Structural Polymorphism of Poly(ethylene oxide)-Poly(propylene oxide) Block Copolymers in Nonaqueous Polar Solvents

The solution properties of water-soluble amphiphiles in nonaqueous polar solvents are important in the elucidation of the effects of solvent quality on self-assembly and also in practical applications where the use of water as a solvent is undesirable. We studied the self-assembly of a poly(ethylene oxide)−poly(propylene oxide) (PEO−PPO) block copolymer (Pluronic P105:  EO37PO58EO37) in formamide (as selective solvent for the PEO block) and present here results on the binary concentration−temperature phase diagram and on the microstructure. In addition to formamide-rich and polymer-rich solution regions, four “gel” regions with different microstructures, stable over a wide temperature range (from 20 °C to more than 90 °C), have been identified and characterized by small-angle X-ray scattering (SAXS). The PEO−PPO block copolymer in formamide exhibits a thermoreversible transition from a micellar solution to a micellar cubic gel (of Pm3n crystallographic structure) at 25−35 wt % polymer concentrations. At h...