Interfacial synthesis of conducting polymers and their nanocomposites and study on their thermoelectric and electrochemical properties

Abstract

Intrinsically conducting polymers are promising materials because of their unique electrical, optical, and redox properties, which enable new applications, including polymer electronics and electrochemical devices. Despite remarkable advances in our understanding of the electrical properties and nanostructures of conducting polymers, the commercial and industrial applications of these materials remain limited. In this thesis, new interfacial synthesis methods are introduced to resolve essential problems associated with conventional oxidative polymerization, such as undesirable side reactions and polymer precipitation. By resolving these problems, the proposed methods produce highly conductive polymers and their nanostructured composites. Thermoelectric and electrochemical properties of the resultant materials were studied. In Section 3.1, the solid/liquid interfacial polymerization of aniline in nonaqueous solution is discussed. Aniline was polymerized in chloroform in the presence of an organic acid and a solid oxidant. The aniline molecules were oxidized on the surface of solid oxidant to yield polyaniline, which simultaneously dissolved into the solution as the chains were doped with the acid. Compared with the conventional aqueous method, the new synthetic route reproducibly provided polymers with a higher molecular weight, lower degree of ortho-substitution, and higher degree of crystallinity, resulting in 2.3 times higher conductivity (580 ± 40 S cm-1) and 6-8 times higher thermoelectric power factor (~24.9 μW m-1K-2). The highly conductive PANI produced using the organic-solvent-based method developed in this work should further expand the applicability of this polymer. Section 3.2 describes the preparation of conducting nanoporous composite membranes via the templated synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT). The polymerization was conducted on the surface of continuous nanochannels of a template membrane using two immiscible solutions, resulting in a several-nanometer-thick coating of PEDOT on the surface of pores. PEDOT was well percolated with only 2.8 vol% in the composite, which led to a high conductivity of ~10-9 S cm-1 at this composition. The continuous nanopores of the template could be maintained with less pore-clogging even at a 20 vol% loading of PEDOT because of the limited amount of the monomer as well as the oxidant in the nanospaces during the polymerization. The loading amount of PEDOT was modulated to 20 vol% by varying the concentration of oxidant solution; as a result, the conductivity and porosity of the composite membranes were controlled. Compared to the pristine template, the conductivity increased to 1.6 S cm-1 and the pore volume decreased from 0.33 to 0.18 cm3 g-1, while the surface area was preserved (23.7 m2 g-1) in the composite membrane with the highest loading of PEDOT. The electrochemical current response and mechanical strength of the membranes were sufficiently high for them to be used in nanofluidic applications involving electrochemistry. Electromodulation of water permeation through the membranes was successfully demonstrated with the aid of an anionic surfactant. Section 3.3 describes the synthesis of a hybrid nanocomposite composed of selenium nanoparticles coated with a thin layer of a conductive polymer, poly(styrene sulfonate) doped PEDOT (PEDOT:PSS), as well as the thermoelectric properties of this nanocomposite. The conductive polymer layer on the surface of the nanoparticles in the composites formed a percolating network between the stacked nanoparticles, resulting in an electrical conductivity similar to or greater than that of the pure polymer. The thermoelectric power factor of the resulting composite was greater than that of an individual polymer or selenium nanoparticles. The electrical conductivity of the composite was further increased by thermal annealing, thereby improving the power factor to 0.15 μW m-1 K-2, which was nine times greater than that of the polymer.