Preparation And Application Of Nanocellulose Composite Hydrogel

Cellulose is the most widely distributed and abundant natural polymer material in nature,with many advantages including wide sources,biocompatibility,degradability and renewability.Nanocellulose,prepared from cellulose,has been used in optically transparent materials,reinforced polymer materials,biomimetic materials,template materials and wearable device hydrogel sensor materials due to their attractive intrinsic properties,such as good degradation ability,high mechanical strengthand,high crystallinity,low thermal expansion coefficient.For traditionalhydrogel materials,poor mechanical properties and single function are the main disadvantages.Therefore,the application of nanocellulose has important practical significance and practical value for the functionality of hydrogels.In this work,the preparation of nanocellulose and the preparation,structure and properties of nanocellulose composite hydrogels were systematically researched.The first part aims to the preparation of nanocellulose by using deep eutectic solvent as the pretreatment solvent for cellulose combined with high pressure homogenization.The innovative research on the recycling and utilization of deep eutectic solvent was carried out.The second part aims to design and develope nanocellulose composite functional hydrogels for flexible wearable sensors.The third part aims to the effects of nanocellulose and graphitic carbon nitride on the properties of composite hydrogels were systematically investigated.Fourth,we are committed to improving the mechanical properties and electrical conductivity of hydrogels,and introducing deep eutectic solvents to obtain hydrogels with high strength and high electrical conductivity.Fifth,the self-healing properties,adhesion and application as transient electronic devices of acidic DES hydrogels are studied.The last part of the work uses photopolymerization to efficiently prepare polyacrylic acid hydrogels,which endow the hydrogels with excellent self-healing properties and conductive properties.Firstly,using absorbent cotton as raw material,nanocellulose(cellulose nanocrystals(CNCs))was prepared using choline chloride/oxalic acid DES system combined with high pressure homogenization.Properties such as crystal structure and stability were studied.The results show that the CNCs are short rod-shaped cellulose nanocrystals with a diameter of 50-100 nm and a length of 500-800 nm,with smooth surface,high crystallinity(77.6%)and good thermal stability.DES has been cycled through at least 3 preprocessing while maintaining its preprocessing capability.Therefore,this method is beneficial to the rapid and green processing of biomass cellulose without affecting the yield and performance of CNCs.Compared with natural cellulose,the thermal stability of CNCs was reduced.The maximum thermal degradation rate of CNCs occurred in the second thermal degradation stage,and the decrease in thermal stability was mainly due to the influence of the carboxyl groups and nanoparticles contained.Then,the nanocellulose prepared above was introduced into polyvinyl alcohol/sodium carboxymethyl cellulose hydrogel by means of freeze-thaw cycles to prepare nanocellulose composite hydrogels(CNC/CMC-Na/PVA hydrogel).with high ductility and toughness.The introduction of CNCs endows the hydrogel with a rigid nanonetwork that exhibits synergistic enhancement of mechanical properties as well as ionic conductivity.The ionic conductivity of the 2.0%CNC/CMC-Na/PVA hydrogel was 0.021 S/cm at room temperature.CNC/CMC-Na/PVA hydrogels were prepared in the DMSO-water binary solvent system,which endowed the hydrogels with excellent freezing resistance,and the hydrogels remained flexible and conductive at-70℃(0.014 S/cm).Moreover,the hydrogel has excellent mechanical properties(stress up to 1.4 MPa,fracture strain up to 1018%),good transparency(90%),freezing resistance(-70℃)and long-term solvent retention and excellent comprehensive properties.At the same time,CNC/CMC-Na/PVA hydrogels have been assembled into sensors for monitoring large and small movements of the human body,exhibiting excellent responsiveness and stability,and having the prospect of being used as human-friendly electronic devices.On the basis of the previous work,g-C3N4 nanosheets and CNCs were introduced into CMC-Na/PVA hydrogels as nanofillers to form more physical cross-linking points(hydrogen bond interactions),thereby increasing the mechanical properties of hydrogels.The effects of the addition of different g-C3N4 nanosheets and CNCs on the structure,mechanical properties,thermal properties and swelling properties of the composite hydrogels(g-C3N4/CNCs-H)were systematically investigated.The maximum mechanical strength of the 1.0%g-C3N4/CNCs-H hydrogel was up to 2.31 MPa and the fracture strain value is about 1560%.In the case of a large number of hydrogen bonds exist inside the g-C3N4/CNCs-H hydrogel,which increases the swelling performance of the hydrogel(the maximum swelling rate is as high as 117.0 g/g).After soaking in NaCl,the 1.0%g-C3N4/CNCs-H hydrogel has good electrical conductivity,and its ionic conductivity reach 0.027 S/cm.The application of gC3N4/CNCs-H hydrogels in flexible sensors displayed excellent sensing properties,stability and sensitivity(GF higher than 1.7).In addition,the flexibility and sensitivity of hydrogels to deformation responses have a great deal of promising as strain sensors for real-time monitoring of various human activities.Compared with ion-impregnated conduction,the ion-polymerization method has the advantages of simple operation and controllable quality.In this part,PVA,g-C3N4 nanosheets,and CNCs were introduced into three choline chloride-based DES with conductive properties to form the hydrogel(DES-hydrogel).The results showed that the DES-hydrogels exhibited excellent electrical conductivity and mechanical properties.The maximum stress value and fracture strain value of DES2-c hydrogel are 2.7 MPa and 1100%,respectively,and the ionic conductivity is as high as 0.139 S/cm.The three groups of DES-hydrogels exhibited different mechanical properties,sensing properties and electrical conductivity owning to the different components of DES.Simultaneously,DES-hydrogels have been assembled into a flexible strain sensor for monitoring human motion,including large-scale motion(finger bending,wrist bending,knee bending,walking,etc.),displaying good stability and excellent sensitivity.Building on the previous section,the work introduced PVA and CMC-Na into acidic DES(consisting of choline chloride and oxalic acid)to form multi-network polymer crosslinks.Through the interpenetration of CNCs and DES networks,an acidic DES composite hydrogel(Acid-DES hydrogel)with high sensitivity,transient(dissolvable)and high self-healing efficiency was finally synthesised.The dissipated energy through the dynamic cross-linked network endows the Acid-DES hydrogel with excellent mechanical properties(maximum tensile stress of 1.34 MPa and elongation at break of 1250%).Excellent self-healing capability(self-healing efficiency up to 90%)protected our strain sensors from large deformations and accidental damage,making the hydrogel recyclable.Additionally,the hydrogels exhibited good and reproducible adhesion behavior.The Acid-DES hydrogel sensor has good solubility in hot water environment and can be used as a transient electronic device.Acid-DES hydrogels have been assembled into flexible strain sensors for monitoring human body movements,including large-scale movements such as finger bending and small-scale movements(pulse beating and speaking),showing good temperature sensitivity,stability and sensitivity.Compared with thermal polymerization,photopolymerization has the many advantages,such as fast reaction speed,energy saving,environmental protection and controllable reaction.In view of this,in the last part of the work,we prepared PVAPAA/CNC hydrogels with polyacrylic acid as the matrix by in-situ photopolymerization of AA in DES.Due to the good compatibility among DES,AA,PVA and CNCs,the permeability of PVA-PAA/CNC hydrogels is as high as 90%.When the content of CNCs was 2 wt%,the mechanical test results showed that the maximum breaking stress of the PVA-PAA/CNC hydrogel was 0.82 MPa,and the breaking elongation was 540%.The PVA-PAA/CNC has excellent adhesion and excellent self-healing properties.Based on the PVA-PAA/CNC hydrogels with good sensing linear correlation,fast response,and good durability,the hydrogels have broad application prospects in the fabrication of strain sensors.As a hydrogel-based sensor,it can not only monitor tiny human pulse signals,but also monitor human movements with large deformations such as fingers and wrists,and has the ability to monitor human movements in real time.