Davide Cristofori

Encapsulation of submicrometer-sized silica particles by a thin shell of poly(methyl methacrylate)

Polymer encapsulation of submicrometer-sized silica particles by synthesis of the polymer shell, poly(methyl methacrylate) under static conditions in a reaction medium free of surfactants and stabilizing agents is described. The Stöber method, a base-catalyzed hydrolysis and condensation of tetraethyl orthosilicate is used for the synthesis of the monodisperse colloidal dispersion of silica particles. The silica particles are subsequently modified in situ with the surface grafting of the silane coupling agent, 3-(trimethoxysilyl)propyl methacrylate. Encapsulation is achieved using tetraethyl orthosilicate as a reaction medium, in which a thermally initiated radical polymerization of methyl methacrylate is promoted in the presence of the modified particles by a seeding method which leads to a thin coating of poly(methyl methacrylate), and hence silica core-shell particles. The complete encapsulation of individual silica spheres by poly(methyl methacrylate) is visually evidenced by TEM microscopy which reveals the presence of a polymer shell coating up to 10 nm. Evidence for the presence of a poly(methyl methacrylate) shell is further corroborated by DSC/TGA, DRIFT-IR and NMR measurements.

High dielectric constant rutile–polystyrene composite with enhanced percolative threshold

TiO2 (rutile) nanocrystals, obtained by hydrothermal synthesis, are coated with polystyrene, grown by RAFT polymerization, and are dispersed into a polystyrene matrix at various concentrations. The morphology of both the polystyrene coating shell and TiO2 filler particles dispersed in the polymeric matrix is investigated. The polymer molecules attached to the surfaces of TiO2 nanoparticles exist in a “brush” regime; rutile nanoparticles self-assemble in chestnut-burr aggregates whose number increases with the filler amount. By increasing the filler concentration, the composites display a high dielectric constant, which is ascribed to the self-assembling of rutile nanoparticles in chestnut-burr aggregates, where a number of rutile crystals share the lateral faces and form capacitive microstructures. The crystals in these aggregates are separated by a polymer thin layer and allow a high percolative threshold, 41% v/v of filler amount, before the formation of a continuous network responsible for the sudden change of the dielectric characteristics. Despite the high content of inorganic filler, the dissipation factor remains low, even approaching the lower frequencies. The material is easily processable because of its polymeric nature and good reproducibility, thanks to the morphology control of the filler particles and their aggregates.

Structural and magnetic properties of mesoporous SiO2 nanoparticles impregnated with iron oxide or cobalt-iron oxide nanocrystals

Magnetic nanocomposites of FeOx@SiO2 and CoFe2O4@SiO2 were prepared via a wet-impregnation route using mesoporous silica nanoparticles as a support matrix. The small pores in the matrix were exploited as nanocavities for controlled growth of the embedded oxide phase, initially examined by introducing different wt% loadings of FeOx in four different samples and sequentially treating them under oxidising and reducing conditions. Comparative examination of the morphological and structural properties of the FeOx@SiO2 compositions shows that a 17 wt% (nominal) loading of the oxide phase, a mixture of Fe3O4 (magnetite) and γ-Fe2O3 (maghemite), is fully embedded within the pores. The 60–70 nm dimensions of the SiO2 nanoparticles are visible in TEM micrographs which reveal a spheroidal shape. TEM also shows a ca. 3 nm size for the crystalline oxide particles embedded within, which agrees with the pore sizes estimated through porosimetric analysis. The measurements for field-cooled (FC), zero-field-cooled (ZFC) magnetizations, and hysteresis loops in the temperature range of 3 K to 300 K reveal that an enhancement in the density of magnetization is obtained for the 17 wt% FeOx@SiO2 sample following reductive thermal treatment. A CoFe2O4@SiO2 nanocomposite prepared with a nominal 14 wt% oxide shows comparable structure and morphology to the 17 wt% FeOx@SiO2 sample, yet superior magnetic properties. The higher density of magnetization in CoFe2O4@SiO2 is attributed to its 40% content of magnetic material in the crystalline phase, versus 6–8% in FeOx@SiO2. Efficient surface functionalisation with APTES, monitored by DRIFT-IR, implies that the magnetic nanocomposites could be used in bio-labelling applications. Data derived from Raman spectroscopy, N2 adsorption/desorption measurements, and TGA are also used to characterise the nanocomposite materials.