Crystal orientations and structures of poly(ethylene-ran-vinyl acetate) films coated onto silicon substrates

The crystal orientations and structures of poly(ethylene-ran-vinyl acetate) (EVA) films coated onto silicon substrates to a thickness of 5-200 nm were investigated by performing isothermal crystallization of the films. Bulk-like isotropic crystal orientations and orthorhombic crystalline phases were observed in films thicker than 50 nm. In the thinner films (<50 nm), the crystal orientations remained edge-on (with the chain axis parallel to the substrate), and two crystal structures, orthorhombic and hexagonal, were observed. The preference for an edge-on orientation was attributed to surface nucleation and repulsive interactions between the crystallizable ethylene components and the substrate. An orthorhombic structure developed at the surface from the primary crystallization during isothermal crystallization. A hexagonal structure formed near the interface as a result of secondary crystallization after cooling to room temperature. These results are discussed in terms of the favorable interactions between the polar vinyl acetate and the substrate, which affect the crystallization of the ethylene component in thin EVA films.