Conformational Change of Phenyl Ring Side Group during Stress Relaxation in Glassy Poly(styrene- c o -acrylonitrile)

We investigated the stress and birefringence Δ n during stress relaxation in poly(styrene- co -acrylonitrile). The Δ n changed its sign from positive to negative during the stress relaxation with increasing relaxation time and temperature in the glassy region. The negative Δ n was found to increase when the stress was released and the specimen was cooled to room temperature. Such characteristic Δ n behavior is ascribed to three relaxation mechanisms consisting of distortion, orientation, and conformational change of the phenyl ring side group. The master curve of the Δ n was nicely explained by assuming that the distortion relaxation is expressed by Kohlrausch−Williams−Watts equation, the orientation relaxation by modified Rouse equation, and the conformational change by a stretched exponential relaxation function. The curve fitting procedures of the master curves revealed that the conformational change of the side group is much slower than the phenyl ring vibration (β relaxation); i.e., the rotational angle of the side group changes from 28° to 48° with a relaxation time of 0.15 s (at 105 °C) during the stress relaxation.