Multi-Scale Chemical Insights into The Static and Dynamic Behaviour of Metallo-Polyelectrolyte Gels

Metallo-Polyelectrolyte Complexes (MPEC) in the gel state represent a class of multifunctional, weakly ionizing materials with intriguing properties deeply rooted in their coordination chemistry and metal ion exchange. We provide a comprehensive exploration of the underlying chemical principles governing MPECs, with a primary focus on the profound effects of metal ion valency and system pH on bonding thermodynamics, association-dissociation kinetics, and global polymer morphology and their regulation of overall gel behaviour. Employing quantum density functional theory (DFT), we estimate the binding energy between the metal cations and the polymer backbone. Hard mono-, di-, and trivalent metal ions exhibit bidentate bridging coordination environments by carboxylate groups and aqua/hydroxy ligands as verified using FTIR, giving stoichiometric charge balance to the complexes. This results in a range of binding energies of -1 to -8 eV. We corroborated these findings with Isothermal Titration Calorimetry (ITC) to probe the solution-phase binding of metal salts to poly(acrylic acid). We demonstrate that the impact of association kinetics at the molecular level propagates to macroscopic behaviour, with higher valency metal ions distinctly influencing relaxation dynamics, thereby increasing gel stiffness and extending the plateau modulus. The role of proton activity in the solution and gel phases – which systematically modulates polyanion charge sparsity and, accordingly, metal binding sites for the formation of these dynamic bonds – can be readily explained using our theoretical treatment. This treatment is in good agreement with our experimental exploration of the MPEC phase space (dissolution, homogeneous gel, and phase separated metal-rich vs. metal-poor gel phase). Guided by theory, our experimental observations unveil the profound influence of pH on polymer relaxation. As charge sparsity decreases, MPEC gels transition towards behaviour reminiscent of monovalent gels, accompanied by critical alterations in network topology. These findings underscore a competition for polyanion (RCOO^-) association between weakly-bound monovalent species in the buffer (eg. Na⁺), strongly-bound, higher-valency metal species (Mⁿ⁺), and the drive for carboxylate protonation when pH < pK_a. This intricate interplay between metal ion valency and pH-regulated charge sparsity fundamentally governs the global thermomechanical response of solvated MPEC gels: from binding site availability to bond dynamics and overall gel morphology. Local chemistry, therefore, has a profound influence on overall thermal, mechanical, and electrical response of metal cross-linked systems. The assumption – and critical role – of solvent in the gel system cannot be overlooked. Our implicit solvent approaches, taken together with DFT and experimental findings on the collective metal-ligand and metal-aqua/hydroxyl ligand complexation character, explain well the bulk-scale observations in the gel phase, providing mechanistic insights into the polymer-metal interactions, transport, and phase-behaviour. This work unravels the intricate & entangled chemistry underpinning MPEC gels, offering critical insights into their valency-driven, pH sensitive, solvent-dependent global responses.