Self-Assembly of Amphiphilic Block Copolymers: The (EO)13(PO)30(EO)13-Water-p-Xylene System

Abstract

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 2 H NMR of heavy water ( 2 H 2 O) 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 structural lengths involved. A rich phase behavior with normal hexagonal, lamellar, bicontinuous cubic, and reverse hexagonal liquid crystalline regions, in addition to three separate liquid phases, was observed at 25 °C. The cubic phase was identified as having a structure associated with the Gyroid minimal surface. A very small liquid region, found between the normal hexagonal and lamellar phases on the binary water-polymer axis, is identified as a melted analogue of a bicontinuous cubic phase often present in this part of the phase diagram. The pure polymer exists at 25 °C as a disordered melt. Structure and segregation are induced by the addition of water. p-Xylene is soluble in the polymer melt but does not induce structure when added alone. For polymer concentrations above ∼50%, a sequence of liquid crystalline phases is observed when the water-to-oil ratio is varied. Here we can identify the volume fraction of apolar components, given by the sum of the PPO and oil volume fractions, as the major parameter governing the phase behavior.