Prediction of nonlinear long-term tensile creep of heterogeneous blends:: Rubber-toughened polypropylene-poly(styrene-co-acrylonitrile)

The proposed predictive format for the tensile compliance, D-b(t), of polymer blends is based on (1) a two-parameter equivalent box model (EBM) and (2) calculations of the phase continuity of components in blends by using modified equations of the percolation theory. The input data encompass (1) selected parameters characterizing the creep of parent polymers and (2) critical volume fractions v(1cr) and V-2cr of constituents in blends (delimiting the interval of phase cocontinuity). To describe the effects of time and stress on tensile compliance, a routinely used empirical equation was found suitable. Parameters characterizing the creep of parent polymers and their blends were extracted from generalized dependencies obtained through the superposition of time dependencies of tensile compliance assessed for a series of applied stresses and reconstructed with regard to the strain-induced free-volume expansion. Reconstructed dependencies can be approximated by a straight line, which simplifies the predictive format. The comparison of experimental and calculated compliance dependencies shows that the proposed format plausibly predicts the blend creep behavior over the studied interval of 0.1-10,000 min. The upswing in blend compliance after a long period of creeping is attributed to the redistribution of stresses acting on the constituents in the course of the creep under a constant load. This format is believed to have allowed the researchers in this study to anticipate (at least semiquantitatively) the creep behavior of intended blends. (C) 2003 Wiley Periodicals, Inc.