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Now showing 1 - 10 of 23
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    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; 24181
    Incorporation 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.
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    A novel mold design for one-continuous permeability measurement of fiber preforms
    (Sage Publications Ltd, 2015) N/A; N/A; Department of Mechanical Engineering; Yalçınkaya, Mehmet Akif; Sarıoğlu, Ayşen; Sözer, Murat; PhD Student; Master Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 110357
    One-continuous permeability measurement experiments allow measuring permeability of a fiber preform within a range of fiber volume fractions by conducting a single unsaturated (a.k.a. transient) flow experiment on a dry specimen at an initial thickness, and a set of saturated flow experiments on the wetted specimen by varying the thickness of the mold cavity. This approach allows quicker database construction and reduces the effect of inherent variation of fabric structure caused by inconsistent labor on permeability. In this study, the drawbacks of previous mold designs are eliminated by using appropriate sealing, gap thickness adjustment mechanism and features that allow straightforward and reliable manual operation. Experiments for three different fabric types are conducted and the results are discussed. It is mainly observed that the unsaturated permeability is higher than the saturated permeability.
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    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; 7129
    Mechanism 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.
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    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; 24181
    Hydrogen 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.
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    Density functional theory study of the electrochemical oligomerization of thiophene: transition states for radical-radical and radical-neutral pathways
    (Elsevier, 2004) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129
    Accurate density-functional-theory (DFT) calculations on oligothiophenes containing up to 6 units have been performed in order to understand the various possible mechanisms of growth. There are two possible mechanisms for this process which involve the coupling of cationic radicals with either another cationic radical or with a neutral oligomer. Most of the electronic properties of these oligomers are well understood, but there is very little known about the nature of the intermediates and the transition states leading to polymerization. The initial oxidation, forming stable intermediates, releasing protons and/or further oxidations are studied in terms of the energetics, changes in the geometry, charge distributions and possible signatures in the vibrational spectra. The radical-radical pathway is found to be the more probable one between two mechanisms. Also the attack of monomers/or shorter oligomers on the longer ones is found to be faster than the coupling of equal size chains.
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    Effect of permeability characterization at different boundary and flow conditions on vacuum infusion process modeling
    (Sage Publications Ltd, 2017) N/A; N/A; Department of Mechanical Engineering; Yalçınkaya, Mehmet Akif; Çağlar, Barış; Sözer, Murat; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 110357
    Permeability characterization of a fabric preform is a key factor that affects the accuracy of process modeling of vacuum infusion. There are various flow types and boundary conditions (such as one-dimensional or radial flow under constant injection pressure or constant injection flow rate during unsaturated or saturated flow regimes) used in permeability measurement experiments in the literature. This study investigates the effect of using different flow and injection boundary conditions in permeability characterization on the results of coupled one-dimensional mold-filling and compaction model. The results of the model are compared with vacuum infusion mold-filling experiments. It is shown that using the permeability measured at constant injection pressure and unsaturated flow results in the closest fill time compared to the experiments for all three types of fabrics investigated in this study.
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    Effect of surface modification of colloidal silica nanoparticles on the rigid amorphous fraction and mechanical properties of amorphous polyurethane-urea-silica nanocomposites
    (Wiley, 2019) Oguz, Oğuzhan; Candau, Nicolas; Bernhard, Stephane H. F.; Heinz, Ozge; Stochlet, Gregory; Plummer, Christopher J. G.; Menceloğlu, Yusuf Z.; N/A; Department of Chemistry; Department of Chemistry; Söz, Çağla Koşak; Yılgör, Emel; Yılgör, İskender; PhD Student; Researcher; 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; College of Sciences; N/A; N/A; 24181
    Colloidal silica nanoparticles (NPs) modified with eight different silane coupling agents were incorporated into an amorphous poly(tetramethylene oxide)-based polyurethane-urea copolymer matrix at a concentration of 10 wt % (4.4 vol %) in order to investigate the effect of their surface chemistry on the structure-property behavior of the resulting nanocomposites. The rigid amorphous fraction (RAF) of the nanocomposite matrix as determined by differential scanning calorimetry and dynamic mechanical analysis was confirmed to vary significantly with the surface chemistry of the NPs and to be strongly correlated with the bulk mechanical properties in simple tension. Hence, nanocomposites with an RAF of about 30 wt % showed a 120% increase in Young's modulus, a 25% increase in tensile strength, a 15% decrease in elongation at break with respect to the neat matrix, which had no detectable RAF, whereas nanocomposites with an RAF of less than 5% showed a 60% increase in Young's modulus, a 10% increase in tensile strength and a 5% decrease in the elongation at break.
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    Entangled polymer dynamics in attractive nanocomposite melts
    (American Chemical Society (ACS), 2020) Şenses, Erkan; Tyagi, Madhu Sudan; Faraone, Antonio; Department of Chemical and Biological Engineering; N/A; Şenses, Erkan; Darvishi, Saeid; Faculty Member; PhD Student; Department of Chemical and Biological Engineering; College of Engineering; Graduate School of Sciences and Engineering; 280298; N/A
    We investigate single chain dynamics of an entangled linear poly(ethylene oxide) melt in the presence of well-dispersed attractive nanoparticles using high-resolution neutron spectroscopy at particle volume fractions as high as 0.53. The short-time dynamics shows a decrease of the Rouse rates with particle loading, yet the change remains within a factor of 2, with no evidence of segment immobilization as often hypothesized. The apparent reptation tube diameter shrinks by approximate to 10% from the bulk at a 0.28 particle volume fraction when the face-to-face interparticle distance approaches the single chain size. The tube diameter is remarkably concentration-independent at higher loadings where all chains are essentially bound to particle surfaces. These direct experimental observations on the microscopic chain dynamics in attractive nanocomposites are distinct from their nonattractive counterparts and account for some of the unusual dynamic behaviors of the nanoparticles as well as rheology in the composites.
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    Hydrophilic polyurethaneurea membranes: influence of soft block composition on the water vapor permeation rates
    (Elsevier Sci Ltd, 1999) Department of Chemistry; Department of Chemistry; Yılgör, İskender; Yılgör, Emel; Faculty Member; Researcher; Department of Chemistry; College of Sciences; College of Sciences; 24181; 40527
    High molecular weight segmented polyurethaneurea (PUU) copolymers based on an aliphatic diisocyanate, bis(4-isocyanatocyclohexyl)methane and mixed hydrophilic and hydrophobic soft segments were prepared. Hydrophilic blocks consisted of poly(ethyleneoxide) (PEO) of molecular weight 1450 g/mol, whereas the hydrophobic blocks were poly(tetramethylene oxide) of molecular weight 2000 g/mol. Ethylene diamine was used as the chain extender. Hard segment contents of the copolymers were kept constant at 18%, whereas PEO contents were varied between 0% and 50% by weight. Water vapor permeation rates (WVPR) of thin films (23-178 mu m) cast from dimethylformamide solutions were determined. In studies performed at 23 degrees C and 50% relative humidity, the relationship between PEO content and WVPR followed an S-shaped curve. For copolymers containing up to about 15% by weight of PEG, WVPR were fairly low. This was followed by a region where WVPR increased continuously for membranes containing between 15% and 30% PEG. Further increase in PEO content above 30% did not influence the WVPR substantially. There was also a dramatic increase in WVPR with an increase in temperature from 23 degrees C to 37 degrees C. Activation energy of permeation was determined to be 91.5 kJ for PUU containing 22.0% by weight of PEG. Equilibrium water absorption levels of PUU containing different levels of PEO in their backbone structures followed a similar trend to that of WVPR. Hydrophilic PUUs showed good tensile properties and mechanical integrity even at very high levels of water absorption.
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    Modification of polyolefins with silicone copolymers. I. processing behavior and surface characterization of PP and HDPE blended with silicone copolymers
    (John Wiley & Sons Inc, 2002) Süzer, Şefik; Department of Chemistry; Department of Chemistry; Yılgör, İskender; Yılgör, Emel; Faculty Member; Researcher; Department of Chemistry; College of Sciences; College of Sciences
    Permanent surface modification of polypropylene and high-density polyethylene was obtained by blending with small (0.1 to 5.0% by weight) amounts of silicone copolymers. A triblock polycaprolactone-b-polydimethylsiloxane copolymer and a multiblock polydimethylsiloxane-urea copolymer were used as modifiers. Blends were prepared in a twin-screw extruder. Influences of the type and amount of the additive on the processing behavior and surface and bulk properties of the resulting systems were investigated. During processing, the additives also acted as very efficient processing aids, increasing the extruder output dramatically, up to 200%. Surface characterization by water-contact angle measurements and X-ray photoelectron spectroscopy clearly showed the formation of silicone-rich surfaces even with very small amounts of additives, such as 0.1% by weight.