Sonochemical synthesis of polyaniline nanofibers.

Conventionally, micro-sized irregular polyaniline (PANI) particles were synthesized by dropwise addition of the ammonium persulfate (APS) solution into the aniline (ANI) solution with mechanical stirring. By replacing the mechanical stirring with an ultrasonic irradiation, PANI nanofibers in diameters of approximately 50 nm and lengths of 200 nm to several micrometers were prepared. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) showed that at the early stage of polymerization, the polymers formed in both the mechanical stirred and ultrasonicated systems are in the form of nanofiber. However, with continuing of the reaction, these primary nanofibers grow and agglomerate into irregular shaped PANI particles in the mechanical stirred system, while in the case of the ultrasonic irradiation, the growth and agglomeration are effectively prevented, preserving thus the PANI nanofibers in the final product. By increasing the APS/ANI molar ratio from 0.5 to 2.5, the aspect ratios of the PANI nanofibers decreased. The PANI nanofibers exhibit higher solubility than the irregular shaped PANI particles. Although the yield, as well as the conductivity of the ultrasonic synthesized PANI nanofibers, was slightly lower than the irregular shaped PANI particles, the ultrasonic synthesis approach is one of the facile and scalable approaches in synthesizing PANI nanofibers in comparison with other ones without use of templates (e.g., the interfacial polymerization and rapid mixing polymerization). UV-Vis and Fourier transformed infrared (FTIR) spectra indicated ultrasound had no significant effect on the chemical structure of the PANI.