LEE- Polimer Bilim ve Teknolojisi-Yüksek Lisans
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ÖgeApplying different surface treatments on different plastic materials, examining the effect on paint test performance and developing a high-performance painting process(Graduate School, 2022-06-23) Çetin, Elif Burcu ; Kızılcan, Nilgün ; 515181007 ; Polymer Science and TechnologyNowadays, the standard plastic part painting process which is used in the automotive industry today, consists of surface cleaning, surface pre-treatment, primer, base coat and clear coat applications. Although high performance engineering polymers have high mechanical & chemical properties and atmospheric resistance, their use in various applications is limited due to their low surface energies. For this reason, the research and development activities of surface pretreatment applications that increase the surface wetting ability in the automotive industry have increased significantly in recent years. Flame, mechanical and chemical etching, corona and plasma application methods are among the actively used and newly developed surface activation processes. In this study, test plates were prepared from polyamide (PA GF15), Acrylonitrile Butadiene Styrene (ABS), Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS), Polypropylene (PP TD10, PP TD17, Primerless PP) polymeric materials that are used in the automotive industry. Various variations of surface pre-treatment, primer, base coat and clear coat were applied to the test plates. These variations were divided by two categories as related to surface pretreatment and primer application.Three different methods were used for surface pretreatment; (1) flame application, (2) atmospheric plasma application, (3) no surface pretreatment application. There are two conditions for primer application; (1) primer application, (2) no primer application. Application distance was determined as a variable parameter in surface pretreatment methods. Other process parameters were kept constant. This application distance was defined as 10 and 20 cm for flame application, 7 and 10 mm for atmospheric plasma application. After each surface treatment, the surface wetting abilities were analyzed by contact angle measurement and the both morphological and roughness paramter change on the surface were examined with scanning electron microscope (SEM) and 3D optical microscope device. The effect of both surface pretreatments and primer applications on paint performance tests was analyzed. Examined paint performance tests were adhesion resistance, water resistance, humidity resistant, thermal cycle resistance, impact resistance and stone impact resistance. The results of these paint performance tests were evaluated for each material as separately and the optimum painting process steps were determined as special to each polymeric material.
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ÖgeDouble layer thermoplastic polyurethane-gelatin electrospun wound dressings for biomedical applications(Graduate School, 2022-06-23) Yıldırım, Arzu ; Güner, F. Seniha ; Taygun Erol, Melek Mümine ; 515191019 ; Polymer Science and TechnologyElectrospinning 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.
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ÖgeEffect of polylactide molecular weight on cellulose nanocrystal dispersion quality(Graduate School, 2022-01-28) Dündar, Anıl ; Nofar, Mohammad Reza ; 515181001 ; Polymer Science and TechnologyToday, plastics are materials that are frequently used for all sectors, especially as a part of daily life, and their consumption is increasing rapidly with a continuous increase, and these materials are often obtained from petroleum-derived fossil resources. The demand for plastic materials is increasing due to reasons such as the excessive use of plastics, the increasing population, the desire of the society to reach higher quality products, and the increase in standards. Especially with the covid 19 pandemic, rapid consumption has inflated this demand. As the usage increased, plastic pollution increased rapidly and studies have been carried out on this subject due to the limited fossil resources. Instead of these petroleum-derived polymers, biodegradable polymers have come to the fore. At this point, one of the the most ideal candidate is the polylactic acid (PLA) polymer since the physical and mechanical properties of PLA are comparable to some petroleum-derived polymers used in the industry. However in spite of all these good and superior features of PLA, it has some shortcomings too such as low viscoelastic properties, slow crystallization rate, brittleness, low thermal stability. For this reason, improving these drawbacks are the main problems that must be overcome in order to use them instead of petroleum-derived polymers. Various studies are carried out in the literature. Copolymerizing using different monomers in the polymerization stage, blending with different polymers without changing their biodegradability, strengthening with micro or nano additives are the methods frequently applied by researchers in the literature. In this study, it was tried to improve the rheological properties of PLA by using a nano additive, and at the same time, how the concept of molecular weight affects the dispersion of this nano additive was investigated. Cellulose Nanocrystals (CNC), which has become popular recently and can improve both the rheological and morphological properties of PLA without affecting its biodegradability, have been used as nanoadditives. CNCs are biopolymers with only crystalline regions after hydrolysis of the amorphous regions in the structure produced from cellulose with acids. Considering its chemical structure, it shows hydrophilic properties because it contains plenty of oxygen and hydroxyl groups. PLA, on the other hand, is a hydrophobic polymer in the ester structure, consisting of long carbon-hydrogen chains and without hydroxyl molecules in its structure. Therefore, they are incompatible with CNC due to their characteristics and their interaction is too weak. In PLA/CNC nanocomposites produced by the melt mixing method, which is the most preferred in the industry, bad results were obtained due to this incompatibility between matrix and the CNC. Thus, nanocomposites with the expected good properties could not be achieved. In order to increase the interactions of CNC and PLA, interface development was achieved by using surface modifiers and the properties of the nanocomposite improved positively. However, nanocomposites produced by the melt mixing method are also very sensitive to high heat due to the ester structure of PLA and can be easily degraded. For this reason, solvent casting is the method used both to avoid this degradation and to produce PLA/CNC nanocomposite without using any surface modifying chemicals. By choosing a suitable solvent that can dissolve both phases, it is possible to better disperse the hydrophilic CNC in the hydrophobic polymer matrix and achieve better results. The solvent chosen for this study is Dimethyl Sulfoxide. Studies in the literature have reported that it disperses CNC better in PLA matrix than solvents such as THF and DMF. In the literatue percolation threshold values were obtained generally between 0.12% - 3%. Based on these results, 0.5% -1% CNC amounts were selected to examine for this thesis. It was also investigated how CNCs would disperse in three different types of PLA: high molecular weight PLA2500HP, medium molecular weight PLA 3001D, and low molecular weight PLA3251D. The mutual point of these polymers is that all of them are semi-crystalline structure leading better results in CNC containing composites compared to amorphous PLAs according to studies and findings in the literature. Within this context, this thesis searches the effect of PLA's molecular weight on the dispersion level of PLA/CNC nanocomposites through the matrix and network formation which is the main stone to get maximized reinforcing effect. When the preparation method was examined, first of all, CNCs were mixed in DMSO solvent for 2 hours in a water bath mixer, so that the CNCs were thoroughly dispersed. Afterwards, PLA granules were added to the prepared DMSO-CNC mixture and the magnetic stirrer was mixed for 4 hours at 90C. The mixtures were poured into petri dishes and kept at room temperature for 2 days, then dried as a film in an oven under vacuum at 85oC for 5 days. Afterwards, the nanocomposites, which were pulverized with the help of a grinder, were kept under vacuum in an oven for 2 more days. In the light of rheology measurements, viscoelastic properties of all molecular weight PLAs improved dramatically with the addition of 1% CNC. The shear thinning behavior of PLA improved with the increase of the complex viscosity. At the same time, this increase can be interpreted that the CNC is well dispersed in the polymer matrix and a network structure is formed with each other. The viscous-like character of unadulterated PLA has turned into an elastic -like material. This significant increase in the storage module with the 1% CNC additive corresponds to the percolation threshold amount, which is the minimum amount required to improve the rheological properties of PLA . In the studies conducted in the literature, these increases are experienced after the CNC is well dispersed and networked in the PLA matrix. On the other hand, 0.5% CNC amount did not give good results for medium and high molecular weight PLA. The reason for this is that because the CNC is not well dispersed in these structures, no network structure can be formed and the viscoelastic properties cannot change. CNCs, which could not enter the longer chains of medium and high molecular weight PLAs, interacted with each other and acted as a foreign substance in the polymer matrix, thus reducing the usual properties. In low molecular weight PLA, on the other hand, it can be interpreted that viscoelastic properties can still improve and this improvement occurs because CNC can more easily disperse between short PLA chains. Similar results were observed for other rheological results. Considering the apparent yield stress of PLA nanocomposites, those containing only 1% CNC showed yield stress, which is in line with the results obtained in other graphs. TGA results showed that the thermal stability was mostly affected with the medium molecular weight PLA. As the CNC amount increased, the degradation temperature of high and low molecular weight PLAs improved by 11oC and 4oC, respectively. No significant change was observed for medium molecular weight PLA. Non-isothermal DSC results showed that the presence of CNC has remarkable effect on the crystallization of PLA. Especially in medium molecular weight PLA, the presence of CNC decreased the cold crystallization temperatures. In high molecular weight PLA, 1% CNC decreased the melting temperature, while it increased the melting temperature in medium molecular weight. No significant difference was observed in low molecular weight PLA.
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ÖgeFabrication of caffeic acid grafted poly(lactide)-b-poly(hydroxyethylmetacrylate) films(Graduate School, 2022-09-19) Düz, Gamze ; Kahveci, Muhammet Übeydullah ; 515191004 ; Polymer Science and TechnologyIn 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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ÖgeHigh internal phase emulsion template method for fast and selective mercury adsorption(Graduate School, 2022-01-06) Yıldırım, Melis Şeval ; Yavuz, Erdem ; 515171009 ; Polymer Science and Technology ; Polimer Bilim ve TeknolojisiThere are many template methods for designing porous polymers. Porous polymers have become highly preferred in the industry due to their easy processability and properties. In this study, it was polymerized using a high internal phase emulsion template (HIPE). HIPE is named by looking at the ratio of the external phase volume to the total volume. If this ratio is greater than 0.74, it can be called HIPE.Macroporous polymers prepared by the HIPE method were used. The main disadvantage of PolyHIPE polymers is that they have a low high area (SBET ~9 m2g-1). To overcome this disadvantage, a hypercrosslinking reaction was performed. High surface area polymers were obtained with the Fiedel Crafts reaction (SBET ~ 594 m2g-1). After the hypercrosslinked polymer was obtained, three different functionalization steps were applied. These are respectively; aldehyde, carboxylic acid and amide functionalization. The main purpose of this study is to obtain a selective, fast adsorbent on mercury. It is amide groups that provide selectivity to mercury. The -CONR2 group was obtained from the -Cl groups. In this study, amide group was obtained from carboxylic acid by using four different amides as propylamine, ethanolamine, aniline, diethylamine and DIC/NHS. With the emulsion templating strategy, hypercrosslinking polymers with different hyper-crosslinking times (15 minutes, 30 minutes, 60 minutes, 22 hours) were obtained, increased to amide groups and adsorption studies were carried out. While the main product, HXL-30min-PHP-CONR2, adsorbs 28 mg/g mercury in the first 2 minutes, it is 40.5 mg/g in the 180th minute when it reaches equilibrium. For these studies, different pH ranges were tried (pH 3, pH 4, pH 5, pH 6, pH 7) and the optimum pH was found to be 7.Isotherm (Langmuir, Freundlich, Dubinin- Radushkevich) and kinetic (pseudo-first order, pseudo-second order, intra-particle diffusion) models were made for HXL-30min-PHP-CONR2. Window and void diameter were calculated by using SEM images with Imagej program. Reuse studies were performed for HXL-PHP-CONR2 using 0.1 M HNO3, and 90% capacity was observed up to the 5th cycle.
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ÖgeInvestigation of the thermal and rheological properties of PET/PBT blends(Graduate School, 2022-01-28) Benli, Emine Büşra ; Nofar, Mohammad Reza ; 515171014 ; Polymer Science and TechnologyThe 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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ÖgeLight induced synthesis and characterization of clickable polyacrylamide hydrogels(Graduate School, 2022-02-17) Bilgiç, Mehmet Bilgehan ; Yağcı, Yusuf ; 515201010 ; Polymer Science and TechnologyHermann 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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ÖgePost-polymerization modification of poly(vinylene sulfide)s(Graduate School, 2023-08-09) Kakaş, İbrahim Ethem ; Günay, Ufuk Saim ; 515211008 ; Polymer Science and TechnologyThe 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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ÖgeProduction of special resins for use in polyurethane(Graduate School, 2022-01-18) Hüten, Tibet ; Kızılcan, Nilgün ; 515191015 ; Polymer Science and Technology ; Polimer Bilim ve TeknolojisiPolyurethane prepolymers, generally reaction products of diols and diisocyanates, are considered an important part of polyurethane chemistry due to their direct contribution to final properties. Isocyanate and polyol react and form a strong, reinforcing urethane bond. Prepolymers are generally prepared with a 2/1 ratio of isocyanates to polyols to ensure an NCO (isocyanate) terminated structure. NCO terminated structure later reacts with chain extenders and crosslinkers to produce polyurethane structure with larger molecular weight. In prepolymer chemistry, isocyanate is considered a reactive component of the reaction medium. Apart from their role as a reactive component, they also arrange the mechanical properties of polyurethane. Therefore, a proper type of isocyanate should be considered for its application. In microcellular elastomeric shoe sole polyurethane chemistry, diphenylmethane diisocyanate (MDI) has high consumption. Crude 4,4'- MDI has been chosen as the core of formulation for its highly reactive para-positioned NCO groups to form more reinforcing urethane bonds. However, a phenomenon called dimerization creates a dilemma on its high consumption in prepolymers. The reactivity of isocyanate is proportional to its rate of dimerization. Since crude MDI is a highly reactive material it has more tendency for dimerization. Dimerization is a phenomenon occupying a reaction between two isocyanates. The formation of dimers in the reaction medium reduces the free isocyanate components to react with polyols, creating a different reaction profile. Remaining free isocyanates in the reaction medium will react with a polyol to form a polyurethane structure with less frequent reinforcing urethane bonds. Therefore, fewer urethane bonds result in weaker mechanical properties. Changes in mechanical properties and reaction profiles are known symptoms of dimerization. Apart from these symptoms, dimers are known for their high melting and freezing points. According to this phenomenon, optimal operation and storage temperature of prepolymer will be in a more narrow spectrum. Prepolymer with a higher melting point will take a longer time to melt. Therefore, a more complicated pathway of processing should be considered. Storage conditions of prepolymers are also affected by dimerization. Dimerization rate is faster in 5 to 40 °C and above 50 °C . Therefore, prepolymers should be stored under 0 °C to evade dimerization. When the freezing point of a prepolymer is high and closer to the dimerization area, its stability during storage will dropdown. Freezing temperature is a key element to observing the storage stability of prepolymers. Isomers of crude MDI (2,4'- MDI and 2,2'- MDI) can be mixed with crude MDI to lower its melting and freezing temperatures for better operation and storage temperatures. Today, two forms of isocyanates named OP50 (50% - 50% mixture of 2,4'- and 4,4'- MDI) and carbodiimide modified MDI used for storage stabilization and improvement on melting of the prepolymer. Isomers of crude MDI have orto positioned NCO groups with low reactivity due to steric hindrance and show less tendency to dimerization. In literature, there is no concise study on the effect of OP50 and carbodiimide modified MDI in both melting and freezing points of prepolymers. Our study aims to investigate the effects of change in isocyanate and polyol components on freezing and melting points of prepolymers while preventing a dramatic change in mechanical properties. Prepolymers with carbodiimide modified MDI in 7-12 % show the lowest freezing temperature, the starting point of freezing, melting temperature, the starting point of melting. Its projections on mechanical properties in polyurethane (PU2) show improvement on elongation at break, tensile strength. Modulus shows a decrease due to loss of crystallinity. Besides from the best formulation, the mechanical properties of polyurethanes made from the other two best prepolymers are also investigated. PU3 which was made from PREP5 did not show a significant loss in mechanical properties same as the PU2. However, PU4 where PREP6 was used, show losses in compression set and tear resistance due to a lower amount of hydrogen and urethane bonding. It has the highest elongation at break value for its low level of crystalline hydrogen bonds. From shelf life observations in jars at 21 °C, found formula withstand freezing for 38 days while reference formula withstands for 19 days. Freezing showed a faster propagation in PREP1 than PREP4. At the end of 150 days, PREP1 completely frozen while PREP4 stayed in liquid form. For this reason, the formulation was found successful to be used in standard polyurethane applications and increasing the shelf life of prepolymers.
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ÖgeThe investigation of thermo-responsive polymers exhibiting LCST with interactions between water molecules by quantum mechanical methods(Graduate School, 2022-12-30) Kütük Can, Fatma ; Tüzün, Nurcan ; Sütay, Berkay ; 515181025 ; Polymer Science and TechnologyThermo-responsive polymers exhibiting the coil-to-globule phase transition at the lower critical solution temperature (LCST) have been the subject of great interest in the development of smart materials. As a result of LCST behavior in aqueous solutions, the polymers undergo a phase separation in water upon heating, resulting in a decreased solubility and a change in conformation from hydrated coil to collapsed globular for diluted solutions. In this work, thermo-responsive polymers in aqueous solutions have been studied in terms of their conformational changes which plays a crucial role in determining the role of functional groups and their interactions with water. Poly (N-isopropylacrylamide) (PNIPAAm), Poly(N-vinylisobutyramide)(PNVIBA), and Poly(2-ethoxyethyl vinyl ether)(PEOVE) exhibiting LCST behavior in water were studied in order to examine structural changes by using Density Functional Theory (DFT). Geometry optimizations were carried out using the B3LYP functional and 6-31G(d,p) basis set, which are known to perform well in the geometry optimizations of organic molecules. The hydrogen bond interaction of the water molecule with the pentamer of these monomers, which is the longest oligomer chain optimized starting from the dimer molecules, was modeled by testing various configurations. Interaction energies were calculated by using different functionals such as B3LYP, LC-wPBE and CAM-B3LYP and 6-311G(2df,2pd) basis set for a quantitative interpretation of the hydrogen bond. The basis set superposition error (BSSE) was also considered in all interaction energy calculations. Interaction energies were also corrected by the addition of zero point energies (∆ZPE). The effect of segment...solvent and segment...segment interactions on the conformation of the polymer chain were investigated in solvent media. The effect of the size of the oligomer on the conformation was also studied. In order to observe conformational changes in the backbones, Radius of gyration (Rg) and Root-mean-square deviation (RMSD) were calculated between the oligomer-water complexes and the unhydrated oligomer chain. The calculations in this study aim to provide insight into the effect of intermolecular interactions on conformational transitions in single chains of thermo-responsive polymers. Based on our calculations, the increasing number of water molecules results in an increase in the strength of the interaction.