Thermal and mechanical properties of cross-linked photopolymers based on multifunctional thiol-urethane ene monomers

Urethane-based multiene monomers were synthesized and photopolymerized with a trifunctional thiol monomer to form highly cross-linked thiol-ene networks. Real-time FTIR was used to monitor the conversion of thiol and urethane ene monomers as a function of irradiation time. For stoichiometric thiol-urethane ene photopolymerizations, monomers reacted in a 1:1 molar functional group ratio, reaching similar to 90% monomer conversion within several seconds. The effects of the ene chemical structure and concentration on thermal and mechanical properties were characterized by DMA, DSC, TGA, tensile, and energy absorption upon nondestructive impact. The temperature at tan delta(max) of the thiol-urethane ene networks was around 39 degrees C and decreased to lower temperatures with the addition of a reactive diluent diallyl ether ene monomer. Tensile and impact results were combined with fracture toughness measurements to elucidate the effect of the urethane and bisphenol A chemical structures. The energy absorption was dependent on the glass transition temperature of the thiol-urethane ene cross-linked networks. Improvement in fracture toughness and tensile properties was observed with the incorporation of 10 mol % of an ene with a bisphenol A structure. Scanning electron micrographs of fractured surfaces were used to interpret the nature of the brittle fracture. All of the properties of the thiol-urethane ene networks were compared to a conventional trithiol-triallyltriazine trione-based matrix.