Magnetic Properties

Magnetic properties of particles strongly depend on the particle size: with decrease of the particle diameter, the nature of magnetism changes: ferromagnetic particles become superparamagnetic (do not show hysteresis) and these properties are temperature-dependent. If magnetic particles are formed inside a polymer matrix, magnetic nanocomposites have unique properties and can be used for development of new magnetic and magnetooptical devices [95]. Magnetic measurements are often used for materials containing magnetically active particles to elucidate their composition and structure. In this way, Co nanoparticle formation in the PS-6-P4VP micelles and HPS was followed with FMR [32, 47]. In the PS-6-P4VP micelles Co nanoparticles were generated by two methods: by reduction of micelles loaded with CoCl2 or by thermal decomposition of Co2(CO)8 in micellar solutions of these block copolymers [47]. In both cases Co nanoparticles are effectively stabilized by the block copoly-mer matrix and do not aggregate. For CoCl2, the formed particles have a diameter below 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 containing the latter particles display remarkably high values of magnetization at rather low Co contents in the polymer (i.e., a tenfold increase of the specific magnetization was obtained). Co2(CO)8 as a precursor results in more complex behavior. The shape and size of the Co nanoparticles formed by thermolysis can be controlled by the ratio of 4-VP/Co.

Both superparamagnetic and ferromagnetic materials can be prepared. For ferromagnetic samples, coercive force is in the range 250-475 Oe depending on the Co content and block copolymer characteristics. Co nanoparticles formed in the PS-6-P4VP micelles display remarkable stability under air (FMR signal remains unchanged for months); this confirms strong adsorption of pyridine units on the nanoparticle surface.

Co nanoparticles prepared in triblock copolymer, PDMS-fo-PCPMS-fo-PDMS (see Section 5.1.1), showed superparam-agnetic behavior; however, some degree of surface oxidation was observed over time, resulting in a decrease in magnetic susceptibility [48].

A novel approach to preparation of magnetic polymeric nanoparticles by synthesis of the magnetite core and polymeric shell [using methacrylate and hydroxyethyl methacrylate (HEMA)] in a single inverse microemulsion was reported in [96]. The polymeric shell size was controlled in the range 80-320 nm by varying the size of the micro-droplets. The technique of synthesizing a core and a shell in a single microemulsion has improved the particle structure and size distribution and allowed formation of the particles, made of a superparamagnetic magnetite core coated with a polymeric shell of PMA-co-PHEMA random copolymer. Magnetite concentration achieved 3.3 wt%. In a modification of this technique reported in [97], magnetite particles coated with PMAA showed ferromagnetic behavior with coercive force of 50 Oe.

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