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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only 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; 110357One-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.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 Electroadhesion with application to touchscreens(Royal Soc Chemistry, 2019) Ayyıldız, Mehmet; Persson, Bo N. J.; N/A; Department of Mechanical Engineering; Şirin, Ömer; Başdoğan, Çağatay; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 125489There is growing interest in touchscreens displaying tactile feedback due to their tremendous potential in consumer electronics. In these systems, the friction between the user's fingerpad and the surface of the touchscreen is modulated to display tactile effects. One of the promising techniques used in this regard is electrostatic actuation. If, for example, an alternating voltage is applied to the conductive layer of a surface capacitive touchscreen, an attractive electrostatic force is generated between the finger and the surface, which results in an increase in frictional forces acting on the finger moving on the surface. By altering the amplitude, frequency, and waveform of this signal, a rich set of tactile effects can be generated on the touchscreen. Despite the ease of implementation and its powerful effect on our tactile sensation, the contact mechanics leading to an increase in friction due to electroadhesion has not been fully understood yet. In this paper, we present experimental results for how the friction between a finger and a touchscreen depends on the electrostatic attraction and the applied normal pressure. The dependency of the finger-touchscreen interaction on the applied voltage and on several other parameters is also investigated using a mean field theory based on multiscale contact mechanics. We present detailed theoretical analysis of how the area of real contact and the friction force depend on contact parameters, and show that it is possible to further augment the friction force, and hence the tactile feedback displayed to the user by carefully choosing those parameters.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 Dynamic pressure control in vartm: rapid fabrication of laminates with high fiber volume fraction and improved dimensional uniformity(Wiley, 2019) Altan, M. Cengiz; Department of Mechanical Engineering; Department of Mechanical Engineering; Yalçınkaya, Mehmet Akif; Sözer, Murat; Master Student; Faculty Member; Department of Mechanical Engineering; College of Engineering; College of Engineering; N/A; 110357The compaction pressure on the fibrous preform is one of the most critical parameters in vacuum assisted resin transfer molding (VARTM), which significantly affects the preform permeability, mold filling time, and final thickness of the fabricated composite. In this study, the compaction pressure on the vacuum bag was controlled during and after the mold filling to achieve rapid impregnation and improve the fiber volume fraction of the laminate. It was shown that the dynamic pressure control (1) enabled the manipulation of the fabric permeability and faster distribution of the resin to decrease the mold filling time, (2) improved the dimensional uniformity of the laminate by reducing the thickness variation, and (3) increased the fiber volume fraction by further consolidating the preform and removing the excess resin. One of the most essential and prominent features of the process was shown to be the resin removal from the inlet by applying external pressure, which reduced the thickness variation in laminates from 15 to 1%. The mold filling time was reduced by 48% compared with conventional VARTM, while achieving a high fiber volume fraction up to 64% and a low void content of below 1%. POLYM. COMPOS., 40:2482-2494, 2019.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.