Molecular dynamics modeling of polymer crystallization from the melt

Molecular pathways to polymer crystallization and the structures of crystal-melt interfaces are investigated by molecular dynamics simulation. We adopt a simplified molecular model for polymethylene-like chains; the chain is made of CH2-like beads connected by harmonic springs, and the lowest energy conformation is a linear stretched sequence of the beads with slight bending stiffness being imposed. Two molecular systems are considered, one is made of 640 chains of C100 and the other is made of 64 chains of C1000, both being placed between two parallel substrates that represent the growth surfaces of the lamellae growing toward each other. The initial melt kept at a sufficiently high temperature above the melting point is rapidly cooled down to various crystallization temperatures, and the molecular processes of crystallization that follow are investigated. In both systems, we clearly observe the growth of stacked chain-folded lamellae from the substrates. The growing lamellae have a definite tapered shape, and they show marked thickening growth along the chain axis as well as usual growth perpendicular to it. The overall crystallization rate is found to be very sensitive to the crystallization temperature, showing an apparent maximum around 320–330 K for C100. We find that the lamellae do not grow keeping pace with each other but grow in independent rates especially at higher temperatures. We also examine the structures of the lateral growth surfaces and find that the growth surfaces are locally flat and the Kossel mechanism of crystal growth seems to be operative. In addition, the fold surfaces are found to be covered with relatively short chain-folds; at least about 60–70% of the folds are connecting the nearest or the next nearest neighbor crystalline stems. No appreciable bond orientational order is found in the undercooled melt of C100 and C1000.