An isothermal model for high-speed spinning of liquid crystalline polymer fibers – coupling of flow, orientation, and crystallization

Abstract Experiments show high-speed spinning of many synthetic fibers is accompanied by the partial crystallization of the initially amorphous melt. This crystallization is induced by some combination of the extensional flow, molecular orientation and extension, and temperature dependence. The crystallization of the material couples back to affect the fiber rheology, through a process of stress-hardening. Recently Forest, Wang and Bechtel have derived isothermal 1-D equations from macroscopic approximations of Doi-type liquid crystalline polymer (LCP) kinetic equations; their equations model the coupling between the extensional flow and the molecular orientation. Here we extend that model to include a fully three-way coupling of flow, orientation and crystallization mechanics, adopting an Avrami crystallization law with an orientation-dependent rate constant as proposed by Ziabicki, and the Kikutani stress-hardening law. We show that there is a significant difference in the fiber diameter profile, orientation and crystallinity with the inclusion of the stress-hardening to the extensional flow. Of interest here also is the role of crystallization in localizing the drawdown into a so-called `neck' region. We illustrate important qualitative and quantitative effects due to the development of crystallization: a pronounced drop in the fiber radius, an increased rapid fluid acceleration, and enhanced molecular alignment with the fiber centerline axis. These three features all occur over the short orientation-induced crystallization lengthscale, and are isolated in the first 20% of the crystallizing region along the spinline for draw ratios typical of high-speed spinning.