Research Outputs
Permanent URI for this communityhttps://hdl.handle.net/20.500.14288/2
Browse
8 results
Search Results
Publication Metadata only Conformational analysis of model poly(ether urethane) chains in the unperturbed state and under external forces(Amer Chemical Soc, 2002) Department of Chemistry; Department of Chemistry; Department of Chemical and Biological Engineering; Yılgör, İskender; Yurtsever, İsmail Ersin; Erman, Burak; Faculty Member; Faculty Member; Faculty Member; Department of Chemistry; Department of Chemical and Biological Engineering; College of Sciences; College of Sciences; College of Engineering; 24181; 7129Conformational features of model poly(ether urethane) (PEU) single chains are investigated in the unperturbed state and under external force. Model PEUs consisting of symmetrical, aromatic, 1,4-phenylene diisocyanate (PDI) hard segments and poly(tetramethylene oxide) (PTMO) soft segments with varying lengths are considered. Rotational potential maps are calculated quantum mechanically using the AM1 parametrization with Gaussian98. Configurations of the chains are generated by the Monte Carlo technique, using the rotational isomeric state formalism and successive matrix multiplication scheme. Unperturbed dimensions, the change in dimensions when a force acts along the end-to-end vector, the stiffness, and toughness of the chain and orientability of segments under external force are characterized and compared with properties of polyethylene. The characteristic ratio of along PEU chain is 5.0, and the Kuhn length for a single block is 9.2 Angstrom, both of which are smaller than the corresponding polyethylene values. The model chains are significantly more ductile and tougher than polyethylene. The orientability of the backbones exhibits a strong even-odd effect, with strongly orientable bonds neighbored by weakly orienting ones. The degree of rigidity of the phenyl group does not propagate far along the chain. The projection of the bond vectors on the phenyl axis decay rapidly with increasing distance of the bond along the chain.Publication Metadata only Effect of chemical composition on large deformation mechanooptical properties of high strength thermoplastic poly(urethane urea)s(Amer Chemical Soc, 2004) Curgul, Sezen; Çakmak, Miko; Department of Chemistry; Department of Chemistry; Department of Chemical and Biological Engineering; Yılgör, İskender; Yılgör, Emel; Erman, Burak; Faculty Member; Researcher; Faculty Member; Department of Chemistry; Department of Chemical and Biological Engineering; College of Sciences; College of Sciences; College of Engineering; 24181; 40527; 179997The effect of composition on the true mechanooptical properties of thermoplastic poly(urethane urea)s was investigated by selectively varying the type and content of soft and hard segments. Real-time stress-strain-birefringence data together with off-line wide-angle X-ray scattering measurements revealed that soft segment and chain extender play dominant roles on the chemical structures of the poly(urethane urea)s. All poly(tetramethylene oxide) glycol-based samples showed the same crystal structure. The samples containing ethylenediamine as the chain extender showed enhanced crystallizability as compared to those with 1,6-diaminohexane no matter which soft segment was used. In general, samples with lower fraction of hard segment exhibited higher crystallizability than their high hard segment counterparts. Long-term holding of poly(ethylene oxide) samples in stretched state was found to increase crystallinity. The strain-induced crystallization in low hard segment content poly(tetramethylene oxide)based samples was only observed at very high deformation levels. on the other hand, crystallization in the samples containing high hard segment was found to evolve gradually over large deformation range. The strain rate has a considerable effect on the crystallization behavior of poly(tetramethylene oxide)based samples. While the low hard segment content poly(tetramethylene oxide) sample experiences decreasing crystallizability as the strain rate increases, its counterpart containing higher fraction of hard segments exhibits opposite behavior. We have investigated linear and nonlinear stress optical behavior and observed that the span of the initial linear stress optical region varied primarily with composition (slope ranging from 0.1 to 2.2 GPa(-1)) and secondarily with the deformation rate. Hysteresis experiments show that there is a considerable loss of energy in cyclic loading of these materials, and hysteresis increases as the chain extender is changed from 1,6-diaminohexane to ethylenediamine.Publication Metadata only Electrospinning of polyurethane fibers(Elsevier Sci Ltd, 2002) Demir, Mustafa Muamer; Department of Chemistry; Department of Chemistry; Department of Chemical and Biological Engineering; Yılgör, İskender; Yılgör, Emel; Erman, Burak; Faculty Member; Researcher; Faculty Member; Department of Chemistry; Department of Chemical and Biological Engineering; College of Sciences; College of Sciences; College of Engineering; 24181; 40527; 179997A segmented polyurethaneurea based on poly (tetramethylene oxide)glycol, a cycloaliphatic diisocyanate and an unsymmetrical diamine were prepared. Urea content of the copolymer was 35 wt%. Electrospinning behavior of this elastomeric polyurethaneurea copolymer in solution was studied. The effects of electrical field, temperature, conductivity and viscosity of the solution on the electrospinning process and morphology and property of the fibers obtained were investigated. Results of observations made by optical microscope, atomic force microscope and scanning electron microscope were interpreted and compared with literature data available on the electrospinning behavior of other polymeric systems.Publication Metadata only Entangled polymer dynamics in attractive nanocomposite melts(American Chemical Society (ACS), 2020) Şenses, Erkan; Tyagi, Madhu Sudan; Faraone, Antonio; Department of Chemical and Biological Engineering; N/A; Şenses, Erkan; Darvishi, Saeid; Faculty Member; PhD Student; Department of Chemical and Biological Engineering; College of Engineering; Graduate School of Sciences and Engineering; 280298; N/AWe investigate single chain dynamics of an entangled linear poly(ethylene oxide) melt in the presence of well-dispersed attractive nanoparticles using high-resolution neutron spectroscopy at particle volume fractions as high as 0.53. The short-time dynamics shows a decrease of the Rouse rates with particle loading, yet the change remains within a factor of 2, with no evidence of segment immobilization as often hypothesized. The apparent reptation tube diameter shrinks by approximate to 10% from the bulk at a 0.28 particle volume fraction when the face-to-face interparticle distance approaches the single chain size. The tube diameter is remarkably concentration-independent at higher loadings where all chains are essentially bound to particle surfaces. These direct experimental observations on the microscopic chain dynamics in attractive nanocomposites are distinct from their nonattractive counterparts and account for some of the unusual dynamic behaviors of the nanoparticles as well as rheology in the composites.Publication Open Access High-throughput screening of COF membranes and COF/polymer MMMs for helium separation and hydrogen purification(American Chemical Society (ACS), 2022) N/A; Department of Chemical and Biological Engineering; Aydın, Sena; Altıntaş, Çiğdem; Keskin, Seda; Researcher; Department of Chemical and Biological Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 40548Hundreds of covalent organic frameworks (COFs) have been synthesized, and thousands of them have been computationally designed. However, it is impractical to experimentally test each material as a membrane for gas separations. In this work, we focused on the membrane-based gas separation performances of experimentally synthesized COFs and hypothetical COFs (hypoCOFs). Gas permeabilities of COFs were computed by combining the results of grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations, and many COF membranes were found to overcome the upper bound of polymeric membranes for He/H2, N2/CH4, H2/N2, He/CH4, H2/CH4, and He/N2 separations. We then examined the structure–permeability relations of the COF membranes that are above the upper bound for each of the six gas separations, and based on these relations, we proposed an efficient approach for the selection of the best hypoCOFs from a very large database. Molecular simulations showed that 120 hypoCOFs that we identified to be promising based on these structure–performance relations exceed the upper bound for He/CH4, He/N2, H2/CH4, and H2/N2 separations. Both real and hypothetical COFs were then studied as fillers in 25 different polymers, leading to a total of 29 020 COF/polymer and hypoCOF/polymer mixed matrix membranes (MMMs), representing the largest number of COF-based MMMs investigated to date. Permeabilities and selectivities of COF/polymer MMMs were computed for six different gas separations, and results revealed that 18 of the 25 polymers can be carried above the upper bound when COFs were used as fillers. The comprehensive analysis of COFs provided in this work will fully unlock the potential of COF membranes and COF/polymer MMMs for helium separation and hydrogen purification.Publication Metadata only Multiscale polymer dynamics in hierarchical carbon nanotube grafted glass fiber reinforced composites(American Chemical Society (ACS), 2019) Krishnamurthz, Ajay; Tao, Ran; Doshi, Sagar M.; Burni, Faraz Ahmed; Natarajan, Bharath; Hunston, Donald; Thostenson, Erik T.; Faraone, Antonio; Forster, Amanda L.; Forster, Aaron M.; Department of Chemical and Biological Engineering; Şenses, Erkan; Faculty Member; Department of Chemical and Biological Engineering; College of Engineering; 280298Carbon nanotube (CNT) grafted glass fiber reinforced epoxy nanocomposites (GFRP) present a range of stiffnesses (MPa to GPa) and length scales (mu m to nm) at the fiber-matrix interface. The contribution of functionalized CNT networks to the local and bulk polymer dynamics is studied here by using a combination of torsion dynamical mechanical thermal analysis (DMTA), positron annihilation lifetime spectroscopy (PALS), and neutron scattering (NS) measurements. DMTA measurements highlight a reduction in the storage modulus (G') in the rubbery region and an asymmetric broadening of the loss modulus (G '') peak in the alpha-transition region. NS measurements show a suppressed hydrogen mean-square displacement (MSD) in the presence of glass fibers but a higher hydrogen MSD after grafting functionalized CNTs onto fiber surfaces. PALS measurements show greater free volume characteristics in the presence of the functionalized CNT modified composites, supporting the view that these interface layers increase polymer mobility. While NS and DMTA are sensitive to different modes of chain dynamics, the localization of functionalized nanotubes at the fiber interface is found to affect the distribution of polymer relaxation modes without significantly altering the thermally activated relaxation processes.Publication Metadata only Nanoscale particle motion reveals polymer mobility gradient in nanocomposites(Amer Chemical Soc, 2019) Narayanan, Suresh; Faraone, Antonio; Department of Chemical and Biological Engineering; Şenses, Erkan; Faculty Member; Department of Chemical and Biological Engineering; College of Engineering; 280298Polymer mobility near nanoparticle surfaces has been extensively discussed; however, direct experimental observation in the nanocomposite melts has been a difficult task. Here, by taking advantage of large dynamical asymmetry between the miscible matrix and surface-bound polymers, we highlighted their interphases and studied the resulting effect on the nanoparticle relaxation using X-ray photon correlation spectroscopy. The local mobility gradient is signified by an unprecedented increase in the relaxation time at length scales on the order of polymer radius of gyration. The effect is accompanied by a transition from simple diffusive to subdiffusive behavior in accord with viscous and entangled dynamics of polymers in the matrix and in the interphase, respectively. Our results demonstrate that the nanoparticle-induced polymer mobility changes in the interphases of nanocomposite melts can be extracted from the length-scale-dependent slow particle motion.Publication Metadata only Tunable affinity separation enables ultrafast solvent permeation through layered double hydroxide membranes(Elsevier, 2019) Ang, Edison Huixiang; Chew, Jia Wei; Department of Chemical and Biological Engineering; Velioğlu, Sadiye; Researcher; Department of Chemical and Biological Engineering; College of Engineering; 200650Membranes are playing increasingly important roles in purification and separation processes due to inherent advantages like facile, low-cost and green compared to the traditional thermal-driven processes. To enhance permeability to further augment the feasibility of membrane-filtration, emerging two-dimensional (2D) materials are promising as building blocks for making organic solvent nanofiltration (OSN) membranes. The key novelty of this study is the demonstration that, by simply altering the divalent cation type in the layered double hydroxide (LDH) crystal structure, the physicochemical activities of the membranes can be significantly enhanced to allow for the permeation of solvent at an ultrafast rate. Results show that the micrometre-thick LDH laminate supported on a nylon substrate not only provided superb solutes rejection, but also enabled nanofiltration permeances in aqueous and organic solvents (namely, acetone) as high as 298 and 651 l m(-2) h(-1) bar(-1), respectively. Both experiments and simulations suggest that the superior performance originates from the interfacial interactions between the solvent and LDH.