Ternary thiol-ene/acrylate photopolymers: Effect of acrylate structure on mechanical properties

Model ternary thiol-ene/acrylate photopolymerization involving acrylate homopolymerization and copolymerization of thiol-ene and thiol-acrylate monomers were monitored by real-time FTIR. In all ternary mixtures, including those prepared with different acrylate concentrations, acrylate conversion was 100%. However, thiol-ene conversions were found to be controlled by their initial concentrations. The influence of acrylate monomer chemical structure on the thermophysical properties of ternary thiol-ene/acrylate systems was studied with DMA, DSC, and the absorbance of a nondestructive impact energy. The addition of acrylate to the thiol-ene system increased the rubbery modulus while the tan delta(max) shifted to higher temperatures. Densely cross-linked, heterogeneous matrix formation was observed with the broadening of tan delta peaks at high acrylate concentrations. The high impact absorption of these ternary thermoset photopolymers was correlated with the dynamic mechanical damping ability of the networks. Acrylates with higher functionality and low molecular weight per double bond are more effective at increasing the glass transition temperature of the thiol-ene polymer network. Fracture behavior of ternary thiol-ene/acrylate networks under impact shows a dependence on the chemical structure of the acrylate, component concentrations, and low-temperature relaxation processes. The ternary matrix formed with a bisphenol A based difunctional acrylate monomer exhibited improved impact energy absorption at room temperature. Finally, tensile properties of polymer networks formed with thiol-ene/acrylate and thiol-acrylate mixtures are given for comparison purposes.