Neutron spin-echo study of the dynamic behavior of amphiphilic diblock copolymer micelles in aqueous solution

We examined the dynamics of amphiphilic polymer micelles of the diblock copolymer HOVE-NBVE (hydroxyethylvinyl ether as a hydrophilic segment and n-butylvinyl ether as a hydrophobic segment) in aqueous solution using the neutron spin-echo (NSE) technique. The time-correlation function obtained for the polymer micelles was well fitted by double exponential function, and the fast and slow modes could be estimated separately. The slow mode was dominant at smaller scattering angles, and showed an excellent linearity in Γ(decay rate) vs q² (scattering vector) plot. The diffusion coefficient evaluated from its slope and the hydrodynamic radius, R_h, by the Einstein-Stoke equation were consistent with the size of the polymer micelle that was estimated by small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and dynamic light scattering (DLS). Hence, this slow mode reflects the translational diffusion of the polymer micelle in aqueous solution. The fast mode, which was 1 order of magnitude faster than the slow mode, was a major factor for larger scattering angle regions in which the SANS curve showed blob scattering. Thus, the fast mode appears to be an internal motion of the polymer micelle, like the breathing mode of the hydrophilic shell (corona) around the polymer micelle.