Phase behavior and microstructure in binary block copolymer/selective solvent systems: Experiments and theory

A combined experimental and theoretical study of phase behavior and structu re in binary systems of block copolymers and a selective solvent is present ed. The block copolymers used here consist of poly(ethylene oxide) (PEO) an d poly(propylene oxide) (PPO), and the solvent is water, selective for the PEO block. The concentration-temperature phase diagrams of (EO)(8)(PO)(47)( EO)(8) (Pluronic L92) and (EO)(11)(PO)(70)(EO)(11) (Pluronic L122) in water have been determined experimentally. These two block copolymers are unique in that they form both normal (H-1, "oil-in-water") and reverse (H-2, "wat er-in-oil") hexagonal lyotropic liquid crystalline structures in binary sys tems with water over the same temperature range. This is the first time the H-2 structure is reported in a binary PEO-PPO-PEO/water system. Thereafter , the predicted phase diagram of (EO)(20)(PO)(69)(EO)(20) in water is given , employing a self-consistent mean-field lattice theory with internal degre es of freedom. The free energy for a number of possible microstructures was calculated as a function of the polymer concentration, and the extensions of the one-phase and two-phase regions were established. The model predicte d the composition and temperature stability ranges of disordered solution, micellar solution, normal hexagonal, lamellar, reverse hexagonal, and rever se cubic ordered phases which appeared at increasing (EO)(20)(PO)(69)(EO)(2 0) concentration, in good agreement with the experimental Pluronic L122 pha se behavior. Moreover, the model provides the volume fraction profiles for the PEO, PPO, and water components in the self-assembled microstructures, i nformation which is not readily accessible from experiments. Finally, an in creased segregation among the species and an increased domain spacing at in creasing polymer length were found. The data are consistent with the scalin g behavior predicted for the domain size in weakly segregating block copoly mer systems.