PREDICTION OF THE SUBYIELD EXTENSION AND COMPRESSION RESPONSES OF GLASSY POLYCARBONATE FROM TORSIONAL MEASUREMENTS

Modeling of the response of solidlike polymers is often difficult, not only due to the highly nonlinear behavior of the materials but also b ecause of the difficulty of obtaining relevant material data in the la boratory. Here, we examine the possibility of using concepts from fini te elasticity theory to describe the isochronal single-step stress rel axation response for a polymer glass (polycarbonate) far below its gla ss transition. Torque and normal force measurements from torsional str ess relaxation experiments are used to obtain isochoric values for the derivatives W-1 and W-2 of the strain energy density function in term s of the deformation invariants at specific time values (isochrones). The values of W-1 and W-2 are then used to determine isochronal values of the Valanis-Landel [Valanis, K. C. and R. F. Landel ''The St-ain-E nergy Function of a Hyperelastic Material in Terms of the Extension Ra tios,'' J. Appl. Phys. 38, 2997-3002 (1967).] (VL) function derivative s omega'(lambda) and to predict the tension and compression responses for different deformations lambda below yield. It is found that, for t he conditions examined, the experimentally obtained tension and compre ssion responses are well described within the VL framework, despite th e fact that polycarbonate is a compressible material. This success sug gests that the set of experiments required to describe the nonlinear b ehavior of glassy materials may be smaller than previously thought. Al so, volumetric measurements in the uniaxial deformations indicate a de nsification of the glass at large deformations and long relaxation tim es, which is consistent with concepts in the literature that invoke me chanically accelerated aging to describe mechanical and structural int eractions in the physical aging of glassy polymers.