Person: ABDURRAHMANOĞLU, SUZAN
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ABDURRAHMANOĞLU
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SUZAN
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Publication Metadata only Preparation of homogeneous hydrogels by controlling the crosslinker reactivity and availability(TAYLOR & FRANCIS INC, 2008) ABDURRAHMANOĞLU, SUZAN; Abdurrahmanoglu, Suzan; Okay, OguzNetwork microstructures of polyacrylamide (PAAm) hydrogels were investigated by static light scattering measurements. The gels were prepared by free-radical crosslinking polymerization of acrylamide (AAm). To suppress the degree of gel inhomogeneity, the crosslinker reactivity during gelation was controlled by decreasing its availability in the reaction system. Our first approach was the addition of the crosslinker N,N'-methylenebis(acrylamide) (BAAm) in one or three portions during the course of the gelation reactions. As a second approach, a slightly water soluble crosslinker, namely ethylene glycol dimethacrylate (EGDM) was used as a crosslinker in AAm polymerization. Due to the low water solubility of EGDM, EGDM phase in the gelation system act as a reservoir of crosslinker so that the crosslinker can be supplied continuously to the aqueous reaction zone during the course of gelation. It was found that the delayed crosslinker addition technique further increases the degree of inhomogeneity of PAAm hydrogels. The results were explained with increasing viscosity of the reaction solution at the time of the crosslinker addition so that the crosslinking reactions are limited to local regions in the reaction system. The second approach, namely use of the slightly water soluble crosslinker EGDM significantly increases the degree of structural homogeneity of PAAm hydrogels.Publication Metadata only Synthesis and characterization of new dextran-acrylamide gels(WILEY, 2007) ABDURRAHMANOĞLU, SUZAN; Abdurrahmanoglu, Suzan; Firat, YurdunDextran-acrylamide gels were synthesized in a single step reaction by using 4,4'-azobis(4-cynovaleric acid) as bi-functional initiator at 60 degrees C. Corresponding acrylamide (AaM) gels in the absence of dextran were also prepared for comparison. Several parameters such as reaction period (6, 12, and 24 h), monomer and crosslinker concentrations were varied and their effects on the properties of gels were investigated. Gels were characterized by their mechanical and swelling behaviors and in terms of structural changes using SEM. It was observed that swelling degree decreased by increasing the monomer concentration due to formation of more crosslinking points that cause tighter network structure. Mechanical measurements showed that elastic modulus of AaM gels was higher than that of dextran-AaM gel which indicating the importance of dextran concentration on the flexibility of the network.Publication Metadata only Unusual swelling behavior of polymer-clay nanocomposite hydrogels(ELSEVIER SCI LTD, 2007) ABDURRAHMANOĞLU, SUZAN; Can, Volkan; Abdurrahmanoglu, Suzan; Okay, OguzThe swelling behavior and the elastic properties of nanocomposite hydrogels have been investigated. The hydrogels were prepared by freeradical polymerization of the monomers acrylamide (AAm), N,N-dimethylacrylamide (DMA), and N-isopropylacrylamide (NIPA) in aqueous clay suspensions at 21 degrees C. Laponite with a radius of gyration in distilled water of 20 nm was used as clay particles in the hydrogel preparation. The reactions with AAm monomer were carried out in the presence of the chemical crosslinker N,N-methylenebis(acrylamide) (BAAm). It was found that the volume of nanocomposite hydrogels immersed in water rapidly increases and attains a maximum value after about one day. Surprisingly, further increase in the swelling time results in the deswelling of the gels until they reach a limiting swelling ratio after about 5 days. This unusual swelling behavior is observable only when the clay concentration in the hydrogel is above the overlap threshold c*. Swelling measurements combined with the elasticity tests show that the effective crosslink density first decreases, but then increases with increasing time of swelling of the hydrogels. The results were explained in terms of the rearrangements of the highly entangled polymer chains and clay particles during the gel volume change. (c) 2007 Elsevier Ltd. All rights reserved.Publication Metadata only Design of high-toughness polyacrylamide hydrogels by hydrophobic modification(ELSEVIER SCI LTD, 2009) ABDURRAHMANOĞLU, SUZAN; Abdurrahmanoglu, Suzan; Can, Volkan; Okay, OguzPolyacrylamide (PAAm) hydrogels possessing a very large extensibility at break have been prepared via micellar crosslinking copolymerization of acrylamide monomer and N,N'-methylenebis(acrylamide) crosslinker in the presence of hydrophobic comonomers. N-butyl-, N-hexyl-, N-octyl-, and N,N-dihexylacrylamides were used as the hydrophobes in the hydrogel preparation. Incorporation of hydrophobes with an alkyl chain length x > 4 results in an increase in the loss factor tan 6 of hydrogels due to the formation of temporary junction zones inside the gel network. The number N-H of the hydrophobes per hydrophobic block together with the alkyl chain length x of the pendant hydrophobic group were used to tune the loss factor of the hydrogels over two orders of magnitude. Tensile mechanical measurements show that increasing N-H or x also increases the degree of toughness of PAAm hydrogels. Keeping constant the hydrophobe level (20 mol%) at an alkyl chain length x=6, increasing N-H from 9 to 30 increased the elongation ratio at break from 125 to 250%. Hydrogels exhibiting a high toughness, i.e., about 300% elongation ratio at break were obtained by modification of PAAm network chains with 10 mol% N-octylacrylamide. (C) 2009 Elsevier Ltd. All rights reserved.Publication Metadata only Homogeneous Poly(acrylamide) Hydrogels Made by Large Size, Flexible Dimethacrylate Cross-Linkers(AMER CHEMICAL SOC, 2008) ABDURRAHMANOĞLU, SUZAN; Abdurrahmanoglu, Suzan; Okay, OguzPublication Metadata only Equilibrium swelling behavior and elastic properties of polymer-clay nanocomposite hydrogels(WILEY-BLACKWELL, 2008) ABDURRAHMANOĞLU, SUZAN; Abdurrahmanoglu, Suzan; Can, Volkan; Okay, OguzNanocomposite hydrogels were prepared by free-radical polymerization of the monomers acrylamide (AAm), NN-dimethylacrylamide (DMA), and N-iso-propylacrylamide (NIPA) in aqueous clay dispersions at 21 degrees C. Laponite XLS was used as clay nanoparticles in the hydrogel preparation. The hydrogels based on DMA or NIPA monomers exhibit much larger moduli of elasticity compared with the hydrogels based on AAm monomer. Calculations using the theory of rubber elasticity reveal that, in DMA-clay or NIPA-clay nanocomposites, both the effective crosslink density of the hydrogels and the functionality of the clay particles rapidly increase with increasing amount of Laponite up to 10% (w/v). The results suggest that DMA-clay and NIPA-clay attractive interactions are stronger than AAm-clay interactions due to the formation of multiple layers on the nanoparticles through hydrophobic associations. It was also shown that, although the nanocomposite hydrogels do not dissolve in good solvents such as water, they dissolve in dilute aqueous solutions of acetone or poly(ethylene oxide) of molecular weight 10,000 g/mol, demonstrating the physical nature of the crosslink points. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 109: 3714-3724,2008