DYNAMIC SMALL-ANGLE NEUTRON-SCATTERING STUDY OF RODLIKE MICELLES IN A SURFACTANT SOLUTION

Small-angle neutron scattering (SANS) experiments have been carried out on a 50 mM aqueous solution of hexadecyloctyldimethylammonium bromide under shear. In the transient experiments, the solutions were abruptly set under a constant shear rate Γ for a time period of the order of minutes. The buildup and the decay of the anisotropic scattering pattern were monitored. The solutions were in a semidilute concentration range, and the applied shear was well above the threshold value at which a shear-induced phase transition had been expected to begin. The stationary scattering patterns show a sharp peak that is superimposed on an anisotropic pattern. The latter is caused by oriented rodlike micelles that were aligned in the shear (type I micelles). It was possible to separate the anisotropic pattern from the total scattering curve and to analyze both structures in a detailed way. The evolution of the structures, after the shear of 600 s-1 was applied, is studied with a time resolution of 7 s. The results suggest that the sharp peak may be due to cylindrical micelles ordered in a hexagonal liquid-crystalline array (type II micelles). The lattice formed under the applied shear and grew in size with time until an equilibrium state was reached after about 2 min. At the same time, type I micelles decreased in number as time grew. The evolution of the structures after the shear had been reduced from 600 to 0 s-1 is also studied with a time resolution of 1.5 s. The transient SANS patterns are found to be related to the transient measurements of optical flow birefringence and viscosity. The measurements were made at temperatures of 30 and 34°C. A comparative study reveals that the development of the sharp peak is significantly diminished and prolonged at the higher temperature. © 1990 American Chemical Society.