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Asymmetric diblock copolymers of polystyrene (PS) and poly(methylmethacrylate) (PMMA), PS(S-b-MMA), having cylindrical microdomains of PMMA, are model systems to generate nanoporous thin films. With controlled interfacial interactions or applied external electric fields, the cylindrical microdomains can be oriented normal to the surface. Exposure to deep UV radiation degrades the PMMA and crosslinks the PS matrix. After rinsing with a selective solvent, a nanoporous film is obtained. By changing the molecular weight, smooth porous films with hexagonal arrays of pores having diameters ranging from 14 to 50nm were obtained. The results show that molecular weight is a convenient, simple means of controlling pore diameter.