Mechanical properties of polysiloxanes: 1. Poly(tetramethyl-p-silphenylene siloxane) homopolymer and tetramethyl-p-silphenylene siloxane/dimethyl siloxane block copolymers

The mechanical behaviour of poly(tetramethyl-p-silphenylene siloxane) poly(TMPS)] homopolymer and random block copolymers of tetramethyl-p-silphenylene siloxane-dimethyl siloxane poly(TMPS-DMS)] of mean DMS block of 12 monomer units have been investigated over a wide range of composition and temperature using a Rheovibron viscoelastometer. The compositions ranged from TMPS/DMS wt % ratio of $\text{100}\text{0}$ to $\text{30}\text{70}$. The temperature intervals spanned from just above ?120°C to the point where melting became evident as dictated by the molecular architecture of each system. Two amorphous relaxation transitions, corresponding to DMS and TMPS phases, were found for copolymers with high TMPS content (?80 wt %). At lower TMPS compositions (?50 wt %) these transitions merge together. All dynamic transitions are a function of composition and crystallinity. The ‘hard’ TMPS phase provides crosslinks and acts as filler for the rubbery amorphous phase. The percentage elongation under tensile loading increases with the DMS amorphous content, which parallels an increase in clarity and decrease in density and crystallinity. A morphological model which depends on composition is proposed for these polysiloxanes. The model advanced is consistent with other physical evidence derived from other techniques. Changes in mechanical behaviour parallel changes in specimen morphology.