Measuring Flow-Induced Crystallization Kinetics of Polyethylene after Processing

The flow-induced crystallization of polymers is a ubiquitous aspect of processing where the effect of flow can dramatically alter end-use properties of polymers. Here, we will describe the development of a new methodology whereby the effect of flow on crystallization kinetics in polyethylene can be determined after processing has occurred. This is the first illustration for rapidly crystallizing polyethylene, which required a different approach than what has been done for other materials. Critically, this approach necessitated an understanding of the self-nucleation and melt memory behavior as well as using thermal contact fluids with low kinematic viscosity. With an optimized protocol, linear low-density polyethylene, LLDPE, can be heated to temperatures just above the melting point of the crystals using fast-scanning calorimetry, and memory of the flow history can be maintained even in the melt. As a result, the polymer can be melted many times without significantly changing the flow-induced crystallization kinetics, which enables measurement of the kinetics over a wide range of isothermal temperatures. We demonstrate the ability to measure the flow-induced enhancement of crystallization kinetics on samples sheared using a parallel-plate rheometer or fabricated as a blown film. For the LLDPE studied here, the isothermal kinetics were four times faster, relative to quiescent crystallization, for samples prepared at the highest shearing rates. As the shear rate decreased, the enhancement in the isothermal crystallization rate decreased. A particularly powerful aspect of the developed methodology is that it can also be used on as-made parts, and we report a twofold enhancement of crystallization kinetics for films manufactured on a pilot-scale blown-film line.