Relaxing, Fast and Slow: Linear Viscoelasticity and Dynamics in Mixed Cross-Linker Metallosupramolecular Networks

Cross-linking of polymer networks using two transition metals with distinctly different dissociation kinetics results in hydrogels with 2-fold viscoelastic relaxation. Here, we use rheology to demonstrate that gradual substitution of labile Fe(III) by kinetically inert Cr(III) complexes in a metallosupramolecular poly(acrylic acid) hydrogel slows down the faster viscoelastic relaxation mode by almost 2 orders of magnitude. We attribute this observation to an idle neighbor effect: Fe(III) associations display a longer lifetime when being located near Cr(III) ions. In an experiment with a constant high amount of Fe(III), spiking additional Cr(III) ions leads to a less pronounced increase of the Fe(III)-associated relaxation time. A microstructural perspective is provided by DLS, indicating possible clustering of metal ions in mixed cross-linker-PAA systems. Our results suggest that in the investigated system the approach to improve mechanical strength by mixing fast dissociating with highly inert coordination complex cross-linkers (“dual dynamics”) comes at the cost of an increasing relaxation time of the fast cross-linker. We thus point out a fundamental effect in the design of dual dynamic networks based on a complex interplay of cross-links with vastly differing dissociation kinetics.