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pubs.acs.org/Langmuir Triboelectricity: Macroscopic Charge Patterns Formed by Self- Arraying Ions on Polymer Surfaces Thiago A. L. Burgo,† Telma R. D. Ducati,† Kelly R. Francisco,† Karl J. Clinckspoor,† Fernando Galembeck,*,†,§ and Sergio E. Galembeck¶ †Institute of Chemistry, University of Campinas, Campinas SP, Brazil 13083-970 §National Nanotechnology Laboratory at the National Center for Energy and Materials Research, Campinas SP, Brazil 13083-970 ¶Chemistry Department, University of Saõ Paulo, Ribeiraõ Preto SP, Brazil 14040-901 *S Supporting Information Article ABSTRACT: Tribocharged polymers display macroscopically patterned positive and negative domains, verifying the fractal geometry of electrostatic mosaics previously detected by electric probe microscopy. Excess charge on contacting polyethylene (PE) and polytetrafluoroethylene (PTFE) follows the triboelectric series but with one caveat: net charge is the arithmetic sum of patterned positive and negative charges, as opposed to the usual assumption of uniform but opposite signal charging on each surface. Extraction with n-hexane preferentially removes positive charges from PTFE, while 1,1-difluoroethane and ethanol largely remove both positive and negative charges. Using suitable analytical techniques (electron energy-loss spectral imaging, infrared microspectrophotometry and carbonization/colorimetry) and theoretical calculations, the positive species were identified as hydrocarbocations and the negative species were identified as fluorocarbanions. A comprehensive model is presented for PTFE tribocharging with PE: mechanochemical chain homolytic rupture is followed by electron transfer from hydrocarbon free radicals to the more electronegative fluorocarbon radicals. Polymer ions self-assemble according to Flory−Huggins theory, thus forming the experimentally observed macroscopic patterns. These results show that tribocharging can only be understood by considering the complex chemical events triggered by mechanical action, coupled to well-established physicochemical concepts. Patterned polymers can be cut and mounted to make macroscopic electrets and multipoles. ■ INTRODUCTION Thales discovered contact electrification 25 centuries ago, and this topic has received attention from many important scientists over the past 200 years.1 However, there is persistent controversy2−6 concerning the nature of the species responsible for imparting charge to electrical insulators, in polymer− polymer or polymer−metal contacts.7−10 Evidence in favor of electrons11−17 or ions18−26 as charge carriers has been presented by different groups in a debate that was much intensified during the past decade. According to many authors, the outcome of contact electrification can be predicted using the triboelectric series which sums up experimental information on the positive or negative signal of charge found in a solid, following contact with any other solid. However, there is no accepted theoretical basis for the triboelectric series, and quantitative data are scarce. Indeed, even the position of different materials in the triboelectric series has been re-examined, from time to time.27 A recent publication28 challenged the concept of triboelectric series, based on experiments made by contacting two surfaces of identical chemical composition, measuring the resulting electric charges and observing electrostatic potential mosaic patterns on these surfaces, by using Kelvin force microscopy. Terris et al.29 and Knorr30 had previously described related microscopy results, showing that surface scratching produces a bipolar or multipolar potential distribution. On the other hand, tribocharging can be sufficiently well-defined to allow macro- scopic particle self-assembly.31 Received: March 23, 2012 Revised: April 19, 2012 Published: April 24, 2012 © 2012 American Chemical Society 7407 dx.doi.org/10.1021/la301228j | Langmuir 2012, 28, 7407−7416 Downloaded via 173.229.12.141 on October 21, 2020 at 22:30:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.PDF Image | Triboelectricity: Macroscopic Charge Patterns Formed by Self- Arraying Ions on Polymer Surfaces
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