Polymerization of new aniline derivatives: synthesis, characterization and application as sensors

Article abstract of DOI:10.1039/d1ra02474d

This work is focused on modifying aniline monomers with various characteristics that allows one to study the effect of the substituent on the respective polymer. A series of new polyaniline (PANI) derivatives based on anortho-substituted aniline derivative, 2-(1-methylbut-2-en-1-yl)aniline, were synthesized and characterized. The structures and composition of the polymers that we synthesized were confirmed by elemental analysis, proton nuclear magnetic resonance (¹H NMR) spectroscopy, carbon nuclear magnetic resonance (¹³C NMR) spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), and ultraviolet-visible spectroscopy (UV). Characterization by FT-IR and UV-visible spectroscopy techniques indicated that the polymers exist in protonated emeraldine forms. Scanning electron microscope (SEM) results revealed that the surface morphology of the resulting polymers changed from a heterogeneous hierarchical to spherical structure upon changing the substituent in the aniline monomers. The polymers are soluble in common organic solvents, so they can be used to make films. The electrical properties of the polymers were studied and their high sensitivity to moisture and ammonia was demonstrated. The results of the studies showed the prospects of using thin polymer films in the design of chemical sensors. The impact of the substituent on the polymer characteristics is rationalized in terms of steric and electronic effects.

Full text of DOI:10.1039/d1ra02474d

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Polymerization of new aniline derivatives:
synthesis, characterization and application as
Cite this: RSC Adv., 2021, 11, 21006
sensors†
a
a
a
*
*
Lyaysan R. Latypova, Anastasia N. Andriianova,
*
Akhat G. Mustafin,
Ilnur N. Mullagaliev,b Shamil M. Salikhov,a Renat B. Salikhovb
and Gulsum S. Usmanovaa
This work is focused on modifying aniline monomers with various characteristics that allows one to study
the effect of the substituent on the respective polymer. A series of new polyaniline (PANI) derivatives based
on an ortho-substituted aniline derivative, 2-(1-methylbut-2-en-1-yl)aniline, were synthesized and
characterized. The structures and composition of the polymers that we synthesized were confirmed by
elemental analysis, proton nuclear magnetic resonance (1H NMR) spectroscopy, carbon nuclear
magnetic resonance (13C NMR) spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), and
ultraviolet-visible spectroscopy (UV). Characterization by FT-IR and UV-visible spectroscopy techniques
indicated that the polymers exist in protonated emeraldine forms. Scanning electron microscope (SEM)
results revealed that the surface morphology of the resulting polymers changed from a heterogeneous
hierarchical to spherical structure upon changing the substituent in the aniline monomers. The polymers
are soluble in common organic solvents, so they can be used to make films. The electrical properties of
the polymers were studied and their high sensitivity to moisture and ammonia was demonstrated. The
results of the studies showed the prospects of using thin polymer films in the design of chemical
sensors. The impact of the substituent on the polymer characteristics is rationalized in terms of steric
and electronic effects.
Received 29th March 2021
Accepted 28th May 2021
DOI: 10.1039/d1ra02474d
rsc.li/rsc-advances
properties. A review of literature data has shown that incorpo-
ration of substituents such as alkyl,15–17 alkoxy,18,19 and hydroxy
groups20,21 at the ortho position of the aromatic ring has
a benecial effect on the solubility of polymers. As a result,
polymers can be soluble not only in high polarity but also in
medium polarity solvents. If sulfo groups are incorporated into
the ring, the polymers become soluble in aqueous solutions.22
The use of this method of PANI modication not only improves
its solubility but also favors the formation of uniform thin
polymer lms.
Recently, lms of electrically conductive polyconjugated
polymers in resistive sensors, which are used to determine
various toxic gases due to high sensitivity, relatively low cost,
and simple design, were intensely studied.23–25
One of the main advantages of sensors based on electrically
conductive polyconjugated polymer lms is that they can
operate at room temperature.26
Introduction
Polyaniline (PANI) is among the most famous representatives of
the class of electrically conductive polymers. It has a unique set
of properties: redox activity, acid–base properties, electronic
and ionic conductivity, and high general stability and thermal
stability.1–4 For this reason, the polymer nds use in energy-
saving devices, for screening electromagnetic radiation, to
obtain antistatic and electrically conductive coatings, and as
a corrosion inhibitor.5–8 There is a signicant potential of PANI
application in medicine and heterogeneous catalysis.9–11
A
considerable drawback of PANI, similarly to all conductive
polymers, lies in its extremely low solubility in the majority of
typical solvents, which signicantly narrows the scope of its
application.12–14 One of the ways to overcome these problems
involves the functionalization of PANI that makes it possible to
increase the solubility while retaining many of its important
The most important feature that makes PANI a promising
material for sensors is that its chemical and optical properties
change due to protonation (doping) and deprotonation
(dedoping) processes that occur when it interacts with compo-
nents that can change the oxidation state of the polymer. PANI
derivatives are efficient materials for application in potentio-
metric and amperometric sensors for various gases (NH3, HCl,
aUfa Institute of Chemistry of the Russian Academy of Sciences, Republic of
Bashkortostan, pr. Oktyabrya 71, Ufa 450054, Russia. E-mail: an.chem17@gmail.com
bBashkir State University, Republic of Bashkortostan, Z. Validi St 32, Ufa, 450076,
Russia
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra02474d
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H2S, etc.) and in pH sensors.27–29 Therefore, the development of mmol) in ethyl acetate (150 mL). The mixture was stirred on
a technology for producing electrically conductive lms based magnetic stirrer under hydrogen. Once the reaction
on PANI derivatives and for creating chemical sensors based completed (TLC monitoring), the reaction mixture was ltered
a
thereon is a problem of current interest.
off and evaporated.
Previously, we performed a study on the synthesis of a new
2-(1-Methylbutyl)aniline (3). Yield: 2.8 g (93%). 1H NMR
functionalized monomer, ortho-(1-methylbut-2-en-1-yl)aniline, (CDCl3, 500 MHz) d: 1.02 (3H, t, 3J ¼ 7.4 Hz, H-40), 1.34 (3H, d, 3J
a
b
followed by conversion into a polymer.30 The resulting poly- ¼ 6.8 Hz, H-50), 1.38–1.49 (2H, m, H-30 , H-30 ), 1.60–1.67 (1H,
b
a
mer is soluble in NMP, DMF, DMSO and moderately soluble in m, H-20 ), 1.71–1.79 (1H, m, H-20 ), 2.84 (1H, m, H-10), 6.76 (1H,
CHCl3, acetone and THF. The presence of a double bond makes d, 3J6–5 ¼ 7.6 Hz, H-6), 6.89 (1H, t, 3J4–3 ¼ 7.6 Hz, H-4), 7.11 (1H,
it possible to involve this monomer in various chemical reac- t, 3J5–6 ¼ 7.6 Hz, H-5), 7.20 (1H, d, 3J3–4 ¼ 7.6 Hz, H-3). 13C NMR
tions followed by conversion to new polymeric products with (CDCl3, 500 MHz) d: 14.36 (C-40), 20.66 (C-50), 20.93 (C-30), 32.71
physicochemical properties that differ from those of the orig- (C-10), 39.28 (C-20), 116.08 (C-6), 119.13 (C-4), 126.21 (C-3),
inal polymer.
126.45 (C-5), 132.07 (C-2), 143.63 (C-1).
The purpose of this work was to synthesize new functional
General procedure for the synthesis of compounds 4 and 5.
monomers based on ortho-(1-methylbut-2-en-1-yl)aniline and Acetyl chloride (2.9 g, 37.2 mmol) was added with stirring in
study how the structure of the initial aniline derivatives affects small portions to a solution of compound 2 (3.0 g, 18.6 mmol) in
the properties of the resulting polymeric products.
anhydrous CH2Cl2 (50 mL). The mixture was stirred for 6–7 h at
room temperature. Once the reaction completed (TLC moni-
toring), the reaction mixture was washed with a saturated
solution of NaHCO3 (3 ꢁ 40 mL) and extracted with CH2Cl2 (3 ꢁ
30 mL). The extracts were combined and dried with MgSO4. The
solvent was removed under reduced pressure and the residue
was separated on a column with Al2O3 (eluent: petroleum ether–
EtOAc 7 : 1).
Materials and methods
Materials
Aniline was distilled under reduced pressure prior to use.
Dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP), N,N-
dimethylformamide (DMF), tetrahydrofuran (THF), toluene,
methanol (MeOH), chloroform (CHCl3), ethyl alcohol (EtOH),
ethyl acetate (EtOAc), benzene, acetonitrile (MeCN), methylene
chloride (CH2Cl2), petroleum ether, hexane, as well as penta-1,3-
diene were distilled prior to use. All the other chemicals were
analytical grade reagents and were used as procured. 4-Chlor-
opent-2-ene31 and 2-(1-methylbut-2-en-1-yl)aniline32 were ob-
tained by known procedures.
N-[2-(1-Hydroxy-1-methylbutyl)phenyl]acetamide (4) and N-{2-
[1-methylbut-1-en-1-yl]phenyl}acetamide (5). Yields: 2.9 g (70%) of
4 and 0.6 g (17%) of 5. For 4, 1H NMR (CDCl3, 500 MHz) d: 0.79
3
a
b
(3H, t, J ¼ 7.4 Hz, H-40), 1.17–1.32 (2H, m, H-30 , H-30 ), 1.49
b
a
(3H, s, H-50), 1.66–1.78 (2H, m, H-20 , H-20 ), 2.02 (3H, s, Ac), 6.90
3
(1H, d, J6–5 ¼ 7.5 Hz, H-6), 7.01–7.04 (2H, m, H-3, H-5), 7.12
(1H, t, 3J4–5 ¼ 7.5 Hz, 3J4–3 ¼ 7.5 Hz, H-4). For 5, 1H NMR (CDCl3,
500 MHz) d: 0.87 (3H, t, 3J ¼ 7.5 Hz, H-40), 1.70–1.76 (2H, m, H-
a
b
3
30 , H-30 ), 1.93 (3H, s, H-50), 2.10 (3H,s, Ac), 5.65 (1H, t, J2 –3 a
¼
0
0
7.3 Hz, 3J2 –3 b ¼ 7.3 Hz, H-20), 6.59–7.06 (2H, m, H-3, H-4), 7.21
Synthesis of the monomers
0
0
3
3
3
General procedure for the synthesis of compound 2. 2-(1- (1H, t, J5–4 ¼ 8.0 Hz, J5–6 ¼ 8.0 Hz, H-5), 8.27 (1H, d, J6–5
¼
13
Methylbut-2-en-1-yl) aniline 1 (2 g, 12.4 mmol) was heated with 8.0 Hz, H-6). For 4, C NMR (CDCl3, 500 MHz) d: 14.12 (C-40),
KOH (0.7 g, 12.4 mmol) for 1 h in a steel reactor at 300ꢀ. The 16.92 (C-30), 21.74 (Ac), 27.97 (C-50), 44.28 (C-20), 80.70 (C-10),
reaction mixture was cooled to room temperature. The product 122.80 (C-6), 124.02 (C-3), 126.06 (C-5), 128.17 (C-4), 128.91 (C-
was decanted from the solid residue and washed with water (2 2), 138.30 (C-1), 160.08 (Ac). For 5, 13C NMR (CDCl3, 500 MHz)
ꢁ 25 mL), extracted with EtOAc (2 ꢁ 15 mL), and dried with d: 14.03 (C-40), 22.62 (C-30), 24.79 (Ac), 25.07 (C-50), 120.09 (C-6),
MgSO4. The solvent was removed under reduced pressure.
123.81 (C-3), 127.60 (C-5), 128.12 (C-4), 131.33 (C-2), 131.94 (C-
2-(1-Methylbut-1-en-1-yl)aniline (2). Yield: 1.9 g (95%). 1H 10), 132.69 (C-20), 134.29 (C-1), 168.08 (Ac).
NMR (CDCl3, 500 MHz) d Z[E]: 1.04 [1.18] (3H, t, 3J ¼ 7.5 Hz, H-
General procedure for the synthesis of compound 6. KOH
a
b
40), 1.92–1.98 [2.29–2.35] (2H, m, H-30 , H-30 ), 2.07 [2.06] (3H, d, (2.9 g, 52.4 mmol) in MeOH (30 mL) was added to a solution of
4
J5 –2 ¼ 1.0 Hz, H-50), 3.78 (2H, br.s, NH2); 5.69 [5.61] (1H, dt, compound 4 (2.9 g, 13.1 mmol) in MeOH (20 mL) and the
0
0
3
J2 –3 a ¼ 7.5 Hz, 3J2 –3 b ¼ 7.5 Hz, 4J2 –5 ¼ 1.0 Hz, H-2 ), 6.79 [6.77] mixture was reuxed until the starting material was consumed
0
0
0
0
0
0
0
(1H, dd, 3J6–5 ¼ 8.1 Hz, 4J6–4 ¼ 1.5 Hz, H-6), 6.86 [6.84] (1H, dt, (TLC monitoring). A saturated NH4Cl solution was added to the
3J4–5 ¼ 7.5, J4–3 ¼ 7.5 Hz, J4–6 ¼ 1.5 Hz, H-4), 7.05 [7.11] (1H, reaction mixture and the reaction products were extracted with
3
3
dd, 3J3–4 ¼ 7.5 Hz, 3J3–5 ¼ 1.3 Hz, H-3), 7.18 [7.15] (1H, ddd, 3J5–6 EtOAc (3 ꢁ 30 mL). The combined organic extracts were dried
¼ 8.1 Hz, J5–4 ¼ 7.5 Hz, J5–3 ¼ 1.3 Hz, H-5). 13C NMR (CDCl3, with MgSO4 and evaporated. The residue was chromatographed
3
3
500 MHz) d: 14.37 [14.28] (C-40), 22.62 [21.77] (C-30), 24.58 [24.59] on Al2O3 (petroleum ether–EtOAc 8 : 1).
1
(C-50), 115.05 [115.51] (C-6), 118.30 [118.35] (C-4), 127.74
2-(2-Aminophenyl)pentan-2-ol (6). Yield: 1.7 g (75%). H NMR
[127.56] (C-5), 128.85 [128.90] (C-3), 131.36 [132.37] (C-20), (CDCl3, 500 MHz) d: 0.91 (3H, t, 3J4 –3 ¼ 7.4 Hz, H-40), 1.16–1.24
0
0
a
b
131.63 [131.35] (C-10), 133.16 [133.11] (C-2), 142.91 [143.16] (C- (1H, m, H-30 ), 1.32–1.41 (1H, m, H-30 ), 1.62 (3H, s, H-50), 1.84–
a
b
1).
1.89 (1H, m, H-20 ), 1.95–2.0 (1H, m, H-20 ), 6.64 (1H, d, 3J6–5
¼
General procedure for the synthesis of compound 3. Pd/C 7.5 Hz, H-6), 6.70 (1H, t, 3J5–6 ¼ 7.5 Hz, 3J5–4 ¼ 7.5 Hz, H-5), 7.04–
(10 mol%) was added to a solution of compound 1 (3.0 g, 18.6 7.07 (2H, m, H-4, H-5). 13C NMR (CDCl3, 500 MHz) d: 14.47 (C-
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General procedure for the synthesis of compound P-MB. The
compound was obtained from 2-(1-methylbutyl)aniline (1.0 g,
6.1 mmol), 1.7 g (7.6 mmol) of (NH4)2S2O8 and 0.2 mol Lꢂ1 HCl
by the procedure used for synthesizing P-PA.
Poly[2-(1-methylbutyl)aniline] (P-MB). Dark brown powder,
yield: 0.8 g (85%). 1H NMR (CDCl3, 500 MHz) d: 0.73–0.94 (m, H-
40), 1.10–1.34 (m, H-30, H-50), 1.43–1.64 (m, H-20), 2.88–3.05 (m,
H-10), 6.95–7.51 (m, Ph). 13C NMR (CDCl3, 500 MHz) d: 14.19 (C-
40), 20.71 (C-30), 21.60 (C-50), 33.48 (C-10), 39.75 (C-20), 125.68–
127.68 (C-3, C-5, C-6), 130.88 (C-2), 143.11 (C-4), 150.45 (C-1).
General procedure for the synthesis of compound P-AP.
Monomer 6 (1.0 g, 5.6 mmol) was dissolved in water (15 mL) and
the solution was oxidized with ammonium persulfate (1.28 g,
5.6 mmol) in water (15 mL). The reaction mixture was kept for
24 h without disturbance at room temperature. Aer that, the
solution was ltered and washed with a large amount of
distilled water. Subsequently, the polymer was washed with
a large amount of petroleum ether. The polymer was dried
under reduced pressure. For the doping procedure, the polymer
was treated with 1 M HCl solution for 24 h. The powder was then
ltered off, washed with petroleum ether and dried under
vacuum.
Fig. 1 Calibration of humidity and ammonia sensors: (1) power supply
unit, (2) container with water or ammonia, (3) sealed cabinet, (4)
polymer film humidity sensor, (5) electronic hygrometer, (6) ammonia
sensor, (7) Arduino UNO, (8) display.
40), 17.80 (C-30), 27.97 (C-50), 42.81 (C-20), 76.76 (C-10), 117.75 (C-
6), 117.77 (C-5), 126.69 (C-3), 128.08 (C-4), 129.99 (C-2), 145.41
(C-1).
Synthesis of polymers
General procedure for the synthesis of compound P-PA.
Monomer 2 (1.0 g, 6.2 mmol) was dissolved in 0.2 mol Lꢂ1 HCl
(50 mL) and the solution was oxidized with ammonium per-
sulfate (1.7 g, 7.5 mmol) in 0.2 mol Lꢂ1 HCl (50 mL). The
solution color changed from yellow to dark brown as the reac-
tion progressed. Aer 24 h, the solution was ltered and washed
with a large amount of 0.2 mol Lꢂ1 HCl. Subsequently, the
polymer was washed with a large amount of petroleum ether.
The polymer was dried under reduced pressure.
Poly[2-(2-aminophenyl)pentan-2-ol] (P-AP). Dark brown
powder, yield: 0.6 g (60%).1H NMR (CDCl3, 500 MHz) d: 0.72–
0.92 (m, H-40), 1.14–1.42 (m, H-30), 1.47–1.68 (m, H-50), 1.75–1.95
(m, H-20), 7.05–7.12 (m, H-6), 7.24–7.32 (m, H-5), 7.36–7.41 (m,
13
H-6). C NMR (CDCl3, 500 MHz) d: 14.21 (C-40), 17.50 (C-30),
28.32 (C-50), 43.72 (C-20), 76.55 (C-10), 123.27–130.17 (C-3, C-5, C-
Poly[2-(1-methylbut-1-en-1-yl)aniline] (P-PA). Dark brown
6) 137.16 (C-2), 139.54 (C-1).
1
powder, yield: 0.9 g (88%). H NMR (CDCl3, 500 MHz) d: 0.80–
1.03 (m, H-40), 1.67–1.92 (m, H-30), 1.98–2.08 (m, H-50), 5.60–5.74
(m, H-20), 7.01–7.35 (m, Ph). 13C NMR (CDCl3, 500 MHz) d: 14.01
Apparatus and characterization
(C-40), 22.75 (C-30), 25.32 (C-50), 127.65–130.23 (C-3, C-5, C-6, C- Proton (1H) and carbon (13C) nuclear magnetic resonance
10), 134.20 (C-20), 135.92 (C-2), 140.07 (C-4), 151.41 (C-1).
(NMR) spectra were measured in CDCl3 using a Bruker Avance
Scheme 1 Synthesis of 2-(1-methylbut-2-en-1-yl)aniline derivatives followed by the production of PANIs.
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spectrouorophotometer in DMSO solution at 298 K. The
microstructure of nanoparticles was studied with a TESCAN
MIRA 3 LMH high-resolution scanning electron microscope
(SEM) equipped with a detector of scattered and backward
scattered electrons. The survey was conducted at 10 kV accel-
erating voltage and under 10ꢂ3 Pa vacuum. For SEM analysis
a drop of the polymer solution was applied onto a substrate by
spin coating. For sensing study the sitall substrate was cleaned
in an ethanol solution and dried for 10 min in an oven at 50 ꢀC.
A polymer lm was applied on top of substrate by spin-coating
from a 0.1 M DMF solution. The spin time was 2 minutes, and
the spin rates for three samples were 700 rpm. The following
equipment was used in the experiments to measure sensor
properties: MASTECH DC POWER SUPPLY HY3005D-2 power
supplies, a DMM 4020 multimeter as the ammeter, an Arduino
Uno programmable controller, a display, as well as DHT-11 and
MQ135 sensors in Fig. 1.
Fig. 2 FT-IR spectra of P-PA, P-MB and P-AP.
Table
assignments
1 The main absorption bands in FT-IR spectra and their
Results and discussion
Synthesis of samples
Bands and peaks (cmꢂ1
)
Assignments
2-(1-Methylbut-2-en-1-yl)aniline (1) was used as the starting
compound. To obtain 2-(1-methylbut-1-en-1-yl)aniline (2),
double bond isomerization was performed in the starting
compound by treatment with KOH at 300 ꢀC to give product 2
in 98% yield. Catalytic hydrogenation of compound 1 on Pd/C
(10%) in ethyl acetate at room temperature gave 2-(1-methyl-
butyl)aniline (3) in 92% yield. 2-(2-Aminophenyl)pentan-2-ol
(6) was obtained due to an unusual reaction in the course of
compound 2 acylation with acetyl chloride. In this synthesis,
we observed that yet another product, N-[2-(1-hydroxy-2-
methylbutyl)phenyl]acetamide (4), was formed along with N-
{2-[1-methylbut-1-en-1-yl]phenyl}acetamide (5). The content of
product 4 in the product mixture was 70%. The N-acetate
group was readily hydrolyzed upon reuxing compound 4 with
KOH in methanol to give tertiary alcohol 6 in 75% yield
(Scheme 1). The structures of compounds 2, 3 and 6 were
3350–3429 cmꢂ1
2773–2968 cmꢂ1
1608–1616 cmꢂ1
1504–1557 cmꢂ1
1368–1393 cmꢂ1
1117–1138 cmꢂ1
H-Bonded N–H groups
C–H stretching of alkyl structures
Quinone-ring stretching
Aromatic C]C
C–N stretching
]N+–/–N+c–
III spectrometer (500 MHz). Chemical shis are represented in
parts per million downeld from tetramethylsilane as the
internal standard. FT-IR spectra were recorded between 200 and
4000 cmꢂ1 in KBr pellets using a Specord M80 spectrometer.
Elemental analysis was performed on a Euro-2000 CHNS(O)
analyzer. UV-vis spectra were recorded on a Shimadzu Spec-
trophotometer 2600 in DMSO solution at 298 K, at wavelengths
ranging from 190 to 900 nm (slit width – 2.0 nm, scanning speed
– fast), using a 1 cm thick quartz tube. Fluorescence spectral
analysis was conducted using an RF-5301 PC Shimadzu
1
identied by H NMR and 13C spectra.
Chemical oxidative polymerization was carried out to
synthesize PANI derivatives; monomers 2 (1 g, 6.2 mmol) and 3
(1 g, 6.1 mmol) were added to 0.2 mol Lꢂ1 aqueous solution of
Table 2 Elemental analysis for the polymers synthesized
C (%)
Calc.
H (%)
Calc.
N (%)
Calc.
Cl (%)
Calc.
O (%)
Calc.
Total (%)
Calc.
Polymera
Found
Found
Found
Found
Found
Found
P-PA
P-MB
P-AP
74.68
73.85
67.78
71.21
71.89
65.44
7.36
8.39
7.70
6.78
8.19
7.37
7.92
7.83
7.19
7.57
7.35
6.87
10.04
9.93
9.11
3.33
4.87
1.19
—
—
8.22
—
—
8.05
100.00
100.00
100.00
88.89
92.30
88.92
a
Repeat unit.
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Table 3 The solubility of substituted polyanilinesa
ꢀ
temperature (20 C). The product was ltered off and washed
with a large amount of water followed by petroleum ether until
the ltrate became colorless. For the doping procedure, the
polymer was treated with 1 M HCl solution for 24 h. The powder
was then ltered off, washed with petroleum ether, and dried
under vacuum (Scheme 1).
Form
P-PA
P-MB
P-AP
Film color
Solubility
Dark-brown
Dark-brown
Dark-brown
DMSO, DMF, NMP,
CHCl3, CH2Cl2, acetone
DMSO, DMF, NMP, CHCl3,
CH2Cl2, acetone, EtOH, MeCN, THF
DMSO, DMF, NMP, CHCl3,
CH2Cl2, acetone, EtOH, MeCN,
THF, EtOAc, benzene, toluene
FT-IR spectra
The structures of the polymers that we synthesized were
conrmed by FT-IR spectroscopy. One can see in Fig. 2 that the
spectra of poly[2-(1-methylbut-1-en-1-yl)aniline] (P-PA), poly[2-
(1-methylbutyl)aniline] (P-MB) and poly[2-(2-aminophenyl)
pentan-2-ol] (P-AP) samples exhibit peaks characteristic of
PANI, in particular, the bands at 3032 cmꢂ1, 3245 cmꢂ1 and
3038 cmꢂ1 corresponding to the stretching vibrations of the ]
a
The solubility of polymers was measured using 100 mg of a sample in
5 mL of a solvent.
HCl (50 mL), and then the polymerization was conducted by
dropwise addition of an ammonium persulfate solution
(monomer/PSA ¼ 1.00/1.25) in 0.2 mol Lꢂ1 aqueous solution of
HCl (50 mL) as the oxidant at 20 ꢀC for 24 h. Then the PANIs was
ltered and the precipitate was washed with petroleum ether
and 0.2 mol Lꢂ1 HCl to remove aniline oligomers. Aer washing
and ltration, the solid was dried in vacuum to evaporate the
solvent from the sample completely.
Monomer 6 (1 g, 5.6 mmol) was dissolved in 15 mL of
distilled water and the solution was oxidized with a solution of
ammonium persulfate (1.28 g, 5.6 mmol) in water. The poly-
merization was continued for 24 h without disturbance at room
C–H bond. The absorption maxima in the regions of 3350 cmꢂ1
,
3420 cmꢂ1 and 3390 cmꢂ1 correspond to the vibrations of the
–N–H bond. The presence of alkyl substituents in P-PA, P-MB
and P-AP is conrmed by the absorption of C–H bonds in the
ranges of 2773–2968 cmꢂ1, 2870–2957 cmꢂ1, and 2829–
2961 cmꢂ1
. The absorption maxima in the regions of
1616 cmꢂ1, 1608 cmꢂ1 and 1613 cmꢂ1 correspond to the
stretching vibrations of the C]N bond in the conjugated
system. These absorption bands are consistent with the pres-
ence of quinonediimine moieties in the polyaniline chain. The
intense narrow absorption maxima in the samples at
Fig. 3 1H NMR spectra of the monomers and the corresponding polyaniline in CDCl3.
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1510 cmꢂ1, 1557 cmꢂ1 and 1504 cmꢂ1 are consistent with the a pure polymer in the emeraldine oxidation state, as demon-
presence of a benzene ring and correspond to its pulsation strated by the comparison with the calculated values.
vibrations. Moreover, the presence of a benzene ring is consis-
tent with the absorption bands at 798 cmꢂ1, 755 cmꢂ1 and
759 cmꢂ1 corresponding to the out-of-plane deformation
Solubility
vibrations of the C–H bonds. The peaks at 1461 cmꢂ1
,
As pointed out in the Introduction, the main purpose for
attaching alkyl chains to the polyaniline backbone was to
increase the poor solubility of unsubstituted polyaniline, espe-
cially in common organic solvents. Typically, when PANI doped
with mineral or organic acids is added to bipolar aprotic
solvents such as DMSO or NMP, a fraction of the polymer
material (presumably oligomers or low molecular mass mate-
rials) dissolves, but a residual mass is always present in the
mixture. Aer the successful synthesis of P-PA, P-MB and P-AP,
the solubility was determined in various organic solvents. P-PA
and P-MB (dark-brown solution) readily dissolved in polar
solvents such as DMSO, DMF, NMP and THF, as well as in less
polar solvents such as chloroform, acetone, and acetonitrile,
but were insoluble in benzene and toluene. It is noteworthy that
P-AP (dark-brown solution) was soluble in all the organic
solvents tested (Table 3).
1496 cmꢂ1 and 1450 cmꢂ1 correspond to the N]N bonds. The
peaks at 1373 cmꢂ1, 1368 cmꢂ1 and 1393 cmꢂ1 correspond to
the stretching vibrations of the C–N bonds. The absorption
peaks at 1124 cmꢂ1, 1138 cmꢂ1 and 1117 cmꢂ1 are due to the
vibrations of the –NH+] groups formed upon protonation of
the PANI's molecular chain (Table 1). This is evidence of the
degree of electron delocalization and the nature of its electrical
conductivity. The spectra of the P-AP polymer contain bands in
the region of 1368–1358 cmꢂ1 indicating the formation of
a tertiary alcohol. The peaks at 594–462 cmꢂ1, 536 cmꢂ1 and
541 cmꢂ1 correspond to various vibrations of the substituted
rings. Thus, it follows from IR spectroscopy data that the
structures of the polymers that we obtained are close to the
emeraldine structure.
Thus, modifying the alkyl in the PANI side chain is an effi-
cient method for enhancing the solubility of the polymer in
Elemental analysis
The results of elemental analysis are shown in Table 2. The total various types of solvents. Such a versatile solubility should be
of carbon, hydrogen, and nitrogen amounts in the polymers advantageous in the preparation of polymer lms for chemical
synthesized was over 90% in all cases. This is consistent with sensors.
Fig. 4 UV spectra (a), emission (b) and normalized excitation (c) photoluminescence spectra of the polymers in DMSO (10ꢂ2 g Lꢂ1).
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Table 4 Optical characterization data of the polymers
Optical properties
Studying the absorption spectra is the best way to perform
a comparative analysis of the effect of various structural factors
on the change in the molecular geometry of the polymer chain.33
It is known that in most cases, PANI and its derivatives manifest
two absorption maxima in the regions of 300 and 600 nm cor-
responding to the p–p* electron transition in the benzoid
moiety and the n–p* electron transition in the quinoid moiety.33
Similarly to literature data, the PANI derivatives synthesized
exhibit two absorption peaks in the electronic spectra (Fig. 4a).
The presence of a substituent at the aromatic ring of the poly-
mer resulted in a hypsochromic shi of the absorption maxima
(by 10–20 nm) in the electronic spectra relative to those of
unsubstituted PANI. The observed changes are due to the steric
effect of bulky substituents, the presence of which favors an
increase in the torsion angle between the aromatic rings and
a decrease in the degree of conjugation (Table 4).16 It was found
that in the series of PANI derivatives in question, the presence
of a hydroxy group in the substituent of P-AP favors the largest
shi in the absorption maximum due to a greater steric effect
compared to Alk-substituted PANI derivatives (P-MB, P-PA) that
manifest smaller hypsochromic shis. Comparative analysis of
literature data on other alkyl-substituted PANI derivatives
conrms the above regularities.16,30,33
Absorption
Sample
l1 (nm)
l2 (nm)
lEx (nm)
lEm (nm)
Ega (eV)
PPA30
P-MB
P-AP
285
300
274
290
—
430
370
306
315
500
450
377
397
1.91
1.90
1.91
2.06
515
523
519
P-PA
Eg ¼ 1240/ledge
a
1H NMR spectroscopy studies
Additional information about the structures of monomers and
polymers was obtained from NMR spectra. Interestingly,
displacement and hydrogenation of the double bond, as well as
the presence of a hydroxy group in the polymers, made these
samples soluble in polar solvent such as CHCl3, which enabled
us to record their NMR spectra. The NMR spectra of monomers
and polymers recorded in CDCl3 are shown in Fig. 3.
Analysis of the 1H NMR spectra recorded at 500 MHz showed
that aromatic protons in the starting monomers appeared in the
d 6.71–7.17 range as sharp peaks with clearly visible coupling
constants, while the corresponding signals in the polymers were
shied downeld to the region of d 7.06–7.62 and appeared as
broadened lines. The signals of the H-4 proton at 7.0 ppm and
of –NH2 at 3.80 ppm from the monomers disappear in 1H NMR
aer polymerization. The protons of the methylene groups in
the side chains are expected to appear at 0.72–5.74 ppm.
Previously, unsubstituted PANI was found to have lumines-
cent properties with a quantum yield of 0.1 only NMP solution,34
whereas the luminescence quantum yields of substituted PANI
derivatives are much higher.35 In addition, the luminescent
Fig. 5 PLE map of the solutions of P-AP (a), P-PA (b) and P-MB (c).
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properties of the resulting material can be controlled by rigidity.36 However, the opposite picture is observed for all the
changing the synthesis conditions and by varying the doping PANI derivatives studied, namely, incorporation of a bulky alkyl
agent. In view of this, uorescence studies of new PANI deriv- substituent increases the luminescence intensity. It was found
atives are very important. At the same time, analysis of changes previously that a PANI polymer should contain more than two
in the excitation and emission spectra would make it possible to sequentially linked benzenoid moieties to make photo-
estimate the effect of the substituent on the structure of the luminescence observable.38 Thus, the highest luminescence
polymer chain and on the optical properties of the compounds intensity is demonstrated by undoped PANI in fully reduced
being studied.
form. Analysis of the set of the PANI derivatives presented in
Fig. 4b shows the excitation and emission spectra of P-AP, P- this work allows one to make the assumption that incorporation
MB and P-PA recorded in DMSO solutions at room temperature. of an alkyl substituent complicates the oxidation of the amino
The uorescent properties of conjugated polymers can be group to an imino group.33 This assumption is conrmed by the
explained in terms of the band theory of semiconductors.36 On increase in the photoluminescence intensity with an increase in
exposure to radiation, electrons pass from the valence band of the substituent volume, that is, in accordance with the steric
a conjugated polymer to the conduction band and then migrate hindrance that it exerts.
back to the polymer chain. An excited electron and an oppo-
The photoluminescence excitation–emission maps (PLE)
sitely charged hole attract each other and undergo recombina- were studied for the series of PANI derivatives obtained in Fig. 5.
tion to emit a photon.37 According to the general concept of the One can see from the PLE maps that the position of the pho-
photoluminescence of compounds comprising a system of toluminescence maximum does not depend on the excitation
conjugated bonds, incorporation of a substituent into the wavelength and shis with an increase in the substituent
aromatic system should have resulted in a decrease in the volume, which also allows one to make the assumption that the
luminescence intensity due to a decrease in the structure
Fig. 6 SEM images of the polymers.
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steric effect has an inuence on the position of the photo-
emission maximum.
Scanning electron microscopy
The structural exibility and the dependence of physicochem-
ical properties on the synthesis conditions are considered to be
among the most important PANI properties. They make it
possible to identify the structure–property relationship for
nanosized materials and develop potential applications adapt-
ed for various types of morphologies, depending on the prop-
erties required.39
Fig. 6 shows the results of scanning electron microscopy. The
supramolecular structure of the samples obtained differs
signicantly from the structure of PANI and its alkyl derivatives
studied previously.39 Unsubstituted PANI obtained in the clas-
sical way has a brillar structure, whereas its ortho-substituted
derivatives are most commonly characterized by a globular
morphology comprising distinct spheres.39 Similarly to the
samples studied previously, the P-AP and P-PA polymers ob-
tained have a globular morphology, in contrast to P-MB which
has a heterogeneous hierarchical structure.
Fig. 7 Plots of the conductivity of P-PA, P-AP and P-MB films on the
air humidity.
humidity is due to an increase in the mobility of the doping ion
that is weakly bound to the polymer chain through van der
Waals forces.42 As the environment humidity increases, the
polymer lm absorbs moisture, which results in polymer
swelling followed by unfolding of the compact spiral-shaped
polymer chain. The formation of a more aligned structure of
the PANI derivative favors the charge transfer along the polymer
chain.43 It is also assumed that saturation of the polymer with
moisture is accompanied by the formation of a complex due to
the transfer of an unshared electron pair from the nitrogen
atom in the polymer to water molecules.43 The results of
measuring the current–voltage characteristics conrm the
assumption that the mobility of charge carriers along individual
polymer chains and across adjacent chains is high due to the
geometrically ordered, closely packed structure of the polymer
chains.44
The observed difference between the supramolecular struc-
tures formed by the polymers may be due to the nature of the
reacting monomer and the effect of the substituent at the
aromatic ring in the polymer chain.
Mw determination of polymers
Molecular weights of the polymers P-MB and P-AP were ob-
tained by gel permeation chromatography analyses. The
samples were dissolved in THF. Weight average MW (Mw) varies
from 59 796 to 67 268 g molꢂ1. Table 5 summarizes the Mw,
number average MW (Mn), and polydispersity index (PDI) of
these samples.
On the other hand, the polymer chains within the micro-
aggregated P-MB polymer lm were much less ordered and
less densely packed, which resulted in a greater number of
structural defects that apparently improve the sensitivity to
ammonia vapor (Fig. 7 and 8). The P-MB lm had the largest
roughness (Table 6).
The sensing properties of polymers
An important advantage of PANI and its derivatives that allows
one to use them in various elds of technology is that their
physicochemical parameters depend on the chemical compo-
sition of the environment.40 Gas sensors are considered to be
one the promising applications of PANI-based polymers.41 In
view of this, the P-MB, P-AP and P-PA polymers obtained were
used as sensitive materials in resistive sensors. The results are
presented in Fig. 7 and 8.
Based on SEM results, it is assumed that P-PA (Fig. 7) should
have the best response to chemicals in air because it has a more
developed surface. P-PA showed the highest lm conductivity in
humid air (Fig. 8). Presumably, an increase in the electrical
conductivity of a polymer lm with an increase in the ambient
Table 5 Molecular characteristics of the polymers
Sample
Mn (g molꢂ1
)
Mw (g molꢂ1
)
PDI
P-MB
P-AP
31 454
22 353
67 268
59 796
2.1
2.7
Fig. 8 Plots of the conductivity of P-PA, P-AP and P-MB films on the
concentration of ammonia vapor.
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Table 6 Surface roughness parameters of samples
aminophenyl)pentan-2-ol]. The polymers vary in solubility,
depending on the chemical structure of the substituents. The
PANI derivatives presented here demonstrated a high photo-
luminescence intensity. The changes in the emission spectra
made it possible to estimate the effect of structural factors on
the molecular geometry of the polymers obtained. However, the
electrical conductivity of modied polyanilines is lower than
that of pure PANI due to the smaller conjugation length. The
considerable increase in the solubility of the presented series of
polymers, i.e., P-MB, P-AP and P-PA, made it possible to form
homogeneous thin lms and use them as active layers in
resistive gas sensors. Varying the substituents in the polymer
structure resulted in sensors with various sensitivity to the
ambient humidity and ammonia concentration. The greatest
response to the moisture variation was obtained for the sensor
based on P-PA. The quality of the lm surface plays an impor-
tant role in the sensor properties of polymers; therefore, the
PANI derivatives were studied by AFM and SEM. It was found
that the polymer with the smallest roughness was most sensi-
tive to the environment quality.
Compound
P-PA
P-MB
P-AP
Average roughness Ra, nm
Root-mean-square roughness Rq,
nm
1.7
2.4
2.5
3
3.7
4.7
Thus, a detailed study of a number of PANI derivatives with
different substituents showed that the physicochemical prop-
erties of the resulting polymers can be tuned, which makes it
possible to use them in various elds of organic electronics.
Conflicts of interest
Fig. 9 Atomic force microscopy of the surface of polymer films.
The authors declare that they have no known competing
nancial interests or personal relationships that could have
appeared to inuence the work reported in this paper.
It is well known that PANI doping with HCl results in an
increase in electrical conductivity. Upon protonation, neutral
PANI molecules acquire protons to form positively charged local
centers located on the nitrogen atoms, which facilitate the
motion of valence electrons from one such center to another,
thus causing the hopping conductivity. When PANI reacts with
ammonia, the molecules of the latter absorb protons from PANI
to form energetically favorable ammonium (NH4+). As a result,
PANI dedoping occurs.
Considering that the carrier mobility depends on the
ordering of lm molecules on the surface45,46 and to explain the
reasons of the strong difference in the conductivity of lms, we
analyzed the lm surfaces on a scanning probe microscope in
atomic force microscopy (AFM) mode in Fig. 9. As one can see
from Fig. 9, the P-PA lm has a more uniform and smoother
surface than the P-AP and P-MB lms. In addition, P-PA lms
have lower roughness.
Acknowledgements
This work was supported by the Ministry of Science and Higher
Education of the Russian Federation as part of the state task no.
AAAA-A19-119020890014-7.
Notes and references
1 M. A. Shishov, V. A. Moshnikov and I. Y. Sapurina, Glass Phys.
Chem., 2011, 37, 106–110.
2 S. Trey, S. Jafarzadeh and M. Johansson, ACS Appl. Mater.
Interfaces, 2012, 4, 1760–1769.
3 V. G. Kulkarni, L. D. Campbell and W. R. Mathew, Synth.
Met., 1989, 30, 321–325.
4 Z. A. Boeva and V. G. Sergeyev, Polym. Sci., Ser. C, 2014, 56,
144–153.
5 S. Bhadra, D. Khastgir, N. K. Singha and J. H. Lee, Prog.
Polym. Sci., 2009, 34, 783–810.
6 Z. Li and L. Gong, Materials, 2020, 13, 548.
7 E. Akbarinezhad, M. Ebrahimi and H. R. Faridi, Prog. Org.
Coat., 2009, 64, 361–364.
Thus, the presented results of studying the surface of lms
with a scanning probe microscope in atomic force microscopy
mode show that the transport layers with higher conductivity
and moisture sensitivity have lower surface roughnesses.
8 E. Song and J. W. Choi, Nanomaterials, 2013, 3, 498–523.
9 M. R. Gizdavic-Nikolaidis, J. R. Bennett, S. Swi, A. J. Easteal
and M. Ambrose, Acta Biomater., 2011, 17, 4204–4209.
Conclusions
Thus, a series of hitherto unknown poly(o-alkylanilines) have
been synthesized in good yields, namely: poly[2-(1-methylbut-1- 10 L. A. Gallarato, L. E. Mulko, M. S. Dardanelli, C. A. Barbero,
en-1-yl)aniline], poly[2-(1-methylbutyl)aniline] and poly[2-(2-
D. F. Acevedo and E. I. Yslas, Colloids Surf., B, 2017, 150, 1–7.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 21006–21016 | 21015

View Article Online
RSC Advances
Paper
11 M. R. Gizdavic-Nikolaidis, J. C. Pagnon, N. Ali, R. Sum, 29 T. F. Wu and J. D. Hong, RSC Adv., 2016, 6, 96935–96941.
N. Davies, L. F. Roddam and M. Ambrose, Colloids Surf., B, 30 A. Andriianova, A. Shigapova, Y. Biglova, R. Salikhov,
2015, 136, 666–673.
I. Abdrakhmanov and A. Mustan, Chin. J. Polym. Sci.,
12 S. Shreepathi and R. Holze, Chem. Mater., 2005, 17, 4078–
4085.
2019, 37, 774–782.
31 V. I. Levashova and T. P. Mudrik, Pet. Chem., 2008, 48, 314–
13 H. Swaruparani, S. Basavaraja, C. Basavaraja, D. S. Huh and
317.
A. Venkataraman, J. Appl. Polym. Sci., 2010, 117, 1350–1360. 32 R. R. Gataullin, I. S. Afon'kin, A. A. Fatykhov, L. V. Spirikhin
14 J. Kim, S. Kwon and D. W. Ihm, Curr. Appl. Phys., 2007, 7,
205–210.
and I. B. Abdrakhmanov, Russ. J. Org. Chem., 2001, 37, 834–
840.
15 R. Bodalia, R. Stern, C. Batich and R. Duran, J. Polym. Sci., 33 A. N. Andriianova, Y. N. Biglova and A. G. Mustan, RSC Adv.,
Part A: Polym. Chem., 1993, 31, 2123–2127. 2020, 10, 7468–7491.
16 L. R. Latypova, A. N. Andriianova, S. M. Salikhov, 34 P. S. Antonel, E. M. Andrade and F. V. Molina, J. Electroanal.
I. N. Mullagaliev, R. B. Salikhov, I. B. Abdrakhmanov and
A. G. Mustan, Polym. Int., 2020, 69, 804–812.
17 A. R. Modarresi-Alam, H. A. Amirazizi, F. Movahedifar,
A. Farrokhzadeh, G. R. Asli and H. Nahavandi, J. Mol.
Struct., 2015, 1083, 17–26.
18 W. F. Alves, E. C. Venancio, F. L. Leite, D. H. Kanda,
L. F. Malmonge, J. A. Malmonge and L. H. Mattoso,
Thermochim. Acta, 2010, 502, 43–46.
Chem., 2009, 632, 72–79.
35 A. N. Andriyanova, D. E. Gribko, T. T. Sadykov,
I. N. Mullagaliev, R. B. Salikhov and A. G. Mustan, Polym.
Sci., Ser. B, 2021, 63, 135–141.
36 C. A. Parker, Photoluminescence of Solutions with Applications
to Photochemistry and Analytical Chemistry, ed. C. A.
Parker,New York, 1969.
37 F. A. Raqi and K. Majid, Synth. Met., 2015, 202, 147–156.
19 G. D'Aprano, M. Leclerc, G. Zotti and G. Schiavon, Chem. 38 J. Y. Shimano and A. G. MacDiarmid, Synth. Met., 2001, 123,
Mater., 1995, 7, 33–42. 251–262.
20 P. Anilkumar and M. Jayakannan, Langmuir, 2008, 24, 9754– 39 H. D. Tran, D. Li and R. B. Kaner, Adv. Mater., 2009, 21, 1487–
9762. 1499.
21 S. M. Sayyah, A. A. Bahgat and H. M. A. El-Salam, Int. J. 40 O. S. Ahmad, T. S. Bedwell, C. Esen, A. Garcia-Cruz and
Polym. Mater., 2002, 51, 291–314. S. A. Piletsky, Trends Biotechnol., 2019, 37, 294–309.
22 J. Huang and M. Wan, J. Polym. Sci., Part A: Polym. Chem., 41 D. Nicolas-Debarnot and F. Poncin-Epaillard, Anal. Chim.
1999, 37, 1277–1284. Acta, 2003, 475, 1–15.
23 R. B. Salikhov, A. N. Lachinov, R. G. Rakhmeev, R. M. Gadiev, 42 M. V. Kulkarni, S. K. Apte, S. D. Naik, J. D. Ambekar and
A. R. Yusupov and S. N. Salazkin, Meas. Tech., 2009, 52, 427–
431.
24 J. Janata and M. Josowicz, Nat. Mater., 2003, 2, 19–24.
B. B. Kale, Sens. Actuators, B, 2013, 178, 140–143.
43 M. V. Kulkarni and A. K. Viswanath, Polym. Eng. Sci., 2007,
47, 1621–1629.
´
25 G. Harsanyi, Sens. Rev., 2000, 20, 98–105.
44 F. W. Zeng, X. X. Liu, D. Diamond and K. T. Lau, Sens.
Actuators, B, 2010, 143, 530–534.
´
26 A. Pique, R. C. Y. Auyeung, J. L. Stepnowski, D. W. Weir,
C. B. Arnold, R. A. McGill and D. B. Chrisey, Surf. Coat. 45 C. D. Dimitrakopoulos and D. J. Mascaro, IBM J. Res. Dev.,
Technol., 2003, 163, 293–299. 2001, 45, 11–27.
27 F. W. Zeng, X. X. Liu, D. Diamond and K. T. Lau, Sens. 46 A. R. Tameev, R. G. Rakhmeev, V. R. Nikitenko,
Actuators, B, 2010, 143, 530–534.
R. B. Salikhov, A. A. Bunakov, A. N. Lachinov and
28 L. Kumar, I. Rawal, A. Kaur and S. Annapoorni, Sens.
Actuators, B, 2017, 240, 408–416.
A. V. Vannikov, Phys. Solid State, 2011, 53, 195–200.
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