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Langmuir Article Figure 1. Macroscopic potential patterns obtained using two tribocharging techniques. (a) PTFE and (b) PMMA rubbed with PE foam disk. PE film abraded with (c) glass spheres and (d) PTFE pellets in a planetary mixer. Red squares indicate polymer area on each image and the x, y axes are in millimeters. Positive and negative domains covering many millimeters or centimeters are observed in every case. See also Supporting Information Figure S1. Work done in the authors’ laboratory over the past twelve years24−26,32−37 showed electrostatic patterns on the surface of every dielectric solid examined, including carefully cleaned, nonscratched surfaces. In some cases, potential patterns could be explained by using microchemical data obtained by using analytical transmission electron microscopy (ESI-TEM) together with Kelvin microscopy: local excess of cations (or anions) creates domains with excess positive (or negative) charge.33 However, in many other cases the species responsible for excess charge still could not be identified by this combination of techniques due either to the nature of the ionic species or to its low concentration. Direct evidence for the participation of ions derived from water[H(H2O)n]+, [OH(H2O)n]−was obtained, analogous to previous evidence 37,38 Fractal dimensions of line or area features of electrostatic potential maps are often larger than fractal dimensions of the corresponding morphology features, showing that charge mobility is generally even lower than the mobility of surface molecules or macromolecular segments, especially in organic polymers. On the other hand, scale symmetry is a universal property of fractal objects, and it implies that potential and charge patterns should be found at any scale. The first experimental demonstration of the formation of macroscopic potential patterns in polyethylene39 (PE) was recently published. The present paper shows for the first time that macroscopic patterns are obtained in many kinds of tribochemical experiments. Furthermore, experimental evidence is provided showing that tribocharges formed by shearing PE and polytetrafluoroethylene (PTFE) are selectively extracted by common solvents. We also show that positively and negatively charged species derive from PE and from PTFE, respectively. Finally, the reasons for macroscopic charge self-arraying into positive or negative domains and the factors for charge stability are discussed, based on Flory−Huggins theory for polymer solutions and on well-established ideas on the orientation of ■amphiphilesatinterfaces. EXPERIMENTAL SECTION Polytetrafluoroethylene (PTFE), poly(methyl methacrylate (PMMA) and polyethylene (PE) were sheets sold for general use and immersed in ethanol for 2 h prior to tribocharging experiments. This is based on preliminary results showing that immersion in ethanol and a few other liquids largely eliminates static charges. Two tribocharging procedures were used: (a) Square sheets (6 × 6 or 2 × 2 cm2) of each polymer were supported on an aluminum holder mounted on a table-top balance and then rubbed with a disk of another polymer (φ = 25 mm or 10 mm) fixed on the chuck of a drilling machine, which was spun at 5000 rpm for 3 s. The pressure exerted by the spinning disk on the sample was adjusted to 1.5 ± 0.25 kPa, by measuring the force applied to the balance. (b) The polymer sample (6 cm × 6 cm) was mounted on top of an aluminum plate that laid on the horizontal table of a planetary lab shaker. Four grams of glass spheres (φ = 1 mm) or PTFE pellets (5 × 5 × 1 mm3) were spread on top of the sample and the setup was shaken for 60 min, at 5 Hz reciprocating frequency and 10 mm amplitude. The scanning apparatus for noncontact electric potential measure- ments and the controlling software were built by Optron (Campinas). Samples are held on a 10-mm-thick aluminum plate and scanned horizontally with a disk-shaped 5-mm-diameter Kelvin electrode connected to a voltmeter (model 347, Trek Inc.). This system can measure potentials within the ±3300 V range and, the spatial resolution (5 × 5 mm2) is limited by the Kelvin electrode dimensions. The electrode moves in the x−y plane, 2 mm above the tested surface as recommended by the electrode manufacturer. Electrostatic potential scans started immediately after all samples had been abraded. Time allowed for the electrode equilibration on each pixel is 3 s. After for hydrophobic/nonpolar water interfaces understand electrostatic charge build-up and dissipation in many experiments.39 This result was further strengthened by the preparation of bulk water with steady excess charge, both positive and negative.40 and helped to 7408 dx.doi.org/10.1021/la301228j | Langmuir 2012, 28, 7407−7416PDF Image | Triboelectricity: Macroscopic Charge Patterns Formed by Self- Arraying Ions on Polymer Surfaces
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