Triboelectricity: Macroscopic Charge Patterns Formed by Self-Arraying Ions on Polymer Surfaces

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.

Tribocharged Polymer Surfaces: Solvent Effect on Pattern Formation and Modification

“…Charged domains were detected by scanning the polymer surfaces with a macroscopic Kelvin electrode. 7 This finding has no precedents in the literature, but it converges with previous results on charge nanopatterning in dielectric surfaces, confirming that the charge distribution on these surfaces follows fractal behavior. …”

“…7,14 Surprisingly, the potential patterns obtained after rubbing PTFE with PE foam disks immersed in ethanol and dried for different times showed only negative potentials when the PE disks were dried only for short times (Figures 1a, 1b, and 1c). Positive domains (Figure 1d) also appeared, but only after longer drying times (1050 s), when ethanol evaporation was complete, as verified by weighing the PE samples to within «0.01 mg (allowing detection of 1% of the total amount of absorbed ethanol).…”

“…These species are probably also associated with well-known peroxides formed by free-radical reactions with atmospheric oxygen. 7,1113 The stability of the polymeric ions is remarkable, and the charged domains are detected for as long as 60 h. However, these species are expected to be rather unstable, and their persistence is understood by considering that their reactive charged sites are protected from the outer atmosphere by self-assembly, as expected for amphiphilic species. Tribo-ions formed by abrading PTFE surfaces with PE foam disks can be more or less selectively removed by immersion of the charged polymer sample in several liquids.…”

“…7 In the presence of low-molecular-weight (MW) absorbed liquids, the positive PE ions or their precursor radicals react so that the volatile compounds carry positive charges away from the surface and into the atmosphere. In this case, only negative charge domain formation is verified on PTFE surfaces.…”

Effect of humidity on contact electrification due to collision between spherical particles

“…1 Later studies indicated that friction/rubbing is not a necessary condition to generate charges on contacting surfaces. 2 We still do not fully understand how and why such charge generation occurs, 3,4 although considerable progress has been made, such as by considering the role of mechanochemistry 6,7 and the difference of the composite between the surface and the bulk, 2,4,5,8 to more closely reveal the CE charging mechanism. Many researchers have suggested that moisture adsorbed on the surface plays a role in CE 9 and the related ambient humidity is also thought to be particularly important for surface charging.…”