Thiol–acrylate based vitrimers: From their structure–property relationship to the additive manufacturing of self-healable soft active devices

Abstract

Thiol-acrylate based vitrimers combine the advantages of dynamic covalent bonds with the salient features of “click” chemistry including low shrinkage stress and homogenous network properties. The poor storage stability of thiol-acrylate formulations can be overcome by adding an acidic organic phosphate as stabilizer, which at the same time acts as an efficient catalyst to induce transesterifications at elevated temperature. At a sufficiently high rate, the dynamic bond exchange reactions lead to topological rearrangements and a related macroscopic reflow of the photopolymer network. This paves the way towards dynamic photopolymers, which are thermally mendable, malleable, reprocessable and have shape memory properties. In this work, a library of thiol-acrylate monomers for the preparation of digital light processable (DLP) vitrimers is systematically studied. In particular, thiol crosslinkers with varying functionality and number of ester moieties are employed and combined with selected mono- and bi-functional acrylates bearing free –OH groups. In addition, vitrimer networks with higher crosslink density are prepared by adding tri- and tetra-functional acrylates to the formulations. In a comprehensive way, the effect of crosslinking density, monomer structure, network architecture and thermal annealing on cure kinetics, mechanical, thermal and vitrimeric properties of the dynamic photopolymers are studied. The results reveal that by appropriate network design, thermal and mechanical properties of the networks can be controlled over a wide range. In particular, a high mobility of the polymer chains together with a high number of –OH and ester groups is beneficial for inducing fast exchange reactions. The proposed thiol-acrylate based vitrimers show a great potential for the personalized fabrication of soft active devices and their applicability in DLP 3D-printed soft actuators with adequate stretchability and thermal mendability is highlighted.