Molecular simulation of polyolefin crystallization: Nucleation Phenomena

The properties and performance of polyolefins depend strongly upon both the molecular scale structure of the chains and the manner in which they are processed to form engineering materials. Much of their utility derives from 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 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 examine surface nucleation and spreading upon surfaces of foreign materials, in an effort to illuminate the design criteria for effective nucleating agents (NA). We envision a “materials genome” of NAs, and use high throughput simulation to screen entire classes of materials for NA efficiency. Lessons learned from the families of diamond-like and 2D, graphene-like materials will be discussed.