Preparation And Applications Of Lignin-based Photoresponsive Liquid Crystalline Polymer Materials

Lignin is a kind of renewable sources, which is extremely abundant. Lignin-based highly valued products have potential applications in several fields. Preparing lignin-based liquidcrystalline materials by modification of lignin and studying the involved scientific problems and technologies were entiltled important theoretical and practical values. Main research products are as following:Hydrogen bond supramolecules were synthesized by self-assembling between carboxyl groups in lignin and pyridyl groups in azo pyridine derivatives. Subsequently, the molecular structure was confirmed by FT-IR, and characteristic peaks around 2490 cm-1、2050 cm-1、1967 cm-1 belonging to hydrogen bond were shown in FT-IR curves. The results of differential scanning calorimeter(DSC), polarizing optical micrograph(POM) and X-ray diffraction(XRD) illustrate that the strong rigidity of lignin limits the movement and alignment of supramolecules. Janus film could be fabricated by hydrogen bond and layer by layer self-assembly. What’s more, the wetting properties on both sides were distinct. The average values of the contact angels on Janus film at the water-side and air-side were measured to be 70.3o and 109.8o, respectively.Lignin-based macroinitiator was synthesized by nucleophilic substitution reaction of lignin and 2-bromoisobutyryl bromide, which was used to initiate Atom Transfer Radical Polymerization of one azobenzene-containing monomer to obtain a series of lignin-graft photoresponsive liquid-crystalline copolymers. The molecular structures and molecular weights of graft copolymers were confirmed by 1H-NMR, FT-IR and GPC. The thermal properties of lignin-graft azobenzene-containing copolymers showed improved behaviors superior to that of unmodified lignin by measurement of thermal gravimetric analyzer(TGA) and differential scanning calorimeter(DSC). Furthermore, the modified lignin-graft azo copolymers exhibited liquid-crystalline behaviors once their average grafting ratio were higher than 72.8% and the molecular weights of lignin-graft azo copolymer were higher than 19500 correspondingly. The liquid-crystalline phases of lignin-graft azobenzene copolymers were shown to be smetic C and the layer spacing were 3.21 nm evaluated by small-angel X-ray scattering(SAXS) and polarizing optical micrograph(POM). Moreover, the photoresponsive properties of lignin-based liquid-crystalline copolymers were investigated using UV-Vis absorption spectroscopy, and the lignin-graft liquid crystalline copolymers in solution were more sensitive than that of as-cast films. Photochemical liquid crystal to isotropic phase transition was successfully obtained upon UV irradiation, and the reversible isotropic to liquid crystal phase transition was exhibited rapidly when being exposed to visible light.Self-healing soft matters have attracted much attention because of their extraordinary performance for extending working life of materials. To elegantly use sunlight to help selfhealing of liquid-crystalline gels composed of one low-mass liquid crystal(5CB) and one hyperbranched lignin-graft-PMMA, both graphite oxide(GO) and one azobenzene compound are doped with a low content as photoresponsive additives. Upon UV irradiation, the azobenzene derivative can induce gel-sol transition of the gel due to the photoinduced molecular cooperative motion, thus surface dents can be repaired. On the other hand, GO functions as the nanoscale heat source because of the photothermal effect under exposure of visible(VIS) or NIR light, heating the gel to induce gel-sol transition for mending surface cracks. In addition, the mechanical properties of the gels are also improved by addition of GO. The NIR-VIS-UV responsive liquid-crystalline gels shows highly effective gel-sol transition upon solar radiation because of the coexistence of both photochemical and photothermal effect. Furthermore, these sunlight-assistant self-healing gels also show anisotropy and orientation just like other liquid-crystalline materials, which would enable them to find various advanced applications with longer service life.