LEE- Polimer Bilim ve Teknolojisi-Yüksek Lisans
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ÖgeLight induced synthesis and characterization of clickable polyacrylamide hydrogels(Graduate School, 2022-02-17)Hermann Staudinger's groundbreaking study on macromolecules was recognized with the Nobel Prize in Chemistry on December 10, 1953. "High Polymers Bring High Honors" became a worldwide headline. Staudinger had identified the molecular blueprints of natural and synthetic "polymeric materials" with high molecular weights. His groundbreaking concept, which involved covalently bonding a large number of small monomer molecules to form macromolecules, ushered in a new age of molecular design of high molecular weight structural and functional polymeric materials. Polymeric materials are unique in their ability to combine an attractive cost to performance ratio, low energy demand during preparation and processing, flexible feedstock supply, simple processing with short cycle times typical of modern industrial mass production, and extraordinary versatility in terms of property profiles, application ranges, and waste recycling. Step-growth (condensation) polymerization and chain-growth (addition) polymerization are the two types of polymerization processes that can be classified kinetically. The propagation methods of polymer chains can also be used to classify these procedures. The propagation of the chains in addition polymerization are formed by radical, cationic, and anionic reactions, whereas the propagation of the chains in step-growth polymerization are formed by polyaddition and polycondensation processes. Polymers can be prepared by several techniques. Among theses techniques, photopolymerization is a rapidly growing technology since it offers several advantages over thermal polymerization. Higher rates of polymerization, temporal and spatial control, environmental benefits from the elimination of volatile organic compounds are some of the advantages of photopolymerization over thermal polymerization. In addition, probability of side reactions such as chain transfer to occur gets lower enabling the synthesis of more ordered macromolecules. Although constituting a minimal amount in the formulation of a photopolymer, photoinitiators play a vital role by absorbing the light in ultraviolet-visible spectral range, typically between 250 and 450 nm, and converting this light energy into chemical energy in the form of reactive intermediates including free radicals, cations and anions, which then initiate photopolymerization. The two types of free radical initiators are Type I (α-cleavage) and Type II (H abstraction). Substituted carbonyl and aromatic compounds such as benzoin and its derivatives, benzyl ketals and acetophenones are the most common Type I (unimolecular) initiators. On the other hand, Type II photoinitiators, also known as bimolecular photoinitiators, are photoiniating systems that include a photoinitiator including benzophenone, thioxanthone, or quinone, as well as a co-initiator such as an alcohol or amine. Since the first definition made by Staudinger in 1920s, in 100 years, polymer science has witnessed remarkable progress. "Smart polymeric materials" are an example of state-of-the-art polymers. Hydrogels belong to the smart polymeric materials subclass and defined as crosslinked polymers which can absorb large quantities of water due to their hydrophilic structure while not being dissolved as a consequence of their crosslinked structure. Today, they are widely used to produce groundbreaking smart gadgets, sensors, and actuators; their capabilities arise from their capacity to react to external stimuli with an observable reaction. According to their source, hydrogels can be classified as natural and synthetic. Some of the most common examples of natural and synthetic hydrogels are hyaluronic acid and polyacrylamide, respectively. One of the most important features of hydrogels is to possess suitable functionalities for post-synthetic modifications that can support their extended applications. Chemical bonds, permanent or temporary physical entanglements, and secondary interactions, such as hydrogen bonds, can all be used to cross-link hydrogels. In physiological conditions, covalent cross-links are often stable. Mechanical properties can be tuned by altering the ratios of reactants in synthetic hydrogels. Toxic chemicals utilized in the manufacturing of these hydrogels, on the other hand, may diminish their biocompatibility. Polyacrylamide, a polymer of the acrylamide monomer, is a colorless hydrogel that is durable, nonresorbable, nontoxic, and nonimmunogenic, as well as hydrophilic, viscoelastic, cohesive, and biocompatible. Due to these attractive properties, polyacrylamide hydrogels are among the most widely studied hydrogels. By applying appropriate modification methods, functional groups which can tune swelling and enhance mechanical properties can be incorporated into polyacrylamide hydrogels. Coined by K. Barry Sharpless in 2001, "click chemistry" is a family of biocompatible reactions utilized in bioconjugation that allows specific biomolecules to be joined to specified substrates. Click chemistry is not a single specific reaction, but rather a method of producing compounds that mimic natural instances and molecules by connecting small modular parts. Click reactions connect a biomolecule with a reporter molecule in a variety of applications. The concept of a "click" reaction has been employed in chemoproteomic, pharmacological, and different biomimetic applications, and it is not confined to biological settings. Following Sharpless' innovative classification of a number of idealized processes as click reactions, the materials science and synthetic chemistry research groups have followed a variety of paths to identify and execute these click reactions. Christopher N. Bowman has reviewed the radical-mediated thiol-ene reaction as an example. This reaction has all of the ideal characteristics of a click reaction: it is extremely efficient, simple to execute, produces no side products, and gives a high yield rapidly. Furthermore, the thiol–ene reaction is frequently photoinitiated, especially for photopolymerizations that result in highly uniform polymer networks, boosting unique spatial and temporal control capabilities of the click reaction. Copper-catalyzed-azide-alkyne-cycloaddition reaction is another model example of a click reaction. In this reaction, an azide is reacted with a terminal alkyne to produce 1,2,3-triazole. This reaction is regiospecific since it only forms 1,4-disubstituted product, can be carried out at a wide range of temperatures and pH values, in a variety of solvents including water. Furthermore, it is 107 times faster than the uncatalyzed reaction, making it an ideal candidate for the modification of hydrogels. In the present work, a facile synthesis of clickable polyacrylamide hydrogel by photopolymerization using acrylamide and propargyl acrylate in the presence of different photoinitiators and in the absence of any crosslinking agent under ultraviolet and visible light is reported. Clickable polyacrylamide hydrogels were synthesized in the presence of Irgacure 2959 (water soluble photoinitiator), BAPO (visible light photoinitiator) and DMPA. Afterwards, gel synthesized by water soluble initiator was irradiated for 3 h, 6 h, 9 h and 24 h. The effect of irradiation time on gel fraction, swelling degree and compressive elasticity was investigated. Thiol-ene, thiol-yne and copper catalyzed azide-alkyne cycloaddition click functionality of the produced hydrogels were confirmed by using respective fluorescent click components. Furthermore, the effect of hydrophobicity on swelling degree was examined by clicking 1,6- hexanedithiol onto hydrogels.
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ÖgeInvestigation of the thermal and rheological properties of PET/PBT blends(Graduate School, 2022-01-28)The use of polymer materials, due to their easy processing and low cost, is becoming increasingly common today and is frequently used in the industry. Polymer blending is a simple, effective, and cost-effective approach to obtaining a new composite with desired combinations of properties without overtly compromising its advantages. Polymer blends are formed by the physical mixing of two or more polymers. Poly(butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) are two of the most important aromatic-aliphatic polyester resins in the industry. PET and PBT have similar chemical structures and can undergo transesterification reactions at high temperatures. Therefore, they form a miscible blend in the amorphous state without the need for compatibilizers to be used. PET resins are difficult to process due to their slow crystallization rate, high heat deflection temperature, low melt strength, and hardness. Thanks to the flexible butylene groups in the PBT structure, it is easier to process because it has a higher crystallization rate and better melt strength. Blending PBT with PET provides a product with good mechanical and high electrical insulating properties and improves machinability, surface appearance, heat deflection temperature, impact strength, and dimensional stability. Through these features, it is used in automotive parts, household and kitchen appliances that require mechanical, high heat, and chemical resistance. These blends are also used to produce visible parts of devices that require a smooth and glossy surface appeal. While blending PET and PBT provides new structures with superior properties, these structures may also have disadvantages such as brittleness and relatively low mechanical properties. Using chain extenders when forming blends can result in improvements in the shear thinning, shear and elongation viscosity, melt elasticity and melt strength behavior of polymers, and thus in the performance of the final product. In this thesis, PET/PBT blends were produced using a twin-screw extruder. The pellets produced with the extruder were then formed into test specimens using injection molding. Multi-functional styrene-acrylic additive with epoxy reactive groups, whose trade name is Joncryl ADR 4368 ®, was used as the chain extender agent. First, PET and PBT samples were extruded. Afterward, 0.75% by weight Joncryl additive was physically mixed with the obtained samples, and pellets were produced in the extruder. While preparing the blends, PET was added to PBT at rates of 25, 50, and 75% by weight. While Joncryl was added to the PET/PBT blends, pellets were produced in the extruder by physically mixing the PBT, PET, and Joncryl additive material. In this thesis, the effects of using different ratios of PET and adding chain extender on PET/PBT blends were investigated. The thermal properties and crystallization behaviors of the developed blend materials were investigated using the differential scanning calorimetry (DSC) method. The rheological properties were investigated using a rotational rheometer. Fourier transform infrared (FTIR) spectroscopy was used to observe the structural changes in the samples.
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ÖgePost-polymerization modification of poly(vinylene sulfide)s(Graduate School, 2023-08-09)The demands for the fast synthesis of a significant number of target compounds could not be met by conventional chemical synthesis. The "click chemistry" was presented as a solution to this problem based on the biosynthesis pathways and the synthesis of natural products. It is revealed in 2001 by a team of chemists at The Scripps Research Institute, led by K. Barry Sharpless. Click reactions are defined as a group of reactions function well in a variety of environments, which are simple to carry out, produce their intended products in extremely high yields with few to no byproducts, are unaffected by the nature of the groups' connections to one another and exhibit great stero- and regio-selectivity. Additionally, click reactions allow for facile product separation ways. The copper (I)-catalyzed azide-alkyne cycloaddition, an enhanced variant of the Huisgen azide-alkyne dipolar cycloaddition, is the most used click technique. Cu(I) toxicity was yet another disadvantage; it is still unsuitable for biological uses. To address this issue, numerous metal-free "click" reactions were developed to polymer chemistry. Among them, thiol-yne click and thiol-ene click reactions became popular for polymer chemists. The Michael addition reaction is a flexible synthesis technique for the effective coupling of a wide range of nucleophiles with electron deficient olefins. When Michael donors are thiols and Michael acceptors are α-β unsaturated carbonyl compounds, the reaction is called as thiol-yne or thiol-ene addition. Thiol-yne and thiol-ene are robust click reactions that are unaffected by water and produce few and harmless byproducts that can be easily removed. Recent developments of base catalysis enabled high yields with high reaction specificity. The heteronucleophilic conjugate addition reaction involving sodium thiophenolate and propiolates to produce unsaturated esters was first found by Ruhemann shortly after Arthur Michael's initial study on the conjugate addition of carbon nucleophiles to unsaturated substrates. Under ambient reaction conditions, quantitative conversions may be seen, which is undoubtedly made possible by the strong nucleophilicity of the thiolate anion and/or the utilization of highly electrophilic alkynes. The synthesis of acetylenic polymers containing heteroatoms presents considerable importance. One method employed to synthesize these polymers is alkyne hydrothiolation, which involves the combination of alkynes and sodium thiolates to produce vinyl sulfides. The concept of the "hydrothiolation" reaction was initially introduced by Truce and Simms in the 1950s. Alkynes can be hydrothiolated with aryl/aliphatic thiols to form vinyl sulfides by radical, nucleophilic or metal-catalyzed routes. By the nucleophilic and metal-catalyzed alkyne hydrothiolation processes both anti-Markovnikov (linear) and Markovnikov (branched) products are produced, while only anti-Markovnikov (linear) products are produced by the radical alkyne hydrothiolation method. The drawback of metal catalyzed route is the requirement of heat and for the radicalic route, the inorganic bases were not suitable to aryl thiols. While the radicalic reaction can be initiated by thermal or UV irradiation, the nucleophile-catalyzed pathway requires an electron-deficient alkyne or alkene with an adjacent electron-withdrawing group, such as an ester, amide, or cyanide. The nucleophilic thiol-ene addition is constrained by the availability of the activated alkenes, but it also benefits from mild reaction conditions, no detectable byproducts, and high conversion is possible under ideal reaction conditions. The development of organobases such as diethylamine, triethylamine, diphenylamine, DABCO enabled the efficient reaction conditions for hydrothiolation of activated alkynes. Thus, it is demonstrated that the hydrothiolation reaction may be carried out under ideal conditions with high yields by using the nucleophilic pathway catalyzed by organobases and adapting it to the polymer science. Tang and colleagues applied the hydrothiolation procedure to polymer science. In the presence of several secondary and tertiary amines, such as diethylamine, triethylamine, diphenylamine, triphenylamine, and morpholine in high concentrations, they carried out polyhydrothiolation of aryl dithiols with arylacetylenedicarboxylates in the presence of organobases. They also discovered that poly(vinylene sulfide)s had molecular weights up to 32.3 kDa in high yields. For organobase selection part a study conducted by Durmaz and coworkers was taken as a reference. Durmaz and colleagues investigated the systematic and selective modification of the electron-deficient triple bond using thiols via nucleophilic thiol-yne interactions. They developed a polyester with an internal electron-deficient alkyne molecule, and on this scaffold, they investigated amino-yne and thiol-yne processes. The effectiveness of several amidine and guanidine bases, as well as various nitrogen and phosphorus-based catalysts, was examined in nucleophilic thiol-yne reactions. In-depth 1H NMR investigations showed that when the nucleophilic catalysts 1,4-diazabicyclo[2.2.2]octane (DABCO) and 4-(dimethylamino) pyridine (DMAP) were utilized in the reactions, only mono thiolation was seen even though 1.2 equivalents of thiols (per alkyne) were applied. High efficiencies, such as 90 and 84%, were demonstrated by DABCO and DMAP. Additionally, the end products showed mixed stereoregularities. The catalysts were then tested using the amidine base 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) and the guanidine bases 1,1,3,3-Tetramethylguanidine (TMG) and 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD). For DBU, TMG, and TBD, respectively, the efficiency for the mono additions was determined to be 83, 86, and 74%. These bases effectively facilitated the thiol-yne reaction. It is interesting that although using the same amount of thiol for each alkyne repeating unit, these catalysts, as measured by 1H NMR, also generated the double addition products (8%, 4%, and 15% for DBU, TMG, and TBD, respectively) in addition to the mono addition products. According to this study, TBD was preferred for double addition of thiols onto electron deficient vinylene sulfide units during the thiol-ene click reaction due to its efficiency and DABCO was preferred for mono addition of thiol onto the electron deficient alkyne moieties during the thiol-yne polymerization reaction due to its efficiency. Poly(vinylene sulfide)s possess activated double bonds. These double bonds can be crosslinked and functionalized with thiols in the presence of organobases. There are several studies about functionalization of electron deficient alkyne units. In this manner, the synthesis of a polymer with electron-deficient alkyne groups was the subject of research by Thayumanavan and colleagues. On this polymer, they applied thiol-yne and thiol-ene addition processes using reversible addition-fragmentation chain-transfer (RAFT) polymerization. The electron-deficient alkyne groups were modified with thiols in their work using Et3N as an organocatalyst. The thiol vinyl ether functional groups of the resultant homopolymer were modified using a potent base, TBD, to introduce thioacetal linkages in the pendant group as well. This work was an example of post-polymer modification of pendant electron deficient alkene units. Post-polymer modification of electron deficient alkene units was also conducted by Durmaz and coworkers. They employed unsaturated polyester (UP) scaffold for this purpose. By esterifying maleic anhydride with 1,4-butanediol an unsaturated polyester (UP) scaffold was created. Then, in the presence of TBD in CHCl3, this polyester scaffold was altered using different thiols. This was an illustration of post-polymer functionalization of electron-deficient alkene units in the main chain. Truong and Dove conducted a separate study on the functionalization of electron-deficient alkene units. In their research, they initiated the reaction between dodecane-1-thiol and PEG32-bispropiolate using a catalytic amount of Et3N. Then, they modified vinylene sulfide with benzylmercaptan in the presence of TBD. The purpose of this study is to synthesize polymers with electron deficient vinylene sulfide units in the main chain and modifying them with various thiols in the presence of an organocatalyst. In this work firstly, propiolic acid and ethylene glycol were reacted in the presence of p-toluenesulfonic acid monohydrate at 105 ℃ in benzene to obtain a symmetrical ester with two activated alkyne moieties. The product, ethane-1,2-diyl dipropiolate was reacted with 1,6-hexanedithiol in the presence of an organocatalyst DABCO at room temperature which yielded poly(vinylene) sulfide. Activated double bonds of the synthesized poly(vinylene) sulfide were modified with various monothiols (1-octanethiol, 2-mercaptoethanol, 2-furanmethanethiol, benzylmercaptane, 2-propanethiol, 1-propanethiol, cyclopentanethiol, methylthioglycolate, 3-nitrobenzylmercaptan, allyl mercaptan and 2-methyl-2-propanethiol) in the presence of a strong basic and nucleophilic organocatalyst TBD, in CHCl3 with various efficiencies at room temperature. Synthesized polymers were characterized with GPC, 1H NMR and 13C NMR.
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ÖgeFabrication of caffeic acid grafted poly(lactide)-b-poly(hydroxyethylmetacrylate) films(Graduate School, 2022-09-19)In this study, we aimed to prepare an antimicrobial, PLA-based partially degradable caffeic acid functionalized film that has the potential to be used in active food packaging. We report a synthetic route for the functionalization of poly(D, L-lactide)-b-poly(2-hydroxyethyl methacrylate) copolymer grafted with caffeic acid. First, we synthesized a dual initiator, namely 2-Bromo-N-(5-hydroxyphenyl)2-methylpropanamide (BNMP) (yield = %58), bearing ATRP initiator and ring opening polymerization. The initiator was synthesized via an amidation reaction between 5-amino-1-pentanol and 2-bromoisobutyryl bromide in the presence of triethyl amine. Firstly, ring-opening polymerization of D, L-lactide was achieved via the hydroxyl group of BNMP using tin(II) 2-ethyl hexanoate (Sn(Oct)2) as catalyst (Mn = 9800 g/mol). This polymerization was carried out by the melt polymerization method. Secondly, 2-hydroxyethyl methacrylate (HEMA) was polymerized through atom transfer radical polymerization (ATRP) using poly(D, L-lactide) macroinitiator yielding the block copolymer, PLA-b-PHEMA. Eventually, the block copolymer was functionalized via a Steglich esterification reaction between caffeic acid and hydroxyl groups of the PHEMA segment to obtain PLA-b-PHEMA-g-CA. The polymers were characterized by size exclusion chromatography (SEC), nuclear magnetic resonance spectroscopy (NMR), ultraviolet-visible (UV-Vis), and infrared (IR) spectroscopies. The grafting degree was calculated by using the regression line of caffeic acid standards at known concentrations and was found 60.7%. The bioactivity of the caffeic acid-functionalized block copolymer was investigated with DPPH for radical scavenging activity and antimicrobial activity against gram-positive (S. aureus.) and gram-negative (E.coli) bacteria. The films were prepared by mixing different amounts of PLA-b-PHEMA-g-CA with a high molecular weight of commercial PLA via solvent casting technique. The mechanical properties of the films obtained from the mixture of copolymer with high molecular weight PLA were also examined. To gain further insight, the thermal and surface properties of the films were evaluated with differential scanning calorimetry (DSC) measurements and water contact angle measurements(WCA), respectively. According to the results, the synthesized PLA-b-PHEMA-g-CA polymer showed bioactivity.
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ÖgeDouble layer thermoplastic polyurethane-gelatin electrospun wound dressings for biomedical applications(Graduate School, 2022-06-23)Electrospinning is the technique to produce nano or micro scale fibers. Since this technique offers high surface area to volume ratio and porous structure, the dressing mimics the extracellular matrix (ECM). This technique gives the opportunity to use synthetic and natural polymers, active agents or essential oils, for that reasons it is a commonly preferred technique to produce wound dressings. Besides, to be able to add more features to the wound dressings double layer electrospun dressings have been made recently. Therefore, top layer which basicly includes synthetic polymers and active agents mimicks the epidermis layer, wheras sublayer with natural polymers and active agents mimicks the dermis layer of the skin. In this study, double layer nanofiber mats were produced by electrospinning technique. Each layer had a St. John's Wort oil (Hypericum perforatum oil) to provide a curative effect on the wound healing process. Top layer, which includes thermoplastic polyurethane (TPU) and Hypericum Perforatum Oil, was elastic, prevented water and pathogen transition and gave mechanical strength to the electrospun mat. On the other hand, the bottom layer which basically includes cold water fish gelatine and Hypericum Perforatum Oil firstly aimed to repair by being a source of collagen. With these properties the top layer and bottom layer mimic the epidermis and dermis layers of the skin, respectively. Besides, plasma treatment was applied as a surface activation technique for the TPU electrospun mat to have better adhesion to hydrophilic gelatin electrospun layer. By this way, better packing was obtained between the layers. Morphological and chemical characterization, water vapor transmission rate, absorption tests and antibacterial property test were done to the dressings. According to the result, a homogenous fiber structure was obtained. The Sample 2 showed antibacterial property and suggested for moderate to highly exudative wounds. On the other hand, Sample 3 which was a plasma treated sample also had homogenous, bead free fiber structure, showed antibacterial activity and suggested for moderately exudative wounds, but it had lower active agent release during the test period due to better packing between the layers with plasma treatment. Sample 3 had lower active agent release during the test period. Therefore the sample could be suggested for long term remedies. Overall results indicated that obtained nanofiber dressings can be a good candidate as a wound dressing material.