Person: TUNCABOYLU, DENIZ CEYLAN
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Publication Metadata only Porous rubber cryogels: effect of the gel preparation temperature(2014-08-01T00:00:00Z) Oztoprak, Zeynep; Hekimoglu, Tugce; Karakutuk, Ilknur; Tuncaboylu, Deniz C.; Okay, Oğuz; TUNCABOYLU, DENIZ CEYLANThis paper examines the effect of the gel preparation temperature (T (prep)) on the physical properties of the rubber-based macroporous organogels prepared by solution crosslinking in benzene at subzero temperatures. Cis-polybutadiene (CBR) and styrene-butadiene rubber (SBR) were used as the rubber components, while sulfur monochloride (S2Cl2) was the crosslinker in the gel preparation. It was shown that T (prep) is an extremely important parameter to adjust the porous structure and thus, the cryogel properties. The networks formed by CBR and SBR showed an aligned porous structure with an exception of honey-comb structured porous SBR cryogels prepared at -2 A degrees C. 10(1)- to 10(2)-mu m sized regular pores of the networks caused by the benzene crystals act as a template during gelation, separated by 10-20 mu m pore walls in thickness. They exhibit fast swelling and deswelling properties as well as reversible swelling-deswelling cycles in toluene and methanol, respectively. The ability of the organogels for the removal of petroleum products from aqueous solutions was also demonstrated using diesel and crude oil as model pollutants. In addition, the reusability of the organogels and their continuous sorption capacities were checked by repeated sorption-squeezing cycles. All the tests showed that the aligned porous organogels are suitable materials for the oil spill cleanup procedures.Publication Metadata only Structure optimization of self-healing hydrogels formed via hydrophobic interactions(2012-11-09T00:00:00Z) Tuncaboylu, Deniz C.; Argun, Aslihan; Sahin, Melahat; Sari, Murat; Okay, Oğuz; TUNCABOYLU, DENIZ CEYLANIn an attempt to mimic self-healing functions in biological systems, we investigate here the optimum design parameters of self-healing hydrogels formed by hydrophobic associations in aqueous solutions of wormlike sodium dodecyl sulfate (SDS) micelles. n-alkyl (meth)acrylates were used as the hydrophobic comonomer (2 mol %) of acrylamide in the gel preparation. Two structural parameters are crucial for obtaining self-healing gels via hydrophobic interactions. One is the length of the alkyl side chain of the hydrophobe, and the other is the surfactant concentration. In addition, hydrophobic methacrylates generate gels with a higher healing efficiency than the corresponding acrylates due to the limited flexibility of the methacrylate backbones, leading to a greater number of non-associated hydrophobic blocks. These free blocks locating near the fracture surface of the gel samples link each other to self-heal the broken hydrogel. The physical gels without SDS are very tough due to their sacrificial bonds that are broken under force and preventing the fracture of the molecular backbone. (C) 2012 Elsevier Ltd. All rights reserved.Publication Metadata only Autonomic self-healing in covalently crosslinked hydrogels containing hydrophobic domains(2013-11-01T00:00:00Z) Tuncaboylu, Deniz C.; Argun, Aslihan; Algi, Melek Pamuk; Okay, Oğuz; TUNCABOYLU, DENIZ CEYLANSelf-healing hydrogels suffer from low mechanical strength due to their reversible breakable bonds which may limit their use in any stress-bearing applications. This deficiency may be improved by creating a hybrid network composed of a combination of a physical network formed via reversible crosslinks and a covalent network. Here, we prepared a series of hybrid hydrogels by the micellar copolymerization of acrylamide with 2 mol % stearyl methacrylate (C18) as a physical crosslinker and various amounts of N,N--methylenebis(acrylamide) (BAAm) as a chemical crosslinker. Rheological measurements show that the dynamic reversible crosslinks consisting of hydrophobic associations surrounded by surfactant micelles are also effective within the covalent network of the hybrid hydrogels. A significant enhancement in the compressive mechanical properties of the hybrid gels was observed with increasing BAAm content. The existence of an autonomous self-healing process was also demonstrated in hybrid gels formed at low chemical crosslinker ratios. The largest self-healing efficiency in hybrids was observed in terms of the recovered elastic modulus, which was about 80% of the original value. (C) 2013 Elsevier Ltd. All rights reserved.Publication Metadata only Dynamics and Large Strain Behavior of Self-Healing Hydrogels with and without Surfactants(2012-02-28T00:00:00Z) Tuncaboylu, Deniz C.; Sahin, Melahat; Argun, Aslihan; Oppermann, Wilhelm; Okay, Oğuz; TUNCABOYLU, DENIZ CEYLANPolyacrylamide hydrogels formed via hydrophobic interactions between stearyl groups in aqueous micellar solution of sodium dodecyl sulfate (SDS) present two faces depending on which state they are. The gels containing SDS micelles exhibit, in addition to the fast mode, a slow relaxation mode in dynamic light scattering (DLS) and time-dependent elastic moduli, indicating the temporary nature of the hydrophobic associations having lifetimes of the order of seconds to milliseconds. The gels where SDS had been removed after their preparation behave similar to chemically cross-linked ones with time-independent elastic moduli, a high degree of spatial inhomogeneity, and a single relaxation mode in DLS. Because of this drastic structural change, the physical gels are insoluble in water with a gel fraction close to unity. In surfactant containing gels, a large proportion of physical cross-links dissociate under force, but they do so reversibly, if the force is removed they reform again. The reversible disengagements of the hydrophobic units building the physical cross-links leads to a self-healing efficiency of nearly 100%, while no such healing behavior was observed after extraction of SDS due to the loss of the reversible nature of the cross-linkages.Publication Metadata only Self-healing hydrogels formed in catanionic surfactant solutions(2013-01-01T00:00:00Z) Akay, Gizem; HASSAN-RAEISI, Azadeh; Tuncaboylu, Deniz C.; Orakdöğen, Nermin; ABDURRAHMANOĞLU, SUZAN; OPPERMANN, Wilhelm; Okay, Oğuz; TUNCABOYLU, DENIZ CEYLANPhysical gels with remarkable properties were obtained by copolymerization of acrylamide with the hydrophobic monomer stearyl methacrylate (C18) in a micellar solution of cetyltrimethylammonium bromide (CTAB) containing up to 15 mol% sodium dodecyl sulfate (SDS). The addition of SDS causes the CTAB micelles to grow and thus enables solubilization of C18. The gels exhibit time-dependent dynamic moduli, high elongation ratios at break (1800-5000%), and self-healing, as evidenced by rheological and mechanical measurements and substantiated by dynamic light scattering. As the size of the micelles in the gelation solution increases, both the degree of temporary spatial inhomogeneity and the lifetime of hydrophobic associations in the gels increase while the elongation ratio at break decreases. Although the physical gels were insoluble in water due to strong hydrophobic interactions, they could be solubilized in surfactant solutions thus providing a means of characterization of the network chains. Viscometric and rheological behaviors of polymer solutions show a substantial increase in the associativity of the network chains with rising micelle size, which results in prolonged lifetime of hydrophobic associations acting as physical cross-links in gels. The internal dynamics of self-healing gels could thus be controlled by the associativity of the network chains which in turn depends on the size of CTAB micelles.Publication Metadata only Tough interpenetrating Pluronic F127/polyacrylic acid hydrogels(2013-05-01T00:00:00Z) Baskan, Tuba; Tuncaboylu, Deniz C.; Okay, Oğuz; TUNCABOYLU, DENIZ CEYLANTough interpenetrating polymer network (IPN) hydrogels with pH- and temperature sensitivity were prepared by crosslinking copolymerization of acrylic acid (AAc) and N,N--methylenebis(acrylamide) in 20 w/v% aqueous solutions of F127 (PEO99-PPO65-PEO99). The presence of F127 within the gel network slightly decreases the elastic modulus while the loss factor significantly increases, revealing increasing energy dissipation in IPN hydrogels. Cyclic compression tests show large mechanical hysteresis in IPN hydrogels due to the reversible formation of ionic clusters and hydrophobic associations of F127 molecules. The dissipative mechanisms created by F127 lead to the improvement in the mechanical performance of IPN hydrogels when compared to the polyacrylic acid (PAAc) gel controls. PAAc hydrogel formed at 10% AAc fractures under a compression of 0.2 MPa at 78% strain, while the corresponding IPN hydrogel sustains up to 7 MPa compressions at 98% strain, leading to an increase of toughness from 31 to 335 kJ/m(3). IPN hydrogels subjected to the heating-cooling cycles between below and above the micellization temperature of F127 show characteristic features of F127 solutions, i.e., increase of the dynamic moduli on raising the temperature, and thermal hysteresis behavior. (c) 2013 Elsevier Ltd. All rights reserved.Publication Metadata only Surfactant-induced healing of tough hydrogels formed via hydrophobic interactions(2014-02-01T00:00:00Z) Argun, Aslihan; Algi, Melek Pamuk; Tuncaboylu, Deniz C.; Okay, Oğuz; TUNCABOYLU, DENIZ CEYLANDifferent reversible molecular interactions have been used in the past few years to generate self-healing in synthetic hydrogels. However, self-healing hydrogels synthesized so far suffer from low mechanical strength which may limit their use in any stress-bearing applications. Here, we present a simple technique to heal mechanically strong polyacrylamide hydrogels formed via hydrophobic interactions between stearyl groups. A complete healing in the hydrogels was achieved by the treatment of the damaged areas with an aqueous solution of wormlike sodium dodecyl sulfate micelles. The micelles in the healing agent solubilize the hydrophobes in the cut surfaces, so that they easily find their partners in the other cut surface due to the hydrophobic interactions. Surfactant-induced healing produces high toughness (similar to 1 MPa) gels withstanding 150 kPa of stress at a deformation ratio of 1,100 %. The healing technique developed here is generally applicable to the physical gels formed by hydrophobic associations.Publication Metadata only Tough and Self-Healing Hydrogels Formed via Hydrophobic Interactions(2011-06-28T00:00:00Z) Tuncaboylu, Deniz C.; Sari, Murat; OPPERMANN, Wilhelm; Okay, Oğuz; TUNCABOYLU, DENIZ CEYLANLarge hydrophobic monomers stearyl methacrylate (C18) and dococyl acrylate (C22) could be copolymerized with the hydrophilic monomer acrylamide in a micellar solution of sodium dodecyl sulfate (SDS). This was achieved by the addition of salt (NaCl) into the reaction solution. Salt leads to micellar growth and, hence, solubilization of the hydrophobes within the SDS micelles. The hydrogels thus obtained without a chemical cross-linker exhibit unique properties due to the strong hydrophobic interactions. They can only be dissolved in SDS solutions demonstrating the physical nature of cross-links. Results of dynamic light scattering, rheological and mechanical measurements show that the hydrophobic associations between the blocks of C18 or C22 units prevent water solubility and flow, while the dynamic nature of the junction zones provides homogeneity and self-healing properties together with a high degree of toughness. When fractured, the hydrogels formed using C18 associations can be repaired by bringing together fractured surfaces to self-heal at room temperature, after which, they again exhibit the original extensibility) of about 3600%. The existence of free, nonassociated blocks in C18 hydrogels is accounted for their high self-healing efficiencies.Publication Metadata only Hierarchically Macroporous Cryogels of Polyisobutylene and Silica Nanoparticles(2010-05-18T00:00:00Z) Tuncaboylu, Deniz C.; Okay, Oğuz; TUNCABOYLU, DENIZ CEYLANOrganic inorganic hybrid materials attract particular interest because of their excellent mechanical properties. Here, we report the synthesis of hybrid cryogels consisting of interpenetrated polyisobutylene and silica networks. The gels were prepared by cross-linking of butyl rubber in cyclohexane containing silica nanoparticles using sulfur monochloride (S2Cl2) as a cross-linking agent. The microstructure of the hybrid networks formed at subzero temperatures exhibits two generations of pores: 10(1) mu m sized large pores due to the cyclohexane crystals acting as a template during gelation and, 10(-1)-10(0) mu m sized small pores between the aggregates of the nanoparticles. The nanoparticles in hybrid cryogels accumulate within the large pores where cyclohexane crystals originally resided. Compared to the organogel networks with an elastic modulus of a few kPa, hybrid networks exhibit a modulus of elasticity around 300 kPa. Hybrid cryogels can be converted into organic cryogels by dissolving the silica component in aqueous hydrofluoric acid, while removing the polymer component by calcination results in porous silica networks with 10(-1) mu m sized pores.