LEE- Polimer Bilim ve Teknolojisi-Yüksek Lisans

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http://hdl.handle.net/11527/19429

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Preparation pyrene bearing asymmetric block copolymer for sensing applications as a fluorescent chemosensor

(Graduate School, 2022) Koramaz, İlayda ; Karagöz, Bünyamin ; 736892 ; Polymer Science and Technology Programme

Herein, a pyrene based fluorescence chemosensor was designed with vicinal diol pendant groups for sensing application of various targeted analyte such as boric acid via aggregation induced emission. In this protocol, first hydrophilic poly(oligo (ethyleneglycol) methacrylate) (POEGMA) macro-CTA was synthesized via RAFT polymerization. Then, poly(oligo (ethyleneglycol) methacrylate)-block-poly(glycidyl methacrylate)-co-poly(4-(1-pyrenyl)-styrene) (POEGMA-b-PGMA-co-PPySt) block copolymer was prepared via chain extension of POEGMA by RAFT polymerization technique. Then, epoxy units of the PGMA segment were opened in two ways to obtain many vicinal diol groups either by reaction with Trizma base or basic aqueous medium. Resulting asymmetric hydroxyl functional block copolymer expected to exhibit chelating units towards to boric acid from the hydroxyl and/or seconder amine units. By this way, the polymer loss the hydrophilicity from this complexation with the targeted analyte which forces to make agglomeration. This agglomeration triggered the formation of excimer emission via π-π interaction of the pyrene units. This novel approach simply provides selective detection of boric acid detection in aqueous medium. Most of the agglomeration induced emission studies carried out by solvent manipulation, in our case, it was thought that the agglomeration induced by the complexation of targeted analyte. The demonstrated route has novelty on different analytes by simply changing the chelating units of asymmetric block copolymers. The critical point of the material is balancing the hydrophilic and hydrophobic ratio of the block copolymer to get the agglomeration after the complexation of the analyte. Overall, a simple sensing method was represented with this pyrene and vicinal diol bearing asymmetric block copolymer. The resulting tailored polymer molecular weights and repeating units were approximately calculated with 1H-NMR and polydispersity index (PDI) was determined from the GPC traces. From the results, repeating units of POEGMA was calculated as 40, MNMR as 12300 g/mol with 1.10 PDI; for POEGMA-b-PGMA-co-PPySt block copolymer these values were 40:16:2 (OEGMA:GMA:PySt), 15200 g/mol with 1.20 PDI. As for two potential chemosensors, tris tethered and NaOH treated block copolymers, MNMR was found as 17100 g/mol with 1.09 PDI and 15500 g/mol with 1.27 PDI respectively. According to GPC and 1H-NMR results, it was understood that the polymerization reactions proceed in a controlled manner successfully. Optical properties of the block copolymers were evaluated by UV-Vis and fluorescence spectrometer. For tris tethered POEGMA-b-PGMA-co-PPySt and POEGMA-b-PGlyMA-co-PPySt block copolymers in THF, maximum absorption and emission values were found as λabs = 347 nm, λems = 403 nm; λabs = 346 nm, λems = 407 nm under optimized conditions, respectively.
Preparation of polyacrylonitrile grafted onto nano-clay for removal of aluminum from aqueous solutions

(Graduate School, 2022) Küçük, Eda ; Şenkal, Bahire Filiz ; 713424 ; Polymer Science and Technology Programme

Heavy metals are extremely carcinogenic and show mutagenic properties when taken into the body. Aluminum (Al) is a toxic metal that is frequently encountered in daily life and can be taken into the body through drinking water, food, drugs and the air we breathe. Aluminum-based pollutants are found in the wastewater of many industrial sectors such as textile, plastic and food. Removal of toxic pollutants from wastewater has a significant role in terms of health and environment. As a result of many studies on the removal of heavy metals from wastewater, new treatment methods have been discovered. Adsorption is used as the most reliable method preferred for Aluminum among these methods. Polymeric clays are preferred more often than natural clays as adsorbent in the removal of heavy metal-derived pollutants such as Al from wastewater by adsorption method. Polymeric adsorbents with high adsorption capacity and large surface area are obtained by polymerizing functional groups in the structure of the clay and polymer types with desired properties. It has been determined by the studies on this subject that the adsorption ability of the clay is increased by forming a new sorbent with a suitable polymer. In this thesis, a new PAN grafted onto nano-clay was synthesized. 1.095 g (0.006 mole) hydrophilic bentonite nanoclay was wetted with 5 mL of distilled water. In a separate beaker, 0.8 g (0.001 mole) ammonium cerium (IV) nitrate used as initiator in redox polymerization was dissolved with 15 mL of distilled water in the presence of 1 mL (0.024 mole) nitric acid. This mixture was added to the wetted clay and the 10 mL (0.152 mole) of acrylonitrile and 15 mL (0.287 mole) of acetonitrile was put the polymerization medium. The graft polymerization was performed at room temperature for 18 h. Pure nano-clay and polymeric nano-clay were characterized by FTIR analysis. In addition, the Aluminum holding capacity of the polymeric nano-clay was characterized spectrophotometrically by sorption studies. In this study, Eriochrome Cyanine R (ECR) was used as a reagent in Al sorption experiments and all analytical measurements were carried out at pH 6 and using a UV-vis spectrophotometer at 535 nm. Al removal experiments were carried out depending on pH. Furthermore, Al removal studies from aqueous solutions using PAN grafted onto nano-clay as adsorbent were carried out at four different temperatures between 25 °C and 50 °C. In addition, according to the results obtained after the sorption study, kinetic models were applied and adsorption isotherm models were also investigated. When we look at the results of the kinetic models applied in this study, high correlation coefficient values here revealed that the data obtained as a result of the experiment showed a better fit with the pseudo second-order kinetic model.
High 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 Teknolojisi

There 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.
Production of special resins for use in polyurethane

(Graduate School, 2022-01-18) Hüten, Tibet ; Kızılcan, Nilgün ; 515191015 ; Polymer Science and Technology ; Polimer Bilim ve Teknolojisi

Polyurethane 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.
Investigation 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 Technology

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.
Effect of polylactide molecular weight on cellulose nanocrystal dispersion quality

(Graduate School, 2022-01-28) Dündar, Anıl ; Nofar, Mohammad Reza ; 515181001 ; Polymer Science and Technology

Today, 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.
Effects of crosslinking and nanosilica on waterborne polyurethane dispersions

(Graduate School, 2022-02-08) Erkahraman, Yunus ; Köken, Nesrin ; 515181034 ; Polymer Science and Technology

Waterborne polyurethane dispersions (WPUDs) are versatile materials that are a type of waterborne polymer colloids, utilize water as a dispersing medium, and contain little or no organic solvent. WPUDs are increasingly replacing solventborne polyurethanes thanks to their superior properties such as very low or no volatile organic compound (VOC), low viscosity, excellent film properties, ease of use, good adhesion, high abrasion resistance, and flexibility. WPUDs are preferred in a wide range of applications and industries, including coatings, binders, adhesives, sealants, automotive, inks, biomaterials, paper, wood, footwear, textiles, and many more. Different ways are used to improve the weak properties of WPUDs, such as poor mechanical strength, poor water and alkali resistance, relatively low heat resistance, and high raw material costs. The most common of these ways are, using crosslinkers in various structures, hybridization with polymers that contain different groups such as acrylics and silicones, and the creation of composites by adding nano or micron size inorganic fillers to the dispersion such as silica, clay, graphene oxide, SiO2, TiO2, Al2O3, Fe2O3, CaCO3. Nanosilicas, which have different types such as colloidal silica, precipitated silica, fumed silica, are widely preferred for making composite materials with WPUD because of different properties such as superior hardness, low toxicity, good stability and dispersion, and low cost. The main purposes of producing WPUD/nanosilica composites using various techniques including in-situ polymerization, blending, and the sol-gel process are to develop composite materials with superior mechanical, thermal, optical, chemical, rheological, or electrical characteristics than pure WPUDs. This study aims to obtain new waterborne PU dispersion (WPUD)/nanosilica composites by using various nano silicas and crosslinkers with different types such as isocyanate, aziridine, and waterborne PU dispersion, and investigate the effects of crosslinkers and nano silica by forming a film from these WPUD/nanosilica composites. Various tests were carried out to determine the thermal properties, crystallization behaviors, optical properties, swelling degrees of the obtained WPUD/nanosilica composite films, and it was aimed to gain the properties such as high water resistance, homogeneous distribution, and mechanical strength to the films and using them as a surface coating material in the future. In this study, we prepared various formulations using polyether-based aliphatic waterborne polyurethane dispersion with different amounts and types of nanosilicas (hydrophobic fumed silica and aqueous silica dispersion) and different crosslinkers such as polyether modified HDI--based polyisocyanate and polyfunctional aziridine. WPUD ratio was determined as 75% by weight, polyisocyanate ratio of 5%, and polyaziridine ratio of 2%, if present in the formulation. While fumed silica ratio ranged between 0.1 and 3% by weight aqueous silica dispersion ratio ranged between 1% and 25% by weight in the prepared formulations. WPUD/silica nanocomposites were prepared using the blending method by mechanical stirrer. Crosslinkers were added just before the tests due to the limited pot life. After pouring the prepared WPUD/silica nanocomposite dispersion into Teflon molds, cured for 4 hours at room temperature and at 50 °C for 8 hours in the oven to form films with a thickness of approximately 150μm. Fourier transform infrared spectroscopy (FTIR) was used to examine the chemical structure of the waterborne polyurethane dispersion, silicas, and crosslinkers used in the formulations, as well as the films formed after curing, to determine the changes in the peaks with the addition of crosslinker and silica, and to compare with the pure WPUD film. Differential scanning calorimetry (DSC) was used to examine the thermal properties of the films and X-ray diffraction (XRD) was used to examine the crystallization behaviors. Scanning electron microscopy (SEM)-EDX spectrometry was used for the morphological and elemental analysis of the films. The transparency of the films was observed with an optical microscope (OM) under the same conditions. The swelling degrees of the films in water were evaluated over weight changes. Finally, the hardness values of the films were measured with the Shore D Durometer. Characteristic polyurethane peaks were seen in the FTIR spectral analysis of the neat WPUD film. New characteristic peaks formed as a result of crosslinking reactions were determined. In addition, the presence of nano silica in the structure was determined by the appearance of characteristic Si-O-Si peaks. According to the DSC results, it was determined that the films contained crystalline structures, and the thermal stability of the films increased with the addition of fumed silica and aqueous silica dispersion. According to XRD data, it was determined that the crystallinity increased slightly with the addition of polyisocyanate, but the crystallinity decreased significantly with the addition of polyaziridine. In addition to these, nano silica additions were also found to reduce crystallinity. Also, It was determined that the films with the lowest crystallinity were the films containing polyaziridine crosslinker and fumed silica. In the neat WPUD film SEM images, it was observed that the surface was a rough surface consisting of nano and micron-sized polyurethane particles, and phase separations were observed on the surface. While the phase separations decreased and the surface became more uniform with the addition of polyaziridine, the phase separations increased with the addition of polyisocyanate. Also, it was observed that the surface became rougher with the addition of silica, as the silica ratio increased in the samples containing fumed silica, the silica agglomerations increased and the uniform structure did not occur. In the EDX measurements, carbon, oxygen, and nitrogen atoms were determined, and the silicon atom was also included in the structure with the addition of silica. According to the swelling measurement results, it was found that crosslinking and the addition of a certain amount of nanosilica reduced the degree of swelling. It was determined that the films with the lowest swelling degree were the films containing polyaziridine crosslinker and fumed silica. As a result of the transparency tests performed with an optical microscope, it was seen that the transparency of the films decreases as the amount of silica increases. In the hardness test results, it was seen that the hardness of the films increases with the addition of a crosslinker and certain amount silica. Also, It was observed that the addition of polyaziridine increased the hardness more than the polyisocyanate.
Light induced synthesis and characterization of clickable polyacrylamide hydrogels

(Graduate School, 2022-02-17) Bilgiç, Mehmet Bilgehan ; Yağcı, Yusuf ; 515201010 ; Polymer Science and Technology

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.
Double 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 Technology

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.
Applying 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 Technology

Nowadays, 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.
Induced crystalline fiber-like structure as reinforcement in PLA products through applied shear in injection molding

(Lisansüstü Eğitim Enstitüsü, 2022-06-27) Eraslan, Kerim ; Nofar, Reza M. ; 515201009 ; Polymer Science and Technology

In recent years, there has been a growing interest in sustainable, biobased, and biodegradable polymers due to the rise of public awareness towards petroleum-based polymers, the requirement to comply with new environmental laws, and the considerations about the ecological impacts of the product over the entire life cycle. Polylactic acid (PLA) is a biopolymer produced from renewable resources, considered an alternative to commodity and engineering applications due to its high strength, stiffness, and production capacity. However, PLA suffers from high brittleness, low melt strength, and slow crystallization, hampering processing and applicability. To improve the mechanical and thermal properties of PLA, methods such as PLA-based nanocomposite development, fiber modification methods, fiber reinforcement, mixing with rigid polymers or plasticizers, and chain branching have been applied. Although these attempts could eliminate the disadvantages of PLA, the biodegradability and natural advantages of PLA should be preserved while also reducing the end-product cost. Furthermore, the additives or fibers must be compatible with the mechanical and thermal recycling processes applied to the PLA products after use. Self-reinforced PLA composites (SR-PLAs) have been introduced to provide high strength and stiffness without traditional reinforcements. Furthermore, the absence of foreign reinforcements such as glass or carbon fiber can contribute to the complete biodegradability and easy recyclability of the composite. Self-reinforced composites are comprised of identical or similar types of polymers in the matrix and reinforcement phases. This concept aims to obtain a solid and stable matrix-fiber interface using polymers with similar chemical structures in both phases. However, the matrix and the reinforcement phases must have a certain melting temperature difference to produce these composites. Thus, while the matrix will be completely molten, the structure of the reinforcing polymer will be preserved. This study aims to determine the manufacturability and mechanical and thermal properties of in-situ SR-PLAs prepared through injection molding. Different types of PLAs were used to provide the required melting temperature difference in the matrix and reinforcement phase. The matrix was formed by an amorphous PLA (aPLA). The reinforcement phase was created by three semi-crystalline PLA grades (cPLA1, cPLA2, and cPLA3) with different crystallizability and molecular weight. To induce isothermal cold crystallization, the cPLAs were initially annealed at 90 oC, between the glass transition and melting temperature of PLA. Then, the aPLA/cPLA blends with different cPLA types and compositions with a weight ratio of 95/5, 90/10, 85/15, and 80/20 were prepared using a dry mixer. Finally, the aPLA/cPLA blends were dehumidified at 50 oC overnight and injection molded. Before processing, an aPLA/cPLA3 blend with a weight ratio of 85/15 was injection molded in different barrel and mold temperatures to determine the process parameters. The differential scanning calorimetry (DSC) analysis revealed non-isothermal cold crystallization peaks for aPLA/cPLA3 samples processed at an average barrel temperature of 160 oC. This was mainly due to the stretching and unfolding of some cPLA crystals during processing. In addition, the tensile results showed slightly lower mechanical properties for aPLA/cPLA3 samples processed at a mold temperature of 20 oC. Therefore, the barrel and mold temperatures were determined as 150 and 40 oC. Next, the thermal and mechanical behavior of neat PLA and aPLA/cPLA blends were studied. The neat PLA did not exhibit crystallinity due to the absence of the cPLA phase. On the other hand, the aPLA/cPLA blends with cPLA contents below 5 wt% also did not exhibit crystallinity. The aPLA/cPLA blends with cPLA contents above 10 wt% showed crystallinities due to unmelted cPLA crystals during processing as the process was carried below the melting temperature of cPLA crystals. Moreover, the crystallinities of aPLA/cPLA3 blends were more pronounced than aPLA/cPLA1 and aPLA/cPLA2 blends due to the higher melting temperature and crystallizability of cPLA3. The single glass transition around 60 oC indicated miscibility between aPLA and cPLAs. The tensile test results revealed that the tensile strength and modulus values of the aPLA/cPLA samples were significantly greater than those of the neat aPLA. Moreover, this reinforcing effect became even more prominent when cPLA3 was used. These enhancements were due to the induced fiber-like structure obtained through the inherent high shear rate of the injection molding process. The strain at break values did not reveal substantial improvements, although aPLA/cPLA samples were slightly more ductile than neat PLA. Finally, the HDT and impact strength analysis of aPLA/cPLA were investigated to improve the processability and applicability of PLA. While the cPLA addition slightly increased the heat deflection temperature (HDT) to 60 oC, it unaltered the impact strength at 12 kJ/m2. Therefore, polybutylene adipate terephthalate (PBAT) was selected to improve the tensile toughness of SR-PLA blends. cPLA3 was chosen as the reinforcing phase in SR-PLAs due to its most prominent enhancement effect on thermal and mechanical properties. The SR-PLA/PBAT blends with a fixed 15 wt. % PBAT was prepared similarly to SR-PLA preparation methods. In addition, 85 wt. % SR-PLAs contained different cPLA3 compositions with a weight ratio of 90/10, 80/20, and 70/30 to observe the effects of cPLA content on the material properties of SR-PLA/PBAT blends. The thermal analysis revealed that the SR-PLA/PBAT had higher crystallization than SR-PLAs. Moreover, the crystallinity was further increased with increasing cPLA3 content. The melting temperature of SR-PLA/PBAT blends was lower than SR-PLAs. This was attributed to the fact that the PBAT droplets could have penetrated between the cPLA crystals, resulting in a less ordered structure. The SR-PLA(80/20)/PBAT and SR-PLA(70/30)/PBAT blends exhibited non-isothermal cold crystallization due to the higher crystallizability of cPLA3. Furthermore, the cold crystallization in PLA(70/30)/PBAT occurred earlier than in PLA(80/20)/PBAT as the higher cPLA3 contents further enhanced the orientation of fiber-like crystals and supported heterogeneous cold crystallization. Finally, the two distinct glass transitions around -35 and 60 oC indicated immiscibility between PLA and PBAT. The stress-strain behavior of SR-PLA/PBAT blends showed that the PBAT addition resulted in a transition from ductile to brittle behavior. The tensile strength and modulus of SR-PLA/PBAT blends were lower than neat PLA. The maximum losses in terms of percentage for tensile strength and modulus were 15 and 25%, respectively. On the other hand, the strain at the break of the SR-PLA/PBAT blends was significantly greater than neat PLA. Moreover, this reinforcing effect was even more significant in higher cPLA3 contents due to the formation of a more uniform blend morphology. As a result, the ductility of neat PLA was increased from 3.9 to 44.6% in SR-PLA(70/30)/PBAT. Furthermore, the impact strength of neat PLA was enhanced to 16 kJ/m2 in all SR-PLA/PBAT blends. Finally, the improvements in blend morphology were demonstrated by SEM images. The microstructure of SR-PLA/PBAT blends revealed that the PBAT droplets were finely dispersed within the SR-PLA. Moreover, higher cPLA3 contents improved the blend melt strength, promoting the breakup of PBAT droplets. Therefore, a more uniform blend morphology corresponded to enhancements in ductility and impact strength.
Industrial scale sustainable nanocomposite production by melt mixing technique

(Graduate School, 2022-08-12) Avcıoğlu, Nesrin ; Nofar, Reza M. ; Doğu, Mustafa ; 515191011 ; Polymer Science and Technolog

Scientists are in search of new biopolymers as alternatives for petroleum-based polymers due to environmental concerns. Polylactic acid (PLA) is a biopolymer that is produced from renewable feedstock and has high performance with biodegradable and sustainable nature. This is while PLA suffers from brittleness, poor processability, and impact strength. The development of PLA composites and blends are possible ways to overcome such drawbacks. Cellulose nanocrystals (CNCs) are good alternative nanoparticles to improve some of these drawbacks compared to other reinforcements for not only providing good mechanical and physical properties but also being low density, biobased and biodegradable filler. Unfortunately, the production of nanocomposites on a large scale with good CNCs dispersion is still a challenge. This study is aimed to develop sustainable polymer nanocomposites by direct melt-mixing on an industrial scale through two approaches that are with and without masterbatch preparation. For this purpose, a corotating twin-screw extruder (Gülnar 1101402) was used and its specifications are 25 mm screw diameter and L/D ratio of 42. This extruder has 8 barrels with volumetric feeders. PLA was reinforced with CNC (1, 2, and 4% (w/w)) in this industrial-scale co-rotating twin-screw extruder. Polyethylene glycol (PEG) was used as an agent that could enhance the interfacial interaction between PLA and CNC to promote dispersion and acts as a plasticizer to overcome the brittleness of PLA. PEG concentration was kept constant at 10%(w/w) throughout all formulations. In this process, PLA was fed in the main feeder and solvent casted (distilled water was used as a solvent) PEG/ PEG-CNC after shredding was fed into the side feeder. For all trials, the vacuum pump that is located after the side feeder was set as on and the screw speed was set as 130 rpm. The temperature profile for each heating zones was 40°C, 170 °C, 175°C, 180°C, 185°C, 180°C, 185°C, 190°C and 185°C. Obtained granules from this process were dried at 50°C in a vacuum oven overnight both for injection molding and compression molding. In the compression molding process, the mold was set at 190°C for 5 minutes and pressure was gradually enhanced to 1.5 tons. At the end of the heating process under pressure, the mold was cooled quickly with water circulation. Samples for rheological characterization, SEM analysis, XRD analysis were prepared with this method. For the injection molding process temperature of the cylinder and temperature of the mold were set as 190°C and 70°C, respectively and residence time was optimized as 15 seconds. Dog-bone samples for the tensile test were obtained with this processing technique. Fabricated nanocomposites were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), rheological analysis, scanning electron microscopy (SEM) and tensile test. It was seen that from the DSC cooling thermogram, the addition of CNC resulted in non-isothermal crystallization behavior and improvement of crystallization capacity and crystallization temperature. When the consideration of the second heating cycle of normally compounded particles, the addition of CNC resulted in decreasing melting temperature. This means that crystal nucleation is improved with the addition of CNC and small crystals tend to reduce of melting point. The highest change of degree of crystallinity and melting temperature in comparison to the neat PLA were observed for PLA/PEG/CNC2 from 39% to 59% and from 177 oC to 172 oC, respectively. Whereas, for the masterbatch preparation method, the crystal growth observed instead of nucleation. This might be related to the agglomeration of the nanoparticles in the MB preparation technique, but the agglomeration could not be understandable from the SEM results. Besides, low melting shoulders are observed for the only normally compounded particles due to the existence of small crystals in the structure. DSC first heating thermograms were only considered for comparison of the results of XRD because of the thermal history of the materials. The neat PLA shows two distinct peaks in 2θ= 16.8 and 19.2. The different amounts of CNC in the nanocomposites result in some changes in diffraction patterns. This shift in the diffraction patterns means some structural changes in the crystallite. Moreover, loading more amount CNC in the nanocomposites leads to an enhancement of the intensity of crystalline peaks. The addition of CNC to the PLA matrix making a positive effect on the crystallite size and the ordering of the material is better in the nanocomposites that are produced from the MB preparation technique. Additionally, the thermogravimetric test of the neat PLA after extrusion, PEG, PLA/PEG blend, and its nanocomposites were performed to understand the thermal degradation behavior of materials. It was observed that neat PLA, PEG, and nanocomposites have shown one step degradation process. However, the PLA-PEG blend showed a two-step degradation process. The initial thermal degradation temperature of each nanocomposite ranges from about 260°C to 290°C. Maximum degradation temperature observed in neat PEG as 412 °C. These TGA curves demonstrate that the thermal stability of PLA decreases with the addition of PEG. The reduction of the thermal degradation temperature of PLA with the addition of the PEG can be related to some impurities after the chemical reaction between the OH- groups of PEG and ester groups of PLA. With the addition of the 1, 2, and 4 % (wt) CNC on the PLA/PEG, the decomposition temperature shifted to the higher temperatures of 261, 268, and 287°C, respectively. Additionally, the materials which were obtained by the MB preparation technique resulted in a thermal degradation temperature of 284 °C for both materials. Whereas PLA has better thermal degradation temperature in comparison to nanocomposites. Moreover, complex viscosity and storage modulus were considered for the rheological behavior of the materials. The results evidenced that neat PLA and its blend and nanocomposites show a Newtonian plateau at low frequencies, and it was observed that shear thinning behavior at high frequencies. Existence of PEG can be triggered transesterification reactions with PLA in the extrusion process, and these reactions were resulted in the degradation of the PLA matrix meanwhile reduction of polymer molecular weight. This reduction resulted in the reduction of PLA's complex viscosity and even addition of CNC could not enhance the complex viscosity. Considering the storage modulus, the addition of PEG made a negative effect on the moduli of PLA in the low and high-frequency range whereas some enhancements were observed in the storage modulus at the low frequencies with the addition of CNC. This improvement could not be obtained for the MB preparation technique due to the agglomerates. Changing of rigid and brittle nature of PLA with the addition of additives was analyzed with the tensile test. Tensile strength and Young's Modulus of neat PLA before extrusion are 50 MPa and 3500 MPa, respectively. However, after extrusion, it has been seen that a significant decrease in these values as about 42 MPa and 1720 MPa, respectively. PEG was selected as not only a processing aid to improve PLA-CNC interaction but also as a plasticizer. Hence, the addition of PEG into the PLA matrix results in an increment for elongation at the break. Besides, Young's modulus as well as the tensile strength of the blend slightly decreased. With the CNC loading, tensile strength and modulus are slightly enhanced. Whereas elongation at break is reduced due to the addition of CNC which is a stiffer phase. The masterbatch preparation aggregates might result in stress concentration points, so tensile properties cannot enhance. Consequently, it needs to be noted that in the twin screw extrusion process, materials were exposed to high shear. Thus, materials can be easily degraded. Even though PEG used as a well dispersing agent, these results were demonstrated that melt mixing is an inappropriate route to produce PLA-CNC nanocomposites. There is no surface treatment of CNC and that can promote the agglomeration of nanoparticles dramatically.
Silk fibroin cryogel-based shape memory organohydrogels

(Graduate School, 2022-08-19) Baş, Yahya ; Okay, Oğuz ; 515191025 ; Polymer Science and Technology

From textile fibers to biotechnology, silk has been a pivotal material for humanity for centuries. Fibroin, one of the two proteins that consist of silk, facilitates the structural integrity and physical endurance of the silk fibers and has been isolated and widely used for several purposes with its features like hydrophilicity, biocompatibility, and mechanical robustness. Polymeric gels are polymer-based, crosslinked wet and soft materials, including pore walls spaced by solvents that dissolve the polymer. Regarding the solvent-separator type that they include, the polymeric gels are mainly categorized into three types: hydrogels, organogels, and aerogels and they include water, organic solvents, and air in their pores, respectively. Hydrogels consist of over 90% water in their structure, most of all, possess unparalleled biomimicry due to their hydrophilic nature. Cryogels are a subclass of hydrogels that are synthesized under 'cryogenic' conditions, meaning under the freezing temperature of the water. Contrary to the conventional hydrogel case, the reason that gelation reaction occurs here is not the heat given to the system, rather it is the increased concentration of gel components in the unfrozen high concentration channels in the reaction system. This influence is called the 'cryo-concentration effect'. This effect provides the cryogels with more reliable mechanical performance and a strictly ordered porous structure that conventional hydrogels lack. Also, the porous morphology of the cryogels mimics the natural extracellular matrix structure of the animals perfectly, according to the scanning electron microscopy (SEM) image comparison of acellularized tissue scaffold and cryogels. Like there are conventional covalent crosslinks between the polymer chains of polymeric gels, there can also be some secondary interactions whose impact becomes significant upon an increased number of them. They regulate the chain conformation with various bindings they form out of many electrostatic interactions. These switchable crosslinks give the materials adjustable mechanical properties, fixing temporary shapes and healing the defects in their structure. The materials having such properties are called 'smart materials'. Smart materials have a variety of application areas such as sustained drug release systems, nanorobotics, biosensors, and tissue engineering. In the scope of this thesis, it was aimed to form an interconnected micro-organogel structure made of n-Octadecyl acrylate (C18A) and acrylic acid (AAc) monomers, within the pores of silk fibroin cryogels which are freeze-dried and swollen in an organogel precursor solution. The effect provides the OHGs their smart material abilities are the reversible dissipation and assembling of crystalline domains formed by the long alkyl tails of C18A monomers, with the repelling influence of water surrounding them and the hydrophobic interactions between them. These weak bonds would disappear upon heat exposure, over the melting temperature of poly(C18A) crystallites, around 50 oC, and would be reformed after the materials cooled down. These switchable crosslinks should provide the OHGs with their altering mechanical (softening and reformability) and shape memory properties. To confirm these assumptions, two sets of samples were prepared and tested. One set of OHG was made of constant fibroin concentration (5 w/v %) cryogel templates and embedded with micro-organogel structures including 4 different C18A monomer mole fractions: 10, 20, 25, and 30 %. The other set of OHG consisted of 4 different fibroin concentration cryogels: 5, 10, 15, and 20 w/v% scaffolds, and each one was filled with the organogel that has a 30% mole fraction C18A. The questioned parameters in each OHG set were the influence of the C18A mole fraction of organogel and fibroin concentration of cryogel, respectively. Initially, cryogel templates having 4 different fibroin concentrations were characterized regarding their swelling properties, pore morphologies and sizes, and uniaxial compression mechanical behavior. It was observed that cryogel porosity has decreased from 92 % to 74% with increasing fibroin concentration from 5 w/v% to 20 w/v%. SEM visualization showed that the average pore diameter decreased from 25.7 μm to 16.8 μm. Fourier Transform Infrared Spectrophotometry (FT-IR) results indicated a 10 % increase in the β-sheet conformation ratio. After the freeze-drying and organogel synthesis within, the OHG porosity was decreased to 45-8 %. The calculated specific pore volume was 17.3 – 2.9 mL/g for cryogels and it was also decreased to a minimum of 0.16 mL/g for OHGs. While Young's modulus was calculated as 0.04 MPa for 5 w/v% cryogels, regarding the increased fibroin concentration and C18A mole fraction, it was raised to a maximum of 17.7 MPa for OHGs with fibroin concentration of 20 w/v%, and C18A mole fraction of 30%. Fracture strains were observed as 10 % lower for the cryogels that became OHGs, while the fracture stresses increased 10 times their initial values. OHGs were subjected to differential scanning calorimetry (DSC) and rheometer tests to investigate this affiliation between temperature and material properties. In DSC testing, the materials were heated up from 0 to 80 oC in two cycles. The melting peaks observed around 54 oC during the heating stage of the test and the crystallization peak at 43 oC during the cooling stage were attributed to the reversible dissipation and regeneration of poly(C18A) crystallites. The first series of OHGs indicated that the increased mole fraction of C18A has increased the number of secondary interactions. This effect was comparatively shown between OHGs with the aid of a parameter called crystallinity index (fcry) which was calculated from the area below the melting peaks of crystallites in the heating stage of DSC tests. fcry describes the fraction of C18A monomers that contributed to the formation of poly(C18A) crystallites. The fcry values that were calculated regarding the melting enthalpies have shown a drop from 15.7 to 2 % when the C18A mole fractions were decreased from 30 to 10%. As a consequence of the bending shape memory tests, also, the OHGs with maximum C18A mole fractions have been seen to be perfectly capable of keeping the shape dictated to them, whereas OHGs with minimum C18A mole fractions were not. The second series of OHGs indicated that, with increasing fibroin concentration and cryogel pores getting narrower, the amount of secondary interactions within the micro-organogel structure gets lower. The calculated fcry values got decreased from 15.7 to 2.9 % with increasing fibroin concentration, from 5 to 20 w/v%. The morphology of cryogels and OHGs was investigated with X-Ray Diffraction (XRD) tests. Growth and sharpening of the peaks corresponding to 2θ = 9.5o, 20o, 24.6o angles with increasing fibroin concentration in cryogels revealed an increase in regular formations such as β-sheets. In the XRD patterns of OHGs, 2θ = 21.6o peaks, which are not seen in cryogels and correspond to the d = 0.41 nm range, revealed that C18A crystallites were formed in the structure. It is thought that the reduction of these peaks with increasing fibroin concentration is due to the difficulty of the orientation required for the formation of crystalline domains in the narrowing pores. Rheometer measurements done with OHGs with 5 w/v% fibroin and 30% C18A mole fraction have exhibited a significant drop in the elastic modulus (G') of the material, from 1.5 MPa to 220 kPA between room temperature and heated to 65 oC state. Once the material cooled down to room temperature again, it recovered its initial G' value, which indicated the existence of the reversibly dissipated and reformed crystalline domains. These results indicated the OHG's reversibly adjustable viscoelastic properties with the influence of heat. Also, all OHG's synthesized have exhibited shape memory capability with around 95% shape recovery ratio each.
Material properties of thermoplastic matrix carbon fiber reinforced high performance composites using automated fiber placement

(Graduate School, 2022-09-02) Akçakaya, Hatice ; Nofar, Reza M. ; 515191006 ; Polymer Science and Technology

Technological developments lead a way through change on material performance requirements. Material science has been one of the oldest areas of interest of humankind since ancient history, even historical milestones has been identified according to the materials used during the time interval. Throughout the years, material science has been evolved and improved such that, nowadays people are able to tailor the material to get desired property of final product. Material engineers deals with finding solutions to make desired final property products [28,31]. Aerospace is one of the engineering fields that requires strong, lightweight, resistant and flexible materials. This is a must since all aircrafts must be lightweight relatively to the other transportation machines. To answer high performance requirements studies on composite materials have significantly increased during the last decades. Composite materials consist of two or more distinguished material having individual properties, to be able to create a composite material an interface between individual components must take place. Composite materials should have improved final properties than the individual components. Composite materials can be divided into sub classes according to the reinforcement and the matrix phase. Matrix phase holds the reinforcement together and protects the reinforcements from external damages, which can be physical, mechanical or environmental conditions. Matrix materials can be polymer, metal, ceramic, reinforcement on the other hand, can be fibers, structures or particles. One of the most commonly used materials for the engineering applications is carbon fiber reinforced polymer matrix composites on critical parts of design for. For the matrix phase, thermoplastic or thermoset matrix are suitable options for such applications. Thermoplastic matrix has several advantages over thermoset matrix for being recyclable, lighter, durable and easier to storage with respect to thermoset matrix. In this study, unidirectional carbon fiber reinforced thermoplastic matrix composite panels consisting of 11 plies are laid down with automated fiber placement (AFP) robot with [[0°/90°]20°]/90°]s orientation to address a typical design approach. AFP is an additive manufacturing method of producing high quality, near absent flaw final product. It requires less time to manufacture parts with AFP than conventional methods. Impregnation means to wet fibers with matrix, prepreg is shortened name for pre-impregnated. To be able to manufacture a composite part, prepregs are laid down on to each other. Conventional way of manufacturing a composite material is hand lay-up of prepregs. If the reinforcement phase is a continuous fiber, then the stacking of prepregs can be named after orientation of fiber. There are two different options of matrix used for this study, one having relatively higher melting temperature than the other. Different matrix material is used to address, whether melting temperature change causes any difference in thermo-mechanical properties or not. In this study 8 panels are manufactured, they are laid down with different lay-up speeds with average of 100 mm/sec and 400 mm/sec. Change of lay-up speed gives an idea of how fast the production rate can be, in order not to lose thermo-mechanical strength and crystallinity of the material. For the thermoplastic materials, consolidation takes place under certain circumstances. Consolidation is a process of solidification of the polymer. In this study, half of the identical panels are post consolidated in the autoclave. By doing so, the effect of the post consolidation is investigated by post consolidating one of two set of the identical panels in autoclave, others left as in situ. The objective is to investigate the trend of approaching post consolidated performance level without any further need of post consolidation by changing the process parameters. In order to be able to evaluate the outcomes of parameter, material and process change, differential scanning calorimetry (DSC), microscope, gas pycnometer and dynamic mechanical analysis (DMA) tests are conducted. Various results are investigated with respect to crystallinity, defect formation, void content, and mechanical performance of the panels. Results has shown that there is no significant effect of lay-up speed and melting temperature of the matrix on crystallinity, whereas post consolidation has a strong influence on both degree of crystallinity and thermo-mechanical properties. Post consolidated panels have 25000 MPa of storage modulus while in situ panels have 15000 MPa. The results are also elaborated with the void content which is relatively decreases with the post consolidation treatment.
Fabrication of caffeic acid grafted poly(lactide)-b-poly(hydroxyethylmetacrylate) films

(Graduate School, 2022-09-19) Düz, Gamze ; Kahveci, Muhammet Übeydullah ; 515191004 ; Polymer Science and Technology

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.
The 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 Technology

Thermo-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.
Catalyzing the inverse vulcanization reaction via 1,3-benzoxazines

(Graduate School, 2023) Shafizada, Ahmad ; Kışkan, Barış ; 807401 ; Polymer Science & Technology Programme

High-performance polymers take a vital role in our daily life by their commercial manufacturing and extensive range of applications in various areas like structural materials, aerospace, electronics, printed circuit boards, coatings and adhesives. Polymeric materials can be categorized as thermosets and thermoplastics according to their behavior against temperature. Thermoplastics are a resin which is solid at the room temperature, but becomes softer by increasing the heat and showing fluid behavior or properties due to the melting point or passing the glass transition temperature. Some of thermoset polymers can be consider as a high-performance polymers because of their unique features such as flame resistance, decent mechanical strength, dimensional stability and durability against many solvents. Although these remarkable advantages, they have some disadvantages like shorth shelf life, being brittle, water adsortion and etc. The common members of thermoset resins are phenol-formaldehyde, urea-formaldehyde, melamine-formaldehyde and etc. Phenol-formaldehyde resins are well-known, the most and wide produced in the thermoset resins family. In the 20th century it has been found that the drawbacks of phenolic resins can overcome by polybenzoxazines via their excellent properties: high char yield, high thermal resistance and stability, zero volumetric change and low toxic substance releasing during curing, cost-effective, low moisture absorption, without any requirement for catalyst and etc. Additionally, the easy synthesis procedure of polybenzoxazines is also one of their striking advantages. Because there are corresponding monomers as a polybenzoxazine precursors which can be prepared from cost-effective raw materials such as primary amines, formaldehyde and phenols. In this study, it is aimed to catalyze the reaction of vinylic monomers with sulfur, known as reverse vulcanization. In general, for reverse vulcanization to occur, elemental sulfur must be heated up to the homolytic bond breaking temperature of 160 °C. The resulting sulfur radicals react with the double bond and trigger the polymerization from there. Although the required temperature for reverse curing is high, side reactions increase at this temperature. Therefore, it is important that inverse vulcanization can be catalyzed at more reasonable temperatures. In this study, benzoxazines with different functionalities will be used as catalysts. All the benzoxaiznes acted as catalyst to reduce inverse vulcanization. Among the benzoxazine monomers pyridine containing benzoxazine reduced inverse vulcation temperature better compared to other benzoxazines. It is known that pyridine-type systems contribute to the homolytic decomposition of sulfur, and that sulfur generates radicals by removing hydrogen from benzoxazines. An effective catalytic system was designed by combining these two approaches in a single structure. In this thesis, the development of benzoxazine-based high-performance catalysts by utilizing the flexibility of benzoxazine chemistry is described. The designed benzoxazine will contain the pyridine structure on it. Thus, it is thought that the catalytic performance for reverse vulcanization will increase. With a design benzoxazine catalyst, an attempt will be made to reduce the required minimum temperature of 160 oC to 130 oC for inverse vulcanization.
Enhancement of interfacial properties for high performance polyethylene fibers produced via gel spinning

(Graduate School, 2023) Ünlü, Oğuz Kağan ; Kılıç, Ali ; 515211011 ; Polymer Science and Technology Programme

Fibers are materials with high length/diameter ratios, having adjustable fineness, mechanical properties. High performance fibers exhibit excellent mechanical properties such as high modulus, high strength, high abrasion resistance, thermal resistance, low density. One of the methods used to produce high performance fibers is gel spinning, in which the chains are partially entangled in liquid-gel form and connection with each other , rather than solution or melt unlike other methods (dry spinning, wet spinning, melt spinning). In this thesis, ultra high molecular weight polyethylene (UHMWPE) was dissolved in paraffin oil and high performance UHMWPE fibers were obtained by gel spinning method. The gel obtained by dissolving UHMWPE in paraffin oil is formed into filaments by passing through the spinnerettes after extrusion, then passes through the quenching and extraction bath, drying and winding in the final stage. High tensile strength, low specific density, great impact resistance, and exceptional chemical resistance are just a few of the excellent qualities of ultra-high molecular weight polyethylene (UHMWPE) fiber. It is frequently utilized in fishing, aircraft, biomedicine, and ballistic, among other things. UHMWPE fiber has a very high degree of crystallinity (>99%) and macromolecular orientation (>95%), which leads to a high modulus and tenacity of UHMWPE fiber. Gel spinning is a difficult and advanced engineering process. UHMWPE concentration is one of the parameters that determine fiber strength which was kept as 8% wt in this study. For this research, n-hexane was used in the extraction bath to remove paraffin oil from spun UHMWPE fibers. Following the solvent extraction procedure, the fibers undergoing hot drawing with different drawn ratios. The differential scanning calorimeter is used to analyze the thermal and crystallization properties of the fibers in their drawn, undrawn, and gel-state forms. Two newly peaks are observed when DR reached to 40. These peaks shows orthorombic-hexagonal transition. An orthorhombic structure represents a prism-like crystal structure with three unequal edges and internal angles, whereas a hexagonal structure resembles a hexagon with six equal edges. The tensile test was used to reveal the effect of drawn ratios on performance and it was revealed that with the increase in drawn ratio, the mechanical strength would increase by a maximum of 322.71%. Simultaneously, this thesis also focuses on investigating the enhancement of interface properties in UHMWPE fiber/epoxy composites.There are numerous surface treatment methods used to improve the coaction between fibers and composite materials, chemical etching and corona discharge are two commonly used methods to enhance the fiber- epoxy interaction. In this thesis study, the optimum corona discharge result was revealed by experiments performed at different voltage and time parameters. Then, the obtained results from corona discharge method were compared with the results obtained when the surface was modified via chemical etching, and it was seen that hydroxyl and carboxyl groups were formed on the surface more effectively in the chemical etching method. Glutaraldehyde, which is used as a cross-linking agent in chemical etching method, can form bonds with OH functional groups due to its chemical structure. Thus, the hydroxyl groups formed on the UHMWPE fibers become able to cross-link with the hydroxyl groups in the epoxy. Tests have shown an increase in both mechanical performance and adhesion.
Energy dissipation and rate-dependent deformation behavior of STF-integrated PU foam nanocomposites

(Graduate School, 2023-06-08) Gündüz, Emre ; Karagöz, Bünyamin ; 515191022 ; Polymer Science and Technology

Designing energy absorptive structures is crucial for safety and these structures withstand several loads such as compression, bending, and impact. Beside to honeycombs as a core material for sandwich structures, polymeric foams receive great attention as structural components with their outstanding effective stress transfer, viscoelastic properties, and increased surface area. Among several polymeric foams, rigid polyurethane (PU) foams are good candidates presenting high strength and lightweight characteristics and tailorability. PU is a particular polymer group that stands out with its unique chemistry, typically synthesized from polyol and isocyanate, which also has a potential to fill the vacancies between rubbers and plastics based on their mechanical thermal and viscoelastic properties. PU foams can have densities that range from 20 to 3000 kg/m3 which provide a wide range of application areas and properties. The physical and mechanical properties of PU foams are closely correlated to their morphological characteristics, which are mainly determined by several key parameters such as foam density, cell density, cell edge length, wall thickness, and thickness to length ratio (t/l). PU foams could be customized by incorporating nano and/or micro-sized reinforcing agents to obtain unique functionality and properties. Combining nanoparticle inclusion with process parameter optimization can result in improved mechanical properties and multifunctionality. Nanoparticles can act as nucleation points and lead to narrow cell edge length, increased cell wall thickness, and higher cell density, yielding advanced mechanical properties. Despite the considerable research into carbon-based nanostructures such as carbon nanotubes (CNTs) and graphene as reinforcing agents in polymeric foams to maximize their energy absorption capabilities and strength, further efforts are required to investigate the potential role of other nanomaterials, such as shear thickening fluids (STFs), which exhibit extraordinary energy absorption properties. STFs as colloid suspensions, at elevated shear rates, form hydroclusters due to particle interaction yielding a drastic viscosity increase. Thus, rapid viscosity change results in an excellent energy absorption characteristic. This study is aimed to improve the compressive and energy absorption properties of PU foams with STF integration while discussing the microstructure/mechanical property relationship. Initially, STFs were fabricated with up to 30 wt.% fumed and spherical silica content, using a mechanical stirrer at 300 rpm and horn sonicator with 30% amplitude, then investigated by a plate rheometer in order to understand the effect of particle geometry and weight fraction on the flow characteristics. The results revealed that 26 wt.% of fumed silica was the optimum suspension for integration to PU foam with the excellent thickening ratio, and viscosity values increased after critical shear rate up to 67.66 times. The 26 wt.% STFs were successfully integrated into rigid PU foams, using a mechanical stirrer prior to the foaming reaction, at 0.5, 1, and 3 wt.%, and the morphological analysis, compression, and cyclic compression tests at various strain rates up to 0.2 s-1 and 10, 40, 80% strains xxii in order to understand rate dependent properties under different deformation region such as plateau and densification, were performed. According to morphological characterizations, cell edge lengths decreased and cell wall thickness increased with increasing until 1 wt.% STF. The maximum t/l ratio, which is a favorable indicator for mechanical strength prediction, was observed with 1 wt.% STF integration into PU foam. The results showed that 1 wt.% STF presented the highest compressive strength and specific compressive strength with 33% and 10.4% increments, respectively. For the energy absorption properties, 1 wt.% STF demonstrated up to 9.4% higher loss factor and 46.9% total absorbed energy regarding cyclic compression tests strain and strain rate parameters. Dynamic mechanical analyses were also carried out, under bending loads with dual cantilever clamps, to develop the microstructure-mechanical property relationship and to examine the viscoelastic properties. Linear viscoelastic region and storage and loss moduli increased with 1 wt.% STF integration. The results were consistent with both the compression and cyclic compression tests, despite differences in the direction and frequency of the applied force. Integration of 1 wt.% STF resulted in a significant increase of approximately 50% in both storage and compression modulus, while no significant changes in loss factor were observed. By enhancing the storage modulus and broadening the linear elastic regime, STF/PU foams can effectively withstand higher levels of load and displacement without permanent deformation. Excessive STF integration, such as above 1 wt. %, caused deterioration in the foam morphology and cellular structure, resulting in lower mechanical properties and strengths, however the 3 wt.% STF integrated foam still exhibited better properties than neat foam. This study showed that the flow properties of STFs are significantly influenced by the silica surface geometry and weight fraction. Furthermore, the study demonstrated that the addition of an optimized amount of STF enhanced the compressive strength, viscoelastic properties, and energy absorption capabilities of PU foams. With the addition of STF, PU foams could be used in a wider range of application areas and their superior mechanical properties could be used to build safer and reliable structures.
Post-polymerization modification of poly(vinylene sulfide)s

(Graduate School, 2023-08-09) Kakaş, İbrahim Ethem ; Günay, Ufuk Saim ; 515211008 ; Polymer Science and Technology

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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