Polymer composition versus low‐temperature characteristics of polysiloxane elastomers

Abstract

Abstract Gehman cold‐flex, volume‐temperature, and stress‐temperature studies were used to investigate the low‐temperature characteristics of polysiloxane copolymers with respect to copolymer composition. A complete range of phenyl methyl‐dimethyl siloxane copolymers were examined as elastomeric compounds. Like other copolymers, the stiffening temperature was dependent upon the composition of the copolymer. Stiffening of the elastomers was due to crystallization in some cases and second‐order transition in others. Copolymers having either low or high amounts of PhMeSO stiffened because of crystallization, while copolymers of intermediate composition (7.5 to 15 mole‐% PhMeSiO) stiffened because of nearness to their respective second‐order transition temperatures. Second‐order transition temperature increased with the PhMeSiO content. For those siloxane polymers and copolymers that crystallized, the process was rapid when compared with crystallization behavior of other polymer systems. Only one minimum low‐temperature stiffening point was found in the phenyl methyl‐dimethyl siloxane copolymer system. The unique low‐temperature characteristics of polysiloxane elastomers are explainable on the basis of three factors. These are: ( a ) very flexible molecules, ( b ) low temperature coefficient of viscosity over a broad temperature range including low temperatures, and ( c ) copolymerizability of dimethyl siloxane with other siloxanes containing bulky pendant groups which lower the stiffening temperature.