Surface Modification Of Cellulose Nanocrystals To Improve The Dispersibility In Liquid Crystals

Liquid crystal is a kind of substance between crystal state and liquid state,which has birefringence and electro-optic effect.Electric field can change the original arrangement of liquid crystal molecules,thus affecting the optical properties of liquid crystal.5CB(4-cyano-4'-pentyl biphenyl)is a typical positive nematic liquid crystal with electro-optic effect.The light shutter based on 5CB can adjust its light transmittance by electric field.Nanoparticles are often used to dope into liquid crystals to improve the contrast and response speed of liquid crystal.As a kind of rod-shaped nanoparticles extracted from wood,cellulose nanocrystals(CNCs)have a huge specific surface area and belong to renewable resources.It is a new research direction with great potential to dope them into liquid crystals.However,there are a lot of polar groups such as hydroxyl and carboxyl groups on the cellulose nanocrystals.When they are doped into the non-polar liquid crystal,they will aggregate due to the poor compatibility between them.Therefore,it is necessary to modify the surface of the cellulose nanocrystals to improve their compatibility.It is a feasible method to modify the surface of cellulose nanocrystals with biphenyl liquid crystal with reactive end groups.Due to the similar structure of biphenyl liquid crystal and 5CB,it can enhance the interaction between the modified cellulose nanocrystals and5CB liquid crystal,and enhance the compatibility.We have synthesized two kinds of liquid crystal units with reactive end groups:4-(eo-2-methoxy)-4'-cyano biphenyl and 4-(2-aminoethoxy)-4'-cyano biphenyl.The sulfonated cellulose nanocrystals were prepared by sulfuric acid hydrolysis,and CNC-g-M1 was prepared by ring opening reaction between the CNCs-SO₃H and 4-(eo-2-methoxy)-4'-cyano biphenyls.The carboxylated cellulose nanocrystals were prepared by potassium permanganate method,and the CNC-g-M2 was synthesized by amidation of the CNCs-COOH with 4-(2-aminoethoxy)-4'-cyanobenzene.The cellulose nanocrystals modified by liquid crystal elements were dispersed into liquid crystal 5CB by liquid phase method.In CNC-g-M1,the grafting ratio of liquid crystal units is high,which has a strong force with 5CB liquid crystal molecules,and can be stably dispersed in the liquid crystal to form a homogeneous mixture.However,due to the limitation of carboxyl content,the grafting rate of the liquid crystal units of CNC-g-M2 is low,and it can not be evenly and stably dispersed in the liquid crystal.The influence of CNC-g-M1 doping on 5CB liquid crystal is characterized by DSC,POM and self built equipment system.It is found that a small amount of CNC-g-M1doping into the liquid crystal will reduce the domain size of 5CB liquid crystal,and enhance the random distribution of its director.Therefore,the scattering effect of 5CB liquid crystal on the light is improved,and the transmittance of light shutter based on CNC-g-M1 doped liquid crystal in OFF state is reduced.At the same time,due to the less doping amount of CNC-g-M1,it has little effect on the transmittance of the light shutter in ON state,and the transmittance of the light shutters based on doped liquid crystal in ON state are more than 75%.The doping of CNC-g-M1 nanoparticles has a strong interaction force on the liquid crystal,which not only increases the threshold voltage and driving voltage of the light shutter,but also destroys the ordered structure of the liquid crystals in a certain extent,making the anisotropic-isotropic phase transition temperature of the liquid crystal decrease.