Effect of an unsaturated-chain extender on the response of epoxy resins to radiation post-curing

Ultra-violet (UV) radiation has been used to produce functionally graded materials by locally modulating crosslink density. Functionally graded adhesives are an important subset of functionally graded materials, but photocuring is of limited use when creating structural adhesive joints, most of which involve opaque adherends. As a result of these and other difficulties generating such materials in practice, the advantages of functionally graded adhesives have been reported theoretically, but experimental investigations remain rare. For this reason, our group has focused on dual cure systems sensitive to post-curing via high energy radiation as a means of locally modulating properties. Here, a series of thermally cured epoxy resins are shown to exhibit different levels of sensitivity toward gamma (γ) irradiation as a function of the concentration of a bisphenol-derived unsaturated chain extender incorporated into an epoxy-anhydride network that contains unsaturations as well. It was observed that addition of the chain extender slowed curing and impacted the thermal properties of the cured epoxy networks but greatly favored radiation-induced crosslinking over degradation given the observed shifts in properties. A theory proposed by Shibayama relating Tg to crosslink density is applied to better understand the structure and crosslinking behavior of thermally cured epoxies before and after radiation post-curing and provides a useful means to predict radiation-induced changes in performance. The development of such heat / γ radiation dual curable formulations, the understanding of their behavior upon irradiation and the ability to predict key properties via a simple analytical model provides a new perspective for the successful generation of functionally graded materials through methods described in our prior work.