Tribocharged Polymer Surfaces: Solvent Effect on Pattern Formation and Modification

Abstract

Commonly used liquids have a powerful effect on the tribocharge patterns formed by rubbing polyethylene (PE) and poly(tetrafluoroethylene) surfaces. Solvents previously absorbed on PE specifically suppress positive tribocharge deposition. When applied to previously tribocharged surfaces, they also behave as charge-suppressing agents or transfer media. All these unprecedented results verify a new mechanochemical model that was recently introduced to explain the hitherto challenging problem of triboelectricity formation.Triboelectricity was discovered by Thales of Miletus more than 25 centuries ago, but it has not yet been properly explained within the framework of atomicmolecular theory.1 This is largely because of the prevailing scientific problems concerning the identity of charge carriers in dielectric solids and liquids, which are well documented in the literature. 26Recent work from this laboratory showed the formation of macroscopic domains with excess charge in many polymers when they are rubbed under well-controlled conditions. 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. 810The identification of macroscopic charge domains allowed the identification of charge-bearing species such as polymer ions: when poly(tetrafluoroethylene) (PTFE) is rubbed with lowdensity polyethylene (PE), both negative and positive domains are formed on both polymer surfaces. Anions in the negative domains are derived from PTFE, while the positive domains contain PE cations. These species are probably also associated with well-known peroxides formed by free-radical reactions with atmospheric oxygen. 7,1113The 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. 7In this work, we describe the effect of exposure of the polymers to common solvents on the formation of charged domains prior to rubbing them. The effect of liquid droplets placed on tribocharged PTFE surfaces is also described, showing charge-pattern modification on polymer surfaces by charge suppression due to reactions between the polymer ions and the liquids and/or ion withdrawal by extraction or other masstransfer processes.Experiments were performed according to this protocol: PTFE sheets were immersed in ethanol for 2 h prior to tribocharging experiments to minimize any pre-existing static charge. Square PTFE sheets (6 © 6 cm 2 ) were placed on a...

Triboelectricity in insulating polymers: evidence for a mechanochemical mechanism

“…39 The fractal dimension obtained from linescans drawn on SEI micrographs in this paper agrees with previous results obtained by atomic force microscopy, as expected considering that both are determined mainly by surface topography. Particle removal by washing surfaces with suitable liquids is usually a simple task and this explains why a large fraction of the tribocharge is easily removed from various polymer surfaces by simple immersion in both polar and apolar liquids.…”

Friction coefficient dependence on electrostatic tribocharging

“…Most important, knowing that polymer friction produces charges that in turn contribute to increased friction coefficients allows us to avoid experimental limitations imparted by electrical charge effects to macro and micromachines27 by using effective charge removal or by suppressing charge formation, using techniques described in recent work from this group3537 or chemical surface modification, as in this work. For most polymers, charged species are easily removed by immersion in ethanol or propanol and charge formation is suppressed by mildly swelling their surfaces with these and other common liquids.…”

“…Moreover, it is now clear that in the tribocharged polytetrafluoroethylene (PTFE)-polyethylene (PE) pair the positive ions derive from mechanochemical transformation of PE chains while the negative ions derive from PTFE35. Both types of ions are selectively extracted by rinsing with common solvents and this is now the basis for a simple but effective technique for static charge removal from polymers and other insulators37.…”

Friction, tribochemistry and triboelectricity: recent progress and perspectives

“…Immersion in liquids is an effective way to remove tribocharge from a surface. 20,136 Figure 8 shows the effect of some liquids on PTFE tribocharged with PE: ethanol is more effective than water, 10 NaCl aqueous solution and n-hexane. The latter removes positive charges better than negative charges.…”

“…The PE film shows weakened mirror image of the PTFE surface, while the PTFE surface also shows lower 40 contrast after the transfer procedure. 136 The observation of contiguous domains with opposite charge allowed a further development that is the identification of polymer ionic fragments as the tribocharge bearing species that will be described in the next section. 45 Actually, using the very first generation of the Kelvin force microscope, Terris and collaborators tapped a tip on a PMMA surface where subsequent electrostatic mapping revealed positive and negative domains in the same area of contact.…”