High-Performance Bio-Based Epoxy Vitrimers and Composites: Synergizing Robustness and Reprocessability via Fractional Free Volume Regulation.

The trade-off between crosslink density and segment mobility challenges bio-based epoxy vitrimers to simultaneously achieve high mechanical robustness and dynamic adaptability. Here, we report that engineering the fractional free volume serves as an effective structural parameter to decouple this longstanding conflict. By designing bifunctional and trifunctional epoxy monomers from renewable resources, we construct dynamic networks integrating rigid conjugated Schiff bases with flexible siloxane segments, enabling deliberate manipulation of free volume characteristics within the hybrid architecture. Molecular dynamics simulations reveal that the bifunctional system, which possesses a moderately higher fractional free volume, simultaneously achieves high crosslink density and enhanced segmental mobility. This unique combination promotes efficient stress dissipation and accelerates bond exchange kinetics, successfully reconciling strength and toughness without compromising network dynamics. Consequently, the optimized vitrimer exhibits exceptional mechanical properties alongside rapid stress relaxation and self-healing capability. When employed as a composite matrix, it enables non-destructive carbon fiber recovery, as well as adhesive-free welding and thermoforming. This work establishes fractional free volume as a practical design lever for creating high-performance, sustainable covalent adaptable networks, thereby providing a route from empirical toughening toward topology-directed property optimization.