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Publication Metadata only 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 SciencesPermanent 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.Publication Metadata only Facile preparation of superhydrophobic polymer surfaces(Elsevier Sci Ltd, 2012) Department of Chemistry; N/A; N/A; Department of Chemistry; Yılgör, İskender; Bilgin, Sevilay; Işık, Mehmet; Yılgör, Emel; Faculty Member; PhD Student; Researcher; Researcher; Department of Chemistry; College of Sciences; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Sciences; 24181; N/A; N/A; N/AA simple and general method has been developed for the preparation of polymeric materials with superhydrophobic surfaces. The process is applicable to a large number of polymers, thermoplastic or thermoset. In this manuscript preparation and characteristics of superhydrophobic surfaces prepared from a segmented polydimethylsiloxane-urea copolymer (TPSU), a polyether based polyurethaneurea (TPU), poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC) and a crosslinked epoxy resin (EPOXY) are discussed. All samples were prepared onto glass surfaces by using a simple, multi-step spin-coating procedure. In the first step a thin film of the desired polymer was coated onto the glass slide. This was followed by spin-coating of two layers of hydrophobic fumed silica using a dispersion in tetrahydrofuran. Finally to obtain a durable surface, a very thin film of the parent polymer was spincoated from a very dilute solution containing 2.5% by weight hydrophobic silica and 0.25% by weight matrix polymer in tetrahydrofuran. Surfaces were characterized by scanning electron microscopy (SEM), which clearly showed the formation of rough surfaces with homogeneously distributed silica particles in 1-10 mu m range. Static water contact angle and contact angle hysteresis measurements proved the formation of superhydrophobic surfaces. Samples displayed static water contact angles larger than 170 degrees and very low contact angle hysteresis of less than 3 degrees.Publication Metadata only Electronic excitations in stacked oligothiophenes(Elsevier, 2001) Yurtsever, Mine; Department of Chemistry; Yurtsever, İsmail Ersin; Faculty Member; Department of Chemistry; College of Sciences; 7129Electronic spectroscopy of oligothiophenes is studied using the semi-empirical ZINDO methodology. Geometry of the neutral, and doubly charged bipolaronic forms of oligomers from 3T to 8T are optimized. Their coordinates are then used to generate various stacked structures up to tetramers. The effects of the length of the chains, number of layers in stacks, stacking distance between chains, deviation from planarity and number and relative positions of bipolaronic forms on the shapes of the electronic excitations are investigated.Publication Metadata only Hydrogen bonding and polyurethane morphology. I. quantum mechanical calculations of hydrogen bond energies and vibrational spectroscopy of model compounds(Elsevier Sci Ltd, 2002) Department of Chemistry; Department of Chemistry; Department of Chemistry; Yılgör, Emel; Yılgör, İskender; Yurtsever, İsmail Ersin; Researcher; Faculty Member; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; College of Sciences; N/A; 24181; 7129Advanced quantum mechanical calculations within ab initio molecular orbital theory and density functional theory were performed using GAUSSIAN98 programs in quantitative determination of hydrogen bond (H-bond) energies between various model compound pairs. Model compounds studied contained functional groups or segments that were similar to those in segmented polyurethanes and polyureas. These model compounds included urea, 1,3-dimethylurea, 1,3-dimethylcarbamate, diethyl ether, methyl acetate and ethyl alcohol. Optimized conformations, H-bond energies and H-bond lengths of the complexes were determined. Quantum mechanical calculations indicated that based on relative magnitudes of H-bond energies, appreciable amount of phase mixing between hard and soft segments in polyether or polyester based polyurethanes and polyureas should be expected. Vibrational spectra of individual compounds and their hydrogen-bonded complexes (with themselves and other compounds) were determined. Correlation between theoretical and experimental spectra was found to be very good.Publication Metadata only 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; 7129Accurate 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.Publication Metadata only 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; 40527High 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.Publication Metadata only Hydrogen bonding: a critical parameter in designing silicone copolymers(Elsevier Sci Ltd, 2001) Department of Chemistry; Department of Chemistry; Yılgör, Emel; Yılgör, İskender; Researcher; Faculty Member; Department of Chemistry; College of Sciences; College of Sciences; N/A; 24181Structure-property relations in polydimethylsiloxane (PDMS) containing segmented copolymers with model hard segments capable of forming hydrogen bonding, such as urea, N-methylurea and urethane have been investigated. High molecular weight silicone containing copolymers with these hard segments were prepared from PDMS oligomers with number average molecular weights ranging from 890 to 3750 g/mol. Due to major differences in the solubility parameters between PDMS and polar hard segments, all copolymers are expected to display good microphase separation. It was demonstrated that mechanical and thermal properties of these copolymers are directly linked to the strength of the hydrogen bonding in the hard segments. As expected, siloxane-urea copolymers displayed much higher tensile strengths when compared with siloxane-N-methylurea and siloxane-urethane copolymers with similar compositions.Publication Metadata only Simple processes for the preparation of superhydrophobic polymer surfaces(Elsevier Sci Ltd, 2016) 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; 24181Two simple processes; (i) spin-coating, and (ii) doctor blade coating of silica/polymer dispersions are described for the preparation of superhydrophobic polymer surfaces. To demonstrate the versatility and broad applicability of the processes, polymeric surfaces modified included a thermoplastic resin, polystyrene (PS) and a thermoset, crosslinked epoxy resin (ER). Micro/nano hierarchical nature of the surface topographies obtained were demonstrated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and white light interferometry (WLI) studies. Roughness factor (r) and average surface roughness (R-a) values, which are critical in obtaining superhydrophobic surfaces were determined for each polymeric system. It was clearly demonstrated that increased (r) and (R-a) values resulted in superhydrophobic behavior with very high static, advancing and receding water contact angles, well above 150 degrees and contact angle hysteresis values of less than 10 degrees. Incorporation of small amounts (1.0% by weight) of a silicone copolymer or a perfluoroether glycol oligomer reduced the contact angle hysteresis in the epoxy resin system well below 10 degrees and produced truly superhydrophobic surfaces.Publication Metadata only Structure-property relationships and melt rheology of segmented, non-chain extended polyureas: effect of soft segment molecular weight(Elsevier Sci Ltd, 2007) Das, Sudipto; Inci, Bora; Tezgel, Ozgul; Beyer, Frederick L.; Wilkes, Garth L.; Department of Chemistry; Department of Chemistry; Yılgör, İskender; Yılgör, Emel; Faculty Member; Researcher; Department of Chemistry; College of Sciences; College of Sciences; 24181; N/ANovel, segmented non-chain extended polyureas were synthesized. Soft segments (SS) were based on poly(tetramethylene glycol) (PTMO) (average molecular weight 1000 or 2000 g/mol) and hard segments (HS) were based on a single molecule of a diisocyanate, which was either 1,6-hexamethylene diisocyanate (HDI), 1,4-phenylene diisocyanate (pPDI) or 1,4-trans-cyclohexyl diisocyanate (CHDI). An increase in the SS molecular weight was found to lead to an increased formation of SS crystallites below 0 degrees C, which increased the low temperature modulus. Both 1K and 2K PTMO-based polyureas showed a microphase separated morphology, where the HS formed thread-like, crystalline structures that were dispersed in the continuous SS matrix. Upon deformation, the HS were found to breakdown into distinctly smaller threads, which oriented along the direction of the strain; this effect was found to be partially reversible and time dependent. Both the 1K and 2K polyureas based on HDI HS were found to be thermally stable and potentially melt-processible.Publication Metadata only 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; 24181The 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.