Phase Behavior and Structure of Ternary Amphiphilic Block Copolymer−Alkanol−Water Systems: Comparison of Poly(ethylene oxide)/Poly(propylene oxide) to Poly(ethylene oxide)/Poly(tetrahydrofuran) Copolymers

Abstract

The phase behavior of amphiphilic copolymer−alkanol−water ternary systems was investigated for triblock copolymers of similar molecular weight and the same hydrophilic block [poly(ethylene oxide), E] but having different hydrophobic blocks [poly(propylene oxide), P, or poly(n-butylene oxide) = poly(tetrahydrofuran), T]. The alkanol used (butan-1-ol) was comparable in terms of chemical composition to the hydrophobic segments. A rich phase behavior was obtained for the polymer with the P middle block (Pluronic F127, E100P70E100): five different one-phase regions, i.e., micellar (L1) and reverse micellar (L2) solutions, and (micellar) cubic (I1), hexagonal (H1), and lamellar (Lα) lyotropic liquid crystalline regions, were detected. The microstructure in the liquid crystalline regions was established from small-angle X-ray measurements; I1 was found to be primitive cubic. The alkanol molecules are most likely anchored with their OH− group at the E−P interface, increasing the apparent volume of the P blocks relative to that of the E blocks, and thus causing a decrease in the polymer layer curvature from spherical to cylindrical. Only a single one-phase region, extending from the water to the alkanol corner, was observed in the E100T27E100 ternary phase diagram. The extent of the one-phase regions decreased for both E100T27E100 and E100P70E100 systems when the alkanol molecular weight increased.