Photocrosslinking-induced phase separation in evaporative solvents: formation of skin layers and microspheres

We report the structure formation of films obtained via photocrosslinking of precursors during the evaporation of solvents. Although most precursor/solvent systems result in uniform dense films after the process, reaction-induced phase separation can occur in solvents with a unique combination of solubility, evaporation rate and ratio of latent heat to heat capacity. Most significantly, for such solvents, the forced convective evaporation under controlled N-2 flow results in a highly hierarchical film morphology, featuring a skin layer on top of a layer of microspheres formed via a nucleation and growth mechanism. For the first time, the skin layer formed during the evaporation was directly observed after the complete evaporation of the solvent. The thickness of the skin layer is dependent on the processing parameters including the N-2 flow rate, UV intensity and precursor concentrations. The skin layer formation could be suppressed by addition of non-solvent, in which case the characteristic morphology resulted from dominant spinodal decomposition. A model is presented that can qualitatively describe the skin layer formation and its dependence on the processing parameters, providing a mechanistic understanding of the photocrosslinking-induced phase separation under evaporative environments.