Polysiloxane copolymers degradation

The thermostability of siloxane-silazane copolymers of both random and block structure is found to be much higher (i.e. 100-200°C) with respect to polysiloxanes. This effect is brought about by introducing only a few silazane entities into the polymer chain. The reasons for the effect are not clear and the mechanism of thermal degradation of polysilazoxanes will require further experimental studies. 

Another important class of linear polysiloxanes are the siloxane copolymers. The replacement of methyl groups by other organic groups improves the specific physicochemical properties of PDMS, and one such improvement is in the area of resistance to thermal degradation, e.g., phenyl-containing polysiloxanes have superior thermal stability compared to PDMS. 

Radiation-induced Degradation.—There have been several reports on radiation effects in polymers,288 including single crystals,287 fluoropolymers,288 polyamides,289 polysiloxanes,270 polyethylene and its copolymers,271 polypropylene,272 polyolefins,273 polystyrene and its copolymers,274 poly(vinyl chloride) and related polymers,275 rubbers,278 polysulphones and other sulphur-containing polymers,277 polycarbonate,278 nylon,279 poly(vinylpyridines),280 and wool.281... 

Epoxies modified with butadiene acrylonitrile copolymers with (32) amine or carboxy end groups have disadvantages. They are susceptible to thermal and oxidative degradation and also have poor hot/wet properties, i.e., they soften when exposed to heat and moisture for long periods. Polysiloxane rubbers such as polydimethylsiloxane have been considered as a possible alternative. 

Figure 4 shows the H-NMR spectrum of the polystyrene that was obtained after the block copolymer was degraded by heating at 120°C with a solution of p-tol-uenesulfonic acid in toluene for four hr. The spectrum is almost devoid of polysiloxane resonances at 0 ppm. Control experiments with polystyrene demonstrated that polystyrene was not degraded by this treatment. 

Figure 4. 200 MHz H-NMR Spectrum of a Polystyrene Recovered After Degrading a Polysiloxane-Polystyrene Block Copolymer with p-Toluene-sulfonic Acid Hydrate.

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