Concentration profiles in phase-separating photocuring coatings
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We directly measured the local composition profiles in phase-separating photocurable thin films using confocal Raman spectroscopy. To avoid light scattering at phase interfaces, we developed a novel technique to replace the solvent with a monomer to match the reflective indices in the cured films. The results indicated that the concentration distribution of the polymer was uniform in solvent-free monomer/initiator binary solutions, while it was spatially nonuniform when the solvent-based films were thermodynamically unstable and promoted reaction-induced phase separation upon UV irradiation on the top surface. In the latter case, the film exhibited a dual-layer structure, in which the polymer concentration was almost uniform near the top surface, while concentration gradients developed near the bottom surface. The thickness of the top layer with a uniform concentration profile increased with the increase in the UV light intensity. These results implied that the propagation of the reaction front and the resulting light-driven transport of the solvent toward the bottom coating layer played key roles in the formation of nonuniform concentration profiles in photocuring solution coatings.
KeywordsUV curing Coating Reaction-induced phase separation Confocal Raman spectroscopy
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