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Yayın Temperature effect on drying and swelling of kappa carrageenan gels: A steady state fluorescence study(Wiley-V C H Verlag GMBH, 2008) Tarı İlgin, Özlem; Pekcan, Mehmet ÖnderA novel technique based on in situ steady state fluorescence (SSF) measurements is introduced for studying drying and swelling Of kappa(-) carrageenan (kappa carrageenan) gels at various temperatures. kappa(-) carrageenan gels were completely dried and then swelled in water vapor. Pyranine was embedded in kappa(-) carrageenan and used as a fluorescence probe. Scattered light intensities, I-SC and fluorescence intensities, I were monitored during the drying and swelling Of kappa(-) carrageenan gels. it was observed that the fluorescence intensity decreased linearly as drying time was increased. A simple model consisting of Case II diffusion was used to quantify the drying processes of the kappa(-) carrageenan gels. This moving boundary model provided packing constant, kappa(o). During swelling, fluorescence intensity increased exponentially as time is increased. The increase in I, was modeled using Li-Tanaka equation from which swelling time constants, tau(c) and cooperative diffusion coefficients, D-c were determined. It was observed that swelling time constants, tau(c) decreased and diffusion coefficients, D-c increased as the swelling temperature was increased. Activation energies for drying and swelling were also obtained and found to be 53.9 and 47.2 kJ mol(-1), respectively.Yayın Molecular alignment during gel formation from methyl methacrylate: An excimer fluorescence study(Taylor & Francis Ltd, 2005-05) Kaya Aktaş, Demet; Erdoğan, Matem; Pekcan, Mehmet ÖnderGlass transition during bulk polymerization was studied in free-radical cross-linking copolymerization (FCC) of methyl methacrylate (MMA) using the steady-state fluorescence (SSF) technique. Naphthalene (N) was used as a monomer and excimer forming probe. Changes in the viscosity of the pregel solutions due to gel formation dramatically enhance both monomer and excimer fluorescent yield of N molecules. The reaction time at which the monomer and excimer intensities exhibit a sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum resulting from the gel effect. This effect was used to study the gelation of MMA, as a function of time, in various N concentrations. The results were interpreted in the view of percolation theory. The gel fraction, beta, and weight average degree of polymerization, gamma, exponents beta=0.40 +/- 0.02 and gamma=1.70 +/- 0.07 were found in agreement with percolation results for both monomer and excimer measurements, respectively.Yayın In situ fluorescence study of swelling, sorption and desorption processes in and out of PAAm gels(Wiley-V C H Verlag GMBH, 2008-05) Evingür, Gülşen Akın; Karslı, Kadir; Pekcan, Mehmet ÖnderSmall molecule sorption and desorption in and out of polyacrylamide (PAAm) gels were studied at various temperatures. Pyranine (P(y)) dissolved in water used as a probe. Fluorescence emission intensity, I(p) from P(y) was monitored for studying sorption and desorption processes. Scattered light intensities, I(SC) from PAAm gel was also monitored to observed structural variations during sorption and desorption process. Li-Tanaka model was applied to produce the swelling time constants, tau(C) and sorption coefficients, D(C) for the swelling processes. on the other hand, sorption and desorption processes were studied and coefficients were produced by using Fickian model. Related activation energies were also calculated from the corresponding physical processes.Yayın Universal behaviour of glass transition exponents in various polymeric systems(VSP BV, Brill Academic Publishers, 2005) Pekcan, Mehmet Önder; Kaya Aktaş, DemetThe fast transient fluorescence (FTRF) technique was used to study the critical exponents during glass transition in free-radical cross-linking copolymerization (FCC). Methyl methacrylate (MMA), ethyl methacrylate (EMA) and various combinations of MMA with EMA were used during FCC experiments. Pyrene (Py) was used as a fluorescence probe and its fluorescence lifetimes from its decay traces were measured during glass transition. Changes in the viscosity of the pre-gel solutions due to glass formation dramatically increased the Py fluorescent lifetimes, which were used to study the glass transition of MMA, EMA and their mixtures as a function of time, at various temperatures and monomer concentrations. The results were interpreted in the view of percolation theory. The critical exponents, beta and gamma, were measured near the glass transition point and found to be around 0.37 +/- 0.015 and 1.69 +/- 0.05, respectively, in all systems studied, which are in good agreement with the static percolation results.Yayın Molecular recognition during sol-gel and gel-sol transition of kappa-iota carrageenan mixtures(Taylor & Francis ltd, 2005-12) Kara, Selim; Arda, Ertan; Pekcan, Mehmet ÖnderSol-gel and gel-sol phase transitions of kappa-iota (kappa-iota) carrageenan mixtures in two types of salts were studied using photon transmission technique. Here, KCl and CaCl2 were chosen as the proper salts for the carrageenan mixtures. Coil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of carrageenan mixtures in monovalent and bivalent cation solutions were studied upon heating and cooling by using photon transmission technique. Photon transmission intensity, It, was monitored against temperature to determine the (c-h) and (h-d) transition temperatures (T-ch and T-nd). Two distinct transition regions were observed in both heating and cooling of the carrageenan-bivatent CaCl2 salt system. At the first step of heating process dimers decompose into double helices by making (d-h) transition. At the high temperature region (h-c) transition takes place. Carrageenan-monovalent KCl salt system presented only (c-h) and (h-c) transitions upon cooling and heating processes. Transition temperatures at the heating and cooling processes were obtained. A hysteresis was observed between (h-c)-(c-h) and (d-h)-(h-d) transitions for the monovalent and bivalent cations respectively.Yayın Cation effects on phase transition of kappa-iota-carrageenan hybrids: a photon transmission study(Taylor & Francis Group, 2007) Pekcan, Mehmet Önder; Kara, Selim; Arda, ErtanPhase transitions of kappa-iota-carrageenan hybrid in various types of salts were studied using photon transmission technique: NaCl, KCI, MgCl2, CaCl2 and KCl+CaCl2 were chosen as the salt solutions for the carrageenan hybrid. Coil-to-helix (c-h), helix-to-dimer (h-d) and back to helix-to-coil (h-c), dimer-to-helix (d-h) phase transitions of carrageenan hybrid in these mono- and bivalent cation solutions were studied upon cooling and heating, respectively. Photon transmission intensity, It, was monitored against temperature to determine the (c-h), (h-d) and (h-c), (d-h) transition temperatures (T-ch, T-hd and T-hc, T-dh) and activation energies (Delta E-ch, Delta E-hd and Delta E-hc, Delta E-dh) of carrageenan hybrids. Two distinct transition regions were observed both during heating and cooling of the carrageenan-bivalent salt systems. During the heating process, at first dimers decompose into helices by making a (d-h) transition. Then at the high temperature region, a (h-c) transition takes place. During cooling, back transitions repeat themselves. However, the carrageenan-monovalent salt system presented only (c-h) and (h-c) transitions during the cooling and heating processes, respectively. A hysteresis was observed between (h-c)-(c-h) and (d-h)-(h-d) transitions for the monovalent and bivalent cation systems respectively.Yayın Universality of sol-gel phase transition of kappa-carrageenan in various salts: a steady state fluorescence study(Taylor & Francis Ltd, 2007-08) Pekcan, Mehmet Önder; Tarı İlgin, ÖzlemThermal phase transitions of kappa-carrageenan in NaCl, KCl and CaCl2 solutions were studied using steady state fluorescence (SSF) technique. Pyranine was introduced as a fluorescence probe for studying sol-gel phase transitions. Scattered light, I-sc and fluorescence intensity, I was monitored against temperature to determine the sol-gel (T-sg) phase transition temperatures. It was observed that T-sg values are strongly correlated to NaCl, KCl and CaCl2 contents. The weight average degree of polymerization, DPw, and gel fraction G, exponents (gamma and beta) were measured and found to be in accord with the classical Flory-Stockmayer model, i.e., gamma and beta were found to be close to 1.0, independent of salt content.
