Use of the supercritical fluid technology for the preparation of nanostructured hybrid materials and design of the interface

Resumo

The use of nanostructured composites is a promising solution for the design of multifunctional materials. However, the lack of coupling or bonding between the organic and inorganic components of most nanostructured hybrid composites often leads to anisotropic macroscopic properties, limiting the use of these materials. Hence, the interaction at the components interphase must be engineered to enhance materials performance. In this PhD Thesis, the sustainable supercritical carbon dioxide (scCO2) technology is used for both the surface modification of inorganic nanoparticles and the preparation of nanostructured hybrid materials. Bifunctional alkoxysilanes, acting as adhesion promoters, are herein investigated for the surface modification of inorganic nanoparticles; particularly, titanium dioxide (TiO2). For the silanization process, scCO2 is used as the solvent of choice for alkoxysilanes. Fundamental studies on the solubility of octyltriethoxysilane in compressed CO2 and on the kinetics of the TiO2 silanization process are performed. Silanization experiments are conducted to ascertain the influence of pressure and temperature in the tunable physicochemical properties of scCO2 and in the process. This information is needed for the engineering control of the characteristics of the silane coatings. Extension of the supercritical silanization process to other sets of alkoxysilanes and inorganic nanoparticles are presented. Further, nanostructured hybrid materials are prepared using scCO2 technology. Prior to obtain the composite materials, the inorganic nanoparticles are surface silanized in order to facilitate the homogeneous distribution of the nanoparticles within the matrix and to improve the filler-organic matrix interaction. Firstly, biopolymeric matrices of either poly(L-lactic acid) or the blend poly(methylmethacrylate)/poly(¿-caprolactone) loaded with nanometric TiO2 or hydroxyapatite, respectively, are processed employing scCO2 as an anti-solvent and the Particles from a Compressed Anti-Solvent technique. Precipitated materials have potential applications in tissue engineering. Secondly, lipid blend matrices of hydrogenated castor oil/glyceryl monostearate loaded with nanometric TiO2 and caffeine are prepared employing scCO2 as a solute and the Particles from Gas Saturated Solutions technique. Precipitated materials have potential applications in sunscreens and pharmaceutical dermal products. Finally, the extension of the silanization scCO2-assisted process to multiscale complex hybrid materials is assessed. The technology is presented for the two-step carbonation-silanization process of cement-based materials. Firstly, the carbonation of cement is accelerated by using scCO2 as the carbonation agent. The carbonation process is followed by the hydrophobic treatment of the carbonated cement using the supercritical silanization method. The supercritical silanization of the carbonated cement confers water repellence to the material. Prepared materials may be potentially used for the confinement of hazardous wastes in a humid environment or as durable construction materials.