Dual-Dynamic Cross-Linked Epoxy Vitrimers for Self-healable, Recyclable Thermoset and, Composite Applications

Abstract

The design of sustainable materials with recyclability and reprocessability is an area that has received much attention due to the increasing amounts of non-degradable thermoset and composite waste. These could be achieved through the incorporation of covalent adaptable networks in cross-linked systems. This work reports the synthesis of a dual dynamic bond-based epoxy system with imine and disulfide bonds in the epoxy monomer. 4-hydroxy benzaldehyde and 2,4-dihydroxybenzaldehyde were transformed into the respective oxiranes by reacting with epichlorohydrin, subsequently followed by reaction with cystamine. The formed mono- and di-oxirane Schiff base molecules incorporated dual dynamic bonds, namely disulfide and imine linkages. Structure-property correlation studies were undertaken for reversible thermosets formed by curing the dual dynamic epoxy molecule (DIMTO) based on 2,4-dihydroxybenzaldehyde with three different diamines (linear, cyclic and aromatic) as curing agents. The epoxy thermosets for the DIMTO/PXDA, DIMTO/IPDA, and DIMTO/HMDA exhibited Tg values of 150 0C, 141 0C, and 110 0C, respectively. These values are comparable to those reported for commercial epoxies based on Bisphenol A diglycidyl ether (DGEBA) system. The dynamic nature of the bonds in the thermosets was confirmed based on their ability to reshape under the influence of temperature. The DIMTO/IPDA system demonstrated good mechanical properties with a tensile strength of 53 MPa. The DIMTO/IPDA system demonstrated excellent chemical degradation using an aqueous acetic acid solution and water at 70 0C for 14 hrs. Epoxy thermoset DIMTO/IPDA was successfully acid-hydrolysed, reshaped, and self-healed above the topology-freezing temperatures.