Onset of Flow-Induced Crystallization Kinetics of Highly Isotactic Polypropylene

Abstract

Brief intervals of shear prior to a temperature quench accelerate crystallization, resulting in much smaller spherulites. Crystallization kinetics of five commercial linear isotactic polypropylenes were investigated, using a rheometer to impose shear and monitor crystallization after quenching. Shear and quench temperatures, shear rate, and duration were all systematically varied. The crystallization rate increases with increasing applied work, up to a value independent of undercooling beyond which the rate remains constant. This saturation is consistent with a maximum number of nuclei, possibly set by the concentration of heterogeneous impurities. The crystallization rate likewise increases with increasing shear rate, saturating at about 1 s–1 for all grades studied. Only chains in the high molecular weight tail, above about 104 kg/mol, are stretched at this shear rate. Faster crystallization after shear was observed for grades with lower isotacticity. Flow-induced crystallization persists even when shear is applied well above the equilibrium melting temperature (187 °C), finally weakening above the Hoffman–Weeks temperature (210 °C), perhaps because flow-induced precursors are no longer metastable.