Complex Coacervates of Linear Polymers and Polymer-Grafted Nanoparticles

While complex coacervates, dense equilibrium liquids produced by the electrostatic association of dissolved polycations and polyanions, have been formed from a variety of macroions including polyelectrolytes, biomacromolecules, and micellar assemblies, reports of their creation from functionalized nanoparticles are scant and do not include arguments for equilibration, fluidity, or sensitivity to added salt. The integration of solid nanoparticles into coacervates would broaden applications in personal care, adhesives, and pharmaceuticals to include reinforced and functional coatings and adhesives, surface cleaning, and polishing formulations. Here, we show how negatively charged silica nanoparticles, which fail to produce coacervates through complexation of their negative surface silanols with a dissolved polycation (polytrimethylaminomethyl methacrylate, pTMAEMA), do form coacervates when the nanoparticles are functionalized with poly(acrylic acid) (pAA) oligomers. The resulting nanoparticle-integrated coacervates resemble those produced from the analogous macroions in free solution, i.e., pAA combined with pTMAEMA, including the formation of liquid droplets, the qualitative shape and polymer mass concentration of the phase diagram, and the macroscopic appearance, amount, and consistency of the coacervate. Compared with their free solution macroion analogs, nanoparticle-integrated coacervates also display a greater salt resistance (the window of the coacervate formation extends to higher concentrations of added salt), along with a remarkable insensitivity of the salt resistance to grafting density and a near constant polymer content in the coacervate. While polymeric coacervates have traditionally tended toward near stoichiometric incorporation of the two oppositely charged species, the oligomer-grafted nanoparticle architecture of these materials results in a charge imbalance in the coacervate, favoring the free polycation by an excess of 100%. This observation is explained by the inability of the pAA corona on the nanoparticles to fill the coacervate volume, meaning that complexation is necessarily confined to a small near-surface region, with the remainder of the coacervate being occupied by uncomplexed pTMAEMA chains in solution. This architecturally imposed charge imbalance provides an opportunity to sequester additional anionic species within the coacervate phase. This study lays the foundation for the development of water-based coacervate systems that incorporate nanoparticulate functionality for a range of mechanical and other advanced functionalities.