Preparation And Properties Of Xylan-based Functional Hydrogels

As one of the main components of agricultural and forestry biomass,hemicellulose accounts for about 20-35% of lignocellulosic biomass and is a biomass resource that can be utilized in a high value way,with the characteristics of low price,renewable,degradable,and good biocompatibility.The main chain structure of hemicellulose contains a large number of hydroxyl groups,and after chemical modification,it can possess unique properties such as hydrophobicity,electrical conductivity,thermoplasticity and stimulus responsiveness,which plays a pivotal role in the construction of hemicellulose-based functional hydrogel materials.However,because hemicellulose is not a homogeneous glycan,but usually a group of complex glycans,and the content and structure of various types of glycans vary greatly with the type of plant fiber raw materials,and its own low molecular weight and poor processing performance and other weaknesses,resulting in the preparation of hemicellulose-based hydrogels in the process of weak glue-forming ability and poor mechanical properties,limiting the application of hemicellulose in the field of hydrogels.In order to further broaden the application scope of hemicellulose-based hydrogels,this thesis prepared and synthesized a series of xylan-based hydrogels with different properties using laboratory alkali-extracted moso-xylan and industrial dissolving pulp by-product xylan as raw materials,characterized the physicochemical properties of the hydrogels,elucidated their preparation mechanism,explored the mechanical properties,electrical conductivity and motion sensing properties of the composite hydrogels,and revealed the structure and properties of xylan-based hydrogels.The theoretical basis and technical support were provided for the high value utilization of hemicellulose.The main findings of this thesis are as follows.(1)To address the problems of low molecular weight and poor processing performance of laboratory alkali-extracted hemicellulose,this part of the study used alkali-extracted low molecular weight Moso-xylan(Mw ~ 3×104 g/mol)as raw material to prepare composite hydrogel materials by graft copolymerization reaction with acrylic acid,and further introduces metal ions to improve the mechanical and electrical conductivity of the hydrogels through the formation of semi-interpenetrating networks by coordination interactions.The maximum tensile stress of the prepared xylan-based semi-interpenetrating network hydrogel reaches 1.4 MPa,the elongation at break is 1136%,and it has certain electrical conductivity(conductivity: 4.76×10-3 S m-1),and its mechanical properties are better than most existing xylan-based hydrogels,which provides a new way to prepare high-strength xylan-based conductive hydrogels.(2)The existing hemicellulose graft polymerization reactions are mainly initiated by organic small molecules,but this part of the study uses lignosulfonate(SL)and iron ions(Fe3+)to construct a biomass-based macromolecular redox catalytic system,which rapidly induces a large number of free radicals from potassium persulfate at room temperature to further trigger the graft copolymerization of vinyl monomer acrylamide(AM)and carboxymethyl xylan(CMX)graft copolymerization to form hydrogels.The addition of Fe3+ is also capable of ligating with carboxymethyl xylan during the gel formation process without external stimuli such as heating and de-oxygenation,which significantly enhances the toughness of the hydrogel with an elongation at break of 460%.In addition,the introduction of SL/Fe3+ can also confer excellent properties such as good electrical conductivity,transparency(~80%)and UV-blocking properties(blocking ratio: 99%)to the gel,which can be used as a potential application for electronic skin motion sensors.(3)In response to the real problem of large molecular weight(Mw ~ 106 g/mol)of dissolving pulp-xylan in industrial viscose fiber enterprises and the difficulty of conversion and utilization,this part of the study can effectively stabilize the two-dimensional material MXene by oxidatively modifying the industrial by-product xylan into water-soluble aldehyde-based xylan(DAX),overcoming the aggregation of MXene nanosheets in water and the easy oxidation The DAX/MXene system can initiate the polymerization and rapid gelation of different vinyl monomers with a gelation time of less than 90 s,and does not require common initiation methods such as light and heat.In addition,DAX/MX/PAM composite hydrogels have excellent mechanical properties(tensile strength ~38 k Pa,elongation at break ~279%)as well as sensing sensitivity,which have potential applications in biomedical and electronic sensing fields.(4)For the effective use of aldehyde-based xylan(DAX)as a nanocolloid stabilizer and its weak redox property,this part of the study used DAX to stabilize liquid metal(LM)to form a nano-ink with a core-shell structure,which did not show significant precipitation at room temperature for up to 7 days and was able to maintain the nano-core-shell spherical structure without deformation.The DAX/LM nano-ink can be printed,written or impregnated with "wipe-on-conduct" characteristics and exhibits good biocompatibility.Due to the weak reducibility of the aldehyde group,the DAX/LM ink can be used as a catalyst to induce a large number of free radicals from potassium persulfate,which can rapidly trigger the polymerization of different vinyl monomers at room temperature(less than 20 seconds),and further utilize the reactivity of the aldehyde group with chitosan to construct a high-performance dual-network gel sensor with good toughness(elongation at break up to 420%).This work not only breaks through the perceived limitations of xylan in the field of flexible wearable materials,but also serves as a guide for the design of future biomass catalytic systems.