The effects of thermal treatment on the antioxidant activity of polyaniline

The thermal stability of chemically synthesized polyaniline (PANI) was examined, including granular (G) polyaniline powders formed conventionally in an HCl medium, and nanorod (NR) samples prepared using a falling-pH synthesis. The samples were examined before and after dedoping (dd) using thermogravimetric analysis (TGA), which showed small mass losses in the 200-300 °C temperature range, and greater mass losses due to oxidative degradation at higher temperatures. Furthermore, samples were treated thermally at 100, 125, 150, 175, 200, 250 and 300 °C for 30 min in air. SEM images did not show any pronounced effect on the morphologies of the samples from thermal treatment up to 300 °C. The ratios of the intensities (Q/B) of the predominantly quinonoid (Q) and benzenoid peaks (B) from FTIR spectroscopic analysis revealed that NR-PANI and NR-PANIdd underwent cross-linking upon thermal treatment up to 175 °C and were oxidized after treatment above 175 °C. G-PANI and G-PANIdd also underwent the same chemical changes with oxidation occurring above 200 °C. The free radical scavenging capacity of the samples was evaluated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay, and was found to be independent of the spin concentrations of the samples. All samples exhibited a rapid decline in free radical scavenging capacity when exposed to temperatures above 200 °C, indicating that any polymer processing should be undertaken at temperatures less than this value to achieve high antioxidant activity.