Controlling contact electrification with photochromic polymers

Article abstract of DOI:10.1002/anie.201003985

Charge switching: Control of contact electrification was achieved using a combination of photoreactive polymers and light. Upon UV irradiation, polymers substituted with photochromic spiropyrans reversibly switch their rate, and depending on chemical structure, their sign of charging by contact electrification (see picture). Consistent with empirical predictions, these photochromic polymers tended to charge positively upon irradiation with UV light.

Full text of DOI:10.1002/anie.201003985

Communications
DOI: 10.1002/anie.201003985
Photochromism
Controlling Contact Electrification with Photochromic Polymers**
Simone Friedle and Samuel W. Thomas III*
Contact electrification, the separation of charge when con-
external electric fields, at 20–25% relative humidity (RH) and
20–228C.
[
1]
tacting materials separate, is important in a number of
[
2]
applications including electrophotography and the benefi-
We prepared the nitrospiropyran-containing methacrylic
[3]
[18]
ciation of coal. Contact electrification also causes adhesion
monomer SPMA in four steps using modified literature
[4]
[19]
of particles that inhibit the performance of equipment, or
procedures.
Photochromic spiropyrans reversibly form
[
5]
discharges that ignite flammable vapors. Controlling contact
electrification, however, remains an unsolved problem.
Chemical approaches to controlling contact electrification
include aggressive treatments like plasma or mineral acids to
chemically modify the surface in a largely undefined
zwitterionic merocyanines (MC) upon UV irradiation (Sche-
[
20]
me 1a).
As summarized in Scheme 1b, we prepared
[
6]
[7]
manner, additives such as charge-control agents, or
covalently modifying surfaces with groups that bear
“
mobile” ions that are not covalently bound (ionic elec-
[
8]
trets). Whether the mechanism of insulator contact elec-
[
8b,9]
[10]
trification involves the transfer of ions,
electrons, or a
is a matter of debate; a correlation
appears to exist, however, between the hydrophobicity of
[
7,11]
combination thereof
[9]
materials that are not ionic electrets and their charging.
Photochromic molecules transition reversibly between
chemical structures upon absorption of light, often with
substantially different degrees of hydrophobicity.
applications (among many) include color-changing eye-
wear,
[
12]
[13]
Their
[14]
[15]
[16]
fluorescence imaging,
and molecular logic.
Herein we describe spiropyran-based photochromic polymers
that reversibly change contact electrification behavior upon
irradiation.
We used a previously described instrument to measure the
[
8c,17]
dynamics of contact electrification.
Briefly, a magnetic
stir plate causes a ferromagnetic steel sphere to roll in a
circular path on an electrically insulating film. Our experi-
ments interrogate the effect of the chemical structure of the
insulating film on contact electrification of the rolling sphere.
With each revolution of the sphere, it passes over an electrode
Scheme 1. a) Photochromic reaction of spiropyran-containing copoly-
mers that reversibly yields hydrophilic merocyanines upon UV irradi-
ation. The label “A” in the main chain represents a non-reactive
comonomer in the random copolymers. b) Synthesis and names of the
four spiropyran-containing random copolymers.
(
connected to an electrometer) that measures charge on the
sphere. When the sphere is far from the electrode, it measures
only the charge on the dielectric close to the electrode.
Because the sphere passes over the electrode repeatedly, we
can determine the rate of contact electrification. We per-
formed studies in a Faraday cage to mitigate artifacts from
copolymers (SPMA-A) of SPMA and either styrene-based
or methacrylate-based comonomers A by AIBN-initiated
radical polymerization in toluene at 658C. The copolymers
contained 20–40 mol% of SPMA. Spin-casting these poly-
mers from 1% solutions (w/v) in toluene onto glass slides at
2
500 rpm gave optically clear films that were 35–80 nm thick.
As expected, spiropyran-containing films turned blue
[*] Dr. S. Friedle, Prof. S. W. Thomas III
Department of Chemistry, Tufts University
(
lmax = 591–596 nm, see Supporting Information) upon irra-
62 Talbot Avenue, Medford, MA 02155 (USA)
diation with UV light for two minutes (200 W Hg/Xe lamp
equipped with a UV bandpass filter) due to formation of MC
moieties. In addition, the films became more hydrophilic as
demonstrated by decreased advancing and receding water
contact angles upon MC formation (see Supporting Informa-
tion). This result is important because the surface of the film
rather than the bulk is responsible for contact electrification.
In contrast, homopolymers without SP groups, such as
Fax: (+1)617-627-3443
E-mail: sam.thomas@tufts.edu
Homepage: http://ase.tufts.edu/chemistry/thomas/
[
**] This work was supported by a DARPA Young Faculty Award (Grant
No. N66001-09-1-2116) and Tufts University. We also thank Dr.
Christopher N. LaFratta for assistance with optics.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003985.
7968
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Angew. Chem. Int. Ed. 2010, 49, 7968 –7971

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Chemie
poly(styrene) and poly(methyl methacrylate), showed no
change in absorbance spectra or contact angles after UV
irradiation.
Before irradiation of the films, steel spheres rolling on the
SPMA-containing copolymer films charged with the same
rates, within the error of the experiments, as when they rolled
on films of homopolymers of the corresponding inert
monomer. For example, as summarized in Table 1, steel
Table 1: Initial rates of charging of rolling steel spheres on polymer films.
Tabulated values are the means of at least eight measurements; standard
deviations are in parentheses.
À1
Copolymer
Rate of charging [pCs
After UV
]
Before UV
irradiation
Unreactive
homopolymer
irradiation
SPMA-nBuMA
SPMA-MMA
SPMA-St
À8 (2)
À4 (2)
À2 (2)
8 (4)
À60 (30)
À57 (20)
À106 (46)
À20 (6)
À9 (4)
À11 (4)
0.7 (0.7)
11 (4)
SPMA-4FSt
Figure 1. Contact electrification of a steel sphere rolling on a film of
SPMA-4FSt: a) The steel sphere charges positively before and nega-
tively after irradiation of the film with UV light for 2 min. b) Real-time
measurement of the change in charge on the steel sphere caused by
irradiation of a film of SPMA-4FSt. UV irradiation began at t=120 s.
spheres charged positively with an initial rate of 11 Æ 4 pCsÀ1
when rolling on poly(4-fluorostyrene) (P4FSt), compared to
À1
À1
8
Æ 4 pCs SPMA-4FSt, but negatively (À9 Æ 4 pCs ) when
rolling on poly(n-butyl methacrylate) (PBuMA) and SPMA-
À1
nBuMA (À8 Æ 2 pCs ) before UV irradiation. Therefore, the
presence of the spiropyran did not have a statistically
significant impact on the rate of contact electrification.
Upon irradiation of the photochromic films for two
minutes, steel spheres rolling on these films developed
negative charge significantly faster than they did before
irradiation (Table 1). Figure 1a shows charging of a steel
chrome nor photochemical reaction of the polymer backbone
caused the consistent change in charging observed upon UV
irradiation.
The fast nature of SP-to-MC conversion enables real-time
monitoring of the change in charging behavior. As shown in
Figure 1b, a sphere rolling on a film of SPMA-4FSt switched
from having positive charge to negative charge less than one
minute after beginning continuous UV irradiation. Our
approach of monitoring the change in characteristic charging
between photoisomers mitigates the common problem of
variability between samples that commonly plagues studies of
sphere rolling on a film of SPMA-4FSt: it charges positively
À1
(
(
+ 8 pCs ) before UV irradiation of the film, and negatively
À1
À20 pCs ) after. Steel spheres rolling on the other photo-
chromic polymers we examined show the same trend: their
rate of negative charging increased by circa one order of
magnitude upon UV irradiation of the photochromic films for
two minutes. In addition, the initial rate of charging correlates
roughly with the absorbance of the MC (see Supporting
Information). The sharp discontinuities in charge accumula-
tion observed are consistent with electrostatic discharge
[
21]
contact electrification.
The switching of charging behavior of these films is
reversible: consistent with the photochromic nature of
spiropyrans, the UV/Vis spectra and contact angles of UV-
irradiated films (duration of UV irradiation: 20 s) reversed to
the initial SP state after thermal (1 h at 608C) or photo-
chemical treatment (irradiation at l > 515 nm for 1 h). Con-
currently, the rate of charging became characteristic of the
unirradiated film. Following this reversal from MC to SP,
additional UV irradiation recovered approximately 80% of
the MC absorbance at 595 nm, decreased the advancing and
receding contact angles, and again caused the sphere to
charge negatively without a statistically significant decrease in
the rate of charging from the first cycle. Figure 2 shows an
example of this photochemically reversible charging behavior
with SPMA-MMA. Although these films show fatigue of
photochromism after a few cycles by monitoring the UV/Vis
spectra, we have demonstrated up to three cycles of this
reversible photochemical control of contact electrification
(reversed either thermally or photochemically; see Support-
[8c]
events between the rolling sphere and dielectric surface.
Exposing corresponding homopolymers that did not include
the photochrome SPMA to identical conditions of UV
irradiation yielded no change in the rate or sign of charging.
Table 1 summarizes that in all cases, steel spheres had a
much stronger tendency to charge negatively after UV
irradiation than before. This observation is consistent with
our expectations: based on the apparent dependence of the
[9]
sign of charging on hydrophobicity we anticipated that a
material contacting the hydrophilic MC would have a
stronger tendency to develop negative charge than one
contacting the non-ionic SP. We observed the same behavior
when the experiment was conducted in an atmosphere of N2
or when the SP group was irradiated selectively at 365 nm.
Therefore, neither oxidative decomposition of the photo-
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nitrospiropyran. This approach has three important charac-
teristics: 1) it uses the intrinsic properties of electrically
insulating materials, on which static charge is notoriously
difficult to control, 2) it is reversible, and 3) it measures the
change in charging between two states without moving the
sample, which we expect to yield increasingly accurate
structure–property relationship studies. In addition, our
ability to tune the light-induced switching of charging with
the choice of non-reactive comonomer is an additional,
readily implemented design parameter for using organic
chemistry to control contact electrification. Ongoing work in
our laboratory is focusing on elucidating the effect of polymer
composition, improving fatigue resistance, and demonstrating
applications of this capability to switch the sign and magni-
tude of contact electrification with light, such as electrostatic
self-assembly or actuation.
Received: June 30, 2010
Revised: July 24, 2010
Published online: September 15, 2010
Keywords: contact electrification · electrostatics ·
.
photochromism · polymers · spiropyran
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