Synthesis And Properties Of Lignin-containing Hydrogels

Lignin is the most abundant biopolymer second only to cellulose. The conversion of lignininto value-added polymer materials is an effective way to utilize the vast quantities of lignin. Theintroduction of lignin into hydrogels favors the the improvements in performances of hydrogelsand offers plenty of advantages, e.g., controlled release of hydrophilic drug, improved mechanicalproperties, improved biocompatibility and biodegrability of the hydrogels, etc. In this work,lignin-containing temperature-sensitive hydrogels were prepared from deacetylated acetic acidlignin (AAL). The properties of the lignin-containing temperature-sensitive hydrogels, as well asthe relationship between the structural characteristics of lignin and the lignin-containingtemperature-sensitive hydrogels, were investigated. The hydrogels for dye adsorption were alsoprepared from AAL. Adsorption behavior of the dye onto the hydrogels was investigated. The dyeadsorption mechanisms were discussed.The temperature-sensitive hydrogel (LGN) was prepared by graft copolymerization of AALand N-isopropylacrylamide (NIPAAm). DSC curves demonstrated that the lower critical solutiontemperature (LCST) of the hydrogels containing different contents of AAL was approximately31°C. The hydrogels shrank in water at temperatures above the LCST and swelled in water attemperatures below the LCST. SEM images suggested that the cross-section of freeze-driedhydrogels was porous and the pore size of the hydrogel increased with increasing the AALcontent. The data of adsorption and release experiments for the model drug (methylene blue)suggested that the controlled drug release can be achieved. The introduction of lignin on theproperties of the hydrogels is controllable.A lignin-based macromer (acrylated lignin, AcL) with vinyl bonds was prepared by acylatingAAL with acryloyl chloride. The temperature-sensitive hydrogel (ALN) was prepared from AcLand NIPAAm through UV photocrosslinking. The DSC curves demonstrated that the lowercritical solution temperature of the hydrogels reduced from29.9°C to31.6°C when the massratio of AcL/NIPAAm increased to0.24:2.2from0.08:2.2. The SEM images showed that thecross-section of freeze-dried hydrogels was porous and relatively high content of the macromer resulted in reduced pore size in hydrogels. The results indicated the AcL acted as hydrophobicmonomer and crosslinker.The pH-sensitive hydrogel (LGA) was prepared by graft copolymerization of AAL andacrylamide (AAm). The adsorption behaviors of methylene blue (MB) onto the hydrogel wereinvestigated. The results showed that the amount of the MB adsorbed was influenced by theinitial solution pH, adsorption time, and initial MB concentration. The kinetic data were found tobe well represented by the pseudo-second-order kinetic model. The equilibrium data were wellfitted to the Langmuir isotherm equation.AAL was fractionated to obtain high molecular weight lignin (LA) and low molecularweight lignin (LB). The adsorption behaviors of methylene blue (MB) onto LA, LB, andundeacetylated acetic acid lignin (UDL) were investigated. The amount of the MB adsorbeddepended on the initial solution pH, adsorbent dosage, adsorption time, initial MB concentration,and temperature. The kinetic data were found to be well represented by the pseudo-second-orderkinetic model. The equilibrium data were well fitted to the Langmuir isotherm equation.Thermodynamic studies indicated the adsorption of MB on to LA was spontaneous, endothermic,and irreversible. The monolayer adsorption capacities of LA, LB, and UDL for MB adsorption at30oC were63.32,40.97, and18.21mg/g, respectively.The lignin-based hydrogel (CSL) was prepared from sulfomethylated AAL in the presenceof epichlorohydrin as crosslinker. The adsorption behaviors of methylene blue (MB) onto thehydrogel were investigated. The results showed that the amount of the MB adsorbed wasinfluenced by the initial solution pH, adsorption time, and initial MB concentration. The kineticdata were found to be fit the pseudo-second-order kinetic model. The equilibrium data were wellfitted to the Langmuir and Freundlich isotherm equation. The monolayer adsorption capacities ofthe hydrogel for MB adsorption can be up to276.53mg/g at30oC.