Rationalizing the Triboelectric Series of Polymers

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 slightly different 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 the inadequacies of the series established empirically. In general, the fundamental mechanisms that underlie the Downloaded via NATL UNIV OF SINGAPORE on December 19, 2020 at 01:38:11 (UTC). contact electrification of insulating materials (e.g., polymers) See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 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

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 materialsthe 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 challengingor perhaps, an impossibletask 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 have been far from (i.e., via the transfer of the mobile counterions during satisfactory: these studies reported results that do not contact),63 oxidation, sulphonation, aminolysis, dehydrogen- conceptually represent the triboelectric series and/or are ation,61 and a large number of other properties and chemical based on incorrectly designed experiments. Specifically, some modification of the materials. In fact, any changes to the experiments involved contacting a series of materials of interest material and experimental conditions is generally expected to with only a single type of reference material.22,38,39 affect the generation of charge by contact electrification.

1474 DOI: 10.1021/acs.chemmater.8b04526 Chem. Mater. 2019, 31, 1473−1478 Chemistry of Materials Communication

Table 1. Charge Density (i.e., the charge per unit contact area) of the Polymers (indicated in the ﬁrst column) after a Contacting with Another Diﬀerent Polymer (indicated in the top row)

aValues in parentheses represent the standard deviation. The polymers are arranged according to the deﬁnition of the triboelectric series as indicated by the red double-headed arrow on the left.

Figure 3. Correlation between the ordering of materials in the triboelectric series established experimentally and Lewis basicity/acidity.

In our study, we focus on understanding the ordering of Through measuring the equilibrium concentration of the polymers in the triboelectric series because polymers are useful molecules by an analytical method (e.g., FTIR), the for many different types of applications and they usually charge equilibrium constant of the reaction can be determined. The highly due to contact electrification. Our hypothesis is that the Gibbs energy of the reaction, ΔG, can thus be calculated from Lewis acidity/basicity of the repeat unit of the polymer is the the equilibrium constant.64 A molecule has a stronger Lewis principle that governs the triboelectric series because of the basicity when ΔG is lower (e.g., more negative), and vice versa. possible interactions between ions generated by contact The second column of Figure 2 lists 10 molecules with their electrification and the polymers. The Lewis basicity of different corresponding ΔG values shown in the third column as molecules can be quantified by allowing the molecules to extracted from ref 64. These 10 molecules were selected interact with a reference molecule that acts as the Lewis acid. because their ΔG values span a reasonably wide range from Previous works have used different types of reference negative to positive. By considering the molecular structures of molecules to establish different scales for quantifying Lewis these 10 molecules as the repeat units of polymers, we basicity. Here, we chose the scale that uses 4-fluorophenol (4- identified a series of 10 polymers as shown in the first column FC6H4OH) as the reference molecule. This scale has been of Figure 2. Consequently, we prepared the 10 polymers (see widely used in literature and is reported to be the most Section 1 of the Supporting Information, SI, for the methods comprehensive scale for Lewis basicity.64 The interaction of 4- and materials) for performing the contact-charging experi- fluorophenol and a Lewis base (B) is shown in reaction 1. ments. In a typical experiment, we first selected 2 out of the 10 4‐FC64 H OH + BF 4 ‐ FC 64 H OH ··· B (1) polymers and discharged them. We contacted the two

1475 DOI: 10.1021/acs.chemmater.8b04526 Chem. Mater. 2019, 31, 1473−1478 Chemistry of Materials Communication polymers 20 times and then measured the charges of the two of the experiments. Understanding the rationale of the polymers using a Faraday cup connected to an electrometer triboelectric series of polymers allows materials to be selected (Keithley, model 6514). We then repeated the experiments by easily for the vast range of applications related to contact selecting another 2 out of the 10 polymers. This experiment electrification (e.g., devices for harvesting energy or sensors). was repeated for all the possible combinations of 2 out of the 10 different polymers. In total, a combination of 10 × 10 ■ ASSOCIATED CONTENT ff di erent polymers was investigated (Table 1). Importantly, we *S Supporting Information fi arranged the polymers in the rst column of Table 1 according The Supporting Information is available free of charge on the fi to their polarity after contact electri cation. This arrangement ACS Publications website at DOI: 10.1021/acs.chemma- fi corresponds to the de nition of the triboelectric series: the ter.8b04526. polymers that tend to charge more positively are placed closer to the top, while polymers that tend to charge more negatively Methods and Materials, examining the cyclic tribo- are placed closer to the bottom of the list. After arranging the electric series reported in a previous study, rationalizing polymers according to their polarity, we found that all the the ordering of the polymers in the triboelectric series entries above the main diagonal of the table have positive with other physical properties, and raw data of charge polarity, while all the entries below the main diagonal have measurements from contact-charging PVAc with other polymers (PDF) negative polarity. This result showed clearly that the triboelectric series has been established: any polymer that is ranked above a contacting polymer charges positively, while ■ AUTHOR INFORMATION the contacting polymer charges negatively. Importantly, the Corresponding Author ordering of the polymers shown in the first column of Table 1 *(S.S.) E-mail: [email protected]. fi corresponds exactly to that shown in the rst column of Figure ORCID 2. Therefore, our result indicates that the ordering of the Linfeng Chen: 0000-0003-2388-8906 polymers in the triboelectric series correlates with the Lewis basicity of the repeat unit of the polymer (Figure 3). The exact Siowling Soh: 0000-0002-4294-6772 correspondence between charge and Lewis basicity derived Author Contributions from the large set of data of 10 × 10 combination of polymers The manuscript was written through contributions of all indicated that the correlation has been established convinc- authors. All authors have approved the final version of the ingly. manuscript. As shown in Table 1, the triboelectric series is a linear list of Notes materials. On the other hand, a previous study has reported the The authors declare no competing financial interest. existence of a triboelectric series that is “cyclic”: the materials in this series cannot be arranged in a linear list.65 The existence ■ ACKNOWLEDGMENTS of this cyclic triboelectric series suggests that the ordering of fi 4 This work was nancially supported by the Ministry of the materials depends on more than one physical property. Education, Singapore, under Grants R-279-000-408-112 and After repeating the experiments conducted in this previous R-279-000-496-114. study65 using the same set of materials, however, we were not able to obtain the cyclic triboelectric series as reported. The ■ REFERENCES series we obtained was linear (see SI, Section 2, for more (1) Chen, L. F.; Shi, Q. F.; Sun, Y. J.; Nguyen, T.; Lee, C. K.; Soh, S. details on the methods and results). 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1476 DOI: 10.1021/acs.chemmater.8b04526 Chem. Mater. 2019, 31, 1473−1478 Chemistry of Materials Communication

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1478 DOI: 10.1021/acs.chemmater.8b04526 Chem. Mater. 2019, 31, 1473−1478