| Conducting polymers and their composites have significant applications in electrostatic materials, electromagnetic shielding materials, artificial muscles, diodes, transistors, etc. It has been recognized that a metal ion having a higher reduction potential than that of a conducting polymer can be chemically reduced by the conducting polymer. Based on polyaniline (PANI) and polypyrrole (PPy), controlled growth and molecule detection properties of metal nanostructures reduced by using conducting polymer nanofibers, membranes and films, as well as prepration and electromagnetic properties of magnetic conducting polymer composites were studied in this thesis.One-dimensional (1D) PANI, PPy and PANI-PPy copolymer nanofibers are prepared by a surfactant-assisted chemical oxidative polymerization. Though spectroscopic properties of PPy nanospheres and nanofibers are almost identical, the electrochemical responses are morphology-dependent: electrochemical process of PPy nanospheres is controlled by semi-infinite diffusion, while that of PPy nanofibers is dominated by barrier diffusion. Effect of chemical composition on the properties of PANI-PPy copolymers can be described as: PANI-PPy nanofibers synthesized with an excess of either PANI or PPy show similar physico-chemical characteristics as the individual homopolymers, whereas nanofibers from an equimolar mixture of An and Py display unique properties. Preparation of metal nanostructures from chemical reduction of metal ions by conducting polymer nanofibers is also discussed. Conducting polymers with different catagories, morphologies and dopants have various reduction potentials and surface properties, which can be accounted for the yield of metal nanoparticles with different morphologies and sizes.PANI porous membranes and dense films are fabricated, and effects of technique parameters such as dopant, conductive additive, external electric field on the nucleation and growth of metal nanostructures are discussed. Different dopants render PANI porous membranes or dense films with different chemical natures and surface properties, leading to different metal structures grown on PANI membranes or films. Addition of conductive additives (graphite or carbon nanotube) in PANI porous membranes can increase the conductivity of the membrane and improve the doping inhomogeneity of the membrane surface, thus homogeneous metal nanoparticles can be produced. Application of an external electric field to the P-G or P-CNT porous membranes can alter the nucleation and growth mechanism of Ag, resulting in a Ag gradient structure comprised of dendrites, flowers and microspheres. It is believed that the formation of such silver gradient is a synergetic consequence of a vertical diffusion and a lateral electrokinetic flow in the field-assisted model when Ag+ ions move to the membrane surface and are reduced by PANI. While in the absence of an electric field, the movement of Ag+ ions is dominated by a simple diffusion process. It is revealed that the Ag dendrites, flowers and microspheres all possess strong surface enhanced Raman spectroscopy (SERS) properties.Both the pre-fabricated gold nanolayer and chemical nature of the PANI porous membrane play pivotal roles in the formation of homogeneous 3D Ag nanosheet structures. The fabricated Au-Ag hybrid structures show strong and uniform SERS enhancement of the absorbed molecules over the whole surface, with a detection sensitivity of 10 ppm, indicating a promise for sensitive detection of chemical and biological analytes. It is revealed that citric acid absorbed onto the pre-fabricated Au or Ag nanoparticles directs the self-assembly of subsequent grown Ag nanoaprticles into nanosheet structures. This recognition makes the fabrication of homogeneous Ag nanosheet structures on undoped PANI porous membranes become available. Moreover, based on the above mechanism, a wire structure consisting of assembled Ag nanosheets is also successfully fabricated, which shows promising SERS recognition of 20 ppm melamine molecules.Current study reveals that Ag nanowires can only be grown on citric acid doped PANI dense films. Ag nanowires with a wide range of morphologies and sizes can be obtained on one single PANI film. Electrical stability of the Ag nanowires is tested in the voltage range of -0.05~+0.05 V and 0~+4 V respectively, and a meltdown voltage of about 1.5 V is determined. Electromigration and surface diffusion can both be accounted for the meltdown or morphology change of the Ag nanowires during the electrical measurements. The nanowires comprised of self-assembled Ag nanoparticles usually have lower electrical conductivities than those with smooth surfaces, due to the presence of growth defects.Hexagonal barium ferrite (BaFe12O19) nanoparticles have been prepared from a reverse microemulsion technique, and optimum sintering condition and Fe/Ba ratio are determined. Using BaFe12O19 nanoparticles and nickel powder as magnetic cores, BaFe12O19-PANI, BaFe12O19-PPy and Ni-PPy composites are prepared through an in situ chemical oxidative polymerization method. There is no obvious chemical interaction between the conducting polymer and inorganic phase, where the inorganic phase simply plays a role of nucleation center during the polymeri- zation of organic monomers. Electrical conductivities, magnetic properties and electromagnetic properties of the composites can be modulated by controlling the relative content of the inorganic phase and organic phase. Electromagnetic properties of the composites result from a combination of magnetic loss of magnetic materials and dielectric loss of conducting polymers. Complex permeability and permittivity of the composites can be tuned by coating the magnetic materials with conducting polymers, leading to a better impedance matching between the compistes and free space as required according to the transmission line theory. Magnetic conducting polymer composites show promising electromagnetic absorption properties in the frequency range of 2~18 GHz, and the absorption band can be finely tuned.
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