4 sonuçlar
Arama Sonuçları
Listeleniyor 1 - 4 / 4
Yayın Time evolution of film formation from polystyrene particles: a percolation approach(Springer, 2005-12) Uğur, Şaziye; Pekcan, Mehmet ÖnderThis work reports the film formation process from surfactant-free polystyrene (PS) latex particles. Steady state fluorescence and photon transmission techniques were used to study the evolution of film formation. The films were prepared from fluorescein (F)-labeled PS latex particles at room temperature and annealed in 2.5-min-time intervals above the glass transition temperature (T-g) of PS. Fluorescence intensity (I-F) from F was measured after each annealing step to monitor the stages of film formation. Evolution of transparency of latex films was monitored by using the photon transmission intensity, I-tr. Drastic increase in I-tr and I-F above the critical annealing times t(r) and t(c) respectively, were attributed to the percolation behavior of the PS material. Critical exponents, beta, of percolation clusters were measured and found to be around 0.31 and 0.37 for Itr and IF measurement, respectively, which were attributed to the site percolation model.Yayın Critical behavior of thermal phase transitions of iota-carrageenan in CaCl2 solution(Elsevier B.V., 2006-07-15) Özbek, Haluk; Pekcan, Mehmet ÖnderIn situ photon transmission method was applied to study thermal phase transitions of ?-carrageenan in CaCl2 solution. Coil-to-helix (c-h) and helix-to-dimer (h-d) transitions were detected upon cooling. An extra dimer-to-dimer (d-d) transition was observed as well during cooling at low-temperature region. Upon heating only dimer-to-helix (d-h) and helix-to-coil (h-c) transitions were detected. Photon transmission intensity, Itr was monitored against temperature to determine several phase transitions and transition temperatures. It was observed that the c-h transition in ?-carrageenan-CaCl2 system performs a crossover between 3D percolation to classical theory. Upon heating, the specific heat exponent over the d-h transition was extracted from the transmitted intensity data and was found to be 0.045.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 Effect of annealing rate on film formation from poly(vinyl acetate) latex particles(Taylor & Francis Ltd, 2008) Arda, Ertan; Kara, Selim; Pekcan, Mehmet ÖnderThe photon transmission technique was used to monitor the time and temperature evolution of 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 various annealing rates above the Tg of PVAc. It was observed that transmitted photon intensity (I-tr) from these films increased at all annealing rates as the annealing time and temperature are increased. The increase in the I-tr is attributed to the increase in crossing density at the junction surface. The healing points (tau(H), T-H) were determined for various annealing rates and the healing activation energy (Delta H) was measured and found to be 43.4 kcal/mol. The interdiffusion (backbone) activation energies (Delta E) and reptation frequencies (nu) of PVAc chains were measured. The averaged value of Delta E was found to be 226.6 kcal/mol. It is observed that Delta E is unchanged but nu is increased as increasing the annealing rates.
