Effects of Chemical Modification of Thiol-Ene Networks on Enthalpy Relaxation

The highly uniform and dense network structure of photopolymerized thiol-enes was chemically modified. and the enthalpy relaxation of the networks was measured. n-Alkyl acrylate and hydroxyl acrylate groups were incorporated into thiol-ene networks using a phosphine-catalyzed Michael addition reaction. The effect of flexible alkyl side chains and hydrogen bonding on sub-T-g relaxation was evaluated without sacrificing network uniformity. Overall both the rate and extent of enthalpy relaxation decreased as a function of the flexible n-alkyl chain length, while hydrogen bonding resulted in enhanced enthalpy relaxation. A trithiol-triene-triacrylate tertiary system was investigated by correlating enthalpy relaxation and network uniformity. A multifunctional acrylate (TMPTA), being capable of homopolymerization as well as thiol-acrylate copolymerization, was incorporated into a thiol-ene network structure, thereby decreasing the network uniformity and significantly affecting the enthalpy relaxation behavior. In all cases, the extent and rate of enthalpy relaxation were directly related to the heat capacity change at the glass transition which defines the enthalpic departure from equilibrium at it given temperature below T-g.