16 sonuçlar
Arama Sonuçları
Listeleniyor 1 - 10 / 16
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 Swelling of iota-carrageenan gels prepared with various CaCl2 content: A fluorescence study(European Polymer Federation, 2008-01-05) Tarı İlgin, Özlem; Pekcan, Mehmet ÖnderIota carrageenan gels prepared with various CaCl2 content were completely dried and then swelled in water vapor. Steady-state fluorescence ( SSF) technique was used to monitor the swelling process of each iota carrageenan gels at various temperatures. Pyranine was used as a fluorescence probe. Apparent fluorescence intensity, I increased as swelling time increased for all gel samples. The increase in I was modelled using Li-Tanaka equation from which the swelling time constants, tau(1) and cooperative diffusion coefficients, D-c were determined. It was observed that D-c increased as the swelling temperature was increased. On the other hand at each temperature, it was seen that D-c decreased as CaCl2 content was increased. Activation energies for swelling were obtained and found to be 60.5, 61.0, 61.5 and 62.8 kJmol(-1) for the gels prepared with increasing amount of CaCl2 content.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 Small molecule diffusion into swelling Iota-Carrageenan gels: A fluorescence study(Taylor & Francis Group, 2007-04) Ataman, Evren; Pekcan, Mehmet ÖnderSmall molecule diffusion into Iota-Carrageenan gel was studied by using steady-state fluorescence (SSF) technique. Pyranine, dissolved in water was used as fluorescence probe. Fluorescence emission intensity, I-p, and scattered light intensity, I-sc, were monitored to study diffusion and swelling processes at various temperatures respectively. Fickian and Li-Tanaka models were elaborated to produce diffusion, D, and collective diffusion, D-0, coefficients. Diffusion and swelling activation energies were also obtained and found to be 20.5 kj mol(-1) and 28.2 kj mol(-1). respectively.Yayın Theory of fluidity of liquids, glass transition, and melting(Elsevier B.V., 2006-03-01) Dimitrov, Ventzislav IvanovThis is a presentation of a rigorous theory of fluidity of liquids, glass transition and melting of solids in the frame of an asymmetric double well potential model. Potential wells are doubled time to time by the local density fluctuations caused by the thermal longitudinal waves. The average frequency of doubling of potential wells is equal to the frequency of the most energetic waves which obey a law similar to Wein's displacement law in black body radiation. Based on the equilibrium thermodynamic theory of fluctuations and the displacement law, a law of linear pre-diffusion mean-square displacement of particles in a solid is derived: the mean-square displacement of molecules within their potential wells increases linearly with temperature. It is shown that when this is broken-down (where the mean-square displacement at a certain temperature rapidly changes its slope as a function of temperature) glass devitrifies and crystal melts, and all possible solid-liquid transitions of a substance occur at the same critical mean-square displacement: any solid (not only crystals) transforms into liquid when the mean-square displacement, as a fraction of the average intermolecular distance, acquires a certain universal critical value - the same for different substances. It is proved that molecules in a liquid perform specific Brownian motion. The average jump distance is a function of temperature and it is much smaller than the nearest intermolecular distances. At a certain temperature, shown to be the Kauzmann temperature, the average jump distance of Brownian motion becomes equal to zero: the supercooled liquid undergoes glass transition. The transition was proven to be a phase transition of the fourth order: the free energy of the system and its first, second and third derivatives are all continuous functions, but its fourth derivative with respect to temperature is discontinuous. Molecular mobility, diffusion and viscosity are obtained as functions of temperature.Yayın Swelling activation energy of kappa-carrageenan in its gel state: A fluorescence study(John Wiley & Sons Inc, 2007-12-15) Tarı İlgin, Özlem; Pekcan, Mehmet ÖnderA steady-state fluorescence technique was employed to study the swelling Of K-carrageenan gels at various temperatures. Pyranine was used as a fluorescence probe. The fluorescence intensity of pyranine was measured during the in situ swelling process of K-carrageenan gels. The fluorescence intensity increased exponentially as the swelling time increased. The increase in the fluorescence intensity was modeled with the Li-Tanaka equation, from which the swelling time constants and cooperative diffusion coefficients were determined. The swelling time constants decreased and the cooperative diffusion coefficients increased as the swelling temperature was increased. The swelling activation energies were measured to be 47.05 kJ/mol.Yayın Reordering of polystyrene gel due to multiple swelling in organic vapor fast transient fluorescence technique study(Elsevier Science SA, 2006-07-31) Erdoğan, Matem; Pekcan, Mehmet ÖnderReordering of disc-shaped polystyrene (PS) gels due to multiple swelling, under organic vapor was studied by using Fast Transient Fluorescence (FTRF) technique. Disc-sbaped polystyrene gels were prepared by free radical copolymerization (FRC) of styrene (S) with ethylene glycol dimethacrylate (EGDM) as a crosslinker. Pyrene (P) was introduced as a fluorescence probe during polymerization. Swelling experiments were performed by using P doped PS gels under chloroform vapor. After each swelling step, gels were left to dry in an oven at 30 degrees C for consecutive reswelling experiments. Decay curves of P were measured and pyrene lifetimes, tau, were determined. It was observed that tau values decreased as swelling recycles were repeated. It was observed that after the fifth swelling step, two different regimes appeared in the swelling processes in PS gels. Swelling time constant, tau(c), and cooperative diffusion coefficients, D-c were determined by using Li-Tanaka equation for each swelling step in both regimes. It was observed that D-c values decreased up to tenth swelling step and then remained unchanged in both regimes.Yayın Dissolution of Al2O3-polystyrene latex composites: A fluorescence study(Taylor & Francis Ltd, 2005) Uǧur, Şaziye; Tepehan, Fatma Zehra; Pekcan, Mehmet ÖnderThis work reports the use of the steady state fluorescence (SSF) technique to study dissolution of a composite film formed from a mixture of Al 2O3 and polystyrene (PS) latex particles. The composite films were cast from dispersion of pyrene (P)-labeled PS particles in Al 2 O3 solution at room temperature and annealed at 280°C temperature for 10 min. Eight different composite films were studied in various latex contents. Toluene was used as dissolution agent. Fluorescence intensities Ip from P were monitored during dissolution. Fickian diffusion was employed to model the dissolution processes. Dissolution coefficients, D d, were measured and found to be increased at high PS content in the composite films.Yayın Effects of annealing on morphology of polymer/polymer (PS/PMMA) blend; a fluorescence study(John Wiley& Sons Inc, 2006-05-05) Uğur, Şaziye; Pekcan, Mehmet ÖnderSteady state fluorescence (SSF) technique conjunction with optical microscopy were used to study the morphology of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blend upon annealing above glass transition in elevated time intervals. The PS/PMMA blends were prepared from dissolution of pyrene (P) and naphthalene (N) labeled PS and PMMA particles, respectively. Monte Carlo Simulations were performed to model the N and P fluorescence intensities (I-N and I-P), Using photon diffusion theory. Number of N and P photons (N-N and N-P) emerging from the front surface of the blend are calculated when only N is excited, where N-P photons are combined of photons from radiative (N-PR) and nonradiative (N-PNR) energy transfer processes. Optical microscopy images were taken at each annealing step to Support Our findings from fluorescence measurements.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.
