Magnetic gelation: a new method for the preparation of polymeric anisotropic porous materials

In this work, we have introduced a novel process, referred to as magnetic gelation, to produce porous polymeric monoliths with controlled anisotropic structure. Magnetite polymer composite nanoparticles have been prepared by dispersing hydrophobic magnetite nanocrystals in an organic monomer phase, followed by its miniemulsification in water in the presence of a polymerizable surfactant, and finally by polymerizing the resulting droplets into solid nanoparticles. The electrostatically charged nanoparticles, after being swollen with additional monomer, are destabilized and gelled by increasing the ionic strength of the suspension using an enzyme catalyzed reaction that decomposes urea and produces ions. During the gelation process, a magnetic field is applied. The resulting gel phase is hardened by polymerizing the added monomer, thus chemically crosslinking the nanoparticles together. The process combines a conventional reaction-limited aggregation process, which leads to disordered fractal porous structures, with magnetically-induced self-assembly, which aligns nanoparticles into strings of particles parallel to the direction of the applied field. By tuning the strength of dipole-dipole interactions among the nanoparticles, which has been achieved by changing the amount of magnetic material encapsulated in each nanoparticle and by varying the intensity of the applied magnetic field, the anisotropy of the final monoliths has been controlled. The extent of material anisotropy has been quantified by performing magnetic torque measurements. Additionally, Brownian Dynamics simulations have been performed and found to reproduce qualitatively and semi-quantitatively the most important features of the magnetic gelation process.