Effect of Cross-Link Density on Photoplasticity of Epoxide Networks Containing Allylic Dithioether Moieties

A new diepoxide monomer (ADTDE) containing an allylic dithioether moiety has been synthesized to extend the suite of monomers that form networks exhibiting radical-mediated photoplasticity. These cross-linked polymers show permanent shape change under load and stress relax only when under the stimulus of visible or UV radiation. The Lewis base-catalyzed reaction of the epoxide groups in ADTDE with tri- and tetrafunctional thiols was monitored by differential scanning calorimetry and infrared spectroscopy. These curing systems were found to be more efficient than anionic, cationic, or anhydride polymerizations. The dynamic mechanical thermal analysis of ADTDE with the tetrathiol showed that the network was rubbery at room temperature which is considered ideal for the demonstration of photoplasticity. The photoinduced creep behavior was shown to be controlled by the presence of active free radicals in the matrix-on cessation of irradiation, the creep was terminated. The stress relaxation of the stretched network specimens during UV irradiation was studied and was found to be similar to that shown previously for thiol-vinyl ether networks. As expected, the cross-link density of networks prepared from ADTDE with the trithiol was lower than that formed with the tetrathiol and this was found to have a significant influence on the rate and extent of photoplasticity observed-the less highly cross-linked ADTDE/trithiol system exhibited more rapid and more extensive photoplasticity since it contained fewer network strands.