Lignin-based hydrogels

Abstract

Currently, limited information is available on lignin-based hydrogels, but its potential in commercial applications presents a promising avenue for the utilization of lignocellulosic materials. Recent studies have employed an array of techniques for crosslinking various types and species of lignin, combined with both synthetic and natural polymers. This MSc thesis elaborated on effects of incorporating kraft lignin supplied in hydrogel production The hydrogels were synthesized through the radical polymerization of kraft lignin and N-isopropylacrylamide (NIPAAm) with N,N’-methylenebisacrylamide (MBAAm) as the crosslinker, and azobisisobutyroitrile (AIBN) as the initiator. The MANOVA and Tukey Post Hoc analyses determined that the reaction temperature, time, NIPAAm content, and pH exhibited significant effects on the responses for both the control and lignin-based hydrogel Taguchi L9 models. The hydrogel samples were optimized using the SN ratio to maximize yield and swelling ratio; these were determined to be samples 3C and 6C, as well as 2L and 9L, for the control and lignin-based hydrogel models, respectively. The structural analysis of the hydrogels was analyzed using 1H NMR, FTIR, and CHNS, and indicated a high content of NIPAAm within the gels. TGA and DSC indicated an increased thermal stability due to the incorporation of N,N’-methylenebisacrylamide and lignin. The glass transition temperature (Tg) of the hydrogels was also found to be present between 141.45degC to 148.60deg.C. In addition, the swelling behavior of the lignin-based hydrogels in water was found to obey pseudo-Fickian diffusion and second order kinetics. This was also found to correlate with the surface properties of the hydrogels determined by surface area analyzer (via BET method); an increase in surface area, as well as pore volume and size, leading to higher swelling rates and capacities, respectively. Moreover, the equilibrium absorption of the hydrogels in methylene blue dye was found to better follow the Freundlich isotherm model, indicating a heterogeneous surface. Furthermore, the oscillatory rheological measurements conducted to determine the viscoeleastic properties of swollen hydrogels. The hydrogels were found to be predominately elastic, for which the amount of energy dissipated was greater for the control samples than for the lignin-based samples. The structure recovery of the control hydrogels indicated a well-developed polymer network, recovering 87.2% to 96.7% of their elastic moduli following 100% deformation. The lignin-based samples, however, were found to withstand a higher amount of applied stress, indicating that lignin improved the flow point of the hydrogels. In addition, the elastic properties of the hydrogels were found to decrease with increasing temperature, exhibiting a lower critical solution temperature (LCST) between 34deg.C to 37deg.C.