(123e) Experimental and Theoretical Study of Bond Dissociation in Associative Polymer Flows

Associative polymers, where large molecules are dynamically linked together with weak interactions such as coordination bonds, hydrogen bonds, ionic bonds, or hydrophobic interactions, represent an intriguing class of molecules intermediate between simple liquids and solids. Although the foundational work in understanding these systems started in the middle of the 20^th century, they remain difficult to quantitatively model, understand, and design at a molecular level today. A large part of this challenge originates from the fact that many of the molecular processes occurring during flow are difficult to understand through the use of relatively coarse-grained experimental and simulation methods.

Here, an optomechanical method is developed to quantify bond breaking and reformation in associative polymer gels and solutions under shear flow. Using simultaneous rheology and fluorescence measurement (rheo-fluorescence), the breaking and reformation of metal-ligand coordination bonds can be detected based on a fluorescent-quenched transition of the ligand as a function of its association state with the metal center. Studies as a function of shear rate enable quantification of the number of bonds broken in steady-state flow and also potentially evolution during start-up and other transient flows. Combined with rheological measurement, this data provides substantial additional molecular input for comparison with theories of transient networks. In particular, it is shown 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.