Real-Time Quantification of Molecular-Level Dynamic Behaviors Underpinning Shear Thinning in End-Linked Associative Polymer Networks

Abstract

Shear thinning of associative polymers is tied to bond breakage under deformation and retraction of dangling chains, as predicted by transient network theories. However, an in-depth understanding of the molecular mechanisms is limited by our ability to measure the molecular states of the polymers during deformation. Herein, utilizing a custom-built rheo-fluorescence setup, bond dissociation in model end-linked associative polymers is quantified in real time with nonlinear shear deformation based on a fluorescence quench transition when phenanthroline ligands bind with Ni²⁺. All of the networks exhibit shear thinning, and the dangling chain fraction increases with the shear rate. However, the number of broken bonds is smaller than that predicted by transient network theories, indicating additional relaxation modes or topological inhomogeneities in the networks. Through tuning counteranion chemistry, networks with similar relaxation times but varying dissociation and association rate constants (k_d and k_a) of Ni²⁺-phenanthroline cross-links are developed. Decreasing k_a contributes to more dangling chain formation, while the effect of k_d is less pronounced. Following force-accelerated bond dissociation of bridging chains, the dangling ends in networks with higher k_a tend to reassociate to form elastically inactive loops, while the dangling chains are preserved in networks with lower k_a. This indicates the critical role of bond reassociation kinetics in dictating shear-induced topological interchange of different chain configurations. Besides reaction kinetics, decreasing network junction functionality results in less shear thinning and broken bonds, originating from the lower amount of bond breakage required to flow and the higher tendency of the dissociated bonds to reform bridging chains.