RHEOLOGY AND PHOTO-CROSS-LINKING OF THIOL-ENE POLYMERS

A dynamic rheological technique, Fourier transform mechanical spectros copy (FTMS), was used to monitor in real time the evolving rheological properties during UV cross-linking of two thiol-ene systems. These sy stems comprised a trifunctional thiol (trimethylolpropane tris(2-merca ptoacetate)) together with a trifunctional allyl monomer (triallyl iso cyanurate) and a tetrafunctional thiol (pentaerythritol tetrakis(2-mer captoacetate)) with the same allyl monomer. FTMS, in conjunction with specially designed quartz plates, provided an in situ method to elucid ate the effects of temperature and monomer functionality on the photoi nitiated polymerization of these systems. It was found that the tetraf unctional thiol system cross-linked at a faster rate than the trifunct ional thiol system over the temperature range (25-50 degrees C) studie d. Moreover, increasing the temperature increased the cross-linking ra tes for both systems. The Winter-Chambon criterion was applied to dete rmine the gel point and the two parameters which characterize the mate rial at its gel point, the gel stiffness, S, and the relaxation expone nt, n. The gel stiffness was found to be greater for the trifunctional thiol system, which was consistent with the higher value of conversio n calculated fi om the Flory-Stockmayer theory of gelation. Relaxation exponents of 0.80 and 0.81-0.82 were determined for the tri- and tetr afunctional thiol systems, respectively, indicating similar fractal st ructures at the gel point. These relaxation exponents were also invari ant over the temperature ranges studied, suggesting that the cross-lin king mechanisms remained unchanged with temperature. From the temperat ure dependence of the gel times, apparent activation energies of 6.6 a nd 14 kcal/mol were calculated for the tri- and tetrafunctional thiol systems, respectively.