Browsing by Subject "RAFT polymerization"
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Item Open Access Effect of brush length of stabilizing grafted matrix on size and catalytic activity of metal nanoparticles(Elsevier, 2020-06-01) Barsbay, Murat; Özgür, T. Ç.; Sütekin, S. D.; Güven, O.The rationale of this work is to investigate the effect of chain length of stabilizing polymer brushes attached to silica surface on the formation and catalytic activity of metallic palladium nanoparticles (Pd NPs). Poly(vinyl pyrrolidone) (PVP) brushes forming a very thin shell were grafted on silica microparticles (PVP@SiO2) via RAFT mediated graft polymerization, thus controlling the molecular weights and structures of PVP grafts. Pd nanoparticles were formed in PVP stabilizing matrix by gamma-induced reduction of polymer-bound Pd(II) ions to yield Pd(0) decorated core-shell particles (PVP-PdNP@SiO2). SEC and TGA results indicated the formation of PVP brushes with different molecular weights on silica substrate. DLS and TEM measurements revealed that particle growth was sterically blocked by the increase in PVP brush length, thereby forming a greater number of small Pd nanoparticles rather than larger ones. PVP-PdNP@SiO2 samples with different PVP chain lengths and Pd sizes were evaluated for their catalytic activity and reusability in the reduction of 4-nitrophenol to 4-aminophenol. Although the nanoparticles formed in the presence of longer grafted chains are smaller, their leakage into the solution has been found to be more effectively prevented by these long grafts. Thus, PVP-PdNP@SiO2 samples with longer PVP grafts showed more stable catalytic activity in repeated reaction cycles. These findings are particularly important for heterogeneous catalysis systems in that they show the effect of the size of surface-bound polymeric stabilizers on metal NP formation and catalytic activity.Item Embargo Ion specificity from small molecules to oligomers and beyond amide-based macromolecules(Bilkent University, 2024-01) Farooq, SobiaPresence of ions in aqueous solution regulate the properties of molecules in the same aqueous environment. Such alteration processes are mainly dependent on the concentration and the identity of ions. In this thesis, two parts of ion specific effects were aimed to be explored. First the synthesis and characterization of PNIPAM oligomers by using both reversible addition-fragmentation chain transfer (RAFT) and radical polymerization methods will be shown. Both of these methods give the control over molecular size of the polymer. Oligomers with charged and neutral end group were synthesized to comparatively investigate ion specific effect. These oligomers were also systematically characterized by using various analytical techniques such as phase transition temperature measurement, 1H-NMR and Gel permeation chromatography (GPC). Such oligomers were employed to investigate the specific ion effects via the salt influence on the Lower Critical solution temperature (LCST). By employing two sodium salts; NaCl (strongly hydrated) and NaSCN (weakly hydrated), it was found that strongly hydrated anions salt-out both charged and neutral oligomers, whereas weakly hydrated anions increase the phase transition temperature with a salting in mechanism. By empirical modeling with a Langmuir-type binding isotherm, a weak binding with a dissociation constant KD = 0.57 M for charged and KD = 1.13 M for neutral oligomers were demonstrated. The second part of this thesis focused on the specific ion effects beyond amide-based macromolecules i.e. hydroxypropyl cellulose (HPC) as a model for sugar-based macromolecules. Eight sodium salts were employed to demonstrate the entire Hofmeister series. Namely; NaSCN, NaI, NaNO3, NaClO4 NaCl, Na2SO4, Na2CO3, NaH2PO4 were measured on the phase transition temperature and 1H-NMR measurements. Salts of weakly hydrated anions; NaSCN, NaI, NaClO4 and NaNO3 showed a salting in mechanism and demonstrate a non-monotonic phase transition behavior. In contrast, salts of strongly hydrated anions; NaCl, Na2SO4, NaH2PO4 and Na2CO3 showed salting out mechanism with a monotonic decrease in the phase transition temperature. Additionally, the site-specific ion-macromolecule interaction was studied by 1H-NMR, and Correlation Spectroscopy (2D-COSY) NMR measurements. Although, the exact binding site cannot be specified, it was concluded that the ion binding site is at the side-chain hydroxypropyl groups and that yields the salting-in effect that was observed for the weakly-hydrated anions.Item Open Access Surface modification of electrospun cellulose acetate nanofibers via RAFT polymerization for DNA adsorption(Elsevier, 2014-11-26) Demirci, S.; Celebioglu A.; Uyar, TamerWe report on a facile and robust method by which surface of electrospun cellulose acetate (CA) nanofibers can be chemically modied with cationic polymer brushes for DNA adsorption. The surface of CA nanofibers was functionalized by growing poly[(ar-vinylbenzyl)trimethylammonium chloride)] [poly(VBTAC)] brushes through a multi-step chemical sequence that ensures retention of mechanically robust nanofibers. Initially, the surface of the CA nanofibers was modified with RAFT chain transfer agent. Poly(VBTAC) brushes were then prepared via RAFT-mediated polymerization from the nanofiber surface. DNA adsorption capacity of CA nanofibrous web surface functionalized with cationic poly(VBTAC) brushes was demonstrated. The reusability of these webs was investigated by measuring the adsorption capacity for target DNA in a cyclic manner. In brief, CA nanofibers surface-modified with cationic polymer brushes can be suitable as membrane materials for filtration, purification, and/or separation processes for DNA.