기판 위에 코팅된 에틸렌-비닐 아세테이트 랜덤 공중합체 박막의 결정 배향과 구조에 관한 연구

Abstract

The applications of polymer thin films on solid substrates have been explored in a variety of industrial fields because they are inexpensive, lightweight, and flexible, and the fabrication of desired molecular architectures is facile. As a new and intriguing topic in polymer physics, crystallization in thin films has received considerable attention in recent decades. In this thesis, the crystal orientations and structures of poly(ethylene-ran-vinyl acetate) thin films coated on substrates were investigated by using atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXRD).In Chapter I, the motivation and background of this study are briefly introduced and a review of the literature on polymer crystallization is presented that focuses primarily on the hexagonal structures of the homopolymers and copolymers of ethylene and also on crystal orientations in polymer thin films.In Chapter II, the crystal orientation and structures of poly (ethylene-ran-vinyl acetate) (EVA) films coated onto silicon substrates were investigated by performing isothermal crystallization of the films as a function of film thickness. 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. Our results confirmed that the edge-on orientation is preferred, regardless of other factors, such as film thickness or crystallization temperature if the interactions between the polymer chains and the substrate are repulsive. 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.In Chapter III, the crystal orientations and structures of EVA ultrathin films (5, 10 nm) spin-coated onto three different substrates (piranha-cleaned, acetone-cleaned, and hexamethyldisilazane-treated Si wafers) were investigated. 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.