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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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    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.
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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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    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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    Time-dependent morphology development in a segmented polyurethane with monodisperse hard segments based on 1,4-phenylene diisocyanate
    (American Chemical Society (ACS), 2005) Sheth, Jignes P.; Klinedinst, Derek B.; Pechar, Todd W.; Wilkes, Garth L.; Department of Chemistry; Department of Chemistry; Yılgör, Emel; Yılgör, İskender; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 40527; 24181
    The time-dependent morphology development in a segmented polyurethane, which was prepared by the reaction of equimolar amounts of 1,4-phenylene diisocyanate (pPDI) and poly(tetramethylene oxide)glycol of (M.) of 975 g/mol, was investigated. No chain extender was utilized during the synthesis, and the resultant monodisperse hard segments constituted 14 wt % of the copolymer. Time-dependent microphase separation and morphology development was studied at room temperature by using solvent-cast films which were heated above the hard segment melting temperature, 55 degrees C, to erase the semicrystalline microphase morphology. Atomic force microscopy showed that, following heat treatment, the hard phase first developed into short rods within 30 min, followed by a growth period during which the short rods grew longer and eventually into a well-defined percolated structure. Morphology development was also followed by FTIR spectroscopy. While the intensity of the free C=O peak at 1730 cm(-1) decreased, the intensity of the hydrogen-bonded C=O peak at 1695 cm-1, which was not present in the original annealed sample, increased with time and began to plateau in similar to 24 h. A time-dependent increase in the storage modulus of the copolymer, following heat treatment, was also noted. This latter change could be described by the Avrami equation, yielding an Avrami exponent of 0.55. Because of the similarity of the copolymer's morphology to that of short fiber reinforced polymer composites, selected models developed for predicting the modulus of such composites could reasonably estimate the initially surprisingly high ambient temperature storage modulus of the copolymer of 0.9 x 10(8) Pa.
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    Understanding the structure development in hyperbranched polymers prepared by oligomeric A2+B3 approach: comparison of experimental results and simulations
    (2005) Ünal, Serkan; Oğuz, Cihan; Gallivan, M; Long, Timothy; Department of Chemistry; Department of Chemistry; Yılgör, Emel; Yılgör, İskender; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; 40527; 24181
    Structure development in highly branched segmented polyurethaneureas based on oligomeric A2+B3 approach was investigated by experimental studies and kinetic Monte-Carlo simulations. In both simulations and experiments, hyperbranched polymers were produced by the slow addition of A2 onto B3. Experimental studies showed strong influence of solution concentration on the gel point and the extent of cyclization in the polymers formed. In polymerizations conducted at a solution concentration of 25% by weight gelation took place at the stoichiometric ratio [A2]/[B3]=0.886. This is somewhat higher than the theoretical ratio of 0.75. In very dilute solutions, such as 5% solids by weight, no gelation was observed although the stoichiometric amount of A2 added well exceeded the theoretical amount for gelation. Both experimental studies by size exclusion chromatography (SEC) and kinetic Monte-Carlo simulations demonstrated a gradual increase in polymer molecular weights as more A2 is added onto B3. This was followed by a sharp increase in the polymer molecular weight as the gel point is approached. A very similar behavior was observed for the polydispersity values of the polymers formed. Kinetic Monte-Carlo simulations performed at different cyclization ratios showed very good agreement with experimental results. 
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    Probing the hard segment phase connectivity and percolation in model segmented poly(urethane urea) copolymers
    (American Chemical Society (ACS), 2005) Sheth, Jignes P.; Wilkes, Garth L.; Fornof, Ann R.; Long, Timothy E.; Department of Chemistry; Yılgör, İskender; Faculty Member; Department of Chemistry; College of Sciences; 24181
    Soluble model segmented poly(urethane urea)s (PUU) with or without hard segment (HS) branching were utilized to explore the importance of hydrogen bonding and chain architecture in mediating the long-range connectivity of the HS phase. The HS content of all the PUU copolymers was 22 wt %, and the soft segment (MW 970 g/mol) was a heterofed random copolymer of 50:50 ethylene oxide:propylene oxide, which possesses a single terminal hydroxyl group (monol). An 80:20 isomeric mixture of 2,4- and 2,6-toluene diisocyanate, 4,4',4"-triphenylmethane triisocyanate and water were utilized during the chain extension step of the synthesis to incorporate HS branching. DSC and SAXS results on the final plaques indicated that the samples were still able to establish a microphase morphology even in the presence of the highest extent of HS branching utilized in the study. The tapping-mode AFM phase image of the PUU sample without HS branching exhibited the presence of long ribbonlike hard domains that percolated through the soft matrix. The long-range connectivity of the HS was increasingly disrupted with higher levels of HS branching. Accompanying such disruption was a systematic mechanical softening of the PUU samples. FT-IR indicated that incorporation of HS branching disrupted the hydrogen-bonded network within the hard phase. These results demonstrate the importance of hydrogen bonding and chain architecture in mediating the long-range connectivity and percolation of the HS and achieving dimensional stability.
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    Synthesis and characterization of block-graft copolymers [poly(epichlorohydrin-b-styrene)-g-poly(methyl methacrylate)] by combination of activated monomer polymerization, nmp and atrp
    (Springer, 2007) Taşdelen, M. Atilla; Biedron, Tadeusz; Kubisa, Przemyslaw; Department of Chemistry; Department of Chemistry; Demirel, Adem Levent; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; N/A; 6568
    Synthesis of block-graft copolymers, namely poly(epichlorohydrin-b-styrene)-g-poly(methyl methacrylate) (PECH-b-St)-g-PMMA) by combination of Activated Monomer (AM), Nitroxide Mediated Polymerization (NMP) and Atom Transfer Radical Polymerization (ATRP) methods was described. For this purpose, first epichlorohydrin (ECH) was polymerized by using BF3THF complex in the presence of 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyl-1-oxy (HTEMPO) via AM mechanism. The resulting stable radical terminated polymer was subsequently used as a counter radical in the NMP of styrene (St) initiated by benzoyl peroxide to yield block copolymers of ECH and St. Finally, the ATRP of methyl methacrylate (MMA) by using chloromethyl groups of the PECH segment as initiating sites resulted in the formation of (PECH-b-St)-g-PMMA). The structures of the intermediate polymers at various stages were characterized by H-1-NMR spectral investigations. The thermal behavior and surface morphology of the copolymers were also investigated by DSC and AFM measurements.
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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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    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.