Person: ABDURRAHMANOĞLU, SUZAN
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ABDURRAHMANOĞLU
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SUZAN
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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 Rheological Behavior of Polymer-Clay Nanocomposite Hydrogels: Effect of Nanoscale Interactions(WILEY, 2010) ABDURRAHMANOĞLU, SUZAN; Abdurrahmanoglu, Suzan; Okay, OguzThis research highlights different viscoelastic responses of polymer-clay nanocomposite hydrogels depending on the type of the monomer used in their preparation. Polymerization reactions of N,N-dimethylacrylamide (DMA), N-isopropylacrylamide (NIPA), and acrylamide (AAm) in aqueous clay (Laponite) dispersions have been investigated by rheometry using oscillatory deformation tests. The gelation profile of AAm polymerization obeys typical gelation kinetics, while a reverse behavior was observed during the DMA or NIPA polymerizations. In the latter cases, after an abrupt increase in elastic and viscous moduli at the start of the reaction, they both decrease continuously during the Whole Worse of the gelation process. Creep-recovery tests performed on the final hydrogels indicate that the time-dependent viscoelastic response of the gels derived from AAm is distinctly different from the other gels. The retardation time of AAm gel is about twice that of DMA or NIPA gels indicating higher mobility of the crosslink zones in the former gel. As a consequence, a larger amount of energy is dissipated during the deformation of nanocomposite hydrogels based on AAm. Different extent of interactions between the clay particles and the monomers explains the results of observations. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2328-2335, 2010Publication 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