Tunning the properties of pH-responsive lignin-based hydrogels by regulating hydroxyl content

Although pH-sensitive lignin-based materials have extensively attracted attentions because of the smart response to pH, it is still a challenge to realize the regulation of the pH-responsive properties. Focusing on this problem, we proposed a new method by tunning the content of hydroxyl groups to adjust the performances of lignin-based hydrogel (L-HG). The modified lignins with the hydroxyl content ranged from 0.31 to 5.91 mmol/g were obtained after acetylation or demethylation. As the results, when increasing the hydroxyl content, the gels became more compact and smooth, and the tensile strength reached 4.10 MPa. The strain was up to 83.2%, and the bending behavior occurred more easily at various pH (3, 5, 9, 11 and 13). While decreasing the hydroxyl content resulted in the impairments of almost all of the above performances. Based on the test of the moisture content, the pH-sensitive bending behavior might be mainly caused by the opposing forces of hydrophilic and hydrophobic interaction. Finally, the application test showed that the trimmed hydrogel could hook up 10 times weight greater than itself under both acid and alkaline conditions. We believe that this work would provide the theoretical guidance for the regulation of pH sensitivity of L-HG.