Configuration selection in the simulations of the crystallization of short polyethylene chains in a free-standing thin film

The configuration of thin films of short polyethylene chains during the crystallization has been investigated using a recently developed Monte Carlo method on a high coordination lattice, which bridges the coarse-grained and the fully atomistic simulations. Thin films select their own crystal configurations, in which the chains have different orientations with respect to the surface normal. Two kinds of crystal structures, with and without grain boundary in the center of the thin films, in which all the chains are parallel to the free surface, have been found after a deep quench from an equilibrated thin film above the melting temperature in the previous simulations [J. Chem. Phys. 116, 2277 (2002)]. However, another crystal configuration shows up after a quench from the thin film formed by increasing the periodic boundary conditions in one direction from a homogeneous melt. In this configuration all the chains are parallel with each other and to the z axis. This configuration was found both in the experiments and in the previous molecular dynamics simulation of a free-standing thin film of a short alkane. It is demonstrated here that the initial configuration does affect the final structure in the simulation of the crystallization of the free-standing thin film. The current configuration has slightly lower enthalpic and entropic energies and higher melting point than the previous ones. The similarities and differences between three configurations during the crystallization, annealing, and melting processes will be discussed. (C) 2003 American Institute of Physics.