Understanding the molecular origin of non-linear rheological behavior in associative polymer networks.

Enabled by the dynamic feature of bonds, associative polymers are an intriguing class of molecules that can construct materials with unique properties, such as toughness, self-healing, injectability and printability. However, understanding the molecular level dynamics underlying network macroscopic behaviors remains challenging. Herein, utilizing a custom-built rheo-fluorescence set up, the process of bond breaking and reformation in associative polymers can be quantitatively monitored under shear flow, based on a fluorescence quench transition when associative ligands bond with a transition metal complex. Specifically, several model associative networks with metal-phenanthroline complexation as dynamic crosslinking sites, consisting of (1) end-functionalized 4 or 8 arm polyethylene glycol chains, or (2) linear side-functionalized polyacrylamide, were systematically investigated. Force-activated bond dissociation along with an overshoot in the fraction of dangling chains are observed across a set of different shear rate. However, the number of broken bonds is remarkably low even at high shear rates for different polymer systems, suggesting that a complex set of relaxation processes plays an important role in relaxation dynamics of the networks.