Associative polymer networks are everywhere: they are used to modify the texture of our food, the rheology of our consumer products, and to develop biomedical materials. They are used in enhanced oil recovery, and they also form the basis for many natural systems in our own bodies. While existing theories of these materials have been successful in reproducing a wide variety of rheological behaviour, we still do not understand the quantitative relationships between molecular structure and mechanical properties at a level necessary for many materials design challenges.
There are many different models for predicting rheological response of associative polymers; while they predict similar mechanical responses, their molecular predictions are highly divergent. To better correlate macroscopic mechanics with molecular behaviour, we have developed a simultaneous rheology and fluorescence measurement (rheo-fluorescence) that detects the breaking and reformation of metal-ligand coordination bonds. Studies as a function of shear rate enable quantification of the number of bonds broken in steady-state flow and transient flows. These measurements show that the number of broken bonds is remarkably low even at high shear rates, suggesting that additional relaxation processes present in more recent transient network theories must be playing a key role in the relaxation dynamics of the network.