The effect of molecular weight distribution on the optical properties of polystyrene in the transition region

Birefringence measurements during relaxation of stress have been made on five narrow molecular weight distribution (anionic) polystyrenes at temperatures of 91.5, 95.0, and 98.5°C. One broad distribution (isothermal) polystyrene was measured at 91.5°C. The birefringence vs. log time curves obtained are observed to pass through a maximum, the magnitude of the maximum being independent of molecular weight above M̄w = 154,000 and increasing with temperature at constant M̄w. Increasing molecular weight heterogeneity causes the birefringence decay curves to shift to later times and broaden slightly, and the magnitude of their maxima to decrease. Decay data previously obtained on a plasticized commercial polystyrene are presented and shown to be in agreement with present findings. No substantial change in energy of activation associated with the decay process (Ea = 150 kcal./mole) is observed with either broadening of molecular weight distribution or with plasticization.