Insights into liquid-crystalline shape-memory polyurethane composites based on an amorphous reversible phase and hexadecyloxybenzoic acid

Abstract

Liquid-crystalline polymers and shape-memory polymers are attractive to many researchers. This paper proposes a strategy to prepare liquid-crystalline shape-memory polyurethane (LC-SMPU) composites having both liquid-crystalline properties and shape-memory properties. A series of LC-SMPU composites are successfully prepared by doping different concentrations of 4-hexadecyloxybenzoic acid (HOBA) into shape-memory polyurethanes (SMPUs). The results show that the HOBA is physically mixed with the polyurethane matrix. The doped HOBA is categorised as HOBA captured by the polyurethane chains and free HOBA in the LC-SMPU composite. A higher content of HOBA results in a higher fraction of free HOBA, and more HOBA molecules enter into the polyurethane matrix at higher annealing temperatures. The resulting LC-SMPU composites keep their intrinsic liquid-crystalline properties and the crystallisation properties of HOBA. The LC-SMPU composites contain a two-phase, separated structure: a HOBA crystalline phase and the polyurethane matrix. Thus, the LC-SMPU composites show a two-stage decrease in modulus. Finally, shape-memory tests show that the LC-SMPU composites exhibit a triple shape-memory effect; in the first step, strain recovery is associated with the glass transition of the amorphous polyurethane matrix, and in the second step, strain recovery results from the melting transition of the HOBA crystal dopants.