Preparation Of Molecular Photocages Derived From Lignin And Regulation Of Its Optical Properties

The main components of wood are cellulose,lignin and hemicelluloses.The complex structure is synthesized between the three components through covalent bond,hydrogen bond and van der Waals force intermolecular force.This natural supramolecular structure gives wood fibers inherent stability.Lignin is the most abundant renewable aryl compound resource in nature.Its aromatic ring polymer structure mainly contains functional groups such as methoxy,phenol hydroxyl,alcohol hydroxyl,etc.Vanillin can be obtained by oxidative degradation of sodium lignosulfonate（LS）.In this paper,optically active nitrobenzyl photocage was prepared from vanillin,which was degraded by sodium lignosulfonate.Nitrobenzyl compound was modified via chemical route to obtain a lignin-based photocage（PCC）.Then,PCC was used as a crosslinking agent to conduct covalent crosslinking of polysaccharides（amideated sodium carboxymethyl cellulose,chitosan and amideated hyaluronic acid,etc.）to form a network structure.This compact network structure effectively solved the problem of poor water resistance of polysaccharide-based polymer composite,greatly improved the thermal stability and tensile strength（～102 MPa）of the materials,and provided favorable guarantee for the materials to be applied in the harsh environment,such as high temperature and high humidity.The polysaccharide based polymer composite membrane has very low permeability of water vapor(4.93 g mm m^-2 24 h^-1 kPa^-1),oxygen(5.8×10^-9 cm³ m m^-2 h^-1 Pa^-1),carbon dioxide(1.63 cm³ m^-2 24 h^-1)and other gases,and could be used as food preservative bag.When the preservative bag was damaged,due to the excellent optical properties of the crosslinking-agent photocage,could be photo-cleaved quickly in a short time upon 375 nm UV irradiation.Then,the photo-cleavage product were treated with acid-base to regenerate polysaccharides.Recycled polysaccharides were used again and again to prepare new preservative bag.It was worth mentioning that this new preservative bag had similar mechanical properties and gas permeability as the original preservative bag.Besides,it also had similar excellent performance in the field of food preservation.It was found that the thermo-stability and mechanical properties of materials could be effectively improved by constructing covalent cross-linked network structure.In addition,the introduction of carbon dots were beneficial to simplify the preparation steps and further improve the strength of composites.Based on the previous project,nitrobenzyl compound was treated by solvent-thermal carbonization in high temperature to obtain lignin-based carbon dots photocage（P-CDs）with excellent optical properties.Next,lignin-based P-CDs was crosslinked with polysaccharides（amideated sodium carboxymethyl cellulose,chitosan and amideated hyaluronic acid,etc.）to form a network structure after being treated with 375 nm UV light,so as to prepare P-CDs film used for photothermal conversion.P-CDs films had excellent photothermal conversion efficiency（46.6%）,good thermal-stability and superior tensile strength（～120 MPa）,which could further be used in varieties of extremely harsh environment.P-CDs films with compact network structures via the P-CDs,which effectively solved the problem of photothermal reagents leakage.Using its superior photothermal conversion properties,we have combined P-CD film with solar-thermal-electric conversion to construct solar-powered TEG,which was stable in all weather conditions and had promise of energy output in a variety of weather conditions.Combined the advantages of carbon dots and intermolecular covalent crosslinking,carried out solvothermal carbonization of nitrobenzyl photocage at high temperature to obtain lignin-based carbon dots photocage（P-CDs）,which havd excellent optical properties.In order to further improve the mechanical strength of the composite,and gave it the property of multi-function,the crosslinking of P-CDs and sodium carboxymethyl cellulose amide was performed to obtain the covalent cross-linked biomass-based polymer network（CMC-CDs）.The CMC-CDs polysaccharide networks was combined with carbon nanotubes（CNTs）,then,the CMC-CDs/CNTs composite was obtained by filtration process.It was found that CMC-CDs/CNTs composite had excellent tensile strength（～910 MPa）and thermal-stability（high glass transition temperature of～315℃）.In addition,the composite had high carrier concentration(47.6×10¹⁹ cm^-3)and mobility(2.937 cm² V^-1 s^-1),which gave it superior conductivity(1440.92 S cm^-1)and electromagnetic incidence shielding efficiency（EMI SE,80 d B）.When the CMC-CDs/CNTs composite was placed in acid,alkali,salt solution,organic solvent and high/low temperature circumstance,it was found that it had good stability,which proved CMC-CDs/CNTs composite could work normally in extremely harsh environment.After the CMC-CDs/CNTs composite was abandoned,the wastes of CMC-CDs/CNTs composite could be cleaved by 375 nm UV light,where,the sodium carboxymethyl cellulose matrix was completely destroyed and the CNTs powder was automatically precipitated.A new CMC-CDs/CNTs composite was re-prepared by combining the recovered CNTs with CMC-CDs matrix.Compared with the original CMC-CDs/CNTs composite,the mechanical properties,electrical conductivity and EMI SE properties of the new composite were consistent.