Epitaxial crystallization of polyethylene

Abstract

Abstract Thin epitaxial layers of polyethylene, formed on the surface of alkali halide crystals by isothermal solution crystallization, were studied via electron microscopy and electron diffraction as a function of molecular weight, solution concentration, and degree of supercooling. Growth of the monoclinic form of polyethylene was found to increasingly predominate over the usual orthorhombic form as the thickness of the deposit decreased in the case of epitaxy on NaCl. This phenomenon was postulated to have occurred because of a favorable packing scheme for interfacial monoclinic layers and interactions between the polymer and the specific Van der Waals forces of the surface. Although this pseudomorphic growth pattern was relatively independent of molecular weight, initial growth rate perpendicular to the surface had a large molecular weight dependence. Concentration and degree of supercooling also were observed to have significant effects on growth rate. Lattice match between polymer crystallite and alkali halide was found to be an important consideration in the determination of growth rate, initial polyethylene crystal structure, and the supercooling required for nucleation. A model for the long-range electrostatic forces emanating from the alkali halide surface was presented and used to explain the "limiting crystal height" phenomenon at low supercoolings.