Gas Sensitive Materials And Gas Sensors Based On Polymer Nanocomposites

Gas sensitive materials of five series of polymer nanocomposites, namely, composites of multi-walled carbon nanotubes (CNTs) chemically modified with hydroxylamine hydrochloride salt and poly(methyl methacrylate) (PMMA), composites of palladium (Pd~0) nanocomposites and CNTs, composites of polymer-grafted CNTs and nano-sized carbon black (CB), composites of CNTs and hyperbrenched polymers (HBP) with three kinds end groups, polyaniline (PANI) and its composites with CNTs, were designed and perpared, and characterized by FT-IR, NMR, GPC, TGA, TEM, SEM, UV-Vis, XRD, AFM and raman spectroscopy. Resistive type gas sensors based on the composites were prepared by the method of dip-coating, in-situ polymerization and electrospinning, etc. Their gas sensing properties towards the vapors of methanol, triethylamine, ammonia, etc. have been investigated at room temperature. The effects of the strutcure, composition and morphology of the composites and the methods of sensor preparation on their gas responses were disscused, and the sensing mechanism was explored.Gas sensing properties of the acid-treated CNTs were investigated towards the vapors of ammonia, methanol and triethylamine, and the sensing mechanism of the p-type semi-conductor was disscused. The CNTs were modified with hydroxylamine hydrochloride salt and formed composites with PMMA. Gas sensors based on the composites were prepared by the method of dip-coating. They exhibited a decreased resistivity unpon exposure to the vapors of reducing ammonia and methanol, indicating responses of an n-type semi-conductor. In addition, the response towards methanol vapor is fast, reversible and reproducible at room temperatue with response and recovery time of 9s and 2s, respectively.The composites of polyvinylpyrrolidone (PVPD)/Pd~0/CNTs, Pd~0/CNTs and chitosan/Pd~0/CNTs were developed by reducing the mixture of palladium salt, polymer and CNTs. Gas sensors based on the composites were prepared by dip-coating. It was found that PVPD/Pd~0/CNTs composite exhibited the sensitivity 1~4 times higher than that of CNTs alone towards methanol vapor at room temperature, while its sensitivity to methanol and ethanol vapors of the same concentration was almost equal. The PVPD/Pd~0/CNTs composites showed the sensitivity 1～2 times higher than that of PVPD/CNTs in the detection of methanol vapor. In addition, its sensitivity to methanol vapor is 2-3 times higher than that to ethanol vapor, suggesting a selective response to methanol vapor. The responses also showed good linearity, repeatability and stability. The composites of Pd~0/CNTs and chitosan/Pd~0/CNTs exhibited a sensitivity of 5% and 7.3%, respectively, towards 2% methane at room temperature, and the responses were recoverable, stable and reproducible. A sensing mechanism taking into account of the catalytic effect of palladium was proposed.Composites of CNTs and CB grafted with polystyrene, poly(4-vinylpyridine), poly(styrene-b-4-vinylpyridine) and poly(styrene-co-4-vinylpyridine) were prepared by a nitroxide mediated "living" free radical polymerization, and used for preparation of gas sensors by dip-coating. The electrical responses of the composites towards low concentration organic vapors of methanol, tetrahydrofuran and chloroform were investigated at room temperature. It was found that the gas sensitive properties of the composites towards the organic vapors depended on the structure, composition, morphology of the composites and the nature of the vapors. The sensitivity of CNTs composites was higher than that of CB composites. The CNTs composites exhibited a sensing mechanism different from that of CB composites. In addition, the CB composite showed different mechanism towards vapors of high and low concentration.Hyperbranched polymers with different end groups were prepared by melt polycondensation, and formed composites with CNTs to prepare gas sensors by dip coating. The response of the composites towards organic vapors of methanol, triethylamine and acetic acid were investigated. It was found that the sensitivity of the composites was closely related to the end groups of the hyperbranched polymers. The chemical reactions between the end groups of the polymers and the vapors resulted in a high sensitivity and low detection limit of their composites with CNTs. A sensing mechanism was put forward based on the interactions of the vapors with the end groups of the hyperbranched polymers.PANI and its composites with CNTs were prepared by in-situ polymerization and deposited on electrodes modified by self-assembled polyelectrolytes to prepare gas sensors. Their electrical responses to triethylamine vapor were investigated. It was found that the responses of in-situ polymerized PANI were fast, highly sensitive, reversible and reproducible, with the detection limit of ppb level. A sensing mechanism of competition between the reactions of organic vapor with PANI and the doping acid were proposed. The PANI/CNTs composite exhibited a sensitivity twice higher than that of PANI alone, which might relate toπ-electron delocalization between PANI and CNTs. Soluble PANI was prepared by polymerization of aniline in the presence of p-toluene sulfonic acid and doping polyaniline base with D-camphor-10-sulfonic acid (CSA). Their sensitivity to triethylamine vapor was related to the doping levels of PANI. Nanofibers of the CSA doped soluble PANI were prepared by electrospinning. It was found that the fibers exhibited better sensing properties than that PANI films in the detection of triethylamine vapor, which was might be due to their higher surface area.