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    Effect of molecular weight on latex film formation: Photon transmission study
    (Wiley-Blackwell, 1998) Pekcan, O; Arda, E
    A UV-visible (UW) technique was used to monitor the evolution of transparency during film formation from hard latex particles. Two different latex films were prepared from particles with high and lon molecular weight (HM and LM) poly(methyl methacrylate) (PMMA) separately and annealed at elevated temperatures in various time intervals above the glass transition temperature (T-g). In both films, a continuous increase in the transmitted photon intensity (I-rf) was observed above 160 degrees C as the annealing temperature was increased. However, the reflected photon intensity (I-rf) first decreased and then increased by showing a minimum in the same temperature range as the annealing temperature was increased. The increase in the transmitted photon intensity (I-tr) is attributed to increase in the crossing density at the junction surface. The activation energies for back-and-forth motion (Delta E-tr) were measured and found to be around 35 and 25 kcal/mol for the reptating polymer chain across the junction surface in the LM and HM films, respectively. The decrease in I-rf was explained by the void-closure mechanism, and the increase in the I-rf above 160 degrees C was again attributed to the increase in the crossing density at thejunction surface. Back-and-forth activation energies (Delta E-rf) were measured to be around 47 and 18 kcal/mol and the void-closure constants (B) were found to be around 24 x 10(3) and 12 x 10(3) K for the LM and HM film samples, respectively. (C) 1998 John Wiley & Sons, Inc.
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    The effect of preparation temperature on phase transitions of N-isopropylacrylamide gel
    (Taylor & Francis Ltd, 2003) Pekcan, O; Kara, S
    The spinodal decomposition of N-isopropylacrylamide (NIPA) gels prepared at various onset temperatures, To, was studied by photon transmission. It was observed that the increase in turbidity is much faster in a gel prepared at higher Ton than at lower To, values, which indicated that the NIPA-water system reaches the spinodal decomposition much faster for a gel prepared at high T-on. It is understood that a NIPA gel prepared at high T-on possesses more heterogeneities which are gained during gelation and has a low spinodal temperature, T-s. However, NIPA gels prepared at lower T-on values go to spinodal decomposition at higher T-s values.
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    In situ photon transmission technique for studying ageing in acrylamide gels due to multiple swelling
    (Elsevier Sci Ltd, 1998) Pekcan, O; Catalgil-Giz, H; Caliskan, M
    In situ photon transmission experiments were performed using an ultra-violet-visible (u.v.-vis) spectrometer during the multiple swelling of polyacrylamide (PAAm) gel in water. The transmitted light intensity, I, decreased continuously as the PAAm gel swelled. The decrease in I, was attributed to the increase in scattered light intensity which might have orginated from the contrast between 'frozen blob clusters' and holes in the swollen gel. It was observed that the correlation length, xi(c), increased by 1/4 power of the swelling time. The sudden increase in the gel relaxation time, tau, was attributed to the rupture of the network structure after the eighth swelling step during multiple swelling of the PAAm gel. (C) 1998 Elsevier Science Ltd. All rights reserved.
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    Measuring backbone activation energy at polymer-polymer interfaces during latex film formation by using a photon transmission method
    (Pergamon-Elsevier Science Ltd, 1998) Pekcan, O; Kemeroglu, F; Arda, E
    A photon transmission method was used to probe the evolution of optical clarity during film formation from hard latex particles. The latex films were prepared from poly (methyl methacrylate) (PMMA) particles, annealed at elevated temperatures in various time intervals above glass transition (T-g). Scanning electron microscopy (SEM) was used to detect the variation in physical structure of annealed films. In order to mimic the latex film formation process, Monte Carlo simulations were performed for photon transmission through a rectangular lattice. The number of transmitted (N-tr) and scattered (N-sc) photons are calculated as a function of disappeared particle-particle interfaces. The increase in the transmitted photon intensity (I-tr) is attributed to the increase in crossing density at the junction surface. The backbone activation energy (Delta E) was measured and found to be around 33.4 kcal mol(-1) for a partially relaxing polymer chain across the junction surface. (C) 1998 Elsevier Science Ltd. All rights reserved.
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    Molecular weight dependent changes with time during latex film formation: a photon transmission study
    (John Wiley & Sons Ltd, 1998) Pekcan, O; Arda, E
    A photon transmission method was used to probe the change with time of transparency during film formation from latex particles. Two different latex films were prepared from high (HM) and low (LM) molecular weight poly(methyl methacrylate) (PMMA) particles, which were annealed at 10 min time intervals at various temperatures above the glass transition T-g. The increase in the transmitted photon intensity I-tr is attributed to the increase in 'crossing density' at the junction surface. The back and forth activation energies (AE) were measured for HM and LM films and found to be around 29 kcal mol(-1) and 53 kcal mol(-1), respectively, for a reptating polymer chain across the junction surface. Monte Carlo simulations were performed for photon transmission through a rectangular lattice. The number of transmitted photons N-tr was calculated as a function of the mean free path of the photons. It was observed that N-tr increased similarly to I-tr as the square of the mean free path of the photons increased. (C) 1998 Society of Chemical Industry.
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    Photon transmission technique for studying film formation from polystyrene latexes prepared by dispersion polymerization using various steric stabilizers
    (Wiley-Blackwell, 1998) Pekcan, O; Arda, E; Kesenci, K; Piskin, E
    A photon-transmission method was used to monitor the evolution of transparency during film formation from various polystyrene (PS) particles which were produced using different steric stabilizers, that is, poly(acrylic acid) (PAA), poly(vinyl alcohol) (PVA), and polyvinylpyrrolidone (PVP). The latex films were prepared from PS particles at room temperature and annealed at elevated temperatures in various time intervals above the glass transition (T-g). To simulate the latex film-formation process, a Monte Carlo technique was performed for photon transmission through a rectangular lattice. The number of transmitted (N-tr) photons were calculated as a function of particle-particle interfaces that disappeared. The increase in the transmitted photon intensity (I-tr) was attributed to the increase in the number of interfaces that disappeared. The Prager-Tirrell (PT) model was employed to interpret the increase in crossing density at the junction surface. The backbone activation energy (Delta E) was measured and found to be around 120 kcal mol(-1) for a diffusing polymer chain across the junction surface for all PS latex films. (C) 1998 John Wiley & Sons, Inc.
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    Real time monitoring of temperature variation and optical transmission during gelation of acrylamide (AAm) at various crosslinker contents
    (Taylor & Francis Ltd, 2004) Kara, S; Pekcan, O
    Free radical crosslinking copolymerization (FCC) of acrylamide (AAm) with various amounts of N,N'-methylenebis(acrylamide) (Bis) is performed at different onset temperatures (T,,,). Variation of temperature, T, and optical transmission, I-tr, are monitored in real time during the gelation process. The decrease in I-tr is modeled by the monomer consumption rate, and the gelation activation energies, DeltaE(G), are measured at each Bis content. During FCC, temperature increases to a certain level then decreases to its initial value as the gelation time increases, which is explained by the heat released during the polymerization process. Decrease in I-tr is attributed to the increase' in scattered light intensity, I-sc, during the formation of interconnected microgels in the system. It is observed that the number of microgels increases as the onset temperature, T-on, is increased. At a given temperature, high Bis content samples produce larger size of microgel particles compared to low Bis content samples.
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    Real-time temperature and photon transmission measurements for monitoring phase separation during the formation of poly(N-isopropylacrylamide) gels
    (John Wiley & Sons Inc, 2002) Kara, S; Okay, O; Pekcan, O
    Phase separation during the formation of poly(N-isopropylacrylamide) (PNIPA) hydrogels was investigated using real-time photon transmission and temperature measurements. The hydrogels were prepared by free-radical crosslinking polymerization of N-isopropylacrylamide (NIPA) in the presence of N,N'-methylenebisacrylamide (BAAm) as a crosslinker in an aqueous solution. The onset reaction temperature T-o was varied between 20 and 28degreesC. Following an induction period, all the gelation experiments resulted in exothermic reaction profiles. A temperature increase of 6.5 +/- 0.6degreesC was observed in the experiments. It was shown that the temperature increase during the formation and growth process of PNIPA gels is accompanied by a simultaneous decrease in the transmitted light intensities I-tr. The decrease in I-tr at temperatures below the lower critical solution temperature of PNIPA was explained by the concentration fluctuations due to the inhomogeneity in the gel network. At higher temperatures, it was shown that the gel system undergoes a phase transition via a spinodal decomposition process. (C) 2002 Wiley Periodicals, Inc.