Thiol-epoxy-acrylate hybrid networks are formed by the combination of thiol-acrylate Michael addition and nucleophilic thiol-epoxy coupling catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in a one-pot synthesis. A stoichiometric balance between thiol groups (multifunctional thiol) and the addition of epoxide (difunctional epoxy) and acrylate groups (difunctional acrylate) is applied in the reactant mixture. Full conversion is achieved for these two combined thiol reactions based on the disappearance of the thiol absorbance peaks from Fourier transform infrared spectroscopy (FTIR), demonstrating the high efficiency of thiol click reactions. With relatively high molecular weight (MW = 2000 g/mol) acrylates, microphase-separated network materials are produced, as evidenced by the presence of two glass transition temperatures from both differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), and also the morphology characterization by scanning electron microscopy (SEM). Crosslink density of the hybrid networks is systematically controlled by substituting the multifunctional thiol with different amounts of difunctional thiols while maintaining stoichiometric balance between reacting groups. By changing crosslink density in the hybrid networks, a wide range of thermal and mechanical properties is obtained, e.g., Young’s modulus can range from 0.4 to 75.7 MPa. The effect of crosslink density on the phase morphology in these materials is also studied, and the correlation between the network morphology and macroscopic properties is discussed. The synthesized hybrid networks show promise in elastomer applications.
Isothermal polymerization kinetics of two-component nucleophilic thiol reactions, including thiol-epoxy and thiol-acrylate reactions catalyzed by DBU, are characterized by DSC to gain insight into the formation of the thiol-epoxy-acrylate hybrid networks. It is observed that at the same DBU loading the thiol-epoxy reaction occurs at a relatively slower rate compared to the thiol-acrylate Michael addition reaction. For example, the reaction of a stoichiometric mixture of a trifunctional thiol and a difunctional epoxy with 0.167 % mol DBU/mol thiol groups achieved full conversion at 60 oC within ~ 5 hr while the reaction between the same thiol and the long chain diacrylate went to complete conversion in ~ 1 hr under the same conditions. The potential correlation between the hybrid network properties and the reaction time scale difference in these two reactions is discussed. More specifically, autocatalytic behavior is observed in isothermal DBU-catalyzed thiol-epoxy reactions and the kinetic parameters of the reaction, including rate constants, reaction order, and activation energy, can be obtained by fitting the experimental data to Kamal autocatalytic model (J. Polym. Eng. Sci. 1974, 14, 232-239). In terms of the DBU-catalyzed thiol-acrylate Michael addition reaction, the overall reaction follows the first-order kinetics. The effect of monomer structure (e.g., chain length of the difunctional acrylate) on the thiol-acrylate Michael addition reaction kinetics is also studied.
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