The aggregation of EOgg-POes-EOge (EO and PO being ethylene oxide and propylene oxide, respectively) in water has been studied. By gel permeation chromatography (GPC) and self-diffusion studies, it was found that micelles are formed in dilute solutions, but only at higher temperatures. The micelles formed have a hydrodynamic radius of approximately 10 nm over a wide temperature range. The residence time of the polymer molecules in the micelles is extremely long (~hours). At higher polymer concentrations, a clear isotropic gel is formed at intermediate temperatures. The extension of the gel region depends strongly on the presence of cosolutes, such as inorganic salts and hydrocarbons; detailed phase diagrams are presented and discussed. Furthermore, the self-diffusion of the polymer and of solubilized oil was studied. The diffusion of the polymer molecules is slow (Dp ~10'n-10~12 m2/s), decreasing with increasing polymer concentration (roughly as Dp ~c*1) up to 20 wt %. In the gel region, the echo-amplitude decay curves reveal a distribution of diffusion coefficients, precluding detailed analyses. The self-diffusion of the oil is comparable to that of the polymer (D0ii ~DP), and several orders of magnitude slower than the diffusion of water (Dw = 10~9 m2/s).

The self-assembly of a polyethylene oxide)-Wocfe-poly(propylene oxide j-biocfe-poly( ethylene oxide) copolymer (Pluronic L64, (EO)i3(PO)3o(EO)i3) in the presence of water and p-xylene was investigated. The phase boundaries were identified using 2H NMR of heavy water (2H20) 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.

The nature of layers of a series of poly(ethylene glycol) monoalkyl ethers (CnEm) adsorbed on silica surfaces has been systematically investigated by means of null ellipsometry. The results show that adsorption remains low until a well-defined concentration ((s»0.6-0.9)cmc) is exceeded. Then, as surfactants in the interfacial region start to self-assemble, it increases abruptly and plateau adsorption is generally observed prior to the cmc. The normal extension of the interfacial aggregates is relatively constant from intermediate to high surface coverage. Increasing the ethylene oxide to hydrocarbon ratio results in a decreased adsorption. The mean optical thickness, on the other hand, is relatively independent of the number of ethylene oxide groups in the surfactant but almost linearly dependent on the length of the hydrocarbon tail. The values obtained for these parameters suggest that the adsorbed layer is built up of discrete surface aggregates, or micelles, with dimensions resembling those observed in bulk solution.A more refined optical model of the adsorbed layer confirms the notion of surface micelles growing with increasing hydrocarbon content. It also points out that the extension of the surface micelles is slightly larger than the measured mean optical thickness. In addition to studies of neat CnEm surfactants, we also examine the adsorption of mixed surfactant systems. Changes observed in adsorption on altering the bulk ratio of two surfactants are well correlated to the bulk micellar surfactant ratio calculated by ideal solution theory.

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