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    Electrical and optical percolations of polystyrene latex-multiwalled carbon nanotube composites
    (Academic Press Inc Elsevier Science, 2010) Kara, Selim; Arda, Ertan; Dolastir, Fahrettin; Pekcan, Oender
    Electrical conductivity and optical transmittance properties of polystyrene (PS)-multiwalled carbon nanotube (MWCNT) composite films were investigated. Composite films were prepared by mixing of various mass fractions of MWCNT in PS-water dispersions. After water evaporates, powder composite films were annealed at 175 degrees C above the glass transition of PS for 20 min. Photon transmission and two point probe resistivity techniques were employed to determine the variations of the optical and the electrical properties of composites. Transmitted light intensity, I(tr) and surface resistivity, R(s) were monitored as a function of MWCNT mass fraction (M). It was observed that, both the surface resistivity and the optical transparency were decreased by increasing the amount of MWCNT added to the polymeric system. Conductivity and optical results were interpreted according to the classical and site percolation theory, respectively. The electrical (sigma) and the optical (op) percolation threshold values and critical exponents were calculated as M(sigma) = 1.8 wt.%, M(op) = 0-0.13 wt.% and beta(sigma) = 2.25, beta(op) = 0.32, respectively. (C) 2010 Elsevier Inc. All rights reserved.
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    A Quartz Crystal Microbalance (QCM) Study on the Formation of Aqueous ?-Carrageenan-Chitosan Composite Bilayers with NaCl and Graphene Oxide
    (Taylor & Francis Inc, 2023) Kara, Selim; Arda, Ertan; Dolastir, Fahrettin; Pekcan, Onder; Evingur, Gulsen Akin
    Biomaterials and thin biofilms play a fundamental role in the medical, food and pharmaceutical industries. The Quartz crystal microbalance (QCM) measurement technique is one of the attractive techniques which have been used for monitoring the thin-film formation process. In the current work described here, kappa-carrageenan and chitosan solutions were used for the layer-by-layer (LBL) deposition of polyelectrolyte multilayers (PEMs) on the gold surface of a 5 MHz AT-cut (thickness-shear mode) quartz crystal in a flow module. A sensitive QCM system was designed to detect 0.1 Hz differences in the resonance frequency (Delta f) and 10(-7) changes in the energy dissipation (Delta D) values, which are the measures of the deposited mass and the film rigidity, respectively. Negatively charged carrageenan and positively charged chitosan biomolecules in the solutions were used to build up sequential and very thin bilayers on the modified gold surface. The effects of NaCl and graphene oxide (GO) addition on the mass of the deposited films were also investigated. Intermolecular interactions between the biopolymer chains, Na+ cations and GO sheets were explained by interactions between molecules, such as electrostatic forces and hydrogen bondings.
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    Thermal Phase Transitions of Agarose in Various Compositions: A Fluorescence Study
    (Springer/Plenum Publishers, 2011) Kara, Selim; Arda, Ertan; Dolastir, Fahrettin; Pekcan, Onder
    The effect of agarose content on thermal phase transitions of the agarose gels was investigated by using Steady State Fluorescence (SSF) method. Scattered light, I(sc) and fluorescence intensity, I(fl) were monitored against temperature during heating and cooling processes to investigate phase transitions. Two regions were observed during the heating and cooling processes. At the high temperature region, double helix to coil (h-c) transition took place. However, during the cooling process coil to double helix (c-h) transitions occurred at low temperature region. Transition energies were determined using the Arrhenius treatment, and found to be strongly correlated with the agarose content in the gel system. Transition temperatures were determined from the derivative of the sigmoidal transition paths and found to be increased by increasing agarose content in both cases.