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Yayın Cation effect on gel-sol transition of kappa carrageenan(Springer, 2008-04) Pekcan, Mehmet Önder; Tarı İlgin, ÖzlemThe steady state fluorescence (SSF) technique was employed to study gel - sol transitions of kappa carrageenan in NaCl, KCl and CaCl2 solutions. Pyranine was used as a fluorescence probe for monitoring these transitions. Scattered light, I-sc and fluorescence intensity, I was monitored against temperature to determine the gel - sol (T-gs) transition temperatures and exponents. It was observed that T-gs values are strongly correlated to the 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.Yayın Comparision of critical parameters of polymerization and gelation processes: A fast transient fluorescence study(World Scientific Publ Co Pte Ltd, 2005-03-10) Kaya Aktaş, Demet; Pekcan, Mehmet ÖnderThe fast transient fluorescence technique was used to study free-radical polymerization and crosslinking copolymerization of styrene. Ethylene glycol dimethacrylate and Pyrene were used as crosslinking agent and fluorescence probe, respectively. The fluorescence lifetimes of Pyrene from its decay traces were measured and used to monitor the vitrification processes at various temperatures. Changes in the viscosity due to polymer formation dramatically enhanced the fluorescent yield of pyrene molecules. This effect is used to monitor the polymerization and crosslinking copolymerization of Styrene as a function of time, at various temperatures. The results are interpreted in the view of static percolation theory. The critical exponents ß and ? for vitrified fraction and average degree of polymerization were found to be 0.39 ± 0.012; 1.62 ± 0.036 and 0.39 ± 0.004; 1.69 ± 0.064 in agreement with percolation results for polymerization and gelation processes, respectively. Activation energies for polymerization and gelation (?EP and ?EG) were measured and found to be 112.0 ± 4 and 86.9 ± 4 kJ mol-1, respectively.
