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Yayın Vapor-induced film formation from low-T-g particles for different solvent compositions(Elsevier Inc, 2006-05-15) Arda, Ertan; Kara, Selim; Saraç, Ayfer; Pekcan, Mehmet ÖnderThe photon transmission method was used to study latex film formation from poly(vinyl acetate) (PVAc) particles induced by two different solvents. Films with the same latex content were prepared from PVAc particles and exposed to vapor of ethanol-water and acetone-water mixtures in various compositions. Transmitted photon intensities, I-tr, from these films increased with increasing vapor exposure time. The increase in I-tr is attributed to the increase in crossing density at the polymer-polymer junction. The Prager-Tirrell model was employed to obtain the back-and-forth frequency, v, of the reptating polymer chain during film formation induced by solvent vapor. It was observed that the produced v values increase as the solvent content is increased for both solvents. Abilities of both solvents to form films were interpreted with the solubility parameters of the solvents and the PVAc.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 Photon transmission study on conformational ordering of iota-carrageenan in CaCl2 solution(Taylor & Francis Inc, 2005-06) Kara, Selim; Pekcan, Mehmet ÖnderCoil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of iota-carrageenan in CaCl2 solution upon cooling were studied using photon transmission technique. Photon transmission intensity, I-iota r was monitored against temperature to determine the (c-h) and (h-d) transition temperatures (T-ch and T-hd) and activation energies (Delta E-ch and Delta E-hd). An extra dimer-to-dimer (d-d) transition was also observed during cooling at low temperature region. However, upon heating dimers disappear to double helices by making dimer-to-double helix (d-h) transition. Further heating resulted double helix-to-coil (h-c) transition at high temperature region. T-dh and T-ch temperatures and Delta E-dh and Delta E-hc activation energies were also determined. It was observed that T-hc and T-ch temperatures and Delta E-dh and Delta E-hd activation energies do not effected by carrageenan content. However, T-hd, T-dh and T-dd temperatures and Delta E-ch and Delta E-hc activation energies were found to be strongly correlated to the carrageenan content in the system.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 Universal behaviour of gel formation from acrylamide-carrageenan mixture around the gel point: A fluorescence study(Taylor & Francis Group, 2006-08) Kaya Aktaş, Demet; Evingür, Gülşen Akın; Pekcan, Mehmet ÖnderThe steady state fluorescence (SSF) technique was used to study the sol-gel transition, for the solution free radical crosslinking copolymerization of acrylamide (AAm) with various carrageenan content. N, N'- methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) are used as crosslinker and an initiator, respectively. Pyranine (8-hydroxypyrene-1, 3,6-trisuffonic acid, trisodium salt, HPTS) was added as a floroprobe for monitoring the polymerization. Pyranine molecules start to bind to acrylamide polymer chains upon the initiation of the polymerization; thus, the spectra of the bonded pyranines shift to the shorter wavelengths. Fluorescence spectra from the bonded pyranines allows one to monitor the sol-gel transition, without disturbing the system mechanically, and to test the universality of the sol-gel transition as a function of some kinetic parameters like polymer concentration. Observations around the critical point show that the gel fraction exponent P obeyed the percolation result for low carrageenan concentrations (< 2.0%) however classical results were produced at higher carrageenan concentration (> 2.0%).Yayın Synergistic effect of the locust bean gum on the thermal phase transitions of kappa-carrageenan gels(Elsevier Sci Ltd, 2009-03) Arda, Ertan; Kara, Selim; Pekcan, Mehmet ÖnderSynergism between K-carrageenan and locust bean gum (LBG) was studied using the photon transmission technique. Synergistic effects in these polymeric mixtures strongly affected the physical properties of the gel structure. The transmitted light intensities, I-tr, versus temperature variations were investigated during the gelation and liquefaction processes. Slight synergistic peaks were detected in gel-sol and sol-gel transition temperatures for high kappa-carrageenan/LBG ratios (approx. 80/2). Moreover, apparent synergistic peaks were observed in gel-sol and sol-gel transition activation energies for the mixtures approximately with the ratios of 80/10.Yayın Poly (styrene) latex/modified na-activated bentonite nanocomposite films: a fluorescence study(Elsevier Science BV, 2008-12) Uğur, Şaziye; Yargı, Önder; Günister, Ebru; Pekcan, Mehmet ÖnderWe studied film formation of composites of surfactant-free polystyrene (PS) nanoparticles and modified Na-activated bentonite (MLB), by steady state fluorescence (SSF) technique. The films were prepared from a mixture of pyrene (P)-labeled PS particles and MLB at various compositions at room temperature. These films were annealed at elevated temperatures above the glass transition (T(g)) temperature of polystyrene for 10 min. Scattered light (I(S)) and fluorescence intensities (I(P)) from P were measured after each annealing step to monitor the stages of film formation. Evolution of transparency of the composite films was monitored by using photon transmission intensity, I(tr) Atomic force microscopy (AFM) was used to detect the variation in physical structure of annealed composite films. The nanocomposite films exhibited a percolation threshold at 20 wt.% MLB content. Below this fraction two distinct film formation stages were observed which are known as void closure and interdiffusion and above this fraction no film formation was detected. At 0-20 wt.% MLB, minimum film formation, T(0), void closure, T(v) and healing, T(h) temperatures were determined. Void closure and interdiffusion stages were modeled and related activation energies were determined. Void closure activation energies decreased as the percent of MLB increased, no variation was observed in backbone activation energies.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 from nano-sized polystrene latex covered with various TiO2 layers(Wiley, 2006-12) Uğur, Şaziye; Sunay, Selin; Elaissari, Abdelhamid; Tepehan, Fatma Zehra; Pekcan, Mehmet ÖnderSteady-state fluorescence technique was used for studying film formation from TiO2 covered nano-sized polystyrene latex particles. The composite films were prepared from pyrene (P)-labeled PS particles by covering them with various layers of TiO2 at room temperature. These films 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 emission intensity, I-P from P was measured after each annealing step to monitor the stages of film formation. Films present significant increase in I-P above the certain onset temperature called minimum film forming temperature, To. However, at higher annealing temperatures, I-P showed a decrease. Increase and decrease in I-P were modeled by void closure and interdiffusion processes and related activation energies were determined, respectively. Dissolution of annealed PS film, with high TiO2 content presented a nice, ordered nanosized ceramic structure, which may predict the construction of nano-layer photonic crystals.Yayın The effect of clay particles on film formation from polystyrene latex(Wiley, 2006-06) Uğur, Şaziye; Alemdar, Ayşe; Pekcan, Mehmet ÖnderFilm formation from surfactant-free polystyrene (PS) latex was performed in the presence of 5% Na-montmorillonite (NaMMT). The composite films were prepared from pyrene (P)-labeled PS particles at room temperature and annealed at elevated temperatures above the glass-transition (T-g) temperature of polystyrene. Scattered light (I-s) and fluorescence intensity (I-p) from P were measured after each annealing step to monitor the stages of composite film formation. Minimum film formation temperature, T-o, and healing temperatures, T-h, were determined. Void closure and interdiffusion stages were modeled and related activation energies were measured. From these results, it was found that the presence of NaMMT in the PS latex film only affects the minimum film formation, but does not affect the void closure and backbone motion activities.
