Synchrotron Studies and Computational Modeling of Flow-Induced Polymer Crystallization in Shear and Extensional Flow

Northwestern University, Evanston IL

Investigators

Abstract

This grant provides funding to support an experimental and computational study of the phenomenon of ?flow-induced polymer crystallization?, referring to the profound impact of processing flows on the rate of solidification. The experimental portion of this work utilizes new capabilities for x-ray scattering studies of polymer structure in well-defined uniaxial extensional flows which involve pure stretching deformations with no superimposed shear. X-ray scattering experiments will be performed at the Advanced Photon Source at Argonne National Laboratory, taking advantage of high brilliance radiation and fast detectors to monitor development of crystallinity and the degree of crystallite orientation as a function of temperature and flow conditions (deformation rate and total applied strain) in several semi-crystalline polyolefins, such as polyethylene, polypropylene and poly(1-butene). The experimental program will also include exploratory studies of polymer crystallization during injection molding. In the computational portion of the work, these data will be used to test and guide refinements of existing models of flow-enhanced nucleation and crystallization. If successful, the results of this research will lead to improved ability to rationally predict the processing of semi-crystalline polymers in flows where extensional (stretching) deformations play an important role. These include high volume processes such as fiber spinning, film blowing and blow molding. By comparing the nature of flow-induced crystallization in both extensional and shearing flows, this work will also provide insights into the fundamental molecular processes that underlie accelerated polymer crystallization during flow. For instance, one class of models is built upon the assumption that flow-induced chain stretch, predicted by molecular rheological models, is the crucial variable governing accelerated nucleation. This hypothesis will be tested directly by designing shear and extensional flow protocols to produce similar molecular stretch using very dissimilar flow types. Finally, this work serves as a prototype for how powerful in situ experimental studies of materials processing may be closely coordinated with process modeling.