Molecular Dynamics Modeling of the Crystal-Melt Interfaces and the Growth of Chain Folded Lamellae

Abstract

The molecular mechanism of polymer crystallization is one of the most difficult problems and has defied innumerable efforts to understand the process over the last fifty years in spite of its great importance both from the academic and the industrial point of view. We have been studying this historical problem by use of the molecular dynamics simulation method. In this chapter of the book, we review our recent work on the crystal growth of polymers with special focus on polymer behavior at the crystal surface, either at crystal-vapor or crystal-melt interfaces. Our starting molecular model is a bead-spring chain, or a wormlike chain, made of methylene-like united atoms; the zigzag structure of polymethylene is here neglected in order to accelerate crystallization. We proceed with stepwise revisions of the model toward the realistic modeling of polymer crystallization from the dense melt. We start our discussion with the crystallization of polymers on a two-dimensional surface, which is a model of the chain strongly adsorbed on the growth surface. Then we treat the three-dimensional process of crystallization of a single chain from a vapor phase: the adsorption to and the ordering on the growth substrate. Lastly, polymer crystallization from the dense melt is investigated. We also report on fiber formation from a highly oriented amorphous state. Various important issues concerning the molecular mechanism of polymer crystallization are discussed in the light of findings from our direct molecular simulations.