LEE- Polimer Bilim ve Teknolojisi-Doktora
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ÖgeDevelopment of novel biopolyamide compounds as a green and sustainable alternative to petroleum derived polymers and their applications in medical field(Graduate School, 2024-06-27)Polyamides, widely known as nylons, represent a crucial category of polymers renowned for their versatile properties and extensive applications. With exceptional strength, durability, and heat resistance, polyamides like polyamide 6 (PA6) and polyamide 6.6 (PA66) have become highly preferred materials across various industries. However, their petroleum-derived nature poses significant environmental challenges, including the depletion of finite resources and the accumulation of plastic waste. Therefore, the global need for sustainable alternatives to petroleum-derived polymers has led to extensive research into polymer materials derived from renewable resources and those that are biodegradable. In response, the development of bio-based polyamides, such as polyamide 5.6 (PA56), and their biocomposites have gained great importance in polymer research. The primary objective of this thesis was to develop novel biopolyamide compounds as green and sustainable alternatives to petroleum-derived polymers. Specifically, the research aimed to achieve the enzymatic synthesis of bio-based polyamide 5.6 using monomers from renewable feedstocks, develop novel PA56 composites that can replace petroleum-derived composites currently used in various industrial applications, and introduce antimicrobial activity to bio-based PA56 for its potential applications in the medical field. The thesis focuses on finding sustainable alternatives to overcome the environmental issues caused by the extensive use of petroleum-derived polymers. It underlines the increasing global demand for polymers and the urgent need to prevent the environmental problems arising from this trend, such as finite resource depletion and plastic waste accumulation. Furthermore, the thesis highlighted the present scarcity of PA66 due to the limited production of its key monomer, hexamethylenediamine. This scarcity has driven researchers to explore innovative solutions to meet the growing demand for durable and high-performance polymers. The thesis discusses the potential of utilizing bio-based diamines derived from renewable biomass sources in polymer synthesis. It proposes the development and adoption of bio-based polyamides, particularly focusing on bio-based polyamide 5.6 synthesized from renewable sources like cadaverine, to address these concerns and fulfill the emerging material scarcity in the polymer industry. The first part of the study focused on the properties of bio-based polyamide 5.6 (PA56). A comprehensive investigation of the physical, mechanical, rheological, thermal, and flammability properties of bio-based polyamide 5.6 identified the unique properties of PA56 by highlighting its strong points compared to those of its petroleum-derived counterparts, such as polyamide 6 (PA6) and polyamide 6.6 (PA66). PA56 exhibited the highest water absorption among these polyamides. This high water absorption capacity of PA56 makes it advantageous for textile applications. Additionally, PA56 demonstrated similar mechanical properties to PA66, making it suitable as a high-performance engineering thermoplastic. Its rheological and thermal properties further suggest its potential in applications requiring high flowability and thermal stability. Overall, a detailed investigation of the properties of PA56 revealed its potential as a sustainable alternative to its petroleum-derived counterparts across various industrial applications. In the second part of the study, enzymatic synthesis reactions, catalyzed by enzymes like lipases, were presented as an efficient and environmentally friendly method for producing polyamides. The enzymatic synthesis of bio-based polyamide 5.6 from dimethyl adipate and cadaverine (pentane-1,5-diamine) monomers was achieved using lipase enzyme as a catalyst. This solvent-free, single-step process produced PA56 without intermediates or by-products. Successful enzymatic synthesis of bio-based polyamide 5.6 by utilizing immobilized lipase from Candida antarctica as a biocatalyst, along with a renewable monomer like cadaverine showed their potential for sustainability. The enzymatic synthesis reactions were conducted at 60 °C, 70 °C, and 80 °C for 8, 16, 24, and 48 hours, with lipase enzyme concentrations of 5%, 10%, and 20% (w/w) to evaluate the effects of temperature, time, and enzyme concentration on the synthesis process. The effects of these parameters were investigated through the monomer conversion rate. The study observed that higher monomer conversions were achieved with the higher enzyme concentration, suggesting a positive relationship between enzyme concentration and monomer conversion rate. However, polymerization periods exceeding 24 hours at temperatures above 60 °C adversely affected monomer conversion. The decrease in monomer conversion at high temperatures and prolonged reaction times is attributed to factors such as enzyme denaturation and product inhibition. The average molecular weights of the synthesized polyamide 5.6 were determined using gel permeation chromatography (GPC). To facilitate this analysis, the polyamide 5.6 was modified through N-trifluoroacetylation reaction, which involved reacting its amine groups with trifluoroacetic anhydride to form N-trifluoroacetylated polyamide 5.6 (N-TFA-PA56). This modification improved the solubility of PA56 in tetrahydrofuran (THF), the eluent used for GPC analysis. The highest number average molecular weight (Mn) and weight average molecular weight (Mw) were obtained as 11,900 Da and 20,300 Da, respectively for the reaction with enzyme concentration of 20% (w/w), a temperature of 70 °C, and a reaction time of 24 hours. Therefore, these reaction conditions were identified as optimal for lipase catalyzed synthesis reaction of bio-based polyamide 5.6. Instrumental analysis techniques including Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were applied to the enzymatically synthesized polyamide 5.6 products for further characterization. These analytical methods verified the successful enzymatic synthesis of PA56 and highlighted its potential as a bio-based alternative to petroleum-derived polyamides. The third part of the study involved developing novel PA56 and PA66 composites with improved properties. Polymer compounding by extrusion method was performed to tailor the properties of polyamide 5.6. Formulation studies employing glass fibers and flame retardants led to the development of novel bio-based composites with improved properties desired for specific applications in various industries. In these formulations, glass fibers and flame retardants enhanced the mechanical and flammability properties of polyamide 5.6, respectively.
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ÖgeNew methods in the synthesis of single-chain polymeric nanoparticles(Graduate School, 2023-12-12)Nanoparticles can be described as nanometer-sized colloidal particles, possessing exceptional optical, electrical, and magnetic properties. Although numerous types of nanoparticles have been utilized, polymeric nanoparticles with controlled characteristics have recently drawn significant attention as a promising research topic within the field of nanoscience. The design of polymeric nanoparticles having controlled size and functionality is essential for application areas such as drug delivery, microelectronics, and catalysis. A variety of conventional methods including emulsion polymerization, interfacial polymerization, self-assembly, solvent evaporation, or supercritical fluid technology have been employed to prepare polymeric nanoparticles. However, size control and the pre-determination of the functional groups remain challenging by utilizing the above-noted methods. To address these problems, single-chain nanoparticles (SCNPs), also known as single-chain polymeric nanoparticles, have emerged as a crucial class of versatile nanomaterials by the contributions of scientists who are inspired by nature. Ever since the intramolecular crosslinking strategy was first introduced, a variety of approaches has been exploited to yield well-defined SCNPs comprising characteristic features. The intramolecular interactions enable the formation of collapsing or folding of an individual polymer chain via various physical or chemical crosslinking reactions. Due to their unique characteristics resulting from nanometer-sized dimensions (1.5-20 nm), self-crosslinked nanoparticles are of considerable scientific value for particular applications such as drug delivery, bioimaging, biosensors, and catalysis. In this regard, this thesis aimed to demonstrate the synthesis of SCNPs by utilizing intramolecular crosslinking approaches conducted under straightforward and robust reaction conditions. The structure of the precursor polymers was prepared using both polycondensation and controlled/living polymerization techniques and the in-chain crosslinking reactions were established by external addition of crosslinking agent under dilute reaction conditions. In the first study, the synthesis and the functionalization of polyester-based SCNPs are demonstrated utilizing Michael addition reactions. For this purpose, condensation polymerization was first performed to yield a polyester precursor containing reactive alkyne units in the main chain. It is widely accepted that the reactive nature of alkyne units is due to the bonding of two electron-withdrawing carbonyl groups, which renders the triple bond highly electron-deficient and thus suitable for Michael addition reactions. Hereby, linear polymer precursor was intramolecularly crosslinked to generate SCNPs through aza-Michael addition reaction in short durations and without using any catalyst. Piperazine, a secondary diamine compound was utilized as a crosslinking agent with varying ratios to adjust the folding degree and thus the size of the self-crosslinked nanoparticles. The folding procedure was conducted at dilute conditions (c = 1.0 mg mL-1 ) to prevent intermolecular interaction between the polymer chains under mild reaction conditions. Later, the feasibility of the functionalization reaction was investigated via thiol-Michael addition since the remaining alkyne units were still reactive toward the nucleophiles. It has been previously shown in the study that the utilization of a nucleophilic catalyst namely, 1,4-diazobicyclo [2.2.2]octane (DABCO) is required to perform the addition reaction of thiol compounds efficiently to the reactive triple bond. Therefore, the obtained SCNPs were functionalized with a thiol nucleophile in the presence of DABCO for 2 minutes at room temperature. In the second study, not only intrachain collapsing but also the further modification reaction was demonstrated by utilizing nucleophilic aromatic substitution reaction. Firstly, a well-defined precursor polymer with controlled molecular weight has been synthesized via ring-opening metathesis polymerization (ROMP) of oxanorbornene monomer comprising dichlorotriazine (DCT) moiety. By taking the benefits of the electrophilic feature of the chlorine atoms on DCT, intramolecular crosslinking has been performed via nucleophilic aromatic substitution reaction with the addition of a dithiol compound as a crosslinker. Double folding strategy and the post-modification reaction of obtained SCNPs were demonstrated by introducing a different dithiol and a thiol compound, respectively, since unreacted chlorine atoms were still present along the structure. Multiple characterization techniques, including NMR, GPC, DLS, and TEM were used to confirm the formation and modification of SCNPs.
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ÖgeRigid polyurethane foams with improved reaction to fire and low emission properties(Graduate School, 2023-03-16)In this work, different combustion modifiers were evaluated in rigid polyurethane/polyisocyanurate foam with regard to their reaction to fire and emission performance. Terminology of combustion modifiers cover both flame retardants and smoke suppressants. The difference between flame retardants and smoke suppressants derive from the action they exhibit during the combustion. Flame retardants delay the combustion action whereas smoke suppressants aid lowering the smoke and harmful compounds generated during the burning of the substance. Having said that it in the research or in the application area, it is possible to see that combustion modifiers and flame retardants definitions can be used interchangeably. Initially, a literature screening was completed to choose the right flame retardants that are commercially available for rigid polyurethane foams. While most of the found candidates are phosphorous based, few examples such as Hexion TL 91-805D polyol that is nitrogen based, was also in scope. In the flow of the study, flame retardants were then classified according to being reactive or not towards isocyanates. This classification is particularly important when evaluating emission performance of the said substances. On the smoke suppressants side, Zinc borate and ferrocene represent non-phosphorous substances that are suitable to incorporate in this particular application. Performance examination of each combustion modifier was completed using 2 methodology. First methodology was determined as the incorporation of one combustion modifier each time at the same weight in the selected formulation. It was then followed by the incorporation of a combination of one flame retardant and one smoke suppressant in rigid Polyurethane foams. DIN 4102 small scale flame device and NBS smoke chamber instruments were chosen to perform the analysis. This method was useful to reveal synergy between different candidates in the foams. Results showed that among the candidates, interaction of Triethyl phosphate and Zinc Borate as well as Triethyl phosphate and Ferrocene created a synergic impact and greatly improved combustion properties in Polyisocyanurate foams. For the investigation of the found results, thermogravimetric and scanning electron microscope characterizations were carried out. It was revealed that Zinc borate creates a thermal barrier and prevents the cell from a complete destruction once the foam is exposed to ignition. While reaction to fire performance was improved, it was detected that addition of Zinc borate has an impact on the reactivity and free rise density of the foams. Gel time occurred to be longer and density of the foams were measured to be higher with respect to reference. This might be explained by Zinc borate acting as an inert filler. Triethyl phosphate-Ferrocene study also put forward interesting results. While addition of a little amount of Ferrocene provided the best fire performance in rigid Polyisocyanurate foams, more than certain amount of Ferrocene incorporation has led the complete burning of the foam. Therefore, for an enhanced smoke and fire performance, Ferrocene amount should be optimized in the formulations. In the latter step, three compound combustion modifier combinations were examined. Loading of oligomeric Triethyl phosphate into the Triethyl phosphate and Zinc borate combination aided to provide superior performance in fire properties with respect to Triethyl phosphate and Zinc borate containing foam. To confirm the results with the same amount of combustion modifiers loading, the second methodology was used: Analyses were successively repeated by adjusting the foams to the same molded density and P% content in the final material. A cone calorimeter was selected to perform the ultimate combustion test and displayed additional parameters such as Total Heat Release, Peak Heat Release Rate, and Total Smoke Production in 11 formulations. The outcome of the cone calorimeter study was evaluated using Triethyl phosphate (mod 1) containing formulation as the reference. In this way, it was possible to confirm the synergism in other formulas. Formulations that surpassed the performance of reference foam were found to be the same as those completed in the laboratory: Triethyl phosphate-zinc borate (mod 3), Triethyl phosphate-ferrocene (mod 12), and Triethyl phosphate-oligomeric Triethyl phosphate-zinc borate (mod 9) combinations. These 3 combinations displayed either a lowered Total Heat Release or Total Smoke Production Rate or both than the reference. In the final stage, further analysis was completed to check the emission properties of these 3 foams and reference using the Headspace gas chromatography-mass spectrometry characterization method. While Triethyl phosphate showed an elevated pique especially in the reference foam due to the high addition amount, Ferrocene also confirmed to migrate because of the sublimation at high temperatures. Cone calorimeter and headspace analysis confirmed that the Triethyl phosphate-oligomeric Triethyl phosphate-zinc borate combination proved to be the most efficient combination with regards to both reactions to fire and emission properties in rigid Polyisocyanurate foams. This result is also proof of how oligomeric substances can enhance the emission properties of end material. As a final word, this study showed that the fire and emission properties of rigid Polyurethane Polyisocyanurate foams can be enhanced through the addition of the right combustion modifiers at the right amount. Said properties are not only governed by the P% content but also synergism and molecular structure might play an important role in improving the properties in polyurethane formulations.
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ÖgeElectrospun polyacrylonitrile based composite nanofibers containing polyindole and graphene oxide(Graduate School, 2023-03-06)Studies on the conductive polymers has gained great interests when the Nobel Prize in Chemistry was awarded by discovery and development of the conductivity of polyacetylene in 2000. Conductive polymers are also called organic metals. They conduct electricity thanks to the conjugated chain structure consisting of consecutive single and double bonds in their structures. Conductive polymers, which are insulating in neutral state, gain conductivity by doping. Polyacetylene, polypyrrole, polythiophene, poly(3,4-ethylenedioxythiophene) (PEDOT) are some of the conductive polymers that have been studied extensively. These polymers can be used in solar cells, super capacitors, chemical and biosensor application areas. Polyindole (PIN) is one of the conductive polymers which can show electrochromic properties with high redox activity, good thermal stability, slow degradation rate and good air stability. Polyindole containing studies have been increasing in recent years and this polymer can be used in the pharmaceutical field, anticorrosion coatings, photovoltaic batteries, supercapacitor applications or anode material in batteries. On the other hand, with the advancement of nanotechnology, it has been found that materials in nano scale show physical and chemical property differences compare to the bulk form. Nanoscale generally includes the range of 1-100 nm. When the size of the particle in the material becomes too small, the electronic structure of the material can change. For example; gold normally does not react, but can be active at the nano level. Nanofibers are fibers with a high length/volume ratio with average diameters in the order of nanometers. In addition to their chemical properties also depending on the surface properties such as morphology and topography, materials can improve and can be used various areas. Due to their low densities, large surface areas with porous structures, it has a wide range of research and application areas of nanofibers such as filtration, tissue engineering, drug release systems, biomedical, textile, energy storage and sensor. Especially in recent years, electrospinning technique has attracted interests by scientists to generate nanofibers because of it is extremely simple, cheap and practical usage. On the other hand, scientists have great expectations since discovering a few atoms thick materials. Graphene oxide (GO) is a two dimensional material with high surface area. It can be semiconductor or insulating material which depends on the degree of oxidation, sheet size, microstructure and among many other factors. Moreover, graphene oxide contains some functional groups (epoxy, hydroxyl, carbonyl, etc.) on the structure which makes the dispersive ability in the solvent. These oxygen containing functional groups enable the development of GO-based composites, especially due to their ability to disperse in the solvent. Unfortunately, nanofiber production from conductive polymers and GO like materials can be limited or not possible by electrospinning method. For this reason, nanofibers of conductive polymers and GO are produced by making blend or composite with a different polymer called as carrier polymer, whose nanofibers can be easily obtained by electrospinning. Polyacrylonitrile (PAN) is one of the carrier polymer which is a very common usage area especially in the textile manufactory and carbon fiber production. Also, PAN fibers are the precursor of high quality of carbon fibers. PAN is choosen as a carrier polymer and PAN based composites are studied in this thesis. In the first part of the study; the oxidative chemical reaction of polyindole has taken place in the presence of FeCl3. Nanofibers were produced by mixing polyindole with polyacrylonitrile in N, N-Dimethylformamide (DMF) solvent at different weight / volume ratios. Polyacrylonitrile and polyindole blends were generated in different proportions of polyindole. Composite fibers were produced from the solutions by adjusting the optimum conditions using electrospinning method. Morphological, thermal properties, spectral analysis of these fibers were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (FTIR-ATR) and Differential Scanning Calorimetry (DSC). Electrochemical characterization of fibers has been studied by Electrochemical Impedance Spectroscopy (EIS). The experimantal data was used to fit the equivalent circuit with Zsimpwin Software. In addition, it was found that the electrochemical properties (such as double layer capacitance, solution resistance and charge transfer resistance) of composite fibers were effected by surface tension and conductivity of solution. Iron, is an important element in the industry, environment, medical applications areas, biological studies and human health. Different methods such as electron spin coulometry and ion selective electrodes are used in the determination of Fe(II). Differently, in this study, Electrochemical Impedance Spectroscopy is presented as an alternative technique to determine Fe(II) concentration. Electroactive behavior of the fiber electrode was investigated by Cyclic Voltammetry (CV). Electroactivity of the nanofiber selected depending on the impedance and morphological properties of the nanofibers was measured with the help of K3Fe(CN)6/K4Fe(CN)6 electrolyte. It was discussed that the presence of Polyindole (PIN) content showed an electrocatalytic activity against K3Fe(CN)6/K4Fe(CN)6. The lowest Fe(II) ion analyte concentration detection limit for the selected electrode was calculated as 1x10-4 mol.l-1. In the second part of the study which is different from the first part, graphene oxide (GO) is choosen as a material to improve the capacitive property of Polyacrylonitrile. Polyacrylonitrile / Graphene oxide (GO) nanofibers were produced by using a rotary collector instead of a fixed collector in the electrospinning device. Thus, thinner, more aligned nanofibers with higher young modulus were acquired. Oxidative stabilization and carbonization applied to composite nanofibers through the thermal process. In particular, the stretching applied to the nanofiber during oxidation determines the mechanical strength and structure of the final product carbon nanofiber to be formed. Therefore, understanding of the oxidation mechanism is an essential part of the production of carbon nanofibers (CNFs). The stress, temperature and application time utilized to the material in oxidation affect the structure of the carbon nanofiber. Oxidation step of electrospun polyacrylonitrile based composite nanofibers was studied and morphological, spectral and electrochemical properties of composite nanofibers were investigated. Morphological and spectral characterizations of composite nanofibers were performed by FTIR-ATR and Raman Spectroscopy, SEM, AFM and Transmission Electron Microscopy (TEM). Mechanical tests were performed with Dynamic Mechanical Analysis (DMA). Thermal behaviours of composite nanofibers were investigated by Thermal Gravimetric Analysis (TGA). Capacitive behavior of nanofibers were performed by EIS and CV. When there is GO in the structure, the ions in the solution can penetrate into the pores which cause the double layer capacitance (Cdl) value increasement. Average pore diameters have been measured with the ImageJ program to be around 38.5 nm and it has been found that the double layer capacitance (Cdl) of PAN nanofibers containing GO is 0.600 µF which is the highest value. Also, it was observed that the capacitive behaviour of carbon nanofiber formed in the presence of graphene oxide improved. PAN/GO carbon nanofibers exhibit potential for capacitive applications in the light of these results. In the third part of the study, X-ray Photoelectron Spectroscopy (XPS) and FTIR analysis methods were used to understand the oxidative stabilization deeply. The thermal oxidative stabilization of polyacrylonitrile has a complex mechanism with the cyclization and dehydrogenation steps. Polyacrylonitrile (PAN) composite nanofibers with GO were fabricated, and thermal oxidation were performed to these nanofibers. The oxidation process were applied at various temperatures (250 0C, 280 0C, and 300 0C) during 1h and 3h. Nanofibers were significantly effected by high temperature with during long duration time. The effect of GO addition into the nanofibers were analyzed by XPS, FTIR-ATR and, EIS. After heat treatment, change in C1s spectra and development of sp2 carbon was detected by XPS. It was concluded that the presence of GO accelerated the oxidation mechanism and developed the final structure.
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ÖgeDevelopment of cornstarch and tannin-based wood adhesives for interior particleboard production(Graduate School, 2022-09-22)Wood-based panel industry is an important industry that produces wood-based panels such as furniture and parquet, which we frequently use in our daily lives, on an industrial scale. Particleboard is one of the most important products frequently produced in the wood-based panel industry. Particleboard is a composite product. It is made by turning the logs into chips and pressing the formed chips in a hot press by mixing with a thermoset resin. Therefore, the thermoset resin used in particleboard production, as well as the wood raw material, is of great importance in terms of the final properties of the final product. Thermoset resins, which are frequently used in the wood-based panel industry, are based on formaldehyde such as urea formaldehyde and melamine formaldehyde. Cost advantage and high reactivity of formaldehyde-based resins are the most important reasons for their frequent preference. Today, one of the most important problems of the wood-based panel industry is formaldehyde emission. Formaldehyde is classified as a carcinogenic substance and threatens human and environmental health. Therefore, there are formaldehyde emission limits that become more restrictive day by day. Some of the factors affecting the formaldehyde emission of wood-based panels are the formaldehyde found in the natural content of the wood raw material, the type of wood raw material used, and hot press conditions. Apart from these, it is known that one of the most important factors affecting plate formaldehyde emission is formaldehyde-based thermoset resins used as binders. It is known that the formaldehyde content of these resins, which are not involved in polymerization and released, and especially when the urea formaldehyde resin is exposed to moisture and temperature, hydrolysis and release of formaldehyde increase the formaldehyde emission values of the boards. In addition to the fact that formaldehyde-based resins cause formaldehyde emission, they are produced from petrochemical raw materials. Formaldehyde emission caused by formaldehyde-based resins and their synthesis from petrochemical raw materials has pushed the industry and academia to resins that can be synthesized with the use of sustainable raw materials that can be an alternative to formaldehyde-based resins in terms of performance and cost. In the doctoral dissertation conducted in cooperation with Kastamonu Integrated Wood Industry, which is a global power in its sector, and Istanbul Technical University, it was worked on the development of resin formulations containing corn starch - tannin to be used in the production of particleboard suitable for internal applications. Physical and chemical properties of synthesized resins were determined. In order to examine the performances of suitable resin formulations, paticleboard production was carried out in laboratory scale. In addition to the physical and mechanical tests of the particleboards, formaldehyde emission tests were performed. The formaldehyde emission of the produced boards was determined by the perforator method. The physical and mechanical results obtained were evaluated by taking into consideration the particleboards for use in interior applications and their properties standard TS EN 312 standard P2 class limit values. Production of laboratory scale particleboards and physical, mechanical and perforator tests of the produced particleboards were carried out with the support of Kastamonu Integrated Wood Industry R&D department.