| Cellulose nanocrystals(CNC)are green nanomaterials derived from natural cellulose.In recent years,the research of this material has been very hot,especially in its unique self-assembly and optical properties.The CNC suspension can self-assemble to form a lyotropic cholesteric liquid crystal when it reaches a certain critical concentration,and it has very special optical properties.It was also found that the helical structure of CNC cholesteric liquid crystal can be retained in its solid film,called CNC iridescent film,by slow evaporation of solvents.The color of CNC iridescent film is a kind of structural color,which can not be duplicated.It has great potential to be used in special optical devices and anti-counterfeiting labels,so,many researchers have done a lot of research in it.However,there are still many problems in this field to be solved.The size of CNC prepared by the most widely used acid hydrolysis method is not uniform,which has a great influence on the phase separation and self-assembly behavior of CNC suspension;In addition,pure CNC iridescent film is hard and brittle and difficult to process and utilize.At present,a method to prepare a color-controllable flexible CNC iridescent film still needs to be developed.Therefore,this thesis studied the size homogenization method of CNC by using microcrystalline cellulose(MCC)as raw material.CNC cholesteric liquid crystal and CNC iridescent film were prepared,and the method of toughening modification of CNC iridescent film was studied.A flexible iridescent film with good flexibility and usefulness was obtained,and the method for controlling its optical properties was briefly studied.The specific research contents are as follows:This topic firstly studied the process of preparing high quality CNC by catalytic hydrolysis of MCC with sulfuric acid;and then the separation effect of centrifugal grading on CNC size was deeply explored.The results showed that the optimum conditions for the preparation of CNC by catalytic hydrolysis of MCC were: sulfuric acid concentration of 64%,reaction temperature of 45 ?,reaction time of 90 min.Under these conditions,a short rod-shaped CNC with a diameter of 10~20 nm and a length of 150~300 nm can be prepared,and the yield of the CNC is relatively high and the suspension is relatively stable.Centrifugal grading has a significant separation effect on the particle size and size distribution of the CNC.As the centrifugal speed increases,the size of the resulting CNC significantly reduces and the distribution is more uniform.A series of nanocellulose products with short rod shape,uniform size and narrow distribution can be prepared with this method,which has important practical application value.The CNC suspension was then concentrated and the critical concentration of cholesteric liquid crystals was explored by varying the concentration of the CNC suspensions.Aiming at the brittleness improvement of CNC iridescent films,two common plasticizers of starch-based materials,formamide and sorbitol,were used to toughen CNC iridescent films,and their toughening effects were evaluated from various angles.The results show that the critical concentration of CNC cholesteric liquid crystal phase is 5.4 wt%.At this time,the beautiful rainbow color and special fingerprint texture pattern of cholesteric liquid crystal can be observed with polarized light microscope(POM);the iridescent films obtained by slowly drying the CNC liquid crystal suspension show a complex polydomain structure,which has the potential to be applied to the field of optical anti-counterfeiting.Both sorbitol and formamide can be used as plasticizers to improve the flexibility of CNC iridescent films,but the plasticizing effect of sorbitol is better than that of formamide.The optimum dosage of sorbitol is 40%.The surface of CNC iridescent films is smooth,uniform and very soft.At the same time,the mechanical properties of CNC iridescent films are the best.Besides,the addition of these two plasticizers has little effect on the liquid crystal properties of the CNC iridescent films.Finally,the influence of the change of ultrasound power,ultrasound time and ion concentration on the optical properties of CNC flexible iridescent films are studied,and the potential application of CNC flexible iridescent films in the field of optical anti-counterfeiting is briefly discussed.This study proved that both increase of ultrasonic power and prolongation of ultrasonic time could increase the pitch of CNC cholesteric liquid crystal,resulting in red shift of CNC flexible iridescent films under POM and UV-visible reflectance spectrum;the electrolyte(NaCl)shields the surface charge of CNC cholesteric liquid crystal and reduces the pitch.Therefore,the CNC flexible iridescent films appear a blue shift.The polydomain structure of CNC flexible iridescent films under POM is unique and non-repeatable,which meets the requirements of modern optical anti-counterfeiting technology.It can be made into anti-counterfeiting label and applied to the field of modern anti-counterfeiting. |
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