Preparation and characterization of polypyrrole film for chemical vapor sensor applications

Abstract

Polypyrrole (PPy) was chemically prepared via an in situ doped polymerization utilizing seven dopant anions (dopant to monomer molar ratio, D/M = 1/12) to stabilize the positive charges on N of pyrrole rings. These dopant anions were found to play important roles on physical, chemical, and electrical properties of PPy as revealed by several techniques, e.g. X-ray photoelectron spectrometer and the custom-made four-point probe conductivity meter. PPys doped with alpha-naphthalene sulfonate (PPy/A) and beta-naphthalene sulfonate (PPy/B) have good pellet appearance, solubility, thermal stability, specific conductivity, and stability in conductivity. PPy/A was chemically synthesized at various D/M ratios. The D/M ratio giving PPy/A with high specific conductivity and stability in conductivity is 1/12. Upon exposure to acetone vapor at 16.7 vol.% in N2, negative changes in specific conductivity, negative changes in specific conductivity of PPys were observed. These changes depend critically on the type of the dopants used. For the sulfonate dopants, negative changes in specific conductivity exponentially depended on the doping level (N+/N) and the initial specific conductivity. It depended linearly on the proportion of the bipolaron species and the ordering and inversely on the proportion of the imine-like nitrogen defect (=N-). PPy/A exhibited the largest specific conductivity decrement: 0.4 S/cm. With a higher D/M ratio, PPy/A contained a smaller amount of moisture content and its response to acetone was enhanced. Various techniques, e.g. an environmental scanning electronmicroscope, were used to investigate the interaction between PPy and acetone molecules. H-bonding, swelling, and reduction reaction by acetone are suggested to cause the decrease in specific conductivity of PPy.