Edwin Moncada

Nanoparticles prepared by the sol–gel method and their use in the formation of nanocomposites with polypropylene

Hybrid layered aluminosilicate nanoparticles (HLNP) containing octadecylamine (ODA) as the organic part, and silica nanoparticles with spherical morphology containing ODA (HSNP) or without ODA (SNP) were prepared by the sol–gel method and used for the formation of nanocomposites with polypropylene. The polypropylene matrices, of different molecular weight and polydispersity, were prepared using polymers obtained via Ziegler–Natta or metallocene catalysts. A strong influence of the morphology and the presence of ODA on the surface of the nanoparticles was found on the formation and characteristics of the nanocomposites. The mechanical properties and thermal stability of these materials were determined and compared with those of nanocomposites prepared with 2:1 phylosilicate clays such as montmorillonite and hectorite in similar polymer matrices. X-ray diffraction, transmission electron microscopy, and the study of mechanical properties showed that the use of HLNP allows nanocomposites with considerably improved mechanical properties to be obtained, compared with nanocomposites prepared with exfoliated clays. In the case of nanocomposites prepared with spherical particles functionalized with ODA (HSNP), materials with high specific strength combined with high elongation before rupture were obtained. The thermal stabilization of polypropylene matrices containing the synthesized nanoparticles (HLNP, HSNP or SNP) occurs about 50 °C higher than that attained with clays.

Determination of the Network Structure of Sensor Materials Prepared by Three Different Sol-Gel Routes Using Fourier Transform Infrared Spectroscopy (FT-IR)

Solid acid-base sensor materials were prepared by encapsulating three pH indicators (alizarin red, brilliant yellow, and acridine) within a silica matrix using a sol-gel approach through three different routes: (1) non-hydrolytic, (2) acid-catalyzed, and (3) base-catalyzed. Raman and Fourier transform infrared spectroscopies were used to evaluate the silica indicator interactions. Because vibrational bands assigned to functional groups present in the indicator molecules were not detected, the main silica stretching mode vSi-o between approximately 1300 and 1000 cm−1 was used to detect the presence of our indicators within the silica matrix. The large band centered at 1100 cm−1 was deconvoluted into four components corresponding to the longitudinal optic and transversal optic modes of the silicon monoxide (SiO)4 and (SiO)6 siloxane rings. Using the component area of each mode, it was possible to calculate the percentage of each structure. Such percentages ranged from 49% to 70% (SiO)6 for the analyzed samples, within a confidence level of 95% (p = 0.05). (The confidence limits were 53–62%.) These results could be related to the pH indicator content, indicating that the quantity of the encapsulated molecule affects the (SiO)6 percentage values. In addition, a comparison with the radius of gyration obtained by small angle X-ray scattering was done. These results indicate that the analyte accesses the receptor elements through the passages between the siloxane rings but not through the siloxane rings themselves.

Quantification of indicator content in silica-based pH solid sensors by diffuse reflectance spectroscopy

The amount of alizarin red, brilliant yellow or acridine encapsulated within silica matrixes as colorimetric pH sensors, produced by three sol–gel routes (acid catalyzed, base catalyzed and non-hydrolytic routes), was investigated using diffuse reflectance spectroscopy in the UV-Vis region. Univariate calibration curves using silica (commercial or sol–gel synthesized) as a blank were linear (R2 > 0.99) but did not allow coherent quantification of the pH indicator, likely due to the matrix effect. The standard addition method using the silica blank (produced by the corresponding sol–gel route) to correct background afforded coherent results with standard deviation between 0.017 and 0.59, depending on the sol–gel route and indicator.

PREPARATION OF NANOCOMPOSITES BY IN SITU POLIMERIZATION

Polyethylene nanocomposites were obtained by in situ polymerization using both montmorillonite (natural clay) and montmorillonite organically modified with octadecylamine (O-Clo). In this proposed methodology the clays were added directly in the reactor together with the catalytic system (metallocene catalyst and methylaluminoxane) and ethylene. The nanocomposites were characterized by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM), and by tensile stress-strain tests. It was found that the catalytic activity increased around 20% when natural clay was used compared with the standard polymer. But when modified clay was used the catalytic activity did not show important changes. On the other hand, XRD and TEM showed that the clays are in a disordered state and well dispersed in the polyethylene matrix. Finally, the nanocomposites showed an increase of about 30% in Young’s modulus compared with the standard polymer

Correlating the Morphological Properties and Structural Organization of Monodisperse Spherical Silica Nanoparticles Grown on a Commercial Silica Surface.

A variety of nanosilicas have been widely used to fabricate rough surfaces with superhydrophobic and superhydrophilic properties. In this context, we prepared mixed silica and mixed nanosilica that were generated by the growth and self-assembly of synthesized monodisperse silica nanospheres (11-30 nm, 363 m(2)  g(-1) ) on the surface of Sylopol-948 and Dispercoll S3030 by using a base-catalyzed sol-gel route. Using this process, the interactions and hierarchical structure between the nano- and microsized synthesized silica particles were studied by changing the amount of tetraethoxysilane. The resulting materials were characterized by BET analysis, small-angle X-ray scattering (SAXS), dynamic light scattering, FTIR spectroscopy, and SEM. The mixed silica presented a higher specific surface area (326 m(2)  g(-1) ), a six-fold higher percentage of (SiO)6 (44-68 %), and a higher amount of silanol groups (14.0-30.7 %) than Sylopol-948 (271 m(2)  g(-1), 42.6 %, and 12.5 %, respectively). The morphological and hierarchical structural differences in the silica nanoparticles synthesized on the surface of commercial silica (micrometric or nanometric) were identified by SAXS. Mixed micrometric silica exhibited a higher degree of structural organization between particles than mixed nanosilica.