Development of Vanillin-Based Crosslinking Agent with Phase-Locked Dynamic Imine Bonds for Shape-Memory Polyurethanes

Scientists have gained so much attention due to one of polyurethane's tuneable and unique characteristics, i.e., the shape-memory effect. But, the simplistic fabrication of such materials persists a challenging work. In the current work, we report the synthesis and detailed characterization of shape-memory polyurethane obtained by incorporating lower concentrations of imine bonds, which readily undergo phase-locking systems beneficial for microphase separation. A symmetric imine moiety (DIM) was designed through the simple Schiff-base reaction of naphthalene diamine and vanillin. The tensile strength of the films reached up to 30 MPa with an %elongation of 87% by tuning the hard segment content and the concentration of imine bonds. Also, in the meantime, the films showed excellent shape memory effect, where the shape could recover its original form within 2 s at the highest loading of DIM. The remarkable balance between the mechanical properties and the shape memory effect could be associated with the two main factors. Firstly, incorporating symmetrical and rigid C--N along with the naphthalene promoted the microphase separation and smoothened the orientation of the chain segments at the strain-induced crystallization at lower temperatures. Secondly, the phase locking occurred at lowered temperatures due to dynamic imine exchange reaction and H-bonding. The phase recovery started, and chains settled at their original positions once the temperature was elevated.