Clarifying the Role of Epoxy Molecules and Base Catalysts for Vitrimer Transformation from Linear Polyesters

Vitrimers are cross-linked materials with associative bond exchange mechanisms, exhibiting sustainable functions such as healability and recyclability. Transformation of commercial polyesters into vitrimers has been recently explored and regarded as an important tool for practical application of the vitrimer concept. However, the mechanism of such vitrimer transformation processes has not been adequately understood. In this study, we thus tackle this point based on the system using amorphous polyesters, multifunctional epoxy cross-linkers, and base catalysts through relatively simple experimental tools, such as size extrusion chromatography, spectroscopy, and rheology measurement. Concretely, we assess chain decomposition due to the presence of base catalysts, chain extension via the epoxy opening reaction, and network formation via bond exchange (i.e., transesterification), which occurs during the melt blending in a twin extruder and subsequent heat treatment. Particularly, we find the generation of some fraction of short sol chains and formation of branched dangling points on the network strands, which may be indications of the occurrence of transesterification. Overall, this study provides some important insights into the vitrimer transformation from polyesters.