Biyoinformatik ve Genetik Bölümü Koleksiyonu

Permanent URI for this collectionhttps://gcris.khas.edu.tr/handle/20.500.12469/46

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Citation Count: 22
Critical Exponents of Gelation and Conductivity in Polyacrylamide Gels Doped by Multiwalled Carbon Nanotubes
(Taylor & Francis Ltd, 2010) Aktas, Demet Kaya; Evingür, Gülşen Akin; Pekcan, Önder
Polyacrylamide (PAM) doped by multiwalled carbon nanotube (MWNT) gels were prepared with different amounts of MWNTs varying in the range between 0.1 and 15 wt%. The PAM-MWNT composite gels were characterized by the steady state fluorescence technique (SSF). The alternative electrical conductivity (AC) of PAM-MWNT composite gels was measured by the dielectric spectroscopy technique. Observations around the gel point t(gel) for PAM-MWNTs composite gels showed that the gel fraction exponent beta obeyed the percolation result. The critical exponent r of AC electrical conductivity for the composite PAM-MWNT gel was also measured and found to be about 2.0 which agrees with a random resistor network. (C) Koninklijke Brill NV Leiden 2010
Citation Count: 15
A Fluorescence Study on Swelling of Hydrogels (PAAm) at Various Cross-Linker Contents
(Wiley, 2009) Aktas, Demet Kaya; Evingür, Gülşen Akin; Pekcan, Önder
Disk-shaped acrylamide (AAm) gels were prepared from AAm with various NN'-methylenebisacrylamide (Bis) contents as cross-linker in the presence of ammonium persulfate as an initiator by free-radical cross-linking copolymerization in water. Polyacrylamide (PAAm) gels were dried before using for swelling experiments. Steady-state fluorescence spectrometer was employed during the swelling of PAAm hydrogels in water. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine from various Bis content gel samples was measured during in situ swelling process. It was observed that fluorescence intensity decreased as swelling has proceeded. Gravimetric and volumetric experiments were also performed. The Li-Tanaka equation was used to determine the swelling time constants tau(c) and cooperative diffusion coefficients D-c from intensity weight and volume variations during the swelling processes. It was observed that swelling time constants tau(c) increased and diffusion coefficients D-c decreased as the cross-linker content was increased. (C) 2010 Wiley Periodicals Inc. Adv Polym Techn 28:215-223 2009
Citation Count: 13
In situ steady state fluorescence (SSF) technique to study drying of PAAm hydrogels made of various cross-linker contents
(Elsevier Science, 2009) Evingür, Gülşen Akın; Aktas, Demet Kaya; Pekcan, Önder
Drying experiments of polyacrylamide (PAAm) gels were performed using steady-state fluorescence (SSF) spectrometer. Pyranine (P) was introduced as a fluorescence probe and intensity of P from various crosslinker content gel samples was monitored. It was observed that the intensity of P increased during the in situ drying processes. Gravimetrical and volumetric experiments were also performed. An empirical model was derived and introduced to determine the desorption coefficient D of water molecules from the drying PAAm gels. It is observed that D increased as the cross-linker content was increased. (c) 2008 Elsevier B.V. All rights reserved.
Citation Count: 0
Phase Transitions in Pure and Hybrid Hydrogels: A Fluorescence Study
(Wiley, 2009) Aktas, Demet Kaya; Evingür, Gülşen Akin; Pekcan, Önder
Recent observations on phase transitions around the critical point showed that the critical exponents differed drastically from percolation results and classical results for pure hydrogel systems depending on monomer concentration. in addition to pure hydrogels the sol-gel phase transition during radical crosslinking copolymerization of acrylamide (AAm) and N-isopropylacrylamide (NIPA) hybrid was studied by using the steady state fluorescence (SSF) technique. NN'-methylenebis(acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator respectively. Pyranine (trisodium 8-hydroxypyrene-136-trisulfonate acid HPTS) was added as a fluoroprobe for monitoring the polymerization. It was observed that pyranine binds to AAm and NIPA chains on the initiation of the polymerization thus the fluorescence spectra of the bonded pyranines shift to shorter wavelengths. Fluorescence spectra of the bonded pyranines allowed to monitor the sol-gel phase transition without disturbing the system mechanically and to test the universality of the sol-gel transition as a function of polymer concentration ratios. The observations around the gel point of PAAm-PNIPA hybrid show that the gel fraction exponent beta obeyed the percolation result.

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