  The structure of spherical micelles of the diblock poly(styrene-block-acrylic
acid) [PS-b-PA] copolymer in water was investigated up to concentrations where
the polyelectrolyte coronal layers have to shrink and/or interpenetrate in
order to accommodate the micelles in the increasingly crowded volume. We
obtained the partial structure factors pertaining to the core and corona
density correlations with small angle neutron scattering (SANS) and contrast
matching in the water. The counterion structure factor was obtained with small
angle X-ray scattering (SAXS) with a synchrotron radiation source. Furthermore,
we have measured the flow curves and dynamic visco-elastic moduli. The
functionality of the micelles is fixed with a 9 nm diameter PS core and a
corona formed by around 100 PA arms. As shown by the SAXS intensities, the
counterions are distributed in the coronal layer with the same density profile
as the corona forming segments. Irrespective ionic strength and micelle charge,
the corona shrinks with increasing packing fraction. At high charge and minimal
screening conditions, the polyelectrolyte chains remain almost fully stretched
and they interdigitate once the volume fraction exceeds the critical value
0.53+/-0.02. Interpenetration of the polyelectrolyte brushes also controls the
fluid rheology: the viscosity increases by 3 orders of magnitude and the
parallel frequency scaling behavior of the dynamic moduli suggests the
formation of a physical gel. In excess salt, the coronal layers are less
extended and they do not interpenetrate in the present concentration range.
