Development of polypyrrole-based sensors for vapors of flammable chemicals

Abstract

Serial issues concerning development of polypyrrole-based sensors for vapors of flammable chemicals were studied in this dissertation work. 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 a-naphthalene sulfonate (PPy/A) and B-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 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, of PPys were observed. These changes depend critically on the type of the type of the dopants used. For the sulfonate dopants, 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 larges specific conductivity decrement: 0.4 S/cm. Various techniques, e.g. an environmental scanning electronmicroscope, were used to investigate the interaction between PPy and acetone molecules. Swelling, H-bonding, and reduction reaction by acetone are suggested to cause the decrease in specific conductivity of PPy. In order to improve the selectivity of PPy toward acetone and toluene, which are flammable components in lacquer over the non-flammable components acetic acid and water, PPy/A with D/M ratio of 1/5, PPy/A5 has been blended with several insulating polymers: PEO; PMMA; HDPE; PS; and ABS, by three different methods namely dry mixing, solution mixing, and coating. The electrical conductivity responses towards liquid of water, acetone, acetic acid, or toluene of PPy/A5/PMMA, PPy/A5/PS and PPy/A5/ABS blends from solution mixing towards acetone and toluene were enhanced but those towards water were diminished, relative to that of pure PPy/A5. However, the sensitivity towards acetic acid was not significantly different from that of pure PPy/A5. The selectivity improvement had been extended focusing on the solution mixing of PPy/A and PMMA. PPy/A with D/M ratio of 1/12 was blended with PMMA by means of solution mixing, with various weight ratios of PMMA. Compared with pure PPy/A, the selectivity ratio of acetone/acetic acid response of PPy/A/PMMA blend with a PMMA/PPy weight ratio of 3.0 was ca. 3.8 times higher. The film was found to be insensitive to moisture unless the relative humidity was lower than 20% RH in which the selectivity ratio was enhanced. The time required to reach the equilibrium signal at relative humidity of 20-30% RH was 6-15 min; it became as high as 20 - 26 min at 50 - 70% RH.