Design of photocurable thiol-epoxy resins for the processing of vitrimers with vat photopolymerisation 3D printing

Vat photopolymerisation 3D printing is a convenient approach to fabricating complex polymeric objects based on the localised photocuring of a liquid resin. Advancing from free-radical mediated curing mechanisms, the current study focuses on the vat photopolymerisation 3D printing of photopolymers formed by an ionic click reaction between thiol and epoxy monomers. In this process, selected epoxy monomers are mixed with a tetra-functional thiol crosslinker and a photobase generator, which releases 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) upon light exposure to initiate the anionic ring-opening reaction. FTIR, photo-DSC and rheology studies reveal a slow cure rate of the thiol-epoxy resins at room temperature, which is significantly accelerated by increasing the temperature to 50 °C during light irradiation. By optimising the resin composition, shelf life and viscosity, printing of objects via digital light processing 3D printing is enabled by using a vat heated to 50 °C. The printed thiol-epoxy networks contain ample -OH and ester moieties, which undergo thermo-activated bond exchange reactions. Here, the photochemically liberated DBN molecules act as catalysts to accelerate the transesterification reaction at elevated temperatures (150–180 °C). Rheological measurements demonstrate the efficient and Arrhenius-like behaviour of the networks' stress relaxation, giving rise to the dynamic nature of the printed photopolymers, which is further confirmed by reshaping experiments.