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Cation effect on thermal transition of ?-carrageenan
(Vsp Bv, 2005) Pekcan, Ö; Kara, S
Coil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of t-carra-geenan in various cation (CaCl2) solutions upon heating and cooling were studied using the photon transmission technique. Photon transmission intensity, I-tr, 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 observed during cooling at low temperature region. However, upon heating dimers directly decompose into double helices by making dimer-to-double helix (d-h) transition. Further heating results in double helix-to-coil (h-c) transition at high temperature region. T-dh and T-hc temperatures and Delta E-dh and Delta E-hc activation energies were determined. It was observed that T-hc and T-ch temperatures and Delta E-ch and Delta E-hc activation energies were found to be strongly correlated to the CaCl2 content in the system.
Cation effects on sol-gel and gel-sol phase transitions of ?-carrageenan-water system
(Elsevier Science Bv, 2003) Kara, S; Tamerler, C; Bermek, H; Pekcan, Ö
Sol-gel and gel-sol phase transitions Of K-carrageenan in pure water and in KCl solution were studied using photon transmission technique. Photon transmission intensity, I-tr was monitored against temperature to determine the sol-gel and gel-sol temperatures (T-sg and T-gs) and activation energies (DeltaH(sg) and DeltaH(gs)). It was observed that T-gs was notably higher than T-sg due to the hysteresis on the phase transition loops. T-gs and DeltaH(gs) values were also higher for gels containing KCl than for those without KCl. The increase in carrageenan content caused an increase in both critical temperatures and activation energies for the gels prepared in pure water and in KCl solution. Increases in the KCl/carrageenan ratio, raised both T-gs and T-sg. Similarly DeltaH(sg) was elevated by the increase in cation content of the gel. These results were interpreted as the formation of stronger gels in the presence of KCl in water. (C) 2002 Elsevier Science B.V. All rights reserved.
Cation effects on swelling of k-carrageenan
(John Wiley & Sons Inc, 2003) Kara, S; Tamerler, C; Pekcan, Ö
Swelling behavior of kappa-carrageenan gels in water and KCl solutions was investigated by photon transmission experiments following the preparation of gels in the presence and absence of externally added K+ ion as a gel promoting agent. Transmitted photon intensity, I-tr, increased continuously during swelling depending on the carrageenan and ion content in the gel. This increase in I-tr was modeled using the Li-Tanaka equation. Both the experimental work and the model showed that the swelling of low carrageenan and ion content gels took less time than that of high ion content gels. It is confirmed that double helices in a swollen gel move much faster in pure water than in KCl solution during swelling processes. Swelling time constants, tau(I), and collective diffusion coefficients, D-o, were measured for the gels swollen in water and KCl solutions. (C) 2003 Wiley Periodicals, Inc.
Drying of heterogeneous hydrogels formed with various water contents
(John Wiley & Sons Inc, 2001) Kara, S; Pekcan, Ö
Drying of acrylamide (AAm) gels was monitored by using a photon transmission technique. These hydrogels are formed from AAm and N,N'-methylenebis(acrylamide) (Bis) with various water contents by the free-radical crosslinking copolymerization (FCC). The transmitted light intensity, I-tr, increased continuously as AAm gels were dried. The increase in I-tr was attributed to the decrease in the scattered light intensity, I-sc, which might be originated from the contrast between frozen blob clusters and holes in the drying gel. A decrease in I-sc was modeled by using Rayleigh's equation where drying times with various exponents were found to be proportional to the size of the holes. It was observed that the radius of holes, xi (c), decreased in various powers of the drying time depending on water content used during FCC. (C) 2001 John Wiley & Sons, Inc.
Effect of molecular weight on packing during latex film formation
(Academic Press Inc Elsevier Science, 2001) Arda, E; Pekcan, Ö
A UV-visible technique is used to study the evolution of transparency during film formation from latex particles. Latex particles with high and low molecular weight (HM and LM) polymethyl methacrylate (PMMA) are used to prepare films. Two sets of films with different latex content were prepared from HM and LM particles separately, by annealing PMMA particles above the glass transition temperature. Transmitted photon intensity, I,,, from HM and LM films increased as the annealing temperature was increased. The increase in the transmitted photon intensity is attributed to the latex content (film thickness) for the annealed film samples. It is suggested that as the latex particles are packed (film thickness is increased) fewer voids or cracks are formed in the films, Positive and negative absorption coefficients are measured below and above 210 and 180 degreesC annealing temperatures for the HM and LM films. Packing coefficients are obtained for films in various latex contents. It is observed that LM particles are packed much easier than HM particles, (C) 2001 Academic Press.
Effects of aging on latex film formation from polystyrene particles
(John Wiley & Sons Inc, 2001) Pekcan, Ö; Arda, E; Kesenci, K; Piskin, E
Aging due to the storage time on latex film formation was studied using the photon transmission method. The UV visible technique was used to monitor the evolution of transparency during film formation from two different polystyrene (PS) particles produced by using two different steric stabilizers, i.e., polyvinyl alcohol and polyvinyl pyrrolidone. The latex films were prepared from PS particles at room temperature before and after aging and annealed at elevated temperatures in various time intervals above glass transition (T-g). The increase in the transmitted photon intensity I-tr was attributed to the increase in the number of disappeared particle-particle interfaces. Relative decrease in transparency and delay in film formation were observed in the aged latex films compared to the nonaged ones. The Prager-Tirrell model was employed to interpret the increase in crossing density at the junction surface. The back and forth activation energies (DeltaE) were measured and found to be dependent on aging for a diffusing polymer chain across the junction surface. (C) 2001 John Wiley & Sons, Inc.
Film formation from nanosized copolymeric latex particles
(Academic Press Inc Elsevier Science, 2001) Arda, E; Özer, F; Piskin, E; Pekcan, Ö
The photon transmission technique was used to monitor the evolution of transparency during film formation from nano-sized copolymeric latex particles. The latex films were prepared from poly(methyl methacrylate-co-butyl methacrylate) (P(MMAco-BMA)) particles which were produced by microemulsion polymerization, These films were annealed at elevated temperatures in various time intervals above the glass transition temperature (T-g) of P(MMA-co-BMA). It is observed that the transmitted photon intensity (I,) from these films increased as the annealing temperature increased. There are three different film formation stages, These stages are explained by the void closure, healing, and interdiffusion processes, respectively. The activation energies for viscous flow (DeltaH approximate to 16 kcal/mol), minor chains (DeltaE(H) approximate to 27 kcal/mol), and backbone motion (DeltaE(b) approximate to 132 kcal/mol) were obtained using various models. Void closure (tau (v), T (v)) and healing points (tau (H), T (H)) were determined. Using the time-temperature pairs, void closure and healing activation energies were measured and found to be 21 and 30 kcal/mol, respectively. (C) 2001 Academic Press.
Fluorescence and UV techniques for studying neck growth and equilibration processes during latex film formation
(Wiley-Blackwell, 1998) Pekcan, Ö; Canpolat, M; Arda, E
Steady state fluorescence (SSF) and UV-visible techniques have been used to study neck growth and equilibration processes during the coalescence of hard latex particles. Latex films were prepared separately by annealing pyrene (P-y) labelled and unlabelled poly(methyl methacrylate) (PMMA) particles above their glass transition temperature. During the annealing processes, the optical clarity of the films increased considerably. Direct fluorescence emission of excited pyrene from labelled latex films was monitored as a function of annealing temperature to detect this change. Void closure temperature (T-c) and time (t(c)) were determined at the point where the fluorescence emission intensity became maximal. Below this point, the increase in fluorescence intensity (I-op) against temperature was used to determine the activation energy for viscous flow (Delta H approximate to 47 kcal mol(-1)). The decrease in I-op above the void closure temperature was used to determine the backbone activation energy (BE approximate to 44 kcal mol(-1)) for the interdiffusing chains. Unlabelled PMMA particles were used to prepare films for UV-vis measurements. The transmitted photon intensity (I-tr) from these films increased as the annealing temperature was increased. This behaviour was also used to determine the backbone activation energy (Delta E approximate to 35 kcal mol(-1)) for the interdiffusing chains. (C) 1998 Society of Chemical Industry..
Fluorescence methods for latex film formation
(Amer Chemical Soc, 2005) Pekcan, Ö; Arda, E
[Abstract Not Available]
Hysteresis during sol-gel and gel-sol phase transitions of ?-carrageenan
(Sage Publications Ltd, 2003) Kara, S; Tamerler, C; Bermek, H; Pekcan, Ö
Photon transmission techniques were used to study sol-gel and gel-sol transitions in kappa-carrageenan-water systems with various carrageenan contents. The photon transmission intensity, I-tr was monitored against temperature to determine critical phase transition temperatures and transition rates. It was observed that the sol-gel transition temperatures, T-sg, were found much lower than the gel-sol, T-gs, transition temperatures based on the hysteresis of the phase transition loops. Sol-gel and gel-sol activation energies were determined and it was found that a carrageenan-water system required less energy for the gel-sol transition than for the sol-gel transition.
In situ monitoring of energy release in real time during gelation of acrylamide (AAm)
(Elsevier Sci Ltd, 2001) Pekcan, Ö; Kara, S
Temperature variation and optical transmission was monitored in real time during the free radical crosslinking copolymerization (FCC) of acrylamide (AAm) and N,N-1-methylenebis(acrylamide) (Bis). Gelation experiments were initiated at various onset temperatures (T-on). During gelation the temperature rose then decreased to its initial value as the gelation time increased. That is explained with the energy release (Gelation activation energy) during the polymerization process. Gelation activation energies, DeltaE(G) were measured and found to be increased as the onset temperature, T-on was increased. It was observed that transmitted photon intensities, I-sc, decreased dramatically at the reaction time, which were attributed to the increase in scattered light intensity, I-sc during the formation of interconnected microgels in the system. Increase in I-sc was modeled using Rayleigh's equation where reaction time was found to be proportional to the volume of the microgels. It was observed that a number of microgels increased as the onset temperature, T-on was increased. (C) 2001 Elsevier Science Ltd. All rights reserved.
In situ photon transmission technique for monitoring formation of hydrogels in real-time at various water contents
(Elsevier Sci Ltd, 2000) Kara, S; Pekcan, Ö
In situ photon transmission technique was used to monitor the free radical crosslinking copolymerization (FCC) of acrylamide (AAm) and N,N'-methylenebis (acrylamide) (Bis) in real-time. Gelation experiments were performed in various water contents at various wavelengths. It was observed that transmitted photon intensity, I-tr, decreased dramatically at a certain gelation time, which is attributed to the increase in scattered light intensity, I-sc, during FCC. Increase in I-sc was modeled using Rayleigh's equation where gelation time was found to be proportional to the volume of the microgels. The change in I-sc is found to be inversely proportional to the thirth power of the wavelength, lambda, fur high water content samples at early times. However, low water content samples produce smaller and constant power of lambda during FCC at all rimes. The sizes of microgel particles and phase-separated domains were estimated. (C) 2000 Elsevier Science Ltd. All rights reserved.
In situ photon transmission technique for monitoring phase separation in real time during gelation
(Springer-Verlag, 2000) Kara, S; Okay, O; Pekcan, Ö
In situ, real time photon transmission technique was used to monitor the formation of heterogeneities during the free-radical crosslinking polymerization of acrylamide in the presence of N,N'-methylenebis(acrylamide) (BAAm) as a crosslinker. Two different gelation experiments were performed with various BAAm and water contents separately at room temperature. It was observed that the transmitted photon intensity, I-tr decreases dramatically at a certain time, in both type of experiments during the gel formation process from interconnected microgels. Decrease in I-tr was interpreted in terms of phase separation during gelation. The extent of phase separation was measured depending on the crosslinker content and on the degree of monomer dilution during the gel formation process.
In situ real-time photon transmission technique for monitoring formation of polyacrylamide gels at various crosslinker contents
(John Wiley & Sons Inc, 2001) Kara, S; Pekcan, Ö
The in situ, real-time, photon transmission technique was used to monitor the free-radical crosslinking copolymerization (FCC) of acrylamide (AAm) and N,N'-methylenebisacrylamide (Bis). Gelation experiments were performed with various Bis contents at various wavelengths. It was observed that the transmitted photon intensity, I-sc, decreased dramatically at a certain reaction time, which is attributed to the increase in scattered light intensity, I-sc, during the formation of microgels in the system. The increase in I-sc was modeled using Rayleigh's equation where the reaction time was found to be proportional to the volume of the microgels. In most cases, the change in I-sc was found to be inversely proportional to the 2.5 power of the wavelength, lambda, from which the size of the microgel particles were estimated during the FCC. (C) 2001 John Wiley & Sons, Inc.
Latex film formation induced by solvent vapor
(Taylor & Francis Ltd, 2001) Pekcan, Ö; Arda, E
A photon transmission method was used to study latex film formation induced by organic solvent vapor. Various films with the same latex content were prepared separately from the poly(methyl methacrylate) (PMMA) particles and exposed to vapor of chloroform-heptane mixture in various percentage volumes of chloroform. Transmitted photon intensities, I-tr from these films increased in time under vapor exposure. The increase in I-tr is attributed to the increase in 'crossing density' at the junction surface. The Prager-Tirrell model was employed to obtained back-and-forth frequency, nu of the reptating polymer chain during film formation induced by solvent vapor. nu values were obtained and found to be strongly correlated with the percent of chloroform in the solvent mixture. At high and low chloroform contents, polymer chains diffuse according to t(1/2) and t(1/4) laws respectively.
Latex film formation study by using photon reflection method
(Wiley-V C H Verlag Gmbh, 2000) Pekcan, Ö; Arda, E
Photon reflection method was used to monitor the evolution of transparency during film formation from latex particles. Latex films were prepared from Poly (methyl methacrylate) (PMMA) particles, which were annealed at elevated temperatures in various time intervals during which reflected photon intensity, I-rf was measured. I-rf first decreased and then increased as the annealing temperature is increased. Decrease in I-rf was explained with the void closure mechanism. Increase in I-rf against temperature was attributed to increase in crossing density at the junction surface. The void closure constant, B and the back and forth activation energy (Delta E-rf) were measured and found to be around 24 x 10(3) K and 49 kcal/mol respectively.
Lattice heterogeneities at various crosslinker contents -: a gel swelling study
(Elsevier Sci Ltd, 2000) Pekcan, Ö; Kara, S
Disk shaped poly acrylamide (PAAm) gels were prepared from acrylamide (AAm) with Various N,N'-methylenebis(acrylamide) (Bis) contents by free radical crosslinking copolymerization (FCC) in water and dried before use in swelling experiments. Photon transmission experiments for these gels were performed using an UV-visible (UW) spectrometer. Transmitted light intensity, I,from the gel increased at very early times when the PAAm gels are immersed in water, then decreased exponentially as the swelling time increased. Decrease in I,was attributed to the lattice heterogeneities which might be originated between frozen blob clusters and holes in the swelling gel. Decrease in I, was modeled using the Li-Tanaka equation from which cooperative diffusion coefficients, D-c were determined for various Bis content gels. It is observed that the D-c values increased as the Bis content increased. (C) 2000 Elsevier Science Ltd. All rights reserved.
Molecular recognition during sol-gel and gel-sol transition of kappa-iota carrageenan mixtures
(Taylor & Francis Ltd, 2005) Kara, S; Arda, E; Pekcan, Ö
Sol-gel and gel-sol phase transitions of kappa-iota (kappa-iota) carrageenan mixtures in two types of salts were studied using photon transmission technique. Here, KCl and CaCl2 were chosen as the proper salts for the carrageenan mixtures. Coil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of carrageenan mixtures in monovalent and bivalent cation solutions were studied upon heating and cooling by using photon transmission technique. Photon transmission intensity, It, was monitored against temperature to determine the (c-h) and (h-d) transition temperatures (T-ch and T-nd). Two distinct transition regions were observed in both heating and cooling of the carrageenan-bivatent CaCl2 salt system. At the first step of heating process dimers decompose into double helices by making (d-h) transition. At the high temperature region (h-c) transition takes place. Carrageenan-monovalent KCl salt system presented only (c-h) and (h-c) transitions upon cooling and heating processes. Transition temperatures at the heating and cooling processes were obtained. A hysteresis was observed between (h-c)-(c-h) and (d-h)-(h-d) transitions for the monovalent and bivalent cations respectively.
Molecular weight effect on latex film formation from polystyrene particles
(Academic Press Inc, 1999) Arda, E; Bulmus, V; Piskin, E; Pekcan, Ö
The photon transmission method was used to probe the time evolution of film formation from latex particles, Two different latex films were prepared from high molecular weight (HM) and low molecular weight (LM) polystyrene particles at room temperature and were annealed at various temperatures in 2.5-min time intervals above the glass transition. The increase in the transmitted photon intensity (I-tr) is attributed to the increase in crossing density at the junction surface. The Prager-Tirrell model was employed to interpret the increase in crossing density at the junction surface. The back and forth activation energies were measured for HM and LM films and found to be around 59 and 87 kcal/mol 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 photons. It was observed that N-tr, similar to I-tr, increases as the square of the mean free path of photons is increased. (C) 1999 Academic Press.
Molecular weight effect on latex film formation induced by solvent vapor
(Academic Press Inc Elsevier Science, 2002) Akkök, B; Pekcan, Ö; Arda, E
Two different sets of powder latex film were prepared from high-and low-molecular-weight (HM and LM) poly(methyl methacrylate) (PMMA). The optical transmission method was used to the study film formation induced by organic vapor. Various HM and LM films with the same latex content were prepared separately from the PMMA particles and exposed to vapor of a chloroform-heptane mixture in various volume percents of chloroform. Transmitted light intensities, I-tr, from these films increased in time under vapor exposure, which was attributed to the increase in crossing density at the junction surface. 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. v values were obtained and found to be strongly correlated with the percent of chloroform in the solvent mixture and the molecular weight of the polymer chain. It is observed that high-molecular-weight chains reptate much more slowerly than low-molecular-weight chains. (C) 2002 Elsevier Science.

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