Browsing by Author "Evingür, Gülşen Akin"

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Citation Count: 16
Cation Effect on Slow Release from Alginate Beads: A Fluorescence Study
(Springer/Plenum Publishers, 2014) Kaygusuz, Hakan; Erim, F. Bedia; Pekcan, Önder; Evingür, Gülşen Akin
In this study spherical alginate beads containing pyranine (P-y) as a fluorescence probe were prepared by ionotropic gelation of a sodium alginate solution. The steady state fluorescence technique was used to study pyranine release from the alginate beads crosslinked with calcium barium and aluminum ions respectively. The slow release of P-y was observed with the time drive mode of the spectrophotometer at 512 nm. Fluorescence emission intensity (I-p) from P-y was monitored during the release process and the encapsulation efficiency (EE) of pyranine from the alginate beads was calculated. The Fickian Diffusion model was used to measure the release coefficients D-sl. It was seen that the slow release coefficients of pyranine from the alginate beads crosslinked with Ca2+ Ba2+ and Al3+ ions increased in the following order: D-sl (Al3+)> D-sl (Ca2+)> D-sl (Ba2+). In contrast the initial amount of pyranine and EE into the beads showed the reverse behavior.
Citation Count: 22
Critical Exponents of Gelation and Conductivity in Polyacrylamide Gels Doped by Multiwalled Carbon Nanotubes
(Taylor & Francis Ltd, 2010) Aktas, Demet Kaya; Evingür, Gülşen Akin; Pekcan, Önder
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 beta 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. (C) Koninklijke Brill NV Leiden 2010
Citation Count: 4
Drying of polyacrylamide-multiwalled carbon nanotube (MWNT) composites with various MWNTs contents: a fluorescence study
(Walter De Gruyter Gmbh, 2013) Evingür, Gülşen Akin; Pekcan, Önder
We studied the drying of polyacrylamide (PAAm)-multiwalled carbon nanotube (MWNT) composites prepared by free radical crosslinking copolymerization in water with a steady state fluorescence technique. Composite gels were prepared at room temperature with pyranine (Py) doped as a fluorescence probe. Drying experiments were performed in air at various MWNT contents by real time monitoring of the Py fluorescence intensity (I) which increased as the drying proceeded. The Stern-Volmer equation combined with the moving boundary diffusion model was used to explain the behavior of I during drying. It was observed that the desorption coefficient (D) increased as the temperature increased. Drying energies (Delta E) were measured for the drying processes for each MWNT content gel by using fluorescence gravimetrical and volumetric methods. It is understood that Delta E values decrease by increasing MWNT content until 1 wt % MWNT and then increase above the level of this threshold value. The energy of drying is strongly correlated with the MWNT content in the composite. Delta E drops to its lowest value at which conducting cluster starts to appear.
Citation Count: 6
Effect of Calcium Ion Concentration on Small Molecule Desorption from Alginate Beads
(Taylor & Francis Inc, 2014) Evingür, Gülşen Akin; Kaygusuz, Hakan; Erim, F. Bedia; Pekcan, Önder
Spherical alginate beads were prepared by ionotropic gelation of sodium alginate through the use of calcium ions. Pyranine (Py) was added to the alginate solution as a small molecule probe for fluorescence studies. Desorption of Py in water from the alginate beads cross-linked with calcium ions was studied by using the steady state fluorescence technique. The fluorescence emission intensity (I) from Py was monitored during the desorption process at 512 nm using the time drive mode of the spectrofluorometer. The increase in I was attributed to Py release from the beads. The Fickian diffusion model was used to calculate the desorption coefficients D which were found to be increased up to 3% (w/v) CaCl2 concentration in the beads and then decreased with a further increase of CaCl2 content. On the other hand the encapsulation efficiency of Py in the calcium alginate beads presented the reverse behavior compared to D. It was observed that when the content of CaCl2 was increased the incubation time t(0) for the start of desorption increased.
Citation Count: 5
Effect of LCST on the swelling of PAAm-NIPA copolymers: a fluorescence study
(Springer, 2012) Evingür, Gülşen Akin; Pekcan, Önder
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. NN'-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 tau (1) and the cooperative diffusion coefficients D (0) from fluorescence intensity weight and volume variations of the copolymers at various temperatures. It was observed that tau (1) first increased up to LCST and then decreased
Citation Count: 10
Effect of multiwalled carbon nanotube (MWNT) on the behavior of swelling of polyacrylamide-MWNT composites
(Sage Publications Ltd, 2014) Evingür, Gülşen Akin; Pekcan, Önder
The purpose of this paper is to discuss the role of multiwalled carbon nanotube in the swelling of polyacrylamide-multiwalled carbon nanotube composites. Swelling experiments were performed in water at various temperatures by real-time monitoring of the decrease in pyranine (Py) and emission light intensity (I-em). The Stern-Volmer equation is modified for low-quenching efficiencies to interpret the behavior of pyranine intensity during the swelling of polyacrylamide-multiwalled carbon nanotube composites. The Li-Tanaka equation was used to determine the swelling time constants tau and cooperative diffusion coefficients D from fluorescence intensity weight and volume variations of the composite at various temperatures. It was observed that when tau decreased naturally D increased by increasing temperatures.
Citation Count: 12
Elastic percolation of swollen polyacrylamide (PAAm)-multiwall carbon nanotubes composite
(Taylor & Francis Ltd, 2012) Evingür, Gülşen Akin; Pekcan, Önder
Polyacrylamide (PAAm)-multiwalled carbon nanotube (MWNT) composites were prepared via free radical cross-linking copolymerization with different amounts of MWNTs varying in the range between 0.1 and 50 wt%. The mechanical properties of swollen PAAm-MWNT composites were characterized by the tensile testing technique. A small content of embedded nanotubes dramatically changes the compressive elastic modulus of the composites. Compressive elastic modulus dramatically increases up to 1wt% MWNT on increasing nanotube content and then decreases presenting a critical MWNT value indicating that there is a sudden change in the material elasticity. The critical exponent y of elasticity below the critical MWNT content 1 wt% is found to be 0.58 which is consistent with the suggestions of percolation in the superelastic percolation network for PAAm-MWNT composite.
Citation Count: 6
Elastic Properties of a Swollen PAAm-NIPA Copolymer with Various NIPA Contents
(Taylor & Francis, 2014) Evingür, Gülşen Akin; Pekcan, Önder
Copolymer based on cross-linked polyacrylamide (PAAm) having N-isopropylacrylamide (NIPA) was prepared and their elastic properties were studied as a function of NIPA contents. NIPA content dependency of the shear modulus S of the PAAm-NIPA copolymers due to volume phase transition was measured using tensile testing technique at 30 degrees C. It was observed that its shear modulus and toughness were found to be strongly dependent on NIPA content. It is understood that the shear modulus was found to increase with NIPA contents keeping constant temperature at 30 degrees C. Elastic properties of the PAAm-NIPA copolymers show compositional dependence.
Citation Count: 3
Elasticity Study of PAAm-kappa C Composite Prepared in Various kappa C Content and Measured at Several Temperatures
(Polish Acad Sciences Inst Physics, 2015) Evingür, Gülşen Akin; Pekcan, Önder
Polyacrylamide (PAAm)-kappa carrageenan (kappa C) composite gels were prepared via free radical crosslinking copolymerization with various (w/v) percentages of kappa C in the range between 0.5 and 3 (w/v)% of kappa C . Elasticity properties such as stress strain and compressive elastic modulus S of these composite gels were studied in various kappa C content and at several temperatures. The content and temperature dependence of the compressive elastic modulus S of the swollen PAAm-kappa C composite gels due to volume phase transition were produced by using tensile testing technique. It is understood that the compressive elastic modulus was found to decrease up to 1 (w/v)% of kappa C and then increase by increasing kappa C contents at constant temperatures. The composite preserves the ability to undergo the volume phase transition and its compressive elastic modulus is found to be strongly dependent on the kappa C content and temperature. It is observed that the compressive elastic modulus increased when temperature is increased up to 40 degrees C and then decreases below this temperature for all composite gels. However PAAm-kappa C composite gel presented lower values for the compressive elastic modulus showing a minima at 40 degrees C for 1 (w/v)% of kappa C content gel.
Citation Count: 15
A Fluorescence Study on Swelling of Hydrogels (PAAm) at Various Cross-Linker Contents
(Wiley, 2009) Aktas, Demet Kaya; Evingür, Gülşen Akin; Pekcan, Önder
Disk-shaped acrylamide (AAm) gels were prepared from AAm with various NN'-methylenebisacrylamide (Bis) contents as cross-linker in the presence of ammonium persulfate as an initiator by free-radical cross-linking copolymerization in water. Polyacrylamide (PAAm) gels were dried before using for swelling experiments. Steady-state fluorescence spectrometer was employed during the swelling of PAAm hydrogels in water. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine from various Bis content gel samples was measured during in situ swelling process. It was observed that fluorescence intensity decreased as swelling has proceeded. Gravimetric and volumetric experiments were also performed. The Li-Tanaka equation was used to determine the swelling time constants tau(c) and cooperative diffusion coefficients D-c from intensity weight and volume variations during the swelling processes. It was observed that swelling time constants tau(c) increased and diffusion coefficients D-c decreased as the cross-linker content was increased. (C) 2010 Wiley Periodicals Inc. Adv Polym Techn 28:215-223 2009
Citation Count: 0
Fluorescence technique for monitoring swelling of PAAm-kappa-carrageenan gel composites
(Shaker Publishing, 2009) Evingür, Gülşen Akin; Aktaş, Demet Kaya; Pekcan, Önder
Steady-state fluorescence (SSF) technique was employed for studying swelling of polyacrylamide (PAAm) gels with various content of kappa- carrageenan. Disc shaped composite gels were prepared by free-radical cross/inking copolymerization of acrylamide (AAm) with various amounts of kappa-carrageenan (kappa C) using N N'- methylenebis (acrylamide) (BIS) as crosslinker in the presence of ammonium persulfate (APS) initiator. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine was monitored during in situ swelling processes of composite gels. It was observed that fluorescence intensity values decreased as swelling is proceeded. The Li-Tanaka equation was used to determine the swelling time constants tau and cooperative diffusion coefficients D from intensity variations during the swelling processes. It was shown that tau decreased and D increased as the kappa C content in the composites are increased.
Citation Count: 1
Gelation electrical conductivity and elasticity of PAM- MWNT
(Cambridge Univ Press, 2011) Evingür, Gülşen Akin; Pekcan, Önder
Polyacrylamide- Multiwalled carbonnanotube (PAM- MWNT) composites were prepared via free radical crosslinking copolymerization with different amounts of MWNT varying in the range between 0.1 and 15 wt. %. PAM- MWNT composite gels were characterized by fluorescence dielectric spectroscopy and the tensile testing technique. A small content of doped nanotubes dramatically changed gelation time conductivity and young modulus respectively. The gel fraction exponent ? of PAM- MWNT composite gels were measured for various monomer and MWNT concentrations and observed that the gel fraction exponent ? agrees best with the percolation theory for various amounts of PAM- MWNT. These polymer systems which are initially of an isolator character are doped with carbon nanotubes of nano dimensions and when the amount of this addition exceeds a critical value (0.3 wt. % MWNT) known as the percolation threshold then composite gel systems with carbon nanotubes become electrically conducting structures with a critical exponent around r=2 which is close to the theoretical prediction of this value in 3D percolated system as known random resistor network. The observed elasticities are decreased above 3 wt. %MWNT with critical exponent around y=0.72 which is indicative of a transition from liquid-like to solid-like viscoelastic behavior. © 2011 Materials Research Society.
Citation Count: 2
Gelation of PAAm-PVP composites: A fluorescence study
(World Scientific, 2014) Evingür, Gülşen Akin; Kaygusuz, Hakan; Erim, F. Bedia; Pekcan, Önder
Hybrid hydrogels are a new class of composite materials. Polyacrylamide (PAAm) hydrogels are mainly produced by free radical crosslinking copolymerization (FCC) of AAm in the presence of N N'-methylene bis (acrylamide) (BIS) as the crosslinker. Pyranine doped PAAm-poly (N-vinyl pyrrolidone) (PVP) composite were prepared with different amounts of PVP varying in the range between 0.0015 and 0.1 gr. It was observed that pyranine molecules as a fluoroprobe bind to AAm and PVP chains upon the initiation of the polymerization causing the fluorescence spectra of the bonded pyranines shift to the shorter wavelengths. The sol-gel phase transition and its universality were monitored and tested as a function of PVP contents. Observations around the critical point show that the gel fraction exponent beta agreed with the percolation result for below 0.025 gr PVP contents. However classical result was observed above 0.0125 gr PVP content.
Citation Count: 1
Investigation of Drying of Poly(N-isoproplacrylamide-co-acrylamide) by Fluorescence Technique
(Wiley-Blackwell, 2013) Evingür, Gülşen Akin; Pekcan, Önder
The steady-state fluorescence technique was performed on drying of various molar percentages of poly(N-isopropylacrylamide-co- acrylamide) [poly(NIPA-co-AAc)] to elucidate the mechanism of temperature-induced phase separation and the effect of monomer content. Poly(NIPA-co-AAc) copolymers were prepared by free radical cross-linking copolymerization. The fluorescence intensity I of pyranine introduced as a probe increased as drying time was increased for all samples. The behavior of I was modeled by using the SternVolmer equation combined with the moving boundary diffusion model. The desorption coefficient D increased as NIPA content was increased at a given temperature. Gravimetric and volumetric experiments also supported the results of the fluorescence technique. The energy E values were measured for the drying processes for each molar percentage of NIPA monomer content by using fluorescence gravimetric and volumetric methods respectively. It is understood that E values decrease by increasing NIPA content until 50% NIPA and then increase after 50% NIPA indicating that the phase separation has occurred at 50% NIPA. (c) 2012 Wiley Periodicals Inc. Adv Polym Techn 32: E231E240 2013
Citation Count: 14
Mechanical properties of graphene oxide-polyacrylamide composites before and after swelling in water
(Springer, 2018) Evingür, Gülşen Akin; Pekcan, Önder
Polyacrylamide (PAAm) hydrogels have been proposed for use as promising biomaterials in biomedical and tissue engineering but their poor mechanical and water-retention properties have hindered their development. Graphene oxide (GO) an excellent nanofiller was added to PAAm to produce mechanically strong GO-PAAm composites. The free radical crosslinking copolymerization method was used with GO content varying in the range between 5 and 50 A mu l of GO. The mechanical properties of the GO-PAAm composites are measured by compressive test revealing a decrease in the shear modulus and toughness of the composites above 8 A mu l of GO by increasing GO content.
Citation Count: 16
Monitoring the gelation of polyacrylamide-sodium alginate composite by fluorescence technique
(Taylor & Francis Ltd, 2012) Evingür, Gülşen Akin; Tezcan, Filiz; Erim, F. Bedia; Pekcan, Önder
Polyacrylamide (PAAm)-sodium alginate (SA) composite was prepared with different amounts of SA varying in the range between 0.06% and 2% (w/v). The PAAm-SA composite was characterized by the steady-state fluorescence technique. Pyranine was added as a fluoroprobe for monitoring the polymerization. It was observed that pyranine molecules bind to AAm and SA chains upon the initiation of the polymerization. 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 and to test the universality of the sol-gel transition as a function of SA contents. Observations around the critical point show that the gel fraction exponent beta and the weight average degree of polymerization exponent gamma agreed with the percolation result for (<0.25% (w/v)) SA contents. However classical results were produced at (<2% (w/v)) SA contents.
Citation Count: 37
Optical energy band gap of PAAm-GO composites
(Elsevier Science, 2018) Evingür, Gülşen Akin; Pekcan, Önder
Graphene oxide (GO) can be used as an electron acceptor for polymers. The generation of the exciton in such materials is intimately related to the optical band gap. Polyacrylamide (PAAm) - graphene oxide (GO) composites were prepared via free radical crosslinking copolymerization with GO content varying in the range between 5 and 40 mu l of GO. The influence of the GO content on the optical properties of the composites is characterized by ultraviolet visible spectrophotometer. The optical band gap of the composite has been calculated by using Tauc's model. The tail width of the localized states has been calculated by using Urbach's relation and the values of Urbach's energy with the variation of GO have been found from 289 to 296 eV. The results demonstrated that GO can be well dispersed in the PAAm-GO composites and consequently the optical band gap has been correlated with the various amounts of GO. (C) 2017 Elsevier Ltd. All rights reserved.
Citation Count: 0
Phase Transitions in Pure and Hybrid Hydrogels: A Fluorescence Study
(Wiley, 2009) Aktas, Demet Kaya; Evingür, Gülşen Akin; Pekcan, Önder
Recent observations on phase transitions around the critical point showed that the critical exponents differed drastically from percolation results and classical results for pure hydrogel systems depending on monomer concentration. in addition to pure hydrogels the sol-gel phase transition during radical crosslinking copolymerization of acrylamide (AAm) and N-isopropylacrylamide (NIPA) hybrid was studied by using the steady state fluorescence (SSF) technique. NN'-methylenebis(acrylamide) (BIS) and ammonium persulfate (APS) were used as crosslinker and initiator respectively. Pyranine (trisodium 8-hydroxypyrene-136-trisulfonate acid HPTS) was added as a fluoroprobe for monitoring the polymerization. It was observed that pyranine binds to AAm and NIPA chains on the initiation of the polymerization thus the fluorescence spectra of the bonded pyranines shift to shorter wavelengths. Fluorescence spectra of the bonded pyranines allowed 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. The observations around the gel point of PAAm-PNIPA hybrid show that the gel fraction exponent beta obeyed the percolation result.
Citation Count: 5
Studies on Drying and Swelling of PAAm-NIPA Composites in Various Compositions
(Wiley-Blackwell, 2011) Evingür, Gülşen Akin; Pekcan, Önder
The steady-state fluorescence technique was introduced for studying the drying and swelling of disc-shaped PAAm-NIPA composites. Disc-shaped gels were formed with various acrylamides (AAm) and N-isopropylacrylamides (NIPA) by free radical crosslinking copolymerization in water. Composites were prepared with pyranine (Py) doped as a fluorescence probe. Scattered light I(sc) and fluorescence intensities I were monitored during drying of these gels. The fluorescence intensity of pyranine increased and decreased as drying and swelling time increased respectively for all samples. The Stern-Volmer equation combined with moving boundary and Li-Tanaka models were used to explain the behavior of I during drying and swelling respectively. It was found that the desorption coefficient D increased as NIPA contents were increased for a given temperature during drying. However the cooperative diffusion coefficient D(0) increased as NIPA contents were decreased during swelling at a given temperature. Supporting gravimetrical and volumetric experiments were also carried out during drying and swelling of PAAm-NIPA composites. It was observed that NIPA contents affect the drying and swelling process. POLYM. COMPOS. 32:928-936 2011. (C) 2011 Society of Plastics Engineers
Citation Count: 12
Superelastic percolation network of polyacrylamide (PAAm)-kappa carrageenan (kappa C) composite
(Springer, 2013) Evingür, Gülşen Akin; Pekcan, Önder
The elasticity of the polyacrylamide (PAAm)-kappa carrageenan (kappa C) composite was determined as a function of (w/v-%) kappa C content at 40 A degrees C. The gel composites studied contained various percentages (w/v-%) of kappa C. The elasticity of the swollen PAAm-kappa C composite was characterized by using the tensile testing technique. This study investigated the elasticity and the percolation threshold of PAAm-kappa C composite as a function of kappa C content. It is understood that the compressive elastic modulus decreases up to 1 (w/v-%) of kappa C and then increases at contents above 1(w/v-%) of kappa C. The critical exponent of elasticity y was determined between 1 and 1.6 (w/v-%) of kappa C and found to be 0.68. The observed elastic percolation threshold is consistent with the suggested values of the superelastic percolation network.