The hydrophilic-lipophilic character of these polymers can The self-assembly behavior in water of a mixture of two poly be tailored by varying their molecular weight and relative (ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) block size. A number of PEO-PPO-PEO copolymers have copolymers, (EO) 13 (PO) 30 (EO) 13 (L64) and (EO) 37 (PO) 58 (EO) 37 been shown to associate in aqueous solutions to form mi-(P105), was explored at 25ะC. The phase boundaries were estabcelles consisting of a PPO core and a corona dominated lished using 2 H-NMR and inspection under polarized light; the by hydrated PEO segments (3-5). Some PEO-PPO-PEO structure of the various lyotropic liquid crystalline (LLC) phases copolymers are also known to form ''gels'' in water (2-6);
was determined with small-angle X-ray scattering, while viscosity however, the rich phase behavior of these polymers has only and differential scanning calorimetry measurements were used to recently been recognized (6-12). Some of the ''gel'' phases probe the isotropic water-rich solution region. Isotropic regions, similar to the neat polymers, are stable at high polymer content. reported in the literature are of cubic structure but most The addition of water induces structure in the amphiphilic block others still lack proper structural identification. Depending copolymer system. An extended lamellar (D) LLC phase is formed on the polymer concentration, cubic, hexagonal, and/or laat 20-25% water content; a hexagonal (E) and a cubic (I) LLC mellar liquid crystalline phases can be formed (6-12). A phases supersede D at higher water contents. In addition to the similar sequence of phases is observed for nonionic surfacabove, a narrow isotropic region (L) is observed on the L64tant-water systems (13).
water binary axis, in equilibrium with the E and the D phases.
While the study of binary amphiphile-water systems is of
The hexagonal and lamellar LLC phases extended all the way great fundamental value, in practice mixtures of amphiphilic from the L64-rich to the P105-rich side of the ternary L64-P105molecules (often containing salts and nonassociating polywater phase diagram; the characteristic hexagonal and lamellar mers) are used to achieve the properties desired for a particustructural dimensions varied linearly with P105 content in the lar application. A logical step in addressing the complexity L64-P105 mixture at a constant water concentration. An isotropic (micellar) solution phase (L 1 ) dominates the high-water content of real life is to study systems of mixed amphiphiles. We corner of the ternary phase diagram. Viscosity measurements in report here the first phase diagram of a ternary system comthis region provided evidence for increased interactions between posed of two PEO-PPO-PEO copolymers and water ( 2 H 2 O). the micelles as the boundary to the LLC phases was approached. 2 H-NMR and small angle X-ray scattering (SAXS) were the