Preparation And Gas Sensitivity Of THTBN / MWNTs-OH PU Conductive Composites

With the continuous development of human civilization and social economy, more and more people are concerned about their own life quality. In the past two decades, however, with the acceleration of urbanization and industrialization processes, indoor pollution has become increasingly serious. The adverse effects of air pollution, especially the influence of the indoor pollutants on human health has caused constant attention. The continuously erected urban buildings make an originally narrow living space become more crowded. In addition, the ventilation in surrounding environments is not smooth, which results in an indoor environment increasingly harsh. The upholsteries, such as furniture, adhesive, marble and floor and so on, is liable to release volatile organic vapors. Researches show that there are some inevitable relations between indoor air pollutants and human health hazards. Based on these descriptions, this study is committed to develop a novel conductive polymer thin film composite, which can be used to detect volatile organic solvent vapor. The specific work includes the following two aspects:1. Novel THTBN/MWNTs-OH polyurethane (PU) conducting composites were designed and prepared via an in-situ polymerization reaction between hydroxyl groups and isocyanate groups in the substance, and subsequently the PU conducting composite thin films were assembled using tetrahydroxyl-terminated poly(butadiene-co-acrylonitrile)(THTBN) as a matrix, hydroxyl-functionalized multi-walled carbon nanotubes (MWNTs-OH) as a conductive carrier,1,6-hexamethylene diisocyanate (HDI) as a coupling agent, and butanediol as a chain extender. The chemical and crystal structures were characterized by Fourier transform infrared spectra (FT-IR) and X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) was used to study the relative content and the binding energy of each element of conductive composites before and after in situ polymerization reaction. Thermogravimetric (TG) analysis was adopted to measure the mass loss to examine the linkage between THTBN and MWNTs-OH in situ polymerization. The morphologies and the dispersion behavior of THTBN/MWNTs-OH were observed and investigated by scanning electron microscope (SEM), transmission electron microscopy (TEM) and UV-vis measurements.2. The conductive composite films were assembled into sensing elements and the vapor responsivity to different volatile organic compounds (VOCs), especially to benzene and toluene vapors, was studied. The influence of the MWNTs-OH loading amount on conducting properties of conductive composites and response to benzene and toluene vapors were investigated. The effect of operating temperature and the different concentrations of solvent vapors on the responsiveness of the conductive composite films was analyzed. The response time and recovery time of the conductive composite films were detected at different vapor concentrations. The reusability of the composite thin films was examined, and the results were compared with the mixing composites. Experimental results showed that the THTBN/MWNTs-OH PU conductive composite films exhibited a strong selectivity to non-polar organic solvents such as benzene and toluene vapors, and the response intensities increase with increasing the concentration of benzene and toluene vapors which display a good linear dependence. The operating temperature had no perceptible effect on the gas sensitivity, and thus the sensing elements can operate at room temperature.The improvement in dispersity and sensing properties of conductive composite film were closely correlated with the chemical linkage of MWNTs-OH in the THTBN matrices through bridging molecules HDI. Repeated test results indicated that the response values of the materials to vapors were almost unchangeable, and there were no baseline drift. Serious baseline drifts, however, appeared in the mixing composite films, and the response value was reduced after repeatedly exposed to solvent vapors. Based on the above analysis, the developed PU conducting thin films had fast response and reversibility, significant reproducibility, low operating temperature and long-term stability. Therefore, they had a possibility as a candidate of volatile organic solvent vapor sensors.