Extensional-flow-induced Crystallization of Polypropylene

Abstract

A filament stretching extensional rheometer was used to investigate the effect of uniaxial flow on the crystallization of polypropylene. Samples were heated to a temperature above the melt temperature to erase their thermal and mechanical histories. The Janeschitz-Kriegl protocol was applied and samples were stretched at various extension rates to a final strain of e = 3.0. Differential scanning calorimetry was applied to crystallized samples to measure the degree of crystallinity. The results showed that a minimum extension rate, corresponding to a Weissenberg number of approximately Wi = 1, is required for an increase in percent crystallization to occur. Below this Weissenberg number, the flow is not strong enough to align the tubes of constrained polymer chains and as a result there is no change in the final percent crystallization. An extension rate was also found for which percent crystallization is maximized. The increase in crystallinity is likely due to flow-induced orientation and alignment of tubes of constrained polymer chains. Polarized-light microscopy verified an increase in number and decrease in size of spherulites with increasing extension rate. Small angle X-ray scattering showed a 7% decrease in inter-lamellar spacing at the transition to flow-induced increase in crystallization. Crystallization kinetics were examined by observing the time required for melts to crystallize under uniaxial flow. The crystallization time decreased with increasing extension rate, even for extension rates where no increase in percent crystallization was observed. These results demonstrate that the speed of crystallization kinetics is greatly enhanced by the application of extensional flow.