Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis An In-Depth Study of Nucleation and Growth Processes During Stöber Silica Synthesis(Izmir Institute of Technology, 2019) Sop, Elif Suna; Polat, MehmetSilica nanoparticles (SNPs) which can be synthesized with high surface area, controllable morphology and desired particle size have gained significant interests in high-end applications such as catalysis, chemical sensors, cosmetics and drug delivery applications. The sol-gel technique is the most commonly applied method for manufacturing these particles owing to its simplicity and suitability for allowing surface modifications to the final product. Though monodisperse amorphous SNPs have been studied extensively, how their formation proceeds through nucleation and growth is still a topic of debate. Over the years, a number of mathematical models have been suggested for the nucleation and growth of SNPs; some suggesting that silica growth occurred through monomer addition while some arguing that aggregation of nuclei/subparticles were the dominant mechanism. Nevertheless, a clear understanding of the nucleation and growth sub-processes is extremely important in control on the size and shape of SNPs for those industrial applications which demand specific morphology and surface properties. The need for a simple, robust and generalized model, both conceptually and mathematically, to understand formation and growth of Stöber silica particles has been the main driving force for this thesis. In this study, silica synthesis was carried out under a wide variety of experimental conditions while determining the size distributions of the formed particles kinetically during different stages of the synthesis in-situ through SEM analysis using an image analysis software. The outcome of the extensive synthesis work was to obtain a clear understanding of how the formation and growth of the silica particles proceed during synthesis. This conceptual understanding of the nucleation and growth processes was then translated into a mathematical model to predict the size of the particles as a function of synthesis time.Doctoral Thesis Experimental and Computational Investigation of Transport Phenomena in Initiated Chemical Vapor Deposition (icvd) Process(Izmir Institute of Technology, 2017) Ateş, Selcan; Ebil, Özgenç; Ebil, ÖzgençAs a polymer thin-film deposition technique, initiated CVD (iCVD) is a heterogeneous process involving gas phase precursors and solid film formation on a solid/liquid substrates at different temperature regions. Obtaining fine-tuned film properties over different substrate geometries at different process conditions is a challenging tasks and requires experimental trials. The major goal of this study is to develop a computational model which describes all relevant transport phenomena occurring in iCVD process, and which is capable to predict the polymer film thickness at different deposition conditions for flat and/or non-flat substrates in a 3D reactor geometry. A Finite Element Analysis (FEA)-based 3D computational model, which can be applied to a variety number of iCVD reactor and substrate geometries, has been developed in the study. To validate the model, reported experimental conditions of 1H,1H,2H,2Hperfluorodecyl acrylate (PFDA) deposition with t-butyl peroxide (TBPO) initiator, and butyl acrylate (BA) deposition with t-amyl peroxide (TAPO) initiator, are applied to the model, respectively. The simulation results of both deposition processes show good agreement with experimental results reported in literature. Presented model successfully describes the relevant transport phenomena, and provides a priori predictions on polymerization rate, and film thickness on complex substrate geometries for a polymerization reaction with known kinetic data. For further studies, presented model can be modified or used as an approach for modeling of other types of CVD systems as well as facilitating process scale-up. The model can also extract valuable polymerization kinetics data provided that a sufficient number of experiments are performed at a specified substrate temperature, and process parameters and measured final film thicknesses are entered to the model.Doctoral Thesis Macromolecular Design of Hydroxyl Functional Linear and Star-Shaped L-Lactide and ?-Caprolactone Biodegradable Polyesters Utilizing Biosafe Catalysts for Biomedical Applications(Izmir Institute of Technology, 2017) Başalp, Dildare; Tıhmınlıoğlu, FundaIn the present study, macromolecular design of homo and copolymers of lactide (LA) and ε-caprolactone (CL) in different structures by the use of biocompatible catalysts and co-initiators were performed to satisfy a need of tailor-made bioassimilable polymeric structures without any hazardous metal contaminants for various medical applications. Linear and star shaped (di, tetra and hexa functional) poly(L-lactide) (PLLA) and poly(ε-caprolactone) (PCL) homo/copolymers were synthesized by using bismuth(III)acetate (Bi(III)Ac) and creatinine as biosafe catalysts and ethylene glycol, pentaerythritol and myo-inositol as co-initiators. The effect of catalyst type on polymer properties was observed by differences in crystalline structure. Crystalline and amorphous linear and star shaped PLLAs were obtained by using Bi(III)Ac and creatinine as catalysts, respectively. The activity of creatinine was very low comparing to Bi(III)Ac and SnOct2 catalysts. The reactivity of LA monomer was found to be higher than that of CL monomer. The high molecular weight polymers having low PDI values were obtained by using Bi(III)Ac catalyst contary to creatinine catalyst. The decrease in glass transition temperatures and molecular weights of synthesized PLLA and PCL homo/copolymers were observed with the increase in amount of co-initiators due to the decrease in chain length and disruption of crystal formation. The cytotoxicity properties of the catalysts and synthesized linear and functional homo/co PLLAs and PCLs were carried out according to MTT assay. Cytotoxicity of Bi(III)Ac was found as lower than that of SnOct2. Creatinine and the synthesized polymers did not show any cytotoxic properties. The observation of no cytotoxic effect of creatinine catalyst results in the biosafe usage of creatinine catalyst instead of toxic SnOct2 for the synthesis of moderate or low molecular weight homo/co PLLAs and PCLs in bioapplications.Doctoral Thesis Development of a Novel Hybrid Process for the Conversion of Cellulose Into High-Value Chemicals by Applying Voltage in Hot Compressed Water(Izmir Institute of Technology, 2017) Akın, Okan; Yüksel Özşen, AslıIn this study, a novel hybrid method of hydrothermal electrolysis implemented for the decomposition of microcrystalline cellulose (MCC) into high value added chemicals such as levulinic acid, 5-hydroxymethylfurfural (5-HMF), and furfural. The hypothesis of the study was that, when direct current (DC) is applied the formation of ionic and radical species can alter the hydrolysis of cellulose. Based on this hypothesis, the purpose of the study was to build an integrated method of hydrothermal electrolysis that can lower energy requirement of cellulose hydrolysis by altering the selectivity. In order to investigate the individual and coupled effect of operating parameters such as reaction temperature (170-200 ℃), time (30-120 min.), electrolyte concentration (1-50 mM H2SO4), constant current (0-2 A), statistical analysis was conducted by a fractional factorial design. Analysis of variance (ANOVA) test was applied to the main hydrolysis products yields of MCC, total organic carbon (TOC) and cellulose conversion. Based on the response surface plots, 1A of current at 200 ºC maximized TOC yield and cellulose conversions to 62% and 81%, respectively. In order to enhance the selectivity, constant voltage (2.5, 4.0 and 8.0 V) was applied at 200℃. Application of 2.5 V increased TOC (54%) and alter the selectivity of 5-HMF (30%) and levulinic acid (21%). The structural changes in solid residues were analyzed by Fourier Transform Infrared Spectroscopy (FTIR) and found that MCC particles functionalized by carboxylic acid and sulfonated groups by application of 2.5 V. Therefore, change in the selectivity values were conducted with the functionalization of MCC particles due to applied voltage under sub-critical conditions.Doctoral Thesis Selective Fructose Dehydration To 5-Hydroxymethylfufural by Heterogeneous Sulfated Catalysts in Different Solvents(Izmir Institute of Technology, 2016) Kılıç, Emre; Yılmaz, SelahattinIn the present study, different sulfated (SO4/ZrO2, SO4/SiO2, SO4/AC, SO4/TiO2SiO2 and SO4/Ti-SBA-15) and zirconium sulfate loaded (ZrSO4/SiO2 and ZrSO4/AC) catalysts were prepared and characterized. Effect of sulfur content (2.5, 3.0 and 3.5 wt. %) in SO4/ZrO2, effect of Ti content (2 and 6 wt. %) in SO4/ TiO2SiO2 and SO4/Ti-SBA-15, also La incorporation in to SO4/TiO2SiO2 and SO4/Ti-SBA-15 were investigated. Prepared catalysts were tested in fructose dehydration using dimethylsulfoxide (DMSO), water and biphasic aqueous solvents which were water-MIBK and water- MIBK-butanol. Effect of reaction temperature (110, 160 and 200 oC) and effect of fructose/catalyst weight ratio (WFr/Wcat = 0.5, 1.0 and 2.0) on activity and selectivity were investigated. Activity tests were performed in a batch reactor under N2 atmosphere. Reusability tests were carried out up to 4 times. Sulfated catalysts showed different activities. However, there was sulfur leaching in all of them except SO4/TiO2SiO2 and SO4/Ti-SBA-15. This was due to chelating bidendate bond formation between S, Ti and S. Sulfur leaching was also observed over zirconium sulfate loaded catalysts. Acidity of the catalysts increased with sulfur and Ti content; and also La addition. Sulfation created Brønsted acid sites. The most active, selective and stable catalyst was found to be SO4/La-TS-6 (95 % HMF selectivity at 58 % fructose conversion at 110 oC) in DMSO. Testing of this catalyst in water gave high amount of byproducts. Introducing second phase (MIBK) to water, improved HMF selectivity significantly (up to 78 %) and reduced conversion (by 7 %). Fructose dehydration kinetics was also investigated over this catalyst in water- MIBK-butanol (the most selective environmental benign solvent). Selectivity to HMF increased with reaction temperature up to 160 oC, above which it dropped. Increase in WFr/Wcat from 0.5 to 2.0, reduced the selectivity to HMF from 98 to 82 % at high fructose conversions (~90 %). Reaction was found to be 1st order in fructose concentration.Doctoral Thesis Preparation and Characterization of Titania Based Powders and Suspensions for Photocatalytic Applications(Izmir Institute of Technology, 2015) Yurtsever, Hüsnü Arda; Çiftçioğlu, MuhsinGrowing environmental concerns related to the extensive use of nonsustainable fossil fuels and increasing energy demand will force mankind, sooner or later, to tap into clean and sustainable sources of energy and artificial photosynthesis is being considered as a promising prospect for this concern. Photocatalysis is expected to make a great contribution to solutions for environmental problems and renewable energy generation. Titania based photocatalytic materials are one of the widely used materials in artificial photosynthesis research due to their unique chemical and optical properties. Recent research have shown that rare earth element (REE) doping improves the photocatalytic activity of TiO2 phases in the visible region. The effects of REE doping level and heat treatment temperature on the microstructure evolution, optical properties and the photocatalytic water splitting/CO2 photoreduction activities of mostly sol-gel derived titania powders were investigated in this thesis. The results of this thesis indicated that REE ions may enter the interstitial voids of TiO2 lattice or segregate on the grain boundaries of TiO2 as a separate phase and RE2Ti2O7 phases were formed for the REEs with relatively smaller ionic sizes. The experimental work conducted revealed the importance of surface area, crystallinity, light absorption, the presence of oxygen vacancies and structural defects in the efficiency of photocatalytic processes. Results have shown that REE doping improves the artificial photosynthesis activity of TiO2 at relatively low doping levels and 700oC heat treatment temperature as the production rate of H2 and CO/CH4 were significantly higher with REE doped TiO2 compared to pure TiO2.Doctoral Thesis Preparation and Characterization of Nanoparticles as Carriers for Gene Delivery(Izmir Institute of Technology, 2014) Uz, Metin; Altınkaya, SacideIn the first part of this thesis, a comprehensive characterization of polyethylene glycol (PEG) modified AuNPs designed for imaging or diagnostic purposed was carried out to investigate the effect of the size, PEG layer conformation and grafting density on the cellular uptake, toxicity and cell cycle phases against prostate (PC3), colon (CaCo2) cancer cell lines and 3T3 Swiss fibroblast cells. It was noticed that the cellular uptake and toxicity profiles of the particles varied depending on the size, surface properties and cell type. The particles were found to show alterations in cell cycle phases by causing DNA damage without apoptotic behavior at certain doses. In the second part of this thesis, efficient multilayer small interfering RNA (siRNA) delivery systems based on gold nanoparticles (AuNPs), cationic pentablock copolymers or fusogenic peptides were developed using cleavable disulfide bonds and electrostatic interactions. siRNA/Polymer (polyplexes) and siRNA/Peptide (peptideplexes) complexes formed by direct electrostatic complexation between siRNA and the cationic pentablock copolymers or peptides were used as controls, respectively. In addition, a conjugate siRNA delivery system based on the cleavable disulfide bonds between siRNA and fusogenic peptide was also proposed as an alternative system. The siRNA activity, toxicity, cellular uptake and intracellular distribution of the developed systems were investigated against luciferase-expressing SKOV3 ovarian cancer cell line. The use of cationic block copolymers or fusogenic peptides in AuNP based multilayer systems and complex systems, provided efficient siRNA condensation and protection from nuclease enzyme and serum protein degradation, in addition to cellular uptake, endosomal escape and siRNA activity in the cytoplasm.Doctoral Thesis Effect of Colloidal Interactions in the Forming of Lead Magnesium Niobate Ceramics(Izmir Institute of Technology, 2007) Deliormanlı, Aylin Müyesser; Polat, MehmetElectrokinetic properties of lead magnesium niobate (PMN) powders in liquid medium are investigated in this thesis. Isoelectric point of aqueous PMN suspensions was determined as a function of solids concentration. Dissolution character of both Pb+2 and Mg+2 cations in PMN structure were examined as a function of pH. Poly(acrylic acid)-poly(ethylene) oxide PAA/PEO comb polymers were tested as the novel dispersant in this system and their effect on the stability and flow behavior of aqueous PMN suspensions was investigated. Stability and flow behavior were mainly studied by rheological measurements and sedimentation experiments as a function of pH and polyelectrolyte concentration. Adsorption of comb polymers onto PMN surface and their effect on the powder dissolution were also investigated. Results indicated that the PAA/PEO comb polymers impart stability to PMN suspensions over a wide pH range and ionic strength where pure PAA fails. In the course of this study PMN powders were consolidated using two different wet shaping methods. First, PMN films were produced using aqueous tape casting method which is widely used in the manufacture of multilayer ceramic capacitors. Secondly, three-dimensional PMN lattices were produced using robocasting method. In the former case, the goal was to provide alternative recipes for the aqueous tape casting process to be utilized in industrial applications since the current technology is based on the organic solvent based techniques. Results showed that it was possible to produce high quality PMN films with thicknesses in the range of 10 to 250 .m using aqueous based tape casting process. In the latter case, the results showed that robocasting is a suitable technique for the preparation of three dimensional PMN ceramics. Based on these results overall conclusion and the contribution of this study can be summarized as follows: The work carried out gives new insights into the manufacturing of PMN based ceramics using aqueous based techniques. Such improvements may bring benefits in the manufacturing of smaller electronic components employed in the personal computers and mobile phones and other consumer products in the near future.Doctoral Thesis Preparation, Characterization of Enzyme Immobilized Membranes and Modeling Og Their Performances(Izmir Institute of Technology, 2010) Yürekli, Yılmaz; Alsoy Altınkaya, SacideThe objective of this thesis study is to prepare active and stable urease (URE) immobilized membranes for the efficient removal of urea and to predict the performances of these membranes under pressure. Two commercially available ultrafiltration membranes namely Poly (acrylonitrile-co-sodium methallyl sulfonate) copolymer (AN69) and polyethyleneimine (PEI) deposited AN69 membranes (AN69-PEI) were used as supporting materials on which urease is immobilized by means of physical adsorption using layer-by-layer self assembly method or chemical attachment using N-ethyl-N.-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC) and Nhydroxysuccinimide (NHS) coupling agents as a zero crosslinker. During physical immobilization (pH 7.4), the effect of polyelectrolyte type on the activity of immobilized urease was compared between PEI and chitosan (CHI) cationic polyelectrolytes where urease was located either on top of the polyelectrolyte layer (AN69-PEI-URE or AN69-CHI-URE) or between two polyelectrolyte layers in a sandwiched form (AN69-PEI-URE-PEI or AN69-CHI-URE-CHI). The results reveal that the amount of urease immobilized on AN69 membranes are similar and slightly higher than the amount adsorbed on the activated AN69 surface by chemical attachment (AN69-C-URE). The maximum reaction rate was observed with AN69-PEI-URE membrane while the maximum retained activity during storage time was determined with AN69-C-URE membrane. Under dynamic conditions, the hydraulic permeabilities of the commercial and urease immobilized membranes were found similar and the highest urea conversion was achieved with the AN69-PEI-URE-PEI membrane. At the end of 450 minutes of filtration under pressure, the catalytic activity of AN69-C-URE membrane was completely preserved. The mathematical model developed can correlate the experimental filtration data quite well.Doctoral Thesis Rheological Behavior of Nanocrystalline / Submicron Ceramic Powder Dispersions(Izmir Institute of Technology, 2009) Çağlar Duvarcı, Özlem; Çiftçioğlu, MuhsinSeveral rheometric techniques were applied to submicron and nano ceramic powder dispersions systematically in this study. The rheological behavior of the dispersions was determined by steady shear and dynamic shear rheology. Dynamic shear rheological techniques are scarcely used for the characterization of ceramic powder dispersions contrary to polymers.The flow behaviors of the submicron and nano dispersions were found to be dependent on the solids content and fructose concentration. The submicron alumina, nano alumina, and nano titania dispersions in fructose solution showed shear thinning behavior and were fitted to the Herschel-Bulkley model.The dynamic shear rheology measurements showed that the solid part of the dispersions was dominant over the liquid part for both submicron and nano powder dispersions. The elastic modulus was higher than the viscous modulus in stress and frequency sweep measurements. The elastic moduli of the dispersions with solids content lower than 40 vol% were dependent on the angular frequency which indicated a gel-like behavior. However, the elastic moduli of the dispersions with 40 vol% solids were independent of angular frequency which indicated a solid like behavior. Further increase in fructose content had significant effects on both steady shear and dynamic shear rheological behavior of the dispersions regardless of particle size. The submicron and nano ceramic powder dispersions can be prepared by using fructose for the regulation of the rheological behavior of ceramic powder dispersions. The characterization of powder surfaces is essential for the effective adsorption of fructose.