Ferrohydrogels are synthesized by incorporation of magnetic CoFe2O4 nanoparticles into a polyacylamide hydrogel network during the polymerization process by utilizing different cross-linking units. Conventional cross-linked ferrohydrogels, using a molecular cross-linker, are compared to those obtained by our new approach where the magnetic particles, surface-functionalized with methacylic groups, serve as sole, multifunctional cross-linkers. Both experimental series are analyzed with regard to their swelling behavior. The novel composite network is examined with respect to the cross-linkage, the network homogeneity, and the network architecture by various experimental techniques.

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A general and versatile method for the functionalization and subsequent modification of single dispersed magnetite nanoparticles by direct reaction of alkoxysilanes on the particle surface is reported. Our data supports the formation of a dense monolayer that is controlled by sterical needs. By selecting the functionality of alkoxysilanes, the surface properties of the particles can be tailored. Depending on the surface functionality, the modified particles can be used as macrocomonomers or macrocrosslinkers, or as macroinitiators for surface-initiated polymerization and thus enable the covalent attachment of polymers.

The complex susceptibility was measured on CoFe2O4 nanoparticle suspensions in the frequency range between 1 kHz and 1 MHz for different values of a superimposed static magnetic field. The maximum in the imaginary part χ″ of the ac susceptibility shifts to higher frequencies with increasing static magnetic field. The shift is theoretically modeled utilizing the magnetic field dependence of the Brownian relaxation time constant and assuming a distribution of hydrodynamic particle sizes. The mean hydrodynamic size as determined from the maximum of χ″ in zero field and the mean core size as obtained from the shift of the χ″ peak with static field agree very well with the data from transmission electron microscopy and dynamic light scattering measurements, respectively. The results indicate that both core and hydrodynamic size distributions can be determined from measurements on nanoparticle suspensions proposed that magnetic dipole-dipole interactions are negligible.

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