Thermo-mechanical properties of urethane acrylate networks modulated by RAFT mediated photopolymerization

Abstract

Reversible addition-fragmentation chain transfer (RAFT) processes in polymer systems have shown great utility in controlling radical polymerizations. Due to the reversible chain transfer mechanism, RAFT agents have been extensively used to synthesize monodisperse linear polymers and block copolymers in solution or emulsion polymerization. On the other hand, little is known about the effects of RAFT agents on the polymerization kinetics and thermo-mechanical properties of cross-linked materials. This work investigates the effect of RAFT agents on photopolymerization behavior and ultimate polymer properties in urethane acrylate systems. Our results indicate that RAFT agent addition has a significantly different impact on network development depending on inherent system characteristics, i.e., systems with glass transition temperature (Tg) below room temperature (LT) or those with Tg above room temperature (HT). When polymerizing with RAFT agents, photopolymerization rate decreases and can be tuned using different concentrations of RAFT agent. The final materials also show increased elongation at break with decreased Young's modulus. However, ultimate thermo-mechanical behavior and mechanical properties are different based on Tg. With RAFT addition decreased Tg is observed from HT polyurethane networks. Tensile toughness is also doubled in comparison to neat HT films. On the other hand, although the Tg of RAFT modified LT polyurethane networks does not change significantly, toughness of the final polymer films varies significantly with RAFT agent concentration. Enhanced toughness is only observed from films with low RAFT agent concentration while toughness decreases with higher loading of RAFT agent. These results demonstrate that the polymer network and thermo-mechanical properties can be modified by introducing RAFT agent which enables dynamic polymer chain rearrangements in both LT- and HT-acrylate network systems.