Tunable Interpenetrating Polymer Network Hydrogels Based on Dynamic Covalent Bonds and Metal-Ligand Bonds

A series of tunable interpenetrating polymer network (IPN) hydrogels are designed by the orthogonal incorporation of two distinct types of reversible bonds, i.e., Schiff base bonds and metal-ligand coordination bonds. Two copolymers based on poly(N,N-dimethyl acrylamide) (PDMA) are synthesized and used as building blocks for the IPN hydrogels. The first one bears ketone pendant groups and is cross-linked by dihydrazide or bishydroxylamine compounds to form, respectively, acylhydrazone- or oxime-based dynamic covalent bond (DCB) networks. The second one bears terpyridine side groups and is cross-linked by the addition of two different transition-metal cations to obtain supramolecular networks based on metal-terpyridine bis-complexes. Several IPN hydrogels are prepared by combining these different types of reversible bonds to investigate how the two subnetworks influence each other. To this end, the influence of the cross-linker nature and of the hydrogel preparation protocol on the rheological properties of these IPNs are also studied in detail. In particular, we show that the obtained IPN hydrogels exhibit a higher modulus compared to the simple addition of the moduli of the single networks, which we attribute to the entanglements between the two networks. We then study how these IPN hydrogels can disentangle and partially relax if one of the reversible subnetworks is composed of cross-links with a shorter lifetime. Finally, pH is used as a stimulus to affect the dynamics of only one of the subnetworks, and the impact of this change on the properties of the IPNs is investigated.