Residual stresses and birefringence in rapidly quenched glassy polymers

In a basic study of the nonequilibrium glassy state, two related areas have been investigated. The first dealt with homogeneous materials and included work on volume aging, and the temperature dependence of the stress and strain optical coefficients and the modulus. The second dealt with nonhomogeneous materials and included the measurement of the frozen-in birefringence in large quenched plastic samples and the correlation of this residual birefringence, and also the stress, within the framework of residual stress theory. The material used throughout the study was a commercial polystyrene.

Volume aging was measured on small, homogeneous samples quenched from 120°C at various rates by measuring density versus time in a density gradient column. The free volume comcept was incorporated into a first-order rate equation which was used to predict the volume-temperature-time behavior.

The stress and strain optical coefficients and the elastic modulus were measured as a function of temperature. The stress optical coefficient is -4800 Br above the glass transition temperature, changing quickly at that temperature to a small positive number (+10 Br at 20°C). The elastic modulus (Young's modulus) is 3.3 GPa, typical of a glass, for temperatures up to 80°C and then drops rapidly to a rubber-like value.

Nonhomogeneous samples were prepared by quenching large slabs into ice water and room air. The transient temperature profiles were measured and correlated with standard heat transfer theory. The birefringence, being an indirect measure of stress, was measured through the thickness of the quenched slabs. In both quenching cases, the birefringence indicated compressive stresses near the surface and tensile stresses in the center of the slab. A simplified form of the residual stress theory of Lee, Rogers, and Woo [J. Am. Cer. Soc., 48, 480 (1965)] was used to predict the stress and birefringence profiles based on the quench conditions. For the birefringence, a simple temperature-dependent strain optical coefficient proved inadequate and it was necessary to use an effective temperature (based on the free volume concept) to specify the temperature at which the material properties were calculated. Using the strain optical coefficient at that temperature, a satisfactory correlation of the birefringence profiles was obtained.