Preparation, Characterization And Vapor-induced Electrical Response Of PMAA/graphite Conductive Composites

As a gas sensitive material, the loading electric conduction polymer composite materials are received wide interest, because they have light weight, low cost, and are easy to shape and suitable for production on a large scale, etc. Compared with carbon black, graphite, as electric conductive filler, has rich source, corrosion resistance and no pollution to equipments, good electric conductivity and thermal conductivity. Moreover, the graphite may be used to carry on suitable orderly-assemblies of sensitive materials as an electric conduction filler. Namely, the graphite with laminated structures can be utilized to prepare polymer/graphite electric conduction composite materials by insertion polymerization. Thus their properties have been greatly improved. Based on the above consideration, flake graphite, expanded graphite and graphite nano-sheets and poly(methyl acrylic acid)(PMAA) as primary materials in this research were adopted to prepare a series of novel PMAA/graphite electric conduction composite materials via a solution mixing process and in-situ polymerization. The vapor response performances were examined to some organic solvent vapors. The characterization of sensitive materials were carried out on a Fourier transformation infrared spectroscope (FT-IR), a scanning electron microscope (SEM), an X-ray diffraction (XRD), thermal analyzer systems (TG-DTA) and so on. Furthermore, we also have discussed relations between response behavior and material microstructures. It is expected to provide some thoughts and references for development of new gas materials.(1). A solution mixing route was used to prepare the PMAA/flake graphite, PMAA/expanded graphite electric-conduction composite materials. It was discovered that both of the materials have certain responsiveness to polar organic solvent vapors like tetrahydrofuran (THF), N, N-dimethylformamide (DMF), chloroform and acetone, and the biggest response intensity reaches 16 times the original values. SEM and XRD analyses certified that the crystal structure of expanded graphite obtained by oxidation, insertion and high-temperature processing of the flake graphite has not been destroyed. And the electrical conductivity is good. In the meanwhile, the expanded graphite has rich porous structures and bigger layer-to-layer, which is extremely advantageous to the absorption of organic solvent vapors. The vapor responsiveness of the electric-conduction thin film prepared from the expended graphite, however, was smaller than that from the flake graphite. This may be the fact that the layer-to-layer distance of the expanded graphite was reduced because of a mutual attraction itself at a mechanical agitation.(2). A in-situ inserted polymerization avenue was utilized to prepare the PMAA/flake graphite and PMAA/expanded graphite composites. The research in gas response behavior indicated that the response intensities of composite thin films obtained in such case to the above organic solvent vapors have remarkable improvements than by the solution mixing, of which the response intensity of PMAA/expanded graphite conductive composite thin films to the chloroform solvent reached 1739 times. This originated from rich porous structures of the expanded graphite. This structure can form more graphite nano-sheets under ultrasonication. Furthermore, the in-situ polymerization can make the graphite nanosheets dispersed in the poly (methyl acrylic acid) more easily. The better the dispersion behavior of the graphite particles in polymer matrices is, the better the gas sensing behavior to the organic solvents is. On the other hand, the graphite carbon atom can form a weak ionic bond with the chlorine atom of chloroform molecules. In addition, a hydrogen bond and an electric-charge repel function may form between the poly (methyl acrylic acid) and the chloroform molecules. This enables the graphite/PMAA composite materials to have a relative high selective response behavior to-chloroform vapor. But the response is not high to other organic solvent vapors. This is an innovation spot of this work.(3). The graphite nano-sheets/PMMA electric-conduction composite=materials were prepared through the in-situ polymerization route using hexadecyl trimethyl ammonium bromide (CTAB) or the stearic acid or poly(vinyl alcohol) (PVA) as a modifier, respectively. FT-IR and SEM analyses demonstrated that the above composites have quite obvious porous morphologies. Among them, the porous structural size of the stearic acid modified graphite nano-sheets/PMAA thin films is even. The responsiveness and reversibility of such thin films are good to THF and chloroform organic solvents. On the contrary, the porous size of the composite membrane is non-uniform in the case of the trimethyl ammonium bromide or PVA as a modifier. And those composites gave relatively poor responsiveness and reversibility to chloroform, THF, DMF and acetone. The results demonstrated that the pore size distribution of the composite thin film had an important impact on response behavior. On the other hand, the N~+ in CTAB and C=O in poly(methyl acrylic acid) exist in form of the ionic bonding, and the mutual linkage between the hydroxyl (-OH) in poly(vinyl alcohol) and the carboxyl groups (-COOH) in poly (methyl acrylic acid) tied the graphite granules up. These hindered the movement of conducting particles, resulting in the reduction in response of the composite thin films.The experimental results proved that it is an effective method to prepare novel conductive polymer sensitive materials with high response selectivity and good reversibility by modifying the graphite and graphite composites...