Design And Application Of Wood Frame Cellulose-based Smart Responsive Materials Driven By Humidity

Humid thermal fatigue occurs when people exercise or work in high temperature,which can seriously damage their health and reduce work efficiency.Humid thermal regulation of the local space around the human body through individual humid thermal management has a great potential in alleviating humid thermal fatigue and meeting personal thermal humid comfort.At present,the research on individual humid thermal management mainly focuses on near-human body cooling,heating and thermal backup integration,smart wearables,etc.Although humid thermal management has high efficiency,it still has disadvantages such as artificial energy supply,large size and heavy quality.Inspired by the phenomenon that pinecone scales close in the humid environment and open in the dry environment,humidity stimuli-responsive smart material is designed for individual humid thermal management,with sweat evaporation as stimulus source,without artificial energy supply,flexible use conditions,and strong competitive advantages.At present,there are mainly two types of problems in materials driven by humidity response:(1)Building double-layer or multi-layer structural materials based on a variety of materials with different humidity sensitivity,but these materials usually have the problem of short service life due to poor interface bonding ability;(2)Humidity driven materials obtained from non-renewable resources,such as graphite-based,polymer-based,usually have the problems of non-renewable raw materials,complex preparation process,small deformation amplitude and poor responsiveness.Therefore,this study designed and constructed two humidity driving materials based on biomass wood rich in hydrophilic functional groups:(1)By means of orderly and controllable stripping of lignin,a lignin gradient coating structure was constructed on the lignocellulose frame with excellent mechanical strength and rich hydrophilic functional groups,and a humidity response driven smart wood was prepared.The wood integrated frame structure effectively solved the interfacial bonding problem existing in heterogeneous structures.(2)The strategy of full lignin stripping was adopted to form the wood cellulose frame,and then the humidity sensitive conductive material(MXene)was introduced into the frame to construct the frame-coated structure multifunctional actuator,giving the composite smart material better humidity response performance and versatility.Finally,smart wood and MXene/cellulose frame composite smart materials are used as a proof of concept of smart clothes for personal humid thermal management.The research conclusions are as follows:(1)With cellulose frame based-lignin gradient distribution structure,smart wood with optimized thickness of 100μm and 2/3 of the thickness of lignin stripped showed the best humidity driving performance and sensitive response(30.2°/s,which was better than the value reported in most literatures:<25.7°/s),high driving force(lifting drive mass ratio:～88 times,much higher than the value reported in the literature:8～53 times),large bending deformation(≥180°),and strong cycle stability(≥10000 times);in addition,it also has the driving deformation programmability endowed by anisotropy,and the smart wood have an angle of 30°,45°,and 60°with the fiber direction can reach 131°,138°,170°;the preparation method is universal,and a variety of wood(pine,cherry,sycamore,black walnut,etc.)have been successfully designed and constructed as humidity response smart materials.Using this kind of humidity driven smart wood concept as smart wood clothes,it shows good humidity dissipation and heat dissipation effect in hot environment and good thermal insulation effect in cold environment.The water vapor transmission is as high as 3203 g/(m~2*24h),which is higher than that of traditional fabrics(for example,polyester(1907 g/(m~2*24h)、cotton(2103 g/(m~2*24h)),etc.);and the tensile strength is up to 189 MPa,which shows great advantages compared with other fabrics or fibers(i.e.,sportswear-11MPa,polyurethane fiber-3MPa,graphene fiber-48MPa);and the transmission rate of ultraviolet light is almost 0,reflecting good UV protection ability.(2)The MXene/cellulose frame composite smart material further endows the humidity-driven smart material with electromagnetic shielding function.The cellulose frame with all lignin removal has a wider channel inside,which promotes MXene to form a connected network inside and on the surface of the cellulose frame,thus giving MXene/cellulose frame composite smart material excellent electrical conductivity and electromagnetic shielding efficiency.When MXene accounts for 42.8%,the composite smart material exhibits a high conductivity of 2.72×10~4 S/m and an EMI SE of 39.8 d B,which can shield 99.99%of the incident electromagnetic interference;and the shielding performance is much higher than other electromagnetic shielding materials reported in the literature(for example,the EMI SE of CNT/NR is 21 d B,and that of MWCNT/polystyrene is 30 d B).At the same time,both MXene and cellulose frame have polyhydroxyl functional groups and the dense structure formed by the combination of the them,resulting in a more sensitive humidity response(74.6°/s),higher driving force(lifting drive mass ratio:～126 times),large bending deformation(≥176°),and strong cycling stability(≥10000 times).Using the concept of composite smart materials as smart clothes,the water vapor transmission is as high as 3395 g/(m~2*24h),and the tensile strength reaches 172 MPa,which is much higher than that of traditional fabrics.It can not only shield electromagnetic radiation in the environment,to protect physical health,but also adjust the thermal humid comfort of the body,which provides new inspiration for the research and development of smart wearable clothes for personal humid thermal management.