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Full-Text Articles in Engineering
Interdiffusion Of Solvent Into Polymer Films Studied By Molecular Dynamics Simulation, Mesfin Tsige
Interdiffusion Of Solvent Into Polymer Films Studied By Molecular Dynamics Simulation, Mesfin Tsige
Mesfin Tsige
Large scale molecular dynamics and grand canonical Monte Carlo simulation techniques are used to study the behavior of the interdiffusion of a solvent into an entangled polymer matrix as the state of the polymer changes from a melt to a glass. The weight gain by the polymer increases with time t as t(1/2) in agreement with Fickian diffusion for all cases studied, although the diffusivity is found to be strongly concentration dependent especially as one approaches the glass transition temperature of the polymer. The diffusivity as a function of solvent concentration determined using the one-dimensional Fick's model of the diffusion …
Molecular Dynamics Simulation Of Crack Propagation In Highly Cross-Linked Polymers Under Uniaxial Deformation, Mesfin Tsige
Molecular Dynamics Simulation Of Crack Propagation In Highly Cross-Linked Polymers Under Uniaxial Deformation, Mesfin Tsige
Mesfin Tsige
The strength of the interface between a structural adhesive and a solid surface is a fundamental issue. We study fracture in highly cross-linked polymer networks (e.g. epoxy) bonded to a solid surface using large-scale molecular dynamics simulations. An initial crack is created by forbidding bonds to occur on a fraction of the solid surface up to a crack tip. The time and length scales involved in this process dictate the use of a coarse grained bead-spring model of the epoxy network. In order to avoid unwanted boundary effects, large systems of up to a million particles are used. Stress-strain curves …
Packing Of Poly (Tetrafluoroethylene) In The Liquid State: Molecular Dynamics Simulation And Theory, Mesfin Tsige
Packing Of Poly (Tetrafluoroethylene) In The Liquid State: Molecular Dynamics Simulation And Theory, Mesfin Tsige
Mesfin Tsige
Molecular dynamics simulations and polymer reference interaction site model theory calculations were carried out on the C48F98 oligomer of poly(tetrafluoroethylene) (PTFE) at 500 and 600 K. The exp-6 force field of Borodin, Smith, and Bedrov, was used in both the simulation and theory. The agreement between theory and simulation was equivalent to earlier studies on polyolefin melts. The intermolecular pair correlation functions of PTFE were shifted to larger distances relative to polyethylene (PE) due to the difference in the van der Waals radii of F and H atoms. A similar shift to lower wave vectors was found in the structure …
Efficiency, Power, And Period At Two Optimum Operations Of A Thermoelectric Single-Level Quantum Dot, Mesfin Tsige
Efficiency, Power, And Period At Two Optimum Operations Of A Thermoelectric Single-Level Quantum Dot, Mesfin Tsige
Mesfin Tsige
We take a single-level quantum dot embedded between two metallic leads at different temperatures and chemical potentials which works as a heat engine. Two optimization criteria were used and their corresponding optimized efficiencies, powers, and periods evaluated. A comparison between similar quantities of the two optimization criteria reveals mixed advantages and disadvantages. We quantify the engine's overall performance by suggesting a figure of merit that takes into account the contribution of each of the three quantities. Based on the proposed figure of merit, one of the optimization criterion presents a clear advantage. This same criterion is found to be invariably …
Determination Of Interfacial Properties Of Polydimethylsiloxane-Water Systems Using Molecular Dynamics Simulations, Mesfin Tsige
Determination Of Interfacial Properties Of Polydimethylsiloxane-Water Systems Using Molecular Dynamics Simulations, Mesfin Tsige
Mesfin Tsige
Polydimethylsiloxane (PDMS) is a main constituent of silicone adhesives, which have a wide use as adhesives. Often these adhesives are used as sealants. The interaction between water and PDMS is of fundamental importance. To improve our understanding at the molecular level, we have performed molecular dynamics (MD) simulations of PDMS in the presence of water, with the long-term goal of studying how water molecules effect debonding at the surface. Knowledge of the basic interfacial properties of a multicomponent system, such as the surface tension, contact angle, and diffusion constant, are essential to obtain the proper dynamic behavior in a molecular …
Simulation Study Of Water/Silicon Oxide Interface, Mesfin Tsige
Simulation Study Of Water/Silicon Oxide Interface, Mesfin Tsige
Mesfin Tsige
The interaction of water with solid surfaces plays a crucial role in many phenomena. The water-silica interface is one of the typical systems encountered in technological and natural materials. Numerous technological applications of silica were found to rely on its specific surface properties. Large scale quantum mechanics (QM) and classical molecular dynamics (MD) simulations are used to study the molecular configurations and wetting properties of water at the interface of different silicon oxide surfaces. In order to understand how the surface coverage of silanols (-SiOH) affects the wetting behavior of the silica surfaces, both crystalline ((001) alpha-quartz (coverage 9.6 nm-2) …
Diffusion Of Water Through Methyl-And Hydroxyl-Terminated Poly (Dimethylsiloxane), Mesfin Tsige
Diffusion Of Water Through Methyl-And Hydroxyl-Terminated Poly (Dimethylsiloxane), Mesfin Tsige
Mesfin Tsige
Both experimental and numerical reports of the diffusion constant of water through poly(dimethylsiloxane) (PDMS) show variances of nearly an order of magnitude. We use molecular dynamics simulations to calculate the diffusion constant for both methyl- and hydroxyl-terminated PDMS chains. We examine the effects of both concentration and chain length. For a single water molecule, we find that diffusion depends strongly on the initial location of the molecule, as the ``caging'' phenomenon reported by Müller-Plathe can occur. At intermediate concentrations, we observe the formation of dimer and trimer water clusters, leading to lower diffusion rates; at concentrations above the reported aggregation …
Understanding The Behavior Of Poly (3-Hexylthiophene) At Liquid/Vacuum Interfaces, Mesfin Tsige
Understanding The Behavior Of Poly (3-Hexylthiophene) At Liquid/Vacuum Interfaces, Mesfin Tsige
Mesfin Tsige
Among semiconducting polymers used in opto-electronic devices, poly(3-hexylthiophene) (P3HT) is one of the better candidates because of its good electrical properties and ease of processing. The performance of these devices strongly depends on the structural, morphological, dynamic and interfacial properties of P3HT. Using molecular dynamics simulation and utilizing two different models - all-atom and united-atom - we have studied the dynamic and structural properties of free-standing P3HT thin films at liquid/vacuum interfaces. To quantify these properties, the temperature and chain-length dependence of surface roughness, interfacial width, surface tension, torsional defects, and several other surface properties have been investigated. The results …
Structure And Dynamics Of Tetrahalomethane Adsorption On (001) Surfaces Of Graphite And Alpha-Quartz, Mesfin Tsige
Structure And Dynamics Of Tetrahalomethane Adsorption On (001) Surfaces Of Graphite And Alpha-Quartz, Mesfin Tsige
Mesfin Tsige
Molecular dynamics (MD) simulations were used to study the structural and dynamic properties of multilayer adsorption of each of three halomethanes, CF4, CF3Cl, and CF3Br, adsorbed onto the (001) surface of either of two atomically flat but chemically and structurally different substrates (graphite and hydroxylated alpha-quartz) at temperatures ranging from 60 to 300 K. Analysis of the data shows a strong influence on the adsorption characteristics of these halomethane films due to the surface characteristics of the chosen substrate. in particular, the nature of the hydroxylation of a-quartz shows a striking ability to alter the affinity with which species adsorb …
Structure And Dynamics Of Fluorinated Alkanes On Silicon Dioxide Surfaces, Mesfin Tsige
Structure And Dynamics Of Fluorinated Alkanes On Silicon Dioxide Surfaces, Mesfin Tsige
Mesfin Tsige
Despite their great promise in various applications, the structure and dynamics of fluorinated alkanes at interfaces is still an open question. In particular, the knowledge from both theoretical and experimental perspectives is very limited when it comes to understanding the interface between these systems and a solid substrate. Molecular dynamics simulations based on the All Atom OPLS model are used to predict the equilibrium structure and dynamics of short fluorinated alkanes on both amorphous and crystalline silicon dioxide surfaces. In order to understand the effect of layer-layer interaction on the ordering of chains in a given layer, the thickness of …
Effect Of Tacticity On The Interfacial Structural Properties Of Adsorbed Polystyrene Thin Films, Mesfin Tsige
Effect Of Tacticity On The Interfacial Structural Properties Of Adsorbed Polystyrene Thin Films, Mesfin Tsige
Mesfin Tsige
We have carried out atomistic Molecular Dynamics simulations to investigate interfacial structural properties of thin films of polystyrene (PS) adsorbed onto solid substrates. The films are made of PS stereoregular chains, that are, isotactic PS or syndiotactic PS. Three types of surfaces of different phobicity and roughness were considered in the present work: hydroxylated silica, graphite and amorphous silica. The structural properties were studied in terms of side chains, end groups and backbone concentration and orientation in the interfacial regions (substrate/film and film/vacuum). Moreover the effect of temperature was investigated by adsorbing the films at different temperatures, below and above …
Conformational Dependence Of Charge Transport And Band Gap In Poly (3-Hexyl Thiophene) Oligomers, Mesfin Tsige
Conformational Dependence Of Charge Transport And Band Gap In Poly (3-Hexyl Thiophene) Oligomers, Mesfin Tsige
Mesfin Tsige
Structural defects will affect the charge transport properties and the band gap in the Poly (3-Hexylthiophene) (P3HT) polymer, a promising electron donor for organic solar cells. In the present work, such effects are modeled by density functional theory (DFT) calculations on P3HT oligomers up to 12 monomer units in planar and non-planar conformations. DFT calculations were performed at B3LYP/6-31++G(d,p) treating both backbone and hexyl chains explicitly. The structural properties of the oligomers change significantly for 2 to 8 unit isolated oligomers but reach asymptotic values by a 10 unit P3HT chain. The dependence of charge transfer integral on chain length …
Investigating The Glass Transition Temperature Of Water Using A Variety Of Models And Methods Via Molecular Dynamics Simulation, Mesfin Tsige
Mesfin Tsige
In everyday life, we think of solid water existing as crystalline ice. The majority of water in the universe, however, exists as an amorphous solid or ``glassy'' form. This glassy form has garnered significant interest and sparked sizable debate in recent years over the nature and conditions surrounding the transition from liquid to amorphous solid, not the least of which centers on the determination of the glass transition temperature. Previous experimental studies have suggested a glass transition temperature below the temperature at which water undergoes homogeneous nucleation, thus making experimental determination of the glass transition temperature difficult because of the …
Inter-Ring And Hexyl Chain Torsional Potentials In Poly (3-Hexylthiophene) Oligomers: Scaling With The Length Of The Conjugted Polymer Backbone, Mesfin Tsige
Mesfin Tsige
Density functional theory calculations are presented for the equilibrium structures and torsional potentials for isolated Poly (3-Hexylthiophene) (P3HT) oligomers up to 12 monomer units (up to 302 atoms). Calculations were performed at B3LYP/6-31++G(d,p) treating both the backbone of thiophene rings and the hexyl chains explicitly. One-dimensional inter-ring torsional potentials were calculated by rotating backbone around the central inter-ring bond and hexyl torsional potentials were calculated rotating n-hexyl group adjacent to the central inter-ring bond for each oligomer. The torsional and electronic properties change significantly for oligomers with 2 to 8 units but reach asymptotic values for a 10 unit P3HT …
Adsorption Of Methane And Propane On A Lj Wall And On Molybdenum Surface: A Simulation Study, Mesfin Tsige
Adsorption Of Methane And Propane On A Lj Wall And On Molybdenum Surface: A Simulation Study, Mesfin Tsige
Mesfin Tsige
Atomic-scale MD simulations were used to study multilayer adsorption as a function of temperature for two different alkanes (CH4 and C3H8) on a fixed Lennard-Jones wall and on the (001) surface of molybdenum. In sets of simulations on the molybdenum surface, the substrate atoms were made to interact via a Lennard-Jones potential in one study and via an embedded-atom model (EAM) potential in the next. The results show that CH4 and C3H8 on the flat wall possess a highly ordered packing arrangement exhibiting a higher degree of order than films adsorbed on the molybdenum substrate. Additionally, the number of ordered …
Molecular Dynamics Simulations Of Highly Cross-Linked Polymer Networks: Prediction Of Thermal And Mechanical Properties, Mesfin Tsige
Molecular Dynamics Simulations Of Highly Cross-Linked Polymer Networks: Prediction Of Thermal And Mechanical Properties, Mesfin Tsige
Mesfin Tsige
We use all-atom molecular dynamics (MD) simulations to predict the mechanical and thermal properties of thermosetting polymers. Atomistic simulation is a promising tool which can provide detailed structure-property relationships of densely cross-linked polymer networks. In this work we study the thermo-mechanical properties of thermosetting polymers based on amine curing agents and epoxy resins and have focused on the DGEBA/DETDA epoxy system. At first we describe the modeling approach to construction of realistic all-atom models of densely cross-linked polymer matrices. Subsequently, a series of atomistic simulations was carried out to examine the simulation cell size effect as well as the role …
Volume And Structural Analysis Of Super-Cooled Water Under High Pressure, Mesfin Tsige
Volume And Structural Analysis Of Super-Cooled Water Under High Pressure, Mesfin Tsige
Mesfin Tsige
Motivated by recent experimental study of super-cooled water at high pressure [1], we performed atomistic molecular dynamic simulations study on bulk water molecules at isothermal-isobaric ensemble. These simulations are performed at temperatures that range from 40 K to 380 K using two different cooling rates, 10K/\textit{ns} and 10K/5\textit{ns}, and pressure that ranges from 1atm to 10000 atm. Our analysis for the variation of the volume of the bulk sample against temperature indicates a downward concave shape for pressures above certain values, as reported in [1]. The same downward concave behavior is observed at high pressure on the mean-squared-displacements (MSD) of …
The Behavior Of Water At The Interface With Polystyrene, Mesfin Tsige
The Behavior Of Water At The Interface With Polystyrene, Mesfin Tsige
Mesfin Tsige
Solid-aqueous interfaces are of great importance in many industrial applications ranging from oil recovery to biotechnology. The behavior of interfacial water differs drastically from that of the bulk liquid and strongly depends on the atomistic details of the surface itself. Molecular dynamics simulations have been used extensively to study the structure and dynamics of the interface between a polymeric thin film and water. Using a fully atomistic molecular dynamics simulation, we have examined the structure and dynamics of water and atactic polystyrene (aPS) chains near the aPS-water interface. In this talk, we present results for the contact angle of water …
Application Of Carbon Nanotubes For Removing Organic Contaminants From Water, Mesfin Tsige
Application Of Carbon Nanotubes For Removing Organic Contaminants From Water, Mesfin Tsige
Mesfin Tsige
This review article focuses on the surface adsorptive properties of carbon nanotubes and their use in the removal of organic environmental contaminants from water. An introduction to the prevalent organic contaminants as well as some emerging contaminants such as pharmaceuticals and personal care products in aquatic environment are briefly discussed. The growth and surface modification techniques of carbon nanotubes and their implication on the adsorption potential of CNTs are examined. Adsorption capacity and affinity of organic contaminants on carbon nanotubes and parameters affecting such adsorption process including the environmental chemistry such as competition from co-existing environmental contaminants are summarized. The …
Interfacial Structure And Dynamics Of The Liquid/Liquid Interface Between Polydimethylsiloxane And Polystyrene, Mesfin Tsige
Interfacial Structure And Dynamics Of The Liquid/Liquid Interface Between Polydimethylsiloxane And Polystyrene, Mesfin Tsige
Mesfin Tsige
Many important phenomena in biology, chemistry and in various fields involve processes that occur at the interface between two immiscible liquids. A molecular level understanding of such interfaces is crucial for insight into the complex dynamics that are observed at such interfaces. In this study, atomistic molecular dynamics simulations were performed to study the structural and dynamical properties of the liquid/liquid interface between two immiscible polymers, polydimethylsiloxane (PDMS) and polystyrene (PS). A series of simulations is carried out to examine the temperature and molecular weight dependence of the orientation of molecules at and away from the interface, intermolecular correlation at …
The Effect Of Chain Stiffness On The Structure And Phase Behavior Of Diblock Copolymer Melts, Mesfin Tsige
The Effect Of Chain Stiffness On The Structure And Phase Behavior Of Diblock Copolymer Melts, Mesfin Tsige
Mesfin Tsige
In block copolymers the covalent bond joining the different immiscible block segments prevents the occurrence of macroscopic phase separation of the different components of a copolymer chain. Instead, the block segments give rise to well-organized periodic domain nanostructures whose size and shape mainly depend on the dimensions of the blocks and the segment-segment interaction parameters. Variations in the stiffness of the different block segments can directly affect the morphology of the system and may result in a very rich phase behavior. To the best of our knowledge, there is no theory at the atomic or molecular level that explains how …
Layered Atomic Structures Of Silver Vanadate Compounds For Low Shear Strength At High Temperatures, Mesfin Tsige
Layered Atomic Structures Of Silver Vanadate Compounds For Low Shear Strength At High Temperatures, Mesfin Tsige
Mesfin Tsige
The aerospace industry has been a strong driving force for the creation of new and effective wear-resistant and lubricious materials at high temperatures (T > 500 \r{ }C). Solid lubricants (SLs) such as graphite and molybdenum disulfide oxidize and, hence, degrade rapidly at T > 350 \r{ }C. The selection of oxides is a clear viable alternative for the choice of SLs when confronting the problem of oxidation. Double metal oxides of the form MexTMyOz, where Me is a noble metal and TM a transition metal, were found to exhibit relatively low coefficients of friction in the 500 to 700 \r{ }C …
Finding The Density Of Objects Without Measuring Mass And Volume, Mesfin Tsige
Finding The Density Of Objects Without Measuring Mass And Volume, Mesfin Tsige
Mesfin Tsige
No abstract provided.
Halomethane Adsorption On Graphite And Silica Surfaces, Mesfin Tsige
Halomethane Adsorption On Graphite And Silica Surfaces, Mesfin Tsige
Mesfin Tsige
Owing to their simple nature and number of practical applications, the adsorption of halomethanes onto various substrates has been a topic of study in materials science for a number of years. While a number of studies have reported on the surface characteristics of halomethane adsorption on simple, neutral substrates such as graphite, less well understood in general is the behavior of similar adsorbates on other, possibly widely different substrates. How the choice of substrate affects the manner in which these compounds are adsorbed as well as the effects of the substrate on the structure of the adsorbate more than a …
Interfacial Properties Of Polydimethylsiloxane-Water Systems, Mesfin Tsige
Interfacial Properties Of Polydimethylsiloxane-Water Systems, Mesfin Tsige
Mesfin Tsige
Polydimethylsiloxane (PDMS) is a main constituent of silicone adhesives, which have a wide use as adhesives. Often these adhesives are used as sealants. The interaction between water and PDMS is of fundamental importance. To improve our understanding at the molecular level, we have performed molecular dynamics (MD) simulations of PDMS in the presence of water, with the long-term goal of studying how water molecules effect debonding at the surface. Knowledge of the basic interfacial properties of a multicomponent system, such as the surface tension, contact angle, and diffusion constant, are essential to obtain the proper dynamic behavior in a molecular …
Molecular Dynamics Simulations Of Halomethane Adsorption On Two Dissimilar Surfaces, Mesfin Tsige
Molecular Dynamics Simulations Of Halomethane Adsorption On Two Dissimilar Surfaces, Mesfin Tsige
Mesfin Tsige
In this study, atomic-scale MD simulations were used to study multilayer adsorption of three different halomethane compounds (CF4, CF3Cl and CF3Br) onto two different surfaces (graphite and hydroxylated alpha-quartz) to show how orientation, layer structure, packing and dynamics vary as a function of temperature and substrate surface characteristics. In this way, non-polar CF4 on hydroxylated alpha-quartz is shown to exhibit a highly ordered packing arrangement only weakly dependent on temperature variation. In contrast, interactions between polar adsorbate groups and hydroxyl groups on the surface give CF3Cl and CF3Br a very different layer structure at the surface. Analysis of the orientations …
Atomistic Molecular Dynamics Simulation Of The Surface Properties Of P3ht Films, Mesfin Tsige
Atomistic Molecular Dynamics Simulation Of The Surface Properties Of P3ht Films, Mesfin Tsige
Mesfin Tsige
In recent years P3HT has attracted much interest mainly because of its potential applications in solar cells, light emitting diodes and field effect transistors. The performance of these devices is strongly dependent on the structural packing, morphology and interfacial properties of the P3HT. In order to improve the devices efficiency, understanding the structural and dynamical properties of P3HT at the atomic level is important. Most studies on P3HT have mainly focused on understanding its bulk properties. However, the orientation of P3HT chains at the polymer/air interface has not been well investigated. Using molecular dynamics simulations we have studied the interfacial …
Time Evolution Of The Structure Of Fluorinated Alkanes Near Solid Surfaces, Mesfin Tsige
Time Evolution Of The Structure Of Fluorinated Alkanes Near Solid Surfaces, Mesfin Tsige
Mesfin Tsige
The study of thin films of physisorbed molecules has attracted much experimental and theoretical interest in the past few decades. This is mainly due to numerous reports of anomalous behavior in the structural and dynamical properties of polymer thin films at interfaces and the expected consequences in their wide range of practical applications. Although great advances have been made in our understanding of the physics and chemistry of polymer melt/solid and polymer melt/vapor interfaces, there remain a number of fundamental as well as practical issues that need to be addressed regarding the influence of interfaces on a polymer film. One …
Temperature Dependent Adsorption Dynamics Of Binary Mixtures Of Halomethanes On Graphite And Alpha-Quartz Surfaces, Mesfin Tsige
Temperature Dependent Adsorption Dynamics Of Binary Mixtures Of Halomethanes On Graphite And Alpha-Quartz Surfaces, Mesfin Tsige
Mesfin Tsige
Using atomistic molecular dynamics simulations, we have investigated the structure and dynamics of binary mixtures of halomethanes (CF4, CF3Cl, and CF3Br) as a function of temperature on two structurally and chemically different surfaces. The initial distribution of the binary mixtures is either they are uniformly mixed or a layer or layers of one component is placed on top of a layer or layers of the other component. As a function of temperature (below and above the melting temperature of CF4) and the two surface types, we observed a marked change on the concentration, dynamics, orientation, and structure of each of …
Reports: G6 47785-G6: Surface Properties Of Fluorinated And Semifluorinated Alkanes, Mesfin Tsige
Reports: G6 47785-G6: Surface Properties Of Fluorinated And Semifluorinated Alkanes, Mesfin Tsige
Mesfin Tsige
No abstract provided.