Polystyrene-block-poly(methyl methacrylate) copolymers containing [4pi + 4pi] anthracene photodimer at the junction point between the blocks (PS-AA-PMMA) can be cleaved to the parent homopolymer blocks upon heating above 130 degreesC or by UV irradiation at 280 nm. PS-AA-PMMA was characterized and then used in thin films to create micro- to macroscale poly(methyl methacrylate) (PMMA) domains in a polystyrene (PS) matrix. Microphase-separated morphologies in thin films were initially achieved by annealing spin-coated films in supercritical carbon dioxide at 80 degreesC. Subsequent heating and/or UV irradiation was used to cleave the copolymer junction point, affecting an in situ conversion of the diblock copolymer to its parent homopolymers. The evolution of phase separation, from microdomains of PMMA in a matrix of PS to macrophase-separated structures, was investigated by SFM. Subsequent washing with selective solvents removed homopolymer, producing raised nanostructures or nanoporous templates.