Impact of pre-crosslinks on the self-transformation performance of thermoplastic polyesters into vitrimers via intermolecular transesterification

Abstract

We recently proposed a concept of self-transformation from thermoplastic polyesters into vitrimers via intermolecular bond exchange as the cross-linking reaction. Key was the use of polyesters bearing hydroxyl side groups, which were cross-linked without additional cross-linkers through intermolecular transesterification in the presence of a suitable catalyst. In our previous study, a linear polyester was synthesized as the starting polymer by reacting dithiol monomers containing ester bonds (2-SH) with diepoxy monomers (2-epoxy) via a thiol–epoxy reaction, generating hydroxyl side groups along the polyester chain. In the present study, we introduce tetra-functional epoxy monomers (4-epoxy) into the starting materials, with the proportion of 4-epoxy limited to be below the experimentally determined percolation threshold. Consequently, the initial polyester still behaves as a thermoplastic, while the 4-epoxy units serve as pre-crosslinks. We evaluate the impact of 4-epoxy on bond-exchange-based material transformation, based on the thermal, mechanical, and rheological analyses. The important findings are improvements in the time required for transformation and enhancements in the mechanical properties of the final cross-linked products, which could contribute to the practical application of the bond-exchange based transformable materials.