Structural polymorphism of amphiphilic copolymers: Six lyotropic liquid crystalline and two solution phases in a poly(oxybutylene)-b-poly(oxyethylene)-water-xylene system

Six lyotropic liquid crystalline and two solution (equilibrium) phases can be formed (at 25 °C) by an amphiphilic diblock copolymer composed of poly(ethylene oxide) and poly(butylene oxide) in the presence of water (selective solvent for PEO) and "oil" (p-xylene, a selective solvent for PBO). Oil-in-water ("normal") micellar solution (L₁), micellar cubic liquid crystals (I₁), hexagonal liquid crystals (H₁), and a lamellar structure (L_α) are formed with an increase of the copolymer concentration along the oil-lean side of the ternary phase diagram. A micellar solution (L₂), micellar cubic liquid crystals (I₂), hexagonal liquid crystals (H₂), and bicontinuous cubic liquid crystals (V₂), all of the water-in-oil ("reverse") morphology, are formed in the water-lean part of the phase diagram with increasing copolymer concentration and in the presence of some water. The oil-in-water [water-in-oil] structures can accommodate up to 0.33 [0.40] g of oil [water]/ g of copolymer. The formation of a reverse micellar cubic phase (I₂), indexed to the crystallographic space group Fd3m (face-centered), is an important feature of this phase diagram as it is one of the first times that such a structure has been identified. The normal micellar cubic structure (I₁) is body-centered (Im3m). The microstructure in the bicontinuous cubic region (V₂) can be described in terms of a multiply-connected amphiphile bilayer consistent with the Gyroid minimal surface (Ia3d). The absence of a "preferred curvature" in the self-assembly of this macromolecular amphiphile distinguishes it from typical low-molecular weight surfactants and bestows this remarkable structural polymorphism.