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

Semicrystalline polymers exhibit metastable interphases, which must simulta neously accommodate molecular connectivity and disorder across the interlam ellar phase. Off-lattice molecular simulations, previously used to study th e {001} interphase in freely rotating chains, are used here to study the in terlamellar phases between the {101}, {201}, and {502} crystal facets (poly mer chains tilted to the lamellar normal by 19 degrees, 34.4 degrees, and 4 1 degrees, respectively). The order-to-disorder transition from the crystal to the amorphous region occurs with an interface approximately 10-12 Angst rom thick, for all cases studied. The interfacial potential energies for th e {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 i n the interlamellar phase shifts away from tight folding as the tilt angle of the chains exiting the crystal increases. Whereas [110] loops dominate t he (001) interface, loop reentry along [200] and [310] directions is more c ommon in the interfaces with tilted chains. The chain length distributions associated with tilted chains more closely approximate the ideal distributi on suggested by a model of Gaussian chains, which indicates that entropy fa vors 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.