Thiol-epoxy/thiol-acrylate hybrid materials synthesized by photopolymerization

Thiol-epoxy/thiol-acrylate hybrid networks with systematic variations (100/0, 75/25, 50/50, 25/75, 0/100, w/w) were prepared by free-radical photoinitiator (ITX) and photo-base generator (TBD center dot HBPh4) induced photopolymerization. The enhanced spectral sensitivity of TBD center dot HBPh4 at long wavelengths (320-500 nm) with aid of ITX was capable of in situ generation of a strong base (TBD) to achieve the relatively fast and essentially quantitative thiol-epoxy click reaction. The ITX/TBD center dot HBPh4 photoinitiating system, which generated both the free radicals and the strong base upon UV exposure, could induce thiol-epoxy/thiol-acrylate hybrid polymerizations. The kinetics investigated with real-time IR indicated that the thiol-acrylate reactions were faster and more efficient than the thiol-epoxy reactions (>95% conversion in a matter of seconds and minutes, respectively), and the thiol conversion increased with an increase in the epoxy concentration. The incorporation of the thiol-epoxy reaction offered several advantages: the polymerization shrinkage decreased with the increase in the thiol-epoxy content due to the low shrinkage factor for the thiol-epoxy system (every mole of epoxy group polymerized produces 6.3 mL of shrinkage). The glass transition temperature of the thiol-epoxy/thiol-acrylate hybrid networks progressively increased as a function of the thiol-epoxy content, also resulting in enhanced mechanical and physical properties. This work will be helpful to make optical and electronic devices with low shrinkage and stress, decreased residual monomers, and improved mechanical properties.