Modeling Phase Separation of Free-Radical Polymerizations in Crosslinked Networks

Herein, a thermodynamics model based on Flory–Rehner theory is used to predict conditions at which phase separation becomes thermodynamically favored in two-stage polymerization systems with wide utility in optical materials, membranes, and other porous materials. Methods for reduction of the Flory–Huggins interaction parameter and adjustment of component volume fractions are introduced to account for the covalent attachment of a growing polymer chain in the second phase to the network backbone formed in the first stage. The model predicts a delay in phase separation with an increase in covalent attachment, enabling larger Flory–Huggins interaction parameters, larger degrees of polymerization, and larger monomer loadings to be used before phase separation becomes favorable as desired in various applications such as holography. The model is validated against coupled FTIR and UV–vis experiments and found to follow experimental phase diagram behavior with a slight underestimation of conversion at which phase separation begins to occur. The model provides a tool for monomer, network, and polymerization selection without the need for extensive trial-and-error experimentation.