The stress‐strain‐birefringence properties of styrene‐butadiene block copolymers

Abstract

Abstract The stress‐strain‐optical properties of three elastomeric styrene‐butadiene block copolymers containing 31, 40, and 49 wt‐% styrene were studied as a function of temperature. The mechanical and the optical properties indicate that these materials are two‐phase systems in which the polybutadiene chains form an elastomeric phase and the polystyrene a glassy phase with the latter providing physical crosslinks. Birefringence measurements indicate that the decrease in modulus and strength of these materials is associated with a decrease in the concentration of elastically effective network chains. The independence of the stressoptical coefficient on temperature suggests that the decrease in concentration of elastically effective chains is not due to the onset of rubber‐like behavior of flow within the polystyrene regions themselves, at least for temperatures below about 70°C. Rather, the decrease seems to be associated with the increased mobility of the polybutadiene chains at higher temperatures which also leads to an increase in the rate of stress relaxation. Birefringence measured during extension and retraction showed that the stress‐strain hysteresis is due to restricted mobility of the polybutadiene chain segments rather than to permanent viscous flow or to a change in the effective network structure of the block copolymers. The ultimate properties of these rubbers correlated well with the effective network structure in undeformed samples.