Gülşen Akın Evingür

Universal behaviour of gel formation from acrylamide-carrageenan mixture around the gel point: a fluorescence study.

Abstract The 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-trisulfonic 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 β obeyed the percolation result for low carrageenan concentrations (<2.0%) however classical results were produced at higher carrageenan concentration (>2.0%).

Critical Exponents of Gelation and Conductivity in Polyacrylamide Gels Doped by Multiwalled Carbon Nanotubes

Polyacrylamide (PAM) doped by multiwalled carbon nanotube (MWNT) gels were prepared with different amounts of MWNTs varying in the range between 0.1 and 15 wt%. The PAM–MWNT composite gels were characterized by the steady state fluorescence technique (SSF). The alternative electrical conductivity (AC) of PAM–MWNT composite gels was measured by the dielectric spectroscopy technique. Observations around the gel point, t gel for PAM–MWNTs composite gels showed that the gel fraction exponent β obeyed the percolation result. The critical exponent r of AC electrical conductivity for the composite PAM–MWNT gel was also measured and found to be about 2.0, which agrees with a random resistor network.

Steady state fluorescence technique for studying phase transitions in PAAm–PNIPA mixture

The steady state fluorescence (SSF) technique was used to study the sol-gel phase transition during free radical crosslinking copolymerisation of various amounts of acrylamide (AAm) and N-isopropylacrylamide (NIPA) mixtures. N,N′-methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and an initiator, respectively. Pyranine (8-hydroxypyrene-1, 3, 6-trisulfonic acid, trisodium salt, HPTS) was added as a fluoroprobe for monitoring the polymerisation. It was observed that pyranine molecules bind to AAm and NIPA chains upon the initiation of the polymerisation, thus the fluorescence spectra of the bonded pyranines shift to the shorter wavelengths. Fluorescence spectra from the bonded pyranines allowed us to monitor the sol-gel phase transition, without disturbing the system mechanically, and to test the universality of the sol-gel transition as a function of polymer concentration ratios. Observations around the gel point of PAAm–PNIPA mixtures show that the gel fraction exponent β obeyed the percolation result.

Drying of PAAm Hydrogels at Various Temperatures: A Fluorescence Study

The steady‐state fluorescence (SSF) technique was employed for studying the drying of polyacrylamide (PAAm) hydrogels. Disc‐shaped hydrogels were prepared by free‐radical crosslinking copolymerization of acrylamide (AAm) with N,N′‐methylene bisacrylamide (BIS) as crosslinker in the presence of ammonium persulfate (APS) as an initiator. Pyranine (P) was introduced as a fluorescence probe and the intensity of pyranine was monitored during in situ drying at various temperatures. It was observed that the fluorescence intensity of pyranine increased during the drying process. A supporting, gravimetrical experiment was also performed. A phenomenological equation was introduced to determine the desorption coefficient, D, of water molecules from the drying hydrogels at various temperatures. The desorption activation energy, ΔE d, values were measured for the drying processes and found to be 91.08 and 36.82 kJ mol–1 by fluorescence and gravimetrical methods, respectively. This difference most probably originates from the origin of the techniques; the fluorescence technique measures the parameters at a molecular level, whereas the gravimetrical technique measures a parameter in the bulk.

Drying of Polyacrylamide Composite Gels Formed with Various Kappa- Carrageenan Content

Drying of polyacrylamide (PAAm)-κ-carrageenan (κC) composite gels were monitored by using steady-state fluorescence technique. Disc shaped gels were formed from acrylamide (AAm) and N, N′- methylenebisacrylamide(Bis) with various κ- carrageenan (κC) contents by free radical crosslinking copolymerization in water. Pyranine (P) was doped as a fluorescence probe, and scattered light, Isc, and fluorescence intensities, I, were monitored during drying of these gels. It is observed that fluorescence intensity of pyranine increased as drying time is increased for all samples. The increase in I was modeled using Stern- Volmer equation and diffusion with moving boundary. It is found that desorption coefficient, D decreased as κC contents were increased. Supporting gravimetrical and volumetric experiments were also carried out during drying of PAAm- κC composite gels.

Effect of LCST on the swelling of PAAm-NIPA copolymers: a fluorescence study

Temperature sensitive copolymers were prepared by free radical crosslinking copolymerization in aqueous solution with different molar percentage of N-isopropylacrylamide (NIPA) and acrylamide (AAm) monomers. N,N′-methylenebis (acrylamide) (BIS) and ammonium persulfate (APS) were used as a crosslinker and an initiator, respectively. The steady-state fluorescence (SSF) technique was used to determine the low critical phase transition temperature (LCST) for PAAm-NIPA copolymers. Swelling experiments were performed in water at various temperatures by real time monitoring of pyranine (Py) fluorescence intensity, I which decreased as swelling proceeded. The Stern–Volmer equation is modified for low quenching efficiencies to interpret the behavior of pyranine intensity during the swelling of PAAm-NIPA copolymers. The Li–Tanaka equation was used to determine the swelling time constants, τ1 and the cooperative diffusion coefficients, D0 from fluorescence intensity, weight and volume variations of the copolymers at various temperatures. It was observed that τ1 first increased up to LCST, and then decreased; naturally D0 decreased up to LCST and then increased upon increasing temperature. It was understood that (LCST) increases as PAAm contents increase in the PAAm-NIPA copolymers.

Energy needs for drying of PAAm- κC composites prepared with various κC contents

Abstract Various polyacrylamide (PAAm) - Kappa Carrageenan (κC) composite gels were prepared from acrylamide (AAm), N,N- methylenebisacrylamide (BIS) and different κC contents by free radical crosslinking copolymerization in water. Pyranine (P) was introduced as a fluorescence probe, and scattered light, Isc, and fluorescence intensities, I, from these gels were monitored during drying at different temperature. The fluorescence intensity of pyranine increased as drying time is increased for all samples. The behavior of I was quantified using the Stern-Volmer equation with moving boundary diffusion model. Desorption coefficient, D increased as temperature was increased for a given κC content. Supporting gravimetrical and volumetric experiments were also carried out during drying of PAAm-κC composite gels. The energy, △E values were measured for the drying processes for each κC content gel by using fluorescence, gravimetrical and volumetric methods, respectively. It is understood that △E values increase by increasing κC content, indicating that energy needs for the drying of high κC content gel is larger than low κC content gel.

Swelling Kinetics of PAAm–κ-Carrageenan Composites: A Fluorescence Technique

The steady-state fluorescence (SSF) technique was introduced for studying swelling of disc-shaped polyacrylamide (PAAm) gels containing various amount of κ−carrageenan (κC). They were prepared by free-radical cross-linking copolymerization. N,N′-methylenebisacrylamide (BIS) and ammonium persulfate (APS) were added as a cross-linker and an initiator, respectively. Composite gels were prepared at 80°C with pyranine as a fluorescence probe. After drying of these gels, swelling kinetics were performed in water at 60°C by real-time monitoring of the pyranine fluorescence intensity, I, which decreased as swelling proceeded. The Li–Tanaka equation was used to determine the swelling time constants, τ 1, and cooperative diffusion coefficients, D 0, from fluorescence intensity, weight, and volume variations of the gels during the swelling processes in all cases. It was observed that τ 1 decreased and D 0 increased as the κC concentrations in the composites were increased indicating that high κC gels swell faster than low κC gels.

Studying On The Small Molecule Diffusion Into Hydrogels: A Fluorescence Study

In situ steady state fluorescence experiments (SSF) were reported during the diffusion of water molecules into PAAm gels at various temperatures. The fluorescence emission intensities, Ip increased continuously as water molecules diffused into gels. Isc, scattered light intensities, from the gel was also monitored to detect the variation in the gel structure during diffusion process. Swelling and diffusion experiments were quantified by Li‐Tanaka and Fickian models.