Molecular simulations df the interlamellar phase in polymers: Effect of chain tilt

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 degrees, 34.4 degrees, and 41 degrees, respectively). The order-to-disorder transition from the crystal to the amorphous region occurs with an interface approximately 10-12 Angstrom 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/m(2), respectively, compared to 140 mJ/m(2) 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.