Coating and structural locking of dipolar chains of cobalt nanoparticles.

Above a critical size, Co nanoparticles aggregate because of magnetic dipole-dipole interaction into chains. Reported in this paper is the coating of such chains by an AB diblock copolymer in a block-selective solvent for the A block. Also reported is the cross-linking of the deposited or anchored B block of the diblock copolymer to lock in the coating and thus the dipolar chain structure. The Co nanoparticles used were prepared from the high-temperature decomposition of Co(2)(CO)(8) using poly(2-cinnamoyloxyethyl methacrylate)-block-poly(acrylic acid) or PCEMA-b-PAA as surfactant. To coat the dipolar chains, the particles and diblock copolymer poly(tert-butyl acrylate)-block-poly(2-cinnamoyloxyethyl methacrylate), PtBA-b-PCEMA, were dispersed in a good solvent for PCEMA and PtBA. Methanol, a precipitant for PCEMA and a good solvent for PtBA, was then added. This induced the collapsing of the PCEMA blocks and the deposition of the PCEMA block of PtBA-b-PCEMA onto the surface of PCEMA-b-PAA-coated Co nanoparticle chains. The dipolar chains remained colloidally stable in solution for steric stabilization provided by PtBA. The coating was cured by photocrosslinking the PCEMA layer. Such "permanent" and solvent-dispersible Co dipolar chains are novel and may have interesting applications.