Study On Design,Preparation And Strain Sensor Application Of Cellulose Nanofibrils Composite Conductive Hydrogels

Cellulose nanofibrils（CNF）are attractive fillers used in conductive hydrogels due to the intrinsically high aspect ratio,specific surface area,mechanical properties and water dispersibility.However,the poor trade-off between mechanical and electrical properties of CNF-based composite conductive hydrogels hardly meets requirements of commercialization in practical application.Thus,herein a series of modified CNF,phosphorylated cellulose nanofibrils（P-CNF）and quaternary ammonium cellulose nanofibrils（QA-CNF）,were prepared by chemical pretreatment-mechanical method.The physicochemical performance of P-CNF and QA-CNF were explored systematically.Subsequently,a series of CNF-based composite conductive hydrogels were proposed which achieve the trade-off between mechanical and electrical properties.In addition,the ionic coordination and enhancement effect of nano materials based on P-CNF and QA-CNF were introduced.Furthermore,the application as flexible strain sensor was investigated.The detailed investigation has been listed as follows.（1）Two kinds of modified cellulose nanofibrils（P-CNF and QA-CNF）with stable and uniformly aqueous-dispersity were prepared via a facile method composed of chemical-pretreatment and mechanical grinding.In comparison to CNF,P-CNF（1.3±0.5 nm）and QA-CNF（1.1±0.4 nm）both possessed less size.In addition,P-CNF and QA-CNF exhibited improved the uniformity and stability of aqueous dispersion due to the increaseing surface polarities,which is mainly contributed to introduction of phosphate and quaternary ammonium groups.（2）Using P-CNF as filler,PAAM-P-Fe³⁺-PPy composite conductive hydrogels which possessed excellent mechanical and electrical properties were designed and prepared by free radical polymerization combining solution immersion method.Consequently,when the concentration of P-CNF and soaking time of pyrrole solution was fixed at 3 wt%and 3 min,the PAAM-P₃-Fe³⁺-PPy₃ hydrogels displayed distinctive mechanical properties,i.e.,tensile strength:2.54 MPa,elongation at break:926%and toughness:17.71MJ·m^-3 as well as good conductivity（995 m S/m）.Such hydrogel-based sensor had furtherly been attached to the human limbs to monitor related physiological motions.Importantly,the electromagnetic shielding property can be endowed.（3）The PAM-QA-MXene composite conductive hydrogels were fabricated by“one-pot”method.The chemical and microstructure were explored,and the mechanical and strain sensing properties were investigated.Besides,the impact of various components on the mechanical and strain sensing properties were discussed.Notably,the optimized PAM-QA_2.0-MX_2.0 composite conductive hydrogels performed the outstanding mechanical performance due to the incorporation of QA-CNF and MXene with a tensile strength of 2.54 MPa,elongation at break of 926%and toughness of 17.71 MJ·m^-3,as well as an electrical conductivity of 221 m S/m.Meanwhile,such hydrogels possessed an appropriate modulus（58.88 k Pa）which is similar to the that of human skin.Then,the incorporation of MXene endowed the obtained hydrogels with high sensitivity and stability,promptly achieving a remarkable mechanical sensitivity（Conductivity:221 m S/m;GF:2.24）.Moreover,the physiological motions and the continuous shooting action could be monitored by such hydrogels-based strain sensor,thus facilitating the wide applications of CNF composite conductive hydrogels such as wearable flexible strain sensor.