Vitrimer composites based on aligned graphite flakes: Accelerated and anisotropic stress relaxation and malleability

Vitrimers have attracted great interest in the past a dozen years and several ways have been developed to improve their appealing dynamic properties caused by the networks rearrangement. However, it is still a great challenge to improve their dynamic properties like stress relaxation and malleability by introducing fillers that commonly restricted chain mobility in vitrimer composites. Herein, vitrimer composites including single-layer epoxy vitrimer (SEV) composites and multi-layer epoxy vitrimer (MEV) composites were prepared by introducing aligned graphite flakes (GFs) into epoxy vitrimer matrix. Different from the suppressing effect of particle fillers in vitrimer networks, the aligned GFs improved the dynamic properties of vitrimer composites, which accelerated the stress relaxation behaviors and optimized the malleable properties. Meanwhile, the vitrimer composites exhibited unique anisotropic stress relaxation and malleability interestingly. For instance, the characteristic time (τ) of stress relaxation decreased from 1190s (reference epoxy vitrimer) to 863s (MEV composites-horizontal) and 773s (MEV composites-vertical) at 160 °C, while the critical temperature (Tv) of malleability decreased from 160 °C (reference epoxy vitrimer) to 155 °C (MEV composites-horizontal) and 143 °C (MEV composites-vertical). Moreover, the activation energy (Ea) of vitrimer composites increased and their prefactor τ0 decreased, resulting in their improved stability at service condition and promoted dynamic properties at malleable state. The accelerated and anisotropic dynamic properties of vitrimer composites was achieved by introducing aligned GFs for the first time.