Gelation and Cross-Linking in Multifunctional Thiol and Multifunctional Acrylate Systems Involving an in Situ Comonomer Catalyst

Dynamic rheology in combination with Fourier transform infrared spectroscopy (FTIR) is used to examine the gelation kinetics, mechanism, and gel point of novel thiol-acrylate systems containing varying concentrations of an in situ catalyst. Gelation, as evidenced from the gel time determined using the Winter-Chambon criterion, is found to occur more quickly with increasing catalyst concentration up until a critical catalyst concentration of 22 mol %, whereupon the gel time lengthens. Such a minimum in gel time may be attributed to changes in the number of available reaction sites and percentage conversion required for gelation. Chemical conversions at the gel point measured for representative samples are consistent with theoretical values calculated using Flory-Stockmayer's statistical approach, confirming our hypothesis. Relaxation exponents of 0.97 and fractal dimensions of 1.3 are calculated for all samples, consistent with coarse-grained discontinuous molecular dynamics (DMD) simulations. The elevated value of n may be due to the low molecular weight prepolymer. The relaxation exponent and fractal dimensions are invariable over all systems studied, suggesting the cross-linking mechanism remains unaffected by changes in catalyst concentration, allowing the gel time to be tailored by simply modulating the catalyst concentration.