Effect of the formulation parameters on the absorption capacity of smart lignin-hydrogels

Hydrogels have become very popular in the last decades and they are widely employed in many application fields. Physically crosslinked hydrogels lack of toxic and expensive reagents and, thus, they enable a greener and a more economical synthesis process. Till date, many biopolymers have been explored for hydrogel synthesis, but lignin has promised to be a potential one for this aim. To this end, lignin and three different molecular weight PVA (Mw = 13,000–23,000 g/mol, 87–89% hydrolyzed; Mw = 83,000–124,000 g/mol, 99+% hydrolyzed and Mw = 130,000 g/mol, 99+% hydrolyzed) were blended in optimized concentrations via two routes of crosslinking. Only the two highest molecular weight PVAs enabled a successful crosslinking. The swelling capacity of the hydrogels in water as well as in four other mediums showed that lignin enhanced the water absorption capacity and the pH and temperature responsiveness. In addition, it was seen that the swelling rate and the lignin waste of the obtained hydrogels were directly influenced by the molecular weight of the employed PVA. Moreover, the re-swelling capacity of the hydrogels suggested that these materials could be re-used. The differences in pore size and morphology in SEM analyses were in accordance with the behaviour of the samples in compression tests. Additionally, lignin promoted dye adsorption capacity.