Enthalpy Relaxation of Photopolymerized Thiol-ene Networks: Structural Effects

Physical aging behavior of photopolymerized thiol-ene networks was investigated by measuring the extent of enthalpy relaxation in terms of network density and molecular structure. The homogeneous network structure of the thiol-enes, having narrow glass transition temperature ranges, showed characteristic temperature and time dependency relationships for enthalpy relaxation. All thiol-ene films annealed at different temperatures (T-a) for I h according to the isochronal method showed maximum enthalpy relaxation peaks at approximately T-g - 10 degrees C by DSC. The extent of enthalpy relaxation as a function of annealing time (t(a)) was obtained by the isothermal aging method. Correlations between the extent of enthalpy relaxation and the heat capacity difference at T-g were made and related to thiol-ene chemical group rigidity and network linking density. Pendulum hardness values for a selected thiol-ene film showed a clear change in hardness upon aging, indicating sub-T-g mechanical relaxation, consistent with the related enthalpy relaxation process.