  When polyelectrolyte-neutral block copolymers are mixed in aqueous solutions
with oppositely charged species, stable complexes are found to form
spontaneously. The mechanism is based on electrostatics, and on the
compensation between the opposite charges. Electrostatic complexes exhibit a
core-shell microstructure. In the core, the polyelectrolyte blocks and the
oppositely charged species are tightly bound and form a dense coacervate
microphase. The shell is made of the neutral chains and surrounds the core. In
this paper, we report on the structural and magnetic properties of such
complexes made from 6.3 nm diameter superparamagnetic nanoparticles (maghemite
gamma-Fe2O3) and cationic-neutral copolymers. The copolymers investigated are
poly(trimethylammonium ethylacrylate methylsulfate)-b-poly(acrylamide), with
molecular weights 5000-b-30000 gmol-1 and 110000-b-30000 gmol-1. The mixed
copolymer-nanoparticle aggregates were characterized by a combination of light
scattering and cryo-transmission electron microscopy. Their hydrodynamic
diameters were found in the range 70 - 150 nm and their aggregation numbers
(number of nanoparticles per aggregate) between several tens to several
hundreds. In addition, Magnetic Resonance Spin-Echo measurements show that the
complexes have a better contrast in Magnetic Resonance Imaging than single
nanoparticles, and that these complexes could be used for biomedical
applications.
