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Synthesis, characterization, optical and electrical properties of nanostructured poly(aniline-co-o-bromoaniline) prepared by in-situ polymerization method
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Synthesis, characterization, optical and electrical properties of nanostructured poly(aniline-co-o-bromoaniline) prepared by in-situ polymerization method
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A. Mahudeswaran a , J. Vivekanandan a , A. Jeeva a , J. Chandrasekaran b , P.S. Vijayanand a,∗ a
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Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam, Erode 638401, Tamil Nadu, India Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, Tamil Nadu, India
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Article history: Received 23 November 2015 Accepted 12 January 2016 Available online xxx
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Keywords: Aniline O-bromoaniline 17 Copolymer 18 19 DBSA 20 Q3 I–V characteristics) 15 16
A new series of novel nanostructured poly(aniline-co-o-bromoaniline) copolymer has been synthesized by in-situ oxidative polymerization method in the presence of an anionic surfactant, dodecylbenzenesulfonic acid and ammonium persulphate as an oxidant. These nanostructured copolymers are highly soluble in common organic solvents such as NMP, DMSO and DMF. The copolymers have been characterized by various techniques such as UV–visible spectra, FTIR, X-ray diffraction, surface morphology and electrical conductivity. The UV–vis spectroscopy confirms the –* transition and n–* transition. FTIR characteristic peaks confirm benzenoid and quinoid ring formation of copolymers. X-ray diffraction pattern shows the amorphous nature of the copolymer and decreased particle size. SEM image reveals smooth spherical agglomerated granular structure with an average size of 100–200 nm. I–V characteristics show that the current and voltage change linearly at equal monomer concentration. The conductivity of the copolymer is found to be in the order of 10−4 –10−7 S/cm. Since the conductivity value of the material is in the range of semiconductor and also due to ohmic nature, it can be used in the field of organic semiconductors. © 2016 Published by Elsevier GmbH.
1. Introduction
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Conjugated polymers have been extensively studied after the discovery of polyacetylene, polyaniline and polypyrrole [1]. Among these, polyaniline conducting polymer have found potential industrial applications in wide range because of its environmental stability, low cost, ease of synthesis and interesting electrical, electrochemical and optical properties [2,3]. It has tunable physical and electrochemical properties like oxidation, protonation, etc., [4–6]. It finds applications in various electronic devices, sensors, rechargeable batteries, conductive coatings, electromagnetic shielding and separation membranes [7,8]. Polyaniline can be easily synthesized through chemical oxidation method and electrochemical method with remarkable conductivity and it has a drawback of poor solubility in various solvents during industrial processing [9]. In order to improve the solubility of polyaniline, several research attempts have been made to increase the solubility by copolymerizing aniline with other substituted aniline derivatives, doping the polymer
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∗ Corresponding author. Tel.: +91 9942637573; fax: +91 4295226666). E-mail addresses: [email protected], [email protected] (P.S. Vijayanand).
with protonic acids, such as camphor sulfonic acid (CSA) [10] and dodecylbenzenesulfonic acid (DBSA) [11,12] and emulsion polymerization processes [13]. The properties of the poly(substituted aniline) depend on the substituent group like electron withdrawing, electron donating groups or less affecting groups like alkyl groups. Electron withdrawing group decreases the electron density in aniline and electron donating group increases the electron density in the phenyl ring of aniline. Surprisingly polyaniline and polypyrrole microtubes were prepared by in-situ doping polymerization in the presence of -naphthalene sulfonic acid as dopant and surfactant [14]. The template free method is a reliable method to synthesize tubular PANI derivatives. In soft template method, it does not need any membrane as a template and the nanostructures are formed by a self assembly process through molecular interactions such as hydrogen bonding, – band stacking and Van der Waals force [15]. Nowadays the nanostructure polymers possess unusual electrical when compared to the bulk counterpart. The different morphology of conducting polyaniline nanostructures like nanowires, nanotubes, nanoribbons and nanospheres are prepared by surfactant oriented template methods [16–20]. These new creative morphology structures greatly depend upon concentration of the monomers, surfactants and also chain length
http://dx.doi.org/10.1016/j.ijleo.2016.01.095 0030-4026/© 2016 Published by Elsevier GmbH.
Please cite this article in press as: A. Mahudeswaran, et al., Synthesis, characterization, optical and electrical properties of nanostructured poly(aniline-co-o-bromoaniline) prepared by in-situ polymerization method, Optik - Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.01.095
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of the surfactants [21]. Therefore in-situ method has unique importance on the main chain alignment especially for enhancement of specific electronic structural properties. Recently our research group has reported about the synthesis of nanostructured poly(aniline-co-m-aminoacetophenone) copolymer in the presence of DBSA surfactant as soft template [22]. It has been found that nanostructured copolymer is more soluble than homopolymer. Nanostructured poly(aniline-co-m-aminobenzoic acid) copolymer has been synthesized in presence of DBSA showed more solubility in N-methyl 2-pyrrolidone due to its plasticizing nature and possessing remarkable conductivity [23]. However, there are no research reports about DBSA doped poly(aniline-co-obromoaniline) copolymer. Hence in this manuscript we discussed the synthesis, characterization and conducting nature of DBSA doped poly(aniline-co-o-bromoaniline) copolymers and the effects of dopant are also studied by comparing the bulk polymer. Our goals are to investigate about how the side chain molecules and DBSA dopant ordering can influence the structural formation and will influence the electronic charge transport characteristics.
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2. Materials and methods
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2.1. Materials used
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Aniline (Ranken India) was purified by vacuum distillation method. o-Bromoaniline and dodecylbenzenesulfonic acid were purchased from and Hi Media (India). Ammonium persulphate and HCl were purchased from E-Merck (India). The solvents NMP, DMSO and DMF were purchased from Hi Media (India) and used as such. Double distilled water was used for the synthesis.
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2.2. Experimental
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A synthesis procedure for the preparation of DBSA doped copolymers of aniline with o-bromoaniline is as follows. 0.326 g (0.01 M) of DBSA is added in 80 ml of distilled water and stirred to form micelle. 0.05 M (0.466 g) of aniline and 0.05 M (0.860 g) of o-bromoaniline are added into the DBSA solution along with 10 ml (1 M) HCl. Then 0.1 M (2.28 g) of ammonium persulphate solution is added drop wise into the solution. The reaction mixture is stirred for 24 h. The temperature of the reaction mixture is maintained between 0 ◦ C and 5 ◦ C. A green colored precipitate is obtained. The obtained precipitate is washed with distilled water and acetone to remove the unreacted monomers and oligomers. The resultant copolymer is 3:3 ratio of aniline and o-bromoaniline respectively. Similar procedure has been adopted by varying the aniline to o-bromoaniline ratio as 3:2 and 3:1 respectively. The samples are dried using vacuum oven at 40 ◦ C and the samples are subjected to different characterization techniques. The synthesis of poly(aniline-co-o-bromoaniline) copolymer is shown in Scheme 1.
2.3. Instrumentation The UV-absorbance spectra of samples were recorded using ELICO SL-218 double beam spectrophotometer by dissolving the samples in dimethyl sulfoxide (DMSO). FT-IR characterization was done using a Nicolet Magna 560 FTIR spectrometer by making pellet with KBr in the range of 400–4000 cm−1 . X-ray diffraction pattern was analyzed using a Bruker GADDS X-ray Diffractometer. The surface morphology of the samples was analyzed by a JEOL JSM-6335F Scanning Electron Microscope. The I–V characteristics and conductivity of the sample were measured using a digital Keithley 6517B electrometer.
Scheme 1. Structure of poly(An-co-o-BrAn) copolymer.
Fig. 1. UV–visible absorption spectra.
3. Results and discussion
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3.1. Solubility
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Solubility is the important parameter of a material that paves the way for the application perspectives. DBSA doped poly(An-coo-BrAn) copolymer is found to be more soluble in various organic solvents such as N-methyl-2-pyrrolidone (NMP), DMSO, tetrahydrofuran (THF) and N,N-dimethylformamide (DMF). The increase in the solubility is due to decrease in the stiffness polymer and also there will be good interaction between the DBSA doped copolymer and the solvent molecules [15]. The presence of surfactant counter ion plays a major role in the solubility of the polymer. Conducting polymers with ordered structure have the advantage of a combination of high conductivity with good solubility properties. Jonforsena et al. reported that the presence of surfactant counter ion in the polymer chain can induce the solubility of the copolymer [24]. 3.2. Absorption spectrum Fig. 1 shows the UV–visible spectra of poly(aniline-co-obromoaniline) and DBSA doped copolymer poly(aniline-co-obromoaniline) for different concentrations of the comonomer. The
Please cite this article in press as: A. Mahudeswaran, et al., Synthesis, characterization, optical and electrical properties of nanostructured poly(aniline-co-o-bromoaniline) prepared by in-situ polymerization method, Optik - Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.01.095
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indicating more number of benzenoid and quinoid units respectively. The peak around 3220 cm−1 is the results of different types of inter and intra molecular hydrogen bonded N–H stretching vibrations of secondary amines. The characteristic sharp peak at 1301 cm−1 confirms the C–N stretching vibration of the aromatic amine units. The mild peak around 2920 cm−1 indicates the C–H symmetric stretching of DBSA. The appearance of broadband 1147 cm−1 is attributed to the electron delocalization on the polymer backbone leading to the conductivity nature of the copolymer [26]. The aromatic C–H out of plane bending is observed at 821 cm−1 indicating that the copolymers are formed through head-to-tail para-coupling [27]. The peak value 505 cm−1 indicates the presence of C–N–C bending vibration of amine group in the copolymer chain interconnecting two benzene rings through nitrogen. 3.4. XRD analysis
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Fig. 2. FTIR spectra of DBSA doped poly(An-co-o-BrAn) copolymer.
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HCl doped copolymer shows two peaks at 445 nm and 620 nm. The peak at 445 nm corresponds to –* transition and the peak at 620 attributes to excitonic transition between highest occupied molecular orbital of the benzenoid ring and the lowest unoccupied molecular orbital in the quinoid ring. The DBSA doped copolymer show two peaks around 330 nm and 615 nm. Both the characteristic peaks undergo a blue shift or hypsochromic shift indicating an increase in the size of substituents on the phenyl ring. When comparing to the HCl doped copolymer, the DBSA doped copolymer need much higher energy for –* electronic transition and little more energy for excitonic transitions and thus absorbing at lower wavelength region. The blue shift can also be ascribed to the presence of higher torsional angles that increases in size of the substituent in the phenyl ring [25]. Thus the lower shift indicates the effect of DBSA molecules doped into the conducting polyaniline copolymer chain and also confirms the formation of nanostructured particles having decreased conjugated bonds. 3.3. FTIR Fig. 2 represents the FTIR spectrum of DBSA doped poly(anilineco-o-bromoaniline) copolymer. The peak at 1039 cm−1 confirms the presence of the C–Br aromatic stretch in the ring. It can be seen from the spectrum that as the concentration of comonomer is increased, the intensity of the absorption peak at 1039 cm−1 also increases. The peak at 1500 cm−1 represents a C = C stretching vibration in the benzenoid ring. The peak at 1581 cm−1 represents C = C in quinoid ring. These two bands clearly suggest that the copolymer mainly composed of polyaniline units. As the concentration of o-bromoaniline is increased, we can observe the increase in the intensity of the peak at 1500 cm−1 and 1581 cm−1
Fig. 3 a and b shows the X-ray diffraction pattern of HCl doped and DBSA doped poly(aniline-co-o-bromoaniline) copolymer respectively. All the samples specify a broad peak showing the amorphous nature of the copolymer. The main diffraction peak at 2 = 20◦ and 25◦ is due to its periodicity parallel and perpendicular to the polymer chain respectively [28]. In DBSA doped copolymer the peak gets broadened indicating that the size of the doped copolymer particles is reduced than HCl doped copolymer [29]. The peaks located at smaller angle are the indications of a layered structure. This is due to the disordering probably due to the size of side chain and the substituents into the dopant molecule. The dopant enhances the formation of layers in polyaniline depending on the size of dopant counter ions and by the interaction of dopant ions into the free space of polymer chain with the covalently attached side chains. The number of associated dopants and free volume provided by covalently bonded alkyl side chains should be taken in to the cooperative stabilization of oriented structures. As the concentration of o-bromoaniline is increased, the diffraction peaks are weakened and sequentially broadened. This may be due to the presence of bromoaniline in the polymer chain that reduces the crystalline nature and further packing of the molecules may be due to irregular shapes inside the micelle. 3.5. SEM analysis Fig. 4a–d shows the scanning electron microscope images of poly(aniline-co-o-bromoaniline) and DBSA doped poly(aniline-coo-bromoaniline) copolymer nanoparticles, respectively. From Fig. 4a and b the agglomerated structure of the copolymer with irregular and coiled particles is observed. In the SEM image of DBSA doped copolymer, we can see a large number nanostructured copolymer of smooth and flat like surfaces, each of them with 100–200 nm
Fig. 3. (a) XRD pattern of HCL doped poly(An-co-o-BrAn) copolymer. (b) XRD pattern of DBSA doped poly(An-co-o-BrAn) copolymer.
Please cite this article in press as: A. Mahudeswaran, et al., Synthesis, characterization, optical and electrical properties of nanostructured poly(aniline-co-o-bromoaniline) prepared by in-situ polymerization method, Optik - Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.01.095
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Fig. 4. (a, b) SEM image HCl doped poly(An-co-o-BrAn) 3:3. (c, d) SEM image of DBSA doped poly(An-co-o-BrAn) 3:3.
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in size with interconnected network of particles with high surface area. It has also been pointed out that these spherical structures are contributed by the rapid diffusion of polymer chains in the micelle. The aniline nucleates produce stacks stabilized by interactions between phenazine-containing oligomers. The solubility of nucleates in water is restricted and resulting in the hydrophobic nucleates and they adsorb on completed polymer granules as droplets and start the growth of new granules on the surface of the former and thus resulting in fused granular morphology. While the first granule is produced by homogeneous nucleation, the further growth of globular structure is heterogeneous in nature. These nucleates randomly agglomerate in the continuous phase resulting in a granular morphology with porous characteristics [30]. Wan et al. reported about the formation of micelles from monomers and organic acids in the reaction mixture and this mixture act as a soft template for the growth of polymer resulting in a uniform and granular structure [31]. Thus the concentration of the surfactant determines the morphology and shape of the copolymer structure.
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3.6. I–V characteristics
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Fig. 5 a shows the I–V characteristics of the HCl doped copolymer and Fig. 5b–d show the I–V characteristics of DBSA doped copolymers. The graph between the current and the voltage is non-linear in nature for HCl doped copolymer and this is the characteristic peak for the semiconductor diode nature. The I–V curve of DBSA doped copolymers is linear and it confirms the direct charge transfer mechanism due to the ohmic behavior i.e. without any breakdown and it is associated with hopping mechanism [32]. It is interesting to see the increase in the linearity with increase in the concentration of o-bromoaniline in the feed and at equal concentration of the aniline and o-bromoaniline the I–V curve is extremely linear.
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3.7. Conductivity
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The conductivity of the DBSA doped poly(aniline-co-obromoaniline) is found to be 1.84 × 10−4 S/cm. The delocalization of charge transfer between the polaron units and conjugated bonds in the polymer backbone is the responsible for the conductivity
Fig. 5. (a) I–V characteristics of HCl doped poly(An-co-o-BrAn) 3:3. (b-d) I–V characteristics of DBSA doped poly(An-co-o-BrAn). Table 1 Electrical conductivity of poly(An-co-o-BrAn) copolymer. Copolymer
Conductivity, S/cm
Poly(An-co-o-BrAn) HCl 3:3 DBSA poly(An-co-o-BrAn) 3:1 DBSA poly(An-co-o-BrAn) 3:2 DBSA poly(An-co-o-BrAn) 3:3
4.6 × 10−5 1.84 × 10−4 4.29 × 10−6 8.19 × 10−7
of the DBSA doped copolymer. The conductivity value depends on the reaction of protons in DBSA with imine nitrogen atoms in the copolymers [33,34]. When the concentration of the o-bromoaniline content is increased in the feed the conductivity value decreases down to 10−7 S/cm and the values are listed in Table 1. The decrease in conductivity may be due to the presence of bromine atoms which are electronegative in nature in the polymer chain may decrease the charge transfer between the polarons and conjugated bonds. It is well known that doping process generates defects (bipolaronic sites) for the conjugated structure, which are essentially responsible for the electric conduction. These samples were one in which we found the good degree of layering of polymer chains and that formation of regular ordering improves the extent of electron delocalization but due to the presence of electronegative brominated aniline at the side chain that reduces the charge carrier considerably and improving the solubility. 4. Conclusion DBSA doped poly(aniline-co-o-bromoaniline) has been successfully synthesized through chemical oxidation polymerization and the polymers were subjected to various analytical characterizations. The characteristic peaks in FT-IR spectra confirm the formation of the benzenoid and quinoid units and formation of the copolymer. Absorption spectra reveal the transition from n to * and to * transition. The amorphous nature of the copolymer is confirmed by X-ray diffraction pattern and the surfactants decrease the particle size. I–V characteristic study shows the ohmic nature of the material. The electrical conductivity value of the copolymer is in the range of 10−4 S/cm and it is found to decrease as the bromoaniline content is increased. However the copolymer finds good solubility and conductivity values are found to be in the semiconductor range. The ohmic nature of the copolymer
Please cite this article in press as: A. Mahudeswaran, et al., Synthesis, characterization, optical and electrical properties of nanostructured poly(aniline-co-o-bromoaniline) prepared by in-situ polymerization method, Optik - Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.01.095
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material may find the path to act as a hole transporting layer in the organic solar cells. Thus dodecylbenzenesulfonic acid makes remarkable impacts over the optical properties, X-ray diffraction pattern, surface morphology, I–V characteristics and conductivity of poly(aniline-co-o-bromoaniline) copolymer. Further work in this direction will extend to synthesize more functional material for advanced optoelectronic and semiconductor applications.
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Acknowledgment
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The authors are thankful to Council of Scientific and Industrial Research (CSIR) for necessary funding (Grant no. 01/2514/11/EMRII) to carry out the research work and the Management of Bannari Amman Institute of Technology for providing necessary laboratory facilities for the work.
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Synthesis, characterization, optical and electrical properties of nanostructured poly(aniline-co-o-bromoaniline) prepared by in-situ polymerization method
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A. Mahudeswaran a , J. Vivekanandan a , A. Jeeva a , J. Chandrasekaran b , P.S. Vijayanand a,∗ a
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Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam, Erode 638401, Tamil Nadu, India Department of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641020, Tamil Nadu, India
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Article history: Received 23 November 2015 Accepted 12 January 2016 Available online xxx
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Keywords: Aniline O-bromoaniline 17 Copolymer 18 19 DBSA 20 Q3 I–V characteristics) 15 16
A new series of novel nanostructured poly(aniline-co-o-bromoaniline) copolymer has been synthesized by in-situ oxidative polymerization method in the presence of an anionic surfactant, dodecylbenzenesulfonic acid and ammonium persulphate as an oxidant. These nanostructured copolymers are highly soluble in common organic solvents such as NMP, DMSO and DMF. The copolymers have been characterized by various techniques such as UV–visible spectra, FTIR, X-ray diffraction, surface morphology and electrical conductivity. The UV–vis spectroscopy confirms the –* transition and n–* transition. FTIR characteristic peaks confirm benzenoid and quinoid ring formation of copolymers. X-ray diffraction pattern shows the amorphous nature of the copolymer and decreased particle size. SEM image reveals smooth spherical agglomerated granular structure with an average size of 100–200 nm. I–V characteristics show that the current and voltage change linearly at equal monomer concentration. The conductivity of the copolymer is found to be in the order of 10−4 –10−7 S/cm. Since the conductivity value of the material is in the range of semiconductor and also due to ohmic nature, it can be used in the field of organic semiconductors. © 2016 Published by Elsevier GmbH.
1. Introduction
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Conjugated polymers have been extensively studied after the discovery of polyacetylene, polyaniline and polypyrrole [1]. Among these, polyaniline conducting polymer have found potential industrial applications in wide range because of its environmental stability, low cost, ease of synthesis and interesting electrical, electrochemical and optical properties [2,3]. It has tunable physical and electrochemical properties like oxidation, protonation, etc., [4–6]. It finds applications in various electronic devices, sensors, rechargeable batteries, conductive coatings, electromagnetic shielding and separation membranes [7,8]. Polyaniline can be easily synthesized through chemical oxidation method and electrochemical method with remarkable conductivity and it has a drawback of poor solubility in various solvents during industrial processing [9]. In order to improve the solubility of polyaniline, several research attempts have been made to increase the solubility by copolymerizing aniline with other substituted aniline derivatives, doping the polymer
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∗ Corresponding author. Tel.: +91 9942637573; fax: +91 4295226666). E-mail addresses: [email protected], [email protected] (P.S. Vijayanand).
with protonic acids, such as camphor sulfonic acid (CSA) [10] and dodecylbenzenesulfonic acid (DBSA) [11,12] and emulsion polymerization processes [13]. The properties of the poly(substituted aniline) depend on the substituent group like electron withdrawing, electron donating groups or less affecting groups like alkyl groups. Electron withdrawing group decreases the electron density in aniline and electron donating group increases the electron density in the phenyl ring of aniline. Surprisingly polyaniline and polypyrrole microtubes were prepared by in-situ doping polymerization in the presence of -naphthalene sulfonic acid as dopant and surfactant [14]. The template free method is a reliable method to synthesize tubular PANI derivatives. In soft template method, it does not need any membrane as a template and the nanostructures are formed by a self assembly process through molecular interactions such as hydrogen bonding, – band stacking and Van der Waals force [15]. Nowadays the nanostructure polymers possess unusual electrical when compared to the bulk counterpart. The different morphology of conducting polyaniline nanostructures like nanowires, nanotubes, nanoribbons and nanospheres are prepared by surfactant oriented template methods [16–20]. These new creative morphology structures greatly depend upon concentration of the monomers, surfactants and also chain length
http://dx.doi.org/10.1016/j.ijleo.2016.01.095 0030-4026/© 2016 Published by Elsevier GmbH.
Please cite this article in press as: A. Mahudeswaran, et al., Synthesis, characterization, optical and electrical properties of nanostructured poly(aniline-co-o-bromoaniline) prepared by in-situ polymerization method, Optik - Int. J. Light Electron Opt. (2016), http://dx.doi.org/10.1016/j.ijleo.2016.01.095
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of the surfactants [21]. Therefore in-situ method has unique importance on the main chain alignment especially for enhancement of specific electronic structural properties. Recently our research group has reported about the synthesis of nanostructured poly(aniline-co-m-aminoacetophenone) copolymer in the presence of DBSA surfactant as soft template [22]. It has been found that nanostructured copolymer is more soluble than homopolymer. Nanostructured poly(aniline-co-m-aminobenzoic acid) copolymer has been synthesized in presence of DBSA showed more solubility in N-methyl 2-pyrrolidone due to its plasticizing nature and possessing remarkable conductivity [23]. However, there are no research reports about DBSA doped poly(aniline-co-obromoaniline) copolymer. Hence in this manuscript we discussed the synthesis, characterization and conducting nature of DBSA doped poly(aniline-co-o-bromoaniline) copolymers and the effects of dopant are also studied by comparing the bulk polymer. Our goals are to investigate about how the side chain molecules and DBSA dopant ordering can influence the structural formation and will influence the electronic charge transport characteristics.
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2. Materials and methods
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2.1. Materials used
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Aniline (Ranken India) was purified by vacuum distillation method. o-Bromoaniline and dodecylbenzenesulfonic acid were purchased from and Hi Media (India). Ammonium persulphate and HCl were purchased from E-Merck (India). The solvents NMP, DMSO and DMF were purchased from Hi Media (India) and used as such. Double distilled water was used for the synthesis.
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2.2. Experimental
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A synthesis procedure for the preparation of DBSA doped copolymers of aniline with o-bromoaniline is as follows. 0.326 g (0.01 M) of DBSA is added in 80 ml of distilled water and stirred to form micelle. 0.05 M (0.466 g) of aniline and 0.05 M (0.860 g) of o-bromoaniline are added into the DBSA solution along with 10 ml (1 M) HCl. Then 0.1 M (2.28 g) of ammonium persulphate solution is added drop wise into the solution. The reaction mixture is stirred for 24 h. The temperature of the reaction mixture is maintained between 0 ◦ C and 5 ◦ C. A green colored precipitate is obtained. The obtained precipitate is washed with distilled water and acetone to remove the unreacted monomers and oligomers. The resultant copolymer is 3:3 ratio of aniline and o-bromoaniline respectively. Similar procedure has been adopted by varying the aniline to o-bromoaniline ratio as 3:2 and 3:1 respectively. The samples are dried using vacuum oven at 40 ◦ C and the samples are subjected to different characterization techniques. The synthesis of poly(aniline-co-o-bromoaniline) copolymer is shown in Scheme 1.
2.3. Instrumentation The UV-absorbance spectra of samples were recorded using ELICO SL-218 double beam spectrophotometer by dissolving the samples in dimethyl sulfoxide (DMSO). FT-IR characterization was done using a Nicolet Magna 560 FTIR spectrometer by making pellet with KBr in the range of 400–4000 cm−1 . X-ray diffraction pattern was analyzed using a Bruker GADDS X-ray Diffractometer. The surface morphology of the samples was analyzed by a JEOL JSM-6335F Scanning Electron Microscope. The I–V characteristics and conductivity of the sample were measured using a digital Keithley 6517B electrometer.
Scheme 1. Structure of poly(An-co-o-BrAn) copolymer.
Fig. 1. UV–visible absorption spectra.
3. Results and discussion
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3.1. Solubility
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Solubility is the important parameter of a material that paves the way for the application perspectives. DBSA doped poly(An-coo-BrAn) copolymer is found to be more soluble in various organic solvents such as N-methyl-2-pyrrolidone (NMP), DMSO, tetrahydrofuran (THF) and N,N-dimethylformamide (DMF). The increase in the solubility is due to decrease in the stiffness polymer and also there will be good interaction between the DBSA doped copolymer and the solvent molecules [15]. The presence of surfactant counter ion plays a major role in the solubility of the polymer. Conducting polymers with ordered structure have the advantage of a combination of high conductivity with good solubility properties. Jonforsena et al. reported that the presence of surfactant counter ion in the polymer chain can induce the solubility of the copolymer [24]. 3.2. Absorption spectrum Fig. 1 shows the UV–visible spectra of poly(aniline-co-obromoaniline) and DBSA doped copolymer poly(aniline-co-obromoaniline) for different concentrations of the comonomer. The
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indicating more number of benzenoid and quinoid units respectively. The peak around 3220 cm−1 is the results of different types of inter and intra molecular hydrogen bonded N–H stretching vibrations of secondary amines. The characteristic sharp peak at 1301 cm−1 confirms the C–N stretching vibration of the aromatic amine units. The mild peak around 2920 cm−1 indicates the C–H symmetric stretching of DBSA. The appearance of broadband 1147 cm−1 is attributed to the electron delocalization on the polymer backbone leading to the conductivity nature of the copolymer [26]. The aromatic C–H out of plane bending is observed at 821 cm−1 indicating that the copolymers are formed through head-to-tail para-coupling [27]. The peak value 505 cm−1 indicates the presence of C–N–C bending vibration of amine group in the copolymer chain interconnecting two benzene rings through nitrogen. 3.4. XRD analysis
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Fig. 2. FTIR spectra of DBSA doped poly(An-co-o-BrAn) copolymer.
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HCl doped copolymer shows two peaks at 445 nm and 620 nm. The peak at 445 nm corresponds to –* transition and the peak at 620 attributes to excitonic transition between highest occupied molecular orbital of the benzenoid ring and the lowest unoccupied molecular orbital in the quinoid ring. The DBSA doped copolymer show two peaks around 330 nm and 615 nm. Both the characteristic peaks undergo a blue shift or hypsochromic shift indicating an increase in the size of substituents on the phenyl ring. When comparing to the HCl doped copolymer, the DBSA doped copolymer need much higher energy for –* electronic transition and little more energy for excitonic transitions and thus absorbing at lower wavelength region. The blue shift can also be ascribed to the presence of higher torsional angles that increases in size of the substituent in the phenyl ring [25]. Thus the lower shift indicates the effect of DBSA molecules doped into the conducting polyaniline copolymer chain and also confirms the formation of nanostructured particles having decreased conjugated bonds. 3.3. FTIR Fig. 2 represents the FTIR spectrum of DBSA doped poly(anilineco-o-bromoaniline) copolymer. The peak at 1039 cm−1 confirms the presence of the C–Br aromatic stretch in the ring. It can be seen from the spectrum that as the concentration of comonomer is increased, the intensity of the absorption peak at 1039 cm−1 also increases. The peak at 1500 cm−1 represents a C = C stretching vibration in the benzenoid ring. The peak at 1581 cm−1 represents C = C in quinoid ring. These two bands clearly suggest that the copolymer mainly composed of polyaniline units. As the concentration of o-bromoaniline is increased, we can observe the increase in the intensity of the peak at 1500 cm−1 and 1581 cm−1
Fig. 3 a and b shows the X-ray diffraction pattern of HCl doped and DBSA doped poly(aniline-co-o-bromoaniline) copolymer respectively. All the samples specify a broad peak showing the amorphous nature of the copolymer. The main diffraction peak at 2 = 20◦ and 25◦ is due to its periodicity parallel and perpendicular to the polymer chain respectively [28]. In DBSA doped copolymer the peak gets broadened indicating that the size of the doped copolymer particles is reduced than HCl doped copolymer [29]. The peaks located at smaller angle are the indications of a layered structure. This is due to the disordering probably due to the size of side chain and the substituents into the dopant molecule. The dopant enhances the formation of layers in polyaniline depending on the size of dopant counter ions and by the interaction of dopant ions into the free space of polymer chain with the covalently attached side chains. The number of associated dopants and free volume provided by covalently bonded alkyl side chains should be taken in to the cooperative stabilization of oriented structures. As the concentration of o-bromoaniline is increased, the diffraction peaks are weakened and sequentially broadened. This may be due to the presence of bromoaniline in the polymer chain that reduces the crystalline nature and further packing of the molecules may be due to irregular shapes inside the micelle. 3.5. SEM analysis Fig. 4a–d shows the scanning electron microscope images of poly(aniline-co-o-bromoaniline) and DBSA doped poly(aniline-coo-bromoaniline) copolymer nanoparticles, respectively. From Fig. 4a and b the agglomerated structure of the copolymer with irregular and coiled particles is observed. In the SEM image of DBSA doped copolymer, we can see a large number nanostructured copolymer of smooth and flat like surfaces, each of them with 100–200 nm
Fig. 3. (a) XRD pattern of HCL doped poly(An-co-o-BrAn) copolymer. (b) XRD pattern of DBSA doped poly(An-co-o-BrAn) copolymer.
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Fig. 4. (a, b) SEM image HCl doped poly(An-co-o-BrAn) 3:3. (c, d) SEM image of DBSA doped poly(An-co-o-BrAn) 3:3.
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in size with interconnected network of particles with high surface area. It has also been pointed out that these spherical structures are contributed by the rapid diffusion of polymer chains in the micelle. The aniline nucleates produce stacks stabilized by interactions between phenazine-containing oligomers. The solubility of nucleates in water is restricted and resulting in the hydrophobic nucleates and they adsorb on completed polymer granules as droplets and start the growth of new granules on the surface of the former and thus resulting in fused granular morphology. While the first granule is produced by homogeneous nucleation, the further growth of globular structure is heterogeneous in nature. These nucleates randomly agglomerate in the continuous phase resulting in a granular morphology with porous characteristics [30]. Wan et al. reported about the formation of micelles from monomers and organic acids in the reaction mixture and this mixture act as a soft template for the growth of polymer resulting in a uniform and granular structure [31]. Thus the concentration of the surfactant determines the morphology and shape of the copolymer structure.
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3.6. I–V characteristics
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Fig. 5 a shows the I–V characteristics of the HCl doped copolymer and Fig. 5b–d show the I–V characteristics of DBSA doped copolymers. The graph between the current and the voltage is non-linear in nature for HCl doped copolymer and this is the characteristic peak for the semiconductor diode nature. The I–V curve of DBSA doped copolymers is linear and it confirms the direct charge transfer mechanism due to the ohmic behavior i.e. without any breakdown and it is associated with hopping mechanism [32]. It is interesting to see the increase in the linearity with increase in the concentration of o-bromoaniline in the feed and at equal concentration of the aniline and o-bromoaniline the I–V curve is extremely linear.
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3.7. Conductivity
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The conductivity of the DBSA doped poly(aniline-co-obromoaniline) is found to be 1.84 × 10−4 S/cm. The delocalization of charge transfer between the polaron units and conjugated bonds in the polymer backbone is the responsible for the conductivity
Fig. 5. (a) I–V characteristics of HCl doped poly(An-co-o-BrAn) 3:3. (b-d) I–V characteristics of DBSA doped poly(An-co-o-BrAn). Table 1 Electrical conductivity of poly(An-co-o-BrAn) copolymer. Copolymer
Conductivity, S/cm
Poly(An-co-o-BrAn) HCl 3:3 DBSA poly(An-co-o-BrAn) 3:1 DBSA poly(An-co-o-BrAn) 3:2 DBSA poly(An-co-o-BrAn) 3:3
4.6 × 10−5 1.84 × 10−4 4.29 × 10−6 8.19 × 10−7
of the DBSA doped copolymer. The conductivity value depends on the reaction of protons in DBSA with imine nitrogen atoms in the copolymers [33,34]. When the concentration of the o-bromoaniline content is increased in the feed the conductivity value decreases down to 10−7 S/cm and the values are listed in Table 1. The decrease in conductivity may be due to the presence of bromine atoms which are electronegative in nature in the polymer chain may decrease the charge transfer between the polarons and conjugated bonds. It is well known that doping process generates defects (bipolaronic sites) for the conjugated structure, which are essentially responsible for the electric conduction. These samples were one in which we found the good degree of layering of polymer chains and that formation of regular ordering improves the extent of electron delocalization but due to the presence of electronegative brominated aniline at the side chain that reduces the charge carrier considerably and improving the solubility. 4. Conclusion DBSA doped poly(aniline-co-o-bromoaniline) has been successfully synthesized through chemical oxidation polymerization and the polymers were subjected to various analytical characterizations. The characteristic peaks in FT-IR spectra confirm the formation of the benzenoid and quinoid units and formation of the copolymer. Absorption spectra reveal the transition from n to * and to * transition. The amorphous nature of the copolymer is confirmed by X-ray diffraction pattern and the surfactants decrease the particle size. I–V characteristic study shows the ohmic nature of the material. The electrical conductivity value of the copolymer is in the range of 10−4 S/cm and it is found to decrease as the bromoaniline content is increased. However the copolymer finds good solubility and conductivity values are found to be in the semiconductor range. The ohmic nature of the copolymer
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material may find the path to act as a hole transporting layer in the organic solar cells. Thus dodecylbenzenesulfonic acid makes remarkable impacts over the optical properties, X-ray diffraction pattern, surface morphology, I–V characteristics and conductivity of poly(aniline-co-o-bromoaniline) copolymer. Further work in this direction will extend to synthesize more functional material for advanced optoelectronic and semiconductor applications.
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Acknowledgment
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The authors are thankful to Council of Scientific and Industrial Research (CSIR) for necessary funding (Grant no. 01/2514/11/EMRII) to carry out the research work and the Management of Bannari Amman Institute of Technology for providing necessary laboratory facilities for the work.
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