Preparation and characterization of thermo-sensitive gel with phenolated alkali lignin

Thermo-sensitive gel exhibits great potential industrial application. It has been widely used in tissue repair, drug release and water purification for its property of phase transition in response to external stimuli, reusability and biocompatibility. In this study, a novel lignin-based thermo-sensitive gel was synthesized with alkali lignin by two steps. Firstly, phenolated lignin (PPAL) was synthesized with purified alkali lignin (PAL) catalyzed by sulfuric acid. Subsequently, thermo-sensitive gel was achieved by thermal polymerization of phenolated alkali lignin and N-isopropylacrylamide (NIPAAm). Furthermore, the prepared hydrogels were characterized in terms of thermal behavior, interior morphology and their swelling behavior. Compared with PAL-based gel, the obtained PPAL-based gel exhibits a higher crosslinking density and lower critical solution temperature (LCST) due to the increase of reaction site and smaller space volume of the hydrophobic side groups grafted on NIPAAm. TGA data revealed that thermal stability of gel was enhanced (50% weight loss at ~380 °C) by using lignin as precursor. SEM images showed that a more regular interior morphology, better compressive strength was also found (PPAL0.05, 11.15 KPa). Furthermore, the swelling ratio of PPAL-based gel was lower than that of PAL-based gel due to its more complex structure.