Dynamic Mechanical Properties of Extruded Rods of Poly(dimethylsilylene) and Polysilane Copolymers having Methyl and Ethyl Substituents

Abstract

The oriented rods of poly(dimethylsilylene) (PDMS), poly(dimethylsilylene-co-methylethylsilylene) (PMMME) and poly(diethylsilylene-co-methylethylsilylene) (PEEME) were prepared by extruding the polymers through a circular tube die in the temperature range where the hexagonal mesophase forms. In general PDMS cannot be processed into materials below its decomposition temperature, but it was found that PDMS could be compression molded at 250°C under nitrogen atmosphere prior to the extrusion. The extruded polysilanes were characterized in detail by dynamic viscoelasticity and wide-angle X-ray diffraction in order to clarify the structure-property relationship. The temperature dependence of wide-angle X-ray diffraction showed that the crystalline phases of PDMS, PMMME, and PEEME were transformed into the hexagonal mesophase at elevated temperatures. The dynamic storage modulus of PDMS amounts to 7.6 GPa at room temperature and 11.2 GPa at liquid nitrogen temperature. The dynamic storage modulus increases with increasing extrusion ratio, and the increase of modulus with extrusion ratio is well correlated with the change of the crystal orientation function. The dynamic storage moduli of the extruded polysilanes were lowered with the rise in temperature owing to the structure relaxation processes, which lie in the lower temperature range. The decrease of modulus with temperature was more marked in PEEME than in PDMS, suggesting that the molecular motion of the ethyl substituents lowered the modulus at room temperature.