P py 2020

R E V I E W
Synthesis and factor affecting on the conductivity
of polypyrrole: a short review
Ai Ling Pang1
| Agus Arsad1
| Mohsen Ahmadipour2
1
UTM-MPRC Institute for Oil and Gas, School
of Chemical and Energy Engineering, Faculty
of Engineering, Universiti Teknologi Malaysia,
Johor Bahru, Johor, Malaysia
2
School of Materials and Mineral Resources
Engineering, Universiti Sains Malaysia,
Engineering Campus, Nibong Tebal, Pulau
Penang, Malaysia
Correspondence
Agus Arsad, UTM-MPRC Institute for Oil and
Gas, School of Chemical and Energy
Engineering, Faculty of Engineering, Universiti
Teknologi Malaysia, 81310 UTM Johor Bahru,
Johor, Malaysia.
Email: agus@utm.my
Abstract
Polypyrrole (PPy) has unique features such as easy synthesis, environmental stability,
and high electrical conductivity (approximately 105 S/cm and even >380 S/cm) for
bulk and thin-film materials. Thus, PPy is applied in numerous well-established appli-
cations, such as in sensors, supercapacitors, and resonators. These applications take
advantage of the unique properties achieved through the structure and properties of
PPy. This article comprehensively elaborates the methods used to synthesize conduc-
tive PPy, along with the important factors affecting its conductivity. Emphasis is
given to versatile and basic approaches that enable control of the microstructural fea-
tures that eventually determine PPy conductivity. Despite the intensive research in
this area, no previous study has presented all possible relevant information about
PPy fabrication and the important factors influencing its electrical conductivity.
K E Y W O R D S
dopant, electrical conductivity, oxidant to monomer ratio, polypyrrole, synthesis method
1 | INTRODUCTION
1.1 | Conducting polymers
Most polymers are generally good insulating materials. Nevertheless,
the emergence of conducting polymers (CPs) in 1976 has developed
enormous opportunities for the development of polymers with unique
properties.1,2
CPs are organic materials that exhibit mechanical and
physical properties related to conventional polymers and the unique
electrical properties of metals.3-5
CPs belong to a class of polymers
that can inherently conduct electricity attributed to conjugation
structures (ie, having π-electron delocalization) in their polymeric
backbone.6-9
CPs possess several advantages, such as corrosion resistance,
easy synthesis, good mechanical and optical properties, high flexibility
in preparation, and tunable conductivity.1,2,10
Moreover, the light-
weight of CPs enables them to be used as replacement for metals,
particularly in weight-sensitive applications.4
Examples of CPs include
polyaniline (PANI), polypyrrole (PPy), poly(3,4-ethylene
dioxythiophene) (PEDOT), polythiophene, polyacetylene, and poly-
phenylene.10-12
The chemical structures of a few CPs are presented in
Figure 1. Generally, PPy is one of the most commonly used CPs
because of its easy synthesis, unique electrical conductivity, and envi-
ronmental stability.14-18
1.2 | PPy
PPy is a heterocyclic and positively charged conducting polymer
(CP) that contains nitrogen in its oxidized form, and it loses its conduc-
tivity and charge when overoxidation occurred.14,19
Besides, PPy is
electroactive in organic electrolyte and aqueous solutions.13
Further-
more, PPy is known for its nontoxic and biocompatibility,20,21
and it is
utilized in wide-ranging applications (eg, supercapacitors/electrodes,
nanocomposites, gas sensors, biosensors, drug delivery, protective
clothing, anticorrosion coatings, actuators, and adsorbents for the
removal of heavy metals and dye.[19,22-25]
) Figure 2 illustrates some of
the general forms of PPy and their applications. The data in Figure 3A
clearly show the progress of research on PPy and the increased num-
ber of publications providing insights into the recognition of PPy as an
important and extensively explored CPs. Meanwhile, Figure 3B–G
show the scanning electron microscope (SEM) micrographs of PPy
Received: 25 November 2020 Revised: 15 December 2020 Accepted: 15 December 2020
DOI: 10.1002/pat.5201
1428 © 2020 John Wiley & Sons Ltd Polym Adv Technol. 2021;32:1428–1454.
wileyonlinelibrary.com/journal/pat

with different morphologies, such as nanoparticle,26
nanotube,27
nanofiber,28
thin film,29
thick film,30
and sponge-like structure,31
respectively. Despite the extensive research on PPy, to the best of
our knowledge, no comprehensive study on the advances made in the
synthesis of PPy and the factors affecting PPy conductivity has been
conducted.
1.3 | Chemistry of PPy
The monomer units in PPy chains are primarily bonded at α-α posi-
tions (Figure 4A); nevertheless, a minimum amount of pyrrole
(Py) monomer is bonded at α-β and β-β positions (Figure 4B,C).17
PPy
is conductive because of the alternating single and double bonds,
which lead to a certain delocalization of electron density in the mole-
cule.17
However, in its undoped state (neutral), PPy is an insulator
with a large band gap of approximately 3.16 eV.1
Chemical or electro-
chemical doping can be used to enhance PPy conductivity. During
doping, PPy is oxidized and a π-electron is removed from the neutral
PPy chain, changing its structure from the benzenoid structure (aro-
matic) to a quinoid form (Figure 4D,E). Subsequently, a polaron forms
(Figure 5B), and with further oxidation, a second electron is eliminated
from the PPy chain that leads to the formation of a doubly charged
bipolaron (Figure 5C). After doping, PPy is converted into an ionic
complex consisting of cations and incorporated counterions.1,17
2 | SYNTHESIS OF PPY
PPy can be easily prepared in solution and shows unique electrical
conductivity, good stability at room temperature, and unique redox
property.32,33
PPy could be synthesized based on the oxidative
FIGURE 1 The chemical
structures of a few CPs.13
Reproduced by permission of Elsevier
FIGURE 2 General forms of PPy
and their applications
PANG ET AL. 1429

polymerization of Py monomer in organic solvents (acetonitrile and
propylene carbonate) and aqueous medium (water and acid solution)
in the presence of iron (III) chloride (FeCl3) ammonium persulfate
(APS) or other oxidizing agents.34,35
Oxidative polymerization is reg-
arded as the oldest polymerization reaction utilized to obtain
π-conjugated polymers.7
Oxidative polymerization may be performed
chemically, electrochemically, or through ultrasonic waves.35,36
The
most commonly used synthesis methods for PPy are chemical and
electrochemical polymerization.37,38
Recently, the ultrasonic-irradia-
tion-assisted polymerization of PPy has also gained research inter-
est.39-41
Other synthesis methods for PPy include vapor-phase
polymerization (VPP), electrospinning, microemulsion polymerization,
mechanochemical polymerization, and photopolymerization. Figure 6
displays the outline of the synthesis methods of PPy. Still, chemical
oxidative polymerization is the most preferred way for industrial
applications because of its capability to produce a massive quantity of
PPy and various PPy structures (ie, nanoparticles, nanofibers,
etc.).42,43
Table 1 presents the various routes for PPy synthesis,
whereas the advantages and disadvantages of PPy synthesis method
are shown in Table 2. Among the synthesis methods of PPy,
ultrasonication polymerization is preferred because of its short reac-
tion time, cost-effectiveness, scalability, and capability to fabricate
homogenous PPy with uniform shape, size, and good
conductivity.40,58,73
2.1 | Chemical oxidative polymerization
PPy synthesized through chemical oxidative polymerization is an easy
and fast process using simple instruments.37,74
Bulk quantities of PPy
FIGURE 3 (A) Annual number of scientific publications related to PPy based on Elsevier data analysis as of 7th September 2020, and (B–G)
SEM micrographs of PPy with different morphologies, that is, nanoparticle,26
nanotube,27
nanofiber,28
thin film,29
thick film,30
and sponge-like
structure,31
respectively. Reproduced by permissions of The Royal Society of Chemistry and Elsevier
FIGURE 4 (A–C) The possible monomer units bonding in PPy, (D) benzenoid and (E) quinoid forms of PPy.17
Reproduced by permission of
Elsevier
1430 PANG ET AL.

fine powders can be obtained by chemical oxidative polymeriza-
tion.10,42
Generally, Py polymerization occurs through the oxidation of
Py monomer by chemical oxidants in aqueous and nonaqueous media.
The common procedure in the chemical oxidative polymerization of
conducting PPy involves the preparation of two separate solutions
(one containing the monomer and the other, the oxidant) subse-
quently mixed together to start polymerization.75
Figure 7A,B shows
the pictogram and flow chart of the general synthesis procedure of
PPy via chemical oxidative polymerization. In short, chemical oxidative
polymerization is a simple and low-cost method to produce PPy in
bulk quantity. However, achieving uniform structure and controlling
film thickness are difficult for PPy prepared by this method.
The most generally accepted chemical oxidative polymerization
mechanism of PPy is the coupling among radical cations, which
FIGURE 5 PPy in (A) undoped state,
(B) polaran, and (C) bipolaran1
FIGURE 6 Outline of the synthesis
methods of PPy
PANG ET AL. 1431

TABLE 1 Various routes for PPy synthesis
Synthesis method Starting material Product form Ref.
Chemical oxidative polymerization Py, FeCl3.6H2O, ethanol, calcium chloride anhydrous,
sodium alginate
Sponge 31
Py, FeCl3.6H2O, Safranin dye, MO, HCl, ethanol Globular, nanofiber,
nanotube
17
Py, FeCl3.6H2O, Safranin (3,7-diamino-2,8-dimethyl-
5-phenylphenazinium chloride; Safranin T, Basic Red
2) and phenosafranine (3,7-diamino-
5-phenylphenazinium chloride), HCl, ethanol
Nanofiber, nanotube 28
Py, FeCl3, APS, SDS Particle 44
Py, FeCl3, PTSA Film 45
Py, APS, L-MGA, CSA, PTSA, HCl, AA, DMPA Nanofiber 46
Py, FeCl3 Nanoparticle 47
Py, IC, KC, APS, methanol, ITO glass slides Film 20
Py, FeCl3, chromic acid, acetone, methanol, glass
substrate
Thin film 48
Py, APS, TiO2 Nanosphere 49
Py, APS, PVDF, acetone, HCl Nanoparticle 50
Py, APS, L-MGA, D-MGA, L-MDGA, ethanol Nanotube, nanofiber 51
Py, APS, CTAB, LA, AgNO3 Nanorod 52
Py, APS, CSA Thin film 29
Py, FeCl3.6H2O, methanol, HCl, glass substrate Thin film 53
Ultrasonic radiation (sonochemical)-assisted Py, FeCl3, MO, EO, HCl, ethanol Nanotube 27
Electrochemical polymerization Py, TEABF4, acetonitrile, chromium trioxide, H3PO4 Nanorod 54
Py, IC, KC, APS, methanol, ITO glass slides Film 20
Py, LiClO4, tetraethylammonium p-toluenesulfonate,
N-[3-(trimethoxysilyl)propyl]pyrrole (organosilane),
acetonitrile, toluene, dichloromethane
Film 55
Py, NaClO4, NaCl, Na2SO4, NaNO3, NaBF4, CTAB,
PVP, SDS, acetone
Thin film 56
Py, LiClO4 Thick film 57
Py, KCl Thick film 30
Py, NSA, polystyrene sulfonate, n-a-Si:H as charge
separation layer, i-a-Si:H as photoabsorber layer
Thin film 58
Py, ß-naphthalene sulfonate, glass/ITO substrates,
CdTe photoabsorber layer
Thick film 59
Ultrasonic radiation (sonoelectrochemical)-
assisted
Py, KCl, HCl, NaDBS (sonoelectrochemical) Thin film 60
Vapor phase polymerization (VPP) Py, FeCl3 Thin film 61
Py, APS, CTAB, ethanol, ITO-coated glass, silicon wafer Thin film 10
Py, FeCl3, methyl alcohol, glass substrate Thin film 62
Py Thin film 63
Py, glass and silicon substrates Thin film 64
Electrospinning Py, APS, NaDEHS, CHCl3, methanol, PEO Nanofiber 22
Py, APS, DBSA Fiber 65
Microemulsion polymerization Py, CTAB, SDS, KPS Nanosphere, nanofiber 66
Py, FeCl3, n-pentanol, n- decane, SDS Nanoparticle 67
Py, HCl, H2O2, SDS Nanoparticle 68
Py, SDS, KPS Nanofiber 69
1432 PANG ET AL.

involves oxidation, deprotonation, and crosslinking reactions.76
The
polymerization mechanism of Py is shown in Figure 8.16
Polymeriza-
tion starts with Py monomers undergoing oxidation and yielding radi-
cal cations. Subsequently, these radical cations join together to form
soluble bipyrroles through deprotonation. During the propagation
stage, the bipyrroles are oxidized again and form oligomers with radi-
cal cations. This coupling effect is a continuous process and results in
the formation of higher oligomers and finally PPy.43,76
Polymerization
is prolonged until Py oligomers become insoluble and form a precipi-
tate in aqueous medium.
Over the past decades, numerous studies on the synthesis of PPy
through chemical oxidative polymerization method have been carried
out.77,78
For instance, the PPy nanoparticles are chemically synthe-
sized using FeCl3.5H2O as oxidant and naphthalene sulfonic acid
(NSA), sodium dodecylbenzenesulfonate (NaDBS), and SDS as surfac-
tants.77
First, the solution of Py and surfactants is prepared with dis-
tilled water prior to the dropwise addition of the oxidant into the
mixture solution. The conductivity of undoped PPy nanoparticles is
2.55 × 10−4
S/cm, and among all the surfactants used, PPy
nanoparticles doped with β-NSA exhibit the highest conductivity
(33.33 S/cm). Their finding shows that the surfactants used may act
as co-dopants in which the anionic ion of the surfactants co-doped
the PPy chains and enhanced its conductivity. Similarly, PPy
nanoparticles are fabricated using Py and FeCl3, and an electrical con-
ductivity of 9.33 × 10−9
S/cm is obtained at room temperature.79
PPy nanotubes could be synthesized through chemical oxidative
Py polymerization with FeCl3 in aqueous medium containing methyl
red sodium salt.80
In this method, factors including reaction tempera-
ture and sodium salt of organic dye (methyl red) may affect PPy nano-
tube conductivity. Decreasing the reaction temperature to −50
C
increases the conductivity to 104 S/cm. Additionally, the presence of
methyl red sodium salt increases the conductivity of PPy nanotubes
with irregular shapes from 1–5 S/cm up to 84 S/cm for polymeriza-
tion through variable dye concentrations at 20
C. Meanwhile, PPy
particles are chemically prepared by rapid mixing of Py monomers and
oxidants (FeCl3 and APS), in which the electrical conductivity of both
oxidants samples is enhanced with increased reaction temperature
from 25
C to 170
C.79
Nevertheless, better performance is observed
for FeCl3 than APS owing to its lower resistivity.
Additionally, chemical oxidative polymerization is used to prepare
PPy by the addition of FeCl3.6H2O solution to Py solution with Safra-
nin and phenosafranin dyes under stirring for a few seconds and leav-
ing them undisturbed at room temperature for 24 h. PPy conductivity
could also be affected by the concentration of dyes during polymeriza-
tion and their morphology.28
The highest conductivity (35 and
10 S/cm) is obtained with 4 mM Safranin and 2 mM phenosafranin
compared with that of standard globular PPy (5 S/cm). Chemical oxi-
dative polymerization in the presence of FeCl3 oxidant is used to pre-
pare PPy thin films.48
For the preparation of PPy thin films, Py and
FeCl3 are dissolved in distilled water and stirred constantly for 30 min
followed by deposition onto a glass substrate by keeping it in FeCl3
solution for 150 min at room temperature. In this method, the ratio of
oxidant to monomer (O/M) can affect the conductivity of Py thin films.
Decreasing the ratio of O/M increases the conductivity of Py thin
films, owing to the increase in polarons and bipolarons at low O/M.
Moreover, dopant addition can change the conductivity of PPy
thin films. The doping of CSA into PPy thin film increases the conduc-
tivity from 4.29 × 10−9
to 1.56 × 10−8
S/cm with increased dopant
content from 10% to 50%.29
For the chemical preparation of CSA-
doped PPy thin films, CSA-doped PPy powder is dissolved in m-cresol
and stirred for 11 h to obtain casting solution followed by spin coating
at 3000 rpm for 40 s on the glass substrates. In a previous work by
John et al.,81
who fabricated PPy films by the dropwise addition of
APS into the solution of Py and dopants (di (2-ethylhexyl) sul-
fosuccinic acid sodium salt, NaDEHS, and dodecylbenzene sulfonic
acid sodium salt, NaDBSA). PPy film with NaDEHS dopant shows
higher conductivity (in the order of 10−1
S/cm) than PPy film with
NaDBSA dopant.
2.1.1 | Ultrasonic radiation
Ultrasonic radiation or ultrasonication is defined as the application of
vibration energy to agitate aqueous suspensions with frequency
TABLE 1 (Continued)
Synthesis method Starting material Product form Ref.
Mechanochemical polymerization Py, APS, acetone, hydrazine hydrate Nanoparticle 34
Py, KPS Nanosphere 70
Photopolymerization Py, APS, methanol, sapphire substrate Thin film 71
Py, FeCl3, APS, PTSA, sodium salt of PTSA, DBSA,
NADEHS, CSA
Thin film 72
Abbreviations: AA, acetic acid; AgNO3, silver nitrate; CdTe, cadmium telluride; CHCl3, chloroform; CSA, camphor sulfonic acid; CTAB, cetyl trimethyl
ammonium bromide; DBSA, dodecyl-benzenesulfonic acid; D-MGA, N-myristoyl-D-glutamic acid; DMPA, diethylenetriaminepenta (methylene-phosphonic
acid); EO, ethyl orange; FeCl3.6H2O, iron (III) chloride hexahydrate; H2O2, hydrogen peroxide; HCl, hydrochloric acid; H3PO4, phosphoric acid; IC,
Ƭ-carrageenan; ITO, indium tin oxide; i-a-Si:H, intrinsic hydrogenated amorphous silicon (i-a-Si:H); KCl, potassium chloride; KC, ĸ-carrageenan; KPS,
potassium peroxydisulfate; LA, lauric acid; L-MGA, N-myristoyl-L-glutamic acid; L=MDGA, N-myristoyl-L-diglutamic acid; LiClO4, lithium perchlorate;
NaClO4, sodium perchlorate; NaCl, sodium chloride; Na2SO4, sodium sulfate; NaNO3, sodium nitrate; NaBF4, sodium tetrafluoroborate; NaDBS, sodium
dodecylbenzenesulphonate; NSA, naphthalene-1-sulfonic-acid sodium salt; n-a-Si:H, hydrogenated amorphous silicon n-doped; NaDEHS, sulfosuccinic
acid sodium salt; MO, methyl orange; PEO, polyethylene oxide; PTSA, p-toluenesulfonic acid; PVDF, polyvinylidenefluoride; PVP, polyvinylpyrrolidone;
SDS, sodium dodecyl sulfate; TEABF4, tetraethylammonium tetrafluoroborate; TiO2, titanium dioxide.
PANG ET AL. 1433

exceeding 20 kHz.82
Traditionally, ultrasonication has been used to
sterilize food and medical supplies, as well as to homogenize and
emulsify proteins and lipids.82
Over the past few years, ultrasonication
has been recognized for its effectiveness in achieving the uniform agi-
tation of chemicals compared with that of high shear mixing and mag-
netic stirring.73,82
Ultrasonication has also been established as an
TABLE 2 Advantages and disadvantages of PPy synthesis method
Method of synthesis Advantage Disadvantage
Chemical oxidative polymerization -Simple and fast fabrication method
-Efficient to produce bulk quantities in the
form of dispersion, powder and coating
-Difficult to obtain homogenous PPy with
uniform structure
-Produce very thick film and difficult to
control the film thickness
-Difficult to synthesize rigid insoluble
polymers
Ultrasonic radiation/ultrasonication - Shorter reaction time
- Produce nanostructure CPs with uniform
shape and smaller size
-Simple, fast, and require less complicated
set-up
-Potential for scale-up
- Provide energy efficiency (without the
need of high temperatures, high
pressures)
- CPs can be damaged if the ultrasonication
is too aggressive and/or too long
Electrochemical polymerization -High rates of accuracy and purity
-Produces very thin films
-Powerful method to modify CPs
morphology and its electrochemical
properties
-Provide higher mechanical resistance and
electrical conductivity to the produced
films
-High efficiency
-Possible to deposit a doped PPy layer onto
exposed photoactive area of substrate to
avoid possible short circuits
-Difficult to scale-up the process.
-Some monomers are theoretically unable
to electropolymerize
-High oxidation potential could lead to
overoxidation of the polymer
-Costly and time consuming method
Vapor phase polymerization (VPP) -Able to achieve high-purity materials and
interfaces
-Polymerization can occur in different types
of substrates
-Does not require the use of any solvents
-Provides uniform and pinhole free coatings
on all kind of surfaces and substrates
with a very accurate precision
-Inexpensive tool
-Rapid process
-Poor adhesion of PPy molecules to the
substrate surface
Electrospinning -The obtained PPy nanofibers exhibits
unique characteristics such as uniform
ultrafine fibers, high surface-to-volume
ratio, tunable porous structures, and
controllable composition
-Only soluble and thermoplastic polymers
are applicable
Microemulsion polymerization - Inexpensive and simple method, does not
require any template or special
equipment
-Require large amount of surfactant or co-
surfactant to produce effective
stabilization
Mechanochemical polymerization -Environmental friendly as no solvent as
reaction medium is needed
-Efficient and cost-effective method
-Generally uncontrollable to temperature,
time, and pressure controlled reactions
- Not suitable for moisture sensitive
systems and low boiling liquids
Photopolymerization - Easy control on the film thickness
-“Sandwich”-type structures can be
fabricated in situ during the deposition
-The obtained layers are porous (ie, they
have a high surface/volume ratio)
-Illumination is required
-Not all CPs can be produced via this
method
1434 PANG ET AL.

effective method of modifying the structural and morphologies prop-
erties of polymer.39,40,83
Ultrasonication is known for its ability to
accelerate the reactions of heretogeneous systems in a solvent and to
generate new chemistry that is inaccessible using other conventional
synthesis methods.84
Additionally, it is regarded as a green energy
source attributed to shorter reaction times and higher yields.73
Other
benefits of this synthesis method are simple processing and setup,
high purity, cost-effectiveness, and production of nanoparticles with
almost uniform shape and size.39,40,85
Ultrasonication is utilized to assist the chemical oxidative
(sonochemical) and electrochemical (sonoelectrochemical) polymeri-
zation of PPy.36
Figure 9A,B shows the basic set-up of an ultrasonic
radiation–assisted chemical oxidative and electrochemical polymeri-
zations. During sonication, ultrasonic waves pass through a liquid
medium, creating a large number of microbubbles that grow and col-
lapse in a few microseconds (greatly changing the pressure and tem-
perature inside the bubbles). This effect is called ultrasonic
cavitation.36,73
The violent collapse of these cavitation bubbles can
FIGURE 7 (A) Pictogram of chemical oxidative polymerization of PPy,48
reproduced by permission of Elsevier, and (B) flow chart of the
general synthesis procedure of PPy via chemical oxidative polymerization
FIGURE 8 Mechanism involved
in the chemical oxidative
polymerization of PPy.16
Reproduced
by permission from Elsevier
PANG ET AL. 1435

produce localized heating and atomization of the aqueous
medium.82
Many studies have been carried out through the sonochemical
and sonoelectrochemical polymerization of PPy. For instance, PPy
nanotube is fabricated through ultrasonication-assisted chemical oxi-
dative polymerization in which Py solution and FeCl3 solution are
mixed in the presence of MO and ethyl orange (EO) dyes followed by
ultrasonication for 2–3 min and leaving undisturbed for 24 h. The con-
ductivity values for PPy nanotubes prepared in the presence of MO
and EO are 92.5 and 6.8 S/cm, respectively, indicating the effect of
dyes on the electrical conductivity of PPy thin films.27
Moreover, electropolymerization under ultrasonic power is
another approach to preparing PPy thin films, in which a series of pro-
pylene carbonate solutions at various ultrasonic powers and in the
presence of various background electrolytes are used. In this method,
the ultrasonic power is found to affect PPy film conductivity.86
Increasing the ultrasonic power to 44 W decreases the conductivity
of PPy thin films, and the chain structure and quality of PPy could
deteriorate at extremely high ultrasonic power. Additionally, at the
same ultrasonic power, PPy films fabricated with sodium p-
toluenesulfonate (TsONa) show the highest conductivity
(116.7 S/cm), whereas PPy films fabricated with Na2SO4 exhibit the
lowest conductivity (6.3 S/cm) at an ultrasonic power of 44 W.
Similarly, electrosynthesized PPy film doped with dodecylbenzene
sulphonate anions (DBS) under the influence of different amplitudes
of ultrasonication can also affect its conductivity. PPy film conductiv-
ity is found to increase with increased ultrasonication amplitude from
20% to 60% which is attributed to the changes in morphology of
deposits and has distinct voltammetric behavior.58
The use of ultrasonication in the field of material science is
expanding because of its ability to fabricate homogeneous PPy with
uniform shape, nanosize, short reaction time, nonrequirement of high
temperatures and pressures, potential for scale-up, and simple
setup.39,40
Nevertheless, if the ultrasonication treatment is too long
or too aggressive, then the PPy formed could possibly be damaged.
2.1.2 | Radiolysis polymerization
The radiolysis of the aqueous solution by gamma (γ)-rays generates
reactive radical species and subsequently initiates the polymerization
of monomers to form polymer.87,88
The preparation of CPs using
γ-radiation was mainly accompanied by the addition of oxidants.89
Indeed, Karim et al.87
obtained an electric conductivity of PPy about
2.42 × 10−2
S/cm by using in-situ γ-radiation-induced chemical oxida-
tive polymerization method with APS as an oxidant.
The advantages of radiolysis polymerization using γ-ray radiation
including: (i) the reactions are simple, (ii) easy to carry out in various
environments, (iii) unnecessary to use chemical additives, and
(iv) possible to adjust radiation dose to fabricate polymers with
diverse properties.88
Despite such advantages, radiolysis polymeriza-
tion is not suitable to be used for organic monomers that exhibit poor
processability, low solubility, and even sometimes insoluble.89
2.2 | Electrochemical polymerization
Electrochemical polymerization refers to the oxidation of the mono-
mer and the growth of polymer chain onto anode, and this process is
irreversible.90,91
Electrochemical Py polymerization can be carried out
in aqueous and nonaqueous solvents.10,90
Electrochemical polymeri-
zation provides several advantages over chemical polymerization
method. First, PPy with unique conductivity grows and directly
attaches onto the electrode surface.90
Second, the PPy thickness can
be controlled by tuning the input parameters (ie, electric potential,
current density, solvent, or electrode position period) passing through
FIGURE 9 Basic set-up of an ultrasonic radiation assisted (A) chemical oxidative polymerization for PPy synthesis,85
and (B) electrochemical
polymerization for PPy synthesis.82
Reproduced by permission from Elsevier
1436 PANG ET AL.

the electrochemical cell.37,42
Additionally, the properties of PPy (such
as conductivities and structures) can be controlled directly in the
preparation. Eventually, the PPy-coated electrode produced from
electrochemical polymerization can be directly used for energy-
storage applications (eg, supercapacitors and batteries), but chemical-
polymerized PPy requires additional steps to fabricate an integrated
electrode by using nonconductive binders, which is usually time con-
suming and diminishes the electrochemical performance.37
Neverthe-
less, in some particular cases, chemical polymerization has some
advantages over electrochemical method, for instance, chemical poly-
merization is still chosen when a large quantity of PPy is needed.42
Therefore, both synthesis methods have particular application areas
for different purposes.
Many studies have reported the synthesis of PPy through electro-
chemical polymerization. For example, Wysocka-Zołopa and
Winkler56
produced PPy thin films with vertically aligned cone-like
structures through electrochemical Py polymerization in aqueous
solutions containing NaClO4 and polymeric or anionic surfactants,
such as PVP and SDS. The influence of the presence of surfactants on
the structural and I-V performance of PPy thin film is investigated.
They obtained cone-shaped and flat PPy films with and without the
use of surfactants, respectively. Results show that the currents
recorded for the oxidation and reduction of the cone-shaped PPy film
are much higher than those for the flat film of PPy formed in the
absence of surfactant. Consequently, the conductivity of the sample
in the presence of surfactant is higher.
PPy films doped with CSA show higher conductivity than PPy
doped with CSA and sodium molybdate (Na2MoO4) by using
electropolymerized PPy film on treated CT3 steel in a homogeneous
solution mixture of Py and CSA for 50 min, with a constant current of
0.9 mA/cm2
, followed by drying at 50
C in a vacuum.92
Meanwhile,
IC- and KC-doped PPy films using electrochemical polymerization
reveal a higher conductivity for KC-doped PPy film (6.74 ± 49 S/cm)
than IC-doped PPy films (6.31 ± 66 S/cm), which may be due to their
porous and nonporous morphology. However, IC-doped PPy films
exhibit higher mechanical properties than KC-doped PPy film.20
In another similar work, Sougueh et al.55
obtained PPy films
through electrochemical polymerization performed from a solution of
acetonitrile containing Py and LiClO4. They noted that the PPy films
deposited onto silane-modified FTO surfaces are less rough, flatter,
more uniform, more homogeneous, and more hydrophobic compared
with those prepared without silane. The presence of a silane layer
under the PPy films results in improved adhesion of electrodeposited
films, rendering them mechanically stable without deteriorating their
conductivity.
Meanwhile, Eslami et al.93
electropolymerized PPy coatings on
rheo-cast Al-4.5% Si alloy and pure aluminum in the presence and
absence of sodium nitrate (NaNO3). Electrodeposition of PPy coating
is promoted on the alloy surface than pure aluminum; hence, the coat-
ings on the alloys exhibit high thicknesses. Additionally, the presence
of NaNO3 increases the thickness of the coating and produces a
coarser, rougher, less homogenous morphology and helps reduce PPy
film conductivity to enhance corrosion protection. Figure 10A,B
shows the pictogram and flow chart of the synthesis procedure of
PPy via electrochemical polymerization for a three-electrode setup.
Basically, in electrochemical synthesis, the electrode oxidation of
Py monomer generates radical cations that then react with other Py
monomers to form oligomers and eventually PPy polymer.37
However,
the preparation of PPy electrochemically is a complex process, and
the mechanism of polymerization is still not completely under-
stood.10,90
The most generally used electrochemical polymerization
mechanism is presented in Figure 11.16
Polymerization starts with a
Py monomer oxidized to form radical cation and then two radical cat-
ions coupled to form dihydromer dication, which upon deprotonation
becomes bipyrrole. Owing to its low oxidation potential, bipyrrole is
further oxidized to form bipyrrole radical cation, which primarily cou-
ples with Py monomer radical and after deprotonation forms
tripyrrole. The process of oxidation, coupling, and deprotonation con-
tinually occurs and results in the growth of the PPy chain.16
According to Gvozdenovic et al.,91
the formation of primary radi-
cal cations through monomer oxidation significantly affects the poly-
merization yield. If radical cations are too reactive, they may react
with the nucleophilic species present in the electrolyte, subsequently
reducing the polymerization yield. By contrast, if they are in highly
stable form, they may diffuse from the anode before any reaction
occurs, so no electroconductive polymer film is produced. Therefore,
to obtain agood electroconductive polymer film, one must use high-
purity chemicals and cautiously prepare electrode surfaces before
synthesis.90
As mentioned earlier, electrochemical polymerization possessed
several benefits compared with chemical oxidative polymerization
method, for example, high-conductivity PPy with uniform structure
can be directly controlled and grown on the electrode surface. More-
over, a very thin PPy film can be produced through electrochemical
polymerization. However, electrochemical polymerization method
shows low potential for scale up and is expensive compared to chemi-
cal oxidative polymerization.
2.3 | VPP
VPP is a technique in which the monomer is introduced to an oxidant-
coated substrate in vapor form, and polymerization subsequently
occurs at the oxidant–vapor interface.94
The term VPP is used inter-
changeably with vapor deposition polymerization.95
Majumdar et al.96
and Hanif et al.97
described VPP as an easy, inexpensive, and effective
method of depositing CP coating onto a particular substrate. The main
advantage of VPP is that it is a solvent-free synthesis in which the
monomers are applied as vapor rather than a solution, and no liquid
acts as transport medium for particle agglomeration.94,95,98
Neverthe-
less, most CPs produced using this method suffers from insufficient
conductivity that limits their use as current path in electronic
devices.94,99
Notably, Ono and Miyata99
performed some of the earli-
est studies on the VPP of PPy by exposing Py monomer vapor to a
polyvinyl alcohol substrate containing FeCl3. Despite the successful
synthesis of PPy by VPP, the electrical conductivity of PPy film greatly
PANG ET AL. 1437

FIGURE 10 (A) Pictogram of electrochemical polymerization of PPy, and (B) flow chart of the synthesis procedure of PPy via electrochemical
polymerization for a three-electrode setup
FIGURE 11 Mechanism involved in the electrochemical polymerization of PPy.16
Reproduced by permission from Elsevier
1438 PANG ET AL.

depends on FeCl3 concentration, polymerization time, and
temperature.
Additionally, many researchers have reported on the utilization of
VPP for depositing Py monomer onto various substrates. For instance,
Kim et al.100
produced PPy films by in situ VPP method under ambient
conditions using FeCl3.6H2O as oxidants. The method started with
the dipping and coating of FeCl3.6H2O onto polymeric substrates
such as polyethylene terephthalate (PET), polyimide (PI), polyvinyl
chloride (PVC), polystyrene (PS), etc, prior to Py vapor exposure for
5 s to 3 min in a reaction chamber. Homogeneous and thin conductive
PPy films are uniformly fabricated at nanolevel thickness on the plas-
tic film substrates. Furthermore, the conductivity of PPy film typically
600 nm thick rapidly increases (up to 6 × 102
S/cm) with prolonged
deposition time. Using VPP method, PPy thin films were prepared on
a glass substrate in a vapor chamber containing FeCl3.6H2O oxidant
and triblock copolymer poly(ethylene glycol-propylene glycol-
ethyleneglycol) (PEG–PPG–PEG) solution.101
It was noted that PPy
thin films with PEG–PPG–PEG were highly conductive (55 S/cm) in
comparison to PPy thin films without PEG–PPG–PEG (7 S/cm).
Meanwhile, Subramanian et al.102
performed VPP of Py monomer
by using various oxidants such as Fe (III) benzenesulfonate, Fe (III)
ethylbenzenesulfonate, Fe (III) dodecyl-benzenesulfonate, and Fe (III)
p-toluenesulfonate coated onto the PET film and silicon wafer. They
observed the conductivity of PPy films using Fe (III) p-
toluenesulfonate exhibits higher conductivity (25 ± 1 S/cm) than
those formed using Fe (III) benzenesulfonate (6 ± 1 S/cm), Fe (III)
ethylbenzenesulfonate (5 ± 1 S/cm), and Fe (III) dode-
cylbenzenesulfonate (3 ± 0.5 S/cm).
In another research, porous free-standing PPy films in the
gas/water interface have been fabricated through VPP method with
FeCl3 as oxidant.103
A uniform and large-area PPy porous film with
controllable thickness and size of pores is obtained. Results show that
the conductivities of PPy films range within 10−3
to 10−2
S/cm. The
thickness and pore size of PPy film increase from 5.3 to 23.0 μm and
1.3 to 7.2 μm with increased concentration of FeCl3 from 0.05 to
0.5 M. Meanwhile, Li et al.98
used a mild VPP method to prepare PPy
films, in which vanadium pentoxide (V2O5) acts as the oxidant and
metal magnesium (Mg) acts as the substrate. Polymerization reaction
time is 12 h. They discovered that the oxidation of Py vapor on the
surface of V2O5 (precoated onto Mg surface) could be performed at
room temperature under normal atmospheric pressure. The micro-
structure of PPy film exhibits wire-like morphology with a diameter
of about 40–50 nm. Moreover, results confirm that the film com-
prises PPy and V2O5 with conductivity ranging within
0.20–1.22 S/cm.
In another work by Wang et al.,104
they utilized one-step VPP to
fabricate electrically conductive PPy coatings having a super-
hydrophobic and superoleophobic surface on fabric substrates in the
presence of fluorinated alkylsilane. The method starts with the dip
coating of fabric substrates in FeCl3/ethanol solution followed by
placing the dried fabric substrate in a Py-saturated nitrogen atmo-
sphere to perform polymerization. The PPy-coated fabrics show a sur-
face resistance of 0.5–0.8 kΩ/cm. Meanwhile, Majumdar et al.96
fabricated PPy-coated filter paper (FP) doped with HCl, CSA, and
PTSA by using VPP method. Initially, FP (substrate) is soaked in 0.1 M
FeCl3 solution for 15 min followed by drying at room temperature.
Then, FP is exposed to Py vapor by mounting inside a round-bottom
flask containing of the Py monomer. Polymerization is carried out for
1 h at 32
C. The conductivity of undoped PPy-FP is
1.78 × 10−5
S/cm and increases after doping. Among all PPy-FP sam-
ples, the highest conductivity is found in HCl-doped PPy-FP with
3.34 × 10−5
S/cm.
The mechanism of VPP closely follows the mechanism of chemi-
cal oxidative polymerization in solution (Figure 8) in which Py mono-
mer reacts with oxidants (FeCl3 of APS) and subsequently leads to
step-growth polymerization. Furthermore, VPP is a two-step process
in which one or more monomer precursors are delivered through the
vapor phase.95,105
Figure 12A–C shows the pictorial diagram of a gen-
eral VPP process in which the first step involves the precoating of the
oxidants on substrate, which is usually performed through wet chem-
istry. The second step (Figure 12B) is the exposure of the precoated
substrate to monomer vapor inside a vacuum chamber at a given tem-
perature (T) and pressure (P). The following step (Figure 12C) involves
washing away excess oxidant and monomer to yield PPy thin films.
Additionally, the flow chart of the general steps in VPP of Py is shown
in Figure 12D.
The main advantage of VPP is that this method does not require
the use of any solvent. However, PPy prepared by VPP demonstrates
poor adhesion to the substrate surface and hence exhibits lower con-
ductivity than PPy prepared by electrochemical polymerization.
2.4 | Electrospinning
Electrospinning is a relatively simple method of fabricating nanofibers
from various natural and synthetic polymers or polymer blends.22,106
Generally, electrospinning is carried out using a high-voltage electro-
static field by pressing a polymer solution or a melt through a needle
or by coating wires, cylinders, and other objects with the polymer melt
or solution drops to create long and fine nanofibers.12,22,106
Figure 13A,B shows the general schematic diagram and flow chart of
electrospinning method.
The electrospun material exhibits unique characteristics such as
uniform ultrafine fibers, high surface-to-volume ratio, tunable porous
structures, and controllable composition.22
However, electrospinning
becomes dangerous or even impossible if conductive solutions or
melts are used as they may form undesired connections between both
high-voltage electrodes, along which the high voltage may discharge,
resulting in flashovers.106
Compared with other synthesis methods,
electrospinning seems to be the only method that can mass-produce
continuous long nanofibers.107
Nevertheless, to assist in fiber forma-
tion, some non-CPs or chemicals (eg, polyethylene oxide, PEO) are
usually added into spinning solution, which may result in decreased
conductivity of electrospun fibers.107,108
Several studies have
reported on the fabrication of PPy nanofibers through
electrospinning.
PANG ET AL. 1439

For instance, Kang et al.65
produced electrically conducting PPy
fiber nonwoven web through needle-based electrospinning. Initially,
the soluble PPy powder is chemically synthesized using APS as oxi-
dant and DBSA as dopant. Subsequently, PPy powder is dissolved in
CHCl3 and DBSA to prepare concentrated PPy solution for
electrospinning. They fabricated PPy fibers through needle-based
electrospinning method, in which a high voltage ranging from 30 to
45 kV is used for the concentrated PPy solution by attaching the posi-
tive and negative electrodes of the power supply to the needle tip
and to the grounded rotating drum collector, respectively. The PPy
nonwoven web is collected on the negative electrode. The electrical
conductivity of electrospun PPy fiber is 0.5 S/cm, which is slightly
higher than those of powder or cast-film forms.
Additionally, Chronakis et al.108
used electrospinning to prepare
conductive PPy nanofibers. The electrospinning of pure PPy
nanofibers (without carrier) and PPy nanofibers (with PEO as a carrier)
is fabricated in the presence of doping agent, NaDEHS). The polymer
solutions are electrospun at room temperature at driving voltages of
FIGURE 12 Pictorial diagram of a general VPP process involving (A) precoating of the oxidants on the substrate, (B) exposure of the
precoated substrate to monomer vapor, (C) washing away excess oxidant and monomer to yield PPy thin film,105
reproduced by permission from
Elsevier, and (D) flow chart of the general steps in VPP of PPy
FIGURE 13 (A) Schematic
diagram of electrospinning
method,107
reproduced by permission
from Elsevier, and (B) flow chart of
electrospinning of PPy
1440 PANG ET AL.

30 kV. They found that the electrical conductivity of the pure PPy
nanofibers is about 2.7 × 10−2
S/cm, whereas for the PPy/PEO
nanofibers are in the range of 4.9 × 10−8
S/cm to 1.2 × 10−5
S/cm.
In another work, Cong et al.22
synthesized soluble PPy with
NaDEHS as dopant and then subjected them to electrospinning with
or without PEO as carrier to fabricate PPy nanofibers. They dissolved
PPy in CHCl3 and performed electrospinning at 10 kV voltage and
0.8 mL/h flow rate. Results reveal that increased content of PEO
enhances the electrospinning process of the fiber web but tends to
reduce the conductivity from 1.44 × 10−3
to
2.45 × 10−4
S/cm. Furthermore, PPy nanofiber conductivity is found
to increase (1.44 × 10−3
to 2.22 × 10−3
S/cm) with increased temper-
ature following a power function like most semiconductors.
The essence of electrospinning is that this method is the only one
that can fabricate large quantities of continuous long PPy nanofibers.
However, some non-CPs or chemicals are usually introduced into the
spinning solution to assist fiber formation, thereby decreasing the
conductivity of the obtained PPy nanofibers. Moreover, another limi-
tation of electrospinning is that this method is applicable only to solu-
ble and thermoplastic polymers.
2.5 | Microemulsion polymerization
A microemulsion generally comprises two immiscible liquids such as
oil and water and surfactant molecules.6,109
The surfactant is used in
a small quantity to stabilize the droplets of oil in water or water in
oil.109
Some researchers have also performed microemulsion polymer-
ization to oil-water (O-W) or water-oil (W-O) interfacial
polymerization.110,111
In microemulsion polymerization, surfactant
micelles may act as soft templates to generate polymeric particles,
and the shape of micelles depends on the surfactant type and opera-
tional conditions.66
Surfactants with longer carbon chains reportedly
produce larger nanoparticles than shorter carbon chains of surfactant
molecules as they provide more inner space to enable necessary poly-
merization.6
Moreover, microemulsion polymerization is found to
increase the yield of PPy nanoparticles, that is, the extent of
π-conjugation along the polymer backbone and the ordered arrange-
ment of macromolecular chains.111
Figure 14A,B shows the pictorial
diagram and flow chart of the synthesis of PPy nanofibers using
microemulsion polymerization.
Several studies have reported the use of microemulsion polymeri-
zation to fabricate PPy. Hazarika and Kumar6
synthesized PPy
nanoparticles through microemulsion polymerization of Py in the pres-
ence of APS as oxidant with various concentration of SDS as dopant.
Initially, they prepared SDS surfactant solution under stirring for 1 h
and then added Py dropwise under stirring for another 2 h. Lastly,
APS is added to the surfactant–monomer mixture, and the solution is
stirred constantly for another 24 h to complete the polymerization.
Results show that the resistance of PPy nanoparticles decreases with
increased SDS concentration from 0.01 to 0.2 M.
PPy nanoparticles have been synthesized using different Py/APS/
SDS ratios and processing temperatures through microemulsion poly-
merization by Santim et al..112
The increase in SDS and APS molar
ratio does not significantly affect PPy nanoparticle conductivity. How-
ever, at similar Py/APS/SDS molar ratios (2:1:2), electrical conductiv-
ity increases from 5.8 × 10−3
to 21.2 × 10−3
S/cm with decreased
temperature from 28
C to 0
C. Sari et al.113
used similar materials and
FIGURE 14 (A) Pictorial diagram of microemulsion polymerization of PPy,111
reproduced by permission from ACS, and (B) flow chart of the
synthesis of PPy nanofibers using microemulsion polymerization
PANG ET AL. 1441

method to synthesize PPy and found that the SDS surfactant acts as
doping agent in the synthesis, and the addition of 1% SDS increases
the electrical conductivity from 0.0016 to 0.2446 S/cm.
Additionally, Rawal and Kaul114
prepared PPy nanowires and
nanoparticles by microemulsion polymerization using different cap-
ping agents, namely, MO and CTAB. The solutions of the capping
agent and FeCl3 are separately prepared in distilled water and then
mixed together under constant stirring. Subsequently, Py is added
dropwise to the microemulsion solution, and the reaction is performed
under nitrogen environment at room temperature for 12 h. They
found that nanowires and nanoparticles are formed with the use of
MO and CTAB in PPy synthesis, respectively. Furthermore, PPy nano-
wires show higher electrical conductivity (4 S/cm) than PPy
nanoparticles with a conductivity of 1.69 × 10−4
S/cm.
Meanwhile, Khadem et al.66
obtained PPy nanostructures
(nanofibers, nanospheres, and string bead) through the emulsion
polymerization of Py in the presence of CTAB and SDS as surfac-
tants, HCl as dopant, and potassium peroxydisulfate (KPS) as initia-
tor. They noticed that by using CTAB instead of SDS, PPy
nanofibers are obtained and sample conductivity increases. More-
over, with decreased reaction temperature from 40
C to 0
C, the
morphology of resulting particles changes from spherical to fibrillar,
and the conductivity of the resulting PPy increases by two orders of
magnitudes. Additionally, PPy nanofibers exhibit higher conductiv-
ity (0.66 S/cm) than the other morphologies (spherical or
string bead).
Microemulsion polymerization is extensively used because of its
low interfacial tension, high thermodynamic stability, and ability to
solubilize otherwise immiscible liquids.112
However, in microemulsion
polymerization method, a large amount of surfactant or cosurfactant
is needed to achieve effective stabilization. Hence, the exploration of
new and biodegradable surfactant systems with improved interfacial
and chemical properties is essential in the microemulsion polymeriza-
tion method.
2.6 | Mechanochemical polymerization
Mechanochemical methods refer to the chemical transformations acti-
vated by mechanical energy, such as compression, shear, or fric-
tion.115
Mechanochemical techniques such as ball milling and hand
grinding are considered as efficient and cost-effective methods for
preparing conductive polymeric nanomaterials through solvent-free
synthesis.70,115
Despite the abovementioned advantages, mechano-
chemical method is generally uncontrollable with regard to tempera-
ture, time, and pressure. Moreover, it is unsuitable for handling low-
boiling-point liquids, moisture-sensitive systems, and heterogeneous
reactions.115
Figure 15A–C shows the pictogram of ball mill and flow
chart of mechanochemical polymerization of PPy.
Mechanochemical methods have been used to fabricate PPy. For
instance, Palaniappan and Manisankar70
synthesized 80 nm spherical
PPy nanostructures through a green mechanochemical route by using
a precleaned mortar. The method starts with the addition of solid KPS
to Py monomer in a molar ratio of 1:1. Then, the mixture is manually
ground immediately for 20 min until the color of the reaction mass
turned deep black, followed by repetitive washing of the product with
water, diethylether, and ethanol. Subsequently, the product was dried
in vacuum oven at 40
C, and the purified dry PPy powder was sub-
jected for further characterization. They concluded that PPy
nanospheres have high current primarily owing to the availability of a
large accessible surface area.
Posudievsky and Kozarenko34
prepared PPy through the mecha-
nochemical treatment of Py/APS in an agate grinding bowl of a plane-
tary ball mill. They performed the synthesis at a rotation rate of
300 rpm for 1 h under argon atmosphere. At a Py/APS ratio of 2.0,
the obtained PPy exhibits the highest conductivity of
6.5 S/cm. Additionally, at a Py/APS ratio of 4.0, the amount of APS in
the reaction mixture proves to be insufficient to realize the polymeri-
zation reaction.
Meanwhile, Abbasi et al.117
separately dissolved FeCl3 and PTSA
in 50 mL of distilled water at 0
C under constant stirring with gradual
addition. Then, both solutions are mixed together, and temperature
was reduced below 0
C. Subsequently, solution of polyethylene glycol
(PEG8000) was added in the mixture solution of FeCl3 and PTSA
under constant stirring. Next, the distilled Py was dropwise added
with constant high stirring. Then, the mixture solution was filtered
and washed with distilled water and acetone for several times, and
finally the polymer was dried under vacuum before subjected to mill-
ing process using a planetary ball mill at 850 rpm to obtain PPy
nanoparticles. The milling process is carried out under different reac-
tion times at room temperature. Notably, the electrical conductivity of
PPy nanoparticles decreases from 3.29 to 1.78 S/cm with prolonged
milling time from 60 to 300 min, respectively.
Apart from VPP, mechanochemical polymerization is another
example of a solvent-free synthesis method for PPy. However, mech-
anochemical polymerization has drawbacks in controlling the reac-
tions for moisture and air-sensitive materials. Moreover,
understanding the mechanism of mechanochemical polymerization
remains ambiguous and requires further exploration in this
research area.
2.7 | Photopolymerization
Photopolymerization is another approach by which monomers can be
polymerized by exposure to illuminations such as ultraviolet (UV) light,
visible light, laser-generating radicals (photochemical reaction), or
holes (photoelectrochemical reaction).118,119
The photopolymerization
of CPs such as PPy is divided into two main categories, that is,
(i) direct photopolymerization through photoexcitation of the mono-
mer itself and (ii) photopolymerization by using photosensitizer/
photoinitiator systems.118,120
However, most of the CPs cannot be
obtained via the former category as it comprises of a more positive
oxidation peak potential compared to the redox potential of the pho-
tosensitizers.118
Figure 16 shows the pictogram of the photo-
polymerization of PPy via photosensitizer-/photoinitiator-mediated
1442 PANG ET AL.

systems. Furthermore, photopolymerization has been found to be
highly useful in achieving microfabrication and coatings directly onto
the substrate where limitations such as temperature and solubility
exist.119
Moreover, this method is developed to solve the over-
oxidation problem of the electrochemical method, and the process
can be well controlled simply by turning the light on or off.118
Some
reports on multiphotonsensitized Py polymerization and Py photo-
polymerization by using ferrocene and iron complexes as electron
acceptors have been published.121,122
Photochemical Py polymerization with different concentrations
of H2O2 and H2SO4 ranging from 0.12 to 0.96 M and 6.8 × 10−4
to
0.19 M shows that the conductivity of PPy particles remain at values
of 10−3
to 10−2
S/cm with increased H2O2 concentration up to
0.96 M. However, with increased H2SO4 concentration, PPy particle
conductivity increases from 1.41 × 10−3
to 4.71 S/cm, respec-
tively.123
In another example, the photoelectrochemical polymeriza-
tion method of depositing PPy onto hydrogenated amorphous silicon
(a-Si:H) shows that Py polymerization onto the substrate surface can
be improved by the assistance of light. Under red laser illumination,
PPy films doped with polystyrene sulfonate exhibits higher conductiv-
ity compared with PPy films doped with NSA, at 0.3 and 0.5 V,
respectively. Additionally, PPy film conductivity deposited under red
laser illumination is higher than that of PPy films deposited under Xe
illumination.60
Meanwhile, Kasisomayajula et al.119
prepared conductive coatings
of PPy and acrylate through simultaneous photopolymerization with
silver nitrate (AgNO3) as oxidant for Py and Irgacure 907 as
photoinitiator for acrylate. The method starts with the independent
polymerization of Py and acrylate monomers followed by conversion
to hybrid coatings through the photopolymerization of Py and acrylate
occurring simultaneously. Acrylate acts as binder in the final coating,
each of which is made by spreading the reaction mixture onto the
substrate with a spin coater and is irradiated under UV light (intensity
of 35 mW/cm2
) for 20 min. The increase in Py/AgNO3 ratio from
32:1 to 4:1 reduces the nucleophilic attacks of water, thereby leading
to Py linear polymerization. Moreover, with increased acrylate per-
centage in the reaction mixture from 15% to 45%, the conductivity of
PPy coating decreases from 1.06 × 10−2
to 12.88 × 10−2
S/cm.
Few studies on the use of photopolymerization have been con-
ducted because of the low yields and inferior conductivity of PPy
obtained through this method compared with chemical and electrochem-
ical methods.119
Moreover, in contrast to chemical and electrochemical
PPy production, photochemical polymerization enables a better control
over size, shape, and physical properties, whereas photoelectrochemical
polymerization is independent of the nature of electrolyte.110
However,
illumination is required and only particular CPs can be produced through
photopolymerization. Additionally, photopolymerization remains a help-
ful method that requires further investigation.
3 | FACTORS AFFECTING PPy
CONDUCTIVITY
Electrical conductivity enables materials to be utilized in electronic
devices or as conducting membranes for sensor application.124,125
The electrical conductivity of PPy is influenced by several crucial fac-
tors, such as type of oxidants, initial O/M, dopants or surfactants,
polymerization temperature, and polymerization time.126-128
Based on
the aid of Taguchi method, Utami et al.127
found that the type of oxi-
dants and O/M are the two key factors that significantly affect PPy
FIGURE 15 Pictograms of (A) a ball mill (horizontal section), (B) various types of ball mill instruments,116
reproduced by permission of The
Royal Society of Chemistry, and (C) flow chart of general steps in mechanochemical polymerization of PPy
FIGURE 16 Pictogram of photopolymerization of PPy via
photosensitizer/photoinitiator-mediated systems
PANG ET AL. 1443

conductivity. Moreover, factors such as the type of organic dyes and
solvents also have a vital effect on PPy conductivity.129,130
Hence,
this article presents for the first time a brief review on the factors
affecting PPy conductivity as mentioned above.
3.1 | Oxidant
A chemical oxidant or oxidizing agent refers to a chemical substance
that can withdraw electrons and initiate polymerization.43
Oxidants
such as aqueous or anhydrous FeCl3 and APS are commonly used in
PPy polymerization.76,90
However, FeCl3 and water are the best
chemical oxidant and solvent for PPy polymerization with respect to
desirable conductivity.10,127
According to Sasso et al.,43
the decompo-
sition of oxidant tends to produce ions that could act as dopant during
chemical PPy polymerization. For instance, when oxidants such as
FeCl3 and APS are used in the synthesis, they form chloride (Cl−
and
ClO4
−
) and sulfate (SO4
−
) ions.43,131,132
In recent years, numerous reports have focused on the effect of
various oxidants in PPy polymerization.14,44,133
For instance, Sub-
ramanian et al.102
used various oxidants including Fe (III)
benzenesulfonate, Fe (III) ethylbenzenesulfonate, Fe (III) dodecyl-
benzenesulfonate, and Fe (III) p-toluenesulfonate to prepare PPy films.
They obtained highest conductive PPy films using Fe (III) p-
toluenesulfonate at about 25 ± 1 S/cm, followed by PPy films with Fe
(III) benzenesulfonate, Fe (III) ethylbenzenesulfonate, and Fe (III) dode-
cylbenzenesulfonate oxidants with conductivity values of 6 ± 1, 5 ± 1,
and 3 ± 0.5 S/cm, respectively.
Utami et al.127
evaluated the influence of FeCl3, APS, and H2O2
oxidants on the conductivity of PPy particles and noticed that PPy
particles prepared using FeCl3 oxidant display the highest conductivity
(0.54 S/cm), followed by APS (0.13 S/cm) and H2O2 (0.02 S/cm),
respectively. Meanwhile, Yussuf et al.44
studied the effect of FeCl3
and APS as oxidants on the electrical properties of PPy particles. They
found that FeCl3 shows better electrical performance than APS at
room temperature. PPy particles prepared using FeCl3 show a lower
resistivity of about 60 Ω compared with that (about 70 Ω) for APS.
Varga and co-workers133
utilized various oxidants (FeCl3.6H2O,
iron (III) sulfate hydrate (Fe2(SO4)3), and APS) for the polymerization
of PPy nanotubes. Results reveal that PPy with FeCl3.H2O exhibits
the highest electrical conductivity of 66 S/cm. By contrast, the con-
ductivity of PPy nanotubes with the other remaining oxidants is
observed to be 10 S/cm. Additionally, Sapurina et al.129
investigated
the effect of FeCl3, Fe2(SO4)3, iron (III) nitrate (Fe(NO3)3), and iron (III)
citrate oxidants on PPy nanotube conductivity. Results show that the
conductivity for PPy nanotubes prepared with FeCl3 is 42.8 S/cm,
which is the highest, followed by 42.6, 35.2, and 1.01 S/cm using
Fe2(SO4)3, Fe(NO3)3, and iron (III) citrate, respectively.
3.2 | Initial O/M molar ratio
Generally, a higher oxidant concentration leads to faster rates of poly-
merization, resulting in the formation of thicker polymer films that are
rougher and less conductive.105
The potential of oxidants with regard
to PPy conductivity depends strictly on the initial oxidant to Py mono-
mer ratio, where an excess of oxidant is essential for electron with-
drawal from Py; however, the amount should not be higher than a
certain level to avoid overoxidation of PPy chains.43
The variation in
O/M significantly affects PPy thin-film conductivity.34,48
Based on the
findings by Vernitskaya and Efimov90
and Ansari,10
the optimum initial
O/M for Py polymerization is 2.25 to 2.4 with the use of FeCl3 as
oxidants.
Several studies have investigated the effect of initial O/M in PPy
polymerization. For instance, Song et al.126
investigated the conduc-
tivity of 1,5-napthalene disulfonic acid (1,5-NDA)-doped PPy
nanoparticles under the influence of various O/M (FeCl3/Py) values.
Results demonstrate that with increased FeCl3/Py molar ratio from
1 to 4, PPy nanoparticle conductivity increases to 10.98 × 10−2
,
13.89 × 10−2
S/cm, 21.74 × 10−2
, and 10 S/cm, respectively. In a sim-
ilar work, Santim et al.112
studied the effect of O/M (APS/Py) on PPy
nanoparticle conductivity. Results show that PPy nanoparticle con-
ductivity increases from 0.6 × 10−3
to 20.4 × 10−3
S/cm with
increased O/M from 0.5 to 1. However, at O/M of 2, PPy nanoparti-
cle conductivity is 2.3 × 10−3
S/cm.
Deng et al.50
investigated the effect of O/M (APS/Py) on PPy
nanoparticle conductivity. PPy nanoparticle conductivity decreases
from 18 S/cm to approximately 0 S/cm with increased O/M from
0.25 to 1.5. Additionally, Effati et al.134
studied the influence of
APS/Py molar ratio on PPy nanoparticle conductivity. They obtained
the resistivity values of 218.78 and 152.17 Ω with increased APS/Py
from 1 to 2, indicating higher conductivity of PPy nanoparticles at
APS/Py molar ratio of 2.
Yussuf et al.44
reported that the optimum value of O/M (FeCl3/
Py) is 2 to achieve higher conductivity of PPy particles. Utami et al.127
noted that PPy particle conductivity decreases from 0.25 S/cm
(O/M = 1:1) to 0.18 S/cm (O/M = 3:1) but then increases to
0.27 S/cm at higher O/M (5:1). Moreover, Bober et al.135
prepared
PPy nanowires in the presence of FeCl36H2O oxidant and acid blue
(AB 25) dye. They observed an increase in PPy nanowire conductivity
from about 45 S/cm to 62 S/cm with increased O/M from 1.5 to 3.
3.3 | Dopant
One of the common ways used to improve the conductivity of com-
posites is through doping.14,136,137
This process generates the positive
or negative polarons/bipolaron as charge carriers, and these charge
carriers are delocalized over polymer chains, thereby facilitating elec-
tronic conductivity.1
The dopants are generally added into the poly-
mers to improve their electrical conductivity and capacitive
performance.138
Various types of dopants can be used to enhance
PPy conductivity, for instance, BSNa, NSA, TsONa, HCl, itaconic acid
(IA), and fumaric acid (FA).14,96,126
Moreover, surfactants such as
NaDBS and SDS also found to be able to act as dopants in the poly-
merization of PPy.77,139
Many studies have investigated the effect of various surfactants
on PPy conductivity. Hoshina et al.139
studied the effect of different
1444 PANG ET AL.

surfactants on the electrical properties of PPy nanoparticles. Results
show that the conductivities of PPy nanoparticles increase from 0.99
to 5.9 S/cm and 4.5 S/cm with the use of 0.2 mmol of NaDBS and
SDS, respectively. However, the use of surfactants (at similar concen-
trations) such as cetyl trimethyl ammonium chloride, benzalkonium
chloride, and polyethylene glycol mono-p-isooctylphenyl ether
reduces PPy nanoparticle conductivity to 0.79, 0.58, and 0.91 S/cm
respectively. Li et al.77
investigated PPy nanoparticle conductivity pre-
pared using various surfactants such as β-NSA, NaDBS, and SDS. PPy
nanoparticle conductivity (2.55 × 10−4
S/cm) increases with the addi-
tion of surfactants of β-NSA (33.33 S/cm), NaDBS (25 S/cm), and SDS
(4.74 S/cm), respectively. Among all the surfactants used, PPy
nanoparticles doped with β-NSA exhibit the highest conductivity.
Furthermore, Song et al.126
synthesized PPy nanoparticles using a
few types of dopants and found that PPy doped with 1,5-NDA shows
higher electrical conductivity than other dopants (HCl, IA, FA, and
PTSA). The electrical conductivities of PPy nanoparticles are
2.9 × 10−2
S/cm (without dopant), and 38.4 × 10−2
, 5.8 × 10−2
,
3.1 × 10−2
, 4 × 10−2
S/cm, and 10 S/cm with HCl, IA, FA, PTSA, and
1,5-NDA, respectively. Meanwhile, Zhang et al.123
investigated the
effect of different H2SO4 concentration on PPy particle conductivity.
They noticed that PPy particle conductivity increases from
1.41 × 10−3
to 4.71 S/cm with increased H2SO4 concentration from
6.8 × 10−4
to 0.19 M, respectively.
In another example, Hazarika and Kumar111
synthesized and com-
pared the electrical conductivity of PPy nanofibers doped with PTSA
and HCl. Results demonstrate that the electrical conductivity of PPy
nanofibers doped with PTSA is higher than that of PPy nanofibers
doped with HCl, and the measured values are 3.3 × 10−1
and
9.7 × 10−2
S/cm, respectively. Mahmoodian et al.75
further reported
the effect of different anionic dopants (NaDBS, SDS, α-NSA,
anthraquinone-2-sulfonic acid sodium salt monohydrate/5-sulfosalicylic
acid dehydrate [AQSANa-SSCA]), and CSA on the electrical properties
of PPy thin films. The thinnest PPy-NaDBS film exhibits the highest
conductivity (18.9 × 102
S/cm), whereas for PPy-CSA, the thickest
films show the lowest conductivity (3.8 × 102
S/cm).
Meanwhile, Rahaman et al.20
chemically synthesized PPy films in
the presence of IC and KC as dopants. The dopants used in the poly-
merization are anionic and soluble in water. PPy films doped with IC
demonstrated are found to have lower conductivity (0.0002 S/cm)
than PPy films doped with KC (0.0007 S/cm), respectively. Majumdar
et al.96
evaluated the effect of different dopants (HCl, CSA, and PTSA)
on the conductivity of PPy film coated onto FP substrate. Results
demonstrate that the conductivities of PPy film increase from
1.78 × 10−5
S/cm (undoped) to 3.34 × 10−5
, 2.49 × 10−5
, and
2.30 × 10−5
S/cm with the use of HCl, CSA, and PTSA dopants,
respectively.
3.4 | Organic dye
Organic dyes are defined as substances that impart color to a sub-
strate through selective absorption of light, and they are generally
soluble in aqueous medium.140
One of the common characteristic of
CPs and organic dyes is the presence of conjugated double bonds in
their molecular structure; thereby, they may interact via ionic bonding,
π-π attractions, and hydrogen bonds.80
Thus, the interaction between
CPs and organic dyes can significantly affect the conductivity of
CPs.80,140
For example, Sapurina et al.129
investigated the effect of several
dyes on PPy nanotube conductivity. Results show that the conductiv-
ity of PPy nanotubes without dyes is 1.55 S/cm and increases to 5.56,
22.1, and 46.8 S/cm with the use of methylene blue, EO, and MO
dyes, respectively. However, PPy nanotubes prepared in the presence
of cresol red, acid green 25, indigo carmine, reactive black 5, and thy-
mol blue demonstrate lower conductivities than PPy nanotubes pre-
pared without dyes. The conductivity values are 1.38, 0.854, 0.128,
0.054, and 2.28 × 10−3
S/cm, respectively. Furthermore, they found
that PPy nanotube conductivity is increased at low concentrations of
MO (below 0.003 M) with a maximum conductivity about
40–50 S/cm. However, the overall conductivity of PPy nanotubes
decreases to about 10–15 S/cm at MO concentrations above
0.003 M.
Li et al.27
investigated the effect of different dyes (MO, MO acid,
and EO) on PPy nanotube conductivity at an O/M of 2. Results indi-
cate that the conductivity of PPy nanotubes without dye is 2.11 S/cm,
whereas that with MO shows the highest conductivity of about
92.5 S/cm, followed by that with EO of about 6.80 S/cm and that
with MO acid of about 6.45 S/cm, respectively. Meanwhile, Bober
et al.135
compared the effect of acid blue 25 and acid blue 129 dyes
on PPy nanowire conductivity at different O/Ms. Results indicate that
PPy nanowire conductivity prepared with AB 25 shows higher con-
ductivity than AB 129 at all O/Ms. For example, PPy nanowire con-
ductivity at an O/M of 3 is about 62 and 40 S/cm with the use of AB
25 and AB 129 dyes, respectively.
Furthermore, Minisy et al.80
found that at similar O/M (Py/FeCl3)
values of 2.5, the conductivity of PPy nanotubes prepared in the pres-
ence of EO, AB 25, and methyl red are about 22, 60, and
64–84 S/cm, respectively. These conductivity values are higher than
those of PPy nanotubes prepared without the presence of dyes (about
1–5 S/cm).
3.5 | Solvent
PPy conductivity is also affected by the types of solvent used in the
preparation.10,130
A higher donor number (DN) of the solvent means
greater nucleophilicity, thereby leading to shorter average life of inter-
mediates; hence, PPy with lower conductivity is obtained.130,141
Ouyang and Li142
examined the conductivity of PPy thick films pre-
pared in different solvents (dimethyl formamide (DMF), trimethyl
phosphate (TMP), dimethyl sulfoxide (DMSO), carbamic acid
(CH3NO2), 1,2-propanediol carbonate (PC), and H2O). They found that
the PPy thick films prepared with DMF and DMSO exhibit the lowest
conductivity (10−4
S/cm), whereas those prepared with PC and
CH3NO2 show the highest conductivity of 60–70 S/cm. Additionally,
PANG ET AL. 1445

T
A
B
L
E
3
Summary
of
PPy
conductivity
Polymerization
conditions
Structure
M
O
O/M
ratio
Dopant
Dye
S
T
(

C)
Time
(h)
Conductivity
(S/cm)
Application
Ref.
Chemical
oxidative
polymerization
in
deionized
water
Nanoparticle
Py
FeCl
3
—
—
—
—
RT
—
0.0209
Electronic
devices
143
Nanoparticle
Py
APS
—
—
—
—
RT
—
0.0135
Chemical
oxidative
polymerization
in
the
presence
of
1,5-NDA
in
deionized
water
Nanoparticle
Py
FeCl
3
1
1,5-NDA
—
—
0
—
10.98
×
10
−2
—
126
Nanoparticle
Py
FeCl
3
2
1,5-NDA
—
—
0
—
13.89
×
10
−2
Nanoparticle
Py
FeCl
3
3
1,5-NDA
—
—
0
—
21.74
×
10
−2
Nanoparticle
Py
FeCl
3
4
1,5-NDA
—
—
0
—
10
Microemulsion
polymerization
in
the
presence
of
SDS
in
distilled
water
Nanoparticle
Py
APS
1:2
SDS
—
—
28
—
0.6
×
10
−3
—
112
Nanoparticle
Py
APS
1:1
SDS
—
—
—
20.4
×
10
−3
Nanoparticle
Py
APS
2:1
SDS
—
—
—
2.3
×
10
−3
Chemical
oxidation
polymerization
in
droplets
by
microfluidic
system
Nanoparticle
Py
APS
1
—
—
—
RT
—
4.57
×
10
−3
Coating
dispersions
and
additive
particles
134
Nanoparticle
Py
APS
2
—
—
—
RT
—
6.57
×
10
−3
Nanoparticle
Py
APS
2
—
—
—
RT
—
6.57
×
10
−3
Chemical
oxidative
polymerization
in
the
presence
β-NSA,
NaDBS,
and
SDS
in
distilled
water
Nanoparticle
Py
FeCl
3
.5H
2
O
—
Undoped
—
—
RT
—
2.55
×
10
−4
—
77
Nanoparticle
Py
FeCl
3
.5H
2
O
—
β-NSA
—
—
RT
—
33.33
Nanoparticle
Py
FeCl
3
.5H
2
O
—
NaDBS
—
—
RT
—
25
Nanoparticle
Py
FeCl
3
.5H
2
O
—
SDS
—
—
RT
—
4.74
Chemical
oxidative
polymerization
in
the
presence
of
HCl,
IA,
FA,
PTSA,
and
1,5-NDA
in
deionized
water
Nanoparticle
Py
FeCl
3
4
Undoped
—
—
0
—
2.9
×
10
−2
Antistatic
coatings,
drug
delivery
system,
batteries
and
sensors
126
Nanoparticle
Py
FeCl
3
4
HCl
—
—
0
—
38.4
×
10
−2
Nanoparticle
Py
FeCl
3
4
IA
—
—
0
—
5.8
×
10
−2
Nanoparticle
Py
FeCl
3
4
FA
—
—
0
—
3.1
×
10
−2
Nanoparticle
Py
FeCl
3
4
PTSA
—
—
0
—
4
×
10
−2
Nanoparticle
Py
FeCl
3
4
1,5-NDA
—
—
0
—
10
Microemulsion
polymerization
in
the
presence
of
SDS
in
distilled
water
Nanoparticle
Py
APS
—
SDS
—
—
28
—
5.8
×
10
−3
—
112
Nanoparticle
Py
APS
—
SDS
—
—
0
—
21.2
×
10
−3
Chemical
oxidative
polymerization
in
the
presence
of
SDS
in
deionized
water
Nanoparticle
Py
FeCl
3
—
SDS
—
—
RT
3
0.0316
Electronic
devices
143
Nanoparticle
Py
FeCl
3
—
SDS
—
—
RT
5
0.0215
Chemical
oxidative
polymerization
in
the
presence
of
1,5-NDA
in
deionized
water
Nanoparticle
Py
FeCl
3
4
1,5-NDA
—
—
0
6
23.25
×
10
−2
—
126
Nanoparticle
Py
FeCl
3
4
1,5-NDA
—
—
0
12
43.48
×
10
−2
Nanoparticle
Py
FeCl
3
4
1,5-NDA
—
—
0
18
10
Nanoparticle
Py
FeCl
3
4
1,5-NDA
—
—
0
24
1
1446 PANG ET AL.

T
A
B
L
E
3
(Continued)
Polymerization
conditions
Structure
M
O
O/M
ratio
Dopant
Dye
S
T
(

C)
Time
(h)
Conductivity
(S/cm)
Application
Ref.
Chemical
oxidative
polymerization
in
HCl
solution
Nanoparticle
Py
APS
0.25
HCl
—
—
5
—
18
—
50
Nanoparticle
Py
APS
0.5
HCl
—
—
5
—
10
Nanoparticle
Py
APS
0.75
HCl
—
—
5
—
9.1
Nanoparticle
Py
APS
1
HCl
—
—
5
—
8
Nanoparticle
Py
APS
1.25
HCl
—
—
5
—
3.5
Nanoparticle
Py
APS
1.5
HCl
—
—
5
—
≈
0
Emulsion
polymerization
in
the
presence
of
NaDBS,
SD,S,
CTAC,
BAC
and
TritonX-100
in
distilled
water
Nanoparticle
Py
FeCl
3
—
Undoped
—
—
RT
—
0.99
—
139
Nanoparticle
Py
FeCl
3
—
NaDBS
—
—
RT
—
5.9
Nanoparticle
Py
FeCl
3
—
SDS
—
—
RT
—
4.5
Nanoparticle
Py
FeCl
3
—
CTAC
—
—
RT
—
0.79
Nanoparticle
Py
FeCl
3
—
BAC
—
—
RT
—
0.58
Nanoparticle
Py
FeCl
3
—
TritonX-
100
—
—
RT
—
0.91
Chemical
oxidative
polymerization
in
the
presence
of
PTSA
in
distilled
water
Particle
Py
FeCl
3
1:1
PTSA
—
—
RT
—
0.54
—
127
Particle
Py
APS
1:1
PTSA
—
—
RT
—
0.13
Particle
Py
H
2
O
2
1:1
PTSA
—
—
RT
—
0.02
Chemical
oxidative
polymerization
in
the
presence
of
SDS
in
distilled
water
Particle
Py
FeCl
3
2
SDS
—
—
25
—
0.016
—
44
Particle
Py
APS
2
SDS
—
—
25
—
0.014
Chemical
oxidative
polymerization
in
the
presence
of
PTSA
in
distilled
water
Particle
Py
FeCl
3
1:1
PTSA
—
—
RT
—
0.25
—
127
Particle
Py
FeCl
3
3:1
PTSA
—
—
RT
—
0.18
Particle
Py
FeCl
3
5:1
PTSA
—
—
RT
—
0.27
Photopolymerization
under
UV
radiation
Particle
Py
H
2
O
2
—
0.00068
M
H
2
SO
4
—
—
RT
—
1.41
×
10
−3
Biomedical
applications
123
Particle
Py
H
2
O
2
—
0.006
M
H
2
SO
4−
—
—
RT
—
1.03
×
10
−2
Particle
Py
H
2
O
2
—
0.05
M
-H
2
SO
4−
—
—
RT
—
2.62
×
10
−2
Particle
Py
H
2
O
2
—
0.097
M-
H
2
SO
4−
—
—
RT
—
2.50
Particle
Py
H
2
O
2
—
0.19
M
H
2
SO
4
—
—
RT
—
4.71
Chemical
oxidative
polymerization
in
the
presence
of
methyl
red
salt
Particle
Py
FeCl
3.
6H
2
O
2.5:1
—
Methyl
red
—
20
—
64–84
—
80
Particle
Py
FeCl
3.
6H
2
O
2.5:1
—
Methyl
red
—
−50
—
104
(Continues)
PANG ET AL. 1447

T
A
B
L
E
3
(Continued)
Polymerization
conditions
Structure
M
O
O/M
ratio
Dopant
Dye
S
T
(

C)
Time
(h)
Conductivity
(S/cm)
Application
Ref.
Chemical
oxidative
polymerization
in
the
presence
of
SDS
in
distilled
water
Particle
Py
FeCl
3
2
SDS
—
—
170
—
0.076
—
44
Particle
Py
FeCl
3
2
SDS
—
—
25
—
0.016
Chemical
oxidative
polymerization
in
the
presence
of
PTSA
in
distilled
water
Particle
Py
FeCl
3
1:1
PTSA
—
—
RT
1
0.28
—
127
Particle
Py
FeCl
3
1:1
PTSA
—
—
RT
8
0.32
Particle
Py
FeCl
3
1:1
PTSA
—
—
RT
24
0.10
Chemical
oxidative
polymerization
in
the
presence
of
DBSNa,
SDS,
α-NSA,
AQSANa-SSCA,
and
CSA
in
deionized
water
Thin
Film
Py
APS
5:1
DBSNa
—
—
5
—
18.9
×
10
2
Semiconducting
layer
in
metal
insulator
semiconductor
field
effect
transistor
devices
75
Thin
Film
Py
APS
5:1
SDS
—
—
5
—
9.43
×
10
2
Thin
Film
Py
APS
5:1
α-NSA
—
—
5
—
6.7
×
10
2
Thin
Film
Py
APS
5:1
AQSANa-
SSCA
—
—
5
—
8.75
×
10
2
Thin
Film
Py
APS
5:1
CSA
—
—
5
—
3.8
×
10
2
Galvanostatic
electrochemical
polymerization
in
the
solution
of
IC
and
KC
Film
Py
APS
—
IC
—
—
0
—
2.2
×
10
−4
—
20
Film
Py
APS
—
KC
—
—
0
—
6.7
×
10
−4
Potentiostatic
electrochemical
polymerization
in
H
2
O,
acetonitrile
and
nitromethane
solutions
Film
Py
—
—
NaClO
4
—
H
2
O
25
—
1.39
Corrosion
protective
layer
for
stainless
steel
130
Film
Py
—
—
NaClO
4
—
Acetonitrile
25
—
9.09
Film
Py
—
—
NaClO
4
—
Nitromethane
25
—
33.34
Chemical
oxidative
polymerization
in
the
presence
of
methyl
orange
template
Nanotube
Py
FeCl
3
.6H
2
O
—
—
—
5
—
66
Electrode
material
in
capacitive
deionization
cell
20
Nanotube
Py
Fe
2
(SO
4
)
3
—
—
—
5
—
4.8
Nanotube
Py
APS
—
—
—
5
—
0.2
VPP
Nanotube
Py
FeCl
3
—
Undoped
—
—
RT
—
1.78
×
10
−5
Low-cost
flexible
gas
sensors
96
Nanotube
Py
FeCl
3
—
HCl
—
—
RT
—
3.34
×
10
−5
Nanotube
Py
FeCl
3
—
CSA
—
—
RT
—
2.49
×
10
−5
Nanotube
Py
FeCl
3
—
PTSA
—
—
RT
—
2.30
×
10
−5
Chemical
oxidative
polymerization
in
water
Nanotube
Py
FeCl
3
1:1
—
0.025MMO
—
20
—
42.8
—
129
Nanotube
Py
Fe
2
(SO
4
)
3
1:1
—
0.025MMO
—
20
—
42.6
Nanotube
Py
Fe(NO
3
)
3
1:1
—
0.025MMO
—
20
—
35.2
Nanotube
Py
Fe(III)-
citrate
1:1
—
0.025MMO
—
20
—
1.01
Chemical
oxidative
polymerization
in
water
Nanotube
Py
FeCl
3
1:1
—
Without
dye
—
20
—
1.55
—
129
1448 PANG ET AL.

T
A
B
L
E
3
(Continued)
Polymerization
conditions
Structure
M
O
O/M
ratio
Dopant
Dye
S
T
(

C)
Time
(h)
Conductivity
(S/cm)
Application
Ref.
Nanotube
Py
FeCl
3
1:1
—
0.03
M
—
20
—
5.56
Nanotube
Py
FeCl
3
1:1
—
0.03
M
EO
—
20
—
22.1
Nanotube
Py
FeCl
3
1:1
—
0.03
M
MO
—
20
—
46.8
Nanotube
Py
FeCl
3
1:1
—
0.03
M
Cresol
red
—
20
—
1.38
Nanotube
Py
FeCl
3
1:1
—
0.03
M
Acid
green
25
—
20
—
0.854
Nanotube
Py
FeCl
3
1:1
—
0.03
M
Indigo
carmine
—
20
—
0.128
Nanotube
Py
FeCl
3
1:1
—
0.03
M
Reactive
black
5
—
20
—
0.054
Nanotube
Py
FeCl
3
1:1
—
0.03
M
Thymol
blue
—
20
—
2.28
×
10
−3
Chemical
oxidative
polymerization
in
deionized
water
Nanowire
Py
FeCl
3
3
—
0.01
M
AB
25
—
RT
24
h
62
—
135
Nanowire
Py
FeCl
3
3
—
0.01
M
AB
129
—
RT
24
h
40
Microemulsion
(oil-water)
interfacial
polymerization
containing
xylene
Nanofiber
Py
APS
1:1
PTSA
—
—
RT
—
3.3
×
10
−1
Supercapacitors,
sensors,
molecular
wires
and
composite
materials
111
Nanofiber
Py
APS
1:1
HCl
—
—
RT
—
9.7
×
10
−2
Chemical
oxidative
polymerization
in
the
presence
of
CTAB
in
HCl
solution
Nanosphere,
Nanofiber
Py
KPS
—
CTAB
—
—
40
—
0.006
—
66
Nanosphere,
Nanofiber
Py
KPS
—
CTAB
—
—
25
—
0.115
Nanosphere,
Nanofiber
Py
KPS
—
CTAB
—
—
0
—
0.658
Chemical
(interfacial)
oxidative
polymerization
Nanofiber
Py
APS
—
HCl
—
—
RT
—
25
×
10
−4
Optoelectronic
devices,
chemical
and
biological
sensors,
batteries
and
molecular
probes
144
Nanofiber
Py
APS
FeCl
3
RT
—
21
×
10
−4
Nanofiber
Py
APS
PTSA
RT
—
6
×
10
−2
Nanofiber
Py
APS
—
CSA
—
—
RT
—
4
×
10
−2
Nanofiber
Py
APS
—
PSSA
—
—
RT
—
21
×
10
−4
Abbreviations:
M,
monomer;
O,
oxidant;
PSSA,
polystyrene
sulfonic
acid;
RT,
room
temperature;
S,
solvent;
T,
temperature.
PANG ET AL. 1449

PPy thick films prepared with H2O and TMP demonstrate a conduc-
tivity of 100
S/cm.
Yan et al.130
investigated the effect of different solvents (water,
acetonitrile, or nitromethane solutions) on the conductivity of elec-
trochemically fabricated PPy thick films. The conductivity for PPy
thick films prepared in nitromethane solutions is the highest at
33.34 S/cm and that of water is the lowest at 1.39 S/cm. Meanwhile,
the conductivity of PPy thick films prepared in acetonitrile is found
to be 9.09 S/cm. These results are expected as nitromethane has the
smallest DN and thus has the weakest interaction with the interme-
diates of Py during polymerization. Thus, the conjugated chain of
PPy formed in water is the longest and the conductivity is the
highest.
3.6 | Temperature
Another factor affecting the electrical conductivity of PPy is reaction
temperature.43,112
Based on Sasso et al.,43
the polymerization kinetics
is slowed down at low temperature, and a relatively linear PPy is sub-
sequently formed. In this case, charge-carrier movement is favored,
which is attributed to the regularity of conjugation. Hence, PPy con-
ductivity obtained at lower temperature is usually higher, as
evidenced by several studies.
For example, Santim et al.112
that with decreased synthesis tem-
perature from 28
C to 0
C, PPy nanoparticle conductivity increases
from 5.8 × 10−3
S/cm to 21.2 × 10−3
S/cm. Meanwhile, Minisy
et al.80
examined PPy nanotube conductivity under the influence of
reaction temperatures. PPy nanotube conductivity increases from
64–84 to 104 S/cm with decreased temperature from 20
C to
−50
C. Moreover, Khadem et al.66
studied the effect of temperature
on PPy conductivity and discovered that with decreased reaction tem-
perature from 40
C to 0
C, the conductivities of PPy increase from
0.006 to 0.658 S/cm, respectively. Additionally, the decrease in tem-
perature changes the morphology of PPy from spherical to fribillar.
Conversely, Yussuf et al.44
observed an opposite trend for the effect
of temperature on PPy particle conductivity. They found that the elec-
trical conductivity of PPy particles prepared with FeCl3 oxidant
increased from approximately 1.6 × 10−2
S/cm to about
7.6 × 10−2
S/cm with increased reaction temperature from 25
C
to 170
C.
3.7 | Time
Polymerization time is another factor that affects PPy conductiv-
ity.42,121,129
Different side reactions can reportedly occur during PPy
synthesis at prolonged polymerization time.43
Hence, the importance
of optimal polymerization time in the synthesis of PPy is to minimize
the disruption of conjugation (ie, overoxidation) of PPy.43
Indeed, by
tuning the polymerization time, the conductivity of the PPy obtained
varies, as reported by several researchers. For instance, Song et al.126
influenced the reaction time on the conductivity of 1,5-NDA doped
PPy nanoparticles. PPy nanoparticle conductivity increases from
23.25 × 10−2
S/cm (6 h) to 43.48 × 10−2
S/cm (12 h) and 10 S/cm
(18 h), respectively. However, at a reaction time of 24 h, conductivity
decreases to 1 S/cm.
Moreover, Utami et al.127
observed an increase in PPy particle
conductivity from 0.28 to 0.32 S/cm with prolonged reaction time
from 1 to 8 h, respectively. However, PPy particle conductivity
decreases to 0.1 S/cm at a reaction time of 24 h. In another example,
Nosheen et al.143
found that PPy nanoparticle conductivity prepared
with APS oxidant for a reaction time of 3 h is 1.25 × 10−2
S/cm and
increases to 1.36 × 10−2
S/cm at a reaction time of 5 h. However,
with increased reaction time up to 16 h, PPy nanoparticle conductivity
is 1.35 × 10−2
S/cm. Table 3 presents the summary of PPy conductiv-
ity from different studies.
4 | CONCLUSION
The urgent demand for polymers with high electrical conductivity is a
crucial issue that has led to the growth and advancement of numerous
application technologies. Accordingly, scientists and technologists
have shown the potential of PPy with high electrical conductivity, low
fabrication cost, and environmental friendly for such applications. The
critical success of new PPy materials lies in their preparation methods,
which control their morphology, particle size, and electrical conductiv-
ity. The synthesis methods elaborated in this overview are designed
to obtain PPy in the form of bulk, nanoparticle, thin film, thick film,
nanowire, and nanotube with high electrical conductivity values.
Given that the oxidant, initial O/M molar ratio, dopant, organic dye,
solvent, temperature, and time play a vital role in their structure, the
materials in the nanometer size range exhibit some remarkable prop-
erties that can be exploited to obtain PPy with a high electrical con-
ductivity. In this review, it is suggested that high conductive PPy can
be synthesized using chemical oxidative polymerization in the pres-
ence of suitable organic dyes. This high-conductive PPy could be used
in sensors, capacitors, energy-storage devices, light-emitting diodes,
and transistor.
ACKNOWLEDGMENT
The authors would like to appreciate Universiti Teknologi Malaysia for
the research grant (Q. J130000.21A2.05E25).
AUTHOR CONTRIBUTIONS
Pang Ai Ling: Conceptualization, Information and data collection, For-
mal analysis, Investigation, Methodology, Writing—original draft, Agus
Arsad: Conceptualization, Investigation, Resources, Supervision, Vali-
dation, Writing—review editing, Mohsen Ahmadipour: Writing—
review editing.
CONFLICT OF INTEREST
No potential conflict of interest was reported by the author(s).
1450 PANG ET AL.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available on
request from the corresponding author. The data are not publicly
available due to privacy or ethical restrictions.
ORCID
Agus Arsad https://orcid.org/0000-0002-0111-1939
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How to cite this article: Pang AL, Arsad A, Ahmadipour M.
Synthesis and factor affecting on the conductivity of
polypyrrole: a short review. Polym Adv Technol. 2021;32:
1428–1454. https://doi.org/10.1002/pat.5201
1454 PANG ET AL.

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