Nanocellulose/Carbon Nanotube Flexible Composite Materials And Properties

With the rapid development of flexible wearable devices,intelligent health monitoring and human-computer interaction systems,the conductive film formed by compounding nanocellulose and carbon nanotubes(CNT)has become an important basic material for the development of flexible electronic devices with good mechanical properties,flexible wearability and processability.However,the strong van der Waals force between CNT makes them extremely prone to agglomeration or entanglement,which hinders their development in the field of flexible electronic sensing.Nanocellulose is considered to be an ideal dispersant to CNT attributed to its one-dimensional nanometer size and adjustable active groups on the surface.However,the efficient dissociation of nanocellulose(especially CNF)and its dispersion mechanism for CNT are still unclear.Inspired by the classic TEMPO oxidation method to prepare CNF,while overcoming the harsh and time-consuming reaction conditions and the expensive reagents used in the preparation process,this study explored a method for the efficient preparation of CNF with potassium permanganate(KMnO4)as the oxidant.Then,using the obtained CNF as a green dispersant,benefit from the size matching between the two and the abundant carboxyl structure on the surface of CNF,a CNF/CNT conductive paste with stable dispersion is obtained.Further,films with excellent electrical conductivity and mechanical properties can be formed by vacuum filtration,and patterned flexible electrodes can also be constructed on different flexible substrates,laying a good foundation for the development and application of flexible sensors.The specific summary is as follows:(1)The fficient harvesting of nanocellulose by KMnO4 oxidation method.Under the selective oxidation of acidic potassium permanganate and the autocatalysis of byproduct MnO2,a series of electron and proton transfer and dehydration reactions occurred in needle bleached kraft pulp(NBKP),and finally achieved high efficiency dissociation,resulting in carboxy-functionalized nanocellulose(denoted as POCNF).More importantly,the washing filtrate(containing Mn2+)can be recycled as an important component in the oxidation reaction to reduce the amount of subsequent KMnO4 addition.The potassium permanganate oxidation treatment can effectively realize the fiber dissociation of NBKP and the functional modification of cellulose molecular structure.The obtained CNF has good nano size and dispersion,high carboxylation degree,and excellent mechanical strength(tensile strength up to 75 MPa).Moreover,due to the good nano size(3-7 nm in diameter,0.32-1.10μm in length)and small particle size distribution,the film structure is dense,showing high transparency(95.39%).The difference of refractive index between the crystalline and non-crystalline regions and the air inside the CNF structure gives the film a unique optical property of high haze(>90%)characteristic,which shows its potential application in flexible devices.(2)The dispersion effect of CNF on CNT and the properties of CNF/CNT conductive paste.Using CNF as a green high-efficiency dispersant,a stable dispersed CNF/CNT slurry was obtained with the assistance of ultrasound.The effect of CNF dosage on the dispersion effect of CNT was studied,and the dispersion limit of CNT in CNF aqueous system was explored.Through stability studies,the results showed that the introduction of a small amount of CNF can effectively disperse CNT,the dispersion concentration of CNT can reach 80%,and the obtained CNF/CNT slurry has a low Zeta potential(lower than-60 mV),it means good stability,even under high-speed centrifugation,it can still maintain good dispersion,and the slurry can maintain stable dispersion for more than three months in a static state.The parental structure of CNF and the electrostatic repulsion between carboxyl groups on the surface play a good role in the dispersion of CNF/CNT system.The micromorphological characterization showed that CNF was coated on the surface of CNT.Combined with molecular dynamics simulation,the mechanism of non-covalent π-π stacking between CNT sp2 carbon lattice and CNF was further analyzed,the binding free energy of CNT and CNF was calculated,and the size matching between them and the correlation between the intrinsic molecular structure and the stable dispersion was revealed.(3)The properties of CNF/CNT composite conductive film.The CNF/CNT conductive slurry was filtered under vacuum to form a film.The improvement of the loose and disordered aggregation state of the pure CNT film by the introduction of CNF and the layer-by-layer self-assembly structure formed between the two components,with the tensile strength of 152.87 MPa.Combined with a series of physical/chemical property analysis methods,the double acting mechanism of hydrogen bond and π-π stacking between CNT and CNF under the induction of carboxyl group on the surface of CNF and the synergistic effect of steric hindrance effect was clarified,and the correlation between good interlayer bonding in the microstructure of composite film and excellent macroscopic mechanical strength/toughness and conductivity was revealed.With the increase of CNF content in the system,the self-assembly of CNF/CNT molecules is more orderly,the hierarchical structure is denser,and the mechanical strength of the composite film is enhanced.At the same time,the ordered as "brick-wall-like" structure.The CNT is uniformly and tightly distributed,providing a good conductive transmission channel,and endowing the CNF/CNT composite film with good conductivity,the conductivity up to 67 S cm-1.The crystalline properties of the composite film showed that the existence of CNT will play a role in heterogeneous nucleation,induce CNF to adsorb and crystallize on its surface.This structure also realizes the modification of the surface hydrophilicity of CNT to a certain extent,which will improve the compatibility of CNF/CNT conductive slurry with various substrates.(4)Pattern design and sensing properties of CNF/CNT-based flexible conductive materials.The CNF/CNT conductive ink was obtained by concentrating the well-dispersed CNF/CNT conductive slurry.The state between fluid and solid can be adjusted by adjusting the shear angle frequency.The ink can be used to build patterned electrodes on flexible substrates such as office paper,filter paper,cotton fabric and polyvinylidene fluoride(PVDF)filter membranes through direct writing,coating,screen printing and impregnation.It shown that CNF/CNT conductive ink has the universality of various forming methods and the compatibility with different substrate materials.The effects of different process conditions(such as direct writing times,immersion-curing times,etc.)and different types of substrates on the conductivity of electrodes were explored.The obtained flexible electrodes all have good conductivity,especially the CNF/CNT/FP conductive electrode obtained by impregnation with filter paper(FP)as a template has high electrical conductivity(90.91 S m-1)and excellent mechanical strength(stress tensile of 11.1 MPa).More importantly,the excellent thermoelectric and deformation/pressure sensing properties of CNF/CNT/FP enable it to be applied to real-time human motion/human signal,combined with its good biocompatibility,it shows great application potential in human health monitoring and patient clinical monitoring.The flexible capacitor assembled by interdigital electrodes printed by screen printing has a high area specific capacitance of 26 mF/cm2 at a scan rate of 50 mV/s,showing good electrochemical response performance of CNF/CNT electrodes,and the flexible capacitor can still maintain the corresponding performance under bending deformation.Combining its structural designability,biocompatibility and good flexibility,CNF/CNT-based electrodes show great potential applications in biological systems.