A SMALL-ANGLE NEUTRON-SCATTERING (SANS) STUDY OF WORM-LIKE MICELLES UNDER SHEAR

The structure of a cationic worm-like cylindrical micelle was investig ated by SANS (small-angle neutron scattering). Intensities from 0.1% b y weight solutions in D2O, at rest and under shear, were measured on t he NIST Cold Neutron Research Facility 30-m spectrometer in the wave v ector range 0.03 less than or equal to Q (nm(-1)) less than or equal t o 2.0. Scattered intensity patterns from the solutions subjected to sh ears equal to or greater than 40 s(-1) showed pronounced anisotropy, b ut such anisotropy could not be detected below this apparent threshold shear. The threshold was characterized by a relaxation time since ani sotropy was detected only after several minutes of shearing. In contra st, the anisotropy was apparent immediately the shear was applied at t he higher shears. The data were analyzed based on the assumption that the micelles behave as rigid rods. Estimates of the radii and length u nder shear are given. Polydispersity in rod length is discussed, and w e argue that it contributes significantly to the scattering patterns.