Modeling and optimization of flow-induced crystallization in polymer processing

Scope and method of study. Variations in polymer processing can impart different polymer microstructures that can lead to different physical properties. The configuration of the polymer chain determines the bulk strength. The effect of a flow field on the molecular configuration was modeled using techniques based on kinetic theory of polymers. A dumbbell model which consisted of two beads joined by a weightless rod was used to describe the polymer molecular configuration. The only forces acting on the bead that were considered were the hydrodynamic force and the force due to Brownian motion. A generalized planar flow field was considered to generate the hydrodynamic force. Once the polymer configuration was obtained, the shape of the die was optimized based on the molecular orientation. The Successive Quadratic Programming by the Han-Powell (SQPHP) method was used as optimization algorithm.; Findings and conclusions. A molecular model based on polymer kinetic theory was successfully developed for predicting the steady state configuration of polymer molecules in a generalized flow field. The behavior of the model for various flow fields was studied using the Weissenberg number as a parameter to characterize the flow field strength. The model predictions were also compared with results from x-ray diffraction experiments using an orientation parameter as the basis of comparison, and the results were in very good agreement. A die optimization technique was developed which combined finite element analysis, the molecular model and the SQPHP method. The optimization technique was tested using a planar diffuser without the molecular model and the results obtained were qualitatively consistent with published results. The optimization technique was then used to generate an optimum die geometry for a given degree of anisotropy using the molecular model. An optimum die geometry was obtained based on the chosen objective function for different values of pressure drop.