Intermolecular Force Measurement And Solution Behavior Study Of Lignin Amphiphilic Polymer

Lignin is the second most abundant resource and the only aromatic polymer in plants,which has great application value.Natural lignin is closely linked with cellulose and hemicellulose.Lignin is degraded and oxidized through industrial processes and its hydrophilicity is enhanced,which is called lignin amphiphilic polymer.Most of the lignin amphiphilic polymer are burned as low-value fuel,which not only causes waste of resources,but also causes environmental pollution.By developing lignin into functional materials,improving the performance of lignin-based materials and realizing the engineering application of lignin,the goal of high-value utilization of biomass resources and reduce of environmental pollution could be realized,which has significant economic,environmental and social benefits.The solution behavior of lignin is closely related to its modification process,material preparation,and application performance.Almost all of the solution behaviors originate from the intermolecular forces between lignin molecules.Therefore,the quantitative measurement of intermolecular forces in lignin-related system,analysis of the environmental dependence of intermolecular force,and the relationship between solution behavior and intermolecular force are important to reveal and understand the functional mechanism of lignin materials,guide the modification of lignin moleules,and develop high-performance lignin-based products.In this work,a set of intermolecular force test method for lignin system has been established completely based on the atomic force microscope?AFM?technology.The quantitative measurement of the intermolecular force of various industrial lignin in different solution environments and the exploration of solution behavior in the corresponding systems are realized,which guides the modification of lignin and the preparation of lignin-based functional materials.The main research progress and achievements are as follows:Based on the AFM technology,the intermolecular forces of acetylated lignin?ACL?,enzymatic hydrolysis lignin?EHL?,and lignosulfonate?LS?in water were measured,which is-2.04±0.55 m N/m,-1.21±0.18 m N/m,and-0.11±0.03 m N/m,respectively.Results demonstrate the solubility of lignin is bigger when the intermolecular force between lignin molecules in solvent is smaller.Based on above conclusion,by weakening the total intermolecular force of lignin,the lignin is dissolved in urea aqueous solution.For instance,the interaction between EHL in 5 mol/L urea aqueous solution?-0.42±0.14 m N/m?decreases by 65%compared with it in water?-1.21±0.18 m N/m?.The ¹H and ¹⁷O NMR spectra demonstrate that hydrogen bonding and lone pair-?interaction are formed between lignin molecule and urea molecule,which weakens the intermolecular hydrogen bonding and?-?interaction between lignin molecules.Such interaction causes the solvation and dissolution of lignin molecules in urea aqueous solution.Moreover,the intermolecular force and solubility of EHL in acetone water,tetrahydrofuran water and ethanol water systems were measured,and the mechanism of solution self-assembly of lignin was explored.Results show that water,acetone,tetrahydrofuran,and ethanol are bad solvents of lignin.The intermolecular force of lignin in the above solvents are large,and the solubilities of lignin are small.A certain proportion of organic solvent-water mixture is the good solvent of lignin.The intermolecular force of lignin in such mixed solvents are small,and the solubilities of lignin are large.For instance,the intermolecular between lignin in water,acetone,and water-acetone mixture are-1.21±0.18 m N/m,-0.75±0.35m N/m,and-0.15±0.08m N/m,while related saturation solubility are 0.01 wt%,0.06 wt%,and 35.6 wt%.The molecular dynamic simulation results demonstrate the water molecules and organic solvent molecules solvate the hydrophilic and hydrophobic groups of lignin,respectively.Such situation results in a small intermolecular force and large solubility of lignin in organic solvent-water mixture.With the increase of water content or organic solvent content,the solvent changes from good solvent to bad solvent for lignin,and the solvation of hydrophilic part or hydrophobic part of lignin molecule disappears gradually,which leads to the aggregation and self-assembly of lignin.Based on the above conclusion,by changing the proportion of acetone and water in acetone-water mixed solvent,the homogeneous lignin colloidal spheres were successfully prepared through the solution self-assembly process.The intermolecular forces between LS are measured quantitatively by AFM in water at pH values of 3,5,7,9,and 11,which are-0.43±0.08 m N/m,-0.25±0.03 m N/m,-0.11±0.04m N/m,-0.08±0.02 m N/m,and 0.37±0.07 m N/m,respectively.The attractive part and repulsive part of intermolecular force are divided and observed.Results show the attractive force decreases and repulsive force increases as pH increases,which results from the enhancement of dissociation degree of-SO₃H,-COOH,and-Ph OH.In order to observe the correlation between intermolecular force and solution behavior of LS,the force-indentation curves of LS are measured,and the inter-and intra-parameters of solution behavior are calculated:the intermolecular distance(L_inter),intramolecular distance(L_intra),intermolecular apparent modulus(E_inter),and intramolecular apparent modulus(E_intra).As pH increase,the enhancement of intermolecular repulsion causes the larger L_inter and E_inter.By comparison,the L_intra remains constant and E_intra increases,which results in the change of LS molecules from relative soft to rigid.AFM is employed to quantitatively investigate the dispersion mechanism of LS/SiO₂ system at different pH conditions.Results show that the total repulsive force between SiO₂ particles in LS solution is stronger than it in water,which results in better dispersion stability of SiO₂particles.For instance,the repulsive force between SiO₂in water and LS solution at neutral condition are 0.23 m N/m and 0.33 m N/m,respectively.Moreover,Derjaguin-Landau-Verwey-Overbeek?DLVO?and DLVO-steric repulsion?DLVO-S?formulas are utilized for the fittings of AFM force-distance curves between SiO₂ in both water and LS solution.Based on these fitting results,electrostatic and steric repulsive forces are respectively calculated,which serves as another evidence that LS provides strong steric repulsion between SiO₂particles.Further studies indicate that adsorbance of LS on SiO₂?Q?,normalized interaction constant?A?and characteristic length?L?are the three critical factors of steric repulsion in LS/SiO₂ system.Based on the above conclusions,quaternized grafted-sulfonation lignosulfonate?QAGSL?dispersant is designed and prepared.Because the long hydrophilic chain and quaternary ammonium group,QAGSL exhibits bigger A,L,and Q,which increases the repulsive force between SiO₂ to 0.61±0.04 m N/m.QAGSL exhibits the good dispersing performance to SiO₂ and real cement particles.