Molecular Simulations of the Interlamellar Phase in Polymers: Effect of Chain Tilt

Abstract

Semicrystalline polymers exhibit metastable interphases, which must simultaneously accommodate molecular connectivity and disorder across the interlamellar phase. Off-lattice molecular simulations, previously used to study the {001} interphase in freely rotating chains, are used here to study the interlamellar phases between the {101}, {201}, and {502} crystal facets (polymer chains tilted to the lamellar normal by 19°, 34.4°, and 41°, respectively). The order-to-disorder transition from the crystal to the amorphous region occurs with an interface approximately 10−12 Å thick, for all cases studied. The interfacial potential energies for the {101}, {201}, and {502} facets are computed to be 100, 70, and 90 mJ/m2, respectively, compared to 140 mJ/m2 for the {001} facet. The topology in the interlamellar phase shifts away from tight folding as the tilt angle of the chains exiting the crystal increases. Whereas [110] loops dominate the {001} interface, loop reentry along [200] and [310] directions is more common in the interfaces with tilted chains. The chain length distributions associated with tilted chains more closely approximate the ideal distribution suggested by a model of Gaussian chains, which indicates that entropy favors tilting of polymer chains away from the lamellar normal. These results are consistent with the frequent observation of {201} oriented interfaces in polyethylene and offer a thermodynamic explanation for the selection of interface orientation in semicrystalline polyethylene.