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Yayın Reversible film formation from PS doped PNIPAM particles in various compositions(John Wiley & Sons Inc, 2008-02) Uğur, Şaziye; Yargı, Önder; Pekcan, Mehmet ÖnderFilm formation from polystyrene (PS) doped poly(N-isopropylacrylamide) (PNIPAM) particles was studied using photon transmission technique. The transmitted light intensity, Itr, was monitored during film formation process. Films were prepared by mixing PS and PNIPAM particles in various compositions ranging from 5 to 50 %. Samples were separately heated and cooled in constant rate at temperatures ranging from 10 to 100 C. The increase and decrease in Itr during heating-cooling cycles were explained by void closure and void reconstruction processes. The corresponding activation energies were measured during the reversible film formation process. Percolation model was used to interpret the distribution of PS particles in PNIPAM lattice.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 Films formed from polystyrene latex/clay composites: A fluorescence study(Springer New York, 2005-07) Uğur, Şaziye; Alemdar, Ayşe; Pekcan, Mehmet ÖnderThis study reports a steady-state fluorescence (SSF) technique for studying film formation from surractant-free polystyrene (PS) latex and Na-montmorillonite (SNaM) composites. The composite films were prepared from pyrene (P)-labeled PS particles and SNaM clay at room temperature and annealed at elevated temperatures in 10-min intervals above glass transition temperature (T-g) of polystyrene. During the annealing processes, the transparency of the fllm improved considerably. Scattered light (I-s) and fluorescence intensity (I-s) from P were measured after each annealing step to monitor the stages of film formation. Evolution of transparency of composite films was monitored by using photon transmission intensity, I-tr Scanning electron microscopy (SEM) was used to detect the variation in physical structure of annealed composite-films. Minimum flIm formation temperature, T-o, and healing temperatures, T-h, were determined. Void closure and interdiff-usion stages were modeled and related activation energies were determined. It was observed that both activation energies increased as the percent of SNaM was increased in composite films.Yayın Fluorescence study on Al2O3-polystyrene latex composite film formation(John Wiley & Sons Inc, 2005-06) Uğur, Şaziye; Salman, Oğuz Umut; Tepehan, Galip Gültekin; Tepehan, Fatma Zehra; Pekcan, Mehmet ÖnderThis work reports a steady state fluorescence (SSF) technique for studying film formation from mixture of Al2O3 and polystyrene (PS) latex particles. The composite films were prepared from dispersion of pyrene (P)labeled PS particles in Al2O3 solution at room temperature and annealed at elevated temperatures in 10-min time interval above glass transition (T-g) temperature of polystyrene. Nine different composites film were studied in various latex contents. Fluorescence intensities (I-P) from P were measured after each annealing step to monitor the stages of film formation. No variations in I-p were detected for the films prepared with higher than 33 wt% Al2O3 content. However films prepared below 33 wt% Al2O3 content show considerable increase in I-P above the certain onset temperature called minimum film forming temperature, To. Healing temperatures T., were determined from the maxima of I-P. Void closure and interdiffusion stages were modeled and related activation energies were determined and found to be 20 and 97 kJ.mol(-1), respectively.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.
