Small-angle neutron scattering (SANS) and rheometry were used to investigate the supramolecular structure formed by a poly(oxyethylene-oxypropylene-oxyethylene) (PEO(100)PPO(65)PEO(100), Plutonic F127) copolymer in aqueous solution over a temperature range of 10-75 degrees 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 (similar to 10(6) 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 similar to 40 degrees 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.