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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 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 Monitoring diffusion of reptating polymer chains by a direct energy transfer method: A Monte Carlo simulation(Wiley-Blackwell, 2002-07-29) Tüzel, Erkan; Kısacıkoğlu, Kemal Batuhan; Pekcan, Mehmet ÖnderA kinetic Monte Carlo method was used to simulate the diffusion of reptating polymer chains across an interface. A time-resolved fluorescence technique in conjunction with a direct energy transfer method was used to measure the extent of diffusion of dye-labeled reptating polymer chains. The diffusion of donor- and acceptor-labeled polymer chains between adjacent compartments was randomly generated. The fluorescence decay profiles of donor molecules were simulated at several diffusion steps to produce mixing of the polymer chains. Mixing ratios of donor- and acceptor-labeled polymer chains in compartments were measured at various stages (snap-shots) of diffusion. It was observed that for a given molecular weight, the average interpenetration contour length was found to be proportional to the mixing ratio. Monte Carlo analysis showed that the curvilinear diffusion coefficient is inversely proportional to the weight of polymer chains during diffusion.Yayın Drying of PAAm hydrogels at various temperatures: A fluorescence study(Taylor & Francis Group, 2007-05) Aktaş, Demet Kaya; Evingür, Gülşen Akın; Pekcan, Mehmet ÖnderThe steady-state fluorescence (SSF) technique was employed for studying the drying of polyacrylamide (PAAm) hydrogels. Disc-shaped hydrogels were prepared by free-radical crosslinking copolymerization of acrylamide (AAm) with N,N'-methylene bisacrylamide (BIS) as crosslinker in the presence of ammonium persulfate (APS) as an initiator. Pyranine (P) was introduced as a fluorescence probe and the intensity of pyranine was monitored during in situ drying at various temperatures. It was observed that the fluorescence intensity of pyranine increased during the drying process. A supporting, gravimetrical experiment was also performed. A phenomenological equation was introduced to determine the desorption coefficient, D, of water molecules from the drying hydrogels at various temperatures. The desorption activation energy, triangle E-d, values were measured for the drying processes and found to be 91.08 and 36.82 kJ mol(-1) by fluorescence and gravimetrical methods, respectively. This difference most probably originates from the origin of the techniques; the fluorescence technique measures the parameters at a molecular level, whereas the gravimetrical technique measures a parameter in the bulk.Yayın Sorption and slow release kinetics of PAAM gels at various temperatures(Freund Publishing House Ltd, 2007-11) Akın Evingür, Gülşen; Pekcan, Mehmet ÖnderSmall molecule sorption and slow release in and out of polyacrylamide (PAAm) gels were studied by using steady state fluorescence (SSF) technique. Pyranine (P-y) dissolved in water used as a probe. Fluorescence emission intensity, I-p from P-y was used to monitor for studying sorption and slow release processes at various temperatures. Sorption and slow release processes were analyzed by using Fickian diffusion model and coefficients were obtained. Related activation energies were also calculated for the corresponding physical processes.
