Structure Regulation And Properties Of Functional Nanocellulose-based Aerogel

Nanocellulose aerogels（CA）hold great potential in photovoltaic,catalysis supporter,thermal insulation,medicinal materials,for their fascinating properties,such as high porosity,excellent mechanical properties,biodegradability,low thermal conductivity,low density,and high specific surface area.The properties of CA are closely related to its microstructure.However,due to the strong hydrogen bond interaction within and between the molecular chains of cellulose nanofibers,they are easy to aggregate in the drying process.Thus,the unique microstructure of CA will be lost,and properties such as specific surface areaa and thermal conductivity will be limited.Therefore,there are still many problems to be solved in regulating the microstructure of CA.In this study,cellulose nanofibers（CNFs）were modified by organosiloxane with different functional tail groups.In the suspension of CNFs,organosiloxane is covalently cross-linked with the hydroxyl group on the surface of cellulose,weakening the hydrogen bond interaction between CNFs.At the same time,functional groups with different polarities（amino group,epoxy group,and carbon-carbon double bond）were added to regulate hydrogen bond intensity in CA,and the influence of hydrogen bond strength on CA microstructure,mechanical properties,specific surface area,etc.were explored.And on the basis of regulating the CA microstructure,MXene and poly（3,4-thiophene ethylene 2 oxygen radicals）-poly（styrene sulfonate）（PEDOT:PSS）was used to modify CNFs and prepare CA with the conductive and thermoelectric conversion performance.The impacts of MXene and CNF contents on the micromorphology,mechanical characteristics,and electrical conductivity of the composite aerogel,as well as the application of the composite aerogel in the field of thermoelectric conversion,were explored.The main contents are as follows:（1）Regulating and Controlling the Microstructure of Nanocellulose Aerogels by Varying the Intensity of Hydrogen Bonds.To create CA with varying hydrogen bond strengths,three types of siloxanes（APTES,GPTMS,and VTMS）with different tail groups（amino group,epoxy group,and carbon-carbon double bond）were employed to alter CNFs.After hydrolysis,the three organosiloxanes have similar chemical structures and are cross-linked to the surface of CNFs by covalent bonds.The strength of the hydrogen bond network in CA can be regulated by changing the functional groups in the tail when the degree of cross-linking of organosiloxane on the surface of CNFs is basically the same.Fourier Transform infrared spectrometer（FT-IR）and solid ¹³C NMR were used to investigate the structure and substitution degree of the modified cellulose.The results showed that APTES,GPTMS,and VTMS had been successfully covalently cross-linked with CNFs,and the substitution degrees were 34%,35%,and 35%,respectively.The varied temperature FT-IR test showed that the hydrogen bond coefficient(F_b-OH)of APTES,GPTMS and VTMS modified CA were 0.41,0.39,0.33 and 0.33,respectively.Field-emission scanning electron microscope（SEM）test showed that when the F_b-OH in CA decreased from 0.41 to 0.33,the aggregation of CNFs was effectively inhibited,the CNFs sheet-like aggregare changed to cellulose nanowires in the microstructure of CA,and the specific surface area of CA increased from 44.36m²/g to 99.37 m²/g.Mechanical recovery and thermal insulation have also been improved.The research in this chapter provides a new method for the structural regulation of CA and improves its application prospect in various fields.（2）Preparation and properties of nanocellulose aerogel with thermoelectric conversion performance:On the basis of regulating the microstructure of CA,MXene,a meteria with excellent conductivity and PEDOT:PSS was added to prepare CA with conductivity and thermoelectric conversion performance.The ratios of P-MXene and CNFs were 1:4,2:3,3:2,and 4:1,respectively,and the prepared CA were named C₁M₄@P,C₂M₃@P,C₃M₂@P,and C₄M₁@P,respectively.With the decrease of P-MXene content and the increase of CNFs content,the microscopic morphology of composite CA changed greatly.When the P-MXene content was 80%,P-MXene inside CA aggregated,forming lager sheet-like structure,and only little CNFs existed.When the content of P-MXene decreased to 60%,the morphology of CA was better.But,when it continued to decrease,CNFs and P-MXene aggregated tightly,which affected its morphology,and other properties.The maximum compressive stress increased from 6.87 to 14.72k Pa,the specific surface area decreased from 163.66 to 42.21 m²/g,and the electrical conductivity decreased from 3.51 to 0.51 m S/cm.From those composite CA,C₂M₃@P had the best properties,whose maximum compressive stress,conductivity and ZT was 9.97 k Pa,1.70 m S/cm,and 9.44×10^-4,respectively.Compared with previous reserch,ZT in this work has been greatly improved,as well as the mechanical and thermal insulation properties,which provide a better prospect in the field of thermoelectric.