Complexation of Zwitterionic Polyelectrolyte and Inorganic Nanocluster Macroion into Mechanically Strong Supramolecular Coacervate and Hydrogel

Complexation of polyanions and polycations has been investigated as the self-assembly of responsive polymeric materials ranging from layer-by-layer polymer thin films to polyelectrolyte complex coacervates and hydrogels. Instead of using two oppositely charged polyelectrolytes in salt solutions, we explore the coacervation of anionic tungsten oxide-based polyoxometalate (POM) macroions and zwitterionic polybetaine polyelectrolytes in LiCl salt solutions. The phase diagram of POM-polybetaine complexes is examined with varied POM-to-polybataine charge ratio and LiCl concentration. We are intrigued to observe a solution-coacervate-hydrogel transition as increasing POM-to-polybetaine charge ratios and broader coacervation region as increasing LiCl concentration. Importantly, the organic-inorganic macroion complexes in solution exhibit much enhanced viscoelastic properties with tunable temperature response in comparison to polyelectrolyte complexes. Bulk POM-polybetaine coacervates exhibit intriguing shear-thickening response, yet the polymer-rich liquid coacervates exhibit similar viscoelastic solid-like properties to those of the hydrogels formed at high POM-to-polybataine charge ratios. Zeta-potential and solution conductivity measurements suggest Li-cation medicated ion pairing between both net negatively charged polybetaine and POMs. The organic-inorganic macroion coacervation could open new material opportunities in controlling charge-driven phase behavior and polyelectrolyte assembly.