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Langmuir Article Figure 4. Left: potential maps of two pieces of PTFE charged by shearing with PE disk. Right: (a) IR reflectance spectra of positive and negative areas from the PTFE piece at left; (b) picture (contrast enhanced) of the PTFE piece mapped at left. Both ions and free radicals formed by chain rupture are high- energy, unstable, short-lived species likely to enter a number of further reactions as, for instance, the well-known trans- formation of carbon radicals (R•) into peroxide radicals (ROO•)56,57 under air, but these are not detected by charge measurements. In the case of PTFE rubbed with PE, the low polarity of carbon−carbon bonds in both polymer chains suggests that homolytic chain breakdown predominates. Further, electron transfer from hydrocarbon to fluorocarbon radicals is expected, following the higher electronegativity of the latter.58,59 This qualitative reasoning is supported by the calculation of Gibbs energies for the formation of positive and negative ions by electron transfer to and from fluorocarbon and hydrocarbon free radicals, shown in Table 2. Following these results, the formation of fluorocarbon anions from fluorocarbon free radicals is spontaneous but the formation of hydrocarbon anions and of both types of cations is not spontaneous. Nevertheless, hydrocarbon cation formation is more favorable than the formation of fluorocarbon cations, and the following redox reaction is the most likely to take place, involving free radicals formed tribochemically −H CH CH C·+−F CF CF C· 222 222 → −H CH CH C+ + −F CF CF C− 222 222 The formation of the macroscopic charged domains is thus assigned to the accumulation of hydrocarbocations and fluorocarbanions, but these species are not expected to be stable under ambient conditions used in these experiments unless they are somehow chemically protected. Indeed, ions formed from polymer chain rupture are amphiphilic; this means the charge located at chain-ends creates a polar environment that tends to self-array at any interface with air, hiding the charged portion of the ion under a layer of unreacted polymer and thus protecting it from atmospheric water, oxygen, and other reactive substances. Thus, unstable species are trapped or occluded by stable, intact chain segments and chain ends that make a lower contribution to surface tension than the high- energy species.50,51 The observation of large nonzero positive or negative potential and its slow decay39 in PE was recently reported but using corona charging devices. A mechanism for contact and triboelectrification of insulating polymers is schematically represented in Figure 6. The relative importance of the various events that can take place depends on the materials used, the history of their surfaces and subsurfaces, their morphology and oxidation state, the kinds of intervening mechanical actions, and the environment. For this reason, the outcome of the contact or friction between two surfaces should be spatially nonuniform,60 creating electrostatic nanopatterns similar to those previously described in the literature28−30,32 and the macroscopic patterns described in the present paper. Following the present results, the complex and stable electric patterns obtained by simple contact or friction of dielectric solids are necessarily dependent on highly fixed and stable charges, since the charge maps do not show significant modifications between successive scans, even across potential gradients as large as 6 MV/m, as, for instance, across the separation lines between positive and negative islands in Figure 1. These conditions are met by the hydrocarbon and fluorocarbon ionic species derived from macromolecules experimentally evidenced in this work. The formation of large domains with excess positive or negative charge, in the case of the PTFE/PE pair, was not 7412 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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