Morphology development in photopolymerization-induced phase separated mixtures of UV-curable thiol-ene adhesive and low molecular weight solvents

The influence of photopolymerization rate, solvent quality, and processing parameters on the photopolymerization-induced phase separated morphology of mixtures of thiol-ene based optical adhesive with mixed solvents of diglyme and water or acetone and isopropanol is described. Upon exposure to UV radiation (∼50 mW/cm², 365 nm) for periods of 10-90 s, homogeneous solutions of 5-10 wt% NOA65 and NOA81 adhesive formed phase separated structures with characteristic sizes ranging from 400 nm to 10 μm, with increased photopolymerization rates leading to smaller feature sizes. In the systems containing diglyme and water, morphologies formed by phase separation at a lower degree of photopolymerization were characteristic of spinodal decomposition, while morphologies formed by phase separation at a higher degree of photopolymerization exhibited characteristics of viscoelastic phase separation. In the systems containing acetone and isopropanol, interactions between evaporation and photopolymerization-induced phase separation led to the development of more complicated morphologies, including three-dimensional sparse networks. These morphologies provide a combination of connectivity and low overall volume fraction that can significantly enhance the performance of many multi-functional structures.