STRUCTURAL POLYMORPHISM OF POLY(ETHYLENE OXIDE)-POLY(PROPYLENE OXIDE)BLOCK-COPOLYMERS IN NONAQUEOUS POLAR-SOLVENTS

The solution properties of water-soluble amphiphiles in nonaqueous pol ar 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-assembl y of a poly(ethylene oxide)-poly(propylene oxide) (PEO-PPO) block copo lymer (Pluronic P105: EO37PO58EO87) in formamide (as selective solvent for the PEO block) and present here results an the binary concentrati on-temperature phase diagram and on the microstructure. In addition to formamide-rich and polymer-rich solution regions, four ''gel'' region s with different microstructures, stable over a wide temperature range (fram 20 degrees C to more than 90 degrees C), have been identified a nd characterized by small-angle X-ray scattering (SAXS). The PEO-PPO b lock copolymer in formamide exhibits a thermoreversible transition fro m a micellar solution to a micellar cubic gel (of Pm3n crystallographi c structure) at 25-35 wt % polymer concentrations. At higher polymer c oncentrations, regions with hexagonal (cylindrical), bicontinuous cubi c, and lamellar (smectic) lyotropic liquid crystalline microstructures are stable. The formation of the bicontinuous cubic structure (consis tent with the Ia3d crystallographic space group and the gyroid minimal surface) in formamide is notable, given the rarity of such structure in PEO-PPO block copolymer-water systems. The change of solvent from w ater to formamide did not diminish the structural polymorphism of the PEO-PPO block copolymer. However, the stability regions of the differe nt structures (and in particular of the micellar cubic) in the case of formamide are shifted to higher polymer concentrations and temperatur es compared to water. These observations can be related to a higher so lubility of both PEO and PPO in formamide compared to water, and a hig her effective PEO/PPO block ratio of the polymer. The interfacial area -per-polymer values (extracted :from SAXS data) in the lamellar and he xagonal structures are 10% and 20% higher, respectively, in the case o f formamide than in water, in corroboration with the phase behavior ob servations.