Dynamic imine bonds based shape memory polymers with permanent shape reconfigurability for 4D printing.

Shape memory polymer (SMP)-based 4D printing combines the advantages of SMP and 3D printing to form active materials with delicate structure. Nowadays, studies of SMP-based 4D printing materials mainly focus on crosslinked (meth)acrylate, of which the permanent shape cannot be changed for their covalent linkage, limiting the usage of 4D printing materials. In this paper, a novel methacrylate monomer with aldehyde group (2-(methacryloyloxy)ethyl 4-formylbenzoate, MEFB) and hyperbranched crosslinker (HPASi) are synthesized to build (meth)acrylate systems (IEMSis) with dynamic imine bonds for 4D printing. The flexible chain structure of HPASi significantly enhances the toughness of IEMSis, which are 33-97-fold higher than that of the one without HPASi (IEM). The addition of HPASi also endows IEMSis good shape memory properties, and the shape fixity ratio and shape recovery ratio of them are 97.5-97.6 % and 91.4-93.7 %, respectively. At the same time, IEMSis can undergo stress relaxation process by dynamic exchanges of imine bonds under relatively mild conditions without catalyst, thus to acquire an ability of permanent shape reconfiguration. The shape retention ratio of IEMSi3 is 84.3 %. In addition, the 4D printed structures displayed here indicate that these 4D printing systems have a myriad of potential applications including aerospace structures, soft robotic grippers and smart electron switches, while the reconfigurability shown by IEMSi3 will expand the scope of application fields of 4D printing materials.