Chemoresponsive viscosity switching of a metallo-supramolecular polymer network near the percolation threshold

The precise manipulation of network percolation, combined with the previously reported effects of the kinetics of cross-linking interactions, provide a mechanism by which to optimize the stimulus-responsive mechanical properties of supramolecular gels. Specific metal - ligand coordinative bonds create cross-links between poly(4-vinylpyridine) in DMSO, and an abrupt change in mechanical properties is observed at a critical concentration of cross-linker. The change in mechanical properties is attributed to the onset of percolation within the network, and bulk mechanical properties are shown to be especially sensitive to external stimuli in the vicinity of the percolation threshold. The reversible control of bulk mechanics is demonstrated, and the magnitude of the response ( changes of up to five orders of magnitude in modulus) is determined by the concentration and dissociation kinetics of the cross-linkers. Combinations of cross-linkers, individually present at concentrations below the percolation threshold, provide a related mechanism by which complex viscoelastic switching can be programmed at the small-molecule level.