Viscoelasticity and Structures in Chemically and Physically Dual-Cross-Linked Hydrogels: Insights from Rheology and Proton Multiple-Quantum NMR Spectroscopy

Hydrogels have received considerable attention as an innovative material due to their widespread applications in various fields. As a soft and wet material, its mechanical behavior is best understood in terms of the viscoelastic response to the periodic deformation, which is closely related to the microscopic chemically/physically cross-linked structures. Herein, a dual cross-linked (DC) hydrogel, where a physically cross-linked network by ionic coordination (Fe3+) is imposed on a chemically cross-linked poly(acrylamide-co-acrylic acid) network, was studied in detail by rheology and proton multiple quantum (MQ) NMR spectroscopy. Rheology experiments revealed the diverse temperature- and strain-frequency-dependent viscoelastic behaviors for DC hydrogels induced by the dynamic Fe3+ coordination interactions, in contrast to the single chemically cross-linked (SC) hydrogels. During the shear experiment, the trivalent Fe3+ complex with moderate/weak binding strength might transform to those with strong binding strength and serve as permanent-like cross-linkages to resist the periodic deformation when a large strain frequency was applied. The viscoelastic behaviors of the DC hydrogels were strongly affected by the monomer ratio (C-AA/C-AA,) and Fe3+ concentrations; however, the chemically cross-linked density did not change with C-AA/C-AAm, while the physically cross-linked density was greatly enhanced with increasing Fe3+ concentrations. Besides, the DC hydrogels have less contents of network defects in comparison to the SC hydrogels. The heterogeneous structural evolution with increasing the Fe3+ concentration and monomer ratio was also quantitatively determined and elucidated by proton MQ NMR spectroscopy. In addition, the moduli (G', G '') of DC hydrogels were almost an order magnitude higher than that of the corresponding SC hydrogels, demonstrating the significant contribution of Fe3+ coordination to the mechanical properties, in consistent with the high activation energy of viscoelasticity for the physically cross-linked network as obtained from the variable-temperature shear rheology experiments. The experimental findings obtained from the rheology and proton MQ NMR experiments can be correlated with and complementary to each other. Herein, a combination of rheology and proton solid-state NMR is well demonstrated as an effective and unique way for establishing the relationship between microscopic structures and macroscopic viscoelastic properties.