Rheology of polyphosphate coacervates

Polyphosphate is a highly water-soluble linear polyanion comprised of phosphate groups. A phase separation happens when divalent cations are added to polyphosphate solutions resulting in the formation of polyphosphate coacervates. Such coacervates could potentially be used as a glass precursor or in a variety of bioapplications including microencapsulation. In all of these applications, viscoelastic properties of polyphosphate coacervates directly affect their usage. Here, we show that these polyphosphate coacervates act as extremely viscous Newtonian liquids at low shear rates, with their viscosity highly dependent on the polyphosphate degree of polymerization (D_p) and type of divalent cations (Ca²⁺, Sr²⁺, or Ba²⁺) used for their preparation. For Ca²⁺-only polyphosphate coacervates, specific viscosity is directly related to D_p^1.46 at 20 °C, similar to highly concentrated polymer solutions where chain entanglement is not important. For very long chain polyphosphate coacervates, however, elastic properties are dominant presumably caused by physical chain entanglement. Replacement of a fraction of Ca²⁺ with Sr²⁺ or Ba²⁺ results in coacervates having significantly more elastic characters and profoundly higher viscosity than their Ca²⁺-only counterparts. Overall, varying polyphosphate chain length and divalent cation type allows one to modify these materials for a desired application.