Formation of chain-folded structures in supercooled polymer melts examined by MD simulations

We report the first computer experiments with supercooled polymer melts of chains long enough to form chain-folded structures. These structures resemble the lamellae of polymer crystals, and the inverse of the lamellar thickness is related linearly to the crystallization temperature as found in experiments. The small- and wide-angle scattering intensities start growing at about the same time. However, the wide-angle scattering signal is very weak at the beginning. We use molecular dynamics (MD) simulations with a coarse-grained polymer model for chain lengths of N = 100 monomers. The model consists of spherical beads connected by harmonic springs with an additional angle bending potential reflecting the torsional states of the underlying atomistic backbone. It demonstrates that chain stiffness alone without an attractive interparticle potential is a sufficient driving force for the formation of chain-folded structures. We discuss the evolution of structure in the homogeneous nucleation regime as well as the melting process. The influence of the finite size accessible in direct MD simulation of polymer crystallization is discussed critically