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 hexagon al, and bicontinuous cubic liquid crystalline regions, as well as wate r-rich (L(1)) and oil-rich (L(2)) solutions, was observed at 25 degree s 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 boun daries were identified using ?H-NMR and inspection under polarized lig ht. Small angle X-ray scattering was employed to ascertain the structu re of the various Liquid crystalline phases formed and to determine th e characteristic lengths involved. The sequence of L(1), normal hexago nal, lamellar, reverse hexagonal, and L(2) phases was observed when re placing water by oil at a constant polymer weight fraction of 0.5. The volume fraction of the apolar components (PPO + oil) can be identifie d as the major parameter governing the phase behavior. The ternary pha se behavior of the PPO-PEO-PPO copolymer is similar to that of the opp osite block sequence PEO-PPO-PEO copolymer (Pluronic L64) of comparabl e molecular weight to 25R4.