Designing Rigid–Flexible Epoxy Resins to Unlock Shape Memory and Enhance Toughness

It remains a challenge to balance the trade-off between toughness, stiffness, and shape memory properties in shape memory epoxy resins (SMEP). In this study, we employed molecular engineering to synthesize stiff–flexible SMEP, by reacting furan-containing epoxy monomers 2,5-bis[(2-epoxymethoxy)methyl]furan (BOF), derived from biomass, with amines containing imine-amine (IA) and 4,4′-dithiodiphenylamine (4-DTDA). The results indicate that the sacrificial hydrogen bonding and the necessary flexible segments confer BOF/4-DTDA/IA (BDI) with a toughness of up to 13.9 MJ/m3 and a stiffness of 3.5 GPa. Furthermore, the introduction of hyperbranched epoxy resin (HER) yields BOF/4-DTDA/IA/HER (BDIH), which maintains a stiffness exceeding 2 GPa while retaining a toughness of 11.2 MJ/m3. Additionally, BDIH exhibits enhanced tensile strength with an increasing strain rate, which we rationalize based on the loss modulus and scanning electron microscopy morphology analyses. The collaborative motion of the rigid–flexible chain segments and the hydrogen bonding contributes to BDI’s excellent shape memory performance (shape fixation ratio (Rf = 99.00%) and shape recovery ratio (Rr = 99.15%). The physical barrier effect and flexibility of the HER further enhance the shape memory properties. In conclusion, the mechanical features and shape memory performance of BDI and BDIH demonstrate the significant potential for applications in smart molds, smart devices, and other advanced technologies.