Synthesis and radiation stability of silicone elastomers.

A wide range of polymer samples based upon polydimethylsiloxane (PDMS) have been subject to gamma irradiation, and the subsequent effects analysed using a variety of techniques. The preparation of a series of blends and block copolymers of PDMS containing small amounts of polystyrene (PS) is described, and their characterisation by spectroscopic and thermal analytical methods is discussed.A PD11S gum gave a G(X) value of 2.8 which is in good agreement with other reported values. Thermal analysis revealed a shift of 20°C. in the glass transition temperature, and disappearance of the exotherm band, upon onset of gelation. Within the sol component of a cross-linked sample, a complex range of reactions are evident which are related to the absorbed dose. It was found with PS-PDMS blends, upon absorption of low doses of radiation, that the amount of gel produced is very much lower than that observed with PDMS. A substantial degree of radiation protection was observed with a 3% W/w PS blend, which required a gelation dose of almost five times that of homopolymer PDMS. A selected number of block copolymers were irradiated and the gelation dose was again found to be far greater than would be expected of PDMS of similar relative molecular mass. Morphological studies allowed calculation of the size of the PS average domain size which increased with % w/w PS in the blend or copolymer. It is proposed that the radiation protection observed with polymers containing PS is related to the PS average domain size. The large surface area/volume ratio found with a 3% w/w PS blend would facilitate a considerable degree of miscibility of PS with PDMS, thereby decreasing the susceptibility of PDMS to cross-link formation.