Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.)
_______________________________________________________________________________________________
How Polymer Science contributes to the understanding of the
Triboelectric Charging of Insulating Materials
Meurig W. Williams
Xerox Corporation, Webster Research Centre, New York 1970 – 1983 (Retired)
Triboelectric, contact, or static charging is well known in nature and is a basis for copiers, laser printers and other
commercial products. These were developed using an empirical approach to its application because its fundamental
understanding still eludes scientists. But pieces of the puzzle are now in place, and these have involved polymer science.
Lack of progress is attributable to the fact that it has long been considered to be a problem in physics. But advances can
now be expected as a consequence of the recent recognition that a multidisciplinary approach will be required to integrate
what is already known into a coherent overall understanding. This will require further involvement of polymer science
and, in particular, polymer mechanochemistry.
Keywords: Triboelectric charging; contact charging; static electricity; polymer ion fragments; polymer
mechanochemistry; polymer surface enrichment
1. Introduction
When two objects are brought into contact and then separated, electric charges are generated at the surfaces. Such
charges are called triboelectric charges, also known as contact or static charges. Triboelectricity is one of the oldest
areas of scientific study, dating back to experiments by the ancient Greek philosopher Thales of Miletus, who
discovered that rubbing amber against wool led to electrostatic charging. However, rubbing is not necessary, because
such charging also results from simple nonfrictional contacts.
The buildup of this electrical potential can lead to electrostatic discharge, with consequences that can range from
discomfort to disaster. Results can be as mild as a jolt we experience by touching a doorknob after walking across a rug
in dry weather, or as dire as the crash of the Hindenburg, which has recently been confirmed to result from a static
charge igniting a hydrogen leak. Such discharges can result in failure of semiconductor devices, which is a matter of
increasing concern in view of the continued miniaturization of such devices, with increasing sensitivity to such charges.
And they are of major concern for NASA because the dry conditions on the Moon and Mars are ideal for triboelectric
charging.
Fig. 1 The Hindenburg explosion as it came in for a landing in New Jersey in 1937.
But not all static is a nuisance. When controlled, triboelectric charging is at work in copiers and laser printers, which
remain its largest and best-known commercial application. Although static electricity is a familiar subject, much still
remains unknown about how and why such charges form. Research across many disciplines of science and engineering,
243
Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.)
_______________________________________________________________________________________________
from physics and chemistry to medicine and meteorology is currently being conducted on triboelectricity’s various
aspects. However, relatively few scientists are engaged in understanding it at a fundamental level.
Contact charge exchange between two metals was established long ago to involve the transfer of electrons and, since
that time, such charging was considered to be a problem in physics. It was reasonable, as a starting point, to invoke the
transfer of electrons as a mechanism for charging between metals and insulating materials such as polymers and glass.
Limited evidence has been provided to support electron transfer as a charging mechanism between metals and polymers,
but many other results contradicted this. It appears to be the consensus that election transfer can be a contributing
mechanism in addition to one or more other mechanisms that also apply to charging between two polymers.
Electron exchange was also postulated to account for charging between two polymers or insulators in general, even
though it was pointed out by prominent scientists such as Whitesides at Harvard University that there was no evidence
to support such a theory [1]. Transfer of mobile electrons trapped in high energy states on an insulator surface to lower
energy states in another insulator on contact was the basis for the Surface State theory. Multiple contacts between
insulating particles having nominally identical compositions occur in nature, as in sand storms, dust devils and ash
grains from volcanic eruptions, and account for explosions in grain silos and coal mines. It has been shown that smaller
particles typically charge negatively and larger particles positively. This has been interpreted by Lacks et al at the Case
Western Reserve University in terms of the Surface State theory, with the transfer of electrons from larger to smaller
particles resulting from simple geometry [2,3].
But this explanation was discounted in 2014 when the theory was experimentally disproved by Jaeger et al at the
University of Chicago using particles of zirconium dioxide silicate [4]. Rigid adherence to that model by many
researchers probably impeded addressing the overall complexity of the problem.
There are several reasons why such little research is being conducted on the basic mechanisms of triboelectricity.
One is its complexity, partly on account of the belief that several different mechanisms can occur simultaneously.
There is also the relatively recent realization that other sciences, in addition to physics, are required, and a range of
multidisciplinary ability is beyond the reach of most research groups. That was pointed out in 2012 in the American
Scientist article: What Creates Static Electricity? Traditionally considered a physics problem, the answer is beginning
to emerge from chemistry and other sciences [5]. Another reason is the current lack of motivation. In the early copiers
introduced by Xerox in 1959, a sufficient level of understanding of triboelectricity was possible through empirical
research. And it was empirical research that resulted in the breakthroughs required for today’s high image quality (by
Eastman Kodak for high speed copiers and printers, and by Canon at the low end). That was also the case for more
recent products based on triboelectricity. These include those for the separation of plastics from waste electrical and
electronic equipment, a product designed for metal alloy identification by generating x-rays [6], and for forensic wipe
sampling using an electrostatic microprobe [7].
2. The role of polymers in the mechanistic understanding of triboelectricity
Six major advances have occurred in the fundamental understanding of how and why triboelectric charging occurs. One
is the elimination of the Surface State theory, already discussed. The others all involved polymer science.
2.1 Ion exchange
Up to the late 1970s it was considered by Xerox, Eastman Kodak, IBM and other corporations that understanding the
fundamental mechanisms of triboelectricity would be of considerable importance for improving copier performance. It
was Kodak in the mid-1970s that made the first major advance that later led to the establishment of a mechanism for
triboelectric charging. They discovered that addition of a quaternary ammonium chloride to the toner resulted in vastly
improved image quality compared to the early Xerox copiers [8]. They postulated the concept that the smaller chloride
ion is mobile, and its facile transfer between the surfaces creates the charging. It was two decades later that the use of
polymers definitively proved that mechanism. McCarty and Whitesides at Harvard University [9] attached a quaternary
cation to a polymer to prevent its transfer. They demonstrated a quantitative correlation between charging and the
amount of transfer of the smaller mobile anion.
2.2 Material transfer with ionic domains
In 2011, it was demonstrated by Grzybowski and his group at Northwestern University that material transfer can be
accompanied by charge exchange on a nanoscopic level when two polymers are pressed together and then separated
[10]. Using Kelvin force microscopy, high resolution analysis of a surface’s electrical properties, they found that,
although each surface develops a net charge of either positive or negative polarity, each surface also supports a random
mosaic of oppositely charged regions of nanoscopic dimensions. This kind of charge exchange was unexpected. For
centuries, it had been assumed that one surface become uniformly positively charged and the other uniform negative.
Various types of spectroscopy and chemical analysis of the surfaces revealed oxidized species, believed to be
responsible for the charging. This mechanism for the first time revealed the driving force for charge exchange. For
polymer material to transfer from one surface to another requires covalent bonds to be broken with the formation of
244
Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.)
_______________________________________________________________________________________________
polymer fragment free radicals at both scission sites. It was suggested that they react with ambient oxygen and water to
form the charged species.
Following up on this landmark discovery, the Grzybowski group reported in 2013 that the surface charges in the
domains are stabilized by association with surface radicals, which are formed by polymer bond scission. And doping the
polymers with free radical scavengers removes the radicals, resulting in destabilization of the domains and rapid
discharge [11]. The business significance of this is immense because it allows for the design of antistatic coatings to
protect semiconductor devices from failure by buildup of static electricity, a matter of increasing concern in view of the
continued miniaturization of such devices, with increasing sensitivity to such discharges.
2.3 Transfer of ionic polymer fragments, and the role of mechanochemistry
In 2012, Galembeck and his coworkers at the University of Campinas, Brazil, took this material transfer mechanism a
step further [12]. Teflon and polyethylene were sheared together, pressed and twisted against each other. After
separation, the team found macroscopic domains or patterns, both positively and negatively charged, analogous to those
found by Grzybowski’s group. Materials extracted from the surfaces with solvents were identified as polymer ions. The
Teflon residues were predominantly negatively charged, and the polyethylene residues were primarily positively
charged. Galembeck’s team proposed this mechanism: High temperature at the frictional points of contact results in
polymer platicization and/or melting. Shear forces cause breaks in the polymer molecules’ chains forming polymerfragment free radicals. Electron transfer from the polyethylene radicals to the more electronegative Teflon radicals
converts these free radicals to positive and negative polymer ions respectively, which are known as ampiphiles.
Charged macroscopic domains form due to a combination of two factors: ampiphiles at interfaces are known to sort
themselves into arrays when they are in the type of polar environment created by the ions, and Teflon and polyethylene
are immiscible.
A comparison of the work of Galembeck and Grzybowski illustrates the complex interaction between polymer
properties and the nature of the contact in affecting the charge exchange mechanism. The contribution of each of the
factors Galembeck identified in the material transfer mechanism depends on the viscoelastic, topographical, chemical
and other properties of the specific polymers used, and also on the nature of the contact. For example, the ease of bond
scission would differ between polydimethylsiloxane (PDMS), a polymer having a silicon-oxygen backbone, employed
by Grzybowski, and the carbon backbone–based polymers used by Galembeck. The degree of melting, or plasticization,
can be expected to be less in light, low-friction contacts than in shear or vigorous rubbing contacts, on account of the
lower temperatures involved, in addition to being affected by inherent polymer properties such as glass transition
temperature (where the material changes its flow properties without any change in molecular structure). But polymerchain scission of a soft polymer such as PDMS can occur at lower temperatures in low-pressure, low-friction contacts
on account of the polymer chains entangling at the interface, which break on separation. Such entanglements are
enhanced in silicon-oxygen backbone polymers by the presence of oligomers (fragments of polymers) and cyclic
oligomers (where the fragments have a ring structure). These substances exist in dynamic equilibrium; they are
modified constantly due to the continual opening and closing of silicon-oxygen bonds, but have no net change. In the
material transfer mechanism the driving force for creation of the charges is the input of mechanical energy during the
contact of the polymers. This work has resulted in the recognition that the complex and emerging field of
mechanochemistry of entangled polymer chains, which can be considered an interface between chemistry and
mechanical engineering, is an important discipline required for further understanding. The study of mechanophores
promises to be useful for further investigations of how polymer chain scission is related to the generation of electrical
charges. These are functional groups, incorporated into a polymer, which respond to mechanical forces in a controlled
manner [13].
2.4 A disputed charging theory based on electrochemistry
Evidence for an electron transfer mechanism for insulator-insulator charging has been provided using electrochemistry.
When two polymers like Teflon and Lucite were rubbed together vigorously, it was found by Liu and Bard at the
University of Texas [14,15,16] and others that the surface charges developed can carry out different chemical redox
reactions such as metal deposition, ion reduction and chemiluminescence, reactions which can be induced by electrons
but not by ions. This conclusion was later challenged by Piperno et al at the Weizmann Institute of Science, Rehovot,
Israel [17] who reported that those findings can be accounted for by a mechanism other than surface electrons and
consistent with surface ionic charges as reported by Galembeck.
2.5 Distortion of a polymer surface exposes different compositions
Central to determining how and why triboelectric charging of polymers occurs is understudying how it relates to
polymer composition. This has been severely hindered by the failure to recognize that 1) polymers (and other insulators
such as glass) are typically not homogeneous as a function of depth beneath the surface, and 2) contact results in
distortion of a polymer surface to an extent depending on the precise nature of the contact (pressing, rubbing, twisting,
245
Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.)
_______________________________________________________________________________________________
etc). Which means that the chemical composition of a polymer at the points of contact is likely to be different for
different types of contact. And also be different from the nominal bulk composition which is used to characterize the
polymer [18,19]. This oversight has undoubtedly resulted from insufficient input from chemists. Many different types
of contact have been employed in innovative experimental designs, but apparently no efforts have been made to study
this factor as a controlled primary variable. Researchers are still reluctant to address this issue. For example, it has
recently been reported that triboelectric charging is related to the degree of distortion of a glass surface [20], but
remarkably there was a failure to mention that different degrees of distortion are likely to expose different chemical
compositions.
The depth beneath a polymer surface that affects its contact charging has been usefully referred to as “charge
penetration depth”, a concept which in principle can be quantitatively determined. A powerful technique for this was
proposed by Williams [21]. Polymer series were designed such that their topmost layers were the same, but they
differed in their compositions beneath that surface in a systematic way. There are two ways to achieve these surfacebulk compositional differences. Fluorinated molecules and polymers have low surface energies and, when added to
other polymers, will migrate to the topmost molecular layers when films are solution cast to allow for thermodynamic
equilibration while drying. Alternatively, ionic components, on account of their high surface energies, will avoid the
topmost layers. The availability of several extremely sensitive methods for analyzing surfaces as a function of depth
adds to the appeal of this approach. These include XPS (x-ray photoelectron spectroscopy), TOF SIMS (Time of Flight
Secondary Ion Mass Spectrometry) and LEIS (Low Energy Ion Scattering).
The use of such series of surface enriched polymers led to the important discovery that contact with other polymers
related to the topmost molecular layers of the films, but with metals related to the compositions of deeper layers. Bare
metal and polymer coated beads were cascaded over inclined films and the charges on the beads determined in a
Faraday Cage, as shown in Fig 2.
Fig. 2
One interpretation would be that contact between two polymers results from material transfer between the topmost
molecular surfaces in accordance with the mechanisms elucidated by Professors Grzybowski and Galembeck. And in a
metal-polymer contact, electrons have access into deeper layers beneath the surface. But there is an alternative
possibility: that the difference results from the relative surface roughness of the metal beads compared to the smoother
polymer coated beads. In which case the rougher metal beads gouge out deeper layers of the polymer film surfaces than
the smoother metal coated beads. It has been suggested that a distinction can be made between these two mechanisms
simply by employing bare metal beads having the same degree of surface smoothness as the polymer coated beads [22].
References
[1]
[2]
[3]
[4]
S. J. Vella, X. Chen, S. W. Thomas, X. Zhao, Z. Suo, G. M. Whitesides, J. Phys. Chem. C 114, No. 48, 20885–20895 (2010).
Available: http://dx.doi.org/10.1021/jp107883u
D. J. Lacks and A. Levandovsky, J. Electrostatics 65, 107–112 (2007). Available: http://dx.doi.org/10.1016/j.elstat.2006.07.010
J. F. Kok and D. J. Lacks, Phys. Rev. E 79, 051304 (2009). Available: http://dx.doi.org/10.1103/PhysRevE.79.051304
Waitukaitis SR, Lee V, Pierson JM, Jaeger HM. Size-dependent same-material tribocharging in insulating grains. Phys. Rev.
Lett. 30 May 2014. Available: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.218001
246
Polymer science: research advances, practical applications and educational aspects (A. Méndez-Vilas; A. Solano, Eds.)
_______________________________________________________________________________________________
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
Williams MW. What Creates Static Electricity? Traditionally considered a physics problem, the answer is beginning to emerge
from chemistry and other sciences. Am Sci. July-August 2012. Available:
http://www.americanscientist.org/issues/feature/2012/4/what-creates-static-electricity
tribogenics.com
Fletcher RA, Electrostatic microprobe for determining charge domains on surfaces. Rev. Sci. Instrum. 86, 113702 (2015).
Available: http://dx.doi.org/10.1063/1.4935168
Jadwin, T A, Mutz, AN and Rubin, BJ. 1975. Electrographic toner and developer composition. United States Patent 3,893,935.
L. S. McCarty and G. M. Whitesides, Electrostatic charging due to separation of ions at interfaces: Contact electrification of
ionic electrets. Angew. Chem. Int. Ed. 27, No. 12, 2188–2207 (2008). Available: http://dx.doi.org/10.1002/anie.200701812
Baytekin HT, Patashinski AZ, Branicki M, Baytekin B, Soh S, Grzybowski BA. The mosaic of surface charge in contact
electrification. Science. 2011;333(6040): 308-12. Available: http://www.sciencemag.org/content/333/6040/308
Baytekin HT, Baytekin B, Hermans TM, Kowalczyk B, Grzybowski BA. Control of surface charges by radicals as a principle
of antistatic polymers protecting electronic circuitry. Science. 2013;341(6152):1368- 1371. Available:
http://dx.doi.org/10.1126/science.1241326
Burgo TAL, Ducati TRD, Francisco KR, Clinckspoor KJ, Galembeck F, Galembeck SE. Triboelectricity: Macroscopic charge
patterns formed by self-arraying Ions on polymer surfaces. Langmuir; 2012. Available:
http://pubs.acs.org/doi/abs/10.1021/la301228j
Polymer mechanochemistry: the design and study of mechanophores. Brantley JN, Wiggins KM, and Bielawski CW. Polymer
International, Volume 62, Issue 1, pp 2-12, January 2013. DOI: 10.1002/pi.4350
C. Liu and A. J. Bard, Nature Materials 7, 505–509 (2008). Available: http://dx.doi.org/10.1038/nmat2160
C. Liu and A. J. Bard, Chem. Phys. Lett. 480, Iss. 4-6,145–156 (2009). Available: http://dx.doi.org/10.1016/j.cplett.2009.08.045
C. Liu and A. J. Bard, Chem. Phys. Lett. 485, Iss. 1-3,231–234 (2010). Available: http://dx.doi.org/10.1016/j.cplett.2009.12.009
S. Piperno, H. Cohen, T. Bendokov, M. Lahav, I. Lubomirsky, Proc. Electrostatics Society of America Annual Meeting on
Electrostatics (2011)
Williams MW. Triboelectric charging of insulating polymers–some new perspectives. AIP Advances. 2012;2: 010701.
Available: http://scitation.aip.org/content/aip/journal/adva/2/1/10.1063/1.3687233
Williams MW. What Creates Static Electricity? Traditionally considered a physics problem, the answer is beginning to emerge
from chemistry and other sciences. Am Sci. July-August 2012. Available:
http://www.americanscientist.org/issues/feature/2012/4/what-creates-static-electricity
Xie L, He PF, Zhou J, Lacks DJ. Correlation of contact deformation with contact electrification of identical materials. J. Phys.
D: Appl. Phys. 2014;47:215-501. Available: http://iopscience.iop.org/0022-3727/47/21/215501
Williams MW. Triboelectric Charging of insulators - Evidence for Electrons versus Ions. IEEE Transactions on Industry
Applications, May/June 2011, Volume 47, Number 3, pp1093-1099.
Williams MW. Triboelectric charging in metal-polymer contacts - How to distinguish between electron and material transfer
mechanisms. Journal of Electrostatics, Volume 71, Issue 1, February 2013, pp 53-54. Available:
http://dx.doi.org/10.1016/j.elstat.2012.11.006
247