(217i) Preparation of Hierarchical Hydrogels By Photocrosslinking Induced Phase Separation in Evaporating Solvents

Reaction induced phase separation (RIPS) offers a convenient way to create polymer microstructures.  Exploiting volatile nature of solvents during the RIPS may provide an unique opportunity to achieve structures that cannot be obtained otherwise. Herein we demonstrated successful fabrications of three different hydrogel films, namely, poly(n-isopropylacrylamide), polyacrylamide and poly(2-hydroxyethyl methacrylate), with highly hierarchical structures by photocrosslinking of precursors during the evaporation of solvents. The resultant hydrogel films feature a dense skin on top of a layer of microspheres. For the first time, the skin layer formed during fast solvent evaporation was directly observed after the complete evaporation of the solvent. The overall morphology of the hydrogels is dependent on the processing parameters including the N₂ flow rate, UV intensity and precursor concentrations, and can be qualitatively understood based on an evaporation-reaction model. The hydrogels prepared by this method have potential applications in drug delivery carriers and ultrafast responsive sensors.