Photo-Plasticity in Thiol-ene Network Polymers - A Review

In this paper we briefly overview the area of stimuli responsive polymers and the subset of photo-plastic polymers which change shape or undergo stress relaxation during irradiation. The chemistry, mechanism and properties of a new class of photo-plastic polymers is presented in detail - these tetrathiol-diene networks contain an allylic dithioether (also termed allylic disulphide) unit in the network strands and when this moiety is attacked by a photogenerated radical, the allylic dithioether undergoes β-cleavage which breaks one network bond and reforms another. Two different methods of integration of the allylic dithioether unit into the backbone are investigated - the use of a cyclic allylic dithioether which copolymerizes with a thiol-ene system or the incorporation of the allylic dithioether moiety in a divinyl ether monomer which can directly copolymerize with the thiol. The photo-plasticity effect of these thiol-ene networks is revealed in both creep and stress relaxation experiments, however, the extent of photo-plasticity is less extensive than expected. The polymerization kinetics and evolution of the rheological properties during photocuring are studied and the slower copolymerization of the monomer containing the allylic dithioether is discussed. The photo-rheology and dynamic mechanical properties of the thiol-ene networks are interpreted in terms of the network structure.