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

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 (L_α) 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.