Browsing by Author "Kara, Selim"

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Citation Count: 6
A comparison of fluorescence and UV-visible spectrometry techniques for thermal phase transitions of agarose gels
(Springer, 2015) Arda, Ertan; Kara, Selim; Mergen, Ömer Bahadır; Pekcan, Önder
In this study thermoreversible phase transitions of high (HMP) and low (LMP) melting point agarose gels were investigated by using the UV-vis and fluorescence spectroscopy techniques. Pyranine was added to the aqueous solution of agarose as a fluorescence-probe. Transmitted light (I (tr)) and fluorescence emission (I (fl)) intensities from the gel samples with different agarose concentrations were monitored during the heating (gel-sol) and cooling (sol-gel) processes. For the both techniques gel-sol (T (gs)) and sol-gel (T (sg)) transition temperatures were determined from the first derivatives of the sigmoidal transition paths. It was observed that the critical transition temperatures obtained from UV-vis and fluorescence data slightly increased depending on the agarose type and concentration and those values were found to be in accord with each other. Transition activation energies were determined using the Arrhenius type equation and were found to be strongly correlated with the agarose content in the gel system. The produced gel-sol (Delta E (gs)) and sol-gel (Delta E (sg)) transition energies from the fluorescence data were found to be lower than that of the UV-vis data which was attributed to the temperature-dependent fluorescence quenching effect.
Citation Count: 19
Critical Exponents of Kappa Carrageenan in the Coil-Helix and Helix-Coil Hysteresis Loops
(Taylor & Francis Inc, 2009) Tarı, Özlem; Kara, Selim; Pekcan, Önder
The steady-state fluorescence technique was used to study coil-helix (sol-gel) and helix-coil (gel-sol) transitions of the kappa carrageenan-water system with various carrageenan contents. Fluorescence (I) and scattered light (I-sc) intensities were measured against temperature to determine critical phase transition temperatures and exponents. It was observed that the coil-helix transition temperatures T-ch were much lower than the helix-coil (T-hc) transition temperatures due to the hysteresis of the phase transition loops. The gel fraction exponent (beta) was measured and found to be in accord with the classical Flory-Stockmayer model.
Citation Count: 32
Effects of GNP addition on optical properties and band gap energies of PMMA films
(Wiley, 2019) Mergen, Ömer Bahadır; Arda, Ertan; Kara, Selim; Pekcan, Önder
In this study the effects of graphene nanoplatelet (GNP) doping on the optical parameters (absorption coefficient alpha and extinction coefficient k) and the optical transition energies (optical band gap and Urbach energies) of poly(methyl methacrylate)/graphene nanoplatelet (PMMA/GNP) composite films were studied. PMMA/GNP composite films with various GNP mass fractions were prepared by spin coating technique. The absorbance (A) changes of the prepared composites were measured by using UV-Vis technique. The alpha and k values of the composites were obtained from UV-Vis data. The observed rapid increase in A values in UV-region were associated with the optical transitions of GNP electrons from valence to conduction band. The direct (E-d) and indirect (E-i) optical band gap energies of the composites were determined by using Tauc method. Both of the band gap energies were decreased when the GNP content in the PMMA matrix is increased. The decrease in the band gap energies was interpreted as evidence of increased conductivity of the composites. Additionally energies of the band tails (Urbach energy) were calculated. It was seen that the Urbach energy levels were increased with GNP content. POLYM. COMPOS. 40:1862-1869 2019. (c) 2018 Society of Plastics Engineers
Citation Count: 81
Electrical and optical percolations of polystyrene latex-multiwalled carbon nanotube composites
(Academic Press Inc Elsevier Science, 2010) Kara, Selim; Arda, Ertan; Dolastir, Fahrettin; Pekcan, Önder
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.
Citation Count: 4
Electrical optical and fluorescence percolations in P(VAc-co-BuA)/MWCNT composite films
(Taylor & Francis Ltd, 2013) Arda, Ertan; Kara, Selim; Pekcan, Önder
Effects of multiwall carbon nanotube (MWCNT) addition on the electrical conductivities optical transparencies and fluorescence emissions of poly(vinyl acetate-co-butyl acrylate) (P(VAc-co-BuA))/MWCNT composite films were studied. Optical transmission fluorescence emission and two point probe resistivity techniques were used to determine the variations of the optical fluorescence and electrical properties of the composites respectively. Transmitted photon intensity (I-tr) fluorescence emission intensity (I-fl) and surface resistivity ((s)) of the composite films were monitored as a function of MWCNT mass fraction (M) at room temperature. All these measured quantities of the composites were decreased by increasing the content of MWCNT in the composite. The conductivity and the optical results were attributed to the classical and site percolation theories respectively. The fluorescence results however possessed both the site and classical percolation theories at low and high MWCNT content regions respectively.
Citation Count: 5
Epidemic models for phase transitions: application to a physical gel
(Taylor & Francis Ltd, 2017) Bilge, Ayşe Hümeyra; Pekcan, Önder; Kara, Selim; Öğrenci, Arif Selçuk
Carrageenan gels are characterized by reversible sol-gel and gel-sol transitions under cooling and heating processes and these transitions are approximated by generalized logistic growth curves. We express the transitions of carrageenan-water system as a representative of reversible physical gels in terms of a modified Susceptible-Infected-Susceptible epidemic model as opposed to the Susceptible-Infected-Removed model used to represent the (irreversible) chemical gel formation in the previous work. We locate the gel point T-c of sol-gel and gel-sol transitions and we find that for the sol-gel transition (cooling) T-c > T-sg (transition temperature) i.e. T-c is earlier in time for all carrageenan contents and moves forward in time and gets closer to T-sg as the carrageenan content increases. For the gel-sol transition (heating) T-c is relatively closer to T-gs
Citation Count: 1
Evaluation of the fractal dimension of polyacrylamide during gelation and swelling
(Elsevier, 2021) Arda, Ertan; Kara, Selim; Pekcan, Önder; Gülşen, Akın-Evingür
[Abstract Not Available]
Citation Count: 5
Fractal Dimensions of kappa-Carrageenan Gels during Gelation and Swelling
(Taylor & Francis Inc, 2018) Kara, Selim; Arda, Ertan; Pekcan, Önder
A photon scattering technique for research on the sol-gel and gel-sol transitions in kappa-carrageenan-water systems with various carrageenan contents (CC) was utilized for characterizing the fractal dimensions during gelation and swelling. It was observed that the scattered photon intensity I-sc increased at all temperatures with an increase in the CC when I-sc was monitored against temperature. Additionally the sol-gel transition temperatures were found to be much lower than the gel-sol transition temperatures causing hysteresis of the phase transition loops. I-sc increased with an increase in CC at all test temperatures which is attributed to the formation of a fractal-like carrageenan gel. After drying the gels were used in swelling experiments where the gels were immersed in water at room temperature reswelling to the original structure. It was observed that I-sc from the carrageenan gels increased as the CC was increased. The fractal dimension d during gelation was found to increase as the gelation temperature was increased. On the other hand the d values during swelling decreased as the swelling time was increased.
Citation Count: 14
GELATION MECHANISMS
(World Scientific Publ Co Pte Ltd, 2012) Pekcan, Önder; Kara, Selim
In this paper, we survey the gelation mechanisms for various polymeric systems which are classified by the type and the strength of the cross-linkages. These are the "irreversible" gels that are cross-linked chemically by covalent bonds and the "reversible" gels that are cross-linked physically by hydrogen or ionic bonds and by the physical entanglement of polymer chains. Some of the natural polymer gels fall into the class of physical gels, among which the red algae that has attracted attention for various applications is discussed in detail. Various composite gels, formed from mixture of physical and chemical gels are also discussed in the last section of the article. Theoretical models describe the gelation as a process of random linking of subunits to larger and larger molecules by formation of an infinite network, where no matter what type of objects are linked, there is always a critical "gel point" at which the system behaves neither as a liquid nor as a solid on any length scale. The Flory-Stockmayer theory and percolation theory provide bases for modeling this sol-gel phase transition. The experimental techniques for measuring the critical exponents for sol-gel phase transitions in different polymeric systems are introduced and the validation of various theoretical predictions are surveyed.
Citation Count: 8
Mathematical Characterization of Thermo-reversible Phase Transitions of Agarose Gels
(Taylor & Francis Inc, 2018) Bilge, Ayşe Hümeyra; Pekcan, Önder; Kara, Selim; Bilge, Ayşe Hümeyra
The thermal phase transition temperatures of high (HMP) and low melting point (LMP) agarose gels were investigated by using UV-vis spectroscopy techniques. Transmitted light intensities from the gel samples with different agarose concentrations were monitored during the heating (gel-sol) and cooling (sol-gel) processes. It was observed that the transition temperatures T-m defined as the location of the maximum of the first derivative of the sigmoidal transition paths obtained from the UV-vis technique slightly increased by increasing the agarose concentration in both the HMP and LMP samples. Here we express the phase transitions of the agar-water system as a representative of reversible physical gels in terms of a modified Susceptible-Infected-Susceptible epidemic model whose solutions are the well-known 5-point sigmoidal curves. The gel point is hard to determine experimentally and various computational techniques are used for its characterization. Based on previous work we locate the gel point T-0 of sol-gel and gel-sol transitions in terms of the horizontal shift in the sigmoidal transition curve. For the gel-sol transition (heating) T-0 is greater than T-m i.e. later in time and the difference between T-0 and T-m is reduced as the agarose content increases. For the sol-gel transition (cooling) T-0 is again greater than T-m but it is earlier in time for all agarose contents and moves forward in time and gets closer to T-m as the agarose content increases.
Citation Count: 0
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.
Citation Count: 8
Sorption and Desorption of PVA-Pyrene Chains in and out of Agarose Gel
(Springer/Plenum Publishers, 2012) Kara, Selim; Gacal, Burçin; Tunç, Deniz; Yağcı, Yusuf; Pekcan, Önder
In situ steady-state fluorescence (SSF) measurement technique was applied to investigation of pyrene labeled Poly(vinyl alcohol) (PVA-Py) molecules diffusion in and out of agarose gels. Gel samples with four different concentration of agarose were prepared. PVA-Py was synthesized by "click" chemistry method and dissolved in water to use in diffusion experiments. The results were analyzed by using Fickian type diffusion model, and it was found that sorption and desorption processes of PVA-Py molecules in and out of agarose gel have two distinct regions for short and long diffusion times. Sorption and desorption coefficients were measured and it was seen that the diffusion rates were much larger at short times and at lower agarose concentrations.
Citation Count: 6
Study of Thermal Phase Transitions in Iota Carrageenan Gels via Fluorescence Technique
(Wiley-Blackwell, 2011) Tari, Ozlem; Kara, Selim; Pekcan, Önder
The effect of carrageenan concentration on thermal phase transitions of the iota carrageenan gels was investigated by using fluorescence technique. During heating and cooling processes scattered light I(sc) and fluorescence intensity I(p) were monitored against temperature to investigate phase transitions. Transition temperatures from the derivative of the transition paths were determined. Two regions were observed during the heating and cooling processes. At the first step of the heating dimers were converted into double helix by undergoing dimer to double helix (d-h) transition. At the higher temperature region double helix to coil (h-c) transition took place. During the cooling process these transitions are arranged in the order of coil to double helix (c-h) and double helix to dimer (h-d). A hysteresis was observed between (h-d) and (d-h) transitions. The critical gel fraction exponents beta were found to be independent of the system by indicating that they all fall into the same universality class. (C) 2011 Wiley Periodicals Inc. J Appl Polym Sci 121: 2652-2661 2011
Citation Count: 5
Thermal Phase Transitions of Agarose in Various Compositions: A Fluorescence Study
(Springer/Plenum Publishers, 2011) Kara, Selim; Arda, Ertan; Dolastir, Fahrettin; Pekcan, Önder
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.
Citation Count: 1
Thermal Phase Transitions of IOTA Carrageenan in CaCl2 Solutions: A Fluorescence Study
(Taylor & Francis Inc, 2011) Tari, Ozlem; Kara, Selim; Pekcan, Önder
The fluorescence technique was employed to study thermal phase transitions of iota (-) carrageenan (IC) in CaCl2 solution. IC gels underwent coil to double helix (c-h) and double helix to dimer (h-d) transitions upon cooling. Upon heating IC gels presented dimer to double helix (d-h) and double helix to coil (h-c) transitions showing hysteresis types of transition paths. Scattered light Isc and fluorescence intensity I were monitored against temperature to determine phase transitions. Transition temperatures were determined from the derivative of the transition paths. The critical gel fraction exponent was measured and found to be in accord with the classic Flory-Stockmayer model.