(725c) Porous Materials From Time Dependent Magnetically Assisted Self-Assembly of Nanoparticles

There is a big interest in the production of porous materials with a well organized pore structure for applications such as chromatography, membrane separation, catalyst supports, bio-inspired materials and scaffolds for cell growth. In many cases, porous materials can be prepared from colloidal suspensions and have usually a random porous structure.

We present a method to create porous ceramic materials with a complex structure. In its simplest realization, the method makes use of magnetic polymer composite nanoparticles, synthesized via free radical miniemulsion, as templates in the formation of silica monolith via the sol gel process. When the nanoparticles are mixed with a silica precursor, a catalyst and a porogen, silica nucleates and condenses on top of the nanoparticles, which are aligned in the direction of the applied magnetic field. After heat treatment the particles are removed and porous anisotropic silica monoliths are obtained. Monoliths with a more complex morphology can be prepared using time-dependent configuration of the magnetic field applied during the sol-gel process. The sol-gel process kinetics has first been investigated by means of rheology, in order to precisely identify the gel time. Then, simple time dependent configurations of magnetic field have been applied. In one case, the sol-gel process was started in the presence of a magnetic field for a time shorter than the gel point, and then and the reaction completed without any magnetic field. In another case, the reaction was started in the absence of a field, which was then turned on at a later time point. An additional variation of the procedure has been explored, in which the reaction is started in the presence of a magnetic field, and the sample is then subject to strong mixing before letting gelation reach completion in the absence of a field. Finally, materials obtained from sol-gel processes carried out in the presence of pulsating magnetic fields with different frequencies have been also prepared. All these procedures aim at controlling the structure through a competition between magnetic dipolar interactions, which align the nanoparticles into strings, and diffusion or shear forces, which disrupt the alignment. All samples have been characterized using SEM microscopy, torque magnetometry, mechanical compression and mercury porosimetry.