Photoinduced phase separation in the presence of a nonreactive volatile solvent provides a simple, light-tunable, and costeffective route for fabrications of porous polymer films. Here, we show that the short-time (<10 s) ultraviolet irradiation of a photoreactive solution film promotes particular asymmetries in the composition and porosity across the thickness. Confocal Raman spectroscopy revealed that the compositional asymmetry is maximum at a certain critical light intensity. We discuss the film formation mechanism based on the light-driven diffusion of the solvent and monomer.
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