A photochemical process for controlling block copolymer (BCP) domain orientation in an area selected manner is described. Polymers with photoswitching surface energy, used as interfacial underlayers adjoining the BCP layer, were synthesized with photoacid labile monomers. The interfacial polymers were designed to be either inherently neutral or preferential to poly(styrene-block-4-trimethylsilylstyrene). Through patternwise exposure to 193nm light and subsequent reaction with photogenerated acid, the wetting characteristics of the underlayer material can be switched from neutral to preferential (N2P) or preferential to neutral (P2N). Thermal annealing of the BCP confined between a patterned N2P or P2N substrate and a neutral top coat results in alternating areas of perpendicular and parallel BCP domains within the same film. If adapted in tandem with directed self-assembly, this approach could enable new routes to customizable patterns for advanced microelectronics and memory devices.