Study On The Self-Assembly Mechanism Of Cellulose Nanocrystal Under Flow Field And Construction Of Its Functional Materials

We have previously demonstrated that vacuum assisted self-assembly technique(VASA)can be used for fabricating iridescent cellulose nanocrystal(CNC)film with highly oriented layered helical structures inside.Compared to traditional employed evaporation induced self-assembly method,which is time consuming and cannot be used for preparing a whole piece of film,the filtration process takes less time and the prepared films show the more uniform appearance in color.However,the self-assembly mechanism of CNC nano-rods during the filtration process has remained unclear.In this study,we experimentally demonstrated that the self-assembly of CNCs under flow field only took place at the interface of filter membrane and CNC suspension.By varying the filtration duration time,the obtained CNCs films all present iridescent color,and the film thickness increased with the increase of filtration duration.The UV-Vis spectra of the samples correspond to different filtration time showed different extinction behavior.Therefore the formation of chiral nematic films from CNC suspensions via vacuum assisted filtration involves a sequential sedimentation of CNC nano-rods on the filter membrane.SEM images of the film samples all present layered helical structures.Through analyzing the DLS profiles,AFM images and transmittance profiles(UV–Vis spectra)of CNC particles in the upper suspension,it is found that the particle size of CNCs as well as the concentration of the CNC dispersions did not increase as solvent was drained out.Since the optical properties(iridescent color)of cholesteric CNC film cannot be reproduced by printing or photocopying,CNCs iridescent films are of latent applications in optical encryption technology.However,solid films formed from pure CNC suspensions are brittle and difficult to handle or integrate within an industrial process.For another,adjusting the coloration of CNC film by modifying the chiral nematic organization is essential for developing its applications as anti-counterfeit materials.At present,new approaches for fabricating iridescent CNC films with both pliability and tunable coloration are still anticipated.In this study,we present a feasible vacuum infiltration method for preparing iridescent CNC films with both pliability and tunable coloration,in which ILs of Amim Cl was incorporated as a plasticizer.Tensile test results manifest that both the strength and modulus of the rigid fragile CNC film were undermined while the strain at break was increased due to the infiltration of Amim Cl.The plasticization effect of Amim Cl makes the film soften and pliable.The gradual red shift in the reflected color with the increasing ILs percent content in the solution,which was monitored by UV-vis spectra of the films,indicates that the proposed method is also effective in tuning the coloration of CNC films.SEM and EDX analysis show the uniform interpenetration of Amim Cl and partial desulfurization in CNC film,which benefits the improvement of film thermal stability.The prepared Amim Cl plasticized cholesteric CNC film can be further toughened via hot-pressing treatment,during which cellulose on skins of CNCs was partially dissolved by Amim Cl and compressed to make a welding layer.The presented method will serve as a significant instance of promising routes to improve the mechanical performances of cellulosics with mesomorphic ordered structure.We have previously made an attempt to fabricate graphene oxide(GO)/CNC hybrid films with or without iridescent optical properties via VASA technique.Based on this work,we fabricated crack-free and large size thermal reduced graphene(TRG)/CNCs iridescent composite film via VASA technique.The composite film has highly ordered,layered chiral liquid crystal structure at submicrometer level and it shows the reversible change in color with the adsorption or desorption of water.This work suggests that,if the interactions betwen CNC and foreigner nano-particles are carefully controlled,VASA technique would be an effective way to fabricate hybrid nanomaterials combining the CNC liquid crystal self-assembled character and the peculiarity of guest nanoparticles.Currently,multilayer security labelling techniques that combine a simple colorimetric system for consumer identification and an additional covert identification authenticated by instruments offer significantly enhanced protection.In particular,colorimetric and fluorometric approaches have attracted a great deal of attention for anti-counterfeiting purposes.In this study,we first made a mixed suspension of carbon dots(C-dots)and CNCs,and then the mixture was subjected to vacuum filtration for preparing composite films.Both structural colors from CNCs assembly and emission colors from C-dots were observed on the composites,which may find applications in anti-counterfeiting devices.Using the same sample preparation process,we incorporated [Eu W10O36 ]9-,a polyoxometalates,into the self-assembly process of CNCs via vacuum filtration for fabricating functional films that combining the colorimetric property of CNC assembly and photochromic function of [Eu W10O36 ]9-.It is found that the addition of polyoxometalates would give rise to the blue shift of the reflected peak wavelength,and the excess addition of polyoxometalates would even destructed the self-assembly of CNCs.In this study,through chemical reduction of the pre-fabricated GO papers obtained via vacuum filtration,RGO papers with controllable lateral contraction and mechanical performances were prepared using p H as a tuning parameter.We discovered that the size stability and mechanical performances of reduced graphene oxide(RGO)papers obtained by chemical reduction of the vacuum filtered GO papers are strongly dependent on the p H value of the starting GO suspensions.Moreover,it is found that RGO paper obtained from GO suspension with p H=7 demonstrated superior performance of both flexural endurance and electrical conductivity.By investigating the structural differences of the prefabricated GO papers and their swelling behaviors during reduction,as well as the unlike microstructures of the obtained RGO papers,we interpreted the origin of the p H-mediated controllable contraction and mechanical performance of RGO papers.The alteration of p H related surface chemistry of GO sheets resulted in the differences in stretch degree,the interlayer d-space,the aggregated microstructure of GO in the dry papers.In addition,because of their unequal C/O ratios,GO papers showed dissimilar degree of swelling at the “swelling while reducing” stage.As a result,the “pulling forces” on graphene sheets induced by water evaporation during ambient drying were dissimilar,which is responsible for the different kinking degree of GO sheets and the conspicuous disparities in contraction rates.The differences in macroscopic mechanical and electrical properties of three RGO papers were ascribed to their dissimilar degree of reduction and the different aggregated structure of RGO sheets.