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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 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.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.
