Thermodynamic Analysis of Photopolymerization-Induced Phase Separation Processes at Subzero Temperatures to Predict Experimental Conditions Leading to Porous Thermosets

Polymeric porous structures impact on a wide variety of applications. A range of strategies have been approached to particularly synthesize cross-linked porous polymers. In this scenario, a variety of cross-linking strategies from reactive precursors can lead to phase-separated morphologies, which can subsequently be used to be replicated in a porous matrix. Polymerization-induced phase separations (PIPS) and photopolymerization reactions are commonly addressed for this purpose, either separately or combined (photo-PIPS), resulting in a variety of morphologies depending on the reaction conditions. However, cross-linking reactions are typically conducted at room/high temperature, constraining to a certain extent the accessed morphologies to those compatible to typical liquid–liquid (L–L) phase-separation events. To extend the underlying principles of photo-PIPS to the subzero temperature domain (cryo-photoPIPS), in which solid–liquid (S–L) equilibria could emerge, giving way to a broader palette of morphologies, a theoretical background is certainly required so as not to grope around in experimental studies based on trial-and-error strategies. In the present study, we tackle this challenge by addressing a thermodynamic approach based on the Flory–Rehner model to rationally predict the low-temperature phase equilibrium behavior of a system subjected to cryo-photoPIPS. The theoretical calculations were experimentally validated using a model photopolymerizable system consisting of poly(ethylene glycol)dimethacrylate (PEGDMA) and a low-molecular weight modifier (cyclohexane, CH), the latter employed to induce phase separation processes during polymerization at 258 K. Phase behavior predictions, dictated by S–L equilibria, were validated by taking into account both kinetic aspects associated with the photopolymerization protocol utilized (continuous or pulsatile) and the composition of the reactive blend, demonstrating the possibility of controlling the resulting morphologies by modifying the relative rates of CH segregation and PEGDMA polymerization.