Shape memory polyurethane - Amorphous molecular mechanism during fixation and recovery

Abstract In this work, we investigate the governing fixation mechanism in a polycaprolactone-diol/2,4,2,6-Toluene diisocyanate/Ethylene glycol based shape memory polyurethane (SMPU). In particular, we test whether strain-driven crystallization, known to govern the SMPU fixation state in semi-crystalline SMPUs, is indeed vital for a significant memory capability. As synthesized, the SMPU exhibits semi-crystalline morphology. However, after heating, the synthesized semi-crystalline SMPU becomes amorphous and retains this state for up to 60 h, so that its crystallization is delayed. Consequently, this enables a complete shape-memory cycle without the effect of crystallization over the shape-memory properties. We show that in the amorphous state, the bulk SMPU displays substantial memory-effect capabilities approaching optimal performance, without contribution from crystallization mechanisms. This effect is further enhanced in electrospun SMPU nanofibers, likely as a result of the high molecular orientation induced by extensional flow. We propose that the dominant fixation and recovery mechanism in such SMPUs is the inhibition of molecular chain mobility, imposed by interlocking of the SMPU hard-segments when the polymer network is cooled below the transition temperature.