Scanning Electric Potential Microscopy Research Articles

Electrostatic Contributions in the Increased Compatibility of Polymer Blends

Successful blending of different polymers to make a structural or functional material requires overcoming limitations due to immiscibility and/or incompatibility that arise from large polymer-polymer interfacial tensions. In the case of latex blends, the combination of capillary adhesion during the blended dispersion drying stage with electrostatic adhesion in the final product is an effective strategy to avoid these limitations, which has been extended to a number of polymer blends and composites. This work shows that adhesion of polymer domains in blends made with natural rubber and synthetic latexes is enhanced by electrostatic adhesion that is in turn enhanced by ion migration, according to the results from scanning electric potential microscopy. The additional attractive force between domains improves blend stability and mechanical properties, broadening the possibilities and scope of latex blends, in consonance with the "green chemistry" paradigm. This novel approach based on electrostatic adhesion can be easily extended to multicomponent systems, including nonpolymers.

Heterogeneity in Styrene−Butadiene Latex Films

Low-Tg styrene-butadiene (SB) latex films were investigated by noncontact atomic force microscopy and scanning electric potential microscopy, revealing a number of different morphologies and electric potential patterns across films cast from the same SB latex dispersions under the same conditions. Surface leveling and charge dispersion throughout the films are, thus, restrained even at temperatures above Tg and the minimum film-formation temperature. An unprecedented electric pattern is observed, in which the particle cores are more positive than the contacting particle outer layers. Different packing patterns, including cubic and hexagonal arrays, coexist in neighboring areas. Zonal centrifugation of the SB latex in sucrose density gradient shows that particles cover a broad range of densities. Thus, film surface heterogeneity is at least partly due to particle heterogeneity. Fractal dimensions of topographic profiles are lower than those of the electric potential profiles, showing that charge mobility is much more restrained than polymer chain motion at the film surface and that it imposes a limit to the charged chain-ends motion.

Skim and cream natural rubber particles: colloidal properties, coalescence and film formation

Cream and skim fractions of freshly tapped natural rubber latex have been studied using atomic force microscopy and scanning electric potential microscopy to elucidate the topology and charge properties in film formation. Elemental distribution maps of the particles have also been obtained using electron energy-loss imaging in a low-energy transmission electron microscope. The two rubber fractions are obtained by centrifugation. The cream fraction is stable while rapid coagulation occurs in the skim fraction. After removal of the coagulum, no further coagulation occurs and the remaining skim rubber particles are stable. The rubber particles from the cream rubber particles contain higher amount of adsorbed protein–phospholipid materials compared to those in the “self-cleaned” skim fraction. This difference in membrane property has a significant impact on the spreading of the cis-1,4-polyisoprene cores, their coalescence and film formation behavior. Coalescence of cream particles appears to be hindered by the membrane materials, forming a rough film that retains the topology of individual particles. Skim particles coalesce more readily, forming relatively smooth films.

Hydrophobic Polymer Modification with Ionic Reagents: Polystyrene Staining with Water-Soluble Dyes

Homopolymer polystyrene films tinted with methylene blue and other hydrophilic cationic dyes are made following a new two-step procedure: dye adsorption in latex particles, followed by polymer plasticization with a suitable solvent. The dyed polymer is soluble in nonpolar solvents, and the dyes do not phase separate, even in nonsolvents for the pure dye salts. Microanalytical data on the particles and films were acquired using energy-loss spectroscopy imaging (ESI-TEM) and scanning electric potential microscopy (SEPM). The results are interpreted considering that (i) the polystyrene latex particles have a mild ionomer character, due to the charged sulfate groups arising from the polymerization initiator and bound to the chain ends; (ii) there is formation of ion pairs made out of dye cation and chain-end sulfate; (iii) the cationic dye self-associates, as evidenced by a pronounced metachromatic behavior. A nonpolar polymer can thus be made compatible with a basic dye without any special chain modificatio...

Aligned Nafion® Nanocomposites: Preparation and Morphological Characterization

AbstractA novel nanocomposite based on Nafion® (ionomer) and a layered silicate is studied. An innovative approach of processing a composite under electric field to induce orientation of the inorganic filler was carried out and accomplished in the present work. The morphology of oriented composites was analyzed by scanning electric potential microscopy (SEPM) and transmission electron microscopy (TEM). Films based on both plain ionomer and composite displayed fine contrast in SEPM, differentiating the phases, which could not be achieved using conventional atomic force microscopy. The orientation of the silicates in the composites is evident from the SEPM picture.AFM of Nafion®/montmorillonite composite films prepared by solution casting under electric field (Image obtained by SEPM).magnified imageAFM of Nafion®/montmorillonite composite films prepared by solution casting under electric field (Image obtained by SEPM).

Polymer electrostatics: detection and speciation of trapped electric charges by electric probe and analytical electron microscopy

AbstractThis work reviews new probe and electron microscopy approaches for the detection of charged domains in insulating polymers, as well as for the identification of the charge‐bearing species: scanning electric potential microscopy (SEPM), electric force microscopy (EFM) and energy‐loss spectroscopy imaging in the transmission electron microscope (ESI‐TEM). The SEPM and EFM micrographs show patterned domains bearing excess electric charges and extending for tens of nanometers, in polymer latex particles and films. The charged species are identified by ESI‐TEM as emulsion polymerization initiator and surfactant residues, as well as the associated counter‐ions. Charged domains are also observed in common thermoplastic polymers, producing unexpectedly large electric potential gradients.

Substrate effect on latex particle self-arraying: a SEPM study

Sub-monolayers of PS-AAM latex particles were examined by scanning electric potential microscopy (SEPM), non-contact atomic force microscopy (AFM) and field-emission scanning electron microscopy (FESEM). The micrographs revealed domains of positive or negative potentials within the individual particles and aggregates, forming multipoles. All the particle multipoles have the same relative orientation in the image plane and they are aligned with the mica a crystallographic axis. Particle alignment is interpreted considering the polarization anisotropy in the mica (001) plane: the deposited particles rotate, until the particle dipole-mica induced dipole interaction energy is minimum.

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.

Electric Charge Clustering and Migration in Latex Films: A Study by Scanning Electric Potential Microscopy

Scanning electric potential microscopy images were acquired from coalesced poly (styrene−butyl acrylate−acrylic acid) latex films having a Tg equal to 15.9 °C. The micrographs show the boundaries between particles even in transparent films (transmittance > 99%), and the boundaries appear as positive domains, relative to the particle cores. Large local electric potential gradients are observed, ca. 1 V/10 nm, but these are smeared when the latex is dialyzed prior to film formation. Electric domain patterns are modified by aging, annealing and exposure to toluene or chloroform vapors. Completely independent patterns are obtained, in each case. The significance of charge distribution to latex cohesion, adhesion and swelling behavior is discussed.

Scanning Electric Potential Microscopy Imaging of Polymer Latex Films: Detection of Supramolecular Domains with Nonuniform Electrical Characteristics

Scanning electric potential microscopy (SEPM) together with noncontact AFM images were obtained from the surfaces of two latex films: the first is a self-arrayed poly(styrene-co-hydroxyethyl methacrylate) macrocrystal, while the other is a thin film formed by coagulation of polystyrene particles at the latex liquid surface, under exposure to chloroform vapor. In both cases, large electric potential differences are detected by the microscope probe between adjacent domains in the films. Domains with positive or negative potentials reach a few hundred nanometers in size. These dielectric films are thus mosaics of electrically charged domains, and their electroneutrality is only observed at the macroscopic level.

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.

Size Effects on the Microchemistry and Plasticity of Stöber Silica Particles: A Study Using EFTEM, FESEM, and AFM-SEPM Microscopies

Two different samples of monodisperse Stober silica particles were examined using three different microscopies: energy-filtered analytical transmission electron microscopy (EFTEM), high-resolution field-emission scanning electron microscopy (FESEM), and scanning probe microscopy, in the noncontact atomic force (AFM) and scanning electric potential microscopy (SEPM) modes. Upon drying the silica dispersions, the larger (ca. 141 nm) particles were only partially deformed by capillary adhesion, whereas the smaller particles (ca. 36 nm) were strongly deformed and closely packed into dense films of a low porosity, which is evidence of their larger plasticity, or superplasticity. Electric potential distribution maps obtained by SEPM showed a significant interparticle as well as intraparticle contrast, especially in the case of the smaller particles. Examination by electron backscattering also revealed a larger contrast among the smaller particles, thus evidencing a nonuniformity of chemical composition. The re...