writer：Ya-ting Peng, Tao Wang, Hang Li, Rong Yang, Jin-chun Li
keywords：Azobenzene, Photoresponse, Liquid crystal, Postcrosslinking
source：期刊
specific source：Acta Polymerica Sinica
Issue time：2020年

A series of main-chain azobenzene liquid crystalline copolyesters containing side hydroxyl group (Az-LCP) were synthesized with 4,4''''-bis(6-hydroxyhexyloxy)biphenyl (BHHBP), 4,4''''-bis(6-hydroxyhexyloxy) azobenzene (BHHAB), diethyl malate (DM) and phenyl succinic acid (PSA) by random copolymerization. Chemical structure of the Az-LCPs was characterized by proton nuclear magnetic resonance (1H-NMR) and gel permeation chromatography (GPC). And the phase transition behavior of the Az-LCPs was characterized by differential scanning calorimeter (DSC), and X-ray diffraction (XRD). Az-LCPs showed a nematic phase with a glass transition temperature (Tg) around room temperature. With increasing BHHAB monomer, the Tg and nematic-isotropic transition temperature of Az-LCPs decreased from 25.6 °C and 96.4 °C to 19 °C and 88.2 °C, respectively, showing a trans-cis photoisomerization effect of Az-LCPs. Then monodomain azobenzene liquid crystalline networks (Az-LCNs) were prepared by uniaxial stretching at nematic phase first, postcrosslinking in the hexamethylene diisocyanate solution for 12 h. Moreover, the orientation degree decreased with increasing BHHAB monomer into Az-LCP due to the decreasing Tg and π-π stacking interaction. While being crosslinked with HDI, the Tg of Az-LCNs increased up to 40 °C, meanwhile, the nematic-isotropic phase transition became broad and almost disappeared as the BHHAB monomer increased. All the Az-LCNs showed a UV-light induced bending and vis-light induced unbending behavior at room temperature except Az-LCN4 which contains 50% BHHAB monomer. Crosslinking duration of Az-LCNs also exhibited an influence on the photoresponsive bending/unbending behavior. With increasing crosslinking duration, the bending angle increased first and then decreased. In addition, the maximum bending angle and photoresponsive speed of Az-LCNs decreased with increasing thickness of the Az-LCNs films. Az-LCP1 with 10% azobenzene content and cross-linking for 12 h exhibits excellent photoresponsive behavior with a high bending angle of 88° and the fastest photoresponsive speed.