Selim Kara

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.

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.

Photon transmission technique for monitoring free radical crosslinking copolymerization in various crosslinker contents

Abstract Photon transmission technique was used to monitor the free radical crosslinking copolymerization (FCC) of acrylamide (AAm) and N , N ′-methylenebis (acrylamide) (Bis) in time. FCC experiments were performed with various Bis contents. 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 macrogel formation from interconnected microgels. Increase in I sc was modeled using Rayleighs equation where gelation time was found to be proportional to the volume of the microgels.

Lattice heterogeneities at various crosslinker contents—a gel swelling study

Abstract 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 (UVV) spectrometer. Transmitted light intensity, I tr 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 tr was attributed to the lattice heterogeneities which might be originated between “frozen blob clusters” and holes in the swelling gel. Decrease in I tr 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.

Hysteresis During Sol-Gel and Gel-Sol Phase Transitions of k-Carrageenan: A Photon Transmission Study

Photon transmission techniques were used to study sol-gel and gel-sol transitions in k-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 photon transmission technique for monitoring formation of hydrogels in real-time at various water contents

Abstract 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 Rayleighs 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, λ , for high water content samples at early times. However, low water content samples produce smaller and constant power of λ during FCC at all times. The sizes of microgel particles and phase-separated domains were estimated.

In situ monitoring of energy release in real time during gelation of acrylamide (AAm)

Abstract Temperature variation and optical transmission was monitored in real time during the free radical crosslinking copolymerization (FCC) of acrylamide (AAm) and N , N ′-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, Δ E G were measured and found to be increased as the onset temperature, T on was increased. It was observed that transmitted photon intensities, I tr , 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 Rayleighs 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.

Cation effect on thermal transition of ι-carrageenan: a photon transmission study

Coil-to-double helix (c-h) and double helix-to-dimer (h-d) phase transitions of ι-carrageenan 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 (ΔE ch and ΔE 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 ΔE dh and ΔE hc activation energies were determined. It was observed that T hc and T ch temperatures and ΔE ch and ΔE hc activation energies were found to be strongly correlated to the CaCl2 content in the system.

Photon transmission technique for monitoring swelling of acrylamide gels formed with various crosslinker contents

Abstract Photon transmission experiments were performed using a UV–visible (UVV) spectrometer during the swelling of polyacrylamide (PAAm) gels. These gels were prepared from acrylamide (AAm) with various N , N ′-methylenebisacrylamide (Bis) contents by free-radical crosslinking copolymerization (FCC) in water and dried before use for swelling experiments. Transmitted light intensity, I tr increased at very early times when PAAm gels are immersed in water, then decreased continuously as swelling time is increased. Decrease in I tr was attributed to the increase in the scattered light intensity, I sc which may originate from the contrast between ‘frozen blob clusters’ and holes in the swelling gel. Decrease in I tr was modelled using the Li–Tanaka equation from which time constants, τ 1 , and collective diffusion coefficients, D o were determined for various Bis content PAAm gels. τ 1 and D o were found to be strongly correlated with the wavelength, λ , of I tr and the Bis content of the gel samples. The correlation between λ and τ 1 and D o predicts the existence of ‘frozen blob clusters’ in PAAm gels.

Critical Exponents of Kappa Carrageenan in the Coil-Helix and Helix-Coil Hysteresis Loops

The steady-state fluorescence technique was used to study coil-helix (sol-gel) and helix-coil (gel-sol) transitions of the kappa carrageenan-water system with various carrageenan contents. Fluorescence (I) and scattered light (Isc ) intensities were measured against temperature to determine critical phase transition temperatures and exponents. It was observed that the coil-helix transition temperatures, Tch were much lower than the helix-coil (Thc ) transition temperatures due to the hysteresis of the phase transition loops. The gel fraction exponent (β) was measured and found to be in accord with the classical Flory-Stockmayer model.