Dynamics of Hydrogels with a Variable Ratio of Permanent and Transient Cross-Links: Constitutive Model and Its Molecular Interpretation

Mechanical reinforcement of hydrogels is important to gel science. “Dual cross-linked” hydrogels with chemical and transient cross-links can exhibit improved mechanical properties compared to the corresponding chemically cross-linked gels. Here, we use model dual poly­(vinyl alcohol) cross-linked gels with a variable ratio of chemical and transient cross-links to study how this ratio affects chain dynamics. Torsion and tensile test results are well described by a constitutive model with only four parameters. The characteristic time of network relaxation (corresponding to the peak of the loss modulus) increases with decreasing transient cross-linker concentration. The characteristic time distribution gradually changes from Maxwell-like to Rouse-like relaxation. These results imply that the Rouse mode of the chains between cross-links controls transient network dynamics. Our work provides a molecular interpretation of network dynamics and guides the optimization of the cross-linking ratios in dual cross-linked gels.