Modeling Flow-induced Crystallization
Rong Zheng, Peter Kennedy
Moldflow
Australia

Keywords: flow-induced crystallization , orientation , rheological behaviour

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We have attempted to model flow-induced crystallization (FIC) of polymer based on crystallization kinetics approximated by the Kolmogoroff-Avrami theory which has been modified with an orientation-dependent Avrami exponent and a differential equation for free-energy-dependent nucleus generation. A two-phase microstructural model is used to calculate the change in the free energy of the flowing melt and the orientation of the semi-crystalline. The FIC model has been illustrated by numerical examples for crystallization of an isotactic polypropylene subjected to short-time shear treatments. Results show:
(1) Both shear rate and shearing time enhance crystallization rate. Given the same total shear strain, with different combinations of shear rates and shearing times, the combination of high shear rate with short shearing time was found to be most effective in accelerating crystallization.
(2) Upon cession of shearing, the number of activated nuclei per unit volume is not reduced to the quiescent value instantaneously. It can be “frozen” in place or slowly relax, depending on the given relaxation time.
(3) The predicted crystallinity-dependent viscosity and PVT behavior show that the solidification occurs at higher temperatures for sheared melts than that takes place under quiescent conditions.