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Synthesis and characterization of polyaniline and poly(aniline-co-o-nitroaniline) using vibrational spectroscopy
⁎Tel.: +964 7504063415. kuestan_ahmad@yahoo.com (Kuestan A. Ibrahim)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Due to the advantages of material abundance and synthetic simplicity, polyaniline can be used as a high capacity cathode material. However, its practical application in battery has been hindered by poor electrochemical utilization and cycling instability. To solve these problems, we synthesized the Polyaniline-co-o-nitroaniline aniline. The copolymers were synthesized for 1:1 and 1:4 M ratios of aniline and o-nitroaniline in acidic medium using ammonium persulfate as oxidant and their properties were compared with that of polyaniline. The prepared samples have been characterized using number of techniques including Raman spectroscopy, FTIR, UV–vis, and conductivity. The polymers showed less electrical conductivity than polyaniline. Unlike polyaniline, the presence of nitro group caused higher frequency dependence of electrical conductivity. The FTIR bands at 1560, 1306 and 1148 cm−1 are corresponding to the polyaniline salt. The Raman band observed in the range of 1100–1140 cm−1 is the characteristic of conductive polyaniline and is due to the charge delocalization on the polymer backbone.
Keywords
Poly aniline
Copolymer
Industrial application
Raman spectroscopy
FTIR
o-Nitroaniline
1 Introduction
There is an explosive increase in the demand for composite materials in the last two decades, which are now available as materials with an unique combination of properties. Polyaniline (PANI) has been in the forefront of the global search for commercially viable conducting polymers because of its unique proton dopability, excellent redox recyclability, chemical stability, variable electrical conductivity (which can be varied by changing the pH at which it is prepared), low cost and ease of synthesis (Khalid and Mohammad, 2007). Thus, the composites based on conducting polymers and insulating polymers have been studied as materials for industrial products, such as rechargeable batteries, conductive coatings, light emitting diodes, gas sensors and antistatic materials (Singh, 2007).
Intrinsically conductive polymers, including polyaniline PANI have the utilization potential in a large number of applications: rechargeable batteries (Sivakkumar and Saraswathi, 2004), sensors, electronic devices (Ogura et al., 1995; Borole et al., 2004), light-emitting diodes (Falcoua et al., 2005), corrosion protection of metals (Karthikaiselvi et al., 2012; Elia et al., 2001), conducting paints and glues, electromagnetic shielding (Kuo et al., 1998; Upadhyay and Ahmad, 2010), antistatic formulations, gas-separation membranes coatings etc. However, the major problem in applying these polymers is their poor processibility by solvent or multitechniques. Most of them are insoluble in common solvents and undergo degradation before reaching the melting point. Also they have poor mechanical strength and are environmentally unstable. Many researchers have succeeded in overcoming these problems (MacDiarmid and Epstein, 1994; Waware and Umare, 2005; Ismail et al., 2008) and are generating polymer composites and blends (Heegar, 2001; Pant et al., 2006; Mzenda et al., 2002) which can show better stability and mechanical strength. Several methods were used to overcome this problem including N-substitutions or ring substitutions of aniline by aliphatic large radicals, block copolymers and doping PANI base with a functionalized protonic acid (Haba et al., 2000; Cao et al., 1992; Yoon et al., 1994). Conducting polymers such as polyacetylene, poly thiophene, and poly aniline (PANI) are semiconductors in their pure form but become highly conducting upon doping (Salih, 2011).
Copolymerization offers a way of improving the processibility of PANI. The properties of the poly(substituted anilines) like toluidine, anisidine, etc. depend on the type of substitution like electron withdrawing, electron donating groups (Scheme 1) or less affecting groups like alkyl groups. Electron withdrawing group decreases the electron density in aniline, electron donating group increases the electron density in the phenyl ring, whereas alkyl group may not affect much to the electron density in aniline only some mild positive inductive effect (+I) may increase the electron density (Upadhyay and Ahmad, 2010).
Copolymers of aniline with m-nitroaniline (poly(aniline-co-m-nitroaniline)) were readily synthesized in various molar ratios of co-monomers by chemical and electrochemical polymerization. The copolymerization rate decreased with an increasing ratio of m-nitroaniline in the monomer (Ding et al., 2012). Chemically oxidation copolymerization of aniline and o-chloroaniline with 4 M ratios has been performed using ammonium persulfate as an oxidant in HCl medium at 0–4 °C (Borkar, 2012) (showed structure of copolymers in Scheme 2).
The effect of different compositions of monomers on the transport properties of poly(aniline-co-o-anisidine) by various techniques such as optical, electrical and magnetic has been investigated and compared with the homopolymers (Umare et al., 2002). Electro chemical co-polymerization of o-anisidine with o-toluidine was carried out in 1 M H2SO4 by cyclic voltammetry. The copolymer compositions were altered by varying the monomer feed ratios during electro synthesis (Borole et al., 2007, 2006) (structure of copolymers as shown in Scheme 3).
Copolymers of o-/m-toluidine with o-nitro aniline were chemically synthesized in various molar ratios of the comonomers by emulsion polymerization. Although o-nitro aniline does not homopolymerize, the copolymers of o-nitro aniline with o-/m-toluidine could readily be synthesized. The copolymers show comparatively higher conductivity, better solubility and higher thermal stability than the homopolymers, ortho- and meta-polytoluidines (Savitha and Sathyanarayana, 2004). The polyaniline family is challenging because of its conductivity and solubility in polar organic solvents that depend not only on the oxidation state but also on the degree of protonation and nature of dopants. It is also observed that polyaniline-co-toluidine. Copolymer has better solubility than homo polymers in various organic solvents (Kumar, 2000; Bilal, 2007). In this work, poly(aniline-co-o-nitro aniline) was prepared and characterized using UV and FTIR spectroscopy emphasizing to study the effect of incorporation of o-nitro aniline on the resulting copolymers.
2 Materials and methods
2.1 Materials
Aniline (Fluka) was double distilled before use. o-nitro aniline (Aldrich) and all the reagents and solvents (ethanol, methanol) were analytical grade and used after further purification. UV–vis spectra were obtained on a Spectroscan80D, UV/visible Spectro-photometer, with quartz cells of 1 cm optical path. Intrinsic conductivity measurements were recorded using a Genard conductivity bridge (1659 RLC Digibridge, USA). Infrared spectra were recorded at room temperature using BIORAD Excalibur (FTS 3000MX series) instrument. Raman spectroscopy measurements were done on an ISA lab-ram model system using an Argon laser beam (∼50 mW 632.28 nm laser diode for excitation). A confocal aperture of 200 μm and a spectral resolution of 5 cm−1 were used.
2.2 Methods
2.2.1 Homo polymerization of aniline
In a typical poly aniline experiment, doubly distilled aniline (10 ml, 0.1097 mol) dissolved in hydrochloric acid (HCl) solution (1 M, 100 ml) was placed in a flask equipped with a dropping funnel and cooled to 0 °C. If monomers were not completely soluble in this medium, an additional HCl solution (1 M, 50 ml) was added.
The oxidizing agent was prepared by dissolving (1.25 g, 0.0548 mol) ammonium persulfate in HCl solution (1 M, 100 ml). The oxidant solution was added drop wise to aniline solution with vigorous stirring at 0 °C. A green precipitate formed after 5–10 min. The reaction mixture was stirred for 4 h at 0 °C and for 20 h at room temperature. The reaction mixture was poured into 200 ml water to complete the precipitation and the polymer was collected on a glass frit, washed with distilled water and HCl solution (1 M) to remove un-reacted monomer. Collected green precipitates of emeraldine hydrochloride were washed with distilled water till these washings were colorless. The filtered and washed precipitates were then treated with aqueous solution of ammonium hydroxide (pH ≈ 9) under stirring for 6–8 h for effective un-doping to obtain emeraldine base. This reaction mixture was filtered, washed with distilled water till washings were neutral to a pH indicator paper. The blue emeraldine base powder obtained was then filtered and washed with a water and methanol mixture and finally dried under vacuum for 48 h.
2.2.2 Copolymerization of aniline with o-nitro aniline (1:1, 1:4)
0.05 mol of each one of doubly distilled aniline and o-nitro aniline were dissolved in HCl solution (1 M, 150 ml) to prepare copolymer 1:1, and then placed in a flask was cooled to 0 °C. Based on the method of homopolymerization of aniline (2.2.1), (0.025 mol of aniline was mixed with 0.1 mol of o-nitro aniline to prepare copolymer 1:4).
3 Results and discussion
The copolymers were synthesized using two different molar ratios of 1:1 and 1:4 of aniline: o-nitro aniline in the comonomers feed. The general structure of the copolymer is shown in Scheme 1. The yield of the copolymers, decreases with increasing o-nitro aniline content in the co-monomer feed. This could be due to the highly electron withdrawing nature of the nitro group which reduces the electron density on the ring and also the steric interferences hindering polymerization. The results agree with those obtained by Upadhyay et al. (2010). Strong intermolecular interaction between nitro groups and the polaronic nitrogen atoms or hydrogen bonding can give rise to a favorable six member chelate ring, forming localized structure resulting in a lower conductivity.
The formation of hydrogen bonds between the oxygen of the –NO2 group and the –NH hydrogen localizes the electron density on the ring leading to further decrease in the rate of polymerization, (Scheme 4). It is of interest to note that o-nitro aniline does not homopolymerize under conditions employed for the polymerization of aniline and its derivatives. This could be due to the high electron withdrawing capacity of the nitro group in nitro aniline reducing the electron density of the ring (Kumar, 2000).
3.1 Raman spectroscopy
Fig. 1 shows the Raman spectroscopy analysis of PANI and poly(aniline-co-o-nitroaniline). The band at 1485 cm−1 has been assigned to an in-plane deformation of the C–C bond of the quinoid ring of the doped PANI and band at 1473 cm−1 corresponds to an in-plane deformation of the C–C bond of the quinoid ring of the doped copolymer (1:1), and (1:4) (Cochet et al., 2000). PANI spectra showed bands at 1471 and 1338 cm−1, which are due to the C⚌N– and C–N– stretching in quinoid and benzenoid rings, respectively. While in case of copolymer (1:1), and (1:4), the same bands were investigated at 1468 and 1320 cm−1, respectively (Perepichka et al., 2004). A small band at 1174 cm−1 has been assigned to an in plane deformation of the C–C bond of the quinoid ring of the polyaniline (Cochet et al., 2000). The bands at 1382 and 1595 cm−1 correspond to C–N+ stretching and C–C stretching of the quinoid ring respectively (Wu et al., 2005). A band at 1487 cm−1 was assigned to the formation of bipolarons (Barnard and Goff, 1999). The Raman shift at 1247 cm−1 corresponds to the C–N stretching in polaronic units (Jain and poorni, 2010). The peak observed in the 1100–1140 cm−1 region is characteristic of conductive polyaniline and is due to the charge delocalization on the polymer backbone (Kim et al., 1988).
3.2 Fourier transform infrared (FTIR)
FTIR spectra of both PANI and doped copolymer, are shown in Fig. 2. The spectra of all the copolymers show the main bands in the region of 1590, 1508 and 1308 cm−1 corresponding to the ring-stretching vibrations of the quinoid and benzenoid rings of aniline and nitro aniline, respectively. Furthermore, the quinoid peak appears split, while the benzenoid band is not split in the spectra. The quinoid band is sensitive to the structure, and hence, two bands at 1595 and 1560 cm−1 arising from quinoid stretching of aniline and nitro aniline ring segments are observed. A band near 1385 cm−1 is assigned to the C⚌N+ stretching adjacent to the quinoid structure while a medium intensity band at 1310 cm−1 is associated with C–N stretching vibration in the alternate units of quinoid–benzenoid–quinoid rings as shown in Fig. 2. The peaks at 1304 and 1210 cm−1 correspond to N–H bending and the symmetric component of the C–C (or C–N) stretching modes. Most of the bands of the copolymers are asymmetric and symmetric stretching modes of the nitro group of o-nitro aniline which occur at 1510 and 1346 cm−1, respectively (Rao et al., 2000).
The band around 1170 cm−1 is attributed to B–(NH+)⚌Q structure which is formed during the protonation process. A band at 1110 cm−1 is assigned to the C–H in plane bending. This confirms the presence of o-nitro aniline segments in the copolymer and also suggests an increase in the conjugation of the nitro group with the ring. The significant lowering of the frequencies can be related to the hydrogen bonds formed between the oxygen of the nitro group and the hydrogen of the amine group (Kumar, 2000).
The bands at 1145 and 831 cm−1 can be attributed to the in-plane and out-of-plane C–H bending modes, respectively (Ding et al., 2012) The strong band at 1145 cm−1 in the base is much intense and broader in the salt spectrum. In addition to the above peaks, the spectrum of the polyaniline exhibits peaks at 3320 cm−1 that could be attributed to NH2 stretching mode and a band located at 1653 cm−1 related to the NH2 bending vibration, while the peak at 684 cm−1 is attributed to NH2 wagging. In addition, the peak at 1580 cm−1 confirms the presence of a protonated imine function, and the band characteristic of conducting protonated form is observed at 1242 cm−1.
3.3 UV–visible spectroscopy
The synthesized polymers were subjected to the physico-chemical characterization by using UV–vis spectroscopy as shown in Fig. 3.
The first absorption band of π–π∗ bond has appeared at 280–295 nm, is assigned to the π–π∗ transition of the benzenoid ring on the basis of the earlier studies on polyaniline and it is related to the extent of conjugation between the adjacent phenyl rings in the copolymer chain. The second absorption band located at 605–635 nm is attributed to the quinoid ring transition (charge transfer from HOMO of the benzenoid ring to LUMO of the quinoid ring) (Tzou and Gregory, 1993). This band is dependent on the overall oxidation state of the polymer. The absorption spectrum of the polyaniline salt doped with HCl shows bands at 326, 433 and 630 nm, with higher conductivity (Yang and Shaolin, 2008).
A characteristic band for polaron-π∗ transition appeared at 450 nm, indicating that the resulting PANI emeraldine salt was in the doped state (Abd Razak et al., 2009). At low concentrations of o-nitro aniline in the comonomers feed, delocalization due to the presence of alternating electron donating and withdrawing groups may not be very effective, resulting in a copolymer with lower conductivity.
At high or low concentrations of o-nitro aniline in the copolymer, steric effects of the substituent may predominate over the electronic effects and lead to a lowering in conductivity. This seems understandable since conductivity is dependent on the orientation of the phenyl rings as well as on the electron density of the ring (Kumar, 2000). The lower conductivity of the copolymer with a higher feed content of o-nitro aniline can be rationalized on the basis of the effects of the nitro groups on the copolymer structure.
3.4 Conductivity of poly aniline and copolymers
It is obvious that the conductivity of the copolymer decreases with increasing concentration ratio of aniline to nitro aniline in the mixture and is lower than that of polyaniline, the influence of weight of dopant material on the conductance of the polyaniline and polyaniline-co-o-nitro aniline is shown in Figs. 4 and 5 and the details of results presented in Tables 1 and 2. It is apparent that the conductance of the polymers increases as the weights of the dopant material increases. This is caused by both increased separation of the copolymer chain due to the presence of side groups and the presence of electron-withdrawing NO2 group in the copolymer chain (Upadhyay and Ahmad, 2010; Yang and Shaolin, 2008). Synthesized copolymers are depending on their synthesis, the dopants, and the concentration of the reactants used. The higher conductivity of polyaniline obtained by the present method could be attributed due to a more homogeneous protonation of the imine nitrogen and better ordered chain conformation of the polymer (Rao et al., 2002). It has been observed that, copolymers possess a lower conductivity as compared with homo polyaniline. This could be related to the presence of nitro group NO in the components. As well, the conductivity decreases with increasing nitro aniline content in copolymers. On the other hand, the conductivity increases with increasing dopant concentration given in Table 1. But also Higher electrical conductivity poly anilineCQ than copolymer (1:1, 1:4), copolymer (1:1) has higher conductivity from copolymer (1:4) by the higher nitro group decrease the conductivity of copolymer. The results are similar to those achieved by Rao et al., 2002.
| Weight (gm) | Conductivity (×10−3 S cm−1) (PANI) | Conductivity (×10−3 S cm−1) (co-polymer 1:1) | Conductivity (×10−3 S cm−1) (co-polymer 1:4) |
|---|---|---|---|
| 0.00 | 0.1225 | 0.130 | 0.035.8 |
| 0.01 | 0.148 | 0.187 | 0.118 |
| 0.02 | 0.163 | 0.334 | 0.168 |
| 0.03 | 0.372 | 0.461 | 0.197 |
| 0.04 | 0.570 | 0.537 | 0.374 |
| 0.05 | 0.668 | 0.656 | 0.482 |
| 0.06 | 0.735 | 0.748 | 0.630 |
| 0.07 | 0.856 | 0.874 | 0.764 |
| 0.08 | 0.955 | 0.979 | 0.916 |
| 0.09 | 1.184 | 1.097 | 1.060 |
| 0.10 | 1.378 | 1.207 | 1.428 |
| Weight (gm) | Conductivity (×10−3 S cm−1) (PANI) | Conductivity (×10−3 S cm−1) (co-polymer 1:1) | Conductivity (×10−3 S cm−1) (co-polymer 1:4) |
|---|---|---|---|
| 0.00 | 0.424 | 0.083.3 | 0.025 |
| 0.01 | 0.506 | 0.127 | 0.060.2 |
| 0.02 | 0.549 | 0.176 | 0.194 |
| 0.03 | 0.656 | 0.318 | 0.258 |
| 0.04 | 0.754 | 0.403 | 0.491 |
| 0.05 | 0.943 | 0.512 | 0.612 |
| 0.06 | 1.015 | 0.672 | 0.670 |
| 0.07 | 1.070 | 0.738 | 0.749 |
| 0.08 | 1.146 | 0.890 | 0.909 |
| 0.09 | 1.249 | 0.999 | 1.079 |
| 0.10 | 1.416 | 1.060 | 1.218 |
4 Conclusion
The o-nitro aniline was copolymerized with aniline by chemical oxidative polymerization with ammonium persulfate as the oxidizing agent in acidic medium using different feed ratios of the comonomers. Although o-nitro aniline does not homopolymerize, these copolymers exhibited higher conductivities and good solubility in solvent DMSO than the homo-polyaniline and this may be attributed to greater electron delocalization and the presence of hydrogen bonding and inter chain linking due to the nitro group of o-nitro aniline. Raman spectra indicate the presence of the quinoid ring in polyaniline structure. UV visible spectra show the presence of different rings, higher conductivity of polyaniline than copolyaniline-o-nitroaniline after doping with aminophenol sulfonic salt and bromophenole sulfonic salt, and that the conductivity of polyaniline doped with bromophenole sulfonic salt is slightly higher than that with aminophenol sulfonic salt.
Acknowledgements
I would like to gratefully acknowledge Dr. Mukhtar H. Ahmed (Department of Chemistry, College of Science, Salahaddin University, Kurdistan region, Erbil, Iraq. Nanotechnology Integrated Biomedical Engineering Center (NIBEC), University of Ulster, jordanstown, BT37 0QB, Belfast, UK.) for recording the FTIR, Raman spectra and helpful discussions. Also I thank Dr. Sheeren and Miss Bayan from the chemistry department for recording the UV–vis and conductivity measurements.
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