Papers by iskender yilgor
Turkish Journal of Chemistry
A comprehensive review of the structure-morphology-property relations in segmented thermoplastic ... more A comprehensive review of the structure-morphology-property relations in segmented thermoplastic polyurethanes and polyureas (TPU) is provided. Special emphasis is given to the influence of the soft segment structure, polarity, and molecular weight, diisocyanate symmetry and the nature, extent, and strength of hydrogen bonding on the morphology and thermal and mechanical properties of TPUs. Experimental results obtained on composition-dependent TPU morphology and properties by various techniques were also compared by the morphology profiles generated by computational methods such as quantum mechanical calculations and molecular dynamics simulations.
ACS Photonics, Apr 7, 2022
In the next decade, we will witness the replacement of a majority of conventional light sources w... more In the next decade, we will witness the replacement of a majority of conventional light sources with light-emitting diodes (LEDs). Efficient LEDs other than phosphors can enhance their functionality and meet different lighting needs. Quantum dots (QDs) have high potential for future LED technology due to their sensitive band-gap tuning via the quantum confinement effect and compositional control, high photoluminescence quantum yield (PLQY), and mass-production capacity. Herein, we demonstrate white LEDs using QDs that reach over 150 lumens per electrical Watt. For that we synthesized green-and red-emitting ZnCdSe/ ZnSe core/shell QDs by low-temperature nucleation, hightemperature shell formation, and postsynthetic trap-state removal. Their cadmium concentration is lower than 100 ppm, satisfying the current EU RoHS regulations, and their PLQY reaches a high level of 94%. The PLQY of QDs is maintained within the device on blue LED via liquid injection, and their integration at optimized optical densities leads to 129.6 and 170.4 lm/W for red-green-blue (RGB)-and green-blue (GB)-based white LEDs, respectively. Our simulations further showed that an efficiency level of over 230 lm/W is achievable using ultraefficient blue LED pumps. The simple fabrication and high performance of white LEDs using QD liquids show high promise for next-generation lighting devices.
Journal of Sol-Gel Science and Technology, Jun 18, 2013
In-situ liquid-state 29 Si nuclear magnetic resonance was used to investigate the temporal concen... more In-situ liquid-state 29 Si nuclear magnetic resonance was used to investigate the temporal concentration changes during ammonia-catalyzed initial hydrolysis of tetraethyl orthosilicate in different solvents (methanol, ethanol, n-propanol, iso-propanol and n-butanol). Dynamic light scattering was employed to monitor simultaneous changes in the average diameter of silica particles and atomic force microscopy was used to image the particles within this time frame. Solvent effects on initial hydrolysis kinetics, size and polydispersity of silica particles were discussed in terms of polarity and hydrogen-bonding characteristics of the solvents. Initial hydrolysis rate and average particle size increased with molecular weight of the primary alcohols. In comparison, lower hydrolysis rate and larger particle size were obtained in the secondary alcohol. Exceptionally, reactions in methanol exhibited the highest hydrolysis rate and the smallest particle size. Ultimately, our investigation elaborated, and hence confirmed, the influences of chemical structure and nature of the solvent on the formation and growth of the silica particles under applied conditions.
Nature food, May 18, 2023
Polymer, 2003
In continuing efforts to understand urea phase connectivity in flexible polyurethane foams and it... more In continuing efforts to understand urea phase connectivity in flexible polyurethane foams and its implications on physical properties, LiCl is used to alter the phase-separation behavior of slabstock foams. Comparisons are also drawn with plaque counterparts, which are prepared using the same polyol, isocyanate, and chain extender (water). LiCl is shown to alter the solid-state phase separation behavior of the foams and the plaques in a similar manner. This is confirmed using multiple characterization techniques, which provide information at different scale lengths. The foams and plaques with and without LiCl are shown to possess a microphase separated morphology with interdomain spacings of ca. 100 Å. SAXS and TEM reveal that addition of LiCl reduces the urea aggregation behavior, typical in slabstock polyurethane foams, leading to a loss in the urea phase macro connectivity. Hard segment ordering, as studied by WAXS and FTIR, is shown to be of a similar nature in the plaque and foam, which do not incorporate LiCl. Addition of LiCl leads to a loss in the segmental packing behavior, or micro level connectivity of the urea phase, in both the plaques and corresponding foams, as inferred from WAXS and FTIR. The LiCl additive interacts with the polyol soft segments in an insignificant manner as shown from FTIR and DMA. In addition, foams containing LiCl are found to possess more intact cell windows due to the influence of LiCl on reaction kinetics as well as its effect on the precipitation of the urea phase. The experimental observations are supported by quantum mechanical calculations using a density functional theory approach, where molecular interactions between LiCl and model ether, urethane, and urea compounds are investigated. Interaction geometries of most stable complexes and their stability energies are calculated. Stability energies of ether/LiCl, urethane/LiCl, and urea/LiCl were determined to be 2 189, 2 617, and 2687 kJ/mol, respectively, reinforcing that LiCl interacts predominantly with urea hard segments and in a minimal manner with the polyol soft segments.
Acs Symposium Series, Jun 8, 1983
Abstract : Novel elastomeric polysiloxane modifiers for epoxy networks have been synthesized and ... more Abstract : Novel elastomeric polysiloxane modifiers for epoxy networks have been synthesized and characterized. In addition, curing studies of conventional epoxy resins incorporating these oligomers have been conducted. Structures were prepared having either epoxide, primary amine and/or secondary amine endgroups. The polymerization step for the siloxanes consisted of a base catalyzed equilibration of the appropriately functional disiloxane and octamethylcyclotetrasiloxane. In the case of hydroxy piperazine terminated modifiers, polymers were first synthesized with epoxy endgroups. These endgroups were then subsequently capped with an excess of piperazine prior to curing. The oligomers were characterized by FTIR, 'H and 13C NMR, GPC, endgroup analysis and vapor pressure osmometry. The secondary amine terminated oligomers basically act as linear modifiers. By contrast, the aminopropyl functional siloxanes produce a crosslinked network. Piperazine terminated oligomers show much better compatibility with the epoxy resins compared to the aminopropyl terminated oligomers.
Acta Otorhinolaryngologica Italica, Apr 1, 2016
In vivo tissue response and durability of five novel synthetic polymers in a rabbit model Biocomp... more In vivo tissue response and durability of five novel synthetic polymers in a rabbit model Biocompatibilità e durata in vivo di cinque nuovi polimeri sintetici testati su coniglio
Macromolecules, May 11, 2021
Achieving a unique combination of stiffness, strength, extensibility and toughness in sol-cast po... more Achieving a unique combination of stiffness, strength, extensibility and toughness in sol-cast poly(urethane-urea) (PU) copolymer films is a challenge since these properties are-in general-mutually exclusive. Here we demonstrate that geometric confinement of the basic building blocks controls stiffness, strength, extensibility and toughness in PU films. Our results suggest that the severity of geometric confinement can be tuned by adjusting (i) soft segment molecular weight (SSMW) and (ii) drying temperature (DT) thanks to their effects on the structure formation via micro-phase separation and/or (confined and/or bulk) crystallization. It is therefore possible to produce (i) soft (no notable confinement) and (ii) stiff, strong, extensible and tough (severe confinement) materials without changing any other parameter except SSMW and DT. The former has a typical physically cross-linked network and shows a well-defined elastomeric behavior with elastic modulus (E) of 5-20 MPa, tensile strength (smax) of 30-35 MPa, extensibility (e) of 1000-1300% and toughness (W) of 90-180 MJm-3. The latter, on the other hand, possesses an elegant hierarchical structure containing tightly packed secondary structures (72-helix, 41-helix and antiparallel b-sheets) and displays an elasto-plastic behavior with E of 400-700 MPa, smax of 45-55 MPa, e of 650-850% and W of 200-250 MJm-3. Hence, our findings may be of interest in designing advanced materials containing synthetic replica of the secondary structures found in protein based materials. The structure formation in the materials with this structural hierarchy is driven by the confined crystallization of helical poly(ethylene oxide) (PEO) chains in subnanometer urea channels, which-to the best of our knowledge-is a new phenomenon has not yet reported in PU literature, and complemented by the "bulk" crystallization of PEO and/or the micro-phase separation. This has enabled us to demonstrate that geometric confinement of the basic building blocks along the multiple length scales controls stiffness, strength, extensibility and toughness in sol-cast PU films. Our findings suggest that the severity of geometric confinement in sol-cast PU films can be tuned by controlling soft segment molecular weight and drying temperature since these two parameters are of great importance for the structure formation through micro-phase separation and/or (confined and/or bulk) crystallization. This, in return, can be utilized to create entirely different morphologies and hence achieve significant variations in mechanical properties without changing any other parameter. To demonstrate, solcast PU films w/o severe geometric confinement are prepared and comprehensively investigated. The films with severe confinement possess a structural hierarchy spanning length scales from angstroms to micrometers, leading to unique combinations of stiffness, strength, extensibility and toughness. Our findings may also be of interest in mimicking certain protein structures containing tightly packed natural design motifs like helices and anti-parallel b-sheets as the structural hierarchy demonstrated here consists of these secondary structures. The structure formation in these films is governed by the confined crystallization of helical poly(ethylene oxide) (PEO) chains in subnanometer urea channels and complemented by the "bulk" crystallization of PEO and the micro-phase separation. The confined crystallization of PEO in subnanometer urea channels is indeed known to create host-guest (inclusion) complexes. However, to the best of our knowledge, it has not yet been demonstrated in PU copolymers. We therefore also show how the a form of these complex crystals is formed and what kind of impacts it has on the structure-property relations of the PU films. 2. EXPERIMENTAL SECTION 2.1. Materials PEO oligomers with number average molar masses, Mn of 2000, 4600 and 8000 g.mol-1 were obtained from Merck. Bis(4isocyanatocyclohexyl)methane (HMDI) with a purity of >99.5% was kindly provided by Bayer. 2-Methyl-1,5diaminopentane (MDAP) chain extender was kindly supplied by DuPont. Dibutyltin dilaurate (DBTDL) catalyst was purchased from Witco. Reagent grade tetrahydrofuran (THF) and dimethylformamide (DMF) were also obtained from Merck. All chemicals were used as received. 2.2. Synthesis The hard segment model compounds (HMDI-MDAP) with molecular weights similar to those of the HSs in the copolymers were prepared as control samples and characterized as given in Figure S1 and S2. The copolymers were synthesized by the two-step polymerization technique, also known as the prepolymer method. The detailed procedure followed was previously reported by our group. 28-32 All reactions were performed in 500 mL threenecked round-bottom Pyrex reaction flasks equipped with a mechanical overhead stirrer, a thermometer and a funnel, with
Twhmcorrespondence should be directed-~ ~ o_ modified materials to be predominantly siloxane even... more Twhmcorrespondence should be directed-~ ~ o_ modified materials to be predominantly siloxane even at quite low bulk levels of the reactive oligomers. Preliminary mechanical ,-property studies are encouraging and detailed morphology-property Lstudies are continuing.
ACS applied polymer materials, Oct 8, 2021
Macromolecules, Apr 1, 1994
The depletion of alkylamine terminal groups at the vacuum-polymer interface is measured for a,@-d... more The depletion of alkylamine terminal groups at the vacuum-polymer interface is measured for a,@-difunctional poly(dimethylsi1oxane) oligomers by X-ray photoelectron spectroscopy. The driving force for this depletion is the high relative surface energy of the amine terminal groups compared to that of the low surface energy poly(dimethylsi1oxane) backbone. The degree of surface end group depletion, within the maximum sampling depth probed (ca. 7 nm), is found to be on the order of 40% for a 960 molecular weight oligomer and decreases slightly with an increase in the oligomer molecular weight. Angle-dependent measurements are applied to determine end-group concentration depth profiles. End-group depletion is largest at the shallowest sampling depths and decays rapidly toward the bulk. The decay profiles cannot be explained by simple monotonic decay functions, consistent with the expected effects of connectivity between the end groups and the chain backbone, but the data are insufficient to prove whether the profiles are oscillatory in nature, as expected from theoretical considerations.
The objective of this work wos to synthesize polyarylether materials and related siloxane copolym... more The objective of this work wos to synthesize polyarylether materials and related siloxane copolymers. A second investigation was to characterize their structure, mechanical behavior and to initiate radiation degradation studies by both particulate radiation as well as UV radiation. The technical approach was to utilize aromatic nucleophiiic displacement reactions to produce highly aromatic polysulfones and related structures. Functional oligomers were also utilized to prepared well-defined siloxane copolymers. The grant number for this research was NASA Research Cooperative Agreement NCC1-63. The grant duration was 12 months. In addition to the principal investigator, other research scientists involved on this project;
Journal of polymer science, Mar 1, 1984
The synthesis of bisphenol A poly(carbonate-ester) copolymers was studied by phase-transfer catal... more The synthesis of bisphenol A poly(carbonate-ester) copolymers was studied by phase-transfer catalysis and modified interfacial polymerization. Only low molecular weight copolymers were prepared directly from dicarboxylic acids, phosgene, and bisphenol A by an interfacial process that involves the use of pyridine as catalyst, HCl acceptor, and weak base. To avoid the use of tertiary amines, which can be difficult to remove from the polymer products, and to produce higher molecular weight copolymers from the same dicarboxylic acid precursors another synthetic method was developed. This more effective method required careful pH control that was achieved by the selective use of the weak-base potassium carbonate in the first stage of the process. Moreover, elevated reaction temperatures (-65-7OOC) and phase-transfer catalysis were used. The carbonate-ester copolymers prepared by this technique had consistently high intrinsic viscosities, little or no anhydride microstructure, and higher degrees of ester unit incorporation than those produced by the pyridine-catalyzed method. These copolymers also had glass transition temperatures (T,) 20-30°C higher than bisphenol A polycarbonate homopolymer. An analytical method for determining quantitatively the amount of ester units in the bisphenol A poly(carbonate-esters) was developed by using Fourier transform infrared spectroscopy (FT-IR). Agreement between this FT-IR method and a quantitative nuclear magnetic resonance (NMR) method was found to be reasonable, especially for copolymers with ester unit percentages lower than 40%.
Macromolecular Chemistry and Physics, Mar 24, 2006
Polymer, Apr 1, 2019
Degumming of Bombyx mori silk cocoons by a novel and mild process using aqueous ammonia and fabri... more Degumming of Bombyx mori silk cocoons by a novel and mild process using aqueous ammonia and fabrication of electrospun polycaprolactone/silk fibroin (PCL/SF) nanofibrous scaffolds is reported. Cocoons were degummed in 0.3% w/w solutions of boiling ammonia (28-30%) for 45 minutes. Degummed SF fibers were dissolved in phosphoric and formic acid (7/3 v/v) mixture, coagulated in methanol, filtered and dried. PCL solutions containing different amounts of SF were electrospun in formic acid, a green solvent. Scaffolds were characterized to confirm the successful incorporation of SF and to demonstrate formation of nanofibrous webs with good biomechanical properties. Cell viability assay was performed by seeding Human BJ fibroblast cells on scaffolds. In vitro analysis showed that the scaffolds produced were non-toxic and incorporation of SF resulted in enhanced cell proliferation. Nanofibrous PCL/SF scaffolds with good biomechanical properties developed through dialysis free processing of silk fibroin can be promising substrates for tissue engineering applications.
Polymer Bulletin, 1985
Bisphenol A-based epoxy resins were modified with either phenolic hydroxyl or aromatic amine func... more Bisphenol A-based epoxy resins were modified with either phenolic hydroxyl or aromatic amine functionally-terminated poly(arylene ether sulphone) oligomers and thermally cured with 4,4' diaminodiphenyl sulphone. The resulting networks displayed significantly improved fracture toughness, with little sacrifice in modulus. The bisphenol A-based polysulphones were molecularly miscible with the epoxy precursors over the entire range of compositions and molecular weights investigated, but developed a two phase structure upon network formation. The molecular weights and composition of polysulphone chemically incorporated into the network were varied and their effect on several important physical properties was investigated. The dynamic mechanical analysis and scanning electron microscopy (SEM) studies showed that it is possible to generate a two-phase morphology in the cured networks wherein polysulphone composite particles are dispersed in the epoxy matrix. Despite the two-phase structure, the modified crosslinked systems are nearly transparent, due to a similarity in component refractive index values. The fracture toughness of these modified networks under plane strain conditions improved significantly with minimal sacrifice of the flexural modulus.
Progress in Organic Coatings, Dec 1, 2018
Microporous poly(lactic acid) (PLA) coatings with fairly homogeneous pore size and distribution w... more Microporous poly(lactic acid) (PLA) coatings with fairly homogeneous pore size and distribution were obtained through spin-coating process. Critical effects of the thermal conductivity of the substrate (copper, glass, insulated glass), solvent type (dichloromethane, chloroform and tetrahydrofuran), polymer concentration (3-15% by weight) and relative humidity of the environment (0-85% relative humidity) on the formation and topography of microporous surfaces were investigated. Characterization of the surfaces was achieved via scanning electron microscopy, white light interferometry and static water contact angle measurements. By tuning the production parameters, PLA coatings displaying microporous surfaces with uniform pore sizes and distribution were obtained. Pore sizes varied from 1 to 7 μm depending on the experimental conditions.
Journal of Polymer Science Part B, Oct 31, 2017
Hydrogen bonding between hard segments has a critical effect on the morphology and properties of ... more Hydrogen bonding between hard segments has a critical effect on the morphology and properties of polyurethanes. Influence of temperature on hydrogen bonded urethane network and melting behavior of a model semicrystalline segmented polyurethane was investigated by experiments and simulations. Polyurethane was synthesized by the stoichiometric reaction between p-phenylene diisocyanate and poly(tetramethylene oxide) (PTMO) with a molecular weight of 1000 g/ mol. Simulations were carried out using dissipative particle dynamics (DPD) and molecular dynamics (MD) approaches. Experimental melting behavior obtained by various techniques was compared with simulations. DPD simulations showed a room temperature microphase morphology consisting of a three-dimensional hydrogen-bonded urethane hard segment network in a continuous and amorphous PTMO matrix. The first-order melting transitions of crystalline urethane hard segments observed during the continuous isobaric heating in DPD and MD simulations (340-360 K) were in reasonably good agreement with those observed experimentally, such as AFM (320-340 K), WAXS (330-360 K), and FTIR (320-350 K) measurements. Quantitative verification of the melting of urethane hard segments was demonstrated by sharp discontinuities in energy versus temperature plots obtained by MD simulations due to substantial decrease in the number of hydrogen bonds above 340 K. V
Macromolecules, Jun 1, 1993
... Claire Jalbert and Jeffrey T. Koberstein' Institute of Materials Science and Dep... more ... Claire Jalbert and Jeffrey T. Koberstein' Institute of Materials Science and Department of Chemical Engineering, University of Connecticut, Storrs, Connecticut 06269 ... and Pose+J developed a theory for the surface tension of polymer melts based upon the Cahn-Hilliard theory of ...
Polymer Composites, Apr 1, 1983
Synthesis and free-radical curing reactions of the bis-glycidylmethacrylate of bisphenol-A (Bis-G... more Synthesis and free-radical curing reactions of the bis-glycidylmethacrylate of bisphenol-A (Bis-GMA) were investigated. Bis-GMA resin was synthesized by reacting Epon 825 with methacrylic acid, followed by characterization using IR and NMR spectroscopy. Various modifiers having reactive double bonds were co-cured with the methacryloxy resins, using a free radical initiator. The networks obtained were compared with a cycloaliphatic amine cured epoxy network. Thermal characterization shows that methacryloxy cured systems are more resistant to mechanical penetration and have higher glass transition temperatures and better stability to thermal decomposition as compared with the conventional diamine cured epoxy networks. Dynamic mechanical experiments and stress-strain tests also indicate that Bis-GMA based networks have higher tensile moduli and lower elongation at break. Using different modifiers such as 2-ethylhexyl acrylate, the tensile and impact properties of the networks can be improved.
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Papers by iskender yilgor