Photoelastic Properties of Polystyrene in the Glassy State. III. Styrene Derivatives and Copolymers

Abstract

The stress-optical coefficient, Young's modulus, and strain-optical coefficient at room temperature (27°C) have been determined for several styrene derivatives, for two copolymer systems (styrene/α-methyl styrene and styrene/methyl methacrylate) over the entire composition range, and for several copolymers of styrene with acrylates and methacrylates of varying composition. The results indicate that steric effects, particularly in the immediate neighborhood of the chain backbone, have a very important influence on the nature of the molecular motions (and, consequently, on the photoelastic behavior) during glassy distortions. The stress-optical coefficient becomes more negative (phenyl rings less free to orient) as a result of α-substitution on the chain or ortho-substitution on the phenyl ring. Substitution in the para-position of the phenyl ring, or increasing the separation of the phenyl ring from the chain backbone makes the stress-optical coefficient more positive (phenyl rings more free to orient); plasticizers (including residual monomer and solvents) or decreasing molecular weight have an effect in the same direction. Copolymer systems show nonideal mixing, photoelastically, indicating that interaction effects between neighboring monomer units along the chain are of significant magnitude. This may provide a means of studying the nature of the monomer sequences in copolymers (e.g., of block and graft copolymers), and also of observing stereospecific structure in polymers.