Tensile yield-stress behavior of poly(vinyl chloride) and polycarbonate in the glass transition region

The yield-stress behavior of two glassy polymers is studied through the glass transition region over a wide range of strain rates. For temperatures below the glass transition temperature, the yield stress behavior could be described as a non-Newtonian flow in agreement with Eyring's theory, if one excepts a narrow range relating to the slowest strain rates. For temperatures above T_g, the yield-stress behavior is still nonlinear but fits the relations based on the concept of free volume.