Preparation And Property Of High Toughness And High Conductivity PVA Composite Hydrogel Modified By Lignin

As a new type of conductive soft material,conductive hydrogel had the characteristics of softness,deformation resistance,conductive,etc.,and had broad application prospects in the field of flexible electronic materials.Polyvinyl alcohol(PVA),which was rich in hydroxyl groups,had good biocompatibility and biodegradability.Therefore,PVA-based hydrogels were widely used in biomedicine,tissue engineering and other fields and expected to be used in flexible conductive hydroxyl material.However,as a flexible electronic material,the mechanical properties and electrical conductivity of PVA hydrogel are relatively poor,which made it difficult to meet the demand.Lignin was the second largest biomass resource in plants after cellulose.It had an aromatic ring structure and was rich in functional groups such as hydroxyl,carboxyl and aldehyde groups.Therefore,using lignin to modify PVA hydrogels could prepare green fully degradable composite conductive hydrogels,but the following key problems need to be solved.Firstly,lignin had large molecular weight,complex structure which resulted in a large number of active functional groups been encapsulated;Secondly,lignin was easy to agglomerate which would make the interface compatibility between lignin and hydrogel matrix been poor;Thirdly,the lignin/composite conductive hydrogel lacked effective conductive medium,resulting in poor conductivity and insufficient conductivity.In response to the above problems,the main research content and conclusions of this article were as follows:(1)In order to reduce the molecular weight of lignin and expose more active functional groups,enzymatically hydrolyzed lignin was pretreated by alkaline depolymerization in this article.Under alkaline conditions,hydroxide radicals breaked the ester bonds,ether bonds and carbon-carbon bonds of lignin macromolecules,turning them into small molecules.The enzymatically hydrolyzed lignin which before and after depolymerization were characterized by FT-IR,GPC and potentiometric titration experiments.The experimental results show that after 8 hours of depolymerization,the relative molecular weight of partial depolymerization of lignin decreased significantly,the average molecular weight decreased from 4500 g/mol to1760 g/mol,and the phenolic hydroxyl group also increased to a certain extent,which increased from 1.83 mmol/g to 3.75 mmol/g.(2)The partial depolymerization enzyme hydrolyzed lignin(DEL)was introduced into the polyvinyl alcohol(PVA)hydrogel to prepare PVA@DEL composite hydrogels(lignin content 1%,2%,5%,10%).Among them,when the addition amount of DEL was 2%,the hydrogel exhibits the better mechanical properties(tensile strength of 1.3 MPa,elongation at break was 525%).Then utilized the adsorption of metal ions by the many polar oxygen-containing functional groups of lignin to introduce silver ions into the hydrogel network and realize in-situ reduction by soaking sodium citrate.The tensile strength of the hydrogel Ag~0@DEL could reach up to 13 MPa,the elongation at break could reach 1200%,and the electrical conductivity could reach 1.0 S/m.At the same time,when it was deformed by pressing or twisting,etc.,the output signal changes significantly,which meaned it has good conductivity sensitivity.(3)Firstly,PVA/CNF hydrogels were prepared by adding CNF(respectively 1%,2%,5%,and 10%)in the PVA hydrogel matrix,When the amount of CNF was 1%,the tensile strength of the hydrogel was 1.6 MPa and the strain reached 650%.Then,DEL was introduced on this basis to form the natural binding force between CNF.Taking advantage of the ability of lignin to absorb silver ions,and through repeated mechanical training,the CNF in the hydrogel produces an oriented structure to achieve the anisotropy of the hydrogel in terms of mechanical and electrical conductivity.After mechanical training,the tensile strength of the hydrogel parallel to the training direction increased from 2.0 MPa to 4.1 MPa,and the electrical conductivity increased from 1.1 S/m to 1.3 S/m,while the tensile strength of the hydrogel perpendicular to the training direction changed little,and the electrical conductivity was reduced to 0.8 S/m.