Rheological modeling of flow-induced crystallization in polymer melts and limitations on classification of experiments

A model has been developed, which provides a coupling between the kinetics of flow-induced crystallization and the relaxation behavior of the high molecular weight chains in a polymer melt. Stretch of the backbone tubes of these chains is found to be the key factor in the formation of nucleation precursors. Orientation is not sufficient. This is demonstrated by combining simulations and experiments with different materials, under varying flow conditions and at different temperatures. Saturation of the number density of flow-induced precursors and resulting nuclei, which is an essential element of the model, is also observed in experiments. The saturated number density is shown as a function of characteristic Weissenberg numbers, based on the reptation time and on the Rouse time of the high molecular weight chains. In some experiments, a large effect of flow is observed while the Weissenberg number based on the Rouse time is smaller than unity. This apparent disagreement with the model is discussed, showing that classification of flow-induced crystallization experiments based on Weissenberg numbers is difficult, if not impossible. When longitudinal growth of precursors is included, the saturation part of the model should be reformulated, depending on whether only chains of high molecular weight or chains of all molecular weights are involved in the growth process. Based on rheological measurements on crystallizing melts, the latter appears to be the case.