Controlling Single Axis Directional Assembly Of Cellulose Nanocrystalsetwork And Optical Applications

Structural colors of optical materials based on the assembly of rod-like cellulose nanocrystals(CNCs)have been widely concerned because of their high optical signal saturation and wide color gamut.Those structural colors are free of the limitations from luminescent materials,such as photobleaching and aggregation quenching.The colors also own responsiveness under stimuli of stress or environmental action.These features establish the core value of the optical materials with the CNC assembly structure in the future optical information engineering.However,the high charge density on CNC surface easily makes the assembly structure chiral,leading to iridescent colors.The diffraction wavelength of those iridescent colors is very sensitive to detection angles and detection positions.Especially when the assembly period of the CNC assembly structure is short,the structural color of the CNC-based optical film varies extremely fast,increasing the difficulty in identifying the information carried by CNC assembly structure and making the information easily misread.Introducing fluorescent substance,such as fluorescent groups or quantum dots,into the CNC-assembly structure can avoid the influence of iridescent colors and produce monochrome luminscence with high quantum efficiency.However,those methods also lose the advantage of CNC assembly on structure colors.Thus,to eliminate the chirality and iridescent colors of the CNC-assembly structure and form single structure color and solid state illumination,this research designed a method to orderedly assemble CNC in a single dimension.The assembling process was studied;along with the emission-enhancement mechanism of Stokes scattering induced by inelastic collision between CNC and photons.The principle of solid-state emision of CNC achiral assembly structure was clarified,as well as its variation rule.Furthermore,the information-coding ability of CNC optical film was studied based on a patterning technique,and the optical concealment condition of the coded information was studied by adjusting the emission spectrum.Meanwhile,this study studied the co-assembling ability of CNC with flexible substrates,and clarified the response of emission properties of the achiral array structure of CNC under mechanical stimuli.The results showed that the degree of chirality elimination of the CNC array structure induced by solvent evaporation was controlled by pre-assembly treatment(ultrasound,concentration adjustment),assembly conditions(temperature)and CNC surface chemical structure adjustment.When the achiral assembled CNC array structure was excitated by light with a wavelength equal to the Bragg diffraction wavelength of the array,the CNC inelastically collided with the photons strongly and achieved solid-state photoluminescence with an emission quantum efficiency of up to 13%.A suitable CNC length and surface chemistry made the excitation wavelength of the solid state emission in the ultraviolet region.After the CNC assembly structure was patterned,the information carried by the pattern was naturally hidden under visible light,and information could be read under specific ultraviolet light.Besides,the co-assembly of CNC with sodium alginate could maintain the achiral assembly and solid-state luminescence ability of CNC.The elongation of the co-assembled films reached 40%,solving the problem of insufficient flexibility of the self-assembled CNC structure.The plastic deformation during the deformation process increased the effective refractive index of the CNC assembly structure,causing that the excitation peak of the CNC array structure blue-shifted and changing the spectral shape.This research realized the chiral elimination of the CNC assembly structure and removed the iridescent colors that caused the CNC optical film information easily misunderstood.Also,by studying the solid-state emission properties of CNC achiral arrays,a information hiding method based on patterned CNC optical film was constructed.Furthermore,this research studied the achiral elimination rule when the CNC was co-assembled with the flexible substrate,and prepared a CNC/alginate co-assembled film responsive to mechanical stimuli.This series of work established the theoretical foundation for the assembly-induced emission of rod-like nanoparticles.