Molecular modeling of crosslink distribution in epoxy polymers

Experimental studies on epoxies report that the microstructure consists of highly crosslinked localized regions connected with a dispersed phase of low-crosslink density epoxy. Because epoxies play a major role in many structural applications, the influence of the crosslink distribution on the thermo-mechanical properties must be determined. But as experiments cannot reliably report the exact number or distribution of crosslinked covalent bonds present in the molecular network, molecular modeling is a valuable tool that can predict the influence of crosslink distribution on thermo-mechanical properties. In this study, molecular dynamics are used to establish well-equilibrated molecular models of an EPON 862-DETDA epoxy system with a range of crosslink densities and distributions. Crosslink distributions are varied by forming highly crosslinked clusters within the epoxy network and then forming additional crosslinks that connect between clusters. Results of simulations on these molecular models indicate that elastic properties increase with increasing levels of overall crosslink density and the thermal expansion coefficient decreases with overall crosslink density, both above and below the glass transition temperature. It is also found that within the range of crosslink distributions investigated, there is no discernible influence of crosslink distribution on the elastic modulus and the linear thermal expansion coefficient of the epoxy. (paper)