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Publication Metadata only 1,3-bis(gamma-aminopropyl)tetramethyldisiloxane modified epoxy resins: curing and characterization(Elsevier, 1998) Department of Chemistry; Department of Chemistry; Yılgör, Emel; Yılgör, İskender; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 40527; 24181Incorporation of siloxane oligomers with reactive organofunctional terminal groups, such as amine, epoxy and carboxy, into the structure of epoxy networks, provides improvements in the fracture toughness, water absorption and surface properties of the resultant systems. 1,3-bis(gamma-aminopropyl) tetramethyldisiloxane (DSX) was used as a model curing agent and modifier in bis(4-aminocyclohexyl)methane (PACM-20) cured diglycidyl ether of bisphenol-A (DGEBA) based epoxy resins. Curing reactions followed by differential scanning calorimetry indicated faster reaction rates between DSX and DGEBA as compared with PACM-20 and DGEBA. Mechanical characterization of the modified products showed improvements in tensile and impact strengths as expected. Glass transition temperatures of these materials showed a decrease with an increase in DSX content.Publication Metadata only A comparative study of the structure-property behavior of highly branched segmented poly(urethane urea) copolymers and their linear analogs(Elsevier Sci Ltd, 2005) Sheth, JP; Unal, S; Beyer, FL; Long, TE; Wilkes, GL; Department of Chemistry; Department of Chemistry; Yılgör, Emel; Yılgör, İskender; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; N/A; 24181The solid-state structure-property behavior of highly branched segmented poly(urethane urea) (PUU) copolymers and their linear analog was investigated. A limited study of their solution theological behavior was also undertaken. The linear PUUs were synthesized by the two-step prepolymer method, whereas the oligomeric A(2) + B-3 methodology was utilized to synthesize the highly branched materials. The soft segments (SS) were either poly(tetramethylene oxide) (PTMO) or poly(propylene oxide) (PPO). All copolymers utilized in this study, with one exception, contained 28 wt% hard segment (HS) content. DMA, SAXS, and AFM studies indicated that the linear as well as the highly branched PUUs were microphase separated. The SS T-g of the highly branched PUUs was nearly identical to that of their respective linear analogs. However, the linear copolymers exhibited broader and less temperature sensitive rubbery plateaus, both attributed to one or both of two reasons. The first is better hydrogen bonding organization of the HS phase as well as greater HS lengths than in the highly branched analogs. The second parameter is that of a potentially higher chain entanglement for the linear systems relative to the branched analogs. Tapping-mode AFM phase images confirmed the microphase morphology indicated by SAXS and DMA. Ambient temperature strain-induced crystallization was observed in the PUU based on PTMO 2040 g/mol at a uniaxial strain of ca. 400%, irrespective of the chain architecture. Stress-strain, stress relaxation, and mechanical hysteresis of the highly branched copolymers were in general slightly poorer than that of their linear analogs. Ambient temperature solution viscosity of the highly branched materials in dimethyl formamide was substantially lower that that of the linear samples of nearly equal molecular weight.Publication Metadata only A DFT study of polymerization mechanisms of indole(Elsevier Sci Ltd, 2002) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Polymerization of unsubstituted indoles is studied by accurate density functional theory calculations. Relative stability of all possible dimers of indole is computed in order to understand the thermodynamics of polymerization. It is observed that 2-position is the most likely site to enhance polymerization. A selected set of trimers and tetramers which use a 2-position for linkages are generated to understand the further growth of polyindole. A study of local minima arising from different distributions of the torsional angles reveals that there are two equally probable conformations and the one with the torsional angle changing signs alternatively is slightly favored. The cyclic structures are also investigated and it is shown that it is possible to generate stable three- and four-membered cyclic structures. Finally, the structures of radical cations and intermediate states are fully optimized and the energetics of these metastable species are used to explain the competing mechanisms of radical-radical and radical-neutral pathways.Publication Metadata only A quantum mechanical study of the electrochemical polymerization of pyrrole(Elsevier Science Sa, 2001) Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Mechanism for the electrochemical polymerization of pyrrole is studied using accurate density functional theory; (DFT) calculations. The primary emphasis is on the structures and stability of intermediates generated during various mechanisms. Structures of the radical cations, which play role in reactions, an optimized to elucidate radical-radical and radical-neutral pathways. The competing probabilities of reactions between various size oligomers are discussed in terms of their thermodynamical stability.Publication Metadata only A theoretical study of structural defects in conjugated polymers(Elsevier Science Sa, 1999) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Accurate ab-initio calculations are performed for pyrrole and thiophene oligomers bonded through alpha and beta carbons. The thermodynamical stabilitiy of all possible binding types including the branched forms of tetramers and pentamers are compared. Employing the probabilities obtained from these calculations, a Monte Carlo type growth scheme is applied to predict branching as functions of the chain length and temperature. A high degree of branching for polypyrrole is reported whereas the linear chains dominate the structure of polythiophene.Publication Metadata only Ag, Co3O4, Ag-Co3O4, and Ag/Co3O4 nanoparticles decorated mesoporous natural phosphate: effect of metal synergy and preparation method on the catalytic reduction reaction(Springer, 2022) Orfi, Hamza; Mekkaoui, Ayoub Abdelkader; Laayati, Mouhsine; Labyad, Salim Adam; El Firdoussi, Larbi; El Houssame, Soufiane; N/A; Department of Chemistry; Sündü, Buse; Metin, Önder; PhD Student; Faculty Member; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Sciences; N/A; 46962Addressed herein is a facile method for the preparation of cobalt-silver (Co-x-Ag-y) nanoparticles decorated mesoporous natural phosphate (m-NP). Various Co-x-Ag-y@NP nanocatalysts were prepared in different forms of Ag and Co3O4 including mono- and bimetallic active sites. The bimetallic nanocatalysts were prepared by two different preparation methods, namely in-situ (Ag-Co3O4) and ex-situ (Ag/Co3O4 core-shell). In the first step, colloidal Co-x-Ag-y nanoparticles were synthesized in solution and their formation was monitored by using UV-Visible spectroscopy. Furthermore, the obtained nanoparticles were characterized by XRD and IR spectroscopy. All nanoparticles were deposited on m-NP using a simple wetness impregnation method followed by a calcination at 500 degrees C. The prepared nanocatalysts were fully characterized by advanced analytical techniques including IR, XRD, XPS, SEM-EDX, FESEM, and TEM. The catalytic reduction of 4-nitrophenol was studied as a model reaction to investigate the effect of synergy created between the metals, oxidation state, catalyst structure, and preparation method on their catalytic activity. Accordingly, reaction kinetics and comparative study of various colloidal Co-x-Ag-y and m-NP supported nanocatalysts in the reduction of 4-nitrophenol was carried out. The optimized conditions were used to study the substrate scope of the catalytic reduction over various nitroarenes.Publication Metadata only Anomalous dilute solution properties of segmented polydimethylsiloxane-polyurea copolymers in isopropyl alcohol(Elsevier, 2006) Ward, Thomas C.; Department of Chemistry; Department of Chemistry; Department of Chemistry; Yılgör, İskender; Yılgör, Emel; Gökçümen, Güneş Ekin Atilla; Faculty Member; Researcher; Undergraduate Student; Department of Chemistry; College of Sciences; College of Sciences; College of Sciences; 24181; 40527; N/APreliminary characterization of amphiphilic segmented copolymers of polydimethylsiloxane and urea 'hard blocks' was conducted by measuring isopropyl alcohol (primarily) dilute solution viscosities via capillary viscometry. The traditional data analysis techniques, which provide for extrapolation of intrinsic viscosities from these experiments, revealed that increasing concentrations of polymer produced lower reduced viscosities rather than the expected higher values. A very approximate data fit reveals negative Huggins and Kraemer constants from these analyses, which are highly unusual. In a solvent such as DMF, a similar polymer having poly(tetramethylene oxide) and urea blocks and measured with identical conditions exhibited the expected behavior, showing increasing reduced viscosities over concentrations in the same range. However, the non-linearity of the data is suggestive of much more complex hydrodynamic, or supramolecular, interactions that are not clarified by the initial research.Publication Metadata only Catalyst effect on the transesterification reactions between polycarbonate and polycaprolactone-B-polydimethylsiloxane triblock copolymers(Springer, 1999) Ardal, D; Department of Chemistry; Department of Chemistry; Yılgör, Emel; Yılgör, İskender; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; N/A; 24181Transesterification reactions between bisphenol-A polycarbonate and polydimethylsiloxane-polycaprolactone block copolymers were studied in melt at 270 degrees C. Influence of the composition of reaction mixture and the catalyst type on the formation and properties of the products obtained were investigated by spectroscopic, chromatographic and thermal methods. Both of the catalysts used, zinc acetate and lanthanum acetylacetonate are very efficient transesterification catalysts for the system studied. GPC and DSC results clearly show the formation of novel polymers displaying combined properties of the polycarbonate and the silicone-ester copolymer.Publication Metadata only Comparison of hydrogen bonding in polydimethylsiloxane and polyether based urethane and urea copolymers(Elsevier, 2000) Department of Chemistry; Department of Chemistry; Department of Chemistry; Department of Chemistry; Yılgör, Emel; Burgaz, Engin; Yurtsever, İsmail Ersin; Yılgör, İskender; Researcher; Undergraduate Student; Faculty Member; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; College of Sciences; College of Sciences; 40527; N/A; 7129; 24181Hydrogen bonding in polydimethylsiloxane and polyether based urethane and urea type segmented copolymers was investigated by infrared spectroscopy, differential scanning calorimetry and quantum mechanical calculations. Hydrogen bonding in model urethane and urea compounds was compared with those of the copolymers, in order to determine the extent of interaction and resulting phase mixing between hard and soft segments in these copolymers. Quantum mechanical calculations were also used to determine the interaction energies due to hydrogen bonding in model urethane and urea compounds. Further, similar calculations were also performed to quantify the interactions between silicone and ether type soft segments, and urea and urethane type hard segments. As expected, these calculations clearly indicated the absence of any interaction between silicones and urea groups, while there was substantial hydrogen bonding between urea groups and the oxygen in the ether type soft segments. Results of FTIR studies and quantum mechanical calculations were in good agreement with thermomechanical behavior and mechanical properties of these copolymers.Publication Metadata only Conformational analysis of model poly(ether urethane) chains in the unperturbed state and under external forces(Amer Chemical Soc, 2002) Department of Chemistry; Department of Chemistry; Department of Chemical and Biological Engineering; Yılgör, İskender; Yurtsever, İsmail Ersin; Erman, Burak; Faculty Member; Faculty Member; Faculty Member; Department of Chemistry; Department of Chemical and Biological Engineering; College of Sciences; College of Sciences; College of Engineering; 24181; 7129Conformational features of model poly(ether urethane) (PEU) single chains are investigated in the unperturbed state and under external force. Model PEUs consisting of symmetrical, aromatic, 1,4-phenylene diisocyanate (PDI) hard segments and poly(tetramethylene oxide) (PTMO) soft segments with varying lengths are considered. Rotational potential maps are calculated quantum mechanically using the AM1 parametrization with Gaussian98. Configurations of the chains are generated by the Monte Carlo technique, using the rotational isomeric state formalism and successive matrix multiplication scheme. Unperturbed dimensions, the change in dimensions when a force acts along the end-to-end vector, the stiffness, and toughness of the chain and orientability of segments under external force are characterized and compared with properties of polyethylene. The characteristic ratio of along PEU chain is 5.0, and the Kuhn length for a single block is 9.2 Angstrom, both of which are smaller than the corresponding polyethylene values. The model chains are significantly more ductile and tougher than polyethylene. The orientability of the backbones exhibits a strong even-odd effect, with strongly orientable bonds neighbored by weakly orienting ones. The degree of rigidity of the phenyl group does not propagate far along the chain. The projection of the bond vectors on the phenyl axis decay rapidly with increasing distance of the bond along the chain.