Temperature and pH Responsive Hydrogels Using Methacrylated Lignosulfonate Cross-Linker: Synthesis, Characterization, and Properties

In this work, biobased hydrogels with temperature and pH responsive properties were prepared by copolymerizing N-isopropylacrylamide (NIPAM), itaconic acid (IA), and methacrylated lignosulfonate (MLS), where the multifunctional MLS served as a novel macro-cross-linker. The network structures of the lignosulfonate-NIPAM-IA hydrogels (LNIH) were characterized and confirmed by elemental analysis, Fourier transform infrared, and C-13 nuclear magnetic resonance. The equilibrium swelling capacity of the LNIH hydrogel decreased from 31.6 to 19.1 g/g with MLS content increasing from 3.7 to 14.3%, suggesting a strong dependence of water absorption of the gel on MLS content. LNIH hydrogels showed temperature sensitive behaviors with volume phase transition temperature (VPTT) around the body temperature, which was also influenced by MLS content. Moreover, all LNIH hydrogels exhibited pH sensitivity in the range of pH 3.0 to 9.1. Rheological study indicated that mechanical strength of the gel also increased with MLS content. The results from this study suggest that lignosulfonate derivative MLS is a potential feedstock serving both water-absorbing moiety and cross-linker for preparation of biobased smart hydrogels.