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Electric field alignment of diblock copolymer thin films
By anchoring random copolymers to the substrates, the interfacial interactions were tuned precisely and dependence of the orientation of lamellar microdomains in thin films on interfacial interactions was investigated quantitatively. A critical film thickness was found, below which a parallel alignment of the lamellar microdomains was seen throughout the film and depends upon the strength of the interfacial interactions. The electric field alignment process is a competition between the applied electric field and surface fields. In the early stages, the surface field dominates. A mixture of orientations of the lamellae was found if the interfacial interactions were not balanced. However, in the presence of lithium ions (210ppm), lamellar microdomains were aligned along the applied electric field direction throughout the film, regardless of the strong interactions of blocks with substrate. For cylindrical microdomain forming diblock copolymer thin films, starting from a poorly-ordered state, surface fields and opposed electric fields biased the cylinder orientation. With time upon annealing, the cylinders are locally disrupted to form ellipsoidal shape microdomains that connected into cylinders in the applied field direction. Starting from an ordered state with cylinders parallel to the surface, the applied field enhanced fluctuations at the interfaces of the microdomains and disrupted cylinders into spheres. This transition is similar to thermoreversible cylinder-to-sphere order-order transition. With time, the spheres deformed into ellipsoids and reconnected forming cylindrical microdomains oriented at ∼45° with respect to the applied field, which subsequently aligned along the field direction. These studies were complemented by studies on an electric field induced disordered sphere-to-cylinder transition in thin films. Under an electric field, the asymmetric diblock copolymer formed spherical microdomains that were deformed into ellipsoids and, with time, interconnected into cylindrical microdomains oriented in the direction of the applied electric field. A route to control the microdomain orientation in three dimensions in diblock copolymer thin films was also studied by use of two orthogonal, external fields. An elongational flow field was applied to obtain an in-plane orientation of the microdomains of the copolymer melt and an electric field, applied normal to the surface, was then used to further align the microdomains.
Xu, Ting, "Electric field alignment of diblock copolymer thin films" (2004). Doctoral Dissertations Available from Proquest. AAI3136798.