Phase behavior of amphiphilic block copolymers in water-oil mixtures: The pluronic 25R4-water-p-xylene system

A rich phase behavior with lamellar, normal hexagonal, reverse hexagonal, and bicontinuous cubic liquid crystalline regions, as well as water-rich (L₁) and oil-rich (L₂) solutions, was observed at 25°C in a ternary system composed of water, p-xylene, and Pluronic 25R4, an amphiphilic block copolymer of ethylene oxide (EO) and propylene oxide (PO) with the structure (PO)₁₉(EO)₃₃(PO)₁₉. The phase boundaries were identified using ²H-NMR and inspection under polarized light. Small angle X-ray scattering was employed to ascertain the structure of the various liquid crystalline phases formed and to determine the characteristic lengths involved. The sequence of L₁, normal hexagonal, lamellar, reverse hexagonal, and L₂ phases was observed when replacing water by oil at a constant polymer weight fraction of 0.5. The volume fraction of the apolar components (PPO + oil) can be identified as the major parameter governing the phase behavior. The ternary phase behavior of the PPO-PEO-PPO copolymer is similar to that of the opposite block sequence PEO-PPO-PEO copolymer (Pluronic L64) of comparable molecular weight to 25R4.