The influence of liquid crystals on the properties of sisal fibre polyurethanes with multi-shape memory effects

This paper aims at elucidating the influence of liquid crystals (LCs) on the properties of shape memory polymers. A novel series of liquid crystalline shape memory polyurethanes, named LC-SF-SMPUs were successfully prepared by chemically cross-linking polyester sisal fibre cellulose (PSF) with polycaprolactone (PCL) and 4,4′-di(6-hydroxyhexyloxy)biphenyl (BP6). The effect of BP6 on the morphology and properties has been carefully investigated. The results show that the cross-linking structures provided a stable network, high elasticity and good thermal stability to the LC-SF-SMPUs, and demonstrate that the obtained LC-SF-SMPUs not only maintained the LC property of BP6, but also enhanced the intrinsic crystallization of PCL. Thus, the LC-SF-SMPUs showed excellent multi-shape memory properties with two crystalline transition temperatures. In the triple-shape memory effect of LC-SF-SMPUs, the first step of strain recovery is associated with the melting transition of the soft segment crystals of the polyurethane matrix, and the second step of strain recovery results from the melting transition of the LC mesogen. The strain fixity improved with increasing BP6-content, due to the enhanced crystallinity. However, the total strain recovery decreased as a result of the lubrication of the BP6 chains. This work demonstrates a new combination of LC properties and multi-shape memory properties, which makes it ideal for many potential applications in new smart devices.