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 Peter Holmqvist,* Paschalis Alexandridis, and Bjo ¨rn Lindman Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received August 16, 1996. In Final Form: December 30, 1996 X 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, E 100P70E100): five different one-phase regions, i.e., micellar (L1) and reverse micellar (L2) solutions, and (micellar) cubic (I1), hexagonal (H1), and lamellar (LR) lyotropic liquid crystalline regions, were detected. The microstructure in the liquid crystalline regions was established from small-angle X-ray measurements; I 1 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 E 100T27E100 ternary phase diagram. The extent of the one-phase regions decreased for both E100T27E100 and E100P70E100 systems when the alkanol molecular weight increased. Introduction Amphiphilic copolymers, and in particular triblock copolymers consisting of a poly(propylene oxide) (P) middle block and poly(ethylene oxide) (E) end blocks (com- mercially available under the Pluronic and Synperonic trade names and also known as poloxamers), exhibit a very interesting solution behavior and have been the subject of many recent studies (see refs 1-4 for reviews). Block copolymers of the E n P x E n type behave in aqueous solutions in many respects like typical non-ionic sur- factants: they are surface active, 5 P being hydrophobic and E hydrophilic, and can form different self-assembled structures (micellar solutions 6,7 as well as cubic, hexagonal, and lamellar lyotropic liquid crystals 8-11 ) depending on the polymer concentration and/or temperature. The characteristic length and time scales related to self- assembly are usually larger than those of typical surfac- tants, since the amphiphilic block copolymers are 10 to 30 times bigger. An advantage of this class of “surfactants”, when compared to typical nonionic surfactants, is the great control over the amphiphilic properties afforded by the variation (during synthesis) of the size and the ratio between the hydrophobic and the hydrophilic blocks; 12 this is especially important in the various applica- tions. 4,13,14 Moreover, poloxamer-type block copolymers exhibit low toxicity 15 and are thus used in pharmaceutical products. 14 Block copolymers consisting of poly(ethylene oxide) and poly(1,2-butylene oxide) (B) 16,17 or poly(n- butylene oxide) (also called polytetrahydrofuran, T, be- cause of the monomer used for their synthesis) 18 are also surface active and self-assemble in solution but are less studied than the E n P x E n copolymers; E n B x E n copolymers became commercially available only recently. 16 Although the aqueous solution behavior of E n P x E n -type amphiphilic block copolymers is well studied 1-3 relatively little is known about their ternary systems with water and organic solvents. 19-22 The hydrophobic blocks of the E n P x E n ,E n B x E n , and E n T x E n amphiphilic copolymers differ in polarity from those commonly found in nonionic sur- factants: in the P, B, and T blocks there is a lot of ether oxygen present which imparts polarity, while the hydro- phobic part of nonionic surfactants is usually an alkyl chain. Because of this polar character, it is not possible to dissolve aliphatic hydrocarbons (which are often encountered in mixtures with nonionic surfactants 23 ) in X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Alexandridis, P.; Hatton, T. A. Colloids Surf. A 1995, 96, 1. (2) Almgren, M.; Brown, W.; Hvidt, S. Colloid Polym. Sci. 1995, 273, 2. (3) Chu, B.; Zhou, Z. Surfactant Sci. Ser. 1996, 60, 67. (4) Alexandridis, P., Lindman, B., Eds. Amphiphilic Block Copoly- mers: Self-Assembly and Applications; Elsevier Science B.V.: Amster- dam, 1997. (5) Alexandridis, P.; Athanassiou, V.; Fukuda, S.; Hatton, T. A. Langmuir 1994, 10, 2604. (6) Malmsten, M.; Lindman, B. Macromolecules 1992, 25, 5440. (7) Alexandridis, P.; Holzwarth, J. F.; Hatton, T. A. Macromolecules 1994, 27, 2414. (8) Malmsten, M.; Lindman, B. Macromolecules 1993, 26, 1282. (9) Wanka, G.; Hoffmann, H.; Ulbricht, W. Macromolecules 1994, 27, 4145. (10) Alexandridis, P.; Zhou, D.; Khan, A. Langmuir 1996, 12, 2690. (11) Zhou, D.; Alexandridis, P.; Khan, A. J. Colloid Interface Sci. 1996, 183, 339. (12) Whitmarsh, R. H. Surfactant Sci. Ser. 1996, 60, 1. (13) Edens, M. W. Surfactant Sci. Ser. 1996, 60, 185. (14) Alexandridis, P. Curr. Opin. Colloid Interface Sci. 1996, 1, 490. (15) Rodriguez, S. C.; Singer, E. J. Surfactant Sci. Ser. 1996, 60, 211. (16) Yang, Y.-W.; Deng, N.-J.; Yu, G.-E.; Zhou, Z.-K.; Attwood, D.; Booth, C. Langmuir 1995, 11, 4703. (17) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1996, 12, 1419. (18) Holmqvist, P. ; Nilsson, S ; Tiberg, F Colloid Polym. Sci., in press. (19) Chu, B. Langmuir 1995, 11, 414. (20) Alexandridis, P.; Olsson, U.; Lindman, B. Macromolecules 1995, 28, 7700. (21) Alexandridis, P.; Olsson, U.; Lindman, B. J. Phys. Chem. 1996, 100, 280. (22) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1997, 13, 23. (23) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: London, 1994. 2471 Langmuir 1997, 13, 2471-2479 S0743-7463(96)00819-0 CCC: $14.00 © 1997 American Chemical Society