Properties And Assembly Regulation Studies Of Crystalline Nanocellulose

Many organisms in nature display bright structural colors,such as the tropical berries,butterfly wings,crustaceans and the compound eye structure of mantis shrimp in nature.This is because they have a special chiral optical structure,which allows them to use circularly polarized light to communicate,such as courtship,feeding,avoiding natural enemies and spreading information.The generation,regulation and application of circularly polarized light are a hot research topic in the field of optical materials.However,the process of preparing chiral photonic crystal materials with circularly polarized light properties is often complicated.As a natural biopolymer,crystalline nanocellulose(CNC)can spontaneously assemble into one-dimensional chiral photonic crystals.This facile assembly method is important for the development and application of circularly polarized optical materials.CNC has a one-dimensional rod-like twisted morphology.When the CNC solution evaporates to a critical concentration,the CNC is able to spontaneously form a chiral nematic liquid crystal with a periodic left chiral helical structure.The one-dimensional chiral photonic crystal structure can be retained in the solid film.This allows the CNC film to selectively reflect left-handed circularly polarized light and transmit right-handed circularly polarized light.However,one-dimensional chiral photonic crystal CNC films always have a large number of structural defects;the tunability of the chiral optical signal of CNC-based chiral photonic crystal films is poor;the self-assembly of CNC cannot form chiral photonic crystal materials with complicated helix structures;structural color of CNC-based chiral liquid crystal microspheres is difficult to achieve.These problems limit the application of CNC in the field of chiral optical materials,such as optical encryption,signal converters and omnidirectional laser emitters.Therefore,how to solve the above problems has certain scientific and application significance.For the question that one-dimensional chiral photonic crystals always have a large number of defects,this thesis firstly explored the characteristics of CNC self-assembly and modulates them to obtain a more uniform one-dimensional chiral photonic crystal structure.Then,trace magnetic guest nanoparticles were introduced into the CNC film;this magnetic chiral composite expanded the chiral optical properties of one-dimensional chiral photonic crystals;they had a magnetic field modulated circular dichroism signal.To address the problem that CNC self-assembly cannot form chiral photonic crystal materials with complicated chiral axis structure,this thesis utilized the local microfield introduced by the magnetic assemblies;this microfield regulated the assembly of CNC into chiral photonic crystal structures with both horizontal and vertical cross helix.Finally,CNC-based chiral liquid crystal microspheres with tunable structural color and circular dichroism were constructed using water-in-oil Pickering emulsions.The above study provides new ideas for the construction of complex chiral photonic crystals and the development of new circularly polarized optical materials.The details are as follows.In the first chapter,this thesis briefly described the theoretical knowledge and property characteristics of photonic crystals and liquid crystals;then the structure,physicochemical properties,extraction methods and CNC-based composites of CNC were introduced in detail;finally,the formation mechanism,chiral optical properties and assembly regulation methods of CNC chiral liquid crystals were highlighted,as well as the related research progress of CNC-based chiral liquid crystal composites.From the second chapter,this thesis started to address the problem of the large number of defects in CNC films of one-dimensional chiral photonic crystals.This chapter investigated magnetic field method and kinetic intervention method to regulate the self-assembly process of CNC.Compared with the kinetic modulation method,the magnetic field-modulated film material exhibited better planar texture,more uniform helical axis orientation,and superior chiral optical properties.The assembly processes of these two approaches were monitored separately;for the magnetic field method,the modulation of the CNC helical axis orientation by the magnetic field occurred at the early stage of the growth and fusion of tactoids,while for the kinetic method,the kinetic intervention reduced the speed of the gelation process and made the helical axis alignment longer.The above studies enrich the understanding of CNC self-assembly and contribute to the development of environmentally friendly and efficient chiral optical materials.In Chapter 3,trace amounts of carboxylated iron tetroxide nanoparticles(MNP)were introduced into CNC to construct magnetically responsive one-dimensional chiral optical materials.Hybrid CNC/MNP suspensions were subjected to an evaporative solvent-induced co-assembly process;magnetic CNC/MNP composite films with one-dimensional chiral photonic crystal structures were obtained.The magnet can modulate the intensity of the circular dichroic signal of this CNC/MNP film;the degree of modulation of the circular dichroic signal was related to the magnetic field strength of the magnet,the loading of MNP in the CNC/MNP composite film,and the direction of the applied magnetic field.This study enriches the chiral optical properties of CNC films and demonstrates the potential of CNC-based chiral photonic crystals as magneto-optical materials.From Chapter 4 onward,this thesis began to address the problem that CNC was difficult to assemble into chiral photonic crystal materials with complicated spiral axis structures.In Chapter 4,this thesis investigated the method of constructing CNC-based chiral optical materials with both horizontal and vertical cross helix structures through magnetic field modulation.Trace amounts of superparamagnetic nanoparticles(Fe₃O₄)were introduced into the CNC suspensions.The external magnetic field directionally modulated the assembly process of the mixed suspension of CNC and Fe₃O₄.The results showed that the CNC around the chain-like Fe₃O₄ assembly had a concentric fingerprint structure,which was vertically embedded in the planar texture of the CNC;this made the planar and lateral sides of the CNC/Fe₃O₄ film showed different colors of left-handed circular polarized light reflection patterns and have different chiral optical properties.This indicated that the CNC-based chiral nematic films with both horizontal and vertical cross spiral axis structures has two-dimensional chiral optical properties.The assembly monitoring of the CNC/Fe₃O₄ system revealed that the Fe₃O₄assemblies could reorient the helical axis of the CNC chiral nematic phase.The studies in this chapter provide new ideas for the design of 2D chiral optical materials with tunable helix.In Chapter 5,CNC-based chiral liquid crystal microspheres were constructed using water-in-oil Pickering emulsions,and the formation conditions of structural color were investigated,resulting in CNC-based chiral liquid crystal microspheres with tunable structural color and circular dichroism.The CNC assembly under spherical domain-limited conditions was dependent on the spatial shape.Hydrophobized zeolite were used as stabilizers to prepare water-in-oil Pickering emulsion microspheres.Under the stabilization of the zeolite(LTA)shell layer,the CNC of the inner phase could self-assemble in the spherical space;its helical axis direction rotated 360^o along the spherical center.The monitoring results of the CNC assembly process showed that it took a very long time for the pitch and photonic band gap of the CNC to shrink to the visible region after forming the 360^o helix oriented concentric fingerprint texture.The reflected structural color of CNC@LTA microspheres could be tuned to the visible range by regulating the assembly conditions.This work has potential applications in CNC-based chiral optical additives,ink printing,and omnidirectional laser emitters.Chapter 6 is the conclusion and outlook of the thesis.