Evolution of chain entanglements under large amplitude oscillatory shear flow and its effect on crystallization of isotactic polypropylene

Abstract

The effect of large amplitude oscillatory shear (LAOS) flow on isotactic polypropylene (i-PP) chain entanglement-disentanglement transition and on the subsequent crystallization behavior was studied by dynamic rheology and polarized optical microscope (POM). i-PP with reduced entanglement density was generated through the application of LAOS flow at 180 °C. The re-entanglement process of disentangled chains was in-situ monitored by time-sweep rheological measurements. Interestingly, the re-entanglement kinetics was substantially slower than expectations based on the linear viscoelastic relaxation time of the fully entangled melt. Moreover, with the adjustment of shear conditions (strain amplitude, frequency and temperature), the entanglement density could be effectively modified. Disentangled chains exhibiting less topological constraints played a distinct role in the subsequent crystallization process. The nucleation density and growth rate of spherulites increased with reducing the entanglement density, resulting in a faster overall crystallization kinetics, compared to that in the fully entangled melt. It is worth to note that LAOS flow could produce a less disentangled melt state, and has a correspondingly weaker influence on the subsequent crystallization behavior, with respect to steady shear flow.