Abstract:
Antioxidants are commonly used as preservatives for protecting foodstuffs from oxidation. Packaging is believed not only to increase the economic value of products but also improve their qualities. Conducting polymers (CPs), such as soluble polyaniline and soluble polypyrrole, have been reported to exhibit antioxidant activity. Development of conducting polymer containing packaging materials, with antioxidant properties, is therefore the focus of this project. Polypyrrole (PPy), polyaniline (PANI) and poly (3,4-ethylenedioxythiophene) (PEDOT) powders were prepared using various amounts of the oxidant ammonium persulfate (APS). Spectroscopic methods, including IR and Raman, were used to identify the structures of the CP powders synthesized at various APS levels. It was found that a high level of APS led to overoxidation of the CP powders. The conductivity and doping level of the CP powders formed using higher concentrations of the APS oxidant were found to be lower than those prepared using low concentrations of APS, which might be due to overoxidation, damaging the CP structures during the preparation processes.
The antioxidant activity of PPy, PANI and PEDOT powders were evaluated using the DPPH and ABTS assays. The results showed that CP powders synthesized at a high concentration of APS presented lower free radical quenching effects, likely also due to overoxidation. The optimum initial ratio of APS to monomer for the synthesis of CP powders with a superior DPPH free radical scavenging was found to be around 0.5 for PPy, 1.5 for PANI and 1.0 for PEDOT, and the antioxidant ranking of the CP powders was as follows: PANI1.5 > PPy0.5 > PEDOT1.0. The reduced forms of PPy, PANI and PEDOT exhibited better radical scavenging abilities than the as-prepared powders. PANI nanotubes, synthesized at high pH, was also found to exhibit stronger free radical scavenging activity than the regular granular PANI formed under acidic conditions. Structural changes of PANI powders after reaction with free radicals were observed in IR and XPS spectra, showing an increase in the ratio of imine (C=N) to amine (C-N) units.
PANI EC films were prepared using a solution method and the antioxidant capacity of the PANI EC films are examined using the ORAC assay. The results showed efficient peroxyl free radical scavenging activity of the PANI EC films. A very good correlation between the ORAC response and the area of tested film was also observed, indicating a homogenous dispersion of active PANI powder across the film. Similar to the results obtained from the DPPH and ABTS assays, reduced PANI presented greater peroxyl radical scavenging activity than the as-prepared powders. The influence of the PANI EC films on the oxidation of Ropufa oil was determined by peroxide value (PV) measurement. After incubation at 60oC for several days, the oil stored in the presence of the PANI EC film was found to exhibit a lower PV than in the absence of a CP film, indicating that the conducting polymer is effective in inhibiting oxidation of fish oil.