Insights into the mechanism of vitrimer reconsolidation via X-ray tomography

Abstract : Vitrimers are a type of crosslinked polymer that can undergo mechanical recycling – one of the simplest and most efficient reprocessing methods. An easy way to implement this method consists of grinding the vitrimer into millimeter-sized particles, followed by compression-molding at a temperature high enough to both exceed the glass transition temperature and enable rapid associative covalent bond exchange, but low enough to avoid thermal degradation. In spite of the facile nature of the process, many open questions remain, including how reconsolidation occurs and how the reconsolidated material behaves. This study uses a design-of-experiments approach to investigate the structural rearrangements of a polybenzoxazine (PBz) vitrimer during mechanical recycling as a function of reprocessing parameters. Through the application of micro-computed X-ray tomography (μCT), it is found that particle size is the most important factor controlling porosity, followed by the compression-molding temperature. However, if the compression-molding temperature and/or time are too high, this can induce cracks in the material due to degradation. The mechanisms of pore closure have been investigated as a function of time by considering the correlation between pore volume and sphericity, with three successive steps revealed. It is further demonstrated that increasing the dynamic bond content in the vitrimer significantly decreases the porosity after reprocessing. Overall, this work shows that, under well-defined preparation and reprocessing conditions, pore-free recycled vitrimers can be obtained, which is highly desirable to retain the mechanical properties of the non-recycled vitrimer. • Vitrimer reconsolidation during mechanical recycling is studied by x-ray tomography • Porosity is mainly controlled by particle size followed by reprocessing temperature • Increasing vitrimers dynamic bond content reduces porosity after reprocessing • Pore closure mechanism involves three successive stages related to pore sphericity