Flow-induced crystallization and birefringence development of semicrystalline polymers in injection molding and fiber spinning process

A novel approach for the numerical simulation of crystallinity and frozen-in birefringence in moldings and fibers of semicrystalline polymers was proposed. The approach was based on the calculation of elastic strain that became frozen when the flow-induced crystallization occurred. The flow effect on the equilibrium melting temperature elevation due to the entropy reduction between the oriented and unoriented melts was incorporated to model crystallization. To find frozen-in elastic recovery during crystallization and entropy change, a nonlinear viscoelastic constitutive equation was used. From the ultimate elastic recovery the crystalline orientation function was calculated. The crystalline and amorphous contributions to the overall birefringence were obtained from the crystalline orientation function and the flow birefringence, respectively. The skin layer thickness and birefringence profiles were predicted and compared with experimental results available from earlier studies in moldings of polypropylenes of various molecular weights obtained at various melt temperatures, holding pressures, holding times, injection speeds and wall temperatures. Crystallizaion kinetic studies were carried out of two polyesters (PBT and PEN) and parameters required for process simulation were obtained. The temperature, diameter, density and birefringence profile in both low- and high-speed spun PET fibers were predicted and compared with the experimental data from literature. Theoretical predictions in moldings as well as fibers were found to be in good agreement with the experimental data.