Acidity’s Impact on Yield, Morphological Structure, and Surface Functionalities of Polyaniline Synthesised via Oxidative Polymerisation

This research aimed to examine how varying the acidity levels during the chemical oxidative polymerisation (COP) of aniline affects the colour, yield, morphology, and surface functionalities of polyaniline (PANI). In the COP method, aniline monomer was first dissolved in hydrochloric acid (HCl) medium; the solution was then drop added with ammonium persulfate to initiate the polymerisation process. To answer the research objective, the medium acidity was manipulated using different concentrations of HCl (i.e., 0 M, 0.01 M, 0.1 M, 1 M and 10 M). The as-synthesised PANIs were then purified and characterised for their physicochemical properties. The findings revealed that reducing the medium pH from 2.35 (in distilled water) to 0.80 (in 10 M HCl) resulted in a colour change from light brownish to dark green, indicating the formation of emeraldine PANI. Furthermore, it was observed that the amount of PANI yield is inversely related to the medium pH. A decrease in medium pH from 2.35 to 0.80 remarkably increased the PANI yield by 3.2 times. Regarding morphological structure, higher medium acidity produced PANI with well-defined globule shapes, while lower HCl concentrations resulted in a mixed morphology including globules, tubes, and plate-like structures. In addition, it was found that the diameter of globule-like PANI has increased from ~266.67 nm to ~508.33 nm, when the HCl concentration was changed from 0 M to 10 M. Apparently, increasing the acidity of the COP medium promotes enlargement of the PANI size. Despite that, there is not much change in the surface functional groups of the formed PANI. Overall, this study offers insights into manipulating PANI properties by adjusting the medium acidity during the COP process. While the impacts of medium acidity on yield, morphology, and functionalities of PANI were revealed, further investigation is required to evaluate the effect on electrical properties.