Carbon Metamorphoses in a Medium with Varying Ionic Concentration
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Carbon Metamorphoses in a Medium with Varying Ionic Concentration V. Bouda, J. Chladek, Czech Technical University in Prague, Faculty of Electrical Engineering, Technicka 2, 166 27 Prague 6 (Czech Republic)
ABSTRACT Carbon black in a polymer melt was used to model self-organized processes. A gradual growth of ionic concentration in the polymer melt induced a complex behavior, including a rapid increase in AC conductivity after a time delay. Colloid theory can provide an effective interpretation. The metamorphoses of the carbon black assembly include soft axial order generation, soft lateral order generation, axial condensation, and lateral condensation of carbon black particles, respectively. Similar processes are expected to be effective in generating complex carbonaceous organic structures in natural water. INTRODUCTION Applications of carbon are in a state of considerable flux as new designs, new products and new materials, such as high strength fibers, low-pressure diamond, and fullerenes or nanotubes, are continuously being introduced. A self-assembly often grows from carbonaceous colloidal particles. Self-assembly involves an equilibrium of noncovalent interactions (mainly van der Waals forces and electrostatic forces) between the surfaces of particles. Their dependence on a dynamic equilibrium makes these architectures reversible and reorganizable under mild conditions. In the case of organic systems based on carbonaceous colloidal particles, a more intricate hierarchy of construction can be invoked. Biological structures formed by self-assembly include thin fibres, membranes, vesicles, DNA, and a wide variety of structures in a cell. In the 21st century self-assembled materials may represent building blocks comparable to those of alloys, plastics, and semiconductors in the 20th century (1). EXPERIMENTAL METHODS AND RESULTS The growth of the self-assembled structure of carbon black (CB) colloidal aggregates has recently been studied by experimental methods (2-5). The CB-network properties have been measured in a polymer melt that supports the freely growing CB-network. These experiments were carried out on low-density polyethylene (LDPE) and CB with a high specific surface area 800 m2/g. The diameter of the spherical CB-primary particles was 27 nm and the average size of their ramified aggregates was about 200 nm. The components were mixed using a Brabender plastograph. The sheets of composites were prepared by compression molding. The samples of composites were then annealed at 190-200°C to induce different states of CB self-assembly in the LDPE melt. Impedance analyses (AC conductivity and permittivity) of samples 0.5 mm thick and 9 mm in diameter at voltage 1 V and frequency 80 kHz in situ and in real time were measured during the process. Tinned copper was applied to the opposite large area surfaces for electrical contacts. The tin concentration in the samples was checked by the Rutherford Back Scattering method (RBS). Optical microscopy, Atomic Force Microscopy (AFM), and Transient Electron Microscopy (
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