LEE- Polimer Bilim ve Teknolojisi-Yüksek Lisans
Bu koleksiyon için kalıcı URI
Gözat
Son Başvurular
1 - 5 / 5
-
ÖgePost-polymerization modification of poly(vinylene sulfide)s(Graduate School, 2023-08-09)The demands for the fast synthesis of a significant number of target compounds could not be met by conventional chemical synthesis. The "click chemistry" was presented as a solution to this problem based on the biosynthesis pathways and the synthesis of natural products. It is revealed in 2001 by a team of chemists at The Scripps Research Institute, led by K. Barry Sharpless. Click reactions are defined as a group of reactions function well in a variety of environments, which are simple to carry out, produce their intended products in extremely high yields with few to no byproducts, are unaffected by the nature of the groups' connections to one another and exhibit great stero- and regio-selectivity. Additionally, click reactions allow for facile product separation ways. The copper (I)-catalyzed azide-alkyne cycloaddition, an enhanced variant of the Huisgen azide-alkyne dipolar cycloaddition, is the most used click technique. Cu(I) toxicity was yet another disadvantage; it is still unsuitable for biological uses. To address this issue, numerous metal-free "click" reactions were developed to polymer chemistry. Among them, thiol-yne click and thiol-ene click reactions became popular for polymer chemists. The Michael addition reaction is a flexible synthesis technique for the effective coupling of a wide range of nucleophiles with electron deficient olefins. When Michael donors are thiols and Michael acceptors are α-β unsaturated carbonyl compounds, the reaction is called as thiol-yne or thiol-ene addition. Thiol-yne and thiol-ene are robust click reactions that are unaffected by water and produce few and harmless byproducts that can be easily removed. Recent developments of base catalysis enabled high yields with high reaction specificity. The heteronucleophilic conjugate addition reaction involving sodium thiophenolate and propiolates to produce unsaturated esters was first found by Ruhemann shortly after Arthur Michael's initial study on the conjugate addition of carbon nucleophiles to unsaturated substrates. Under ambient reaction conditions, quantitative conversions may be seen, which is undoubtedly made possible by the strong nucleophilicity of the thiolate anion and/or the utilization of highly electrophilic alkynes. The synthesis of acetylenic polymers containing heteroatoms presents considerable importance. One method employed to synthesize these polymers is alkyne hydrothiolation, which involves the combination of alkynes and sodium thiolates to produce vinyl sulfides. The concept of the "hydrothiolation" reaction was initially introduced by Truce and Simms in the 1950s. Alkynes can be hydrothiolated with aryl/aliphatic thiols to form vinyl sulfides by radical, nucleophilic or metal-catalyzed routes. By the nucleophilic and metal-catalyzed alkyne hydrothiolation processes both anti-Markovnikov (linear) and Markovnikov (branched) products are produced, while only anti-Markovnikov (linear) products are produced by the radical alkyne hydrothiolation method. The drawback of metal catalyzed route is the requirement of heat and for the radicalic route, the inorganic bases were not suitable to aryl thiols. While the radicalic reaction can be initiated by thermal or UV irradiation, the nucleophile-catalyzed pathway requires an electron-deficient alkyne or alkene with an adjacent electron-withdrawing group, such as an ester, amide, or cyanide. The nucleophilic thiol-ene addition is constrained by the availability of the activated alkenes, but it also benefits from mild reaction conditions, no detectable byproducts, and high conversion is possible under ideal reaction conditions. The development of organobases such as diethylamine, triethylamine, diphenylamine, DABCO enabled the efficient reaction conditions for hydrothiolation of activated alkynes. Thus, it is demonstrated that the hydrothiolation reaction may be carried out under ideal conditions with high yields by using the nucleophilic pathway catalyzed by organobases and adapting it to the polymer science. Tang and colleagues applied the hydrothiolation procedure to polymer science. In the presence of several secondary and tertiary amines, such as diethylamine, triethylamine, diphenylamine, triphenylamine, and morpholine in high concentrations, they carried out polyhydrothiolation of aryl dithiols with arylacetylenedicarboxylates in the presence of organobases. They also discovered that poly(vinylene sulfide)s had molecular weights up to 32.3 kDa in high yields. For organobase selection part a study conducted by Durmaz and coworkers was taken as a reference. Durmaz and colleagues investigated the systematic and selective modification of the electron-deficient triple bond using thiols via nucleophilic thiol-yne interactions. They developed a polyester with an internal electron-deficient alkyne molecule, and on this scaffold, they investigated amino-yne and thiol-yne processes. The effectiveness of several amidine and guanidine bases, as well as various nitrogen and phosphorus-based catalysts, was examined in nucleophilic thiol-yne reactions. In-depth 1H NMR investigations showed that when the nucleophilic catalysts 1,4-diazabicyclo[2.2.2]octane (DABCO) and 4-(dimethylamino) pyridine (DMAP) were utilized in the reactions, only mono thiolation was seen even though 1.2 equivalents of thiols (per alkyne) were applied. High efficiencies, such as 90 and 84%, were demonstrated by DABCO and DMAP. Additionally, the end products showed mixed stereoregularities. The catalysts were then tested using the amidine base 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) and the guanidine bases 1,1,3,3-Tetramethylguanidine (TMG) and 1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD). For DBU, TMG, and TBD, respectively, the efficiency for the mono additions was determined to be 83, 86, and 74%. These bases effectively facilitated the thiol-yne reaction. It is interesting that although using the same amount of thiol for each alkyne repeating unit, these catalysts, as measured by 1H NMR, also generated the double addition products (8%, 4%, and 15% for DBU, TMG, and TBD, respectively) in addition to the mono addition products. According to this study, TBD was preferred for double addition of thiols onto electron deficient vinylene sulfide units during the thiol-ene click reaction due to its efficiency and DABCO was preferred for mono addition of thiol onto the electron deficient alkyne moieties during the thiol-yne polymerization reaction due to its efficiency. Poly(vinylene sulfide)s possess activated double bonds. These double bonds can be crosslinked and functionalized with thiols in the presence of organobases. There are several studies about functionalization of electron deficient alkyne units. In this manner, the synthesis of a polymer with electron-deficient alkyne groups was the subject of research by Thayumanavan and colleagues. On this polymer, they applied thiol-yne and thiol-ene addition processes using reversible addition-fragmentation chain-transfer (RAFT) polymerization. The electron-deficient alkyne groups were modified with thiols in their work using Et3N as an organocatalyst. The thiol vinyl ether functional groups of the resultant homopolymer were modified using a potent base, TBD, to introduce thioacetal linkages in the pendant group as well. This work was an example of post-polymer modification of pendant electron deficient alkene units. Post-polymer modification of electron deficient alkene units was also conducted by Durmaz and coworkers. They employed unsaturated polyester (UP) scaffold for this purpose. By esterifying maleic anhydride with 1,4-butanediol an unsaturated polyester (UP) scaffold was created. Then, in the presence of TBD in CHCl3, this polyester scaffold was altered using different thiols. This was an illustration of post-polymer functionalization of electron-deficient alkene units in the main chain. Truong and Dove conducted a separate study on the functionalization of electron-deficient alkene units. In their research, they initiated the reaction between dodecane-1-thiol and PEG32-bispropiolate using a catalytic amount of Et3N. Then, they modified vinylene sulfide with benzylmercaptan in the presence of TBD. The purpose of this study is to synthesize polymers with electron deficient vinylene sulfide units in the main chain and modifying them with various thiols in the presence of an organocatalyst. In this work firstly, propiolic acid and ethylene glycol were reacted in the presence of p-toluenesulfonic acid monohydrate at 105 ℃ in benzene to obtain a symmetrical ester with two activated alkyne moieties. The product, ethane-1,2-diyl dipropiolate was reacted with 1,6-hexanedithiol in the presence of an organocatalyst DABCO at room temperature which yielded poly(vinylene) sulfide. Activated double bonds of the synthesized poly(vinylene) sulfide were modified with various monothiols (1-octanethiol, 2-mercaptoethanol, 2-furanmethanethiol, benzylmercaptane, 2-propanethiol, 1-propanethiol, cyclopentanethiol, methylthioglycolate, 3-nitrobenzylmercaptan, allyl mercaptan and 2-methyl-2-propanethiol) in the presence of a strong basic and nucleophilic organocatalyst TBD, in CHCl3 with various efficiencies at room temperature. Synthesized polymers were characterized with GPC, 1H NMR and 13C NMR.
-
ÖgeFabrication of caffeic acid grafted poly(lactide)-b-poly(hydroxyethylmetacrylate) films(Graduate School, 2022-09-19)In this study, we aimed to prepare an antimicrobial, PLA-based partially degradable caffeic acid functionalized film that has the potential to be used in active food packaging. We report a synthetic route for the functionalization of poly(D, L-lactide)-b-poly(2-hydroxyethyl methacrylate) copolymer grafted with caffeic acid. First, we synthesized a dual initiator, namely 2-Bromo-N-(5-hydroxyphenyl)2-methylpropanamide (BNMP) (yield = %58), bearing ATRP initiator and ring opening polymerization. The initiator was synthesized via an amidation reaction between 5-amino-1-pentanol and 2-bromoisobutyryl bromide in the presence of triethyl amine. Firstly, ring-opening polymerization of D, L-lactide was achieved via the hydroxyl group of BNMP using tin(II) 2-ethyl hexanoate (Sn(Oct)2) as catalyst (Mn = 9800 g/mol). This polymerization was carried out by the melt polymerization method. Secondly, 2-hydroxyethyl methacrylate (HEMA) was polymerized through atom transfer radical polymerization (ATRP) using poly(D, L-lactide) macroinitiator yielding the block copolymer, PLA-b-PHEMA. Eventually, the block copolymer was functionalized via a Steglich esterification reaction between caffeic acid and hydroxyl groups of the PHEMA segment to obtain PLA-b-PHEMA-g-CA. The polymers were characterized by size exclusion chromatography (SEC), nuclear magnetic resonance spectroscopy (NMR), ultraviolet-visible (UV-Vis), and infrared (IR) spectroscopies. The grafting degree was calculated by using the regression line of caffeic acid standards at known concentrations and was found 60.7%. The bioactivity of the caffeic acid-functionalized block copolymer was investigated with DPPH for radical scavenging activity and antimicrobial activity against gram-positive (S. aureus.) and gram-negative (E.coli) bacteria. The films were prepared by mixing different amounts of PLA-b-PHEMA-g-CA with a high molecular weight of commercial PLA via solvent casting technique. The mechanical properties of the films obtained from the mixture of copolymer with high molecular weight PLA were also examined. To gain further insight, the thermal and surface properties of the films were evaluated with differential scanning calorimetry (DSC) measurements and water contact angle measurements(WCA), respectively. According to the results, the synthesized PLA-b-PHEMA-g-CA polymer showed bioactivity.
-
ÖgeDouble layer thermoplastic polyurethane-gelatin electrospun wound dressings for biomedical applications(Graduate School, 2022-06-23)Electrospinning is the technique to produce nano or micro scale fibers. Since this technique offers high surface area to volume ratio and porous structure, the dressing mimics the extracellular matrix (ECM). This technique gives the opportunity to use synthetic and natural polymers, active agents or essential oils, for that reasons it is a commonly preferred technique to produce wound dressings. Besides, to be able to add more features to the wound dressings double layer electrospun dressings have been made recently. Therefore, top layer which basicly includes synthetic polymers and active agents mimicks the epidermis layer, wheras sublayer with natural polymers and active agents mimicks the dermis layer of the skin. In this study, double layer nanofiber mats were produced by electrospinning technique. Each layer had a St. John's Wort oil (Hypericum perforatum oil) to provide a curative effect on the wound healing process. Top layer, which includes thermoplastic polyurethane (TPU) and Hypericum Perforatum Oil, was elastic, prevented water and pathogen transition and gave mechanical strength to the electrospun mat. On the other hand, the bottom layer which basically includes cold water fish gelatine and Hypericum Perforatum Oil firstly aimed to repair by being a source of collagen. With these properties the top layer and bottom layer mimic the epidermis and dermis layers of the skin, respectively. Besides, plasma treatment was applied as a surface activation technique for the TPU electrospun mat to have better adhesion to hydrophilic gelatin electrospun layer. By this way, better packing was obtained between the layers. Morphological and chemical characterization, water vapor transmission rate, absorption tests and antibacterial property test were done to the dressings. According to the result, a homogenous fiber structure was obtained. The Sample 2 showed antibacterial property and suggested for moderate to highly exudative wounds. On the other hand, Sample 3 which was a plasma treated sample also had homogenous, bead free fiber structure, showed antibacterial activity and suggested for moderately exudative wounds, but it had lower active agent release during the test period due to better packing between the layers with plasma treatment. Sample 3 had lower active agent release during the test period. Therefore the sample could be suggested for long term remedies. Overall results indicated that obtained nanofiber dressings can be a good candidate as a wound dressing material.
-
ÖgeHigh internal phase emulsion template method for fast and selective mercury adsorption(Graduate School, 2022-01-06)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.
-
ÖgeProduction of special resins for use in polyurethane(Graduate School, 2022-01-18)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.