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Langmuir Article potential measurements, polymers were placed within a Faraday cup for charge measurement using an electrometer (model 6514, Keithley Instruments). Tribocharged PTFE surfaces were analyzed using three different techniques: infrared microreflectance (ATR/IR), pyrolysis, and electron-energy loss spectroscopy (EELS). Negative and positive areas were cut out from previously tribocharged samples using a grounded metal blade and ATR/IR spectra of a 50 μm2 area were acquired with a Smiths IlluminatIR II instrument coupled to an Olympus BX51 microscope, using ZnSe windows, 64 scans and 4 cm−1 resolution. Pyrolysis experiments were done on tribocharged PTFE surfaces (2 × 2 cm2) with a PE disk (φ = 10 mm) at 320 °C. EELS spectra and energy-filtered transmission electron microscopy images (EFTEM) were obtained from extracts of tribocharged domains using a Carl Zeiss CEM-902 transmission electron microscope. Elemental images were obtained using the three-window method and the energy- selecting slit was set at 303 eV for C, 544 eV for O, and 694 eV for F. The images were acquired using a Slow Scan CCD camera (Proscan) and processed in the iTEM Universal TEM Imaging Platform. Simulation Methods. Evaluation of surface charge density on the samples follows the procedure described in previous papers.26,39 Each pixel on the surface map is a square with 5 mm sides and this is further subdivided in a 500 × 500 pixel matrix, where virtual charges are placed. The electrostatic potential (VT) measured 2 mm away from the matrix plane is generated by all charges (qi) weighted by the distance r from the charge to the measuring point, and can be calculated, using a C++ code for equation of superposition principle defined as follows: nnq VT=∑V= 1 ∑i i=1 4πε0 i=1 ri (1) The number of excess charges per pixel is adjusted by trial and error, until the calculated and measured potentials match, within experimental error. The standard free energies for hydrocarbon and fluorocarbon anion or cation formation from the respective free radicals were calculated by (U)B3LYP/6-31+G(d,p) computational model. Geometries were fully optimized and the vibrational frequencies were calculated. The c■alculations were made by Gaussian09 software.41 RESULTS AND DISCUSSION Every tribocharging experiment made in this laboratory led to nonuniformly charged polymer surfaces, forming macroscopic potential and charge patterns or mosaics. Figure 1 shows potential patterns recorded by scanning PTFE and PMMA surfaces that were previously tribocharged by rubbing with a spinning PE foam disk, under controlled pressure and speed. Electrostatic potential on many pixels is in excess of ±3 kV, showing that the PTFE surface contains segregated positive and negative fixed charges arranged within macroscopic (positive or negative) domains in the millimeter−centimeter range, which is rather counterintuitive and is not expected within the conceptual framework of the triboelectric series. Charge surface concentration that produces 3 kV potential at a 2 mm electrode−sample distance is 254 charges/μm2. This means that the average distance between ions accounting for excess local charge is in the 60 nm range. Assuming that the area occupied by an ion is in the 0.1−1 nm2 range, the fraction of surface area occupied by the excess ionsthose that account for excess charge in the macroscopic domainsis less than 0.03%, and the volume concentration of net excess charges in the surface layer is ca. 10−7 mol L−1. At this low concentration, electrostatic repulsive interactions are very low, following the classical Debye−Hückel theory for electrolyte solutions. Potential maps are also shown for PE films tribocharged with glass balls and PTFE pellets on top of the table of a planetary mixer. Again, nonuniform charging is observed: positive domains predominate on the PE surface scratched with PTFE, but they are scarce on PE that contacted glass balls. These experiments were repeated many times, and a set of maps obtained by abrading PTFE with PE foam or PE with PTFE is in Supporting Information Figure S1. The maps themselves are not reproducible in detail, but the patterns obtained are reproducible: tribocharged PTFE potential maps always show two large adjacent areas, one positive and the other negative. The overall charged area is larger than the area of the spinning disk: the diameter of the latter is 1.5 cm, while the charged areas are inserted within squares reaching as much as 3 cm each side. This shows that charges produced by even short-lasting friction quickly spread to areas adjacent to those where mechanical action actually took place. Few other smaller areas with charge excess are eventually observed, but the most often found pattern is a dipole formed by spread charges. On the other hand, PE maps also show positive/negative domains or positive domains only. Nevertheless, the pixel potentials in the PE maps are much lower than in PTFE, showing that both positive and negative charges are preferentially deposited on PTFE. Net charge on each sample can be calculated from the respective potential map by summing up the contributions made by every map pixel as described in a previous paper,39 and it can also be directly determined using a Faraday cup. Results in Table 1 show that the two methods agree within 10% or Table 1. Net Charge Determined by Faraday Cup and by Summing up the Contributions Made by All Pixels in the Potential Maps Applying the Superposition Principle materials charge in Faraday charge calculated by superposition cup/Coulomb principle/Coulomb PTFE × PE foam PMMA × PE foam PE film × glass balls 5.51 × 10−9 6.78 × 10−9 −3.25 × 10−8 6.01 × 10−9 6.61 × 10−9 −3.07 × 10−8 7409 dx.doi.org/10.1021/la301228j | Langmuir 2012, 28, 7407−7416 better, which is very good considering the complete independence of the two procedures, the low spatial resolution of the potential measurements, and the fractal geometry of charge distribution. Moreover, this shows that potential maps account for all the charges incorporated in each sample. Sequentially recording electrostatic potential maps from the same sample and plotting the potential of any given pixel as a function of time allows an assessment of the stability of charge patterns. A typical result is presented in Figure 2, showing that local potentials are rather stable. Plots for pixels well within each domain do not show significant variations, while those in borderline areas show noise that is probably due to deviations in positioning the scanning electrode. Only one pixel showed a definite trend of potential decrease, and this is tentatively assigned to nonuniformity of the contributing charged species, which means some charge-bearing species may lose charge to the atmosphere or to surrounding areas faster than others. Measured decrease in potential can also be due to charge penetration in the sample, because then the contribution to electrode potential would decrease. Considering all the data in Figure 2, charge penetration in PTFE is negligible in the present time scale but it can be faster in one or another area, due to the presence of pores or some other sample defect.PDF Image | Triboelectricity: Macroscopic Charge Patterns Formed by Self- Arraying Ions on Polymer Surfaces
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