Influence of Network Topology on the Viscoelastic Properties of Double Dynamics Hydrogels

Dual-cross-linked networks (DCNs), interpenetrating polymer networks (IPNs), and IPN-derived double networks (DNs) are increasingly utilized to fabricate hydrogels with unique mechanical properties. However, the relationship between the topology of these networks and the resulting dynamics is rarely compared and little understood. To tackle this shortcoming, this work presents a systematic investigation of the viscoelastic properties of DCN, IPN, and DN hydrogels as well as their corresponding single networks by oscillatory shear rheology using both frequency and strain sweeps. All the hydrogels are based on the same orthogonal combination of a supramolecular interaction: zinc(II)-terpyridine bis-complexes, and of a reversible covalent bond: oxime, as cross-links. To understand the contribution of each sub-network to the properties of the DCN, IPN, and DN hydrogels, the corresponding single networks, i.e., cross-linked by only one type of bond, are first studied in detail. All double dynamics hydrogels have a plateau modulus much higher than the sum of the plateau modulus of the single networks, evidencing a synergetic effect between the sub-networks. However, the origin of this modulus increase varies according to the network topology. We also show that the relaxation behaviors of the DCN, IPN, and DN hydrogels are influenced by the dynamics of the corresponding single dynamic networks. Finally, the strain sweeps reveal that, for all network topologies, the amplitude of deformation at which the linear viscoelastic region of the double dynamics networks stops is governed by the oxime network, while the metallo-supramolecular network governs the amplitude of deformation up to which the sample can resist before starting to break.