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Yayın Study on swelling of hydrogels (PAAm) at various temperatures by using fluorescence technique(Springer US, 2007-10) Kaya Aktaş, Demet; Akın Evingür, Gülşen; Pekcan, Mehmet ÖnderSteady-state fluorescence (SSF) technique was employed for studying swelling of polyacrylamide (PAAm) hydrogels. Disc-shaped gels were prepared by free-radical crosslinking copolymerization of acrylamide (AAm) with N, N'- methylenebis (acrylamide) (BIS) as crosslinker in the presence of ammonium persulfate (APS) as an initiator. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine was measured during in situ swelling process at various temperatures and it was observed that fluorescence intensity values decreased as swelling is proceeded. 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 is observed that swelling time constants, tau(c) decreased and diffusion coefficients, D-c increased as the swelling temperature is increased. The swelling activation energies, Delta E were measured from the intensity, weight and volume variations and found to be 10.7, 32.2 and 64.1 kJ mol(-1), respectively.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.
