The effect of kinetic chain length on the mechanical relaxation of crosslinked photopolymers

This work investigates the effect of kinetic chain length on the network structure of multi-functional (meth)acrylates. Two chain transfer agents, dodecanethiol and m-toluenethiol, were added to a series of crosslinking free radical polymerizations to decrease the kinetic chain length. Using near-IR spectroscopy and dynamic mechanical analysis, the effect of chain transfer on the polymerization rate and network properties (i.e. Tg and modulus) was investigated. The results suggest that macroradical chain length, which has been shown to play a role in the termination kinetics of crosslinking systems, may also impact the network properties of the formed polymer. With the addition of only 0.5–1 wt% chain transfer agent, differences in the polymerization rate and mechanical properties were observed in the crosslinking methacrylate systems. The polymerization rate was significantly suppressed and the Tg of the cured network was found to decrease by up to 10 °C, depending on the monomer formulation. The largest differences in the mechanical properties occurred in the systems with the lowest crosslinking density and diminished as the crosslink density of the cured polymer increased. Furthermore, the differences were less dramatic in the multi-EGDMA systems, that have some inherent ability to chain transfer, and were not discernable in the transfer dominated diacrylate systems.