Relaxing, Fast\nand Slow: Linear Viscoelasticity and\nDynamics in Mixed Cross-Linker Metallosupramolecular Networks

Cross-linking of polymer networks using two transition\nmetals with\ndistinctly different dissociation kinetics results in hydrogels with\n2-fold viscoelastic relaxation. Here, we use rheology to demonstrate\nthat gradual substitution of labile Fe(III) by kinetically inert Cr(III)\ncomplexes in a metallosupramolecular poly(acrylic acid) hydrogel slows\ndown the faster viscoelastic relaxation mode by almost 2 orders of\nmagnitude. We attribute this observation to an idle neighbor effect:\nFe(III) associations display a longer lifetime when being located\nnear Cr(III) ions. In an experiment with a constant high amount of\nFe(III), spiking additional Cr(III) ions leads to a less pronounced\nincrease of the Fe(III)-associated relaxation time. A microstructural\nperspective is provided by DLS, indicating possible clustering of\nmetal ions in mixed cross-linker-PAA systems. Our results suggest\nthat in the investigated system the approach to improve mechanical\nstrength by mixing fast dissociating with highly inert coordination\ncomplex cross-linkers (“dual dynamics”) comes at the\ncost of an increasing relaxation time of the fast cross-linker. We\nthus point out a fundamental effect in the design of dual dynamic\nnetworks based on a complex interplay of cross-links with vastly differing\ndissociation kinetics.