Understanding the character of photoresponse in glassy polymeric materials with molecular modeling

Highly cross-linked thermoset polymers have several excellent properties for use in structural applications, such as high glass transition temperature, high elastic modulus and yield stresses, and good environmental stability. However, these rigid materials also tend to experience brittle failure and poor ductility. Here, we consider azobenzene-loaded polymer glasses as a route to developing rigid resins and adhesives that rapidly change mechanical properties on irradiation. Although experiments and simulations have shown that photoactive groups can trigger strong property changes in polymeric materials, the vast majority of work in responsive polymers has been carried out on soft, compliant elastomers and solvent-swollen gels. Further, the response time of photo-activated soft materials is often on the order of minutes or hours. We explore the isomerization of photo-activated azobenzene incorporated in deeply glassy epoxy-amine networks. Using large-scale atomistic molecular simulations, we show that the character of the local environment near each responsive group strongly alters the "waiting time" between the photo-stimulus and individual molecular photo-isomerizations. Importantly, the median wait time for isomerization can be varied by orders of magnitude by tuning the strength of interactions between the dispersed azo-compounds and the surrounding matrix. Interestingly, the molecular simulations predict a wait time distribution having a power-law decay with exponents near unity, where the extent of the power-law distribution grows with decreasing temperature or increasing density, up to a point where the median time diverges with density. Overall, we find that the density and energies act as key predictors of isomerization events in the glass. These results suggest that future efforts to correlate isomerization with descriptions of the local packing and covalent environments will be central for a rational chemical design of responsive resins.