Phase-Separated Thiol-Epoxy-Acrylate Hybrid Polymer Networks with Controlled Cross-Link Density Synthesized by Simultaneous Thiol-Acrylate and Thiol-Epoxy Click Reactions

Thiol-epoxy-acrylate hybrid polymer networks (HPNs) are formed by the combination of nucleophilic thiol-acrylate Michael addition and thiol-epoxy coupling reactions catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in a one-pot synthesis. A stoichiometric balance between thiol groups (multifunctional thiols with a thiol functionality greater than two) and the addition of epoxide (difunctional epoxy) and acrylate groups (difunctional acrylate) is applied in the reactant mixture. Full conversion is achieved based on the disappearance of the thiol absorbance peak from Fourier transform infrared spectroscopy, demonstrating the high efficiency of thiol-click reactions. With relatively high molecular weight (MW = 2000 g mol^-1) acrylates, novel phase-separated, thiol-based hybrid materials are obtained, as evidenced by the presence of two glass transition temperatures from both differential scanning calorimetry and dynamic mechanical analysis as well as morphology characterization by scanning electron microscopy. Cross-link density of the hybrid networks is systematically controlled by substituting the multifunctional thiols with different amounts of difunctional thiols while maintaining a stoichiometric balance between reacting groups. By changing cross-link density in the HPNs, a wide range of thermal and mechanical properties can be obtained; e.g., Young's modulus can range from 1.5 to 75.7 MPa. The effect of cross-link density on the phase morphology in these materials is also discussed.