Converting an electrical insulator into a dielectric capacitor: End-capping polystyrene with oligoaniline

Article abstract of DOI:10.1021/cm304057f

We report a simple and low-cost strategy to enhance the dielectric permittivity of polystyrene by up to an order of magnitude via incorporating an oligoaniline trimer moiety at the end of the polymer chains. The oligoaniline-capped polystyrene was prepared by a copper-catalyzed click reaction between azide-capped polystyrene and an alkyne-containing aniline trimer, which was doped by different acids. By controlling molecular weight of polystyrene, the end-capped polymers can be induced to form nanoscale oligoaniline-rich domains embedded in an insulating matrix. Under an external electric field, this led to an increase in dielectric polarizability while maintaining a low dielectric loss. At frequencies as high as 0.1 MHz, the dielectric permittivity and dielectric loss (tan δ) were ～22.8 and ～0.02, respectively. This strategy may open a new avenue to increasing the dielectric permittivity of many other commodity polymers while maintaining relatively low dielectric loss.

Full text of DOI:10.1021/cm304057f

Article
pubs.acs.org/cm
Converting an Electrical Insulator into a Dielectric Capacitor: End-
Capping Polystyrene with Oligoaniline
Christopher G. Hardy,† Md. Sayful Islam,‡ Dioni Gonzalez-Delozier,† Joel E. Morgan,§ Brandon Cash,†
Brian C. Benicewicz,† Harry J. Ploehn,*,‡ and Chuanbing Tang*,†
‡
†Department of Chemistry & Biochemistry and Department of Chemical Engineering, University of South Carolina, Columbia,
South Carolina 29208, United States
§Department of Chemistry and Chemical Biology and Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic
Institute, Troy, New York 12180, United States
ABSTRACT: We report a simple and low-cost strategy to enhance the dielectric
permittivity of polystyrene by up to an order of magnitude via incorporating an
oligoaniline trimer moiety at the end of the polymer chains. The oligoaniline-
capped polystyrene was prepared by a copper-catalyzed click reaction between
azide-capped polystyrene and an alkyne-containing aniline trimer, which was
doped by different acids. By controlling molecular weight of polystyrene, the end-
capped polymers can be induced to form nanoscale oligoaniline-rich domains
embedded in an insulating matrix. Under an external electric field, this led to an
increase in dielectric polarizability while maintaining a low dielectric loss. At
frequencies as high as 0.1 MHz, the dielectric permittivity and dielectric loss (tan
δ) were ∼22.8 and ∼0.02, respectively. This strategy may open a new avenue to
increasing the dielectric permittivity of many other commodity polymers while
maintaining relatively low dielectric loss.
KEYWORDS: dielectric polymers, energy storage, oligoaniline, nanodomain, polystyrene
fluorinated comonomers to prepare random fluorinated
copolymers such as poly((vinylidene fluoride)-r-(chlorotri-
fluoroethylene)) (P(VDF-CTFE)).26,28,29,33−37 Though these
random copolymers are capable of high breakdown strength, fast
energy discharge rates, and relatively low dielectric loss (e.g., tan
δ ∼0.02 at 1 kHz), their dielectric permittivity drops sharply at
high frequency.34
Recently, a class of “molecular composites” has been
developed, in which a conductive π-conjugated macromolecule
is directly attached to the polymer backbone.19,38−43 Delocaliza-
tion of electrons across the π-network can produce high
interfacial polarization upon charge displacement, ultimately
resulting in large dielectric responses. The Wang group was the
first to attach oligoaniline octamer moieties to the ends of a
ferroelectric polymer.41 This resulted in a “dumbbell-shaped”
copolymer containing terminal oligoaniline units. The addition
of 10 wt % oligoaniline units increased the dielectric permittivity
from 12 to 85 at 1 kHz. However, the addition of more than 10 wt
% aniline resulted in significant increases in dielectric loss,
presumably because of electron conduction across the film.
Stoyanov et al. prepared a block copolymer in which one domain
was complexed with polyaniline.39 While there were improve-
ments in permittivity (from 2 to 8 at 1 kHz and from 2 to 7 at 1
MHz) between 1.0 and 1.8 wt % polyaniline, additions of above
INTRODUCTION
■
Dielectric polymer film-based capacitors have shown promise in
applications including portable electronic devices, hybrid electric
vehicles, pulse power devices, and energy storage because of their
light weight, low cost, and excellent processability.1−13
Particularly, pulse power devices require accumulating much
energy over a relatively long period of time and releasing it very
quickly, thus increasing the available instantaneous power.14−24
Insulating commodity polymers play an important role in
dielectric capacitors since most have very high dielectric
breakdown strength, high volume availability, and low cost.
Among various dielectric polymers, biaxially oriented poly-
propylene (BOPP) is the industrial standard polymer for
fabrication of capacitors because of its high breakdown strength
(>700 MVm−1) and low dielectric loss (tan δ ∼0.0002 at 1 kHz).
However, BOPP has a low dielectric permittivity (εr = 2.2),
ultimately leading to low energy densities (ca. 1−1.2 J/cm3).25,26
Similarly, many other commodity dielectric polymers including
polystyrene, polyethylene, polyvinyl chloride, and polycarbonate
have high breakdown strength (∼ 450−850 MV/m) and low
dielectric permittivity (εr ∼2.0−5.0 at 1kHz).14,15
There is a crucial need to develop dielectric polymers with high
dielectric permittivity while maintaining low dielectric loss.
Poly(vinylidene fluoride) (PVDF) and its polymer derivatives
have shown great promise since they have both high dielectric
breakdown strength (500−700 MVm−1) and moderate
permittivity (εr = 10−20 at 1.0 kHz).26−33 Recent studies have
focused on modifying the chemical structure of PVDF with bulky
Received: December 19, 2012
Revised: February 7, 2013
Published: February 17, 2013
© 2013 American Chemical Society
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2.0 wt % polyaniline resulted in abrupt increases in conductivity.
At this point complexation of the polyaniline with the polymer
backbone was exhausted, and likely resulted in continuous
conductive pathways across the film. Cui. et al. prepared a
poly(ethylene oxide)-polyoligoaniline alternating copolymer
that contained oligoaniline repeat units in the polymer main
chain.40 The copolymer films showed high dielectric permittivity
(εr ∼ 70 at 1 MHz), but also exhibited extremely high dielectric
loss (tan δ = 2.72).
Scheme 1. Oligoaniline End-Functionalized Polystyrene and
Its Contribution to Increasing Dielectric Permittivity
Clearly, chemically integrating the conductive domain into the
polymer chain inhibits aggregation, thus reducing dielectric loss.
However, the loading content of the conductive domain remains
limited, since increases in the π-conjugated fraction eventually
result in conductivity abruptly increasing to high levels. To
address this issue, we have recently taken a new approach to
developing dielectric materials by creating interfacially domi-
nated polymeric materials based on nanophase-separated block
copolymers.44 While the minor block forms nanodomains with
high dielectric polarizability, the majority matrix block insulates
the conductive domains to avoid percolation and minimize
interdomain conduction. Under an external electric field,
electronic conduction induces nanodipoles along the phase
boundary because of space charge accumulation at the domain
interfaces. Specifically, we prepared a series of diblock
copolymers in which the major fraction was an insulating
poly(methyl acrylate) block while the minor fraction had a side
chain containing a sulfonic acid moiety, which was complexed
with an oligoaniline trimer through supramolecular interactions.
We observed both enhanced dielectric properties (εr = 11 at 1
MHz) and decreased dielectric loss (tan δ = 0.5 at 1 MHz) for the
oligoaniline-complexed diblock copolymer compared to the
uncomplexed diblock copolymer (εr = 5 at 1 MHz and tan δ = 2.7
at 1 MHz). However, this approach was limited, as the sulfonic
acid on the side chain of the block copolymer was the only
possible dopant for oligoaniline.
In this paper, we report a new, simple, and low cost approach
that could be generalized to enhance dielectric permittivity of
many commodity polymers, which has not yet been considered
for high performance dielectric capacitor materials. This
approach is based on capping the ends of polystyrene chains
with oligoaniline through a click reaction between azide-
terminated polystyrene and an alkyne-containing aniline trimer.
The oligoaniline is then doped with various acids, including large
organic acids such as dodecylbenzenesulfonic acid (DBSA) and
camphorsulfonic acid (CSA). Because of the chemical
incompatibility, it is expected that highly polar oligoaniline will
self-assemble into nanoscale domains (i.e., a few nm) dispersed
in a nonpolar polystyrene matrix (Scheme 1). Such highly
polarizable nanodomains would make a positive contribution
toward increasing the overall dielectric permittivity. Indeed, we
observed that a small fraction of oligoaniline increased the
dielectric permittivity of polystyrene by up to an order of
magnitude while the dielectric loss remained low.
chromatography (GPC) was performed at 50 °C on a Varian system
equipped with a Varian 356-LC refractive index detector and a Prostar
210 pump. The columns were STYRAGEL HR1, HR2 (300 × 7.5 mm)
from Waters. HPLC grade dimethylformamide (DMF) with 0.01 wt %
LiBr was used as eluent at a flow rate of 1.0 mL/min. Polystyrene
standards were used for calibration. Mass spectrometry was carried out
on a Waters Micromass Q-Tof mass spectrometer, with a positive ion
electrospray as the ionization source. UV−vis spectroscopy was carried
out on a Shimadzu UV-2450 spectrophotometer, scanning mono-
chromatic light in the range of 190−900 nm. A quartz cuvette with a
path length of 10.00 mm was used, and the solvent was DMF. FTIR
spectra were recorded on a PerkinElmer Spectrum 100 FTIR
spectrometer equipped with a Universal ATR sampling accessory.
Thermal transitions of the polymers were measured by differential
scanning calorimetry (DSC) using a TA Instruments Q2000 in a
temperature range from −70 to 150 °C at heating and cooling rates of 5
°C/min under constant nitrogen flow at a rate of 50 mL/min. Samples
(between 3 and 8 mg) were placed in aluminum hermetic pans and
sealed. The data were collected on the second heating run.
Small-Angle X-ray Scattering (SAXS) data were acquired on a Bruker-
AXS Nanostar-U instrument equipped as follows: copper rotating anode
X-ray source (wavelength, λ = 0.154 nm, 6 KW supply 0.1 × 1 mm
filaments) operated at 50 KV, 24 mA; Montel focusing optic; collimating
assembly of 3 pinholes: (1) 750 μM, (2) 400 μM, and (3) 1000 μM,
spacing (1-to-2) 925 mm, (2-to-3) 485 mm; extended sample chamber
with x-y stage (where the beam is the z axis), secondary beam path
1050−1060 mm; beam path between focusing optic and detector under
vacuum (<0.1 mBar); 2-dimensional detector: Hi-star, multiwire
proportional chamber, 1024 × 1024 pixels; control software: Bruker
SAXS v. 4.1.36; detector flood-field and spatial calibrations use 55Fe
source; sample-to-detector distance calibrated using silver behenate.
Bulk film samples were placed in a hole of copper spacer (1 mm thick)
and then sandwiched between two sheets of Kapton films. The samples
were then placed in the evacuated sample chamber at room temperature
with a typical exposure time of 20 min. Data were integrated over the full
circle of azimuthal angle values in the 2D SAXS scattering images with an
EXPERIMENTAL SECTION
■
Materials. All reagents were purchased from Alfa Aesar and Aldrich
and used as received unless otherwise noted. Styrene was distilled before
use. Hydroxy-terminated oligoaniline trimer (OANI−OH) was
prepared according to a reported procedure.44 Difunctional bromine-
and azide-terminated polystyrene was prepared according to a
procedure previously reported.45
Characterization. 1H NMR (300 MHz) and 13C NMR (100 MHz)
spectra were recorded on a Varian Mercury 300 spectrometer with
tetramethylsilane (TMS) as an internal reference. Gel permeation
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Scheme 2. Synthesis and Doping of Oligoaniline-Functionalized Polystyrene
increment of 0.01 degree in 2θ. Finally, the intensity I(q) was plotted
against q = 4π/λ sin(θ/2).
kHz. The maximum applied field strength ranged from 15 to 300 kV/
cm, depending on film thickness and the sample conductivity. Stored
energy density W = ∫ E dD was determined by numerical integration of
the D-E data.
̂
Films for dielectric characterization were prepared by dissolving
polymer samples in toluene (67 mg/mL) and casting in heavy-gauge
aluminum pans. The solvent was removed by evaporation at 65 °C
under slightly reduced pressure (635 mmHg absolute) for 24 h,
producing films with uniform thickness without solvent bubbles, cracks,
or other defects. Film thicknesses were measured at multiple positions
with a micrometer; measured thicknesses ranged from 2 to 25 μm. Strips
of aluminum pan bearing copolymer films were cut using scissors; the
aluminum pan served as the bottom electrode for dielectric measure-
ments. Circular gold electrodes (area 1.13 cm2) were deposited on the
films’ top surfaces by sputter coating in an argon atmosphere through a
shadow mask.
The films’ complex impedance using an impedance analyzer (Agilent
model 4192A LF).46−48 Measurements were carried out at low applied
voltage (typically 10 mV) and varying frequency (typically 102 to 1.2 ×
107 Hz) for 3−5 specimens of each sample to ensure reproducibility. A
parallel RC circuit model expected to describe a “leaky” capacitor was
used to determine the real and complex parts of the relative permittivity
and the loss tangent from measured values of impedance magnitude and
phase angle.
Synthesis of Oligoaniline-Alkyne (OANI-Alkyne, 2). 5-Hex-
ynoic acid chloride was prepared by heating 5-hexynoic acid (8.0 mL, 73
mmol) in thionyl chloride (8 mL, 110 mmol). After refluxing for 12 h,
the product was collected by vacuum distillation. Hydroxy-terminated
oligoaniline trimer (9.42 g, 34.1 mmol) was dissolved in 30 mL of dry
tetrahydrofuran (THF), and the flask was purged with nitrogen.
Triethylamine (7.1 mL, 51 mmol) was added, and the solution was
cooled to 0 °C. A solution of 5-hexynoic acid chloride (5.48 g, 37.5
mmol) in 10 mL of dry THF was added over 30 min. After stirring at
room temperature overnight, the reaction mixture was filtered and
concentrated to dryness. The solids were dissolved in dichloromethane
and extracted with water twice. The aqueous layers were combined and
extracted with dichloromethane three times. The organic layers were
combined and stirred over anhydrous sodium sulfate. The solution was
filtered, and the filtrate was concentrated to dryness. The resulting solids
were stirred in refluxing hexanes overnight. The red/brown liquid was
filtered, leaving a dark purple solid. The product was collected, vacuum-
dried, and analyzed by NMR, FT-IR, and mass spectrometry. Yield: 10.2
1
Polarization measurements at higher applied voltages were carried
out using a Premier II ferroelectric polarization tester (Radiant, Inc.)
using the same film specimens prepared for impedance testing. Films
made from pure polystyrene (Aldrich, 192,000 g/mol) were also
characterized. Polarization data (D vs E) were acquired for applied
voltages ranging from 1 to 199 V and cycle frequencies of 100 Hz and 1
g, 81.0%. H NMR (2, DMSO-d6, δ): 7.92 (s, 1H, Ph-NH-Ph-NH-),
7.88 (s, 1H, Ph-NH-Ph-NH), 6.65−7.19 (m, 13H, Ph), 2.84 (t, 1H, C
CH), 2.61 (t, 2H, OC(O)CH2), 2.25 (td, 2H, CH2CCH), 1.78 (quin,
2H, CH2CH2CH2). 13C NMR (2, DMSO-d6, δ): 172.21 (CO),
145.28, 143.12, 142.79, 136.95, 129.54, 122.70, 120.16, 120.07, 118.79,
115.96, and 115.52 (Cq of Ph), 84.08 (CH2CCH), 72.39 (CH2C
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CH), 32.79 (COCH2), 23.85 (CH2CH2CH2), 17.58 (CH2CH2C).
FTIR (cm−1): 3388, 3294, 3052, 2965, 2916, 1736, 1600, 1511, 1380,
1310, 1225, 1197, 1167, 1144, 861, 817, 749, 692. MS (EI), m/z calcd for
C24H22N2O2: 370.17; found: 370.
Synthesis of Oligoaniline-Terminated PS (OANI-PS-OANI, 5).
Oligoaniline groups were added onto the end of the PS polymer chains
through a click reaction with oligoaniline-alkyne (2) and the terminal
azide groups from polymer 4. Cu(I)Br (0.1 equiv) was charged into a
round-bottom flask and purged with nitrogen for 30 min. OANI-alkyne
(2, 2 equiv), N3-PS-N3 (4, 1 equiv N3), and PMDETA (0.15 equiv) were
added to a pear shaped flask, dissolved in THF, and bubbled with
nitrogen for 30 min. The mixture in the pear shaped flask was transferred
to the round-bottom flask and stirred at room temperature overnight.
The reaction mixture was concentrated to dryness, dissolved in
dichloromethane, and extracted with water three times. The organic
layer was dried over anhydrous sodium sulfate, filtered, and
concentrated. The solution was then precipitated into methanol two
times. The solid product was collected by filtration and vacuum-dried
overnight. Products 5a and 5b were analyzed by 1H NMR and FTIR. 1H
NMR (3a, CD2Cl2, δ): 6.2−7.2 (br, Ph), 3.28 (br, OC(O)CH2), 3.16,
(br, NtriazoleCH(Ph)CH2), 2.61 (br, CH2CCtriazole), 2.46 (br,
CH2CH2CH2), 1.1−2.4 (br, CH2CHPh). FTIR (cm−1): 3391, 3027,
2923, 2849, 1732, 1659, 1601, 1495, 1451, 1023, 906, 756, 697 .
Doping of OANI-PS-OANI with HCl (6), DDBS (7), and CSA (8).
Fractions (0.2 g) of polymers 5a and 5b were dissolved in dry DMF (3
mL) and passed through microfilters (pore size 0.2 μm). HCl, DBSA, or
CSA (50 equiv per OANI group) was added to the polymer solutions.
Ammonium persulfate (50 equiv per OANI group) was also added to
each solution. The solutions were then stirred at 70 °C for 48 h. Once
cooled, dichloromethane (40 mL) was added, and the mixture was
extracted with deionized water three times. The organic layer was stirred
over anhydrous sodium sulfate, filtered, and concentrated to dryness. A
small sample of each doped polymer was taken for analysis by UV−vis
spectroscopy. The remainder of the sample was dissolved in toluene (3
mL).
1
Figure 1. H NMR spectra of hydroxy-terminated (1) and alkyne-
terminated (2) oligoaniline.
60% monomer conversion to limit coupling termination
reactions and to ensure that all polymer chain ends contained
a bromine atom. Both difunctional PS polymers had low
polydispersity indices (PDI < 1.1). The final molecular weight
could be accurately determined by GPC analysis, as the system
was calibrated using PS standards. The final molecular weight
and PDI are shown in Table 1. The terminal bromine groups on
difunctional PS homopolymers 3a and 3b obtained by ATRP
were converted to azide groups by reaction with sodium azide.45
The transformation from bromide to azide end groups was
confirmed using FTIR, as a sharp band appeared at 2094 cm−1,
which is typical for an azide stretching mode.
To prepare the oligoaniline-terminated PS, a click reaction was
performed on the azide-terminated PS (4a and 4b) with the
oligoaniline-alkyne (2) using copper(I) bromide and PMDETA
in THF. The excess oligoaniline-alkyne and residual copper
bromide were removed by extraction with water followed by
precipitating into a large excess of methanol two times. The
disappearance of the alkyne stretch at 3294 cm−1 and the azide
band at 2094 cm−1, the appearance of a band at 1504 cm−1, which
is typical of a triazole group, and the appearance of a small, broad
peak at 3391 cm−1 from the amine groups of the oligoaniline
confirmed the addition of the oligoaniline onto the PS chain end
(Figure 2).
Doping with Acids. The oxidation states of the oligoaniline
trimer were previously investigated by UV−vis spectrosco-
py.44,50−54 Briefly, when oligoaniline is in the fully reduced form,
only one absorption peak at 310 nm is observed in a solution of
DMF. When an oxidant (e.g., ammonium persulfate) and an acid
dopant are added, the oxidized oligoaniline displays a peak
around 570 nm due to the charge transfer from the benzenoid
ring to the quinoid ring. Additionally, the peak that was at 310 nm
shifts to 301 nm. As these peaks are very prominent, UV−vis was
again used to confirm the oxidation and complexation of
oligoaniline when doping with acids. Polymers 5a and 5b were
doped with HCl (6a and 6b), dodecylbenzenesulfonic acid
(DBSA, 7a and 7b), and camphorsulfonic acid (CSA, 8a and 8b),
as summarized in Table 2. An excess of acid as well as ammonium
persulfate were added to the polymer solution (5a and 5b) and
stirred at 70 °C for 48 h to ensure that all oligoaniline moieties
were oxidized and doped with the corresponding acid. Removal
of excess acids is crucial, as it has been previously shown that any
free acid can result in increased dielectric loss in final dielectric
materials because of ionic conduction.44 To ensure that all excess
free acids and remaining ammonium persulfate were removed,
the polymer solutions were dissolved in dichloromethane and
extracted with deionized water three times. The organic layer was
RESULTS AND DISCUSSION
■
Synthesis of Oligoaniline-Terminated PS. Oligoaniline-
terminated polystyrene (OANI-PS-OANI, 5) was prepared as
outlined in Scheme 2. To add an alkyne-group onto the termini
of the oligoaniline moiety, 5-hexynoic acid was refluxed in oxalyl
chloride, effectively converting the acid group to an acid chloride.
The resulting 5-hexynoic acid chloride was then reacted with the
hydroxy-ended oligoaniline trimer (1) under basic conditions to
give an alkyne-terminated oligoaniline trimer (OANI-alkyne, 2).
The purity of 2 was confirmed by NMR, FTIR, and mass
spectrometry. Besides the appearance of the alkyl chain protons
from the addition of the hexynoic acid group in the proton NMR
between 1.78 and 2.84 ppm, we also observed the disappearance
of the hydroxide proton from compound 1 at 8.91 ppm and a
shift of the amine protons from 7.65 ppm and 7.48 ppm for
compound 1 to 7.92 ppm and 7.88 ppm for compound 2 (Figure
1). FTIR analysis showed the appearance of sharp bands at 3388
and 1736 cm−1 for compound 2, correlating to an alkyne group
and an ester group, respectively. These results from proton NMR
and FTIR, along with mass spectrometry, confirmed that product
2 was successfully prepared through the halide displacement
reaction.
Polystyrene was prepared by atom transfer radical polymer-
ization (ATRP) using a difunctional initiator so that both ends of
the PS would contain bromine atoms.45,49 Specifically, dimethyl
2,6-dibromoheptanedioate was used as the difunctional initiator,
and the molar ratio of [initiator]:[Cu(I)Br]:[PMDETA] was
1:1:1.1. Both high (3a) and low (3b) molecular weight PS
homopolymers were prepared by adjusting the feed ratio of
monomer to initiator. Both polymerizations were stopped below
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Table 1. Preparation of Bromide and Azide End-Functionalized Polymers
a
b
entry
polymer
[monomer]:[initiator]
Mn, g/mol (NMR)
Mn, g/mol (GPC)
PDI (GPC)
3a
3b
4a
4b
Br−PS−Br
Br−PS−Br
N3−PS−N3
N3−PS−N3
480:1
98:1
29,000
6,200
29,800
6,300
1.05
1.04
1.06
1.08
29,800
6,300
a
b
1
Calculated by H NMR using monomer conversion. Calculated by GPC calibrated by polystyrene standards.
Figure 2. FTIR overlay for oligoaniline-alkyne (OANI-alkyne, 2),
bromide- (3b), azide- (4b), and oligoaniline- (5b) end functionalized
polystyrene.
Figure 3. UV/vis spectra for polymers 5b−8b.
then dried over sodium sulfate, filtered, and vacuum-dried. Small
samples of the final doped polymers (6a−8b) were analyzed by
UV−vis. As an example of confirmation of the doping process,
the UV−vis spectra for the lower molecular weight doped
polymers 5b, 6b, 7b, and 8b are shown in Figure 3. Polymers 5a,
6a, 7a, and 8a exhibited similar UV−vis spectra, as listed in Table
2. The oxidation and doping process was clearly observed with
the shift of the π−π* peak from 310 nm to around 300 nm, as
well as the appearance of peaks around 390 and 525 nm.
Dielectric Properties. The dielectric properties of oligoani-
line-capped PS (OANI-PS-OANI), undoped and doped with
various acids, were characterized using impedance spectroscopy
and polarization testing. Impedance measurements yield the
relative permittivity as a function of frequency (Figure 4) for
polymers 3a−8b. The higher molecular weight (∼30,000 g/mol)
Br-terminated polystyrene 3a has a relative permittivity of about
2.7, nearly independent of frequency. Upon converting the end-
group from Br to OANI units, the relative permittivity for
polymer 5a increases to a value of about 3.5, again constant
across the 103−106 Hz frequency range. Polymer 6a, in which the
OANI units are doped with HCl, shows a slight increase in
relative permittivity (εr = 3.9−4.1 between 103−106 Hz)
compared to polymer 5a. In the same frequency range, the
permittivity of polymer 7a (OANI-PS-OANI doped with DBSA)
increases to values between 6 and 9; polymer 8a, (OANI-PS-
OANI doped with CSA) shows greater enhancement of
permittivity with values in the 8.8−12 range.
Lower molecular weight oligoaniline-capped PS polymers
show similar trends but larger enhancement in permittivity. The
lower molecular weight (∼6,000 g/mol) Br-terminated poly-
styrene 3b has a nearly constant permittivity of about 4.3.
Undoped polymer 5b has permittivity value of about 3.6. HCl-
doped polymer 6b has permittivity values around 8; DBSA
doped polymer 7b had permittivity between 13.3 and 20; and
CSA doped polymer 8b had permittivity values between 22.6 and
24.2 across the range of 103−106 Hz. The permittivity of polymer
8b is nearly 1 order of magnitude higher than that of polystyrene
homopolymer, indicating the significant impact of the oligoani-
line chain end when doped by the large organic acid, CSA. The
greater enhancement in permittivity for the lower molecular
weight OANI-PS-OANI polymers can be attributed to their
higher fraction of aniline. The weight percents of the
oligoaniline/acid complex relative to the total molecular weight
of the polymers are summarized in Table 2.
As shown in Figure 5, the loss tangents for all polymers,
including the acid doped polymers, remain below 0.6 across the
range 103−106 Hz. For CSA-doped polymers 8a and 8b with
highest permittivity, the dielectric loss at frequency 0.1 MHz was
Table 2. Preparation of Oxidized and Doped Oligoaniline-Ended PS
a
b
entry
polymer
oxidant
none
dopant
absorbance peaks (nm)
wt % OANI/acid
5a
5b
6a
6b
7a
7b
8a
8b
5a
5b
5a
5b
5a
5b
5a
5b
none
none
HCl
310
1.81%
7.94%
1.93%
8.41%
2.84%
12.02%
2.56%
10.91%
none
310
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
(NH4)2S2O8
300, 395, 524
300, 395, 524
298, 385, 520
298, 385, 520
299, 383, 529
299, 383, 529
HCl
DBSA
DBSA
CSA
CSA
a
b
Values from UV−vis. Calculated assuming complete doping of oligoaniline.
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only 0.05 and 0.02, respectively. This was substantially lower
than other oligoaniline-containing ferroelectric copolymers as
previously reported.39−41
Past experimental and theoretical studies have shown that
bulky organic acids can have a large effect on the conductivity in
polyaniline and oligoaniline films.55,56 A recent study utilizing
density functional theory (DFT) found that organic acid CSA
has much stronger interactions with the nitrogen atoms of
oligoaniline than HCl, resulting in more stable complexes.57 This
suggests that organic acids produce more charge transfer
between the dopant and the oligoaniline complex, allowing for
greater electron transfer, and ultimately enhanced conductivity.
As shown in Figure 6, polymers 7a, 7b, 8a, and 8b, which contain
Figure 4. Relative permittivity versus frequency for polymers (A) 3a−8a
and (B) 3b−8b.
Figure 6. Conductivity versus frequency for (A) polymers 3a−8a and
(B) polymers 3b−8b.
oligoaniline units doped by large organic acids (DBSA and CSA),
display much higher levels of electrical conductivity than HCl-
doped polymers 6a and 6b. Specifically polymers 7a, 7b, 8a, and
8b display conductivities 2 orders of magnitude greater than 6a,b
at low frequencies (103 Hz), and an order of magnitude greater at
high frequencies (106 Hz). These higher levels of conductivity
directly correlate to higher levels of permittivity across the range
of 103−106 Hz (Figure 4).
In addition, DBSA- and CSA-doped polymers 7a, 7b, 8a, and
8b display relative permittivity that decreases noticeably with
increasing frequency. Bulky DBSA and CSA anions complexed
with oligoaniline create relatively large dipoles that may undergo
orientational polarization and contribute to the permittivity,
especially at low frequencies. Orientational polarization might be
responsible for the energy dissipation observed at low
frequencies for polymers 7a, 7b, 8a, and 8b (Figure 5).
Orientational polarization relaxes at higher frequencies (>104
Hz), where the enhanced dielectric responses likely result
Figure 5. Loss tangent (dielectric loss) versus frequency for (A)
polymers 3a−8a and (B) polymers 3b−8b.
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primarily from electronic polarization. Again, organic acids
DBSA and CSA facilitate greater charge separation and local
space charge buildup at the interface between conducting and
insulating segments. These results are consistent with previous
work which utilized large organic acids to dope polyaniline and
oligoaniline-containing polymers to prepare highly conductive
aniline-based films.41,58
Figure 7 shows results from polarization testing at low to
moderate voltages (1−199 V), in contrast to the impedance
results obtained at very low applied voltage (typically 10 mV). As
expected, the polarization curves for PS homopolymer are nearly
linear with low hysteresis. In contrast, all acid-doped OANI-
capped PS polymers show significantly enhanced dielectric
polarization compared to PS homopolymer, as evidenced by the
slopes of the D-E curves in Figure7. This shows that acid-doped
OANI-capped PS polymers have much higher energy storage
capacity than PS homopolymer.
Figure 8 shows the stored energy density of acid-doped OANI-
capped PS relative to that of pure PS measured at the same
electric field polarization. The higher molecular weight polymers
(6a, 7a, 8a), containing 2−3% OANI (Table 2), have stored
energy densities that are 4−8 times higher than that of the PS
homopolymer. For lower molecular weight polymers (7b, 8b),
doped with DBSA or CSA and having 11−12% OANI, the
relative energy densities increase further, to 10−12 times higher
than that of PS. However, the relative energy density of polymer
6b (HCl-doped, 8.41% OANI) decreases relative to that of
polymer 6a, although it is still more than twice as large as the
energy density stored in PS homopolymer at the same applied
field strength. This trend can be seen in Figure 7A, in which the
D-E curve for polymer 6b has a smaller slope than that of
polymer 6a.
The D-E curves in Figure 7 also show that all acid-doped
OANI-capped PS polymers manifest more nonlinearity and
hysteresis than PS homopolymer. In general, the energy loss
percentage (not shown here) increases with OANI content and
maximum applied electric field, and decreases with increasing
polarization cycle frequency.
The significant enhancement of the permittivity of polystyrene
by the chain-end group could be explained by the presence of
oligoaniline-rich domains dispersed in the polystyrene matrix.
The formation of these nanoscale domains would significantly
enhance the interfacial area of highly polarizable nanodipoles.
This hypothesis is further supported by the higher permittivity of
low molecular weight PS compared to that of higher molecular
weight PS when doped with same reagents, as the weight fraction
of oligoaniline plus dopant in the lower molecular weight PS was
in the range of 8−12 wt %, which was sufficient to have nanoscale
phase separation between chain ends and the polystyrene matrix
(Scheme 1). However, this phase separation would be much less
prominent in high molecular weight PS as the weight fraction of
oligoaniline plus dopant was only around 2 wt %, which would
lead to totally disorganized systems. To support this hypothesis,
SAXS measurement was carried out on polymers 6a−8b. For the
high molecular weight polymers 6a, 7a, and 8a, no ordered peaks
were observed, as shown in Figure 9A. Given that polymers 6a,
7a, and 8a had only 2 wt % oligoaniline/acid dopant, these
polymers probably formed homogeneous systems. However, for
the low molecular weight polymers 6b, 7b, and 8b, a weak
correlation peak at the 5 nm length scale (d = 2π/q) was observed
(Figure 9B). Since there were no additional higher order peaks
present, it can be concluded that these polymers did not form
well-ordered nanodomains of oligoaniline/acid dopant complex,
Figure 7. Dielectric polarization versus applied electric field for PS and
OANI-capped PS doped with (A) HCl, (B) DBSA, and (C) CSA. All
measurements carried out with 100 Hz cycle frequency.
but rather disordered domains with broad interfaces between
them and polystyrene matrix. Nevertheless, these results suggest
that these highly polarizable nanodomains led to significant
enhancements in dielectric permittivity.
CONCLUSION
■
In conclusion, we prepared oligoaniline end-functionalized
polystyrene polymers via click chemistry between azide-ended
polystyrene and alkyne-containing oligoaniline. The oligoaniline
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AUTHOR INFORMATION
Corresponding Author
*E-mail: tang4@mailbox.sc.edu (C.T.), ploehn@cec.sc.edu
(H.J.P.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Office of Naval Research (award
N000141110191) and the University of South Carolina.
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