Polyaniline copolymers with solvent-mimic side chains

Article abstract of DOI:10.1002/pola.27648

We investigated new polyaniline copolymers with solvent-mimic side chains for enhanced processability in various solvents. The solvent-mimic side chains, benzyloxypropoxy (BOP), phenoxybutoxy (POB), and dihydroxypropoxy (DHP), were introduced into copolym

Full text of DOI:10.1002/pola.27648

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Polyaniline Copolymers with Solvent-Mimic Side Chains
Seung-Chul Lee, Chan-Keun Jung, Hoon-Joo Jung, Suck-Hyun Lee, O.-P. Kwon
Department of Molecular Science and Technology, Ajou University, Suwon 443–749, Korea
Correspondence to: O.-P. Kwon (E-mail: opilkwon@ajou.ac.kr)
Received 19 January 2015; accepted 24 March 2015; published online 11 April 2015
DOI: 10.1002/pola.27648
ABSTRACT: We investigated new polyaniline copolymers with
solvent-mimic side chains for enhanced processability in vari-
ous solvents. The solvent-mimic side chains, benzyloxypro-
poxy (BOP), phenoxybutoxy (POB), and dihydroxypropoxy
(DHP), were introduced into copolymers and used with nonpo-
lar aromatic and polar alcoholic solvents, respectively. Com-
pared to a polyaniline homopolymer, polyaniline copolymers
with a small amount of side chains (<4 mol %) exhibit different
physical properties, including film-forming ability. This can be
attributed to the solvent-mimic side chains strongly interacting
with the solvent and/or the polyaniline backbone. Especially, in
nonpolar aromatic solvents, polyaniline copolymers with non-
polar aromatic BOP and POB side chains exhibit good film-
forming ability leading to high electrical conductivity, while the
polyaniline homopolymer did not form a film. Therefore, intro-
ducing solvent-mimic side chains in conducting polymers is a
very attractive method of enhancing their processability and
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2015 Wiley Periodicals, Inc. J. Polym.
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KEYWORDS: conducting polymers; conjugated polymers; electri-
cal conductivity; polyaniline; processability; solvent-mimic side
chains; structure-property relations
low electrical conductivity in other solvents.37 In addition, m-
cresol is very toxic and acidic (pKa 5 10)43 and can seriously
damage skin.44 Moreover, in multilayer structured devices,
polar aromatic p-conjugated materials are easily dissolved in
m-cresol having polar phenolic aromatic characteristics.
Therefore, developing polyanilines with high processability
and electrical conductivity in a solvent other than m-cresol
(i.e., nonpolar or nonacidic solvents) is one aim in this field.
INTRODUCTION Recently, solution-processable electronic
devices including flexible electrodes, embedded resistors,
thermosets, and Electro Magnetic Interference (EMI) shield-
ing packages have been in high demand owing to their con-
siderable commercial potential.1–8 They possess advantages
such as low-cost, flexibility, as well as easy and large-area
processability.9–11 Among solution-processable conducting
materials,12–24 organic conducting polymers are very attrac-
tive owing to several advantageous physical proper-
ties.12–20,25,26 For example, conducting polyanilines show
various advantages such as facile synthesis on a large scale
for mass production and also exhibit high electrical conduc-
tivity and high flexibility.27–36 In conducting polymers, their
physical properties and processability strongly depend on
the dopant and solvent combination owing to primary and
secondary doping effects, solubility, and dispersion abil-
ity.37–42 Therefore, to obtain the desired physical properties
and processability, control of the chemical structure of the
conducting polymers together with the dopant-solvent com-
bination is very important.
To improve processability in various solvents, the chemical
structure of polyaniline needs to be modified. However, in
many cases, such approaches showed that the enhanced proc-
essability was compensated by a decrease in electrical con-
ductivity.45–53 For example, polyaniline copolymers with
alkoxy and alkyl substituents exhibited high solubility in vari-
ous common organic solvents. However, with the exception of
protic m-cresol, their electrical conductivity was significantly
lower (<1 S/cm) in other organic solvents.45–48 Up to now,
the effect of different substituents on polyaniline on its electri-
cal performance and processability has not yet been fully opti-
mized. In addition, the incorporation of side chains that have
a chemical structure similar to those of the solvents has rarely
been reported.
A well-known dopant–solvent combination for polyaniline is
protic camphorsulfonic acid (CSA) in polar alcoholic aromatic
solvent m-cresol, which exhibits high electrical conductivity
owing to high solubility and efficient strong secondary doping
effects with extended main-chain conformation.27,37,40,41 How-
ever, with the exception of m-cresol, the dopant CSA exhibited
In this work, we designed and synthesized new polyaniline
copolymers bearing solvent-mimic side chains. As shown in
Figure 1, the polyaniline copolymers BOPA, POBA, and DHPA
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1H NMR (400 MHz, CDCl3): d 7.749 (d, 2H, Ar), 7.302–7.206
(m, 5H, Ar), 5.272 (s, 2H, ACH2), 4.151 (t, 2H, ACH2), 3.486
(t, 2H, ACH2), 2.411 (s, 3H, ACH3), 1.931 (m, 2H, ACH3).
Synthesis of tert-Butyl 2-(3-(benzyloxy)propoxy)
phenylcarbamate
tert-Butyl-2-hydroxyphenylcabamate
was
synthesized
according to literature.39 tert-Butyl-2-hydroxyphenylcarba-
mate was dissolved in THF followed by the addition of
potassium-tert-butoxide. After stirring for 1 h, 3-(benzylox-
y)propyl-4-methylbenzenesulfonate (TsOR1) dissolved in
THF was added dropwise using a dropping funnel, and
then, the solution was stirred for 12 h at 40 ꢀC. Ethyl ace-
tate was added to remove precipitated potassium-tert-
butoxide and the solution was separated using dichlorome-
thane and water. A yellow viscous product was obtained
using column chromatography (ethylene acetate:n-hexane 5
1:4). Yield 5 64%.
FIGURE 1 Newly synthesized polyaniline copolymers with
solvent-mimic side chains.
bear small amounts (<4 mol %) of the solvent-mimic side
chains, benzyloxypropoxy (BOP), phenoxybutoxy (POB), and
dihydroxypropoxy (DHP) groups, respectively. The BOP and
POB side chains were used with nonpolar aromatic solvents
and the DHP side chain was used with polar alcoholic nonar-
omatic solvents. For comparison, unsubstituted polyaniline
(denoted here as “PANI”) was also investigated. We exam-
ined the influence of solvent-mimic side chains on the physi-
cal properties of the copolymers, including their electrical
conductivity. We found that in nonpolar aromatic solvents,
BOPA and POBA copolymers, which have nonpolar aromatic
side chains, exhibit good film-forming ability leading to high
electrical conductivity (>10 S/cm). In contrast, polyaniline
homopolymer PANI did not form a film when an identical
process was followed.
1H NMR (400 MHz, CDCl3): d 8.003 (s, 1H, ANH), 7.261–6.786
(m, 9H, Ar), 4.482 (s, 2H, ACH2), 4.082 (t, 2H, ACH2), 3.588
(t, 2H, ACH2), 2.067 (m, 2H, ACH2), 1.466 (s, 9H, AC(CH3)3).
Synthesis of 2-(3-(Benzyloxy)propoxy)benzene Aminium
Chloride
tert-Butyl 2-(3-(benzyloxy)propoxy)phenylcarbamate was
dissolved in methanol and 4.0 M HCl in 1,4-dioxane was
added. The solution was stirred for 1 h. Next, methanol and
1,4-dioxane were removed under vacuum. Diethyl ether was
added to the solution, which was stirred for 2 h. The precipi-
tated product was dried in a vacuum oven at 95 ꢀC.
Yield 5 90%.
1H NMR (400 MHz, CD3OD): d 7.475–7.05 (m, 9H, Ar), 4.827
(s, 2H, ACH2), 4.285 (t, 2H, ACH2), 3.487 (t, 2H, ACH2),
2.073 (m, 2H, ACH2).
EXPERIMENTAL
Materials and Characterization
All chemicals were purchased from Aldrich or TCI and used
without further purification. 1H NMR spectra were recorded
Synthesis of tert-Butyl-2-(4-
phenoxybutoxy)phenylcarbamate
tert-Butyl-2-hydroxyphenylcarbamate and potassium-tert-
butoxide were dissolved in THF and then the solution was
stirred for 1 h. 4-Bromobutyl phenyl ether was dissolved in
THF and added to the solution by dropwise using a dropping
on
a Varian 400MHz instruments. Chemical shifts are
reported in ppm and are referenced against (CH3)4Si. UV–
vis–NIR absorption spectra were measured with a Jasco V-
670 spectrophotometer. Scanning electron microscopy (SEM)
images were obtained by JSM-6700F. X-ray diffraction (XRD)
spectra were recorded on Ultima III. Elemental analysis (EA)
was performed using LECO CHN-900.
ꢀ
funnel. After stirring for 12 h at 40 C, potassium-tert-butox-
ide was precipitated using ethyl acetate and removed by fil-
tration. The resulting solution was separated using
dichloromethane and water. A yellow viscous product was
obtained using column chromatography (ethylene acetate:n-
hexane 5 1:4). Yield 5 62%.
Synthesis of 3-(Benzyloxy)propyl-
4-methylbenzenesulfonate (TsOR1)
First, 3-benzyloxy-1-propanol (16.7 g, 0.1 mol) was dissolved
in tetrahydrofuran (THF) (250 mL) and then sodium hydride
(60%) (6.0 g, 0.15 mol) was added. The solution was stirred
for 1 h until the solution color changed from yellow to
brown. After adding tosyl chloride, the solution was stirred
for 12 h. Next, the solution was filtered to remove residual
sodium hydride and the THF was evaporated. The residue
was separated using dichloromethane and water. A yellow
product was obtained using column chromatography (ethyl-
ene acetate:n-hexane 5 1:4). Yield 5 80%.
1H NMR (400 MHz, CDCl3): d 8.145 (s, 1H, ANH), 7.095–
6.845 (m, 9H, Ar), 4.063 (t, 4H, ACH2), 2.003 (m, 4H, ACH2),
1.530 (s, 9H, AC(CH3)3).
Synthesis of 2-(4-Phenoxybutoxy)benzene
Aminium Chloride
tert-Butyl-2-(4-phenoxybutoxy)phenylcarbamate was dis-
solved in methanol. After addition of 4.0 M HCl in 1,4-diox-
ane, the solution was stirred for 1 h and concentrated by
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evaporation. Diethyl ether was added to the solution and
then stirred for 2 h. The obtained product was dried using a
ꢀ
vacuum oven at 95 C. Yield 5 90%.
1H NMR (400 MHz, CD3OD): d 7.250–6.885 (m, 9H, Ar),
4.237–4.064 (t, 4H, ACH2), 2.073–2.006 (m, 5H, ACH2,
ANH).
Synthesis of 2-(2,3-Dihydroxypropoxy)phenyl Acetamide
2-(2-Oxiranylpropoxy) acetanilide was synthesized according
to literature.54 Then, 2-(2-oxiranylpropoxy) acetanilide was
dissolved in 2.0 M formic acid. After stirring at 80 ꢀC for
2 h, the solution was concentrated by evaporation and then
purified by reprecipitation using ethanol. Yield 5 77%.
1H NMR (400 MHz, DMSO): d 9.02 (s, 1H, ANH) 8.01, 799
(d, 1H, Ar), 7.05–6.97 (m, 2H, Ar), 6.87, 6.86 (d, 1H, Ar),
5.16, 5.15 (d, 1H, ACH2), 4.72–4.68 (m, 1H, ACH), 4.07, 4.06
(d, 1H, ACH), 3.87, 3.85 (d, 2H, ACH2), 2.08 (s, 3H, ACH3).
Synthesis of 2-(2,3-Dihydroxypropoxy)benzene Aminium
Chloride
FIGURE 2 Synthetic route of monomers for (a) BOPA and
First, 2-(2,3-dihydroxypropoxy)phenyl acetamide was added
to water followed by 4.0 M aqueous HCl. Then the solution
was stirred at 70 ꢀC for 24 h. The resulting solution was
evaporated and reprecipitated using ethanol. A white prod-
uct was obtained with a yield of 83%.
POBA and (b) DHPA polyaniline copolymers.
strongly depend on the solvent characteristics as well as the
dopant.27,37,40,41 Here, solvent-mimic side chains with a
chemical structure similar to that of the solvent were incor-
porated into polyaniline copolymers. The nonpolar aromatic
BOP and POB side chains in the BOPA and POBA copolymers,
respectively, were chosen for nonpolar aromatic solvents,
typically toluene. The polar nonaromatic DHP side chain
with diol groups in the DHPA copolymer was chosen for
polar nonaromatic alcoholic solvents, typically methoxyetha-
nol (MeEtOH). The synthetic route of substituted monomers,
2-(3-(benzyloxy)propoxy) benzene aminium chloride for
BOPA copolymer, 2-(4-phenoxybutoxy)benzene aminium
chloride for POBA copolymer, and 2-(2,3-dihydroxy propox-
y)benzene aminium chloride for DHPA copolymer, is shown
in Figure 2.
1H NMR (400 MHz, D2O): d 7.33–7.28 (m, 1H, Ar), 7.25, 7.23
(d, 1H, Ar), 7.04, 7.01 (d, 1H, Ar), 6.96–6.92 (m, 1H, Ar),
4.11–4.03 (br, 1H, ACH), 4.01–3.99 (br, 2H, ACH2), 3.64–
3.54 (br, 2H, ACH2).
Synthesis of New Polyaniline Copolymers
The polyaniline copolymers were synthesized by using the
self-stabilized dispersion polymerization (SSDP) method.27,28
Aqueous 4 M HCl and chloroform were added to the reaction
and then the temperature was maintained at 0 ꢀC. After 30
min, the aniline monomer and substituted monomer bearing
solvent-mimic side chains were dissolved in the heterogene-
ous biphasic solution. The solution was stirred for 30 min to
make a uniform emulsion. Next, ammonium persulfate (APS)
dissolved in aqueous HCl was added dropwise using a drop-
ping funnel over 2 h. After being stirred for 12 h, the result-
ing solution was filtered and washed using water and
methanol. The filtered polyaniline copolymers doped with
HCl, which is emeraldine salt (ES) form, were dried in a vac-
uum oven. The dedoping process was performed using an
ammonium hydroxide aqueous solution. After 24 h, the poly-
aniline copolymer, which is emeraldine base (EB) form, was
filtered and then dried in a vacuum oven.
For the synthesis of polyaniline copolymers BOPA, POBA,
and DHPA, we used the SSDP method with a biphasic system
comprising CHCl3 and water [see Fig. 3(a)].27,28,39 This
method was used to inhibit an ortho-coupling reaction and
reduce structural defects.27,55 In polyaniline copolymers, the
feeding ratio of substituted monomer to unsubstituted ani-
line monomer is <4 mol %. Increasing the molar ratio of the
substituted monomer increases its solubility (and also proc-
essability and film-forming ability), but also decreases the
electrical conductivity.39,45–53 The polyaniline copolymers
here denoted by BOPAm, POBAm, and DHPAm, in which m
presents the feeding ratio of substituted monomers (see
Table 1 and Fig. 1). For BOPA2 copolymers, the feeding
molar ratio aniline:2-(3-(benzyloxy)propoxy)benzene ami-
nium chloride is 100:2.0. For POBA2 and POBA4 copolymers,
the feeding molar ratio aniline:2-(4-phenoxybutoxy)benzene
aminium chloride is 100:2.0 and 100:4.0, respectively. For
DHPA0.5 and DHPA1 copolymers, the feeding molar ratio
RESULTS AND DISCUSSION
Design and Characterization of New Polyaniline
Copolymers
Figure 1 shows the chemical structure of newly synthesized
polyaniline copolymers BOPA, POBA, and DHPA. In conduct-
ing polymers, their electrical conductivity in relation to pri-
mary and secondary doping effects and processability
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aniline:2-(2,3-dihydroxy propoxy)benzene aminium chloride
is 100:0.5 and 100:1.0, respectively. For comparison, unsub-
stituted polyaniline PANI was also synthesized under similar
reaction conditions.
In the biphasic system, polyanilines were mainly polymer-
ized at the interface between the CHCl3 and water phases, as
reported in Refs. [27 and 28. The oxidant APS created radi-
cals in water phase due to significantly higher solubility of
APS in water compared to organic phase.28 In addition, the
aniline monomers and substituted monomers locate at the
interface as shown in Figure 3(a).28 The nonpolar BOP and
POB and highly polar DHP side chains can act as self-
stabilized agents in nonpolar/polar biphasic systems during
polymerization. To confirm this hypothesis, the morphology
of polyaniline copolymer powders with ES form doped with
HCl, prior to the dedoping process, were measured by SEM.
As shown in Figure 3(b), all five polyaniline copolymers
exhibited thin skin morphology with a balloon shape. In
addition, the surface in contact with the organic nonpolar
CHCl3 phase is very smooth, while the surface in contact
with the polar aqueous phase is relatively rough. Interest-
ingly, compared to BOPA and POBA, which has nonpolar side
chains, the morphology of DHPA copolymers containing
highly polar side chains exhibited an increasingly well-
developed balloon shape [see left SEM images in Fig. 3(b)].
This can be attributed to differences in the amphiphilicity of
the side chains; that is, compared to the BOP and POB side
chains, DHP side chains are more polar and can stabilize the
interfaces more. This result could also be attributed to the
large difference between molar ratio of substituted mono-
mers in the copolymers and feeding ratio in reaction; this is
discussed later.
The physical properties of the synthesized polyanilines are
listed in Table 1. As-synthesized polyaniline copolymer pow-
ders have a nonconducting EB form. To analyze the molar
ratio of the substituted monomer to the unsubstituted ani-
line monomer in as-synthesized polyanilines, we carried out
EA of C, H, and N.39
The molar ratio of the substituted monomer in BOPA2 is
smaller than the feeding ratio, that is, unsubstituted aniline
monomer:substituted monomer 5 100:1.3 (see Table 1). In
contrast, the molar ratio of the substituted monomer in DHPA
copolymers is higher, with the ratio of unsubstituted aniline
monomer:substituted monomer at 100:3.5 for DHPA0.5 and
100:14 for DHPA1. This is due to nonpolar BOP side chains
preferring to be located in the nonpolar organic layer, while
the polar DHP side chains prefer the polar aqueous layer,
resulting in a higher stabilizing effect [see Fig. 3(a)]. This
agrees well with the morphology observed in the SEM images
in Figure 3(b). As shown in Figure 3(b), a high molar ratio of
substituted monomer having polar side chains in DHPA
copolymers resulted in an increasingly well-developed bal-
loon shape compared with low molar ratios of the substituted
monomer having nonpolar side chains in BOPA copolymers.
FIGURE 3 (a) Schematic diagram of the self-stabilizing effect of
monomers with side chains in a biphasic water/chloroform sys-
tem. (b) SEM images of as-synthesized particles of polyaniline
copolymers doped with HCl: from top to bottom BOPA2,
POBA2, POBA4, DHPA0.5, and DHPA1. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
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TABLE 1 Summary of the Physical Properties of Polyanilines
Feeding
Ratio
Substituted
Inherent
Viscosity
Conductivity,
CSA/m-Cresola,b (S/cm)
Ratio (n:m)
Conductivity (S/cm)
BOPA2
POBA2
POBA4
DHPA0.5
100:2
100:2
100:4
100:0.5
100:1.3
100:6.9
100:11
100:3.5
1.17
1.90
1.78
0.82
330 (25 ꢀC)
192 (25 ꢀC)
171 (25 ꢀC)
87 (25 ꢀC)
138 (60 ꢀC)
108 (70 ꢀC)
24 (25 ꢀC)
28 (70 ꢀC)
307 (25 ꢀC)
11 (DBSA/toluene)a
15 (DBSA/toluene)a
2.8 (DBSA/toluene)a
1.0 (AMPSA/MeEtOH)c
DHPA1
PANI
100:1
–
100:14
–
1.24
1.39
0.6 (AMPSA/MeEtOH)c
5.0 (DBSA/toluene)c
2.6 (AMPSA/MeEtOH)c
223 (60 ꢀC)
a
c
Casting film using the four-line probe method.
Stirring temperature of solution.
Pellet using the four-point probe method.
b
Similar to the DHPA copolymers, POBA copolymers exhibit
higher molar ratio of the substituted monomer relative to
unsubstituted aniline monomer in copolymer than feeding
ratio in reaction: unsubstituted aniline monomer:substituted
monomer 5 100:6.9 for POBA2 and 100:11 for POBA4. The
reason of higher molar ratio in POBA copolymers is also
attributed to the polarity of side chains. The POB side chain
possesses higher polarity than BOP side chain due to higher
polarity of phenoxy group at the end of POB side chain than
benzyloxy group at the end of BOP side chain. However, the
POB side chain is still less polar than DHP side chain. As the
order of polarity of side chains is BOP < POB < DHP, the
molar ratio of substituted monomer relative to unsubstituted
aniline monomer also shows identical order with polarity
(i.e., BOPA < POBA < DHPA, see Table 1).
polymerization, which may result in a low molecular weight.
Inherent viscosity related to molecular weight was measured
in H2SO4 solution at 30 C. As listed in Table 1, the inherent
viscosity of BOPA, POBA, and DHPA copolymers is relatively
high, similar to that of PANI. Therefore, the steric hindrance
of solvent-mimic side chains in BOPA, POBA, and DHPA does
not play a crucial role during polymerization owing to the
high flexibility of the BOP, POB, and DHP side chains.
ꢀ
Small amounts of solvent-mimic BOP, POB, and DHP side
chains in the copolymers do not strongly affect their thermal
stability and electronic structure. The thermal stability of
BOPA and DHPA copolymers was confirmed by thermogravi-
metric analysis (TGA). As shown in Figure 4, the initial
weight loss temperature of both BOPA and DHPA EB forms
is over 400 ꢀC, which is similar to that of unsubstituted
PANI. The POBA copolymers also exhibit high stability with
Introducing large side chains in aniline monomers can result
in large steric hindrance between the monomers during
ꢀ
high initial weight loss temperature of over 300 C.
FIGURE 5 UV–vis–NIR absorption spectra of PANI and polyani-
line copolymers (EB form) in NMP. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.com.]
FIGURE 4 TGA curves of PANI and polyaniline copolymers (EB
form). [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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perature. The conducting ES films were prepared on glass
using a casting method. Electrical conductivity measure-
ments were carried out at room temperature using a four-
line probe method. Four gold-wire electrodes were attached
to the ES films using carbon paste with a spacing of 1 mm.
As listed in Table 1, BOPA2 copolymers with CSA/m-cresol
exhibited an electrical conductivity of 330 S/cm, which is
similar to the electrical conductivity of unsubstituted PANI
synthesized under similar experimental conditions (307 S/
cm). The reason for the high electrical conductivity of BOPA2
could be the small amount of BOP side chains (1.3 mol %).
This is also supported by the UV–vis–NIR absorption spectra.
As shown in Figure 6(a), both BOPA2 and PANI exhibited a
well-developed free-carrier tail at the NIR–IR region. XRD
patterns of polyaniline films doped with CSA in m-cresol are
shown in Figure 7. As expected, both BOPA2 and PANI films
exhibit high crystallinity; strong diffraction peaks at
2h 5 ꢁ15ꢀ, 20ꢀ, and 25ꢀ. The intensity of the diffraction
peak at around 2h 5 25ꢀ (ꢁ3.5 Å) is related to strong p–p
stacking of the polyaniline backbone.28,37
In POBA copolymers, the electrical conductivity decreases
with increasing molar ratio of substituted monomer: 192 S/
cm for POBA 2 and 171 S/cm for POBA4 (see Table 1). The
electrical conductivity of POBA copolymers is lower than
BOPA copolymer. This is supported by the UV–vis–NIR
absorption spectra and XRD patterns. Compared to BOPA
copolymer, POBA copolymers exhibit lower absorption of
free-carrier tail at the NIR–IR region as shown in Figure 6(a)
and lower crystallinity as shown in Figure 7. However, the
electrical conductivity of POBA2 copolymers is still in the
same order of electrical conductivity of PANI and BOPA.
FIGURE 6 UV–vis–NIR absorption spectra of polyaniline solu-
tions doped with CSA in m-cresol. The solution was stirred at
room temperature or at 60 ꢀC (70 ꢀC). [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
In contrast, compared to PANI, DHPA copolymers, which
have polar DHP diol side chains, exhibit a lower electrical
Figure 5 shows the UV–vis–NIR absorption spectra of polya-
niline copolymers and PANI in N-methylpyrrolidone (NMP)
solution. The wavelengths of the absorption maximum in the
substituted copolymers BOPA and DHPA are practically iden-
tical with those of PANI: 328 and 642 nm for BOPA2, 326
and 634 nm for DHPA0.5 and DHPA 1, and 328 and 638 nm
for PANI. The wavelengths of the absorption maximum in
POBA copolymers are also similar with slight blue shift: 320
and 611 nm for POBA2, and 314 and 595 nm for POBA4.
Therefore, small amounts of solvent-mimic BOP, POB, and
DHP side chains do not significantly affect the p-conjugated
characteristics and thermal stability of polyaniline.
Influence of Side Chains on Physical Properties
The interactions between polyaniline, solvents and dopants
have a significant influence on the chain conformation of pol-
yaniline.37,40,41,56 First, newly synthesized polyaniline copoly-
mers were doped with
a well-known dopant–solvent
FIGURE 7 X-ray diffraction patterns of polyaniline films doped
with CSA in m-cresol. For preparation of films, the solution
was stirred at room temperature or at 60 ꢀC (or 70 ꢀC). [Color
figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
combination: CSA and m-cresol. For preparation of conduct-
ing ES films, polyaniline EB forms were ground with CSA
dopant for 30 min in the solid state. The ground powder
was added to m-cresol and stirred for 7 days at room tem-
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FIGURE 8 Schematic illustration of BOPA copolymer in toluene solution. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
conductivity (87 S/cm for DHPA0.5 and 24 S/cm for DHPA1,
see Table 1). As shown in Figure 6(b), the UV–vis–NIR
absorption spectra show not only a free-carrier tail at the
NIR–IR region but also a localized polaron band at NIR
region (around 730–1100 nm) for both DHPA copolymers.
The XRD pattern shows that compared to PANI, DHPA copol-
ymer films exhibit a lower crystallinity. In DHPA0.5 and
DHPA1 prepared at room temperature, the intensity of dif-
fraction peaks at 2h 5 ꢁ15ꢀ, 20ꢀ, and 25ꢀ is smaller than
that of PANI (see Fig. 7).
decreases with increased stirring temperature with values of
307 S/cm for room temperature and 223 S/cm for 60 ꢀC.
Therefore, strong hydrogen bonding occurs with the highly
polar diol group in the DHP side chains significantly inhibi-
ting the doping process.
To expand this hypothesis, DHPA1 copolymers with an
increased number of highly polar DHP side chains
(n:m 5 100:14) were also examined. In contrast to the
DHPA0.5 copolymer, the effect of stirring temperature was
not significant in the DHPA1 copolymer. The electrical con-
ductivity of the DHPA1 films prepared at both stirring tem-
peratures (room temperature and 70 ꢀC) is very similar:
24 S/cm for room temperature and 28 S/cm for 70 ꢀC. As
shown in Figure 7, XRD patterns of the DHPA1 films pre-
pared at both stirring temperature (room temperature and
The reason for the low electrical conductivity of DHPA
copolymers may be related to the formation of strong hydro-
gen bonds between the highly polar diol groups in the DHP
side chains and polar polyaniline backbone; this results in an
inhibition of the doping process. To confirm the hydrogen-
bond effect, DHPA copolymers were doped at temperature
above room temperature. DHPA0.5/CSA ground powder was
ꢀ
70 C) are also very similar. It can be seen that the hydrogen
bonds between large amounts of the highly polar diol group
(14 mol %) and the polyaniline backbone are very strong
and increasing the stirring temperature could not break the
hydrogen bonds.
ꢀ
stirred at an elevated temperature (60 or 70 C) in m-cresol.
The electrical conductivity of the films was measured at
room temperature. As listed in Table 1, DHPA0.5 copolymers
prepared at higher temperature exhibited higher electrical
conductivity; 87 S/cm for 25 ꢀC, 138 S/cm for 60 ꢀC, and
108 S/cm for 70 ꢀC. This electrical conductivity behavior of
DHPA0.5 agreed with the absorption spectra shown in Figure
6(b). Compared to DHPA0.5 prepared at room temperature,
As processability and doping efficiency of polyanilines
strongly depend on the dopant–solvent combination, newly
synthesized BOPA, POBA, and DHPA copolymers were inves-
tigated in different dopant–solvent combinations. The sol-
vents and dopants were chosen to have chemical structures
similar to those of the side chains. For BOPA and POBA hav-
ing nonpolar aromatic side chains BOP and POB, respectively,
nonpolar aromatic solvent toluene and dopant dodecylbenze-
nesulfonic acid (DBSA) having nonpolar aromatic dodecyl-
phenyl ring were used. Figure 8 shows schematic illustration
of the combination of BOPA/DBSA in toluene. For DHPA hav-
ing polar nonaromatic DHP side chain having diol groups,
polar nonaromatic alcoholic solvent MeEtOH and dopant 2-
acrylamido-2-methyl-1-propane sulfonic acid (AMPSA) hav-
ing a polar nonaromatic amide group were used. In many
reports, an excess amount of dopant (i.e., a molar ratio of
aniline:sulfonic acid > 2:1) also acts as a dispersion agent,
providing an increase in electrical conductivity and
ꢀ
DHPA0.5 prepared at higher temperature (60 C) exhibited a
decreasing localized polaron band at NIR region (around
730–1000 nm).
As shown in Figure 7, the XRD patterns show similar behav-
ior; the crystallinity of a DHPA0.5 film prepared at high tem-
ꢀ
perature (60 C) significantly increases compared to the one
prepared at room temperature. The intensity of the diffrac-
tion peak at around 2h 5 25ꢀ, which is related to the p–p
stacking of the polyaniline backbone, increases with stirring
temperature. For comparison, PANI was also examined under
the same conditions. In contrast to DHPA0.5, PANI films pre-
pared at a higher temperature did not exhibit increased elec-
trical conductivity; in this case, the electrical conductivity
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tion of BOPA and POBA copolymers, DHPA copolymers and
PANI homopolymers exhibited poor film-forming ability with
both DBSA/toluene and AMPSA/MeEtOH. After casting the
solution, only BOPA and POBA copolymers doped with DBSA
in toluene formed good-quality films with a large size, while
DHPA and PANI formed powder-like films with cracks. The
high film-forming ability of BOPA and POBA copolymers is
attributed to solvent-mimic side chain BOP and POB groups.
The nonpolar aromatic BOP and POB side chains have chemi-
cal structure similar to that of toluene and the DBSA dopant
has a nonpolar aromatic phenyl ring (see Fig. 8). The interac-
tions between side chains (BOP and POB), toluene, and the
dopant DBSA may result in increased solubility and leading
to good film-forming ability.
The electrical conductivity of BOPA and POBA films prepared
by the casting method was measured using a four-line probe
method. With the exception of BOPA and POBA, DHPA and
PANI with DBSA/toluene and AMPSA/MeEtOH did not form
films. Because of poor film-forming ability, a four-point probe
method was used to measure the electrical conductivity of
DHPA and PANI, whose pellets were prepared by pressing
the powder-like films. The electrical conductivity of the
investigated polyanilines in DBSA/toluene and AMPSA/
MeEtOH is listed in Table 1. As expected from the film-
forming ability above, BOPA2 and POBA2 films with DBSA/
toluene exhibit the highest electrical conductivity (11 and
15 S/cm, respectively), while DHPA pellets with AMPSA/
MeEtOH exhibit the lowest electrical conductivity (<1.0 S/
cm). As shown in Figure 9, this trend of electrical conductiv-
ity is also observed in the UV–vis–NIR absorption spectra; a
free-carrier tail is well developed in the BOPA and POBA
films, while the localized polaron band at 800 nm is domi-
nant in DHPA. Chemical modification of polyanilines with
alkoxy and alkyl substituents for increasing processability
results in low electrical conductivity (<1 S/cm) in many
cases45–48; the electrical conductivity of the BOPA2 and
POBA2 films with DBSA/toluene (>10 S/cm) is relatively
high. As well as good film-forming ability, the high electrical
conductivity of BOPA2 and POBA2 is very promising.
FIGURE 9 UV–vis–NIR absorption spectra of polyaniline solu-
tions doped with (a) DBSA in toluene and (b) AMPSA in
methoxyethanol (MeEtOH). [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
processability compared to doping conditions with molar
ratio of aniline:sulfonic acid 5 2:1.57–59 However, owing to
the excess amount of dopant, additional processes for elimi-
nating residual dopants are needed and often exhibit
decreased mechanical stability (i.e., decreasing hardness and
easy gelation).60,61 Here, the molar ratio of dopant to aniline
(aniline:sulfonic acid 5 2:1) was used. The ES solution was
prepared by adding ground powder of EB/dopant into the
corresponding solvent; the solution was stirred for 7 days at
room temperature. The conducting ES films are prepared by
either using a casting method on glass or a pelletizing
In large-scale industrial production, basket milling is a
widely used mixing technique. Therefore, we also examined
BOPA2 and POBA2 copolymers using the basket milling pro-
cess. In a basket mill, the ground powder of BOPA2 EB and
DBSA was mixed at 2000 rpm with a bead (0.8 mm diame-
ter) for 5 h in toluene. For POBA2 EB/DBSA, xylene was
used. BOPA2 and POBA2 casting films, prepared by basket
milling, also exhibit good film-forming ability (see Fig. 10)
and maintain high electrical conductivity (11 S/cm for
BOPA2 and 14 S/cm for POBA2).
method. For measuring electrical conductivity,
a four-
line probe method was used for casting films and four-
point probe method for pellets. For comparison, unsubsti-
tuted PANI was also examined in both solvent–dopant
combinations.
Although both BOPA, POBA, and DHPA copolymers possess
solvent-mimic side chains for the corresponding solvents and
dopants, the processability and electrical conductivity behav-
iors are different. As discussed above, BOPA and POBA
exhibited good film-forming ability with high electrical con-
ductivity, while DHPA exhibited poor film-forming ability
with low electrical conductivity. In DHPA copolymer films
The physical properties of the investigated polyanilines with
DBSA/toluene and AMPSA/MeEtOH are listed in Table 1.
Prepared by the casting method, BOPA2, POBA2, and POBA4
copolymers doped with DBSA in toluene exhibited good film-
forming ability, which is essential for various applications
that require solution-based films. In contrast, with the excep-
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ability, leading to high electrical conductivity, while the poly-
aniline homopolymer did not form a film under identical
conditions. In contrast, owing to the strong interaction of
polar DHP side chains with the polyaniline backbone, DHPA
copolymers exhibited low processability and low electrical
conductivity compared to PANI. Therefore, introducing
solvent-mimic side chains is a very attractive method for
enhancing processability and other physical properties.
ACKNOWLEDGMENTS
This work has been was supported by the National Research
Foundation of Korea (NRF) grant funded by Ministry of Science,
ICT and Future Planning (No. 2014R1A5A1009799) and the
Fundamental R&D Program for Core Technology of Materials
funded by the Ministry of Trade, Industry and Energy, Republic
of Korea.
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