Inez Valéria Pagotto Yoshida

Silicon oxycarbide glasses from silicone networks

Abstract Rubbers resulting from the hydrosilylation reaction between poly(methylsiloxane), and 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcycletetrasiloxane, have been pyrolyzed to silicon oxycarbide ceramics, with ceramic yield varying as a function of the mixture composition. The rubber–ceramic conversion process was investigated by 13 C and 29 Si nuclear magnetic resonance and infrared spectroscopies, thermogravimetric analysis and density measurements. The composition of the ceramic products was determined by elemental and X-ray fluorescence analyses and their structural characterization was done by 29 Si solid-state magic angle spinning nuclear magnetic resonance. The structure of the ceramic products is, actually, a mixture of SiO 4 , SiCO 3 , SiC 2 O 2 , SiC 3 O, SiC 4 sites randomly distributed in the bulk. The C–C/SiC x O 4− x composites were prepared by infiltration of the siloxane mixture into the bidirectionally reinforced carbon plate. The composites obtained after pyrolysis showed an improved resistance to oxidation in relation to untreated carbon plates.

Hybrids of perfluorosulfonic acid ionomer and silicon oxide by sol-gel reaction from solution: Morphology and thermal analysis

NAFION®/silicon oxide hybrids were prepared from solution by hydrolysis/polycondensation of alkoxy silanes. Transparent, but brittle films were obtained when TEOS was used as the inorganic polymer precursor, showing a lamellar structure by transmission electron microscopy. Part of the TEOS was substituted by TMDES to increase the film flexibility. For substitution higher than 10% phase separation was clearly observed by scanning and transmission electron microscopy. The thermal analysis of NAFION® and hybrids with different inorganic contents showed two main endothermic transitions, one assigned to the NAFION® ionic clusters and the other to the melting of the perfluorinated matrix. The cluster transition temperature decreases as the TEOS content increases up to 50 wt%, but than increases as the TEOS content reaches 80 wt%.

Investigation on kinetics of thermal decomposition in polysiloxane networks used as precursors of silicon oxycarbide glasses

In this study, polysiloxane networks prepared by hydrosilylation or hydrolysis/condensation reactions were considered to be potential precursors for Si–C–Osystems. Different precursors had different pyrolytic properties, which was essentially due to their molecular architecture. The kinetics parameters, such as the activation energy, E (kJ/mol) involved in the polymer-to-ceramic conversion, were investigated by thermogravimetry using a multiple heating rate kinetic method. The relationships between the molecular architecture and the precursor composition were compared to that of a linear poly(dimethylsiloxane) precursor. Solid-state 29 Si nuclear magnetic resonance, infrared spectroscopies, density measurements, and X-ray diffraction measurements were made on the final samples. These products were typically amorphous, with a molecular structure formed by a random distribution of different silicon sites and variable amounts of free carbon residue. 2002 Elsevier Science B.V. All rights reserved.

Phase separation in PMMA/silica sol-gel systems

Abstract The mechanism of phase separation of sol-gel alkoxy-derived silica systems obtained by acid catalysis in the presence of poly(methyl methacrylate) (PMMA) and tetrahydrofuran was investigated by light scattering. Spinodal decomposition (SD) was confirmed for systems with intermediate PMMA/alkoxy compositions occurring with a simultaneous viscosity increase. The initial stages of phase separation followed the linear theory proposed by Cahn. For systems with low PMMA content, a behaviour typical of nucleation and growth was detected. In this case gelation took place after demixing. The morphology of the final PMMA/silica hybrid material was investigated by scanning and transmission electron microscopy with element-specific analysis. A matrix/disperse domains morphology was also verified for systems which demixed by SD, indicating that phase coarsening occurs. A micellar/lamellar microstructure was observed inside the separated phases.

Poly(phenylsilsesquioxane)s : Structural and morphological characterization

Poly(phenylsilsesquioxane) (PPSQ) polymers that were obtained from different synthetic routes were comparatively studied. The polymers were characterized by infrared and solid-state 29Si NMR spectroscopies. According to the results of X-ray diffraction and thermogravimetric analyses, the materials richest in silanol showed a less organized network and lower weight loss temperature. The morphology of the products was influenced by the preparation conditions. PPSQ, with a morphology rich in spherical particles, was achieved with an n-hexadecyltrimethylammonium bromide template in the reaction medium, whereas the morphology of this polymer obtained in the absence of the template was featureless. Small-angle X-ray scattering analyses revealed that the PPSQ samples showed a predominance of surface-fractal behavior.

Hybrids of SiO2 and poly(amide 6-b-ethylene oxide)

Hybrids of poly(amide-6-b-ethylene oxide), Pebax®, and silica were prepared by using a sol-gel process. The inorganic phase was grown by hydrolysis-condensation of tetraethoxysilane, TEOS, by acid catalysis, in presence of Pebax/n-butanol or Pebax/formic acid:tetrahydrofuran (1 : 1 w/w). Pebax/silica hybrids, with different compositions, were obtained and characterized by stress-strain tests, dynamic-mechanical analysis, X-ray diffraction, transmission electron and scanning electron microscopy. The effect of the preparation conditions on the hybrid properties is discussed.

Surface modification of cotton nanocrystals with a silane agent

The research herewith aims at obtaining cellulose nanocrystals with a reduced hydrophilic surface character using a silane with isocyanate groups (isocyanatepropyltriethoxysilane), which are very reactive to hydroxyl groups and thus, are readily able to react with the low quantity of free hydroxyl groups present in the cellulose nanocrystal surfaces, therefore, promoting surface modification. Cellulose nanocrystals were obtained by hydrochloric acid hydrolysis of cotton fiber and were characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR) and solid state 29Si nuclear magnetic resonance (NMR) and their morphologies were investigated by scanning and transmission electron microscopy techniques. The nanocrystals presented a needle-like geometry with a 10 nm approximate diameter and a 166 nm average length. FTIR, 29Si NMR and silicon mapping images showed that nanocrystal surface chemical modification was successfully achieved. Also, the results confirm that the chemical modification occurred mainly at the nanocrystal surface, keeping the morphological integrity of the nanocrystals. The applied methodology for surface modification of the cellulose nanocrystals provided nanofillers with more appropriate surface characteristics that allow the dispersion in polymeric matrices and the adhesion at filler-matrix interface to be obtained. This may result in a better performance of these nanocrystals as reinforcing agents of hydrophobic polymer matrices.

Polycyclic silicone membranes. Synthesis, characterization and permeability evaluation

In this work, membranes based on polycyclic silicone networks were synthesized from different cyclic siloxanes, through hydrosilylation reactions. These membranes showed good thermal stability, glass transition higher than conventional silicone networks and the absence of pores and phase segregation, with roughness surface. The nanomorphology of the materials was studied by small angle X-ray scattering. Some samples produced practically no scattering and some produced patterns that are typical of non-particulate systems. The gas permeability behavior of the membranes was investigated, showing permeability coefficients higher than those observed for some organic polymers, and selectivity coefficients higher than those found for PDMS based membranes.

Microstructural characterisation of monolithic ceramic matrix composites from polysiloxane and SiC powder

Abstract The microstructure of ceramic matrix composites (CMCs) prepared by moulding α-SiC powder in the presence of preceramic polysiloxane as a binder was investigated. During pyrolysis, the powder/polymer system was converted into a ceramic body, in which the shrinkage and the porosity contents could be tailored. Different amounts of polymeric binder and also different firing temperatures were found to have strong influence on the microstructural parameters, such as average pore size, pore size distribution and overall porosity, as determined by both mercury porosimetry and nitrogen adsorption techniques. These parameters were related to shrinkage rate. The morphological characterisation of the composites obtained at 1000 °C, performed by field emission scanning electron microscopy, showed a uniform distribution of the α-SiC, in the SiCxO4−x phase, as well as SiC whisker particles in the pores of the CMCs. Additional characterisation of the ceramic phase, obtained from preceramic polysiloxane, was performed by 29Si and 13C nuclear magnetic resonance and infrared spectroscopies and also by X-ray diffraction. The results suggested a random distribution of silicon sites, SiCxO4−x with 0≤x≤4, in the ceramic bulk, at 1000 °C. This phase evolved to β-SiC at higher temperatures.

Poly(borosiloxanes) as precursors for carbon fiber ceramic matrix composites

Ceramic matrix composites (CMCs), constituted of a silicon boron oxycarbide (SiBCO) matrix and unidirectional carbon fiber rods as a reinforcement phase, were prepared by pyrolysis of carbon fiber rods wrapped in polysiloxane (PS) or poly(borosiloxane) (PBS) matrices. The preparation of the polymeric precursors involved hydrolysis/condensation reactions of alkoxysilanes in the presence and absence of boric acid, with B/Si atomic ratios of 0.2 and 0.5. Infrared spectra of PBS showed evidence of Si-O-B bonds at 880 cm-1, due to the incorporation of the crosslinker trigonal units of BO3 in the polymeric network. X ray diffraction analyses exhibited an amorphous character of the resulting polymer-derived ceramics obtained by pyrolysis up to 1000 °C under inert atmosphere. The C/SiBCO composites showed better thermal stability than the C/SiOC materials. In addition, good adhesion between the carbon fiber and the ceramic phase was observed by SEM microscopy