Study the Effects of Supramolecular Interaction on Diffusion Kinetics in Hybrid Hydrogels of Zwitterionic Polymers and CNTs

Hybrid hydrogel systems of functionalized multi-walled carbon nanotubes and zwitterionic sulfobetaine copolymer of thermo-responsible poly(N-isopropylacrylamide) (PNIPAM) are synthesized and testified against different existing diffusion kinetic models in neutral, acidic, and basic environments. Evaluating these hybrid hydrogels’ swelling patterns at different pH indicates that changing pH can tailor the diffusion mechanism without affecting its diffusion kinetics. Interestingly, it is also revealed that diffusion kinetics and diffusion mechanisms are two distinct phenomena and can be viewed separately for supramolecular hybrid hydrogel systems. Elovich kinetic model best describes the swelling kinetics of only PNIPAM hydrogel. Diffusion kinetics of hybrid hydrogel of PNIPAM and CNTs or PNIPAM, zwitterion, and CNTs follow pseudo-first-order, while pseudo-second-order-kinetics best describe hydrogels with only zwitterions. Evaluation of diffusion mechanism at different pH reveals that hydrogels with only zwitterions follow anomalous non-Fickian diffusion, whereas only CNTs follow normal Fickian diffusion irrespective of pH; combining both systems leads to a dual-stage diffusion mechanism at pH 2, which in turn reduces as the pH increases. These observations suggest that synergistic effects of supramolecular interactions can provide a novel property profile to hybrid hydrogels, requiring attention while designing such a system where diffusion is a decisive parameter.