(567b) Magnetic Gelation
Magnetic nanocomposites, based on magnetite nanocrystals and polymer matrix, are of great interest for many fields (e.g. material science, medicine, diagnostic). A possible application in material science is for example the production of porous material with controlled pore size and pore size distribution, which can be used for magnetic separation or applications in chromatography.
For this work, we introduce a novel technique to create porous materials with controlled structure that we call magnetic gelation. First of all, magnetic colloids, composed of magnetite nanocrystals dispersed into a polystyrene matrix, and stabilized by SDS, have been prepared via miniemulsion polymerization. Due to the superparamagnetic behavior of the nanocrystals embedded in the polymer matrix, the magnetic colloids, which can be prepared in a size range from 70 to 300 nm, develop strong and reversible dipolar interactions only in the presence of an external magnetic field.
We have then studied the gelation behavior of dispersions of these nanoparticles in presence and absence of an external magnetic field. The nanoparticels, which are stabilized by means of electrostatic interactions, are partially destabilized through the addition of controlled amount of electrolytes. In the absence of magnetic fields, the particles self-assemble into random fractal clusters, which eventually percolate to form a colloidal gel, at particle volume fraction of a few percents. Instead, when an external magnetic field is applied, the particles align themselves in columnar structures in the direction of the field. If the external magnetic field is applied when the gelation process has already begun, both particles and clusters are aligned in the direction of the magnetic field. By tuning the time at which the field is applied, different structures and characteristic pore sizes in the gel are obtained. To be able to study the final structure of the obtained materials, their structure is fixed by a procedure called reactive gelation . The reactive gelation process consists in swelling the polymer particles with a small amount of monomer and crosslinker and let them react after the gelation process to transform the weak physical bonds keeping the particles together in the gel matrix into much stronger covalent bonds. The structures so obtained have been characterized by means of scanning electron microscopy.
 Marti N., Quattrini F., Butté A., Morbidelli M., MACROMOLECULAR MATERIALS AND ENGINEERING, 290, 221-229 (2005)