STRUCTURE AND RHEOLOGY STUDIES OF POLY(OXYETHYLENE-OXYPROPYLENE-OXYETHYLENE) AQUEOUS-SOLUTION

Small-angle neutron scattering (SANS) and rheometry were used to inves tigate the supramolecular structure formed by a poly(oxyethylene-oxypr opylene-oxyethylene) (PEO(100)PPO(65)PEO(100), Plutonic F127) copolyme r in aqueous solution over a temperature range of 10-75 degrees C and a concentration range of 10-20 wt %. At copolymer concentrations of le ss than 12.5% the solutions are Newtonian fluids. Gels with an ordered structure (cubic packing of spherical micelles) are observed over a w ell-defined temperature window when the copolymer concentrations are g reater 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 g els. Near, but outside, the gel phase boundary the solutions are non-N ewtonian (shear thinning). However, SANS shows these solutions contain domains having the same structure as that in the gel phase. The tempe rature window for ordered structures and non-Newtonian behavior become s wider with increasing copolymer concentration. The degree of overlap of the micellar shells increases with increasing copolymer concentrat ion at a given temperature and reaches a maximum at similar to 40 degr ees C at a fixed polymer concentration. The yield stress is due to rep ulsive interactions of PEO chains in the overlapped micellar shell. On the basis of SANS and rheology results, the proposed mechanism of gel ation is that it involves repulsive interactions among close-packed sp herical micelles, rather than aggregation or transitions in micelle mo rphology to rods or lamellae.