Preparation Of Polypyrolle And Its Nanocomposites And Investigations On Their Humidity And Gas Sensing Properties

In this paper, a series of novel gas and humidity sensitive materials of polymer nanocomposites were prepared, including vapor phase polymerized polypyrrole (PPy) and its binary and ternary composites with palladium (Pd) nanoparticles and carbon nanotubes (CNTs), PPy nanoparticles and nanofibers fabricated by using soft templates, composites of crosslinked and quaternized poly(4-vinylpyridine)(QC-P4VP) with PPy and carbon black (CB) nanoparticles. The nanocomposite sensitive materials were characterized using SEM, TEM, AFM, FT-IR, UV-vis, XRD, TGA and cyclic voltammetry, and their electrical responses to humidity, NH3and other gases have been investigated at room temperature. The effect of the composition, structure, morphologies and preparation conditions on the sensing characteristics of the gas sensors and humidity sensors based on the composites was discussed. Moreover, the sensing mechanisms were explored.PPy doped with polystyrenesulfonic acid (PPy-PSSA) and its composite with Pd nanoparticles (PPy/Pd) were prepared by vapor phase polymerization. The effect of the preparation conditions on the gas sensing properties of PPy-PSSA and PPy/Pd was examined, including polymerization time, polymerization temperature, the concentration and ratios of reactants, polymer additives. It was found that incorporation of Pd nanoparticles into PPy-PSSA significantly improved its sensitivity to NH3. The sensitivity of PPy/Pd to50-2000ppm NH3was13.1-58.9%, which was higher than that of PPy alone by a factor of two. PPy/Pd exhibited fast response to1000ppm NH3, with the response time and recovery time of14s and148s respectively. The size of Pd nanoparticles and the morphology of the composite film had a great effect on the sensing properties of PPy/Pd. In addition, a sensing mechanism was proposed that Pd nanoparticles promoted the charge transfer between PPy and NH3.Carbon nanotubes grafted with sodium polystyrenesulfonate (PSS-CNT) were prepared by a nitroxide mediated "living" free radical polymerization, while the composite of PSS-CNT with Pd nanoparticles (PSS-CNT/Pd) was prepared by in-situ reduction of palladium salts. Moreover, the composites of PPy/PSS-CNT and PPy/PSS-CNT/Pd were fabricated by vapor phase polymerization. The effect of polymerization time, polymerization temperature, the concentration of oxidant, and the contents of CNTs and Pd nanoparticles on the sensing properties of the composites was investigated. PPy/PSS-CNT showed much higher sensitivity to NH3than PPy and PSS-CNT separately, which could be ascribed to the interactions between CNTs and PPy. Furthermore, PPy/PSS-CNT/Pd exhibited faster response and higher sensitivity towards NH3in comparison to PPy/PSS-CNT, and the sensitivity of PPy/PSS-CNT/Pd to5ppm NH3was as high as6.2%. In addition, it displayed linear electrical response towards NH3over5-20ppm and20-100ppm, respectively. Meantime, the response of the nanocomposite to NH3was reversible, repeatable, and selective with respect to vapors of organic solvents. It was proposed that there might operate a synergetic effect among PPy, PSS-CNT and Pd nanoparticles, which enhanced sensitivity to NH3of the nanocomposites.Water-dispersible polypyrrole nanoparticles (PPy-NPs) were prepared using PSSA as a soft template. The effect of the preparation conditions, including polymerization temperature, concentrations of oxidant and doping agent on the gas sensing properties of PPy-NPs was investigated. The sensitivity of PPy-NPs towards1ppm NH3was as high as2.5%, indicative of its potentials for detection of ppb level of NH3. The response time and recovery time of PPy-NPs upon exposure to1000ppm NH3were16s and118s respectively. Furthermore, the PPy-NPs were featured with good reversibility, repeatability and good selectivity over vapors of organic solvents in the response towards NH3.Polypyrrole nanofibers (PPy-NFs) were fabricated by using the fibrillar complex of methyl orange and FeCl3as a soft template. The effect of structure and morphology of PPy-NFs on its gas sensing properties was investigated. In addition, the effect of the after-treatment processes, including immersion in the solution of ammonium persulfate and thermal treatment in air, on the sensing properties of PPy-NFs was explored. It was found that the sensitivity of PPy nanofibers to NH3was higher than that of PPy nanoparticles. Furthermore, it was revealed that the after-treatment could change the structure and morphology of PPy-NFs, thus affect its sensing properties.QC-P4VP/PPy was fabricated via vapor phase polymerization of pyrrole on QC-P4VP, and QC-P4VP/CB was obtained by a nitroxide mediated "living" free radical polymerization. The effect of composition, structure and morphology on their humidity sensing properties was studied. The results indicated that both composites exhibited much reduced impedance at low relative humidity (RH) due to the introduction of PPy and CB, thus could detect very low humidity (<10%RH). QC-P4VP/PPy exhibited linear impedance response to relative humidity over0-60%RH, with a sensitivity as high as6.89x103Ω/%RH. While QC-P4VP/CB showed very wide sensing range of0-97%RH, suggesting its ability to realize full range detection of humidity. The humidity sensing mechanisms of QC-P4VP/PPy and QC-P4VP/CB were explored by using complex frequency spectra analysis. It was proposed that the humidity sensing behaviors of the composites were related to the contributions of both ionic and electronic conduction at different humidity levels.