Exploration on the microwave‐assisted synthesis and formation mechanism of polyaniline nanostructures synthesized in different hydrochloric acid concentrations

Abstract

Under microwave‐assisted synthesis, polyaniline (PANI) products with multiple nanostructures were synthesized by the oxidative polymerization of aniline and ammonium peroxodisulfate in the different concentrations of hydrochloric acid solutions. The structural analysis of PANI using FTIR, UV, and XPS indicated that phenazine‐like oligomers were produced in acid‐free and low acidic systems. Moreover, long linear PANI chains were obtained in the presence of highly acidic solutions. The morphology of PANI observed by SEM and TEM showed that nanoscale structures, including stacked sheets, nanotubes, branched nanofibers, and uniform nanofibers, occurred respectively in acid‐free solution, low acidity, medium and high acidity systems, effectively regulating by acidity. The formation mechanism of PANI nanostructures was explored here. The sheets were formed by the oligomers containing flat phenazine rings that can be stacked together with strong π–π interactions. Furthermore, nanotubes were fabricated by the self‐curling of thin sheets consisted of phenazine‐like oligomers with numerous linear units in the chains. The nanofibers are supposed to form by the linear PANI chains and the secondary growth during aniline polymerization caused the branch formation on the nanofibers. All results indicate that acidity, rather than microwave assistance, is the critical factor that determines the polymerization mechanism and the final nanostructure. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 3357–3369

Polymerization of polyaniline under various concentrations of ammonium peroxydisulfate and hydrochloric acid by ultrasonic irradiation

“…The main features of the samples are rather similar. As shown in Figure 2(a), peaks at 1558, 1477, 1296, and 1226 cm −1 corresponded to quinonoid ring stretching, benzenoid ring (B) stretching, CN stretching of secondary aromatic amine, and CN stretching in B‐NH‐B‐NH‐B unit, respectively, which concurred well with former literatures and announced the successful synthesis of PANI 29,32,40,41 . The peak at 1646 cm −1 corresponded to vibration of CC group, and the maxima at 977 cm −1 was ascribed to CH bend vibration of out‐of‐plane of alkenes, 42 which were from phenazine‐like structures 29 .…”

“…As shown in Table 1, with the increase in concentration of HCl, the E.C of the synthesized PANI also increases. The E.C value of 0.5 S/cm for 2 M HCl was found to be much higher than those reported by Qiu et al 32 (0.162 S/cm) who synthesized PANI by the assistance of microwave, and that of Reza et al 31 (0.36 S/cm) who applied direct mixing method. This confirms the advantage of ultrasonic irradiation over other techniques.…”

“…Furthermore, peaks at 909 corresponded to CH out‐of‐plane deformation vibration, while the one at 780 cm −1 was ascribed to monosubstituted or 1,2‐disubstituted benzenoid ring 43 . In addition, the peak at 1075 cm −1 was due to vibration of CN in amine, 42 and that at 869 cm −1 was ascribed to CH out‐of‐plane deformation for 1,4‐disubstituted benzenoid ring 32 . It is worthful to note that the peak at 1075 cm −1 was not from sulfur acid, which was wrongly corresponded by Qiu et al, 32 cause the same peak was found by Trchov et al 29 at spectrum of PANI samples polymerized by different oxidant not containing sulfur element, like potassium dichromate, iron (III) chloride and silver nitrate.…”

“…When the HCl concentration was 1 M, the intensity of this peak was relatively weak. Maxima at 872 and 794 cm −1 corresponded to CH out‐of‐plane deformation for 1,4‐disubstituted ring 32,42 . In addition, the peak at 1042 cm −1 was due to vibration of CN in amine 42 .…”

“…In addition, the peak at 1075 cm −1 was due to vibration of CN in amine, 42 and that at 869 cm −1 was ascribed to CH out‐of‐plane deformation for 1,4‐disubstituted benzenoid ring 32 . It is worthful to note that the peak at 1075 cm −1 was not from sulfur acid, which was wrongly corresponded by Qiu et al, 32 cause the same peak was found by Trchov et al 29 at spectrum of PANI samples polymerized by different oxidant not containing sulfur element, like potassium dichromate, iron (III) chloride and silver nitrate. Furthermore, the broad peak at 1115 cm −1 shown in Figure 2(b) corresponded to the vibration mode of NH + = Q in the ES form, as could been seen from the spectra, the intensity of this peak increased with the increase of HCl concentration, demonstrating the increasing protonation of PANI.…”