Exploring the Properties of Polyelectrolyte Complexes: Formamide as a robust Alternative to Water

S.A. Hassoun, Z.A. Digby, J.B. Schlenoff

Florida state University

Plasticizers are small molecules widely used to soften brittle or glassy polymers.1 Polyelectrolyte complexes or coacervates, PECs, are blends of oppositely-charged polymers. Formed by spontaneous phase separation from mixed solutions of polyelectrolytes, PECs retain a high weight percent of water when in contact with aqueous solutions. This water is essential to all the physical properties of PECs, including mechanical properties and ionic conductivity.

In addition to effective plasticization by water, PECs can also be softened by breaking the charge pairing between positive, Pol⁺, and negative, Pol^-, polyelectrolyte repeat units. This “saloplasticity” is reversible and allows glassy PECs to be processed into many bulk formats.

Other than water, the selection of small molecules available for plasticizing PECs is surprisingly small

Therefore, using a solvent with a higher dielectric constant than water should result in PECs with weaker interactions. Formamide, with a dielectric constant of 109 was chosen as the solvent as it had previously been shown to dissolve individual polyelectrolytes. The purpose of this work was to probe the interactions and material properties of PECs with formamide and water/formamide mixtures. Rheological experiments demonstrate a dramatic shift in T_g to lower temperatures in all systems used, as well as a drop in modulii. Turbidimetry experiments reveal lower stabilities in formamide compared to water, including dissolution of a water-insoluble PEC. ATR-FTIR experiments demonstrate the stability of PECs in formamide. Calorimetry experiments reveal nearly athermal complexation events and lower hydration enthalpies.