Sulphonated polyaniline/MWCNTs nanocomposite: preparation and promising thermoelectric performance
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Here is concise description of in situ prepared polyaniline and its nanocomposite with multi-walled carbon nanotubes followed by sulfonation. Thus, prepared materials were characterized by Fourier transform infrared spectroscopy, X-ray diffraction analysis, field emission scanning electron microscopy and electro-thermal analysis. Incorporation of ultrasonicated multi-walled carbon nanotubes significantly increased the electrical conductivity due to π–π interaction of polyaniline with multi-walled carbon nanotubes and its back to back sulfonation further rendered fortification. Finally, as-prepared nanocomposite showed greater electrical conductivity as well as improved thermal stability in terms of DC electrical conductivity retention under isothermal and cyclic aging conditions.
KeywordsPani/MWCNTs nanocomposite In situ polymerization Sulfonation Isothermal and cyclic aging
The publication flood over 150 years old polyaniline and about 35 years aged rapidly rising carbon nanotubes [1, 2] evidently shows, how they have matured over the past years with a very broad spectrum and wide ranging from engineering and technological fields to their commercial and economical aspects till date. However, in spite of their astonishingly swift progress, both the polyaniline (Pani) and multi-walled carbon nanotubes (MWCNTs) have some limitations regarding their solubility and processibility . To combat these problems, synergistic nanocomposite formulations of these constituents have been the burning desire of the researcher’s world due to their outstanding mechanical and electronic properties . Despite intractable improvement ensued so far, backbone stiffness of polyaniline, limited stability of MWCNTs dispersions in water remains an ongoing challenge complicating its processing, management and ultimately the scope of their applications of these materials . On account of these and to optimize their efficiency for use in various applications such as high strength nanofibres, sensors, nanoelectronic wires, it has become of immense interest to attach some functional parts on the constituents [6, 7, 8]. These modifications (covalent or non-covalent) either by organic or inorganic species in general and especially by sulfonic groups considerably enhance their stability and solubility while strong surface acidity making it highly useful for sophisticated electronic applications and excellent catalyst support for highly dispersed metal nanoparticles [9, 10, 11, 12].
For this, various approaches followed to date are hydrothermal method , microwave-enhanced chemical modification , indirect chemical modifications , etc. which set in functional moieties like –SO3H, –COOH, etc. on MWCNTs and generally at elevated temperature. But, herein we have tried to attach functional group on polyaniline as well as over its nanocomposites with MWCNTs by concentrated H2SO4 at ice temperature 0–5 °C. Detailed studies of DC electrical conductivity retention were also done under isothermal and cyclic aging conditions to ascertain their stability for potential applications in latest sophisticated technologies.
Monomer “aniline” from E-Merck India Ltd. was purified by distilling twice before use. Multi-walled carbon nanotubes (MWCNTs) used in this study was purchased from Iljin Nano Tech, Seoul, Korea (diameter and average length were about 10–20 nm and 20 μm, respectively). Potassium persulphate (PPS), HCl (AR grade) and methanol were purchased from CDH India Ltd. and were used as received. Double distilled water was used in all the experimental procedures and washing.
Preparation of Pani(EB) and Pani(EB)/MWCNTs nanocomposite
The nanocomposite of Pani(EB)/MWCNTS was prepared by in situ oxidative polymerization of aniline. Aniline (5 mL) was dropwise added to 200 mL of 1 M HCl under continuous stirring. The suspension of ultrasonicated nanoparticles of MWCNTs (0.15 g) in 100 mL of 1 M HCl was added into aniline solution. The solution which act as an oxidant was obtained by dissolving 14.8 g of K2S2O8 in 200 mL of 1 M HCl. The polymerization of aniline was affected by dropwise adding the oxidant solution into the reaction mixture and stirring continuously for about 22 h. The resultant greenish black slurry was filtered and washed thoroughly with 2.5 L double distilled water to remove excess acid as well as oxidant until filtrate became colorless. Thus, prepared nanocomposite was dedoped by aqueous ammonia (1 M) solution to convert it into emeraldine base (EB) form. Pani(EB)/MWCNTs were dried at 70 ± 5 °C for 2 h in an air oven, converted into fine powder and was stored in an airtight sample tube for further investigations. Pani(EB) was also prepared using the same method.
H2SO4 treatment of Pani(EB) and Pani(EB)/MWCNTs nanocomposite
Thus, prepared Pani(EB) and Pani(EB)/MWCNTs nanocomposite were treated by sulfuric acid using the earlier reported method . As-prepared Pani(EB) and Pani(EB)/MWCNTs nanocomposite were treated with H2SO4 (1 M) in an ice bath 0–5 °C under constant stirring for 4 h. The resultant greenish suspension was filtered, washed with water followed by 500 mL of methanol and dried in an air oven at 70 ± 5 °C The as-prepared Pani(EB) and Pani(EB)/MWCNTs nanocomposite were assigned the names as S-Pani and S-Pani/MWCNTs, respectively.
Results and discussion
Preparation of S-Pani and S-Pani/MWCNTS nanocomposite
FTIR spectroscopic studies
In case of H2SO4-treated form of Pani (S-Pani) as well as their nanocomposite (S-Pani/MWCNTs), almost all the vibrational peaks are much diffused. The characteristic peaks at 1578 and 1090 cm−1 may be attributed to SO3 asymmetric and symmetric stretching mode of vibration, respectively. Furthermore, peaks at 952 and 805 cm−1 correspond to the out-of-plane bending in a meta-substituted aromatic ring.
X-ray diffraction studies
In case of H2SO4-treated form of Pani (S-Pani) (Fig. 5c), as well as their nanocomposite (S-Pani/MWCNTs) (Fig. 5d), almost all the diffraction peaks are much diffused with very low intensity, indicating enhancement in amorphosity. Thus, results of XRD seem to support the possibility of sulfonation of these in situ products which was also implied by FTIR results.
Scanning electron microscopy (SEM) studies
DC electrical conductivity
The electro-thermal studies’ as-prepared in situ products can be measured only ES form of Pani(ES) and Pani(ES)/MWCNTs. While EB form of Pani as well as Pani/MWCNTs shows conductivity after sulfonation to form S-Pani and S-Pani/MWCNTs were very well done by standard 4-in-line probe technique. From the electrical conductivity measured, these materials were observed to behave as semiconductors within the temperature range of 40–150 °C. It was observed that the addition of MWCNTs has caused enhancement in electrical conductivity in as-prepared Pani(ES)/MWCNTs nanocomposite. Since both the Pani and MWCNTs are good conductors, the enhancement in DC electrical conductivity may be credited to the additive synergism of both the constituents interacting at molecular level. Further treatment with H2SO4 also caused promising augmentation in electrical conductivity of S-Pani as well as S-Pani/MWCNTs. However, the higher electrical conductivity of S-Pani/MWCNTs compared with S-Pani can basically be attributed to the extra presence of MWCNTs in the nanocomposite and its back to back internal doping with sulfonic groups further rendered enrichment.
DC electrical conductivity retention
For better understanding of effect of accelerated aging on the DC electrical conductivity retention of these as-prepared materials, they were examined by isothermal as well as cyclic accelerated aging techniques.
Stability under isothermal aging conditions
Thus, the combined effect of incorporation of MWCNTs into Pani following sulfuric acid treatment enhances the stability in terms of retaining electrical conductivity leading to better stabilization of S-Pani/MWCNTs.
Stability under cyclic aging conditions
Likewise, S-Pani as well as S-Pani/MWCNTs showed a regular decrease in loss in electrical conductivity in subsequent cycles causing great improvement in thermal stability. This seems to be because of the development of highly strong covalent bond between –SO3H groups of H2SO4 with Pani.
Conclusion with future prospects
In summary, we have successfully prepared Pani and its nanocomposite with MWCNTs via oxidative polymerization. Furthermore, the effective supplementation of sulfuric acid has brightly improved their electrical properties. After detailed studies of characterizations and electrical properties, finally as-prepared S-Pani/MWCNTs nanocomposite was observed to possess greater electrical conductivity and better isothermal stability in terms of electrical conductivity retention than Pani and S-Pani. This increment is attributed to the addition of ultrasonicated MWCNTs in the polymer matrix, and its back to back treatment with sulfuric acid further rendered enrichment. Thus, it may be postulated as a universal approach to prepare nanocomposite with enhanced electrical conductivity as well as enriched thermal stability which may find better realistic applications in modern electronic devices and seems to be a replaceable alternate even for metals in next generation.
One of the authors, Mahfoozurrahman Khan, acknowledges with thanks the financial support from University Grants Commission (UGC) New Delhi.
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