Poly(ethylene oxide)-containing amphiphilic block copolymers in ternary mixtures with water and organic solvent: effect of copolymer and solvent type on phase behavior and structure

Abstract

The phase behavior and microstructure in ternary systems composed of an amphiphilic copolymer, an aqueous solvent (‘water’) and an organic solvent (‘oil’) are the subjects of an ongoing investigation by our research group. Results (at 25°C) for a number of different block copolymers, consisting of poly(ethylene oxide) (E) as the hydrophilic block and poly(propylene oxide) (P), poly(1,2-butylene oxide) (B), poly( n-butylene oxide)  poly(tetrahydrofurane) (T) or poly(styrene) (S) as hydrophobic blocks, and for the p-xylene, n-butyl acetate and butan-1-ol organic solvents are reviewed here. The effects on self-assembly of the copolymer and ‘oil’ type are thus revealed. Oil-in-water (‘normal’) micellar solutions (denoted L 1) and lyotropic liquid crystalline regions of normal micellar cubic (I 1), normal hexagonal (H 1) and lamellar (L α) structure can be formed with an increase of the copolymer concentration along the oil-lean side of the ternary phase diagrams (water being a selective solvent for the E blocks). When ‘oil’ is the selective solvent for the hydrophobic blocks, micellar solutions (L 2) and lyotropic liquid crystalline regions of micellar cubic (I 2), hexagonal (H 2) and bicontinuous cubic (V 2) structure, all of the water-in-oil (‘reverse’) morphology can be formed in the water-lean part of the phase diagram with increasing copolymer concentration, provided that some water is present. Such a notable structural polymorphism (which cannot be attained by low molecular-weight surfactants) is attributed to the macromolecular nature of the amphiphilic block copolymers, which allows fine-tuning of the interfacial curvature by varying the degree of swelling of the hydrophilic and hydrophobic blocks. The location of the different phases in the ternary phase diagram depends primarily on the relative amount of hydrophobic-hydrophilic material. When the ‘oil’ is not a selective solvent for the hydrophobic blocks, the liquid crystalline structure at water-lean compositions is lost. Crystalline/glassy precipitates are formed when the oil is a ‘bad’ solvent for the hydrophobic blocks, and isotropic solutions can be obtained when the oil is a ‘good’ solvent. In the case of a σgood’ solvent, the liquid crystalline phases formed at oil-lean compositions swell with ‘oil’.