Modification of the Microstructure in Block Copolymer−Water−“Oil” Systems by Varying the Copolymer Composition and the “Oil” Type: Small-Angle X-ray Scattering and Deuterium-NMR Investigation

Abstract

The study addresses the effects of block composition on the self-assembly (and resulting microstructure) of amphiphilic block copolymers in the presence of selective solvents (“water” and “oil”) by examining the ternary phase behavior and structure of two copolymers, E 20 P 70 E 20 and E 100 P 70 E 100, having the same block architecture (E x P y E x ) and P:poly(propylene oxide) middle-block size but different E:poly(ethylene oxide) end-block sizes (Pluronic P123, E 20 P 70 E 20, contains 30% E and Pluronic F127, E 100 P 70 E 100, 70% E). A characterization (using SAXS and deuterium NMR) of the ternary isothermal (25 °C) E 20 P 70 E 20 −butyl acetate−water and E 20 P 70 E 20 −butanol−water systems is presented first. A variety of lyotropic liquid-crystalline (LLC) phases are thermodynamically stable in the former (butyl acetate) system, both of the “normal” (oil-in-water) and of the “reverse” (water-in-oil) morphology. In the latter (butanol) system, the reverse LLC phases are not stable but are replaced by an extensive water-lean solution region under the influence of the amphiphilic character of butanol. Following the presentation of the E 20 P 70 E 20 −“oil”−water systems, the microstructure afforded by E 20 P 70 E 20 is compared to that of E 100 P 70 E 100 (Holmqvist, P.; Alexandridis, P.; Lindman, B. Macromolecules 1997, 30, 6788). In the E 20 P 70 E 20 phase diagrams, the lamellar structure (of zero interfacial curvature) is the most extensive. The high-E (hydrophilic) content of E 100 P 70 E 100, however, favors oil-in-water LLC structures with high interfacial curvature. The copolymer−water side of the E 100 P 70 E 100 ternary phase diagrams is dominated by the micellar cubic LLC structure; for cylindrical and lamellar structures to form, significant amounts of oil are needed. No reverse LLC phases are formed by E 100 P 70 E 100 in the presence of water and either butyl acetate or butanol. The normal hexagonal phase in the E 100 P 70 E 100 systems occurs in approximately the same composition range as the lamellar phase in the E 20 P 70 E 20 systems.