Improving Electrical Conductivity, Thermal Stability, and Solubility of Polyaniline-Polypyrrole Nanocomposite by Doping with Anionic Spherical Polyelectrolyte Brushes
© Su. 2015
Received: 10 May 2015
Accepted: 30 June 2015
Published: 25 July 2015
The extent to which anionic spherical polyelectrolyte brushes (ASPB) as dopant improved the performance of polyaniline-polypyrrole (PANI-PPy) nanocomposite was investigated. Different characterization and analytical methods including Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric analysis (TGA), scanning electron microscopy (SEM), and X-ray diffraction (XRD) confirmed that ASPB serving as dopant could improve the comprehensive properties of PANI-PPy nanocomposite. It was different from dopants such as SiO2, poly(sodium-p-styrenesulfonate) (PSS), and canonic spherical polyelectrolyte brushes (CSPB) which only enhanced the performance of PANI-PPy nanocomposite on one or two sides. The electrical conductivity of (PANI-PPy)/ASPB nanocomposite at room temperature was 8.3 S/cm, which was higher than that of PANI-PPy (2.1 S/cm), (PANI-PPy)/PSS (6.8 S/cm), (PANI-PPy)/SiO2 (7.2 S/cm), and (PANI-PPy)/CSPB (2.2 S/cm). Meanwhile, (PANI-PPy)/ASPB nanocomposite possessed enhanced thermal stability and good solubility. In addition, the effects of polymerization temperature, the molecular weight of grafted polyelectrolyte brushes, and storage time on electrical conductivity were discussed.
During the last several decades, conducting polymers have been the subject of numerous investigations due to their excellent physical and chemical properties originating from their unique π-conjugated system [1–3]. Among the conducting polymers studied, polyaniline (PANI) and polypyrrole (PPy) are of particular interest because of their promising electrical conductivity, high environmental stability, interesting redox properties, magnetoresistance (MR) behaviors, and electrochemical performances [4–10]. In comparison with numerous reports about PANI and PPy, researches on copolymers of aniline and pyrrole are still far from enough. Because the copolymer may overcome the shortcomings of a single π-electron in the homopolymer, obtaining composites with excellent property , studies on copolymerization of aniline and pyrrole gradually attract people’s attention. Electrochemical [12, 13] and chemical oxidative polymerization methods [14, 15] are the most common methods used in the synthesis of conducting copolymers. However, the large-scale application of PANI-PPy composite is sometimes limited by the difficulty of insolubility and infusibility of the material which can lead to poor electronic conductivity and mechanical properties. Therefore, the improvement of the comprehensive properties of PANI-PPy nanocomposite is of significance.
To date, most of the published research on this topic has been developed to improve the performance of conducting polymers on certain aspects. Xin et al.  prepared poly(aniline-co-pyrrole) nanocomposite by chemical oxidation polymerization using iron(III) chloride hexahydrate (FeCl3·H2O) as an oxidant and sodium dodecylbenzenesulfonate (SDBS) as a surfactant. They found that the nanocomposite had high electrical conductivity by selecting proper conditions for the synthesis process. Li et al.  proposed a method to prepare poly(pyrrole-co-aniline) nanofibrils using a template by chemical copolymerization technique. It was reported that the length, diameter, and thickness of copolymer nanofibers were controlled by using AAO as a template, and the copolymer nanofibers had good thermal stability. Moreover, as for PPy, enhanced mechanical properties and reduced flammability were obtained by doping with epoxy resin . However, since the properties of conducting polymers are mutually restraining, the improvement of their comprehensive performance is an important and difficult task.
In recent years, conducting polymers doped with polyelectrolyte have achieved outstanding progress . Wu and coworkers  developed PPy, which exhibited excellent electrical conductivity and solubility by using different concentrations of water-soluble polystyrene sulfonate (PSS). The reason for this may be that de-doping does not easily happen for doped ions due to the large size of the polyelectrolyte, so the electrical conductivity of conducting polymers is more stable. Meanwhile, the entanglement effect of the long flexible chains of the polyelectrolyte can effectively hinder the growth of copolymer chains, helping to enhance its solubility. It is undoubtedly a good reference for the development of conducting polymers with excellent performance. In addition, in order to improve the thermal stability, magnetoresistance, and processing performance of conductive nanocomposites, besides organic materials just like MWNTs/PANI , inorganic materials were used, such as PPy/Fe3O4 [22, 23], PPy/SiO2 [24, 25], and PPy/Co3O4 . In view of this, highly branched anionic spherical polyelectrolyte brushes (ASPB), consisting of polyelectrolyte chains affixing to the surface of spheres, may be a novel dopant of conducting polymers which can improve the performance of conducting polymers by introducing the brush polymers with certain functional groups.
This paper presented a facile method for the synthesis of (PANI-PPy)/ASPB nanocomposite by chemical oxidative polymerization. On the basis of a previous work , the advantage of ASPB serving as dopant in the synthesis of (PANI-PPy)/ASPB nanocomposite was evaluated. Compared to the PANI-PPy nanocomposite doped with PSS, SiO2, and canonic spherical polyelectrolyte brushes (CSPB), (PANI-PPy)/ASPB nanocomposite exhibited good performances of electrical conductivity, thermal stability, and solubility. Moreover, since electrical conductivity was an important performance for conducting polymers, the effects of polymerization temperature, the molecular weight of grafted polyelectrolyte brushes, and storage time on electrical conductivity were also studied.
Aniline and pyrrole (Sinopharm Chemical Reagents Co., Ltd, Shanghai, China) were distilled under reduced pressure before use. Ammonium persulfate (APS, 98 %) was purchased from Sinopharm Chemical Reagents Co., Ltd, Shanghai, China. Other chemicals and solvents including hydrochloric acid (HCl, 36–38 %) and ethanol were analytical reagents and were used without further purification. The ASPB (D z ≈ 100 nm, M w = 500–2000 g/mol) consisting of modified SiO2 cores and PSS brushes were prepared by surface-initiated polymerization . CSPB (D z ≈ 100 nm, M w = 2000 g/mol) were composed of SiO2 cores and poly(diallyldimethylammonium chloride) (p-DMMPAC) brushes.
Synthesis of (PANI-PPy)/ASPB Composite
Fourier transform infrared spectroscopy (FTIR) was obtained using a Nicolet AVATAR 360FT spectrometer (USA). The chemical composition of (PANI-PPy)/ASPB nanocomposite was inspected by energy-dispersive X-ray diffraction (EDX) spectroscopy attached to a scanning electron microscope (SEM) which was used to investigate the morphology of samples. It was recorded on a Quanta 200 microscope (FEI, Netherlands) operated at 30 kV. X-ray diffraction (XRD) measurements were performed on a Shimadzu “XRD-6000” instrument (Japan) operating at a voltage of 40 kV and a current of 40 mA with CuKα radiation, λ = 1.54060 Å. The samples were measured in a continuous scan mode at 5°–50° (2θ) with a scanning rate of 5°/min. Thermo-gravimetric analysis (TGA) was carried out on a SETSYS-1750 instrument at a heating rate of 10 °C/min under nitrogen atmosphere.
The solubility of the samples can be reflected by the conductivity of the saturated solution (T = 25 °C, pH = 6). It is measured with a DDS-12A digital conductivity meter (Hubei Provincial Institute of Measurement and Testing). The specific process is as follows: 10 mg of samples was dissolved in 4 mL of ethanol with stirring and heating to boiling, then the supernatant and soluble impurities were removed. The process was repeated twice. After 10 mL of ethanol was added and heated to boiling for the purpose of fully dissolving the samples, the samples were placed in a bath at constant temperature for 20 min to precipitate the solid. To avoid the effect of the solid particles suspended in the electrode on experimental results, 3 or 4 mL of supernatant was added into the beaker. Additionally, the conductivity of the saturated solution of ethanol used as reference was required to be measured.
Results and Discussion
In order to demonstrate that the conducting composites are composed of copolymers of aniline and pyrrole, instead of a simple blend of the homopolymers, the contrast FTIR spectra of the copolymers and blends are also shown in Fig. 2b. As can be seen from the figure, most characteristic absorption peaks for the blends also appear in the spectrum of the copolymers. However, compared with the blends which have only one C–N stretching vibration at 1130 cm−1 (Fig. 2b (a’)), two C–N stretching vibrations (1203 and 1097 cm−1) and a carbonyl group (1701 cm−1) are displayed (Fig. 2b (a)), which is consistent with the literature .
Electrical conductivity is determined using a RTS-4 four-point probe which is a simple apparatus for measuring the resistivity of conductive composites. Results indicate that the room-temperature electrical conductivities of PANI-PPy, (PANI-PPy)/PSS, (PANI-PPy)/SiO2, and (PANI-PPy)/CSPB nanocomposites are 2.1, 6.8, 7.2, and 2.2 S/cm, respectively, while (PANI-PPy)/ASPB nanocomposite shows a high value of electrical conductivity (8.3 S/cm). The increase in magnitude of the electrical conductivity is consistent with the FTIR results.
Evaluation of the Effects of Dopants
Effect of dopant species on the performance of conductive nanocomposites
Electrical conductivity (S/cm)
Conductivity of saturated solution (μS/cm)
Exothermic peak (°C)
It can be observed from the table that the contributions of each dopant on electrical conductivity, solubility, and thermal stability of PANI-PPy nanocomposite follow the order ASPB > SiO2 > PSS > CSPB, ASPB > PSS > CSPB > SiO2, and ASPB > SiO2 > PSS > CSPB, respectively. When doped with CSPB, the performances of conductive nanocomposites are not significantly improved except their solubility, indicating CSPB are not suitable as the dopant of conducting polymers. It may be due to the repulsive interaction between their cationic charge of brush layers and radical cation of conducting copolymers, so that CSPB do not enter into the conducting copolymer chains. For (PANI-PPy)/SiO2 nanocomposite, the addition of SiO2 may increase the structure regularity of conducting polymers, so the electrical conductivity and thermal stability of conductive composites improve. However, the insolubility of SiO2 in most solvents also makes it powerless in improving the solubility of conducting copolymers. Furthermore, the main advantage of PSS is that it can increase the solubility of PANI-PPy nanocomposite. On the one hand, its flexible polymer chains in the copolymer system hinder the growth of the copolymer chain, making it smaller and shorter. On the other hand, the hydrophilicity of PSS also promotes the water solubility of conducting copolymers. But studies have shown that it has no obvious effect on improving the thermal stability. For ASPB consisting of a SiO2 core and PSS brushes, just a combination of the advantages of SiO2 and PSS is conducted. The thermal stability and solubility of copolymers are enhanced by SiO2 particles and PSS chains, respectively. Therefore, ASPB are an excellent dopant which can improve the comprehensive performances of PANI-PPy nanocomposite.
Effect of Polymerization Temperature and the Molecular Weight of Grafted Polyelectrolyte Brushes on Electrical Conductivity
Effect of Storage Time on Electrical Conductivity
This paper proposes a simple and effective method to improve the comprehensive performance of conductive composites. By doping the ASPB, the electrical conductivity, thermal stability, and solubility of PANI-PPy composite are enhanced. The electrical conductivity of (PANI-PPy)/ASPB nanocomposite is 8.3 S/cm, which is higher than that of (PANI-PPy)/CSPB (2.2 S/cm), (PANI-PPy)/PSS (6.8 S/cm), and (PANI-PPy)/SiO2 (7.2 S/cm). Furthermore, the influences of polymerization temperature, the molecular weight of grafted polyelectrolyte brushes, and storage time on the electrical conductivity of (PANI-PPy)/ASPB nanocomposite are investigated. Results show that the long grafted chains and low reaction temperature help to improve the electrical conductivity of conductive composites.
This work is supported by Innovation Program of Shanghai Municipal Education Commission (15ZZ112).
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