Study On The Self Assembly Of Highly Transparent Cellulose Nanofibril/Montmorillonite Nanocomposite Films

Cellulose Nanofibers(CNF)films have a broad and bright application prospect in electronic devices,food packaging and other fields due to their excellent mechanical flexibility,strong tensile strength,high optical transparency,biodegradability and biocompatibility.However,existing problems of poor inflammability,barrier properties and thermal stability must be solved before the practical application of CNF thin films in those fields.Incorporating montmorillonite(MMTs)into CNF film is a promising method to overcome above problems.Nevertheless,the directly addition of bulk MMTs will significantly deteriorate the mechanical and optical properties of the resulting film and can't meet the requirement of practical uses.The reason is that the layered MMTs are prone to aggregate and result in the emergence of disordered structure in the composite film and the decline of interfacial interaction force,thus preventing the effective transfer of the excellent properties of CNF and layered MMTs into the film.Therefore,addressing this contradiction will provide a scientific guideline for the construction of high performance CNF-MMT nanocomposite films,and promote the functionalization of CNF films and their practical applications.To avoid the easy flocculation of monolayer MMTs in water,this dissertation selects amphiphilic CNFs as a green dispersant to prepare monolayer MMTs dispersion with high stability,and the dispersive mechanism of CNFs for monolayer MMTs is elucidated,which could provide raw material for the construction of highly transparent CNF-MMT nanocomposite films.First,the influence of dosage of CNF on the yield and colloidal stability of monolayer MMTs in water are studied.The results show that the yield and colloidal stability of monolayer MMTs improve significantly with increasing dosage of CNF.When the addition of CNF is 50 wt%,the aqueous monolayer MMTs dispersion achieved highest yield and optimal stability(yield is 62.2%,and TSI is 3.02).The dispersive mechanism of CNF for monolayer MMTs in water is uncovered by atomic force microscope(AFM)and quartz crystal balance(QCM-D).It is demonstrated that CNFs mainly adsorb on the surface of MMTs through hydrogen bonding,and provide electrostatic repulsion and steric repulsion to maintain the stability of monolayer MMTs in water,and the dosage of CNFs have an influence on the ratio of static electricity and steric hindrance.When the dosage of CNFs is low(2 wt%),CNFs only provide van der Waals forces.With the dosage of CNFs increase,the electrostatic repulsive forces increases and gradually becomes dominant force.Second,the influence of the lamellar structure of MMTs and the intermolecular forces existed in CNF-MMT on the mechanical and optical properties of the nanocomposite film are studied.The result shows that the mechanical,optical,and barrier properties of the nanocomposite film prepared by monolayer MMTs are significantly improved(a stress of 132 MPa,a toughness of 1.720 MJ/m~3,a light transmittance of 90.3%,a WVTR of 41.0 g·mm/m~2·day)compared with the nanocomposite film prepared by bulk MMTs(a stress of 30 MPa,a toughness of 0.243 MJ/m~3,a light transmittance of 70.0%,a WVTR of 78.6 g·mm/m~2·day).Through AFM and SEM analyses,monolayer MMTs are more easily to form orderly stack than bulk MMTs,and orderly structure is more beneficial to the spread of light,dissipation of stress,and the prevention of gases.Furthermore,from the point of enhancing the intermolecular forces between CNF and MMT,we introduce positively charged PEI into the film to enhance its properties.When the additive amount of PEI is 0.5%,the tensile strength of the nanocomposite film improves significantly from 135 MPa to 175 MPa,and the WVTR reduces from 41.0 g·mm/m~2·day to 20.9 g·mm/m~2·day.Nonetheless,as the amount of PEI is further increased,the performance of the composite film decreases.Therefore,PEI can significantly enhance the interaction force between CNFs and MMTs through ion bonds,but excess PEI will lead to strong interaction and result in the disorder of the inside orderly structure.Finally,we investigate the ionic conductivity of CNF-MMT nanocomposite film and its potential application in supercapacitor as a separator.Results improve that the ionic conductivity of the nanocomposite film increases with the increase of the MMTs content.When the content of MMTs is 30 wt%,the ionic conductivity of the composite film reached the maximum value of 0.78 s/cm in 1 mol/L Na~+solution.Compared with commercial non-woven separator,supercapacitor with CNF-MMT(30wt%MMT)nanocomposite film was improved by 23%.To sum up,this dissertation significantly improve the mechanical,optical and barrier properties by preparing high quality monolayer MMTs to facilitate the formation of orderly structure in film and introducing PEI to enhance the intermolecular forces in film.This breakthrough provides a theoretical guidance for the preparation of high performance CNF-MMT composite films;it is conducive to promoting the application of green transparent and flexible nanocellulose films in the fields of electronic devices and packaging so as to realize the sustainable development of human society.