Construction And Performance Modulation Of Nanocellulose Reinforced Conductive Composite Hydrogels

Conductive hydrogels,a class of three-dimensional cross-linked polymer networks with excellent electric conductivity,possess huge potential to be applied in the field of electronics,bionic skin,sensors,etc.However,traditional conductive hydrogels usually have poor mechanical properties and are lack of functional diversity,which severely restricts their application.On the other hand,with the development of the nanotechnology in the field of cellulose science,nanocellulose with unique nano-scale,high elastic modulus,high specific surface area,high crystallinity and easy surface functionalization has received numerous attention.In respond to the need of efficiently conversing biomass resources to value-added products,nanocellulse can be ultilized as reinforcement agent to develop composite materials.This dissertation is focusing on fabricating conductive nanocellulose reinforced composite hydrogels and illuminating the formation mechanism and structure-property relationship of the composite hydrogels.Potential applications of the composite hydrogels in the field of strain sensors and supercapacitors are also discussed.The main results can be summarized as follows:?1?Based on the effect of nano-reinforcement,cellulose nanofibrils?CNF?and tannic acid?TA?were chosen as nanofiller and functional additives to fabricate poly?vinyl alcohol??PVA?/borax/CNF/TA composite hydrogel?PBCT?with moldability,stretchability,self-healing,adhesiveness,conductivity,antioxidant and antibacterial activities.Dynamic borate ester,hydrogen bonds and chain entanglements among PVA,borate,CNF and TA contributed to the formation of 3D network within the PBCT hydrogels.With the participation of CNF,the PBCT hydrogels showed good mechanical stability.Due to the reversible borate ester bonds and hydrogen bonds,the PBCT hydrogels showed favorable moldability and self-healing property within 10 s.With the addition of TA,the hydrogels could be stretched up to 2000%and exhibited good properties of scavenging both DPPH and ABTS·⁺free radicals and effective antibacterial activity against S.aureus.?2?Based on the theories of energy dissipation and metal coordination,mechanical hydrogels were fabricated by introducing quaternary ammonium salt functionalized CNF?QCNF?and iron ions(Fe³⁺)into polyacrylic acid?PAA?hydrogel.The composite hydrogels consisted of two types of networks,which were covalent crosslinking PAA and non-covalent crosslinking among the components?coordinate bonds,hydrogen bonds and electrostatic interaction?.The composite hydrogels with incorporation of QCNF and Fe3⁺exhibited improved mechanical properties.One can note that the PAA/QCNF/0.4mol%Fe Cl₃ hydrogel with QCNF content of1.00%and Fe Cl₃ content of 0.4mol%showed optimal mechanical performance with toughness,elastic modulus and fracture elongation up to 7.47 MJ/m³,0.35 MPa and 838.8%,respectively.Meanwhile,the hydrogels showed excellent KCl absorption.With an increase in the immersion time,the swelling ratio increased significantly and then reached equilibrium.The swelling hydrogels possessed increasing ionic conductivity with increased swelling ratio.Moreover,the conductive composite hydrogels can be employed as strain sensors.With the tensile strain of380%,relative resistance change of the hydrogel reached 201.4%.After manual stretch-unloading cycles,the relative resistance change of the hydrogel strain sensors remained stable,revealing remarkable cycle stability.?3?Inspired by the anti-freezing mechanisms of anti-freezing solution,Li Cl was introduced into polyacrylamide?PAM?/CNF hydrogel?AF?,which resulted in anti-freezing property and high conductivity.With the incorporation of CNF,the AF hydrogels showed good stretchability and compression performance.Differential scanning calorimetry?DSC?results showed that the phase transition temperature of AF hydrogelsdecreased with the increasing concentration of Li Cl.Especially,the AF50%hydrogel maintained its elastic property at-80°C and could withstand various deformations?e.g.,twisting,bending,compression?.Benefiting from Li Cl,the AF hydrogels exhibited satisfied ionic conductivity as high as 9.97 S/m.As the temperature decreased from 25°C to-40°C,the AF50%hydrogel possessed desirable ionic conductivity of2.25 S/m.A supercapacitor consisting of the AF hydrogels sandwiched between two CNT/PPy film electrodes was assembled.After cooling from 25°C to-20°C,the areal capacitance of the device decreased from 110.2 m F/cm² to 36.9 m F/cm²,revealing wide working temperature range.The assembled device was flexible and could bear various bending angles with nearly unchanged capacitive behaviors.Furthermore,the device maintained about 96%of its original capacitance after 10000 charge-discharge cycles at 25°C,indicating long-term cycling stability.?4?Inspired by the catalytic mechanism of catechol groups and metal ions,an adhesive,anti-freezing and conductive hydrogel?PCLT?was developed via incorporating of TA,CNF and Li Cl into PAM hydrogel.TA and Li⁺could efficiently activate ammonium persulfate?APS?to generate radicals and trigger the free-radical polumerization.SEM images exhibited the typical three-dimensional network structures of PCLT hydrogels.With the increase of TA concentration,the compression strength and tensile strength of PCLT hydrogels decreased,while the elongation at break increased.Notably,the elongation at break of PCLT12%hydrogel was as high as 836.7%.Moreover,the ionic conductivity of PCLT hydrogel increased from 8.11 S/m to 11.66 S/m at 25°C with an increase of TA concentration from 1.5%to 12%.When the environment temperature decreased,the ionic conductivity of PCLT 12%hydrogel reached 1.25S/m at-40°C,revealing good conductivity and low temperature tolerance.The PCLT hydrogels can directly stick on the surface of various hydrophilic of hydrophobic substrates.The shear tensile test results showed that the adhesive strength of PCLT12%attached to hogskin,glass,PTFE and silicon rubber reached 1.81 KPa,7.29 KPa,8.71 KPa and 6.74 KPa,respectively.Furthermore,the PCLT hydrogel can be employed as wearable strain sensors to monitor human motions?e.g.,fingers,wrists,elbows?.