(621a) Molecular Simulation of Crystallization of Chain Molecules from the Melt

The properties and performance of commercial polymers depend strongly upon both the molecular scale architecture of the chains and the manner in which they are processed to form engineering materials. Polyolefins in particular owe much of their utility to the spectrum of semicrystalline morphologies that can be realized. The development of semicrystalline morphology is controlled through processes of crystal nucleation and growth as the polymer is cooled from the melt, in either the presence or absence of process flows. Due to the small spatiotemporal scales involved, however, it remains a challenge to examine these processes directly and determine their underlying physical mechanisms.

In this talk, we describe the use of molecular dynamics and Monte Carlo simulations to characterize nucleation and growth of a new crystal phase from the polymer melt. Homogeneous nucleation is analyzed in terms of classical nucleation theory using a method based on mean first passage times (MFPT) of crystalline clusters of varying size. This MFPT analysis is then applied in two dimensions within layers near a crystal surface, to examine the so-called secondary (or surface) nucleation theory for chain molecules; within this analysis, crystal growth can be decomposed into sequential and simultaneous processes of surface cluster nucleation and spreading of the crystal cluster across the growth front. Finally, we introduce the effects of flow fields typical of polymer processes, and examine the phenomenon of flow-enhanced nucleation (FEN). In accord with experiments, substantial acceleration of the nucleation rate is observed as the system approaches a Weissenberg number (Wi) of unity. The acceleration is consistent with reduced entropy of the flow-oriented melt, but one also observes significant amplification of the rate pre-factor above Wi=1, indicative of enhanced transport effects. Application of these approaches to the study of crystallization of polymers under other conditions of contemporary interest appears promising, and may serve to shed new light on fundamental questions within the field of polymer crystallization in engineering applications.