Study On The Structural Properties Of Shape Memory Polyurethane/p-alkoxybenzoic Acid Liquid Crystal Composites

Shape memory polyurethane(SMPU) has attracted more and more attention because of a broad adjustable transition temperature, large deformation, good biocompatibility, active and convenient shape change in response to external condition. However, general SMPU with only one functionality has not met the growing trend of the new muti- functional materials. Liquid crystals have been widely used in photovoltaic devices, information storage and nonlinear optical materials because of its unique photo electromagnetic properties. Therefore, the research was carried out in this study to better combine the shape memory properties and liquid crystal characteristics. Thus, materials with many functionalities are developed.The research focuses on the studies of the influences of different structures and content of the mesogen on the structures and properties of the shape memory polyurethane liquid crystal composites. SMPU with a 40wt% hard segment content was synthesized by choosing polyethylene glycol 6000(PEG6000) as soft segment, liquefied 4,4'-diphenylmethane diisocyanate(MDI) and 1,4-butanediol(BDO) as hard segment. And it was served as the substrate material Three series of shape memory polyurethane liquid crystal composites were prepared by incorporating 4-alkoxy benzoic acid with different molecular structure and liquid crystal features, such as 4- hexadecyloxy benzoic acid(HOBA), 4-dodecyloxy benzoic acid(DOBA), 4-octocyloxy benzoic acid(OOBA) into SMPU with solution blending method. For each series, the structures and properties were characterized systematically. This study might lay the foundation for the multi- functional properties, multiple responsiveness,efficient actuating way of this kind of composites.Firstly, the structures and properties of the SMPU-HOBA composites were investigated. The results indicate that the dimerization through hydrogen bonding creates along lath-like structure with a three-ring core in the composite, retaining the basic structure of HOBA to form the liquid crystal. Meanwhile the crystallization ability of soft segment and hard segment are both enhanced. However, the crystallization rate of hard segment decreases with the increase of HOBA content. The thermal stability of SMPU is also improved as the incorporation of HO BA. Morphology analysis shows that there are hard segment phase of SMPU, soft segment phase of SMPU, and HOBA phase in the composites. The mesogenic behaviors of the composites are similar to that of HOBA. The composites show a broader mesogenic temperature range, and the small spherulite of hard segment above the clearing point was also observed. Moreover the crystallization transition of soft segment of SMPU and crystallization transition of HO BA endowed the composites with triple shape memory properties.Secondly, the structures and properties of the SMPU-OOBA composites were characterized. The results show that OOBA in the composites also retains the dimeric structure, and it improves the the crystallization ability of soft segment and hard segment of SMPU. The stability of the crystal is increased after heating treatment. Mesogenic features results indicate that the mesogenic behaviors and phase transition behaviors of the composites are similar to that of OOBA. At lower temperature the crystallization of soft segment of SMPU and crystallization of OOBA were observed in the composites. With the temperature increasing, the crystal of soft segment of SMPU and OOBA gradually melted. The OOBA entered into the liquid crystal phase, exhibiting nematic liquid crystal texture. Meanwhile, the spherulite of hard segment of SMPU appeared gradually. DMA results show that the crystal melting transition of soft segment of SMPU and OOBA lead to the two stages of modulus changes. Therefore, SMPU-OOBA composites also perform triple shape memory properties. The shape fixation ratio decreases with the increase of OOBA content, as the lubrication of OOBA resulted in a permanent deformatio n. However, the liquidity and crystallizability of OOBA made the composites show self-healing behaviors.Finally, the structures and properties of the SMPU-DOBA composites were studied. DOBA in the composites retains the dimeric structure to form the liquid crystal characters, which is similar to that of HOBA and OOBA. The introduction of DOBA into the composites enhanced the crystallizability of soft segment and hard segment of SMPU. The isothermal crystallization kinetics results reveal that the crystallization rate of hard segment decreases with the increase of the content of DOBA in the SMPU-DOBA composite. POM results showed the crystallization and mesomorphic transition process. Meanwhile, the small spherulite of the hard segment above the clearing point was still observed. DOBA was scattered in the matrix SMPU with a lower DOBA content in the composites. Thus, it is difficult to form a single DOBA phase. However, DOBA can form a scattered phase and show individual liquid crystal features with a moderate DO BA content in the composites. Also triple shape memory behaviors were demonstrated in the SMPU-DOBA composites.