Browsing by Author "Pekcan, Önder"

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Citation - WoS: 7
Citation - Scopus: 6
Investigation of Pst-Mwcnt Concentration on Epoxyacrylate Photopolymerization and Conductivity of Polymer Films
(Elsevier Science SA, 2013) Doğruyol, Zekeriya; Temel, Gökhan; Doğruyol, Sevnur Keskin; Pekcan, Önder; Arsu, Nergis
Photopolymerization kinetics and conductivity changes of epoxyacrylate composites for various loading modified PSt-MWCNT weight fractions changing from 0.0025 to 0.2 wt.% were evaluated by performing photo differential scanning calorimetry (photo-DSC) and four point conductivity measurements. 0.2% PSt-MWCNT additive polymeric films had their electrical conductivity boosted by 6% more than non-additive polymeric films. (C) 2012 Elsevier B.V. All rights reserved.
Citation - WoS: 1
Citation - Scopus: 1
The Role of Pyranine in Characterization of Paam-Kappa C Composites by Using Fluorescence Technique
(Springer/Plenum Publishers, 2011) Evingür, Gülşen Akın; Pekcan, Önder
Polyacrylamide (PAAm) doped by kappa-carrageenan (kappa C) gels were prepared with various amounts of kappa C varying in the range between 0 wt.% and 3 wt.%. Steady-state fluorescence (SSF) technique was employed for studying sol-gel transition and swelling of PAAm-kappa C composite gels which were prepared by free-radical crosslinking copolymerization. Pyranine was introduced as a fluorescence probe. 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 allow one to monitor the sol-gel transition and to test the universality of the sol-gel transition as a function of some kinetic parameters like polymer concentration. Observations around the gel point t (c) for PAAm-kappa C composite gels showed that the gel fraction exponent beta obeyed the percolation result for low kappa C (< 2.0 wt. %) however classical results were produced at higher kappa C (> 2.0 wt.%). On the other hand fluorescence intensity of pyranine was measured during in situ swelling process at various amounts of kappa C and it was observed that fluorescence intensity values decreased as swelling is proceeded. Li-Tanaka equation was used to determine the swelling time constants tau and cooperative diffusion coefficients D.
Citation - WoS: 4
Conductivity Percolation of Carbon Nanotubes in Polyacrylamide Gels
(Intech Europe, 2011) Pekcan, Önder; Evingür, Gülşen Akın
[Abstract Not Available]
Citation - WoS: 13
Citation - Scopus: 13
In Situ Steady State Fluorescence (ssf) Technique To Study Drying of Paam Hydrogels Made of Various Cross-Linker Contents
(Elsevier Science, 2009) Evingür, Gülşen Akın; Aktas, Demet Kaya; Pekcan, Önder
Drying experiments of polyacrylamide (PAAm) gels were performed using steady-state fluorescence (SSF) spectrometer. Pyranine (P) was introduced as a fluorescence probe and intensity of P from various crosslinker content gel samples was monitored. It was observed that the intensity of P increased during the in situ drying processes. Gravimetrical and volumetric experiments were also performed. An empirical model was derived and introduced to determine the desorption coefficient D of water molecules from the drying PAAm gels. It is observed that D increased as the cross-linker content was increased. (c) 2008 Elsevier B.V. All rights reserved.
Citation - WoS: 6
Citation - Scopus: 7
Drying of Polyacrylamide Composite Gels Formed With Various Kappa- Carrageenan Content
(Springer/Plenum Publishers, 2011) Evingür, Gülşen Akın; Pekcan, Önder
Drying of polyacrylamide (PAAm)-kappa-carrageenan (kappa C) 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 kappa- carrageenan (kappa C) contents by free radical crosslinking copolymerization in water. Pyranine (P) was doped as a fluorescence probe and scattered light I (sc) 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 kappa C contents were increased. Supporting gravimetrical and volumetric experiments were also carried out during drying of PAAm- kappa C composite gels.
Citation - Scopus: 1
Effect of Temperature and Graphene Oxide on the Swelling of Paam-Go Composite Gels
(Avestia Publishing, 2019) Osma, Büşra; Akın Evingür, Gülşen; Pekcan, Önder
Graphene oxide (GO) is a two dimensional carbon material with similar one-atom thickness, and is a light material having extremely high strength and thermal stability [1]. Thus, GO is an efficient filler for the enhancement of the electrical, mechanical and thermal properties of composite materials [2]. We focused on GO as a nanofiller in polyacrylamide hydrogels and PAAm-GO composites to investigate the effect of temperature and graphene oxide on the swelling. Polyacrylamide (PAAm) hydrogels have been proposed for use as promising biomaterials in biomedical and tissue engineering. The composite gels were prepared by free radical crosslinking copolymerization with GO content varying in the range between 8 and 50 μl of GO. The effects of temperature and graphene oxide on the swelling of the composites were studied. The swelling experiment was performed in the distilled water. Decreasing in pyranine (Py) as a fluorescence probe and emission light intensity (Iem) were monitored by steady state fluorescence spectroscopy. Since the increase in Isc corresponds to the increase in turbidity of the swelling composite gel, the corrected fluorescence intensity, I was introduced to analyze the swelling processes. The Stern-Volmer equation combined with Li-Tanaka models was used to explain the behaviour of I during swelling processes. The cooperative diffusion coefficients and time constants were calculated as a function of temperature and GO, respectively.
Citation - WoS: 6
Citation - Scopus: 6
Temperature Effect on the Swelling of Paam-Kappa Composites
(Wiley-Blackwell, 2012) Evingür, Gülşen Akin; Pekcan, Önder
The steady-state fluorescence (SSF) technique was used for studying swelling of disc-shaped polyacrylamide (PAAm)-kappa-carrageenan (kappa C) composites which were prepared by free-radical crosslinking copolymerization at 80 degrees C. Pyranine was introduced as a fluorescence probe during polymerization. Swelling experiments were performed in water at various temperatures by real-time monitoring of the pyranine (Py) fluorescence intensity I which decreased as swelling proceeded. Stern-Volmer equation is modified for low quenching efficiencies to interpret the behavior of Py intensity during the swelling of PAAm-kappa C composites. The Li-Tanaka equation was used to determine the swelling time constants tau(1) and cooperative diffusion coefficients D(0) from fluorescence intensity weight and volume variations of the composites at various temperatures. It was observed that tau(1) first decreased up to 40 degrees C and then increased
Citation - WoS: 3
Citation - Scopus: 4
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 - WoS: 2
Citation - Scopus: 2
Fractal Features and Structural Morphological Optical Characteristics of Sol-Gel Derived Silica Nanoparticled Thin Films
(Polish Acad Sciences Inst Physics, 2018) Uysal, Bengü Özuğur; Pekcan, Önder
Nanostructured silica films using a simple and effective sol-gel spin coating technique were synthesized and the influence of ammonia/sol ratios on the particle size and thickness of this film was investigated. In addition fractal dimensions of the prepared films were determined using the scattering response technique. The samples were characterized by atomic force microscopy and UV-vis spectroscopy. Comparing optical method and image analysis of atomic force microscopy micrographs the fractal dimension of silica nanoparticled thin films was determined. The fractal dimensions of the films verified by atomic force microscopy analysis were found to be around 2.03 which is very close to the values (2.0358 2.0325 and 2.0335) obtained using optical method. As a result of these findings precise determination of the nanoparticled silica thin films fractal dimension using both optical and surface analysis methods was realized.
Citation - WoS: 12
Citation - Scopus: 14
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 - WoS: 5
Citation - Scopus: 6
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 - WoS: 22
Citation - Scopus: 22
Gelation Mechanisms
(World Scientific Publ Co Pte Ltd, 2012) Pekcan, Önder; Kara, Selim
In this paper, we survey the gelation mechanisms for various polymeric systems which are classified by the type and the strength of the cross-linkages. These are the "irreversible" gels that are cross-linked chemically by covalent bonds and the "reversible" gels that are cross-linked physically by hydrogen or ionic bonds and by the physical entanglement of polymer chains. Some of the natural polymer gels fall into the class of physical gels, among which the red algae that has attracted attention for various applications is discussed in detail. Various composite gels, formed from mixture of physical and chemical gels are also discussed in the last section of the article. Theoretical models describe the gelation as a process of random linking of subunits to larger and larger molecules by formation of an infinite network, where no matter what type of objects are linked, there is always a critical "gel point" at which the system behaves neither as a liquid nor as a solid on any length scale. The Flory-Stockmayer theory and percolation theory provide bases for modeling this sol-gel phase transition. The experimental techniques for measuring the critical exponents for sol-gel phase transitions in different polymeric systems are introduced and the validation of various theoretical predictions are surveyed.
Citation - WoS: 4
Citation - Scopus: 3
Electrical Optical and Fluorescence Percolations in P(vac-co-bua)/Mwcnt Composite Films
(Taylor & Francis Ltd, 2013) Arda, Ertan; Kara, Selim; Pekcan, Önder
Effects of multiwall carbon nanotube (MWCNT) addition on the electrical conductivities optical transparencies and fluorescence emissions of poly(vinyl acetate-co-butyl acrylate) (P(VAc-co-BuA))/MWCNT composite films were studied. Optical transmission fluorescence emission and two point probe resistivity techniques were used to determine the variations of the optical fluorescence and electrical properties of the composites respectively. Transmitted photon intensity (I-tr) fluorescence emission intensity (I-fl) and surface resistivity ((s)) of the composite films were monitored as a function of MWCNT mass fraction (M) at room temperature. All these measured quantities of the composites were decreased by increasing the content of MWCNT in the composite. The conductivity and the optical results were attributed to the classical and site percolation theories respectively. The fluorescence results however possessed both the site and classical percolation theories at low and high MWCNT content regions respectively.
Citation - WoS: 4
Citation - Scopus: 3
Polymer/Carbon Nanotube Composite Film Formation: a Fluorescence Study
(Wiley-Blackwell, 2014) Yargı, Önder; Uğur, Saziye; Pekcan, Önder
In this study the effect of multi-walled Carbon nanotube (MWNT) on film formation behavior of Polystrene (PS) latex film was investigated by using steady state fluorescence technique. Films were prepared by mixing of pyrene (P)-labeled PS latex with different amounts of MWNTs varying in the range between 0 and 20 wt%. After drying MWNT containing films were separately annealed above glass transition temperature (T-g) of PS ranging from 100 to 270 degrees C for 10 min. In order to monitor film formation behavior of PS/MWNT composites Scattered light (I-s) and fluorescence intensities (I-P) from P were measured after each annealing step to monitor the stages of film formation. At 0-20 wt% range of MWNT content films minimum film formation (T-o) void closure (T-v) and healing (T-h) temperatures were determined. Void closure and interdiffusion stages were modeled and related activation energies were determined. It was observed that while void closure activation energies increased backbone activation energies decreased as the percent of MWNT is increased in the composite films. POLYM. COMPOS. 35:817-826 2014. (c) 2013 Society of Plastics Engineers
Citation - WoS: 6
Citation - Scopus: 6
Slow Release of Trapped Homopolymers From a Swelling Polymeric Gel: a Fluorescence Study
(Taylor & Francis Inc, 2008) Erdoğan, Matem; Yağcı, Yusuf; Pekcan, Önder
In-situ steady-state fluorescence experiments were performed for studying slow release of pyrene-labeled polystyrene chains from polystyrene gels formed by free-radical crosslinking copolymerization. Atom transfer radical polymerization was used to produce the pyrene end-capped polystyrene chains. In order to load the pyrene end-capped polystyrene chains into the gel disc-shaped gels were left in toluene solutions of pyrene end-capped polystyrene chains of various molecular weights. These swollen gels were redried in air and then immersed in pure toluene solution for monitoring slow release from the gel. These reswelling experiments were performed at room temperature in real time by monitoring the pyrene emission intensity using steady-state fluorescence measurements. Slow-release diffusion coefficients were measured and found to decrease as the crosslink density of the gels increased. It was observed that higher molecular weight pyrene end-capped polystyrene chains released much faster than low molecular ones during the slow-release process.
Citation - WoS: 4
Citation - Scopus: 4
Diffusion Energies of Oxygen Diffusing Into Polystyrene (ps)/Poly (n-Isopropylacrylamide) Composites
(Wiley-Blackwell, 2012) Yargı, Önder; Ugur, Saziye; Pekcan, Önder
Diffusion coefficient of oxygen penetrating into polystyrene (PS) latex/poly (N-isopropylacrylamide) (PNIPAM) microgel composite films were measured using Fluorescence technique. Three different (5 15 and 40wt%) PS content films were prepared from PS/PNIPAM mixtures. Diffusivity of PS/PNIPAM composite films were studied by diffusion measurements which were performed over the temperature range of 24-70 degrees C. Pyrene was used as the fluorescent probe. The diffusion coefficients (D) of oxygen were determined using the SternVolmer fluorescence quenching method combined with Fickian transport and were computed as a function of temperature for each PS content film. The results showed that D values were strongly dependent on both temperature and PS content in the film. Diffusion energies were measured and found to be dependent on the composition of the composite films. Copyright (C) 2011 John Wiley & Sons Ltd.
Citation - WoS: 1
Citation - Scopus: 2
Swelling Kinetics of Paam-Kappa Composites: a Fluorescence Technique
(Taylor & Francis Inc, 2011) Evingür, Gülşen Akın; Pekcan, Önder
The steady-state fluorescence (SSF) technique was introduced for studying swelling of disc-shaped polyacrylamide (PAAm) gels containing various amount of kappa-carrageenan (kappa C). They were prepared by free-radical cross-linking copolymerization. N N'-methylenebisacrylamide (BIS) and ammonium persulfate (APS) were added as a crosslinker and an initiator respectively. Composite gels were prepared at 80 degrees C with pyranine as a fluorescence probe. After drying of these gels swelling kinetics were performed in water at 60 degrees 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 tau(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 tau(1) decreased and D(0) increased as the kappa C concentrations in the composites were increased indicating that high kappa C gels swell faster than low kappa C gels.
Citation - WoS: 8
Citation - Scopus: 7
Fractal Dimensions of Kappa-Carrageenan Gels During Gelation and Swelling
(Taylor & Francis Inc, 2018) Kara, Selim; Arda, Ertan; Pekcan, Önder
A photon scattering technique for research on the sol-gel and gel-sol transitions in kappa-carrageenan-water systems with various carrageenan contents (CC) was utilized for characterizing the fractal dimensions during gelation and swelling. It was observed that the scattered photon intensity I-sc increased at all temperatures with an increase in the CC when I-sc was monitored against temperature. Additionally the sol-gel transition temperatures were found to be much lower than the gel-sol transition temperatures causing hysteresis of the phase transition loops. I-sc increased with an increase in CC at all test temperatures which is attributed to the formation of a fractal-like carrageenan gel. After drying the gels were used in swelling experiments where the gels were immersed in water at room temperature reswelling to the original structure. It was observed that I-sc from the carrageenan gels increased as the CC was increased. The fractal dimension d during gelation was found to increase as the gelation temperature was increased. On the other hand the d values during swelling decreased as the swelling time was increased.
Citation - Scopus: 1
Application of fluorescence technique for understanding film formation from polymer latexes and composites
(Elsevier, 2021) Uğur, Ş.; Pekcan, Önder
This chapter summarizes the application of fluorescence technique to understand all aspects of film formation using both pure polymer latexes and polymer nanocomposites. Transient fluorescence, steady state fluorescence, and photon transmission techniques were used in conjunction with scanning electron microscopy/atomic force microscopy to learn this process with different latex coatings. Polystyrene latexes and poly(methyl methacrylate) latexes labeled with fluorescence probes (such as pyrene, fluorescein, and naphthalene) were used as the model polymer matrixes. We also introduced theoretical models to describe film formation stage by stage including void closure, healing, and interdiffusion and produced the activation energies related to the process. Furthermore the contributing parameters to the film formation process are identified including annealing time-temperature, solvent vapor, film thickness, particle size, and different filler materials and ratios (clay, TiO2, carbon nanotube, etc.) in both pure latex and composite systems. © 2021 Elsevier B.V. All rights reserved.
Citation - WoS: 6
Citation - Scopus: 9
Particle Size Effect on the Film-Forming Process of Ps/Pba Composite Latexes
(John Wiley & Sons Inc, 2010) Uğur, Şaziye; Sunay, M. Selin; Elaissari, Abdelhamid; Pekcan, Önder
In this work the effect of hard particle size and blend ratio on the film formation behavior of hard polystyrene (PS) and soft poly(n-butyl acrylate) (PBA) latex blends was studied by means of steady-state fluorescence and UV-visible techniques in conjunction with atomic force microscopy. Three different sets of latexes were synthesized: PBA latex (diameter 97 nm) pyrene (P)-labeled large PS (LgPS