Preparation And Study Of Chiral Liquid Crystal Films Based On Cellulose Nanocrystal

Recently,cellulose nanocrystal(CNC)has attracted more researchers because of its excellent mechanical properties,high specific surface area and great optical properties.When the concentration of CNC suspension reaches the critical value,cholesteric liquid crystal will be formed.After the film is prepared,the left-handed structure of cholesteric liquid crystal will be retained in the film,and the film will produce bright and beautiful structural color.There are several kinds of liquid crystal and its applications include display screen,storage equipment,holographic projection and sensors,etc.However,CNC has a rigid structure and high modulus.The film will crack during the drying process when the film is prepared with CNC suspension.And the cholesteric liquid crystal structure of the film will be destroyed when the film is immersed in water,which greatly limits the application of CNC liquid crystal film.Therefore,to solve the issues above meanwhile retaining the structural color of CNC liquid crystal film,some flexible materials can be added.After addition,the CNC liquid crystal composite film has the ability of humidity response and stress response.In this paper,using aqueous solutions of glycerol(Gly)and polyvinyl alcohol(PVA)and waterborne polyurethane(WPU)in CNC suspension,CNC/(PVA:Gly)and CNC/WPU composite films with chiral nematic structure were prepared by evaporation induced self-assembly(EISA)method.The prepared composite films have excellent flexibility and different stimulus response ability.The specific work is as follows:Firstly,the effect of acidolysis time on pitch and color of pure CNC cholesteric liquid crystal films were studied.Using cotton linters as the raw material,CNC with different particle size were made by the sulfuric acid hydrolysis controlled by different reaction times.Transmission electron microscopy(TEM),Zeta potential and nanoparticle size analyzer,total reflection Fourier infrared spectrometer(FT-IR),X-ray diffraction(XRD),ultraviolet visible near infrared spectrophotometer(UV-vis-NIR),scanning electron microscopy(SEM)and polarizing microscope(POM)were used to characterize the structure and properties of the CNC suspension and films.The effect of acidolysis time on the pitch of cholesteric liquid crystal was studied.The results show that when the acidolysis time increases,the particle size of CNC decreases,the absolute value of zeta potential increases,the surface negative charge increases,and the electrostatic repulsion between the rod-like particles increases.Therefore,the pitch of the cholesteric liquid crystal increases and the reflection wavelength of the film redshifts,which eventually lead to the color of the film changing from blue to red.Secondly,new CNC/(PVA:Gly)chiral liquid crystal composite films were introduced to overcome the flaws of pure CNC films.The composite films can be prepared by adding Gly and PVA to the CNC suspension through the EISA method.The colors of the CNC composite films can be controlled by different proportions of CNC to(PVA:Gly).The mechanical properties,thermal properties,optical properties and structure of the composite films were tested by universal testing machine,TGA,SEM,POM,UV-vis-NIR and circular dichroism(CD),respectively.The results show that the left-handed structure of CNC can be completely preserved after adding(PVA:Gly).When the content of the(PVA:Gly)increases,the tensile strength and Young's modulus of the composite films decrease,however,the flexibility of the composite films improves greatly and the thermal decomposition temperature increases.The pitch of the cholesteric liquid crystal increases,the colors of the films change from blue to red,the color and reflection wavelength redshift with the increase of(PVA:Gly)content.The composite film will be transparent after swelling in the water.And the color of film will back to its original color after dehydration.This property can be applied to humidity response or information encryption.Thirdly,the effect of the solid content of WPU and the mass ratio of CNC and WPU suspension on the color of the composite films was studied.First of all,the softer WPU was synthesized by adjusting the ratio of soft and hard segments of WPU.The CNC with suitable acidolysis time was choosed,and WPU was added to the CNC suspension.The chiral nematic structure of CNC/WPU composite films was obtained by EISA.The structure,optical properties and thermal properties of the composite films were characterized by SEM,POM,UV-vis-NIR,CD and TGA,respectively.The results show that the addition of WPU does not change the left-handed structure of CNC chiral liquid crystals.Increasing the solid content of WPU or decreasing the mass ratio of CNC and WPU suspension can make the reflection wavelength generate bathochromic shift,the color of the composite films change from blue to red and the pitch of cholesteric liquid crystal increase.The structural color of composite films and multi domain structure of composite films can be found under POM.The elongation at break of the composite film increases with the increase of WPU content.Finally,the color of the composite films will change greatly when a slight external force is applied.This property can be applied to the stress sensor used for damage detection.