Substrate effects on crystal orientations and structures of poly(ethylene-ran-vinyl acetate) ultrathin films

The crystal orientations and structures of poly(ethylene-ran-vinyl acetate) (EVA) ultrathin films (5, 10 nm) spin-coated onto three different substrates (piranha-cleaned, acetone-cleaned, and hexamethyldisilazane-treated Si wafers) were investigated by using atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXRD). The crystal orientations were found to be edge-on for all three substrates, and two crystal structures, orthorhombic and hexagonal, were observed. As the polar nature of the substrate decreases, the hexagonal structure disappears, and the orthorhombic structure becomes dominant. The preference for an edge-on orientation is attributed not only to the repulsive interactions between the polar vinyl acetate components and the nonpolar ethylene components but also to the wetting conditions, i.e., to the interactions between the polar vinyl acetate component and the substrates. The formation of the hexagonal structure arises from the gauche defects of the ethylene chains. From these results, we discuss the interfacial properties that determine the preferred crystal orientations in thin films, and the effects of the interactions between the polar vinyl acetate components and the substrates on the chain conformations of the ethylene segments.