Small-angle neutron scattering (SANS) and rheometry were used to investigate the supramolecular structure formed by a poly(oxyethylene−oxypropylene−oxyethylene) (PEO100PPO65PEO100, Pluronic F127) copolymer in aqueous solution over a temperature range of 10−75 °C and a concentration range of 10−20 wt %. At copolymer concentrations of less than 12.5% the solutions are Newtonian fluids. Gels with an ordered structure (cubic packing of spherical micelles) are observed over a well-defined temperature window when the copolymer concentrations are greater than 17 wt %. The SANS results show that the aggregation number of the micelles is independent of temperature and concentration. Low-yield stresses, very high zero shear viscosities (∼106 P), and shear thinning are the major rheological characteristics of the gels. Near, but outside, the gel phase boundary the solutions are non-Newtonian (shear thinning). However, SANS shows these solutions contain domains having the same structure as that in the gel phase. The temperature window for ordered structures and non-Newtonian behavior becomes wider with increasing copolymer concentration. The degree of overlap of the micellar shells increases with increasing copolymer concentration at a given temperature and reaches a maximum at ∼40 °C at a fixed polymer concentration. The yield stress is due to repulsive interactions of PEO chains in the overlapped micellar shell. On the basis of SANS and rheology results, the proposed mechanism of gelation is that it involves repulsive interactions among close-packed spherical micelles, rather than aggregation or transitions in micelle morphology to rods or lamellae.

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Small-angle neutron scattering (SANS) was used to investigate the micellar structure formed by a poly-(oxyethylene -oxypropyleneoxy eth y lene) (PEO 13PP03oPEO 13, Pluronic L64) copolymer in D20 over a temperature range of 8.4-35.0 "C. The intermicellar interactions were corrected by using an equivalent hard sphere approximation with an equivalent hard sphere radius. The aggregation number of the micelles decreased with decreasing temperature. The micellar scattering behavior could be well described by a coreshell structure. Based on the core-shell model, the volume fraction of the polymer segments in the micellar shell was less than 0.2. The micellar molar mass became larger after having introduced an organic solvent (xylene) into the system and increased with increasing amount of solubilized xylene. The maximum amount of solubilized xylene in the micelle was of the order of 0.3-0.4 xylene molecule per PO unit. Micellar Structure of PEOPPOPEO Aqueous Solution 9 /a-= 2500, .

Laser light scattering was used to investigate the micellization, the supramolecular formation, and the pregelation process of a PEO-PPO-PEO copolymer (Pluronic L64) in water/o-xylene mixtures over the temperature range 21-45 °C. Large aggregates could coexist with small micelles and unimers at high concentrations. It was found that the micellar size remained essentially the same from dilute to concentrated solutions. However, differing from PEO-PPO-PEO copolymers in pure water, the secondary large aggregates were formed by micelle association, not close-packing. The copolymer concentration and the amount of solubilized water determined both the number and the size of the secondary large aggregates. While the effect of temperature on the micelles was much smaller than that on the large aggregates, the amount and the size of the large aggregates decreased with increasing temperature. An open-association mechanism is proposed for the equilibrium between the micelles and the secondary large aggregates.