Preparation Of Wood Gel Materials And Their Anisotropic Applications

Polymer gels（hydrogels,aerogels,carbon gels,etc.）are commonly consisted of three-dimensional network with physical,chemical and covalent cross-linked polymer chains.Polymer gel materials have been widely used in biomedical,cosmetic,food,catalyst,energy storage and sensing applications.In the context of sustainable development,good recycling and utilization of petroleum-based polymer materials should be accompanied by the development of renewable materials to replace petroleum-based products.As one of the most widely used natural polymer materials on earth,cellulose exhibits the advantages of environmental friendless,good biocompatibility and low cost,and it holds great potential in the preparation of functional gel materials.With the bottom-up method,the use of cellulose obtained from the separation and purification procedures to prepare gel materials is complicated and high energy demanded.In contrast,the direct preparation of cellulose gels from wood by top-down methods has attracted a lot of attention.In this paper,with balsa wood as the original material and delignification wood as the skeleton,polymer monomers,metal ions,inorganic MXene fillers and metal-organic frameworks materials（MOFs）were introduced into the wood channels to prepare wood gels with anisotropy,mechanical properties and excellent sensing properties.The effects of anisotropy on mechanical and sensing properties were systematically compared,and the ion transport pathways and sensing mechanisms in the gels were analyzed.Inspired by the mode of ion transport in the wood gel and the anisotropy of the wood,the MOF-modified wood gel was applied as a filter for air purification,ultimately enabling the full utilization of the wood in different directions.The main findings are as follows:（1）Highly anisotropic,flexible,and conductive wood hydrogels were prepared by using delignified wood as a framework with a designed double cross-linked strategy.By integrating acrylamide/acrylic acid and Fe³⁺ions into the delignified wood matrix,the mechanical performance and ionic conductivity can be largely enhanced.The mechanical properties of wood along different tangential directions were compared and the effects of wood anisotropy analyzed.The optimal sample demonstrated a tensile fracture strength of 8.2 MPa along the tree growth direction,high conductivity of 0.040 S m^-1.The hydrogel was assembled into a sensor that can successfully detect signals generated by human movement and analyzed the differences in electrical signals generated by compression along different directions.The excellent sensing performance is mainly due to the deformation produced by compression which causes the ions to move rapidly along the oriented pores of the wood,resulting in a change in current.This work provides a new direction for the design of wood-based human motion sensors.（2）To solve the problem of wood hydrogels being prone to freezing at low temperatures and low electrical conductivity,the introduction of high concentrations of metal ions can achieve high electrical conductivity and frost resistance.Here,delignified wood substrate was directly converted into wood hydrogels using an inorganic Zn Cl₂ solution system.Zn Cl₂ has dual roles in promoting the exfoliation and dispersion of MXenes,and simultaneously inducing the rearrangement of cellulose fibers.An extensive hydrogen bonding network between cellulose and MXene can be constructed,and the ionic coordination between Zn²⁺ions and hydroxyl groups serves as the secondary cross-linked network for the composite hydrogel.Enabled by the MXenes and Zn²⁺ions,as-prepared wood hydrogel presents well-preserved anisotropy,superior mechanical performance（tensile stress:0.8 MPa,compressive strength:1.44 MPa）,high conductivity(0.26 S m^-1)and extreme temperature tolerance.Wood hydrogels show great potential as sensors for health monitoring applications such as human motion detection,temperature response and light response.Furthermore,the mechanism of sensing is mainly the anisotropic pore channels of the wood,and the MXene lamellae that provides the ion transport pathway.This study provides a simple strategy for developing wood hydrogel sensor systems with high conductivity and wide temperature range response.（3）Due to the strength of the delignified wood,the prepared wood hydrogels also possess a high mechanical strength and therefore the wood hydrogels are not effective in detecting signals generated by weak movements.Here,a simple and sustainable design is proposed to directly prepare elastic carbon aerogel with a layered structure from the wood.Zeolitic imidazolate framework（ZIF-L）was deposited on a wood aerogel matrix without lignin and hemicellulose by in situ growth.The presence of ZIF-L significantly reduced the volume shrinkage of aerogel during carbonization,and the carbonized ZIF-L improved the contact area and provided electron transport path.The prepared aerogel has a stress of 42 k Pa under 80%strain,and the elastic strength and volume remain both above 98%after 200 times of compression,indicating that the aerogel has excellent compressibility and fatigue resistance.Based on this particular structural design,as-obtained aerogel shows excellent sensitivity as pressure sensors to detect subtle signals.In addition,the carbon aerogel can be directly assembled into an all-solid-state symmetric supercapacitor that shows promising electrochemical performance and could be potential candidates for high-performance supercapacitors in wearable devices.（4）In the previous study,the compressibility of wood in the direction perpendicular to the growth direction was mainly utilized.In combination with the ion movement and MOF modifications,here,wood gel filters with a hierarchical structure were prepared using MOFs for the modification of delignified wood.The pores of the wood along the growth direction were utilized as pore channels for capturing air pollution particles.ZIF-8,MOF-199 and Ui O-66 were deposited on the filters through in situ growth approach,thereby leading to a uniform MOF dispersion and forming strong capture sites.The ZIF-8 functionalized wood filter exhibited the highest PM removal performance,with PM_2.5 and PM₁₀ removal efficiencies of 89.8%and91.6%,respectively.The superior PM capture ability of ZIF-8@Wood is attributed to the surface property of the MOF and ordered channels/pores of the wood.Finally,the gas transport pathways and PM capture mechanisms within the wood gel filter are discussed in detail.This cost-effective and environmentally friendly filtration device provides a novel material platform for PM removal,and it could be extended for hazardous gas capture and volatile organic compound adsorption.