Regulation Of Microstructures And Aggregates Of Lignin Amphiphilic Polymers

As the only biomass contains aromatic rings in nature, lignin has high potential to partlyreplace the fossil resources. Industrial lignins and their derivatives have been widely used asmacromolecule surfactants in energy, construction, agriculture, and many other areas becausethey are abundant, environment-friendly and cheap. In this work, the aggregation behaviors oflignin amphiphilic polymers were studied, as well as the regulation of their conformations. Atthe same time, lignin-based nanoparticles were prepared. The results are as follows:Four NaLS fractions with P.D.I.＜1.2were obtained by filtration, ultrafiltration, gelpermeation chromatography (GPC) separations and used to investigate the solution behaviorsof NaLS by DLS and SLS. DLS results showed that there was a slow mode in the dynamicdistributions of NaLS in addition to the fast mode corresponding to NaLS molecules. Bothhindered motion of NaLS chains induced by electrostatic interaction and NaLS aggregatescontribute to the slow modes in NaLS’s dynamic distributions. Mw, SLSwas much larger thanMw, GPCbefore the slow modes were removed. When the slow mode was eliminated, Mw, SLSwas just slightly larger than Mw, GPC, which was caused by the conformation differencebetween NaLS and GPC standards. However, this Mw, SLSwas closer to the real Mwof NaLS.Based on the Dzand Mw, SLSdetermined by DLS and SLS respectively, the molecularconformation of one NaLS fraction was fitted by Perrin formulation and it found to be anoblate ellipsoid with axis ration of3.5.Aggregation behaviors and microstructures of NaLS under different pH conditions,temperatures and ionic strengths were investigated by DLS, SLS, fluorescence spectrometry(FS), Zeta potential and atomic force microscopy. Results showed that NaLS aggregated inacidic solution, and the degree of aggregation decreased with increasing pH. When solutionpH was increased, the intermolecular aggregates of NaLS were disaggregated and thestructure of NaLS changed from compact to loose. As a result, the apparent Mw, SLSof NaLSdecreased, the ratio of Rg/Rhincreased, the adsorption thickness and the surface roughness ofNaLS on quartz slides decreased. NaLS molecules started to aggregate above a criticaltemperature, which was about38℃. When the temperature of NaLS solution was above38°C, the apparent Mw, SLShad a sharp increase, the disappeared slow mode in DLS analysisreappeared, intensity ratio (I1/I3) of pyrene in FS suddenly decreased, indicating that NaLSmolecules started to form new aggregates. The adsorption of NaLS on a solid substrate andthe corresponding surface roughness increased significantly. As the ionic strength of solutionwas increased, NaLS tended to shrink and aggregated a little due to the gradual elimination of electrostatic interaction.Alkali lignin was chemically modified by acetylation, and then used to prepare uniformcolloidal spheres by adding water into acetylated lignin (ACL)/THF solution. Theself-assembled structure and colloid formation mechanism of ACL were investigated by DLS,SLS, transmission electron microscope, scanning electron microscope, X-ray photoelectronspectroscopy, fourier transform infrared, elemental analysis, and contact angle measurement.Results showed that ACL colloidal spheres were obtained from gradual hydrophobicaggregation of ACL molecules, induced by continuously adding water into the ACL/THFsolution. ACL molecules started to form colloidal spheres at a critical water content of44vol%, when the initial concentration of ACL in THF was1.0mg mL-1. The colloidizationprocess was completed at a water content of67vol%. An excessive amount of water wasadded into the dispersions to “quench” the structures of ACL colloidal spheres. The ACLcolloidal spheres had hydrodynamic radius of110nm with a polydispersity (μ2/Γ2) of0.022.The average aggregated number in each colloidal sphere and the average density wereestimated to be1.0×105and0.187g cm-3. These ACL colloidal spheres could whiten alkalilignin without further reaction.Alkali lignin was also modified via atom transfer radical polymerization grafting of2-(diethyl-amino)ethyl methacrylate (DEAEMA). These lignin-g-DEAEMA samples could beeasily dispersed in water upon CO2bubbling. They could also be quickly precipitated out bybubbling N2. The CO2/N2-switchability of dispersion/precipitation was correlated to the graftdensity and chain length of DEAEMA units. This gas-switchable feature rendered the ligninmaterials a wide range of potential applications. Demonstrated in this work was the use oflignin-g-DEAEMA as surfactant for decane/water-Pickering emulsion. This lignin-basedPickering emulsion could be demulsified and re-emulsified by bubbling CO2and N2. Theemulsification/demulsification processes were highly reversible and easily repeatable.