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The alignment of thin films of symmetric diblock copolymer of polystyrene and poly(methyl methacrylate), PS-b-PMMA, in an electric field was studied as a function of film thickness and interfacial energy using small-angle neutron scattering and transmission electron microscopy. There is a competition between the applied electric field, aligning the microdomains normal to the surface, and surface fields that tend to align the microdomains parallel to the surface. For films with thickness t < 10L₀, where L₀ is the equilibrium period of the copolymer in the bulk, interfacial interactions are dominant, and the lamellar microdomains orient parallel to the substrate surface regardless of the applied electric field. If t > 10L₀, interfacial interactions become less important, and lamellar microdomains in the center of the films could be oriented in the direction of the applied field, i.e., normal to the surface. Transmission electron …