Communication Cite This: Chem. Mater. 2019, 31, 1473−1478 pubs.acs.org/cm Rationalizing the Triboelectric Series of Polymers Xuan Zhang, Linfeng Chen, Yan Jiang, Weichun Lim, and Siowling Soh* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore *S Supporting Information ontact electrification is the phenomenon in which static electronic components (e.g., “tribotronics”),13,14 filtration,15,16 C charge is generated when two materials come into and sensors.17,18 Therefore, there is a great need to select the contact and are then separated. After contact, one material appropriate types of materials for useful applications related to charges positively and the other charges negatively (Figure 1). contact electrification or for the fabrication of noncharging This phenomenon is ubiquitous: almost all types of materials surfaces for circumstances in which contact electrification is 1,3,19,20 (e.g., metal, semiconductor, inorganic materials, and polymers) undesirable. charge upon contact. Hence, contact electrification has a vast The triboelectric series is widely used by researchers and range of influences in our lives.1 In many of our daily activities, engineers for the selection of materials for applications related fi we frequently encounter slight annoyances as a result of static to contact electri cation. It ranks a list of materials according charge (e.g., the adherence of dust particles on charged to their tendency to charge positively or negatively (Figure 1). surfaces and the sticking of clothes after drying). The A material that is ranked more positively than a contacting phenomenon is also problematic for many types of industries material charges positively while the contacting material (e.g., petrochemical, chemical, pharmaceutical, semiconductor, charges negatively, and vice versa. This series is purely and packaging). For example, particles charged by contact empirical: it is established based on contact-charging experi- electrification may adhere onto the walls of vessels and hinder ments (i.e., measuring the polarity of the materials after ff 2 contact). Although it has been established experimentally for e ective heat transfer. In addition, excessive accumulation of 21 static charge can cause electrostatic discharge (e.g., sparks). more than 250 years, the fundamental principle that Electrostatic discharge can damage equipment and is known to underlies the ordering of the materials in the series is still 3 unknown. Understanding the fundamental principle will enable cost the electronic industry billions of dollars per year. researchers and engineers to predict the polarity of charge Importantly, it can also cause explosion of flammable gas, generated on materials by contact electrification; hence, there liquid, and solid. On the other hand, many applications rely on will no longer be a need to perform the contact-charging contact electrification; a few representative examples include experiments. This prediction is necessary because the tribo- electrophotography,4 electrostatic separation, electrostatic 5 6 electric series typically contains only a few common coating, microfluidics (e.g., manipulation of droplets), and 22 7 materials. In addition, the triboelectric series is not always electrostatic self-assembly. In particular, there has been a lot exactly reproducible; different research groups usually report ff from https://pubs.acs.org/doi/10.1021/acs.chemmater.8b04526. slightly di erent ordering of the materials (likely due to the − stochastic nature of the phenomenon).2,22 24 Hence, it is important to understand the fundamental principle because of Downloaded by NATL UNIV OF SINGAPORE at 03:41:05:757 on May 31, 2019 the inadequacies of the series established empirically. In general, the fundamental mechanisms that underlie the contact electrification of insulating materials (e.g., polymers) are not well understood. At the molecular level, it is still unclear whether the charge species that transferred from one material to the other is an electron, an ion, or a small (e.g., nano- or microscopic) quantity of charged material. For example, through a series of experiments, Bard and Liu have suggested that the charge species is an electron.25,26 Others Figure 1. Scheme illustrating a commonly reported triboelectric series (e.g., Whitesides and co-workers), however, suggest that the of polymers4 and the process of contact electrification. PVA: charge species may be an ion (e.g., the aqueous ions, H+ and poly(vinyl alcohol). PMMA: poly(methyl methacrylate). PAN: − 2,4,27 polyacrylonitrile. PS: polystyrene. PVC: polyvinyl chloride. PTFE: OH ). polytetrafluoroethylene. Because the fundamental mechanisms are still unknown, it has been challenging to rationalize the ordering of materials in the triboelectric series. Previous studies have proposed of excitement in these recent few years to develop devices ff based on contact electrification (e.g., the general class of di erent physical properties for explaining the triboelectric triboelectric nanogenerator; TENG). A diverse range of interesting devices have been fabricated, including for Received: October 26, 2018 harvesting of energy (e.g., from human motion or from the Revised: February 14, 2019 − environment such as wind, rain, or waves),8 12 control of Published: February 15, 2019 © 2019 American Chemical Society 1473 DOI: 10.1021/acs.chemmater.8b04526 Chem. Mater. 2019, 31, 1473−1478 Chemistry of Materials Communication Subsequently, the ordering of the materials in the triboelectric series was based on the amount of charge generated on the materials after contact. However, the triboelectric series is constructed based on the polarity of charge by contacting the different combinations of two materialsthe amount and polarity of charge are, in fact, two independent quantities. Many factors can affect the amount of charge generated but not the polarity; examples include surface roughness (i.e., different areas of contact result in different amounts of charge generated), hygroscopicity (i.e., surfaces that are more hygroscopic tend to charge lesser), and other factors. The same conclusion is observed from results of this work (see Table 1). In addition, some studies used reference materials that are metallic for contacting the series of materials of interest.22,34,35,39 On the other hand, the triboelectric series consists mostly of insulating materials. In the field of contact electrification, the charging behavior of contacts between a metal and an insulator is regarded as fundamentally different from contacts between two insulators;43 hence, it is unclear whether these results can represent the triboelectric series of insulating materials. In some other studies, the experiments conducted involved mechanical fracturing of the materials (e.g., using high-energy ball milling). On the other hand, contact electrification usually involves forces that are much smaller (e.g., contact or rubbing). Fracturing results in massive breakage of bonds that is probably not representative of contact electrification.36,37 In addition, most of the previous studies that seek to rationalize the ordering of materials in the triboelectric series did not present a convincing set of experimental results to support their conclusions. For example, most studies − − investigated only a few types of materials28 30,32,40 42 and/or concluded with orderings of the materials that are different − from common expectations22,31,32,36,38 41 (e.g., triboelectric series that are widely reported and used3). Some studies did not perform the contact-charging experiments for verifying their results.29,33,36 Because the results given by the previous studies have not been satisfactory, it is widely regarded in the field of contact electrification that there is currently no explanation for the − ordering of materials in the triboelectric series.44 47 In fact, it is generally considered a challengingor perhaps, an impossibletask to predict the triboelectric series and contact-charging behaviors due to the complex nature of the Figure 2. Polymers selected for investigating the correlation between phenomenon.45,48,49 Lewis basicity and the ordering of materials in the triboelectric series. The phenomenon is complex because a vast range of factors The first column shows the polymers. The second column shows the are known to affect the charge generated on surfaces by molecules that represent the repeat units of the polymers. The third contact electrification. Some examples include the method of column shows the Gibbs energy (indicative of the Lewis basicity) of contact (e.g., rubbing or sliding),50 pressure of contact, the molecules extracted from ref 64 shown in the second column. temperature, and humidity.51 A large variety of factors related to the materials have also been discussed; these include the size − 52 51 53,54 series, such as the work function,28 35 energy levels of the of materials, surface roughness, crystalline structure, 55 19 56,57 molecular orbitals,35 ionization potential,22 potential to lose an softness, conductivity, hydrophilicity/hydrophobicity, 4 4,34,35,41,58−60 electron from an anion,37 dipole moment,38,39 donor zeta potential, acidity/basicity, Hammett sub- 61 number,22,40 surface tension (via an electron-donating stituent constant, modification of nonionic functional groups 62 parameter),41 and dielectric constant.42 However, the con- (e.g., nitro or halide), modification of ionic functional groups clusions from these previous studies