Self-assembly Properties Of Amphoteric Lignin And Its Performance As Functional Materials

Biomass has attracted extensive attention for its advantages of abundant,eco-friendly and renewable.Lignin,the only biomass component rich in aromatic benzene ring structures is presenting non-toxic,biodegradable and biocompatible character.Owning to its antibacterial,anti-oxidation and UV absorption functions,it is an ideal raw material for preparing biomass-based functional materials.Alkali lignin?AL?is a by-product of pulp and paper industry,consisting of hydrophobic phenylpropane skeleton and various hydrophilic functional groups,such as phenolic hydroxyl,alcoholic hydroxyl,carboxyl,and methoxy groups.It has been widely used in pesticide dispersants,cement water reducer,coal water slurry dispersant and so on.However,limited by its wide distribution of molecular weight,poor water solubility,and easy aggregate,alkali lignin displays poor compatibility with other materials and have the bottleneck of its application in functional materials.The study of modification of alkali lignin to prepare high value-added functional materials is of great significance for the utilization of biomass resources.This topic has important theoretical significance and application value.Amphoteric polymer is charged macromolecule carrying both acidic and basic groups.The acidic groups are mainly carboxyl and sulfonic acid groups,and the basic groups are tertiary ammonium and quaternary ammonium groups.It possesses unique physicochemical properties such as isoelectric point,pH sensitivity,anti-polyelectrolyte effect and complexation effect.It has been widely applied in oil field development,protein purification,drug controlled release,enzyme recovery and heavy metal chelating.In this work,AL was quaternized to prepare amphoteric lignin?AML?.Based on the regulation of electrostatic and hydrophobicity interaction,the orderly assembly of AML in selective solvents can be realized.The regulatory mechanism of AML microstructure and it's self-assembly mechanism in solution are elucidated.Self-assembled colloidal spheres could control the release of drug depends on its pH-responsive properties.Exploiting the self-assembly properties of AML at solid-liquid interface and in the solution,AML was applied to encapsulate TiO₂ by self-assembly which can endow TiO₂ with excellent UV-blocking performance and safety.The self-assembly properties of amphoteric AML at the oil/water interface were studied,and a lignin-based Pickering emulsion with dual pH responsive characteristics was prepared.Finally,a positively charged lignin-based silver nanocomposite was developed by using AML as reducing agent and stabilizer.The synergistic antibacterial mechanism between the AML and silver nanoparticle was investigated.Our work will not only open up a new way for functional utilization of industrial lignin,but also develop and improve the controllable structure theory of natural random polymer.Moreover,it has important theoretical and practical application value for the green development of functional materials.The main conclusions are as follows:?1?AL was modified by 3-chloro-2-hydroxypropyltrimethylammonium chloride?CHMAC?to prepare amphoteric lignin?AML?.The structure of AML was investigated by FT-IR,¹H-NMR and elemental analysis.Quaternary ammonium group was mainly grafted on the phenolic hydroxyl groups of AL.By changing the amount of CHMAC and AL,the amphoteric lignin with different pH sensitive properties could be prepared,and its self-assembly characteristics at the solid-liquid interface were affected by the pH value of solution.Under the acidic condition,the molecular configuration was more curly and the dense adsorption film on the solid-liquid interface was formed.With the increasing of pH,AML form a large aggregate due to the intramolecular and intermolecular electrostatic interaction,which could form a loose and soft adsorption film.As the pH value further increases,the phenolic hydroxyl ionized,the hydrophilicity of AML increased,and the aggregation degree decreased,resulting in formation of dense adsorption film.?2?The microstructure of AML was regulated by sodium dodecylbenzene sulfonate?SDBS?,which can complexed with AML by electrostatic and hydrophobic interaction force to weaken the?-?interaction between the lignin rings.AML/SDBS self-assembled into pH-responsive lignin-based colloidal spheres?LCS?in an ethanol/water mixture,which exhibited rich phase behaviors responding to pH changes.Ibuprofen?IBU?was used as a drug model and more than 74.44%of it could be encapsulated in LCS via hydrophobic interactions.The in vitro release behavior of IBU was pH-dependent and exhibited controlled release properties.More than 75%of IBU could be preserved in simulated gastric fluid?SGF,pH 1.2?,and over 90%could be released smoothly in simulated intestinal fluid?SIF,pH 7.4?,which meeting the release requirements of oral administration.?3?The uniform LCS@TiO₂ composite microsphere was successfully prepared by self-assembly of AML/SDBS and TiO₂ in ethanol/water mixture.Compared with TiO₂nanoparticles,the hydrophobicity of the LCS@TiO₂ was increased.Thus the compatibility with the hydrophobic cream was improved.The SPF value of the cream containing 20 wt%LCS@TiO₂ was 62.14,which was better than that of the SPF 50 commercial sunscreen cream.More importantly,the lignin could effectively eliminate the free radicals produced by TiO₂ so that the photocatalytic activity of TiO₂ was significantly weakened.Therefore,the composite microspheres could be an effective and safe UV-blocking.?4?A lignin-based Pickering emulsion with dual pH-responsive was designed on the basis of the amphiphilic properties and pH-responsiveness of AML.Under neutral conditions,AML was insoluble in water and self-assembled into aggregate.In the emulsification process,AML aggregate was sheared into lignin particles,stabilized at the oil/water interface,and could act as emulsifier for oil-in-water Pickering emulsions.With the addition of acid or base,AML dissolved in the aqueous phase,emulsions were demulsified,and the emulsification and demulsification process was controllable.The emulsion had the highest stability at a pH of 7and 0.1wt%AML.The Pickering emulsion possesses excellent stability to temperature,salt,and ultraviolet light.Lignin-based Pickering emulsion can not only load and control the release of curcumin but also effectively prevent its photodegradation,the degradation rate is less than 30%even after 72 h UV irradiation.?5?AML was used as reducing agent and stabilizer to prepare lignin/silver nanoparticles composite.The average particle size of silver nanoparticles was 20 nm.The positively charged lignin/silver nanoparticles composite had excellent antibacterial activity against escherichia coli and staphylococcus aureus with a minimum inhibitory concentration of 30ppm.Lignin effectively enhanced the photothermal stability of silver nanoparticles.QCM-D and optical microscopy test results showed that the positively charged AML had a strong binding force with bacteria and can effectively capture it,which could enable the silver nanoparticles to contact bacteria rapidly and kill bacteria.The lignin/silver nanoparticles composite had good compatibility with polyurethane,and when the added amount was 0.5%by weight of the polyurethane,the prepared polyurethane film could completely inhibit the growth of bacteria.