Cobalt nanoparticles in block copolymer micelles: Preparation and properties

The preparation and properties of Co nanoparticles in polystyrene(PS)-poly-4-vinyl-pyridine(PVP) micelles were studied. Elementary Co was generated by two methods : (i) by reduction of micelles loaded with CoCl2, and (ii) by thermal decomposition of Co2(CO)8 in micellar solutions of such block copolymers. Co particles formed by both processes are effectively stabilized by the block copolymer matrix and do not aggregate. For CoCl2 as a Cosource, the formed particles have a size less than 1 nm. Thermal treatment of such dried polymers at 200 °C for 2 h leads to spherical particles of 3–5 nm in size. The polymeric hybrid materials prepared in this way display remarkably high values of magnetization at rather low Co contents in the polymer, i.e., we obtain a tenfold increase of the specific magnetization density. Co2(CO)8 as a Co source, results in a more complex behavior. Co2(CO)8 dissolves in the solvent as well as in the micelle core where it is converted to an cationic—anionic complex involving the 4-VP units. The shape and size of the Co nanoparticles formed by thermolysis can be controlled by the balance of 4-VP/Co and can be varied from spherical particles in the limit of lower Co loads being mainly attached to the micelle core to a star-like and cubic morphology in case of excess of Co2(CO)8. Both superparamagnetic and ferromagnetic materials can be prepared. For ferromagnetic samples coercive force varies from 250 to 475 Oe depending on Co content and polymer sample.