(274d) Real-Time Quantification of Molecular-Level Dynamic Behaviors Underpinning Shear Thinning in End-Linked Associative Polymer Network.

Associative polymers have been regarded as an intriguing class of materials for soft and self-healing electronics, injectable biomedical scaffolds, and 3D printing inks. Fundamentally, it remains challenging to quantitatively correlate macroscopic rheological behaviors of polymer networks with their molecular-level behaviors, which involve a complex interplay between junction and strand dynamics resulting from continuous breakage and reformation of associative bonds. As the ubiquitous flow behavior that associative polymers experience during processing or under deformation, shear thinning is tied to force-activated bond breakage and the retraction of dangling chains as predicted by transient network theories. However, the detailed time-dependent molecular behaviors and how the structural dynamics of a network influence these processes remain elusive. Herein, utilizing a custom-built rheo-fluorescence set up, the amount of bond breakage and reformation in a series of model end-linked associative polymers are quantified in real-time with their non-linear rheological behaviors, based on a fluorescence quench transition when associative phenanthroline ligands bound with transition metal Ni²⁺. Specifically, the exchange dynamics of Ni²⁺-phenanthroline junction are tuned by introducing different counter anions as regulators, including bis-trifluoromethanesulfonimide, chloride and acetate with increasing coordinating ability with Ni²⁺. Network topology is modulated by using star poly (ethylene glycol) with different numbers of arms. Across a set of different shear rates, stress overshoot occurs along with the beginning of dangling chain fraction increase, providing experimental support of the existence of force-activated bond dissociation. Degree of shear thinning increases with shear rate and is accompanied by breaking more bonds. Interestingly, it is found that the amount of bond breakage shows large difference for gels with similar dynamic bond equilibrium constant, yet different association rate. This highlights the importance of dissociation/association kinetics of junction beyond its thermodynamic equilibrium in dictating the topological translation from bridging chains to dangling chains under non-linear extensional flow in associative polymer networks, while junction functionality shows inconspicuous effects. A thermodynamic model with coupled equilibrium reactions was developed to map the entire distribution of different chain configuration (loop, bridging and dangling) and relaxation time of the associative network as a function of regulator strength and concentration, which matches with the trend captured in experiments. Through direct measurement of time-dependent molecular events together with rheological behaviors enabled by custom-built opto-mechanical tools, the obtained fundamental mechanistic insights paved the way for establishing theoretical framework for transient polymer network. Furthermore, this work identifies the key molecular design spaces in regulating the dynamic interchange of different chain configuration within polymer network under deformation, which can be broadly leveraged to advance inverse design of polymeric materials with desired processability and tunable properties for different application scenarios.