SELF-ASSEMBLY OF AMPHIPHILIC BLOCK-COPOLYMERS - THE (EO)(13)(PO)(30)(EO)(13)-WATER-P-XYLENE SYSTEM

The self-assembly of a poly(ethylene oxide)-block-poly(propylene oxide )-block-poly(ethylene oxide) copolymer (Pluronic L64, (EO)(13)(PO)(30) (EO)(13)) in the presence of water and p-xylene was investigated. The phase boundaries were identified using H-2 NMR of heavy water ((H2O)-H -2) and inspection under polarized light. Small-angle X-ray scattering was employed to ascertain the structure of the various liquid crystal line phases formed and to determine the structural lengths involved. A rich phase behavior with normal hexagonal, lamellar, bicontinuous cub ic, and reverse hexagonal liquid crystalline regions, in addition to t hree separate liquid phases, was observed at 25 degrees C. The cubic p hase was identified as having a structure associated with the Gyroid m inimal surface. A very small liquid region, found between the normal h exagonal and lamellar phases on the binary water-polymer axis, is iden tified as a melted analogue of a bicontinuous cubic phase often presen t in this part of the phase diagram. The pure polymer exists at 25 deg rees C as a disordered melt. Structure and segregation are induced by the addition of water. p-Xylene is soluble in the polymer melt but doe s not induce structure when added alone. For polymer concentrations ab ove similar to 50%, a sequence of liquid crystalline phases is observe d when the water-to-oil ratio is varied. Here we can identify the volu me fraction of apolar components, given by the sum of the PPO and oil volume fractions, as the major parameter governing the phase behavior.