BIREFRINGENCE OF AMORPHOUS POLYMERS .4. LARGE DEFORMATION OF POLYSTYRENE NEAR ITS GLASS-TRANSITION TEMPERATURE

The stress, f, and the birefringence, DELTA-n, were measured for polystyrene at 98-degrees-C under large deformations; after an instantaneous elongation (0.6-2 %) and during an elongation at constant speed (initial rate of elongation = 5.5 X 10(-4)-2 X 10(-3) s-1). The stress-optical rule was not valid. The data were analyzed in terms of the modified stress-optical rule, in which each of f and DELTA-n was assumed to be a sum of two terms and the stress-optical rule was assumed valid separately for each set of components: f = f(R) + f(G) and DELTA-n = C(R)f(R) + C(G)f(G), where C(R) and C(G) are material constants. The slowly relaxing component of Young's relaxation modulus, E(R) = f(R)/e, was independent of the strain, e, while the rapidly relaxing component, E(G) = f(G)/e, decreased with strain. At a constant speed of elongation, one component of stress, fR, increased with time and was close to that evaluated from E(R) with linear viscoelasticity theory. The other component, f(G), exhibited a marked overshoot and then decreased with time. This component was much smaller than that derived from E(g) through a linear viscoelasticity relation. The yield phenomenon at a constant speed of elongation is due to the behavior of the G component, and the slow increase of stress at long times is due to the increase of the R component, presumably associated with the deformation of polymer segments.