木质素接枝偶氮苯液晶聚合物的合成与表征及光响应性能研究

Preparation and Photoresponsive Properties of Lignin-graft Azobenzene-Containing Liquid Crystalline Copolymers

以天然高分子木质素为原料,通过亲核取代反应将木质素改性成为大分子引发剂,引发偶氮苯单体的原子转移自由基聚合(ATRP),得到一系列木质素基光响应聚合物.接枝后的木质素的热稳定性明显改善,且平均接枝率达到72.8%时才能表现出液晶行为.小角X射线散射和偏光显微镜的结果表明所形成的液晶相为近晶C型,层间距为3.21nm.在此基础上,用紫外-可见吸收光谱(UV-Vis)对木质素基液晶聚合物的光响应性进行研究,发现溶液中的光响应性比膜状态的光响应速率快.在紫外光的辐照下,木质素基液晶聚合物表现出快速的光致液晶-各向同性相变行为.

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 an azobenzenecontaining monomer to obtain a series of lignin-graft photoresponsive liquid crystalline copolymers. The molecular structures and molecular weights of the graft copolymers were confirmed by1H-NMR,FTIR and GPC. The thermal properties of lignin-graft azobenzene-containing copolymers showed improved behaviors superior to that of unmodified lignin by measurements 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 the smetic C phase and the layer spacing was 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 the as-cast films. Photochemical liquid crystal to isotropic phase transition of the copolymer was successfully obtained upon UV irradiation,and the reversible isotropic to liquid crystal phase transition was exhibited rapidly when it was exposed to visible light.