Materials World Network: Spatio-Temporal Development of Structure during Flow-Induced Crystallization of Polyolefins

California Institute Of Technology, Pasadena CA

Investigators

Abstract

This integrated program between the California Institute of Technology (Caltech) and the Technical University of Einthoven (TU/e) aims to establish predictive models of flow-induced changes in polymer crystallization kinetics and morphology. Polymers are essential players in the ongoing materials revolution. Semicrystalline polymers comprise over two-thirds of the annual production of all synthetic polymers and are in the midst of a renaissance, as recently developed metallocene catalysts allow new combinations of final properties through finer control of the molecular structure. Unfortunately, new synthetic capabilities take many years to deliver new products: extensive trial and error is required because structure-processing-property relations remain poorly understood. Processing alters the rate, form and anisotropy of crystallization: it can accelerate crystallization by orders of magnitude, and it has a profound effect on material properties, such as mechanical strength and gas permeability. This collaborative project will provide principles and computational models that are needed to enable design of macromolecules with processing dynamics in mind. Research at Caltech has revealed early events in flow-induced crystallization and their origin in the dynamics of the melt. Flow can open a kinetic pathway to nucleation, such that the rate of nucleation tracks the rate of molecular motion in the melt. Oriented precursors formed during flow template subsequent oriented growth; the distance between the precursors governs the time for completion of the oriented structure. To translate these new experimental findings into design tools that can be put into practice, corresponding advances in theory and modeling are needed. Therefore, this collaborative program links Caltech's experimental capabilities with what is widely regarded as the world's leading team in modeling polymer processing: the Eindhoven group led by Han Meijer. Their powerful capabilities to model structure development during processing will be extended to connect molecular specifications of the polymer with the kinetic parameters that describe the formation of crystallization precursors and the growth of crystallites on them. In turn, the expanded model will predict trends as a function of molecular parameters, which will be tested experimentally. Integrated modeling and experiment will elucidate the fundamental mechanism by which processing affects polymer structure development as a function of resin molecular characteristics and the imposed flow and thermal history. Technologically, models that incorporate fundamental knowledge of the molecular processes involved in flow-induced crystallization could revolutionize design of semicrystalline polymers and optimization of polymer processing. The Materials World Network award enables this coherent program of simulation and experiment, which will provide a new generation of design tools that will accelerate new product development in polyolefins the largest segment of the thermoplastics industry, consumed at a rate of over 70 million metric tons per year. This award is co-funded by the Division of Materials Research and the Office of International Science and Engineering