Carlos A. R. Costa

Scanning electric potential microscopy imaging of polymers: electrical charge distribution in dielectrics

Scanning electric potential images of polymer surfaces are presented and compared to standard non-contact AFM images. Samples used were a latex film with a well-known distribution of chemical constituents and thus of ionic electrical charges, as well as finished industrial products. Topography and electric potential images show a variable degree of correlation, thus evidencing the independence of topographic and electrical features of the samples, in the micro- and nanoscopic scales. Domains with non-zero negative or positive electric potentials are observed, extending for a few tenths of a micron and creating an electric mosaic in the otherwise neutral polymers. Large electric potential gradients are observed, e.g. in a HDPE film.

Adhesive and Reinforcing Properties of Soluble Cellulose: A Repulpable Adhesive for Wet and Dry Cellulosic Substrates

This work reports, for the first time, the excellent performance of an aqueous alkaline solution of cellulose as an adhesive for wet and dry cellulosic substrates. Uniaxial tensile tests of filter paper and sulfite writing paper strips bonded with this adhesive (5% cellulose and 7% NaOH aqueous solution) show that failure never occurs in the joints but always in the pristine substrate areas, except in butt joint samples prepared with sulfite paper. Tensile test also shows that paper impregnated with cellulose solution is stronger than the original substrate. X-ray microtomography and scanning electron microscopy reveal that dissolved cellulose fills the gaps between paper fibers, providing a morphological evidence for the mechanical interlocking adhesion mechanism, while scanning probe techniques provide a sharp view of different domains in the joints. Additionally, bonded paper is easily reconverted to pulp, which facilitates paper reprocessability, solving a well-known industrial problem related to deposition of adhesive aggregates (stickies) on the production equipment.

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.

Determination of High-Frequency Dielectric Constant and Surface Potential of Graphene Oxide and Influence of Humidity by Kelvin Probe Force Microscopy.

We use Kelvin probe force microscopy (KPFM) and capacitance coupling (dC/dz) to study the electrical properties of graphene oxide (GO). We propose using the dC/dz signal to probe the high frequency dielectric constant of mono- and few-layer GO. Our measurements suggest that the dynamic dielectric constant of GO is on the order of εGO ≅ 3.0 ε0, in the high frequency limit, and independent of the number of GO layers. The measurements are performed at a humidity controlled environment (5% of humidity). The effects of increasing humidity on both the dC/dz and KPFM measurements are analyzed.

Avaliação da degradação térmica e fotooxidativa do ABS para fins de reciclagem

The aim of this work is the evaluation of thermal and photo-oxidative degradation of the terpolymer acrylonitrile-butadiene-styrene, ABS, used in internal automotive components. Injection molded specimens were aged by ASTM D794, ASTM G24 and ASTM G53 standards. The aged test specimens were studied by means of dynamic mechanical analysis and non-contact atomic force microscopy. The results revealed that the transitions of the glass and rubber phases were affected by the degradation. The area under linear loss modulus-temperature curves was related to tensile properties in function of the aging time and standard test method. The degradation of the matrix had a higher influence on the mechanical properties. Both, PB and SAN phases, were more affected when the ASTM G24 standard test method was used. Microscopy images showed that the different ageing methods cause different changes on the surface roughess. The results showed that an ABS that had lost 50 % of rupture elongation has this property almost regenerated (90 %) after being reprocessed.

Predicting Ligand-Free Cell Attachment on Next-Generation Cellulose–Chitosan Hydrogels

There is a growing appreciation that engineered biointerfaces can regulate cell behaviors, or functions. Most systems aim to mimic the cell-friendly extracellular matrix environment and incorporate protein ligands; however, the understanding of how a ligand-free system can achieve this is limited. Cell scaffold materials comprised of interfused chitosan–cellulose hydrogels promote cell attachment in ligand-free systems, and we demonstrate the role of cellulose molecular weight, MW, and chitosan content and MW in controlling material properties and thus regulating cell attachment. Semi-interpenetrating network (SIPN) gels, generated from cellulose/ionic liquid/cosolvent solutions, using chitosan solutions as phase inversion solvents, were stable and obviated the need for chemical coupling. Interface properties, including surface zeta-potential, dielectric constant, surface roughness, and shear modulus, were modified by varying the chitosan degree of polymerization and solution concentration, as well as the source of cellulose, creating a family of cellulose–chitosan SIPN materials. These features, in turn, affect cell attachment onto the hydrogels and the utility of this ligand-free approach is extended by forecasting cell attachment using regression modeling to isolate the effects of individual parameters in an initially complex system. We demonstrate that increasing the charge density, and/or shear modulus, of the hydrogel results in increased cell attachment.

Dataset for "Predicting Ligand-Free Cell Attachment on Next Generation Cellulose-Chitosan Hydrogels"

There is a growing appreciation that engineered biointerfaces can regulate cell behaviors, or functions. Most systems aim to mimic the cell-friendly extracellular matrix environment and incorporate protein ligands; however, the understanding of how a ligand-free system can achieve this is limited. Cell scaffold materials comprised of interfused chitosan–cellulose hydrogels promote cell attachment in ligand-free systems, and these data were used to demonstrate the role of cellulose molecular weight (MW) and chitosan content and MW in controlling material properties and thus regulating cell attachment. Included are the data used for determining gel shear moduli; data used in cell attachment regression models and for determining average cell attachment; combined DAPI and FITC images of 24 hour cell morphology studies; data used for determining median cell aspect ratio, cell aspect ratio distribution, and median cell area; UV-vis adsorbance data; confocal microscopy data used for calculating chitosan penetration into cellulose gels; FTIR spectra data and fitted curves; scanning probe microscopy data used for determining gel surface roughness and capacitive coupling; and measurements of chitosan MW and gel surface zeta-potential.

A New and Simple Procedure for Electric Potential Mapping by Phase Contrast in AFM

Electrostatic phenomena have been intensively studied after the 17th and 18th centuries with contribution from many scientists like Coulomb, Faraday, Volta, Ampere, Maxwell, DuFay and others. Based on electrostatic principles, many important technologies have been developed, such as electrostatic painting [1], electrospinning [2] and photocopying [3]. However, even considering this long history and the great number of resources available and widely used in the study of electric phenomena in insulators, there are still many open questions, for example, how can charge carriers in insulators be detected, identified and quantified? Electrostatic charging of insulators is poorly known and it often goes out of control because the charge carriers are unknown, in almost every case, resulting in many practical problems, including serious recent industrial accidents. Different authors have put forward new proposals but a persistent problem is the difficulty to produce repeatable and predictable electric potential patterns [4,5]. The study of electrostatic patterns on dielectrics and other solids has been greatly improved by the

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.