Formamide as a Robust Alternative to Water for Plasticizing Polyelectrolyte Complexes

Polyelectrolyte complexes, PECs, are glassy and brittle when dry but may be plasticized with water. Although hydrated PECs contain a high proportion of water, many still exhibit a glass transition in the 0 to 100 °C range. The apparently unique effectiveness of water as a plasticizer of PECs has been an obstacle to further developments in applications and in fundamental studies of PEC properties. In this work, it is shown that formamide is an excellent and even superior solvent for plasticizing PECs, substantially decreasing glass transition temperatures relative to those of hydrated PECs when formamide is used as a solvent instead. The affinities of PECs for water and formamide, indicated by the (exothermic) enthalpies of solvent swelling of dry PECs, are comparable. Ion transport dynamics revealed similar lifetimes, about 1 ns, of charge pairs within a PEC solvated with water compared to formamide, despite the differences in their dielectric constants. Ion transport dynamics, which depend on the mobility of pendant groups, have lower cooperativity than those of the polymer backbone. The use of formamide is a significant experimental variable for reducing the glass transition temperature/viscosity of complexed polyelectrolytes and can turn a solid-like hydrated complex into a fluid-like coacervate.