Preparation Of Carboxylated Cellulose Nanocrystals And Study Of Its Chiral Nematic Liquid Crystals

Cellulose chiral nematic liquid crystals(N*-LCs)are chiral periodic materials with a supramolecular structure,which have the special left-handed spiral structure that selfassembled by cellulose nanocrystals(CNCs)and have attracted wide attention in recent years with its optical properties such as excellent optical rotation,selective reflectivity,and circular dichroism.However,as so far,only the sulfated-based CNCs prepared by the sulfuric acid hydrolysis are prone to form N*-LCs,while the carboxylated CNCs prepared by traditional oxidation method are complicated and difficult to get N*-LCs.To the problems,in this thesis,the carboxylated CNCs prepared by two one-step oxidation methods and their N*-LCs behavior have been studied systematically.Firstly,ammonium persulfate(APS)-oxidized cotton cellulose nanocrystals(cAOCNs)and tunicate cellulose nanocrystals(t-AOCNs)were successfully prepared by one-step oxidation with APS.The c-AOCNs and t-AOCNs were characterized by Fourier transform infrared spectrometer(FT-IR),scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffractometer(XRD)and conductivity titration(CT).Simultaneously,polarizing optical microscope(POM)and ultravioletvisible-near infrared reflectance spectrometer(UV-Vis-NIR)were used to observe the suspensions of AOCNs and their dried films.And the unique fingerprint texture of N*-LCs were all observed,among which the N*-LCs of t-AOCNs were the first observation in this field.The results showed that c-AOCNs and t-AOCNs were both cellulose I phase;the average particle size was 144.5 nm and the carboxyl content was 0.73 mmol/g of c-AOCNs,furthermore the critical concentration to form lyotropic N*-LCs was 4.1 wt%,and the pitch(P)was 15.6 ?m;The t-AOCNs were 224.2 ± 162.5 nm in length and 19.0 ± 4.7 nm in width,in which aspect ratio could reach 150,the carboxyl content was 0.40 mmol/g,in addition the critical concentration of lyotropic N*-LCs was 3.5 wt% and the P was 35.3 ?m.The dried films of c-AOCNs and t-AOCNs conformed to the Bragg reflection and showed excellent optical characteristics.Secondly,TEMPO oxidation system was used to prepare carboxylated CNCs,and ultrasound with high power in water was used to replace the acid hydrolysis step,and TEMPO system oxidized cotton cellulose nanocrystals(c-TOCNs)were obtained by one-step oxidation.The c-TOCNs were characterized by FT-IR,XRD,SEM and CT,and the suspensions with different concentrations of c-TOCNs were observed by POM.It was found that the average particle size was 113.1 nm and the carboxyl group content was 1.03 mmol/g of c-TOCNs.Moreover,c-TOCNs suspension could form N*-LCs,the critical concentration was 7.9 wt% and the P was 11.7 ?m.Finally,the liquid crystal self-assembly capabilities of c-AOCNs and c-TOCNs prepared by two one-step oxidation methods were compared.It was found that the critical concentration of c-AOCNs to form N*-LCs was significantly lower and the P is much higher than c-TOCNs due to the larger aspect ratio and the lower carboxyl content.Both two one-step oxidation methods greatly simplified the preparation process of carboxylated CNCs,expanded the preparation method of cellulose N*-LCs,and greatly promoted the preparation of cellulose N*-LCs as a template in chiral composite materials and photonic films.