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The phase behavior and structure of binary amphiphilic polymer-water systems have been studied as a function of polymer concentration and temperature for three poly(ethylene oxide)-b-poly(propylene oxide)b-poly(ethylene oxide) (PEO-PPO-PEO) copolymers of different composition, (EO)6(PO)34(EO)6 (L62), (EO)13-(PO)30(EO)13 (L64), and (EO)37(PO)58(EO)37 (P105), by using 2 H-NMR and small-angle X-ray scattering (SAXS). The number of lyotropic liquid crystalline (LLC) phases formed increases with the poly(ethylene oxide) content and the molecular weight of the polymers in the order L62 < L64 < P105. Only a lamellar LLC phase is formed by L62, while P105 is capable of self-assembling with increasing polymer concentration into (body-centered close-packed) cubic, hexagonal, and lamellar LLC phases. Upon heating, the LLC phases of the L62-water and L64-water systems swell with water; no such swelling is detected for the P105-water system. The thermal stability of the LLC regions increases in the order cubic < hexagonal < lamellar and L62 < L64 < P105. An increase of the temperature results in a decrease in the interfacial area and an increase in the periodicity of the L62 and L64 lamellae. In the P105-water system, the structural dimensions in the lamellar and hexagonal LLC regions are not much affected by temperature. Both the lamellar periodicity and the block copolymer interfacial area decrease with increasing polymer content, for all polymers. The factors influencing the self-assembly mode of amphiphilic copolymers are discussed, and the phase behavior of the PEO-PPO-PEO copolymers in water is compared to that of nonionic surfactants.

We report on a ternary isothermal system consisting of a poly(ethylene oxide)/poly(propylene oxide) (PEO/PPO) amphiphilic block copolymer, "water", and "oil" (where "water" and "oil" are selective solvents for the different blocks), which exhibits the richest structural polymorphism ever observed (in equilibrium) in mixtures containing amphiphiles (such as block copolymers, surfactants, or lipids). The microstructure resulting from the self-assembly of the PEO/PPO block copolymer can vary from normal (oil-in-water) micelles in solution, through all types of normal and reverse (water-in-oil) lyotropic liquid crystals (normal micellar cubic, normal hexagonal, normal bicontinuous cubic, lamellar, reverse bicontinuous cubic, reverse hexagonal, reverse micellar cubic), to reverse micelles, as the relative volume fraction of the apolar ("oil"-like) components increases over that of the polar ("water"-like) components. The structure in the liquid crystalline phases has been established with small-angle X-ray scattering; both the normal and the reverse bicontinuous cubic structures are consistent with the Ia3d crystallographic space group (and the Gyroid minimal surface), while the normal and reverse micellar cubic structures are consistent with the Im3m and Fd3m space groups, respectively. The self-assembly of amphiphilic block copolymer in selective solvents described here provides a link between the self-assembly of surfactants in water (and oil/cosurfactant) and the self-assembly of block copolymers in the absence of any solvent. Furthermore, the ability of the PEO/ PPO amphiphilic block copolymers to attain diverse microstructures is of great importance to numerous practical applications, especially since such copolymers are commercially available (as poloxamers, Pluronics, or Synperonics).

Aqueous solution properties of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) amphiphilic copolymer (Pluronic L64: EO13PO30EO13) were studied in the presence of various alkali halide salts (LiCl, KCl, NaCl, NaBr, and NaI), sodium thiocyanate (NaSCN), and urea ((NH2)2CO). Differential scanning calorimetry (DSC) was employed for the determination of both the unimer-to-micelle transition (critical micellization temperature, CMT) and the phase separation (cloud point, CP). DSC is particularly useful in the case of Pluronic L64 where the detection of the CMT by optical techniques is hindered by the presence of a hydrophobic impurity. The presence of LiCl, KCl, NaCl, and NaBr decreased both CMT and CP (in the order Cl -> Brand Na + > K + > Li + ), whereas addition of NaSCN and urea resulted in a CMT and CP increase (in the order NaSCN > urea). NaI appeared to be an intermediate case as it decreased the CMT but increased the CP. Variation of the anion type (rather than the cation) is a more effective means of modulating the CMT and CP. This is the first study where CMT and CP values were simultaneously determined, and led to the important observation CMT (no salt)-CMT(salt) ) CP(no salt) -CP(salt). Both the micellization and the phase separation of the PEO-PPO-PEO copolymer in water are endothermic; the micellization (microphase separation) enthalpy was much larger than the (macro-) phase separation enthalpy (demonstrating the dominance of the PPO-water interactions over the PEO-water interactions) and increased with increasing NaCl and NaBr concentrations and decreasing NaI and urea concentrations. The salt effects on the solution behavior of the PEO-PPO-PEO polymer were correlated to the ion radius and the solvation heat of the salts.

The surface tension of aqueous solutions of seven polyethylene oxide)-Z>Zoc&-poly(propylene oxide)-6Zoefc-poly(ethylene oxide) (PEO-PPO-PEO) Pluronic copolymers, covering a wide range of molecular weights (3400-14600) and PPO/PEO ratios (0.19-1.79), was determined over the 10-5-10% w/v concentration range, at two temperatures (25 and 35 °C). Two breaks (changes in slope) were observed in the surface tension vs log concentration curve for most of the copolymers. The low-concentration break, occurring at bulk copolymer concentrations of approximately 10-3%, is believed to originate from rearrangement of the copolymer molecules on the surface at complete coverage of the air/water interface. The breaks at the high-concentration part of the surface tension curve occurred at concentrations that correspond to the critical micellization concentration values as determined by a dye solubilization technique. The surface area per copolymer molecule, A, increased as a function of the number of EO segments, ZVeo, obeying a scaling law (A « ZVeo1/2) similar to that of lower molecular weight C¿E, nonionic surfactants. The surface activity of PEO-PPO-PEO block copolymers was compared to that of a PPO-PEO-PPO block copolymer and a PEO-PPO random copolymer and literature values for PEO and PPO homopolymers, in an attempt to probe the effect of molecular architecture on the orientation of the copolymer at the air/water interface. The presence of the PPO block in the center of the copolymer molecule resulted in a copolymer headgroup (PEO) surface area smaller than that of the PEO homopolymer of comparable molecular weight, indicating desorption of PEO segments from the air/water interface and/or tightly packed segments.

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, E20P70E20 and E100P70E100, 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, E20P70E20, contains 30% E and Pluronic F127, E100P70E100, 70% E). A characterization (using SAXS and deuterium NMR) of the ternary isothermal (25 °C) E20P70E20−butyl acetate−water and E20P70E20−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 E20P70E20−“oil”−water systems, the microstructure afforded by E20P70E20 is compared to that of E100P70E100 (Holmqvist, P.; Alexandridis, P.; Lindman, B. Macromolecules 1997, 30, 6788). In the E20P70E20 phase diagrams, the lamellar structure (of zero interfacial curvature) is the most extensive. The high-E (hydrophilic) content of E100P70E100, however, favors oil-in-water LLC structures with high interfacial curvature. The copolymer−water side of the E100P70E100 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 E100P70E100 in the presence of water and either butyl acetate or butanol. The normal hexagonal phase in the E100P70E100 systems occurs in approximately the same composition range as the lamellar phase in the E20P70E20 systems.