Synthesis And Properties Of Chiral Liquid Crystal Polymers Containing Fluorescent Groups

In this paper,two liquid crystal monomers cholesteryl 4-(allyloxy)benzoate(MA)and cholesteryl 4-(hex-5-en-1-yloxy)benzoate(MB)were designed and synthesized.With polymethylhydrosiloxane as the main chain,MA and MB were grafted with brominated olefin 3-bromo-1-propene and 6-bromo-1-propene with different segment length,respectively,to obtain chiral liquid crystal polymers PA-PC series.Methyl 2-(6hydroxy-3-oxo-3H-xanthen-9-yl)benzoate(MC)was introduced into chiral liquid crystal polymers PA-PC series by Williams etherification reaction,and the corresponding fluorescent chiral liquid crystal polymers FPA-FPC series were prepared.The structures of the intermediates,monomers and polymers were characterized by Fourier transform infrared(FTIR)spectroscopy,and ultraviolet-visible(UV-vis)spectroscopy.It was confirmed that all the chemical structures were in accordance with the molecular design.The liquid crystal textures of the intermediates,monomers and polymers were analyzed by polarizing microscope(POM).It was found that the monomers MA and MB showed typical oily streaks textures in the heating cycle and focal conic textures in the cooling cycle,which belonged to cholesteric liquid crystals.All liquid crystal polymers showed colorful Grandjean textures during heating and cooling cycle,and all liquid crystal polymers were confirmed to be cholesteric liquid crystals by X-ray diffraction(XRD)analysis.At the same time,we used the polarimeter to test the specific optical rotation of all polymers,and found that the optical rotation direction of all liquid crystal polymers is left-handed,and the specific optical rotation of the same series of liquid crystal polymers shows a certain linear law.The thermal stability of all chiral liquid crystal polymers was characterized by thermogravimetric(TG)analyzer,the thermal weight loss temperatures of 5%of all liquid crystal polymers were higher than their respective clear transition temperatures,indicating that all polymers did not decompose before the isotropic state and maintain excellent thermal stability.The thermal properties of monomers and polymers were analyzed by differential scanning calorimeter(DSC),and it was found that for chiral liquid crystal polymer precursor PA-PC series,the glass transition temperature increased with the decrease of liquid crystal side chain content,while the clear transition temperature increased at first and then decreased.The longer the liquid crystal side chain is,the lower the glass transition temperature and clear transition temperatures are.The glass transition temperature increases with the increase of the content of fluorescent side chain,while the clear transition temperature increases at first and then decreases.The longer the fluorescent side chain is,the lower the glass transition temperature and clear transition temperature are.In addition,the introduction of fluorescent groups can significantly increase the glass transition temperature and clear transition temperature of liquid crystal polymers.Finally,in order to analyze the fluorescence properties of liquid crystal polymers with fluorescent groups,we analyzed all the polymers by photoluminescence(PL)spectroscopy.The maximum excitation wavelengths of all fluorescent chiral liquid crystal polymers were about 468 nm,and reached the visible range,which provides a practical possibility for future practical applications.