Preparation And Application Of Wood-based Anisotropic Composite Hydrogels

Hydrogel can be used as strain sensor,which can accurately convert external deformation into recordable electrical signal,and is widely used in personalized medical monitoring,human-computer interaction and other fields.Although a great deal of research work has been carried out on the design of multifunctional hydrogels in recent years,it is still a great challenge to integrate excellent anti-swelling,adhesion,mechanical properties and strain sensitivity into a single hydrogel.From the perspective of"green"chemistry,wood hierarchies in nature can not only serve as biomimetic samples,but also give hydrogels unique structural characteristics.In this paper,combined with the structural characteristics of natural wood,chemically treated anisotropic wood cellulose skeleton was introduced into the polymer network,and wood-based anisotropic composite hydrogels with excellent anti-swelling,mechanical properties and sensing properties were prepared.The effects of lignin removal degree,gel network composition and gel network crosslinking density on the properties of anisotropic wood-based composite hydrogels were analyzed.Firstly,the chemical treatment processes of the anisotropic wood cellulose skeleton were compared,and the wood-based composite hydrogel(WCH)was prepared by introducing PAM into the skeleton.Then,N-isopropylacrylamide(NIPAM)was introduced into WCH as substrate to adjust its network structure,and an anisotropic wood-based composite hydrogel with interpenetrating network structure was prepared.Finally,WCH was used as the substrate,and the cross-linking density of the gel internal network was regulated.The anisotropic wood-based composite hydrogels with different cross-linking density were prepared.By analyzing and comparing the physical and chemical properties of composite hydrogels,the following research results were obtained:(1)Introducing the anisotropic wood cellulose skeleton to prepare the wood-based anisotropic composite hydrogel can effectively give the hydrogel excellent anti-swelling and mechanical properties.After alkali treatment for 7 h,the cellulose content in the anisotropic wood cellulose skeleton increased to 56.18%,hemicellulose content decreased to 4.4%,and lignin content decreased to 24.85%,providing a good substrate for the subsequent preparation of WCH.The tensile strength of WCH along the longitudinal direction was 19.8 MPa,and that along the perpendicular direction was 2.7 MPa,which were 39.6 times and 5.4 times that of PAM hydrogel respectively.The WCH weight change rate(42%)after equilibrium swelling was much lower than that of isotropic hydrogel PAM(383%).Since the surface of WCH contains chemical functional groups such as hydroxyl groups of cellulose and free amino groups in hydrogels,it has good adhesion to polar substrate surfaces and can be reused.In addition,WCH can be used as a strain sensor for monitoring bending and pressure.(2)By adjusting the composition structure of the gel network,wood-based composite hydrogels with interpenetrating network can effectively improve the anti-swelling and mechanical properties of anisotropic wood-based hydrogels.With the introduction of PNIPAM,the tensile strength of composite wood hydrogels reached 19.9 MPa along the grain direction.The introduction of PNIPAM chain effectively reduced the entropy elasticity of the molecular chain,and further improved the anti-swelling of the composite hydrogel.After the swelling equilibrium,the weight change rate of the composite hydrogel was only 13.3%,and the size change rate of the composite hydrogel was 2.8%and 2.3%,respectively,which were better than that of WCH(9%).The anisotropic swelling property was confirmed by finite element analysis.In addition,WCH can be used as a strain sensor to monitor finger bending and articulation signals.(3)Anti-swelling and mechanical properties of wood-based anisotropic composite hydrogels can be effectively improved by adjusting the crosslinking density of polymer network.With the densification of the crosslinking network structure,the tensile strength,elastic modulus and toughness of composite wood-based hydrogels increased from 4.64 MPa to 14.73 MPa,from 0.71MPa to 1.25 MPa,and from 16.56 k J/m~3 to 61.35 k J/m~3.With the increase of crosslinking density of composite wood-based hydrogel network,its weight change rate decreases from 18.4%to 13.7%at swelling equilibrium,and its anti-swelling performance is better,which can be verified by finite element analysis of anti-swelling performance.In addition,with the increase of crosslinking density,a high-density hydrogen bond was formed between the composite hydrogel and the hydrophilic polar wood,which increased the adhesion strength of the composite hydrogel surface from 7.63 k Pa to9.13 k Pa,and its sensing performance was also better.The anisotropic wood-based composite hydrogels obtained in this paper have excellent anti-swelling,adhesion,mechanical properties and strain sensitivity,and have broad application prospects as strain sensors,which also provides a simple strategy for the multi-functional and intelligent application of natural balsa wood.