Entity

Time filter

Source Type


Alaghemandi M.,University of Duisburg - Essen | Alaghemandi M.,Ruhr University Bochum | Spohr E.,University of Duisburg - Essen | Spohr E.,Center for Computational science and Simulations
Chemical Physics Letters | Year: 2013

A model nanoengine based on endgrafted Poly(N-isopropylacrylamide) (PNIPAM) on graphene-like sheets is proposed. The nanoengine consists of a water-filled slab and four PNIPAM chains, which are at one end grafted to one of the slab walls and on the other end to a mobile square graphene 'piston'. The basis of the reciprocating motion of the piston is the reversible coil-to-globule transition of polymer chains when changing the temperature of the aqueous environment. Molecular dynamics simulations have been used to investigate the behavior of the proposed system at the full atomistic level. At temperatures below the lower critical solution temperature (LCST) PNIPAM chains are swollen and the nanopiston is in an expanded open state. Above the LCST, the PNIPAM chains are shrunken and the piston is retracted. The studied nanopiston exhibits an amplitude of approximately 10 Å when the temperature is reduced from 310 to 300 K or increased from 300 to 310 K with a frequency of about 10 9 rotations per minute; however the efficiency is very low. © 2013 Elsevier B.V. All rights reserved. Source


Ilhan M.A.,University of Duisburg - Essen | Spohr E.,University of Duisburg - Essen | Spohr E.,Center for Computational science and Simulations
Journal of Electroanalytical Chemistry | Year: 2011

Proton conductivity in fuel cell membrane materials such as Nafion® decreases dramatically with decreasing water content. At very low water content proton transport is thought to occur through narrow necks, which can be either static or fluctuatively formed temporarily. In the present work we investigate the properties of hydrogen bonding and protons in a one-dimensional narrow model pore by using ab initio Car-Parrinello molecular dynamics. The pore consists of eight suitably arranged CF3-CF3 and four CF 3-SO3H entities and is filled with water at varying water content λ (the ratio between the number of water molecules and the number of sulfonic acid groups) between 2.5 and 4.5. Proton mobilization in this pore occurs in two steps. First, around λ = 3 sulfonic acid groups dissociate to form sulfonate groups and hydronium ions which form mostly contact ion pairs. Second, increasing the water content to λ = 4.5 leads to an increase of the population of Zundel-like H5O2+ configurations with more or less symmetrically shared protons. Simultaneously, the number of hydrogen bonds increases and the hydrogen bond network becomes more liquid-like. © 2010 Elsevier B.V. All rights reserved. Source


Alaghemandi M.,University of Duisburg - Essen | Spohr E.,University of Duisburg - Essen | Spohr E.,Center for Computational science and Simulations
Macromolecular Theory and Simulations | Year: 2012

Using molecular dynamics simulations with an OPLS force field, the lower critical solution temperature (LCST) of single- and multiple-chain PNIPAM solutions in water is investigated. The sample containing ten polymer chains shows a sudden drop in size and volume at 305 K. Such an effect is absent in the single-chain system. Large fluctuations of the physical properties of a short single-chain prevent any clear detection of the LCST for the chosen model system, at least on the time scale of 200 ns. The results provide evidence that a critical number of PNIPAM monomer units must be present in the simulated system before MD simulations are capable to detect conformational changes unambiguously. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source


Alaghemandi M.,University of Duisburg - Essen | Alaghemandi M.,Ruhr University Bochum | Spohr E.,University of Duisburg - Essen | Spohr E.,Center for Computational science and Simulations
RSC Advances | Year: 2013

Structure and dynamic behavior of the thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAM) endgrafted onto the inner surface of a simple cylindrical pore model that resembles a carbon nanotube (CNT) with a diameter of 8.4 nm is studied as a function of temperature, of surface-polymer interaction strength, and of pore water content. A free PNIPAM chain in water shows thermo-responsive behavior with a lower critical solution temperature (LCST) of about 305 K. We have investigated two different strengths of PNIPAM-pore interactions. In the strong interaction case, which corresponds to force-field parameters taken without change from the AMBER force field, the endgrafted PNIPAM chain collapses onto the surface at all temperatures studied and hence does not adopt a brush structure. In the weak interaction case the PNIPAM-pore interaction strengths were scaled by a factor of 10, and the temperature-responsive behavior of the PNIPAM chain re-emerges. End-to-end distances, radii of gyration, density profiles, number of hydrogen bonds, and radial distribution functions demonstrate the temperature-dependent structural changes of endgrafted PNIPAM in the pore. Analysis of the translational motion of water molecules in the pore shows that the ratio of the water self diffusion coefficient in a pore with a free pore surface relative to the self diffusion coefficient in a pore containing an end-grafted PNIPAM molecule is less strongly reduced above the LCST than below the LCST, where the chain is in a more extended state. This journal is © 2013 The Royal Society of Chemistry. Source


Lorbeer L.,University of Duisburg - Essen | Lorbeer L.,Center for Computational science and Simulations | Alaghemandi M.,University of Duisburg - Essen | Alaghemandi M.,Center for Computational science and Simulations | And 2 more authors.
Journal of Molecular Liquids | Year: 2014

We report first results of a systematic study of the properties of thermo-responsive polymer chains of poly(N-isopropylacrylamide) (PNIPAM), which are endgrafted onto the inner surfaces of a slab pore of approximately 9 nm width. We have systematically varied the strength of the PNIPAM-surface interaction energy to estimate the variation of the extent of the thermo-responsive effect on different surfaces. For weak to intermediate PNIPAM-surface interactions, the MD simulations show thermo-responsive behavior as characteristic changes of the radius of gyration and other measures of the polymer structure and polymer-water interactions, when comparing simulations below (at 280 K) and above (at 320 K) the lower critical solution temperature of PNIPAM, which is at 305 K. When the PNIPAM-surface interactions become stronger, the polymer loses its thermo-responsive behavior and is adsorbed flatly on the pore walls at both investigated temperatures. © 2013 Elsevier B.V. All rights reserved. Source

Discover hidden collaborations