COMPUTER-SIMULATION OF THE CRYSTAL-MELT INTERFACE IN N-ALKANE WITH IMPLICATION FOR POLYMER CRYSTALLIZATION

A crystal of n-alkane interacting with an environment phase, dense vapour or liquid, is investigated by Monte Carlo simulation; the molecular adsorption process and the structure of the crystal/melt interface are simulated. The crystal is made up to 441 molecules bounded by typical low-index surfaces {110} and {100}; it is surrounded by an amorphous environment phase of 520 molecules. Each molecule is assumed to be rigid and to undergo rotation around and translation along the molecular axis as well as transverse displacement. A reference lattice restriction is imposed for the crystal, while an off-lattice model is adopted for the environment. Remarkable disorder is found at the surfaces of the crystal especially on the {100} surface. The surface disorder is found to be much enhanced by the presence of the environment molecules interacting with the crystal. At the same time, an appreciable order in the melt is observed near the {110} surface. These observations suggest that the excess entropy for the {100} surface is appreciably larger than that of the {110}. The relative magnitudes of the surface energies for the {110} and the {100} surfaces are therefore expected to interchange at higher temperatures. The crystallization of polymers is discussed on the basis of these findings.