Influence of network heterogeneity on the nanoscale mechanical properties of epoxy resins

The presence of structural heterogeneities in glassy epoxy networks can strongly affect the adhesiveness and mechanical properties of epoxy-based materials. Recent studies have demonstrated that the size and architecture of structural heterogeneities can be monitored during the curing process, thus providing an effective way to control multiple physical properties of epoxy resins. However, because the size of heterogeneities is believed to be at the nanoscale, obtaining insights into the direct correlation between the local behavior of heterogeneities and the preparation conditions remains challenging. Here, we combine two recently developed atomic force microscopy (AFM)-based methods, bimodal amplitude and frequency modulation and nanoscale dynamic mechanical analysis (nDMA), to directly visualize and quantify the nanoscale mechanical properties of epoxy resins obtained via different curing conditions. Our AFM maps clearly show a correlation between the curing conditions and the heterogeneous behavior of the resulting epoxy networks, which is in excellent agreement with those predicted from macroscopic measurements. Notably, our nDMA results revealed a relationship between network heterogeneity and the glass transition behavior of the epoxy resins. A combination of AFM-based AM-FM and nDMA methods evidences the effect of the curing temperature on the structural heterogeneities in epoxy networks: a lower pre-curing temperature provides a less heterogeneous network, which in turn can control the thermal and mechanical properties of resultant epoxy resins.