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Molecular dynamics study of the melt morphology of polyethylene chains with different branching characteristics adjacent to a clay surface.

Conformations of model high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE) chains with different intramolecular branch distributions adsorbed on a relaxed octahedral surface of kaolinite, a major clay mineral, at 463 K (190 degrees C) were studied by using classical molecular dynamics (MD) simulation. Prior to the MD simulations, first-principle density functional theory (DFT) calculations were carried out to relax the inorganic surface that was created by cleaving the corresponding kaolinite crystal structure. The high-temperature MD simulation results showed that an ordered polyethylene region with a thickness of about one to three layers of chain segments developed rapidly near the clay surface. On the other hand, chain segments in the far field slowly evolved into another ordered region with a higher degree of order than the one adjacent to the surface. It was observed that the melt morphology in the far field depends on the architecture of the chains. Also, in between the two ordered regions, a region that contained no apparent order formed. The above observation is attributed to the fact that the mobility of chain segments adjacent to the surface was greatly reduced as a result of their strong affinity for the surface, while those in the far field were not. Despite the fact that the results are for the melt state, they suggest that nucleation and lamellar growth of polymer chains nearby an inorganic surface may proceed from the chain segments in the ordered region in the far field rather than from the organic/inorganic interface. This is because chain segments in the three described regions, upon cooling, should not have sufficient thermal energy to reorient themselves drastically to form a single lamella under normal crystallization conditions. However, it should be noted that the above speculation is made based on a rather short equilibration time (approximately 10 ns) used in the simulations.