(420d) Interrelating Relative Reactivity, Network Topologies, and Thermomechanical Properties in Vinyl-Ester Thermosets Using All-Atom Molecular Simulations

In the formation of thermoset networks from vinyl esters (VE) and styrene, it is difficult to control the average molecular weight between crosslinks (Mc) independently from the overall VE:styrene composition. Exploiting differences in reactivity of C-C double bonds on VE vs. styrene, one can shift kinetics toward faster formation of styrene-styrene relative to VE-styrene linkages, but the outcomes in terms of network topology are not well understood. In this work, we developed an ad hoc crosslinking algorithm to build all-atom models of random thermoset networks of VE/styrene in which bond reactivities are tunable. We assess the impact of changing relative reactivity on network topology and thermomechanical properties. We find that at fixed VE:styrene, glassy state Young’s modulus and the glass-transition temperature are not sensitive to relative reactivity, which agrees with existing experimental work [Zaiee and Palmese, J. Polym. Sci. B: Polym. Phys. 37:725, 1999]. Additionally, we investigated the mechanical behavior of the atomistic VE/Styrene network topology and obtained the master curve for the Young’s modulus for various network topologies by applying the time-temperature superposition principle (TTSP) on its apparent Young’s modulus vs. strain rates at different temperatures. The Williams-Landel-Ferry and Vogel-Fulcher-Tammann equations were adopted to numerically determine the shift factors which were used to generate the master curve. The master curve from the simulation showed a good agreement with the experimental master curve [Shan, Robertson and Reifsnider, J. Appl. Polym. Sci. 80: 917–927, 2001]. Our work demonstrated the connection between simulated Young’s modulus at high strain rate to experiments which can be applied to redict the quasi-static Young’s modulus at room temperature. Furthermore, similar master curves for the VE/Styrene network structures with different Mc were obtained to study the effect of Mc on the master curve.