Önder Pekcan

Cation effects on sol–gel and gel–sol phase transitions of κ-carrageenan–water system

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, Itr, was monitored against temperature to determine the sol � /gel and gel � /sol temperatures (Tsg and Tgs) and activation energies (DHsg and DHgs). It was observed that Tgs was notably higher than Tsg due to the hysteresis on the phase transition loops. Tgs and DHgs 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 Tgs and Tsg. Similarly DHsg 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. # 2002 Elsevier Science B.V. All rights reserved.

Free-radical crosslinking copolymerization of styrene and divinylbenzene: real time monitoring of the gel effect using fluorescence probe

The steady-state fluorescence and dilatometric techniques were used to study the free-radical crosslinking copolymerization of styrene (S) and commercial divinylbenzene (DVB) in bulk. Pyrene (Py) was used as a fluorescence probe for the in situ polymerization experiments. The time required for a sudden increase in the fluorescence intensity of Py was recorded for various DVB content and temperature. The monomer conversions and the gel points were recorded by dilatometry. To interpret the experimental data, a kinetic model was developed for the S-DVB copolymerization system. It was shown that both the pendant vinyl groups and Py molecules are affected by the reaction medium in a similar way, their mobility decreases as the DVB concentration increases or the temperature decreases. The reaction time at which the Py intensity in the fluorescence spectra exhibits a sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum resulting from the gel effect. The results show that the fluorescence technique can be used to follow the onset of the gel effect in S-DVB copolymerization.

Healing and interdiffusion processes at particle-particle junction during film formation from high-T latex particles

Abstract When amorphous polymers of same type are brought into contact at temperatures above the glass transition, the interface gradually heals and junction boundaries eventually disappear. Here we used a steady-state fluorescence technique to examine the healing and interdiffusion of polymer molecules during annealing of latex film above the glass transition. The film was prepared from a mixture of naphthalene-(donor) and pyrene-(acceptor) labelled poly(methyl methacrylate) latex particles. We report that during the annealing processes the transparency of the film changed considerably. Healing temperature (TH) and time (τH) were measured when the latex film became almost transparent. Above TH interdiffusion of polymer chains was observed by detecting the energy transfer from excited naphthalene to pyrene molecules. Healing and diffusion activation energies were calculated and found to be 9.84 kcal mol−1 and 30 kcal mol−1, respectively, for the annealed latex film samples.

Real time monitoring of polymerization rate of methyl methacrylate using fluorescence probe

The steady-state fluorescence technique was used to study the polymerization rate and the autoacceleration due to the gel effect in free-radical polymerization of methyl methacrylate. Pyrene was used as a fluorescence probe for the in situ polymerization experiments. The times required for the onset of the gel effect were recorded for various polymerization temperatures. A simple kinetic model was used to validate the experimental data. The results show that the fluorescence technique can be used to follow the onset of the gel effect and the activation energy during the polymerization processes.

Fluorescence technique for studying the sol-gel transition in the free-radical crosslinking copolymerization of methyl methacrylate and ethylene glycol dimethacrylate

Abstract The steady-state fluorescence technique was used to study the free-radical crosslinking copolymerization of methyl methacry- late and ethylene glycol dimethacrylate in the absence and presence of toluene at 75°C. A sol-gel phase transition was observed and monitored by an excited aromatic molecule during the gelation of the above system. Bond percolation theory was employed to quantify the results. The critical exponent and gel point were found to be around 0.45 and 0.36 both in bulk and solution polymerization.

Electrical and optical percolations of polystyrene latex-multiwalled carbon nanotube composites.

Electrical conductivity and optical transmittance properties of polystyrene (PS)-multiwalled carbon nanotube (MWCNT) composite films were investigated. Composite films were prepared by mixing of various mass fractions of MWCNT in PS-water dispersions. After water evaporates, powder composite films were annealed at 175 degrees C above the glass transition of PS for 20 min. Photon transmission and two point probe resistivity techniques were employed to determine the variations of the optical and the electrical properties of composites. Transmitted light intensity, I(tr) and surface resistivity, R(s) were monitored as a function of MWCNT mass fraction (M). It was observed that, both the surface resistivity and the optical transparency were decreased by increasing the amount of MWCNT added to the polymeric system. Conductivity and optical results were interpreted according to the classical and site percolation theory, respectively. The electrical (sigma) and the optical (op) percolation threshold values and critical exponents were calculated as M(sigma)=1.8 wt.%, M(op)=0-0.13 wt.% and beta(sigma)=2.25, beta(op)=0.32, respectively.

In situ fluorescence experiments to test the reliability of random bond and site bond percolation models during sol-gel transition in free-radical crosslinking copolymerization

Sol-gel phase transition during the free-radical crosslinking copolymerization of methyl methacrylate and ethylene glycol dimethacrylate was studied using the steady-state fluorescence method. Copolymerization was performed both in the absence and presence of toluene at 75°C. Sol-gel phase transitions were monitored by observing the direct intensity of an excited pyrene methyl pivalate during in situ fluorescence experiments. Random bond and site bond percolation models were employed to quantify the results of bulk and solution polymerizations, respectively, during the sol-gel phase transition. Gel points were determined and the β exponents were found to be around 0.37 and 0.38 during gelation in bulk and solution polymerizations, respectively.

Photon diffusion study in films formed from high-T latex particles

Abstract The steady-state fluorescence technique was used to study the evolution of transparency during film formation from high-T latex particles. The latex films were prepared from pyrene-labelled poly(methyl methacrylate) particles and annealed in 10 min time intervals above the glass transition temperature (Tg). Scanning electron microscopy was used to detect the variation in physical structure of annealed films. Monte Carlo simulations were performed for photons diffusing the latex films and the number of emitted and scattered photons was calculated. The crossing density of polymer chains at the particle-particle interface was found to depend linearly on ( time ) 1 2 . The activation energy for back-and-forth motion of a reptating polymer chain was measured and found to be 29 kcal mol−1. The corresponding frequencies of a reptating chain were between 1.5 and 42 s−1 above Tg.

Healing and photon diffusion during sintering of high-T latex particles

A steady-state fluorescence technique was used to examine the annealing of films formed from high- T latex particles above the glass transition temperature. The films were prepared by sintering pyrene-labeled poly(methyl methacrylate) latex particles. During the annealing processes, the transparency of the film changed considerably. Direct fluorescence emission from excited pyrene was monitored as a function of annealing temperature to detect these changes. Scanning electron microscopy in conjunction with Monte Carlo simulations of photon diffusion in latex film were used to interpret the fluorescence results. Healing temperature and time were measured at the point where the fluorescence emission intensity becomes maximum. This was associated with the longest optical path of a photon in latex film during healing of particle-particle boundaries. Healing activation energy was measured and found to be 10 kcal/mol.

Effect of molecular weight on latex film formation : Photon transmission study

A UV-visible (UVV) 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 low molecular weight (HM and LM) poly(methyl methacrylate) (PMMA) separately and annealed at elevated temperatures in various time intervals above the glass transition temperature (Tg). In both films, a continuous increase in the transmitted photon intensity (Itr) was observed above 160°C as the annealing temperature was increased. However, the reflected photon intensity (Irf) 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 (Itr) is attributed to increase in the “crossing density” at the junction surface. The activation energies for back-and-forth motion (ΔEtr) 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 Irf was explained by the void-closure mechanism, and the increase in the Irf above 160°C was again attributed to the increase in the crossing density at the junction surface. Back-and-forth activation energies (ΔErf) were measured to be around 47 and 18 kcal/mol and the void-closure constants (B) were found to be around 24 × 103 and 12 × 103 K for the LM and HM film samples, respectively.