Polymeric Macroporous Nanocomposites using highly concentrated emulsions as templates

Resumo

The design of nanocomposites, which typically consist of polymeric matrices with embedded particles having at least one characteristic length in the nanometer range, has recently been the focus of a great attention. Nanocomposites, in a large variety of morphologies and distinct compositions are already on the market. In this context, the construction of novel porous nanocomposites, exhibiting hierarchical structures, will allow the development of innovative advanced materials with promising applications in many fields: catalysis, gas/liquid storage, gas purification, etc. The use of highly concentrated phase emulsions (HIPEs) as templates has been shown to be an effective route for the preparation of macroporous polymers. Specifically, the use of suitable surface-modified inorganic oxide nanoparticles as emulsions stabilizers (so called Pickering emulsions) provide an alternative approach to the classical surfactant-based systems, to obtain such hybrid organic-inorganic nanocomposite porous materials. Thanks to the ability of finely-divided solids to adsorb spontaneously at liquid-liquid interfaces, any functionality coming from the nanoparticles can be imparted to the materials, in a single-step preparation method. It has been described that, the resulting macroporous nanocomposites, obtained in Pickering highly concentrated emulsions, typically exhibit closed-cell structures and rather large pore sizes. These drawbacks are frequently overcome by combining simultaneously surfactants and particles. Nevertheless, there is a lack of systematic study on how the interactions between these two emulsifiers influence the final physicochemical properties of the materials obtained. For instance, contrary to what is often expected, the addition of particles to a surfactant-stabilized emulsion or inversely, the addition of surfactant to a particle-stabilized emulsion can negatively influence its stability, rather than enhance it. The main objective of this research work was to investigate the formation of polymeric macroporous nanocomposites with embedded functional nanoparticles, using W/O highly concentrated emulsions as templates. For this purpose, two kinds of nanoparticles with interesting functionalities have been used: - Superparamagnetic iron oxide nanoparticles - Titanium dioxide photocatalytic nanoparticles Styrene and the crosslinker divinylbenzene are used in the emulsion continuous phase, as a model monomer system. The emulsions are processed into macroporous materials by free-radical polymerization of such a continuous phase. In all cases, nanoparticles are included in the systems, and three different types of emulsions are studied using different approaches: (a) HIPEs stabilized with surfactants and prepared by the phase inversion method, containing nanoparticles inside the continuous phase. (b) HIPEs stabilized with nanoparticles, in absence of surfactant and prepared by the drop-wise addition method. The use of Pickering emulsions to obtain nanocomposite materials constitutes a very novel approach, recently first described. (c) HIPEs stabilized primary with nanoparticles with increasing amounts of surfactant molecules, prepared by the drop-wise addition method. In this work, special emphasis has been given to the study of the individual contribution of either nanoparticles or surfactants on the (in)stability of the resultant highly concentrated emulsions. Moreover, the precise role that each emulsifier plays, focusing on their interactions and the related emulsion stabilization mechanisms, has been investigated. In addition, we have study the arrangement of the nanoparticles in the resulting porous nanocomposites, with respect to several parameters such as particle size or surfactant concentration. Likewise, the main physical properties of the materials, such as macroporous structure, porosity, permeability or mechanical strength, have been compared. Finally, the magnetic properties and photocatalytic activity of the nanocomposite materials, which contained iron oxide and titanium dioxide nanoparticles, respectively, have been characterized.