Maria do Carmo Vasconcelos Medeiros da Silva

Supramolecular ionics: electric charge partition within polymers and other non-conducting solids

Electrostatic phenomena in insulators have been known for the past four centuries, but many related questions are still unanswered, for instance: which are the charge-bearing species in an electrified organic polymer, how are the charges spatially distributed and which is the contribution of the electrically charged domains to the overall polymer properties? New scanning probe microscopies were recently introduced, and these are suitable for the mapping of electric potentials across a solid sample thus providing some answers for the previous questions. In this work, we report results obtained with two of these techniques: scanning electric potential (SEPM) and electric force microscopy (EFM). These results were associated to images acquired by using analytical electron microscopy (energy-loss spectroscopy imaging in the transmission electron microscope, ESI-TEM) for colloid polymer samples. Together, they show domains with excess electric charges (and potentials) extending up to hundreds of nanometers and formed by large clusters of cations or anions, reaching supramolecular dimensions. Domains with excess electric charge were also observed in thermoplastics as well as in silica, polyphosphate and titanium oxide particles. In the case of thermoplastics, the origin of the charges is tentatively assigned to their tribochemistry, oxidation followed by segregation or the Mawell-Wagner-Sillars and Costa Ribeiro effects.

Improved latex film-glass adhesion under wet environments by using an aluminum polyphosphate filler

—Non-crystalline aluminum polyphosphate (A1PP) was added as a filler to poly(vinyl acetate) latex and the resulting dispersion was used both as a glass coating and as a glass-to-glass adhesive. Latex-coated glass samples were subjected to a grid adhesion test following the ASTM D 3359 standard, in which the samples are immersed in water. A1PP improves the latex polymer film adhesion to glass, as evidenced by coating film and adhesive joint stabilities. This is explained in terms of the formation of Al3+ ion bridges between the polymer-polyphosphate bicontinuous composite film and the glass surface.

Latex coagulation induced by alcohol vapors: an unusual factor for colloidal instability

Abstract A polystyrene (PS) latex prepared by redox initiation is coagulated by exposure to the vapors of lower alcohols (methanol to 2-butanol) as well as to alcohol–water (>30% w/w) solutions, but not to the vapors of higher alcohols and many other solvents. Latex particle size determinations by PCS, under various exposure conditions, show that the vapor-induced coagulation takes place at the liquid latex surface, rather than within the liquid, but the effect of co-solvent alcohols on particle diameters under non-coagulating concentrations is significant (∼10%). Two major fractions of the latex are discerned by centrifugation in density gradients as well as by SEM examination of dry coaguli formed under ethanol vapor.

A plasticidade dos amorfos: fazendo pigmentos brancos com fosfato de alumínio

A new white pigment made out of nano-structured non-crystalline aluminum phosphate was recently launched as an industrial product. Pigment opacifying properties are not intrinsic to aluminum phosphate but they arise as the result of a rare hollow particle nano-structure. This is in turn derived from the core-and-shell structure of amorphous aluminum phosphate precipitated under well-defined conditions. The new pigment is a product of the often neglected chemistry of non-crystalline ionic solids that can probably be a rich source of new successful products. The text describes a short account of the R&D activities, from the initial ideas to the present.

Particle and Polymer Microchemistry and Electric Domain Mapping

New microscopy techniques are increasing accessible, yielding hitherto unavailable information on the spatial distribution of chemical constituents in sub-micron and nanosized particles, as well as in polymer films and other materials; this work describes four relevant examples. First, elemental distribution maps of a polystyrene latex obtained by electron spectroscopy coupled to transmission microscopy (ESI-TEM) show that particle C/S ratio is highly variable, evidencing the large differences in polymer Mws, in different particles. Second, electric potential maps of latex macrocrystals obtained by scanning electron potential microscopy (SEPM) show negative large islands dispersed in a positive continuum, with large electric potential gradients. Backscattered electron imaging (BEI) evidences a core-and-shell structure of silica particles: the particles are made out of smaller domains with variable average atomic number, concentrated at the particle outer layers. The fourth example is a complex columnar structure of electric domains, in crystalline alumina, obtained by SEPM.