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Yayın Film formation from TiO2-polystyrene latex composite: a fluorescence study(Taylor & Francis Group, 2007-03-01) Uğur, Şaziye; Sunay Yapışkan, Münüre Selin; Tepehan, Fatma Zehra; Pekcan, Mehmet ÖnderThis work reports the use of the steady state fluorescence (SSF) technique for studying film formation from TiO2 covered polystyrene (PS) latex particles. The composite films were prepared from pyrene (P)-labeled PS particles by covering them with TiO2 at room temperature and then annealed at elevated temperatures in 10 min time interval above glass transition (T-g) temperature of polystyrene. Five different composite films were studied in various TiO2 layer contents. Fluorescence intensities Ip from P were measured after each annealing step to monitor the stages of film formation. Films showed considerable increase in Ip above the certain onset temperature called minimum film forming temperature, T-0. Void closure and interdiffusion stages were modeled and related activation energies were determined and found to be 23.12 and 92.80 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 Percolation approach to film formation from surfactant-free polystyrene particles(Taylor & Francis, 2005-09) Uğur, Şaziye; Pekcan, Mehmet ÖnderIn this study, a film formation process from surfactant-free polystyrene (PS) latex particles is reported. Steady state fluorescence (SSF) and photon transmission (UVV) techniques were used to study the evolution of film formation. The latex films were prepared from pyrene (P)-labeled PS particles at room temperature and annealed at time intervals of 2.5 min above the glass transition temperature (T-g) of PS. During the annealing processes, the transparency of the film changed considerably. Fluorescence intensity (I-0P) from P was measured after each annealing step to monitor the stages of film formation. Evolution of transparency of latex films were monitored by using photon transmission intensity, I-tr. A drastic increase in I-tr and I-0P above the critical annealing times, t(r) and t(c) were attributed, respectively to percolation behavior of PS material from one side to the other side of the latex film. Critical exponents, beta of percolation clusters were measured and found to be around 0.35 and 0.25 for I-tr and I-0P measurements, respectively.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 Film formation stages for poly(vinyl acetate) latex particles: a photon transmission study(Springer-Verlag, 2006-07) Kara, Selim; Pekcan, Mehmet Önder; Saraç, Ayfer; Arda, ErtanPhoton transmission technique was used to monitor the evolution of transparency during film formation from poly(vinyl acetate) (PVAc) latex particles. The latex films were prepared below the glass transition temperature (T-g) of PVAc. These films were annealed at elevated temperatures in various time intervals above the T-g of PVAc. It is observed that transmitted photon intensity (I-tr) from these films increased as the annealing temperature is increased. It is seen from I (tr) curves that there are two film formation stages. These successive stages are named void closure (viscous flow) and interdiffusion. The activation energies for viscous flow (Delta H) and backbone motion (Delta E (b)) were obtained by using well-defined models. The averaged values of the backbone (Delta E (b)) and the viscous flow activation energies (Delta H) were found to be 188.6 and 5.6 kcal/mol, respectively. The minimum film formation (tau (M),T (M)) and healing points (tau (H),T (H)) were determined. Minimum film formation (Delta E (M)) and healing activation energies (Delta E (H)) were measured using these time-temperature pairs. Delta E (M) and Delta E (H) were found to be 32.5 and 28.3 kcal/mol, respectively.Yayın Reversible film formation from nano-sized PNIPAM particles below glass transition(Springer-Verlag, 2007-01) Uğur, Şaziye; Elaissari, Abdelhamid; Yargı, Önder; Pekcan, Mehmet ÖnderReversible film formation process from nano-sized Poly(N-isopropylacrylamide) (PNIPAM) microgel particles were studied during heating-cooling cycles at various rates. Photon transmission technique was used and transmitted photon intensity I (tr) was monitored during heating-cooling cycles. The increase and decrease in I (tr) during heating and cooling was explained with the void closure and void reconstruction processes, and the corresponding activation energies were measured. It was observed that PNIPAM microgels required less energy during reconstruction of voids than their closure.
