Formation Mechanism of a Heterogeneous Network in Epoxy Resins

In general, the network structure formed in epoxy resinsis heterogeneous,and its extent is dependent on the curing temperature. Based on Fourier-transforminfrared spectroscopy in conjunction with coarse-grained moleculardynamics simulations, we herein discuss the origin of mesoscopic heterogeneityin an epoxy resin composed of a typical epoxy base and diamine hardener,which react with each other in two steps. The rate balance of thefirst and second step reactions, which led to chain extension andbranching, respectively, depended on the curing temperature; at ahigher temperature, the second-step reaction became faster. Sincethe reaction proceeded more slowly at lower temperatures, a homogeneousdomain was formed as unreacted substances were incorporated into thenetwork. At higher temperatures, on the other hand, the reaction proceededrapidly before unreacted substances were incorporated into the network,leaving isolated small fragments and nanoscale voids or free spacesin the domain. We also found that an actively reacting domain witha length scale of several tens of nanometers was formed even at aconversion lower than the gel point, regardless of the curing temperature.The connection of domains was also key to better understanding thetemperature dependence of the hierarchical heterogeneity. The finalnetwork was found to be denser and more homogeneous at a lower temperature,while heterogeneous with many free spaces at a higher temperature.Our molecular picture of the network formation drawn by combiningexperimental and simulation techniques advances our current understandingof the heterogeneity formed in epoxy resins.