The protective effect of phenyl group on the crosslinking of irradiated dimethyldiphenylsiloxane

The effect of 60Co γ-radiation on copolymers of polydimethyldiphenylsiloxane (PDMDPS) and mixtures of polydimethylsiloxane (PDMS) and PDMDPS has been studied. The molecular weight, determined by light scattering, increased with the absorbed dose, then gelation occurred and eventually at very high doses a brittle glass-like material was formed. Copolymers and mixtures containing the same amount of phenyl content have shown almost identical behaviour and G-values determined from light scattering and solubility measurements have been found in excellent agreement. Aromatic substitution reduced remarkably the crosslinking efficiency and the protection coefficient has been found to increase almost exponentially with increasing phenyl concentration. Previously published data on polyisobutylene-styrene copolymer (degraded system) and on solutions of polyvinyl alcohol-thiourea (crosslinked system) have been analysed and they have shown similar changes of the protection coefficient with mol concentration of the protector (styrene or thiourea). It was estimated that this phenyl protection was extended over 6 monomer units of dimethylsiloxane (DMS) for low phenyl concentrations up to 10 mol% approximately; then the degree of protection decreased continuously and at 40 mol% phenyl concentration was extended over two or three monomer units of DMS. The results were discussed in view of a new mechanism of estimating the protection efficiency, taking into account the overlapping of the protection ranges of the protector. (author)