Radical-mediated thiol-ene chemistry has become popular in the development of advanced polymer networks. It offers salient attributes of a click reaction, for instance, high yields, high efficiency, negligible formation of side products and insensitivity to oxygen. More precisely, photoinduced thiol-ene reactions benefit from mild reaction conditions and allow for spatial as well as temporal tuning of material properties owing to the step-growth mechanism of the photopolymerization between alkene and thiol.
Photo-induced thiol-ene reactions enhance the homogeneity of network structures resulting in low polymerization shrinkage stress together with narrow glass transition regions. Based on these advantages, thiol-ene chemistry is getting increased attention in the manufacture of micro-gadgets and is majorly used in soft imprint technology. The uniform structure allows for high resolution of the inscribed patterns and even permits the synthesis of nanostructured polymer networks that are necessary for electronic components such as wires and resistors.
Researchers led by Dr. Sandra Schlögl at the Polymer Competence Center Leoben in Austria studied the attributes of uniform thiol-ene networks in comparison to thiol-acrylate networks that were characterized by a higher heterogeneity reference to the combined step and chain growths in the process of polymerization. Their work is published in Advanced Engineering Materials.
Selected thiol esters were prepared that bear two and four functional thiol groups. In view of the ‘ene’ component 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione as well as tris[2-(acryloyloxy) ethyl] isocyanurate were applied to enable a direct correlation between thiol-ene and thiol-acrylate networks. Polymerization was then done upon UV exposure at wavelengths between 250 and 470nm.
The authors used FT-IR spectroscopy in order to determine the crosslink kinetics of the various resin formulations. This gave an insight into the yield of homopolymerrization in thiol-allyl and thiol-acrylate systems. In addition, the authors performed low-field NMR analysis in order to get information on the network structure. Mechanical attributes were however determined by tensile tests.
Gel fractions were estimated from equilibrium swelling analyses and were correlated with NMR experiments. In view of the molecular structure of the monomers over the dynamics of the polymer networks to the ultimate mechanical properties, the authors obtained a correlation of process factors and related thermo-mechanical performance of thiol-acrylate and thiol-ene systems.
The research team successfully prepared binary thiol-acrylate and thiol-allyl networks through radical-mediated thiol-ene chemistry. In the thin film, the photo-induced step-growth mechanism was determined by the reactivity of both the ene and the thiol component. For thiol-acrylate systems, the reaction kinetics was majorly influenced by the simultaneous chain growth of the acrylate monomers, and the homopolymerisation of the ene component played a role in thiol-allyl networks.
FT-IR measurements showed an increase in the yield of homopolymerisation of acrylate as well as allyl monomer when thiols with either low thiol functionality or with low reactivity were applied in the thiol-ene photopolymerization. The study found that the homopolymerisation yield correlated with the heterogeneity of the networks and the thermo-mechanical properties, for instance, glass transition temperature and tensile characteristics. The study further applied the conversion of soluble monomers into insoluble polymers to synthesize negative-toned photoresists with a 25µm structure size.
Melahat Sahin, Santhosh Ayalur-Karunakaran, Jakob Manhart, Markus Wolfahrt, Wolfgang Kern and Sandra Schlögl. Thiol-Ene versus Binary Thiol–Acrylate Chemistry: Material Properties and Network Characteristics of Photopolymers. Advanced Engineering Materials 2017, 19, No. 4, 1600620.Go To Advanced Engineering Materials