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 crosslinker 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.