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Güner, Pınar Tatar

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Pınar Tatar

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Güner

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Now showing 1 - 10 of 11
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    Poly(p-phenylene methylene)-based block copolymers by mechanistic transformation
    (2011) Taşdelen, Mehmet Atilla; Beyazıt, Selim; Güneş, Deniz; Bıçak, Niyazi; Tatar, Pınar; Yağcı, Yusuf; Department of Chemistry; Department of Chemistry; Demirel, Adem Levent; Güner, Pınar Tatar; Faculty Member; Teaching Faculty; Department of Chemistry; College of Sciences; College of Sciences; 6568; 188227
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    Effect of anions on the cloud point temperature of aqueous poly(2-ethyl-2-oxazoline) solutions
    (Amer Chemical Soc, 2012) Department of Chemistry; Department of Chemistry; Güner, Pınar Tatar; Demirel, Adem Levent; Teaching Faculty; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 188227; 6568
    Poly(2-alkyl-2-oxazoline)s have recently gained attention in especially biological applications due to their lower critical solution temperature being close to the body temperature and their biocompatibility. The understanding of how cloud point temperature (T-c) depends on the salt concentration and the molecular mechanisms responsible for such behavior are important to tune T-c as desired by the applications. In this paper, we report the effect of a series of sodium salts on T-c of aqueous poly(2-ethyl-2-oxazoline) (PEOX) solutions by dynamic light scattering. PEOX samples having four different molecular weights were investigated, and the results were compared with those of poly(N-isopropylacrylamide) (PNIPAM), the mostly investigated and used thermoresponsive polymer. Kosmotropic anions decreased T-c linearly while chaotropic anions increased T-c nonlinearly with salt concentration. The contributions of different mechanisms to T-c change have been discussed. Our results indicate that the dominant mechanism is the dehydration of PEOX for divalent kosmotropic anions (CO32-, SO42-, S2O32-) and direct binding for chaotropic anions (NO3-, I-, ClO4-, SCN-). For the remaining monovalent kosmotropic anions (H2PO4-, F-, Cl-, Br-), a combination of dehydration and surface tension mechanisms was in effect. The additional contribution of the surface tension mechanism for the monovalent kosmotropic anions was inferred for different molecular weight PEOX samples and also for PNIPAM. With PEOX molecular weight decreasing from 500 000 to 5000 g/mol, T-c decreased less with salt concentration which was attributed to the contribution of the surface tension mechanism. For PEOX samples, the decrease of T-c with kosmotropic anion concentration was faster compared to PNIPANI due to differences in their chemical structure. Our results show that the molecular mechanisms of interactions between PEOX chains and specific anions can simply be inferred from determination of T-c by a common technique-dynamic light scattering.
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    Hydrogen bonded multilayers of poly(2-ethyl-2-oxazoline) stabilized silver nanoparticles and tannic acid
    (Pergamon-Elsevier Science Ltd, 2017) N/A; N/A; Department of Chemistry; Department of Chemistry; Department of Chemistry; Hendessi, Saman; Güner, Pınar Tatar; Miko, Annamaria; Demirel, Adem Levent; PhD Student; Teaching Faculty; Teaching Faculty; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; College of Sciences; N/A; 188227; 163509; 6568
    The formation of H-bonded multilayers of PEOX stabilized Ag-NPs and TA by layer-by-layer (LbL) process is reported. Ag-NPs were synthesized in aqueous solutions by chemical reduction. Four different molar mass PEOX (5K, 50K, 200K, 500K) was used to stabilize the Ag-NPs in dispersion. The effect of PEOX molar mass, PEOX concentration, pH and time on the temporal stability of Ag-NP dispersions was investigated. All dispersions showed bimodal size distribution. Individual PEOX stabilized Ag-NPs had sizes less than 10 nm. The size of agglomerates depended on the PEOX molar mass and was larger than 50 nm. The agglomerates consisted of individual Ag-NPs in the mesh of bridging PEOX chains. The kinetic stability of the dispersions also depended on PEOX molar mass. Ag-NP dispersions without any PEOX were not stable. PEOX-5K and PEOX-50K stabilized Ag-NP dispersions were stable for 6 days, while PEOX-200K and PEOX-500K stabilized dispersions were stable up to 2 weeks. The amount of Ag-NPs in the dispersion increased with PEOX concentration. Decreasing pH from 9.9 in as-prepared dispersion to 4.5 increased the individual Ag-NP size slightly from 10 nm to 16 nm. All PEOX stabilized dispersions were stable up to 8 days at pH < 9.9. PEOX(Ag-NP)s were used as H-accepting component and TA as H-donating component in the preparation of LbL films. The growth profiles and the pH-induced disintegration of the multilayers were similar to those of PEOX/TA multilayers. All multilayers showed linear growth profiles and were stable up to pH 8.5. SEM and AFM images of the multilayers showed homogeneous distribution of Ag-NPs in the films. The loading capacity of Ag-NPs in the multilayers was determined by XPS to be 1-3%. The understanding of the effect of different molar mass PEOX in stabilizing Ag-NPs in dispersions and in forming uniform Ag-NP loaded pH-responsive LbL films are important in controlling the size and loading capacity of Ag-NPs in multilayers. Ag-NP loaded LbL films of PEOX and TA have potential biomedical applications as antibacterial coatings which can release Ag+ ions from stable multilayers below the critical pH and disintegrate by releasing all Ag-NPs at the critical pH.
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    Formation of poly(2-ethyl-2-oxazoline) fibers in aqueous solutions
    (American Chemical Society (ACS), 2012) Department of Chemistry; Department of Chemistry; Department of Chemistry; Güner, Pınar Tatar; Miko, Annamaria; Demirel, Adem Levent; Teaching Faculty; Teaching Faculty; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; College of Sciences; 188227; 163509; 6568
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    In vitro antibacterial and cytotoxicity assessment of magnetron sputtered Ti1.5ZrTa0.5Nb0.5W0.5 refractory high-entropy alloy doped with Ag nanoparticles
    (Elsevier, 2022) N/A; N/A; N/A; Department of Chemistry; Department of Chemistry; Department of Chemical and Biological Engineering; Department of Chemistry; N/A; Alamdari, Armin Asghari; Hashemkhani, Mahshid; Hendessi, Saman; Güner, Pınar Tatar; Acar, Havva Funda Yağcı; Kavaklı, İbrahim Halil; Ünal, Uğur; Motallebzadeh, Amir; PhD Student; PhD Student; PhD Student; Teaching Faculty; Faculty Member; Faculty Member; Faculty Member; Resercher; Department of Chemical and Biological Engineering; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Sciences; College of Engineering; College of Sciences; N/A; N/A; N/A; N/A; 188227; 178902; 40319; 42079; N/A
    The aim of the current study is to develop and evaluate antibacterial and biocompatible refractory high-entropy alloy (RHEAs) film using RF magnetron sputtering technique. For this purpose, Ti1.5ZrTa0.5Nb0.5W0.5 RHEA film and its Ag nanoparticle doped analog (with an Ag content of 9 atomic %) were deposited on Ti6Al4V substrate. The microstructural characterization revealed the homogenous distribution of the constituent elements and amorphous structure of the deposited films. The antibacterial activity of Ag nanoparticles doped RHEA film was compared with that of undoped film and uncoated Ti6Al4V. The results indicated that doping Ag nanoparticles reduced the colony forming unit of P. Aeruginosa and S. Aureus bacteria by 98.5% and 90.9%, respectively. In addition, healthy C2C12 mouse muscle myoblast cells adhered and proliferated perfectly on the surface of antibacterial Ag nanoparticles doped RHEA film with no indication of toxic effect, demonstrating promising biocompatibility in addition to its strong antibacterial property.
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    Synthesis and characterization of polysulfone-g-poly(2-alkyl-2-oxazoline)s
    (Taylor and Francis Ltd, 2013) Ates, Sahin; Yagci, Yusuf; Department of Chemistry; Department of Chemistry; Güner, Pınar Tatar; Demirel, Adem Levent; Teaching Faculty; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 188227; 6568
    Novel amphiphilic graft copolymers of polysulfone (PSU) possessing hydrophilic poly(2-oxazoline) (POx) side chains were synthesized via the 'grafting from' method. The cationic ring-opening polymerizations of 2-methyl-2-ozaxoline, 2-ethyl-2-ozaxoline, and 2-propyly-2-ozaxoline were initiated by chloromethylated polysulfone macroinitiator (with 15% benzylchloride content), synthesized by chloromethylation of PSU, in the presence of sodium iodide. The effects of reaction time and type of monomer on the yield, and degree of polymerization were investigated. The obtained polymers were characterized by proton nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry analyses. Thin films of graft copolymers were investigated by atomic force microscopy and contact angle measurements. The observed morphologies of thin films were consistent with the amphiphilic nature of the graft copolymers. The films showed temperature dependent surface hydrophobicity due to thermo responsiveness of POx.
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    Photoinduced grafting of polystyrene onto silica particles by ketene chemistry
    (Wiley, 2012) Yılmaz, Görkem; Kumbaracı, Volkan; Talınlı, Naciye; Yağcı, Yusuf; Department of Chemistry; Department of Chemistry; Güner, Pınar Tatar; Demirel, Adem Levent; Teaching Faculty; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 188227; 6568
    This study describes a novel route for incorporation of polymers onto silica particle surfaces to prepare polymer/silica nanocomposites. Photoinduced formation of ketene functionalities at the polystyrene chain ends offers possibility of such ligation through ketene chemistry.
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    Revisiting the crystallization of poly(2-alkyl-2-oxazoline)s
    (Wiley, 2016) erbraeken, Bart; Schlaad, Helmut; Schubert, Ulrich S.; Hoogenboom, Richard; Department of Chemistry; Department of Chemistry; Demirel, Adem Levent; Güner, Pınar Tatar; Faculty Member; Teaching Faculty; Department of Chemistry; College of Sciences; College of Sciences; 6568; 188227
    Poly(2-alkyl-2-oxazoline)s (PAOx) exhibit different crystallization behavior depending on the length of the alkyl side chain. PAOx having methyl, ethyl, or propyl side chains do not show any bulk crystallization. Crystallization in the heating cycle, that is, cold crystallization, is observed for PAOx with butyl and pentyl side chains. For PAOx with longer alkyl side chains crystallization occurs in the cooling cycle. The different crystallization behavior is attributed to the different polymer chain mobility in line with the glass transition temperature (T-g) dependency on alkyl side chain length. The decrease in chain mobility with decreasing alkyl side chain length hinders the relaxation of the polymer backbone to the thermodynamic equilibrium crystalline structure. Double melting behavior is observed for PButOx and PiPropOx which is explained by the melt-recrystallization mechanism. Isothermal crystallization experiments of PButOx between 60 and 90 degrees C and PiPropOx between 90 and 150 degrees C show that PAOx can crystallize in bulk when enough time is given. The decrease of Tg and the corresponding increase in chain mobility at T > T-g with increasing alkyl side chain length can be attributed to an increasing distance between the polymer backbones and thus decreasing average strength of amide dipole interactions. (C) 2015 Wiley Periodicals, Inc.
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    PublicationOpen Access
    Optimum folding pathways of proteins: their determination and properties
    (American Institute of Physics (AIP) Publishing, 2006) Department of Chemical and Biological Engineering; Güner, Pınar Tatar; Arkun, Yaman; Erman, Burak; Teaching Faculty; Faculty Member; Faculty Member; Department of Chemical and Biological Engineering; College of Engineering; 188227; 108526; 179997
    We develop a dynamic optimization technique for determining optimum folding pathways of proteins starting from different initial configurations. A coarse-grained Go model is used. Forces acting on each bead are (i) the friction force, (ii) forces from bond length constraints, (iii) excluded volume constraints, and (iv) attractive forces between residue pairs that are in contact in the native state. An objective function is defined as the total attractive energy between nonbonded residues, which are neighbors in the native state. The objective function is minimized over all feasible paths, satisfying bond length and excluded volume constraints. The optimization problem is nonconvex and contains a large number of constraints. An augmented Lagrangian method with a penalty barrier function was used to solve the problem. The method is applied to a 36-residue protein, chicken villin headpiece. Sequences of events during folding of the protein are determined for various pathways and analyzed. The relative time scales are compared and scaled according to experimentally measured events. Formation times of the helices, turn, and the loop agree with experimental data. We obtain the overall folding time of the protein in the range of 600 ns-1.2 mu s that is smaller than the experimental result of 4-5 mu s, showing that the optimal folding times that we obtain may be possible lower bounds. Time dependent variables during folding and energies associated with short- and long-range interactions between secondary structures are analyzed in modal space using Karhunen-Loeve expansion.
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    PublicationOpen Access
    Self-assembled poly(2-ethyl-2-oxazoline) fibers in aqueous solutions
    (Royal Society of Chemistry (RSC), 2012) N/A; Department of Chemistry; Güner, Pınar Tatar; Miko, Annamaria; Schweinberger, Florian F.; Demirel, Adem Levent; Teaching Faculty; Teaching Faculty; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; 188227; N/A; N/A; 6568
    Poly(2-ethyl-2-oxazoline) (PEOX) formed self-assembled fibers in aqueous solutions above the cloud point temperature (T-c) through a slow crystallization process. The fiber formation above T-c happened both in pure water and in the presence of salting-in (SCN-) and salting-out (CH3COO-) ions. The crystal structure and the melting temperature of the PEOX fibers were determined.