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Cite This: Chem. Mater. 2019, 31, 1473−1478

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

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* Supporting Information

electronic components (e.g., “tribotronics”),^13,14 filtration,^15,16 ontact electrification is the phenomenon in which static and sensors.^17,18 Therefore, there is a great need to select the

C_{charge is generated when two materials come into}

appropriate types of materials for useful applications related to contact electrification or for the fabrication of noncharging surfaces for circumstances in which contact electrification is undesirable.^1,3,19,20 contact and are then separated. After contact, one material charges positively and the other charges negatively (Figure 1). This phenomenon is ubiquitous: almost all types of materials (e.g., metal, semiconductor, inorganic materials, and polymers) charge upon contact. Hence, contact electrification has a vast range of influences in our lives.¹ In many of our daily activities, we frequently encounter slight annoyances as a result of static charge (e.g., the adherence of dust particles on charged surfaces and the sticking of clothes after drying). The phenomenon is also problematic for many types of industries (e.g., petrochemical, chemical, pharmaceutical, semiconductor, and packaging). For example, particles charged by contact electrification may adhere onto the walls of vessels and hinder effective heat transfer.² In addition, excessive accumulation of static charge can cause electrostatic discharge (e.g., sparks). Electrostatic discharge can damage equipment and is known to cost the electronic industry billions of dollars per year.³ Importantly, it can also cause explosion of flammable gas, liquid, and solid. On the other hand, many applications rely on contact electrification; a few representative examples include electrophotography,⁴ electrostatic separation, electrostatic coating,⁵ microfluidics (e.g., manipulation of droplets),⁶ and electrostatic self-assembly.⁷ In particular, there has been a lot
The triboelectric series is widely used by researchers and engineers for the selection of materials for applications related to contact electrification. It ranks a list of materials according to their tendency to charge positively or negatively (Figure 1). A material that is ranked more positively than a contacting material charges positively while the contacting material charges negatively, and vice versa. This series is purely empirical: it is established based on contact-charging experiments (i.e., measuring the polarity of the materials after contact). Although it has been established experimentally for more than 250 years,²¹ the fundamental principle that underlies the ordering of the materials in the series is still unknown. Understanding the fundamental principle will enable researchers and engineers to predict the polarity of charge generated on materials by contact electrification; hence, there will no longer be a need to perform the contact-charging experiments. This prediction is necessary because the triboelectric series typically contains only a few common materials.²² In addition, the triboelectric series is not always 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 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 (e.g., Whitesides and co-workers), however, suggest that the charge species may be an ion (e.g., the aqueous ions, H⁺ and

OH⁻).^2,4,27

Figure 1. Scheme illustrating a commonly reported triboelectric series of polymers⁴ and the process of contact electrification. PVA: poly(vinyl alcohol). PMMA: poly(methyl methacrylate). PAN: polyacrylonitrile. PS: polystyrene. PVC: polyvinyl chloride. PTFE: 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 different physical properties for explaining the triboelectric of excitement in these recent few years to develop devices based on contact electrification (e.g., the general class of triboelectric nanogenerator; TENG). A diverse range of interesting devices have been fabricated, including for harvesting of energy (e.g., from human motion or from the environment such as wind, rain, or waves),⁸⁻¹² control of

Received: October 26, 2018 Revised: February 14, 2019 Published: February 15, 2019

© 2019 American Chemical Society

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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 materials^{28−30,32,40−42} and/or concluded with orderings of the materials that are different from common expectations^{22,31,32,36,38−41} (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 phenomenon.^45,48,49 The phenomenon is complex because a vast range of factors are known to affect the charge generated on surfaces by contact electrification. Some examples include the method of contact (e.g., rubbing or sliding),⁵⁰ pressure of contact, temperature, and humidity.⁵¹ A large variety of factors related to the materials have also been discussed; these include the size of materials,⁵² surface roughness,⁵¹ crystalline structure,^53,54 softness,⁵⁵ conductivity,¹⁹ hydrophilicity/hydrophobicity,^56,57 zeta potential,⁴ acidity/basicity,^{4,34,35,41,58−60} Hammett substituent constant,⁶¹ modification of nonionic functional groups (e.g., nitro or halide),⁶² modification of ionic functional groups (i.e., via the transfer of the mobile counterions during contact),⁶³ oxidation, sulphonation, aminolysis, dehydrogenation,⁶¹ and a large number of other properties and chemical modification of the materials. In fact, any changes to the material and experimental conditions is generally expected to affect the generation of charge by contact electrification.

Figure 2. Polymers selected for investigating the correlation between Lewis basicity and the ordering of materials in the triboelectric series. The first column shows the polymers. The second column shows the molecules that represent the repeat units of the polymers. The third column shows the Gibbs energy (indicative of the Lewis basicity) of the molecules extracted from ref 64 shown in the second column.

series, such as the work function,²⁸⁻³⁵ energy levels of the molecular orbitals,³⁵ ionization potential,²² potential to lose an electron from an anion,³⁷ dipole moment,^38,39 donor number,^22,40 surface tension (via an electron-donating parameter),⁴¹ and dielectric constant.⁴² However, the conclusions from these previous studies have been far from satisfactory: these studies reported results that do not conceptually represent the triboelectric series and/or are based on incorrectly designed experiments. Specifically, some experiments involved contacting a series of materials of interest with only a single type of reference material.^22,38,39

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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)

a

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

4‐FC₆H₄OH + B F 4‐FC₆H₄OH···B

(1)

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polymers 20 times and then measured the charges of the two polymers using a Faraday cup connected to an electrometer (Keithley, model 6514). We then repeated the experiments by selecting another 2 out of the 10 polymers. This experiment was repeated for all the possible combinations of 2 out of the 10 different polymers. In total, a combination of 10 × 10 different polymers was investigated (Table 1). Importantly, we arranged the polymers in the first column of Table 1 according to their polarity after contact electrification. This arrangement corresponds to the definition of the triboelectric series: the polymers that tend to charge more positively are placed closer to the top, while polymers that tend to charge more negatively are placed closer to the bottom of the list. After arranging the polymers according to their polarity, we found that all the entries above the main diagonal of the table have positive polarity, while all the entries below the main diagonal have 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 the contacting polymer charges negatively. Importantly, the ordering of the polymers shown in the first column of Table 1 corresponds exactly to that shown in the first column of Figure 2. Therefore, our result indicates that the ordering of the polymers in the triboelectric series correlates with the Lewis basicity of the repeat unit of the polymer (Figure 3). The exact correspondence between charge and Lewis basicity derived from the large set of data of 10 × 10 combination of polymers indicated that the correlation has been established convincingly. of the experiments. Understanding the rationale of the triboelectric series of polymers allows materials to be selected easily for the vast range of applications related to contact electrification (e.g., devices for harvesting energy or sensors).

ASSOCIATED CONTENT

* Supporting Information

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The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.chemma- ter.8b04526.

Methods and Materials, examining the cyclic triboelectric series reported in a previous study, rationalizing the ordering of the polymers in the triboelectric series with other physical properties, and raw data of charge measurements from contact-charging PVAc with other polymers (PDF)

AUTHOR INFORMATION

Corresponding Author

*(S.S.) E-mail: [email protected].

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ORCID

Linfeng Chen: 0000-0003-2388-8906 Siowling Soh: 0000-0002-4294-6772

Author Contributions

The manuscript was written through contributions of all authors. All authors have approved the final version of the manuscript.
As shown in Table 1, the triboelectric series is a linear list of materials. On the other hand, a previous study has reported the existence of a triboelectric series that is “cyclic”: the materials in this series cannot be arranged in a linear list.⁶⁵ The existence of this cyclic triboelectric series suggests that the ordering of the materials depends on more than one physical property.⁴ After repeating the experiments conducted in this previous study⁶⁵ using the same set of materials, however, we were not able to obtain the cyclic triboelectric series as reported. The series we obtained was linear (see SI, Section 2, for more details on the methods and results). A linear list suggests that the ordering of the materials in the triboelectric series is based on only one physical propertyour results showed that this

property is the Lewis basicity. Lewis basicity measures the ability of the polymer to share its electrons with a molecule; importantly, this molecule can be an ion for the case of a charged surface. The ability to gain either positive or negative charge may be due to the varying extent of the polymers to share their electrons with ions of the respective polarity. Therefore, the established correlation strongly suggests that the fundamental mechanism that underlies contact electrification is ion transfer (e.g., not due to electron transfer). Besides Lewis basicity, we also tried to correlate the ordering of the 10 polymers in the triboelectric series with other physical properties, such as the ionization energy, dipole moment, and dielectric constant (see SI, Section 3, for more details on the methods and results); however, we are not able to identify other convincing correlation. Referring to the Lewis basicity of the repeat unit of the polymer is thus a convenient way to predict the position of the polymer in the triboelectric series without needing to perform the experiments. This correlation enhances our fundamental understanding of a phenomenon that is largely based on empirical results and is often affected by the many uncertainties and irreproducibility

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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This work was financially supported by the Ministry of Education, Singapore, under Grants R-279-000-408-112 and R-279-000-496-114.

REFERENCES

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