 1980 Lipowitz, 1987 Laffon, 1989 Porte, 1989), although any thermodynamically stable compounds have not been found in the Si-O-C system. The formation of such an amorphous silicon oxycarbide phase was also suggested during the pyrolysis of organics-substituted polysiloxane gels to form SiC (Zhang, 1990 Babonneau, 1990 Bums, 1992). 

As for the effect of pyrolysis atmosphere on the conversion of polysiloxanes, vacuum was preferred to Ar atmosphere to develop the correct composition and microstructure in the case of preparation of oxycarbide coating films (Colombo, 1994). 

Fujimoto and co-workers [37] studied the pyrolysis products of polysiloxanes including their stereo and structural isomers. Separation was achieved by GC on a fused silica capillary column. Mass fragmentation data was obtained on pyrolysis products formed at 600 °G. 

Scheme 34. The pyrolysis pathway of a liquid-crystalline polysiloxane Reprinted from [a.302] with permission from Elsevier...

The ability of the complementary Py-GC-MS technique to differentiate between linear and branched polysiloxanes has been investigated. Erzin and co-workers [56] investigated the ability of Py-GC-MS to detect trace levels of silicone polymer in recycled paper, and to differentiate between linear and branched polysiloxanes. The mass spectrometer had a mass range of 100-700 m/z and was operated by single ion monitoring and by selected ion data collection to enhance resolution and detection. A pyrolysis temperature of 750 °C was used. Silicone polymer scraped from the backing sheet was used for calibration. Thermal desorption GC-MS at 225 °C was used for the analysis of volatile components. It was concluded that concentrations of parts per million, and possibly as low as parts per billion could be measured. 

Polysiloxanes. Siloxane rubbers are readily cured by heating in the presence of an appropriate peroxide and the technology is well-established (26). Indeed, this crosslinking chemistry has recently been extended into ceramic applications (27). Vinyl-substituted siloxanes were found to be very effective binders for SiC powder. With only about 1 wt% 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane (Lupersol 101), the [(MeSiOi 5)o.5o(PhSiOi 5)0.25 (CH2=CHMe2SiOo.5)o.25] polysiloxane cured to an infusible solid after one hour at 180 C (27). Subsequent pyrolysis gave P-SiC, and dense ceramic monoliths could be prepared using these polymer systems as SiC binders. 

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