2. 1302 Siddhartha Samanta et al. / Materials Today: Proceedings 2 (2015) 1301 – 1308
weight, environmental stability etc. Among those, polyaniline (PANI) attracts much interest as intrinsically
conducting polymer due to advantages like easy synthesizability, better doping ability, architecture flexibility,
environmental stability, wide applicability etc. [1]. However, like the other conducting polymers solution
processability of PANI is also a great challenge due to its rigid rod like structure. To improve its processability much
effort have been made by chemical and electrochemical polymerization of aniline with ortho and meta-substituted
groups, viz., –OCH3, -CH2CH3, -CH3, -SO3H, -Cl, -F, -I etc. Although, in general, polymers of ortho or meta
substituted aniline give identical structure with better processability, ortho-isomer is preferred over the meta-isomer
for polymer synthesis as the former gives better yield and conductivity than the latter [2]. Moreover, the polyaniline
derivatives have some lower conductivity than that of PANI itself due to repulsion factor [2,3].
The synthesis of polyaniline derivatives containing free active functional groups (e.g., -NH2, -OH etc.) in the
backbone chain can offer better solution processability, better doping by various dopant and good performances
towards various applications. Thus recently, polymers have been synthesized chemically or electrochemically by
using ortho or meta –NH2 (phenylenediamine) and –OH (aminophenol) substituted aniline [4-12]. Unlike other
derivatives of aniline, both the amine and hydroxyl substituent in these two monomers can be oxidized
simultaneously with amine group of aniline. Thus the polymers have been reported to be insoluble in organic
solvents resulting poor processability as well as conductivity. Aromatic amine polymers have attracted much interest
of researchers due to their unique multifunctionality, involving variable conductivity, strong electroactivity, colorful
electrochromism, good optical properties, magnetic activity, environmental stability and thermal stability [13,14].
Phenylenediamine as a monomer had been used very little to synthesize polymer and [15] mostly, hydrochloric acid
was used for the chemical synthesis of polymers with different oxidants, e.g., persulfate including Na+
, K+
, and NH4
+
persulfates, iodine, H2O2, ceric ammonium nitrate, cupric nitrate etc. [4-8]. In most cases the researchers are reported
that the poly(o-phenylenediamine) (PoPD) are insoluble in organic solvents due to ladder structure. A few reports
are also focused that the polymer having ladder and/or open ring structure, are slightly soluble in dimethyle
sulfoxide (DMSO), N,N-dimethyl formamide (DMF), N-methyl pyrrolidone (NMP), tetrahydrofuran (THF) and
acetone [13]. The soluble components of PoPD are assigned to low molecular weight fractions in the literature. On
the basis of spectral evidence, the ladder type structure of polyphenylenediamine is the more acceptable one [13].
This ladder PoPD exhibits a low yield, poor solubility, low molecular weight and brittle film forming properties,
which hampers further study of the polymer structure and other properties.
The synthesis of processable polymer containing free –NH2 functional groups from phenylenediamine monomer
is important for stable doping or potential application purpose with the help of such type of polar functional groups,
just like polyaminophenol [16-19]. However, after an extensive literature survey it was appeared that
polyphenylenediamine was not a well-established polymer and there were controversies about polymerization
mechanism, final structure and characteristics of the polymer [13]. Thus there is enough scope to investigate the
structure, polymerization mechanism and characteristics of the polyphenylenediamine. In this communication we
report the synthesis of polyphenylenediamine by oxidative chemical polymerization using ammonium persulfate
(APS) as an oxidative initiator in varying pH medium. The polymer was characterized by molecular weight
measurement, ultraviolet visible spectroscopy, Fourier transform infrared spectroscopy, proton magnetic resonance
spectroscopy and thermogravimetric analyses, DC-conductivity measurement. The properties of synthesized
polyphenylenediamine were explained on the basis of their structures.
2. Experimental
2.1. Materials
Crystalline synthesis grade ortho-phenylenediamine (Loba Chemicals, India.) was used as received. Hydrochloric
acid (S. D. Fine Chemicals, India), DMSO (Merck, India), DMF (Merck, India), THF (Merck, India), NMP (Merck,
India), acetone (Merck, India), Disodium hydrogen phosphate (Merck, India), Sodium carbonate (Merck, India),
sodium hydroxide (Merck, India) etc. were used without further purification. Double distilled water was used for
synthesis and washing purposes.
3. 1303Siddhartha Samanta et al. / Materials Today: Proceedings 2 (2015) 1301 – 1308
2.2. Synthesis of polymer
Poly(o-phenylenediamine) (PoPD) was synthesized from o-phenylenediamine (oPD) by ammonium persulfate
(APS) as an oxidizing agent in varying pH of reaction medium according to the described method [20]. The
polymerization medium was prepared by using disodium hydrogen phosphate and citric acid buffer and the required
pH, viz., 1, 2, 4, 6 was adjusted by adding aqueous dilute hydrochloric acid or sodium hydroxide solution. First
1.445 g (20 mM) oPD monomer was dissolved in 25 ml pH medium at room temperature with constant stirring.
Separately prepared 3.42 g (30 mM) APS solution in 10 ml same pH medium was added into the above solution in
one lot and the mixtures were stirred continuously for 10 h at room temperature. The drop wise addition of oxidant
solution was avoided as it was lead to lower molecular weight of resulting polymer [13]. The solid polymer mass
was filtered and it was washed with dilute HCl as well as water for several times till the filtrate became colorless
from light brown color. Finally the synthesized polymers were dried at 70-80 °C in an oven for about 12 h.
2.3. Solubility test and film casting
The polymer powder was ground well by using a mortar and pestle. For solubility testing, this polymer powder
(0.05-0.1 g) was taken in a test tube containing 2 ml experimental solvent. Then the test tube was shaken well for
20-30 min continuously by using a mechanical shaker at room temperature (30 °C). For polymer film casting, about
0.7-0.8 g PmAP was dissolved in 15-20 ml of the solvent and this solution was poured on a Petri dish of diameter 10
cm. Then the Petri dish was placed inside a vacuum oven maintained at 80 °C for 7-8 h for the evaporation of the
solvent.
2.4. Doping of the polymer
The synthesized PoPD powder was doped with very dilute hydrochloric acid (1 M) by solution doping technique.
The PoPD powder was dipped in hydrochloric acid for 24 h at room temperature. Then the powder was filtered and
washed with diethyl ether for 2-3 times. The PoPD doped with hydrochloric acid was dried in a vacuum desiccator
for a day. The well dried hydrochloric acid doped PoPD powder was made into a thin pellet of thickness about 0.29-
0.3 mm using a steel die having 1 cm diameter in a hydraulic press under a pressure of 5 tons.
2.5. Characterizations
In order to get an idea about functional groups present, the FTIR spectra of synthesized polymers powder were
recorded in Thermo Nicollet (model number Nexus 870) IR Sspectrometer. The spectrum was recorded in the pellet
form by mixing the dry powder material with KBr (Aldrich, 99%, FTIR grade) with in the region 400 cm-1
to 4000
cm-1
. The UV-vis spectra of the polymers were recorded using about 0.1 % DMSO solution (or suspension) using
Mikropack UV-VIS-NIR, DH 2000. The DMSO solvent was taken as a reference. For structural analyses of the
soluble synthesized polymers, 1
HNMR spectra of the samples were recorded in BRUKER (400 MHz) instrument
using d6-DMSO as solvent. The weight average molecular weights of the synthesized samples were determined by
the dynamic light scattering (DLS) method using Zetasizer Nano Series Nano-ZS, Malvern. Very dilute polymer
suspension in DMSO was used after filtering through a Fluoropore filter (Millipore Corp.). The weight loss of the
polymers powder was observed by thermogravimetric analysis (TGA) in a NETZSCH TG 209 F1 instrument under
nitrogen atmosphere. The weight loss was recorded from room temperature (30 °C) to 500 °C at a heating rate of 10
°C/min. A standard four-point probe (Schroder 1990) system with a Keithley 2400 programmable current
sourcemeter was used to measure the electrical conductivity of the doped polymer pellet. Conducting silver paste
adhesive was used at the contact point between the wires and polymer pellet. The voltage changes were measured
against an applied current and then conductivity ( ) of the sample was calculated using the equation [21]:
Where I, V and s are applied current, voltage measured against applied current and distance between the two
probes, respectively.
4. 1304 Siddhartha Samanta et al. / Materials Today: Proceedings 2 (2015) 1301 – 1308
3. Results and discussion
3.1. Structure of the synthesized polymer
The structure of the synthesized polymers was analyzed by spectroscopic analyses. The characteristic vibrational
bands obtained from FTIR spectra of the polymers are shown in Figure 1. In general, for all the samples a broad
band was appeared in the region 3690 cm-1
to 2250 cm-1
due to the stretching of aromatic C–H, hydrogen bonded –
NH2, and –NH– groups. The free –NH2 groups in the polymer back bone were hydrogen bonded with nearest
nitrogen of –NH– group present in the polymer chain and so –NH2 absorption band was appeared at about 3225 cm-1
as a broad peak. There were pair of bands at 1615, 1525 and at 1373, 1243 cm-1
due to stretching vibrations of
quinoid C-C, benzenoid C-C and quinoid C-N, benzenoid C-N [22] bonds respectively. Two distinct types of
spectral pattern were observed for the polymer synthesized in pH 1 or 2 medium and that synthesized in pH 4 or 6.
The characteristic vibrational bands obtained from FTIR spectra of polymer synthesized in pH 1 or 2 was less
intense with slightly lower wave number than that of the other type, though almost identical spectra was observed
for both the types. This was because the oligomers were formed in higher pH (4 or 6) while polymers were formed
in pH 1 or 2 medium. Probably, the polymer synthesized in pH 1 was ladder polymer and therefore it was insoluble.
As shown in Figure 2, in UV-VIS spectra only one absorption maxima with in the region 420-450 nm was
observed for all the samples. The absorption band was attributed to a — * transition associated with the phenazine
unit conjugated to long pairs of bridging nitrogens [15]. The peak for synthesized samples in pH 1, 2, 4, and 6 was
appeared at 431, 439, 425, and 419 respectively. That means, the peak was somewhat red shifted for synthesized
sample with decreasing order of the pH of the polymerization medium other than pH 2, which had highest red
shifting. From the UV-vis spectra, the formation of oligomers were confirmed in higher pH (4 or 6) while polymers
were formed in pH 1 or 2 medium. Moreover, in terms of conjugated electronic structure the polymer synthesized in
pH 2 medium was the distinct one.
The 1
HNMR spectra of samples synthesized in pH 2 and 4 or 6 (identical spectra was observed) are shown in
Figure 3. The 1
HNMR spectral analysis of polymer synthesized in pH 1 medium was not performed as it was
insoluble in organic solvent. In 1
HNMR (Figure 3) spectrum of the polymer synthesized in pH 2, chemical shift for
three aromatic hydrogens were appeared at 6.8 δ (multiplet), 7.5 δ (multiplet) and 7.8 δ (multiplet). As the signals of
three aromatic hydrogens was observed in 1
HNMR spectrum, the ladder structure having only two hydrogens was
not formed or formed very little. Other two strong singlet peaks were appeared at 6.23 δ and 6.9 δ for the –NH–
groups of ladder units and terminal as well as free –NH2 functional groups, respectively. The broad peaks at 5 δ
(doublet) and 5.6 δ (doublet) was appeared probability due to presence of some phenazine type of impurity. The
structure of the polymer synthesized in pH 2, confirmed from the 1
HNMR spectral analysis, was just like polyaniline
derivative with free –NH2 functional groups. However, some ladder unit might also be present with in the polymer
backbone. The 1
HNMR spectrum of sample synthesized in pH 4 or 6 was identical with that of compound 2,3-
diaminophenazine [23]. In the spectrum (Figure 3) chemical shifts were appeared at 7.5 δ (multiplet) and 7.8 δ
(multiplet) for two aromatic hydrogens with 6.23 δ (singlet) for terminal –NH2 groups and 6.9 δ (singlet) for the –
NH– groups of ladder units. From the derivation of the 1
HNMR peak it was estimated that the structure obtained
from pH 4 or 6 was oligomer having average weight average molecular weight 400 D rather than to be exactly
dimer. However, the weight average molecular weight determined by DLS method for the polymer synthesized in
pH 2 was 8 KDa.
5. 1305Siddhartha Samanta et al. / Materials Today: Proceedings 2 (2015) 1301 – 1308
4000 3500 3000 2500 2000 1500 1000 500
(d)
(c)
(a)
(b)
%Transmitance
Wavenumber (cm-1)
300 400 500 600 700 800
(a)
(d)
Absorbance(a.u.)
Wavelength (nm)
(c)
(b)
Fig. 1. The FTIR spectra of PoPDA synthesized in Fig. 2. The UV-VIS spectra of PoPDA in pH
pH medium (a) 1, (b) 2, (c) 4, and (d) 6. medium (a) 1, (b) 2, (c) 4, and (d) 6.
Fig. 3. The 1
HNMR spectra of PoPDA synthesized in pH medium (a) 2, and (b) 4 or 6.
3.2. Mechanism of polymerization
To explain the structural differences with varying pH medium, a product-oriented mechanism is proposed and
shown in Scheme 1. It has been reported that, two separate oxidation potential for oPD appears in higher pH and the
separation of the oxidation potential is gradually increased with the increasing pH. It was expected that both the –
NH2 groups were protonated and ladder structure was formed due to oxidative polymerization through both the
groups (Scheme 1). However, structure like polyaniline derivative with free –NH2 functional groups with few ladder
unit was formed in pH 2, as only one –NH2 group was protonated (Scheme 1) at that pH [20]. Exactly identical
polymer was found with in pH 3 medium. The by-product sulfuric acid, which was formed due to reduction of APS
oxidant during polymerization, was consumed by the used buffer solution. In pH 4 or 6 both the –NH2 groups were
was weekly protonated and the ladder type dimer or oligomer was formed in less acidic pH as acidic medium is a
necessary condition for the propagation of the chain [24,25].
6. 1306 Siddhartha Samanta et al. / Materials Today: Proceedings 2 (2015) 1301 – 1308
NH2
NH2
in pH 1
or low in pH 2 or pH 3
in pH 4
or pH 6
NH3
+
NH3
+
NH3
+
NH2
NH2
NH2
NH2
NH2
APS APS APS
NH3
+
NH2
NH2
NH2
Polymerization
Polymerization Polymerization
H
N
N
H
H
N
N
H
NH2
NH2n
OligomerLadder polymer
H
N
NH2
Open ring polymer
n n
Scheme 1. Proposed polymerization mechanism
3.3. Properties of synthesized polymers
The appearance of the synthesized mass in different pH medium was also different, viz., black mass obtained in
pH=1; deep brown in pH=2 and yellowish brown in other pH. Moreover, the synthesized polymers other than in
pH=1, which was insoluble, were soluble in strong acidic medium, DMSO, DMF, NMP and sparingly soluble in
dilute acid, tetrahydrofuran. Interestingly, the polymer synthesized in pH=2 was soluble in those organic solvents
but only brittle mechanically weak film was cast form DMSO or DMF. The polymer synthesized in pH 2 was
soluble due to having open ring structure like polyaniline derivative with free –NH2 functional groups while the
ladder polymer synthesized in pH 1 was insoluble. The ladder type dimer or oligomer formed in pH 4 or 6 was
soluble due to low molecular weight.
In order to known the thermal behaviours of synthesized samples, TGA traces was recorded and shown in Figure
4. An initial 3-5 % weight loss was observed for all the samples up to 110 °C due to the release of moisture present
within the sample. The moisture content for polymers synthesized in pH 1 and 2 was higher than that of oligomers
synthesized in pH 4 and 6, as moisture release was easy from low molecular weight oligomers. In the TGA traces of
all the samples a weight fall was observed at around 300 °C, which might be consider as degradation temperature for
the samples due to the release of some decomposition products by the breaking of substituted –NH2 function groups
of the polymers. However, the rate of weight fall for the ladder polymer synthesized in pH 1 having no free –NH2
function groups was quite slow. Due to low molecular weight, this rate of weight fall for the oligomers synthesized
in pH 4 and 6 was observed very fast than that of polymer synthesized in pH 2.
The I–V characteristics of the hydrochloric acid doped polymers synthesized in pH 1 and 2 at room temperature
are shown in Figure 5. From these characteristics better conducting behavior was confirmed for open ring polymer
than that of ladder polymer. The linearity was observed in I–V plots of the polymers and therefore the Ohmic
conduction mechanism was followed in these polymers. The average DC conductivity of the polymer doped with
hydrochloric acid was observed 8.8 x 10-5
S/cm for the synthesized open ring polymer (synthesized in pH 2) while it
was only 1.7 x 10-7
S/cm for the synthesized ladder polymer (synthesized in pH 1). Because of the nonplaner
conjugate ladder polymer synthesized in pH 1, the electron flow in the chain-length direction is disturbed and
therefore it showed less conductivity than that of open chain polymer synthesized in pH 2. However, the oligomers
synthesized in pH 4 or 6 were also nonconducting even after doping by hydrochloric acid.
7. 1307Siddhartha Samanta et al. / Materials Today: Proceedings 2 (2015) 1301 – 1308
100 200 300 400 500 600 700 800
0
20
40
60
80
100
(d)
(c)
(b)
%Residue
Temperature (
o
C)
(a)
0.0 0.2 0.4 0.6 0.8 1.0
0
50
100
150
200
(a)
Voltage(mV)
Currenet ( A)
(b)
Fig. 4. The TGA traces of PoPDA synthesized in Fig. 5. The I-V characteristic of HCl doped
pH medium (a) 1, (b) 2, (c) 4, and (d) 6. PoPDA in pH medium (a) 1 and (b) 2.
4. Conclusion
In order to study the influence of pH of the reaction medium on the structure and property of conducting poly(o-
phenylenediamine), the polymer was synthesized from o-phenylenediamine using ammonium persulfate (APS) as an
oxidizing agent in varying pH of reaction medium. From the structural analysis it was confirmed that ladder type
polymer was formed in pH 1 while open ring type amine derivative of polyaniline structure was obtained in pH 2 or
3. However, the ladder type dimer or oligomer was formed in pH 4 or 6. The polymerization mechanism is proposed
in this communication to explain the structural difference at different pH medium. Due to the presence of free polar
–NH2 functional group substitution the open ring polymer synthesized in pH 2 was well soluble in polar organic
solvent like DMSO and DMF. The open ring polymer showed better conducting behaviour and the average DC
conductivity of the polymer doped with hydrochloric acid was observed 8.8 x 10-5
S/cm for the synthesized open
ring polymer while it was only 1.7 x 10-7
S/cm for the synthesized ladder polymer. Thus, the polymer synthesized in
pH 2 medium was the distinct one in terms of its structure and properties than the others.
Acknowledgements
The authors gratefully acknowledge the financial support provided by the Department of Science and Technology
(DST), Government of India for this work.
References
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![1302 Siddhartha Samanta et al. / Materials Today: Proceedings 2 (2015) 1301 – 1308
weight, environmental stability etc. Among those, polyaniline (PANI) attracts much interest as intrinsically
conducting polymer due to advantages like easy synthesizability, better doping ability, architecture flexibility,
environmental stability, wide applicability etc. [1]. However, like the other conducting polymers solution
processability of PANI is also a great challenge due to its rigid rod like structure. To improve its processability much
effort have been made by chemical and electrochemical polymerization of aniline with ortho and meta-substituted
groups, viz., –OCH3, -CH2CH3, -CH3, -SO3H, -Cl, -F, -I etc. Although, in general, polymers of ortho or meta
substituted aniline give identical structure with better processability, ortho-isomer is preferred over the meta-isomer
for polymer synthesis as the former gives better yield and conductivity than the latter [2]. Moreover, the polyaniline
derivatives have some lower conductivity than that of PANI itself due to repulsion factor [2,3].
The synthesis of polyaniline derivatives containing free active functional groups (e.g., -NH2, -OH etc.) in the
backbone chain can offer better solution processability, better doping by various dopant and good performances
towards various applications. Thus recently, polymers have been synthesized chemically or electrochemically by
using ortho or meta –NH2 (phenylenediamine) and –OH (aminophenol) substituted aniline [4-12]. Unlike other
derivatives of aniline, both the amine and hydroxyl substituent in these two monomers can be oxidized
simultaneously with amine group of aniline. Thus the polymers have been reported to be insoluble in organic
solvents resulting poor processability as well as conductivity. Aromatic amine polymers have attracted much interest
of researchers due to their unique multifunctionality, involving variable conductivity, strong electroactivity, colorful
electrochromism, good optical properties, magnetic activity, environmental stability and thermal stability [13,14].
Phenylenediamine as a monomer had been used very little to synthesize polymer and [15] mostly, hydrochloric acid
was used for the chemical synthesis of polymers with different oxidants, e.g., persulfate including Na+
, K+
, and NH4
+
persulfates, iodine, H2O2, ceric ammonium nitrate, cupric nitrate etc. [4-8]. In most cases the researchers are reported
that the poly(o-phenylenediamine) (PoPD) are insoluble in organic solvents due to ladder structure. A few reports
are also focused that the polymer having ladder and/or open ring structure, are slightly soluble in dimethyle
sulfoxide (DMSO), N,N-dimethyl formamide (DMF), N-methyl pyrrolidone (NMP), tetrahydrofuran (THF) and
acetone [13]. The soluble components of PoPD are assigned to low molecular weight fractions in the literature. On
the basis of spectral evidence, the ladder type structure of polyphenylenediamine is the more acceptable one [13].
This ladder PoPD exhibits a low yield, poor solubility, low molecular weight and brittle film forming properties,
which hampers further study of the polymer structure and other properties.
The synthesis of processable polymer containing free –NH2 functional groups from phenylenediamine monomer
is important for stable doping or potential application purpose with the help of such type of polar functional groups,
just like polyaminophenol [16-19]. However, after an extensive literature survey it was appeared that
polyphenylenediamine was not a well-established polymer and there were controversies about polymerization
mechanism, final structure and characteristics of the polymer [13]. Thus there is enough scope to investigate the
structure, polymerization mechanism and characteristics of the polyphenylenediamine. In this communication we
report the synthesis of polyphenylenediamine by oxidative chemical polymerization using ammonium persulfate
(APS) as an oxidative initiator in varying pH medium. The polymer was characterized by molecular weight
measurement, ultraviolet visible spectroscopy, Fourier transform infrared spectroscopy, proton magnetic resonance
spectroscopy and thermogravimetric analyses, DC-conductivity measurement. The properties of synthesized
polyphenylenediamine were explained on the basis of their structures.
2. Experimental
2.1. Materials
Crystalline synthesis grade ortho-phenylenediamine (Loba Chemicals, India.) was used as received. Hydrochloric
acid (S. D. Fine Chemicals, India), DMSO (Merck, India), DMF (Merck, India), THF (Merck, India), NMP (Merck,
India), acetone (Merck, India), Disodium hydrogen phosphate (Merck, India), Sodium carbonate (Merck, India),
sodium hydroxide (Merck, India) etc. were used without further purification. Double distilled water was used for
synthesis and washing purposes.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)