Designing Rigid–Flexible\nEpoxy Resins to Unlock\nShape Memory and Enhance Toughness

It remains a challenge to balance the trade-off between\ntoughness,\nstiffness, and shape memory properties in shape memory epoxy resins\n(SMEP). In this study, we employed molecular engineering to synthesize\nstiff–flexible SMEP, by reacting furan-containing epoxy monomers\n2,5-bis[(2-epoxymethoxy)­methyl]furan (BOF), derived from biomass,\nwith amines containing imine-amine (IA) and 4,4′-dithiodiphenyl­amine\n(4-DTDA). The results indicate that the sacrificial hydrogen bonding\nand the necessary flexible segments confer BOF/4-DTDA/IA (BDI) with\na toughness of up to 13.9 MJ/m<sup>3</sup> and a stiffness of 3.5\nGPa. Furthermore, the introduction of hyperbranched epoxy resin (HER)\nyields BOF/4-DTDA/IA/HER (BDIH), which maintains a stiffness exceeding\n2 GPa while retaining a toughness of 11.2 MJ/m<sup>3</sup>. Additionally,\nBDIH exhibits enhanced tensile strength with an increasing strain\nrate, which we rationalize based on the loss modulus and scanning\nelectron microscopy morphology analyses. The collaborative motion\nof the rigid–flexible chain segments and the hydrogen bonding\ncontributes to BDI’s excellent shape memory performance (shape\nfixation ratio (<i>R</i><sub><i>f</i></sub> =\n99.00%) and shape recovery ratio (<i>R</i><sub><i>r</i></sub> = 99.15%). The physical barrier effect and flexibility of\nthe HER further enhance the shape memory properties. In conclusion,\nthe mechanical features and shape memory performance of BDI and BDIH\ndemonstrate the significant potential for applications in smart molds,\nsmart devices, and other advanced technologies.