The impact of liquid crystal fillers on structure and properties of liquid-crystalline shape-memory polyurethane composites I: 4-dodecyloxybenzoic acid

Abstract

Aiming to elucidate the influence of liquid crystal fillers on the structure and properties of liquid-crystalline (LC) shape-memory polyurethane (SMPU) composites, a series of LC-SMPU composites are successfully prepared by adding various concentrations of 4-dodecyloxybenzoic acid (DOBA) to shape-memory polyurethane containing 40 wt% hard segment content (SMPU40). The structure, morphology, thermal properties, liquid crystalline properties, and shape memory properties are systematically investigated. The results demonstrate that DOBA is successfully incorporated into the polymer matrix of SMPU40 without interrupting the dimerization structure of DOBA that allows it to exhibit LC properties. The incorporated DOBA not only improves the crystallizability of the soft segment but also promotes the crystallizability of the hard segment. The isothermal crystallization kinetics further reveal that the crystallization rate of the hard segment decreases as the DOBA content increases, and the crystallization of the LC-SMPU composites nucleates in a similar manner. The LC-SMPU composites are composed of a DOBA phase and an SMPU phase, which further forms soft phase–hard phase microphase separation structures. The composite tends to only maintain the nematic LC properties of DOBA due to the interruption of polyurethane chains. However, the LC-SMPU composites exhibit good triple-shape memory effects. The first step of strain recovery is associated with the melting transition of soft segment crystals of the polyurethane matrix, and the second step of strain recovery is resulted from the melting transition of the DOBA crystals.