| With the in-depth study of liquid crystal chemistry,the design and synthesis of branched liquid crystal macromolecules has become a mainstream research direction in the field.Branched liquid crystal compound is a new type of liquid crystal material,which connects multiple liquid crystal arms to central atoms or groups.Azo branched liquid crystal compounds,as a kind of liquid crystal with unconventional structure,will show good and stable liquid crystal performance and regularly changing liquid crystal interval with the change of central group and flexible interval length of liquid crystal arm.At the same time,because of azo group,when heated or irradiated by light with different wavelengths,these compounds will possess optical activity and produce photoisomerization.On the basis of consulting relevant literatures at home and abroad,pmethoxyazobenzene liquid crystal arms are designed and synthesized in this paper,which are three azo liquid crystal units with different rigidity and strength.Six two-arm rod-like azo liquid crystal macromolecules,six three-arm branched azo liquid crystal macromolecules and 12 four-arm branched azo liquid crystal macromolecules were prepared by connecting them to different central groups through reaction.This paper is divided into the following three parts:The first part is the synthesis and performance study of p-methoxyazobenzene liquid crystal arm.P-methoxyphenol was synthesized by diazotization and coupling reaction,and then reacted with adipic acid,octanedioic acid and sebacic acid to prepare three kinds of liquid crystal arms with different flexible chain lengths,namely,pmethoxyazobenzoyloxy acid LCA-O-n(n=6,8,10).The results of FT-IR and 1H-NMR analysis showed that the synthesized compound conformed to the molecular design.The performance of the compound was tested by differential scanning calorimetry(DSC),polarizing microscope(POM)and ultraviolet-visible spectrophotometer(UV-Vis).The results showed that:P-methoxyazobenzoyl acid(LCA)Their melting point,clear point when heating up and isotropic-liquid crystal phase/crystal transition temperature when cooling down decrease with the increase of flexible chain length.In the second part,liquid crystal macromolecules with ethylene glycol,glycerol,phydroxybenzoic acid and 3,5-dihydroxybenzoic acid as the central groups were designed and synthesized.By comparing the properties of two-arm traditional rod-like liquid crystal and three-arm branched liquid crystal,the influence of structure on liquid crystal properties was analyzed.Ethylene glycol and glycerol react with liquid crystal arms to form two-arm rod-like and three-arm branched liquid crystal macromolecules respectively,while p-hydroxybenzoic acid and 3,5-dihydroxybenzoic acid connect the liquid crystal arms to the central group through esterification to form intermediate products,which then react with monophenol to form two-arm rod-like and three-arm branched liquid crystal macromolecules.Structural analysis shows that the synthesized liquid crystal compounds conform to the molecular design.DSC,POM and UV-Vis are used to test the properties of two-arm rod-like and three-arm branched liquid crystal macromolecules.The results show that 12 groups of liquid crystal macromolecules belong to photosensitive thermotropic nematic liquid crystal compounds.Under UV irradiation,the same series of compounds do not shift red or blue with the increase of carbon number in flexible chain.The liquid crystal interval of ethylene glycol(YEC-n),glycerol(GY-n),p-hydroxybenzoic acid(DJJ-n)and 3,5-dihydroxybenzoic acid(EJJ-n)series compounds decrease with the increase of carbon number of flexible chain of liquid crystal arm.The flexible chain n=6 has the best liquid crystal performance,which can reach the highest value when the liquid crystal interval is raised and lowered.In the third part,four-arm branched liquid crystal macromolecules with pentaerythritol,gallic acid,tartaric acid and citric acid as central groups were designed and synthesized.Among them,pentaerythritol and liquid crystal arms are esterified to form four-arm branched liquid crystal macromolecules,while gallic acid,tartaric acid and citric acid firstly connect the liquid crystal arms to the central group through esterification to form intermediate products,and then react with monophenol to form four-arm branched liquid crystal macromolecules.Structural analysis shows that the synthesized liquid crystal macromolecules conform to the molecular design.DSC and POM are used to test the properties of the liquid crystal macromolecules.The results show that all four series of branched liquid crystal macromolecules belong to photosensitive thermotropic nematic liquid crystal compounds.With the increase of the length of flexible interval,the melting point of the series decreases.At the same time,with the increase of the carbon number of the flexible spacer,the clear point(Ti)decreases in the process of heating and cooling,and the liquid crystal interval becomes shorter in the process of heating and cooling.The greater the rigidity of the flexible chain,the better the liquid crystal performance,and the highest value can be reached when the liquid crystal interval is raised and lowered.Among them,the liquid crystal interval of MSZS-n series compounds decreases with the increase of carbon number of flexible chains of liquid crystal arm,and the influence of light induction weakens with the lengthening of flexible chain.MSZS-6 has the best liquid crystal performance,and the liquid crystal interval can reach 78.7℃ when heating up and 119.0℃ when cooling down. |
