Crystallization of thin and ultra-thin polymer films.

摘要

Thin and ultra-thin polymer films have received great interest due to their technological applications in the modern micro-electronic devices. It has been demonstrated that the presence of the interface and the confined geometry greatly affect configuration of the chains, segmental mobility and thermal transitions in the polymer thin films. In the semi-crystalline polymer thin films, the degree of crystallinity, crystallization dynamics, crystal morphology and lamellar orientation are different from their bulk counterparts.; In this dissertation, a comprehensive study in the crystallization and melting of spun-coated thin and ultra-thin films of polyethylene-vinyl acetate)(EVA), linear low and medium density polyethylene (LLDPE and LMDPE) on selected substrates was made. The morphology, the lamellar orientation and the melting point of the films were found to have strong dependence on the film thickness and the substrate. On the strongly attractive substrate such as silicon, the film crystallized as spherulite with densely organized edge-on lamellae for the films thicker than 100nm, the lamellae packed more loosely as the film thickness decreased, until finally exhibited a dense branching morphology with mainly flat-on lamellae when the film is thinner than 15nm. A large degree of crystallinity was observed in the film as thin as 15nm. On the less absorbent substrate such as polyimide, the films crystallized in a randomly organized lamellae structure, which showed little dependence on the film thickness. On all the substrates, the melting point of the film decreased with the film thickness decrease when the film was thinner than a critical thickness, the value of which varied for different substrates. The biggest Tm depression observed was between 12°C and 38°C, depending on the substrates. The finite size effect and the reduced melting entropy caused by the confined movement of the polymer chains due to the substrate absorption were suggested to be the main reasons for the sharp Tm depression. The chain orientation inside the crystals was measured by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, which showed that on the silicon substrate, the chains oriented parallel to the substrate with b axis in the radiation direction of the fibrils of the spherulite, a axis perpendicular to the film surface.