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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 …
Structure And Dynamics Of Squalane Films On Solid Surfaces, Mesfin Tsige
Structure And Dynamics Of Squalane Films On Solid Surfaces, Mesfin Tsige
Mesfin Tsige
Interfaces between liquids and solids or gases play important roles in a number of different technologies and have thus been the main focus of many theoretical and experimental studies. The discontinuous nature of physical properties at interfaces makes it demanding to develop a general theory, and most existing theories are therefore based on macroscopic continuum equations. Now, for the first time, simulations are opening up the field of interfacial problems. Starting from models which have been developed and validated for bulk polymers it is now possible to treat interfaces at the atomistic or molecular level. In the present study, molecular …
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 …
Effect Of Chain Stiffness On The Morphology Of Diblock Copolymer Melts, Mesfin Tsige
Effect Of Chain Stiffness On The Morphology Of Diblock Copolymer Melts, Mesfin Tsige
Mesfin Tsige
The authors have studied the microphase separation of symmetric diblock copolymers with variable block stiffness and different block chain lengths using coarse-grained molecular dynamics simulations. The simulation results show that for symmetric diblock copolymers, a combination of chain length and relative stiffness between the blocks may play the major role in determining the equilibrium morphology of the system. When the variation in stiffness between blocks is small, the equilibrium morphology of the diblock system is found to be lamellar; this is also the case for systems with small chain lengths, regardless of the difference in block stiffness. However, in systems …
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 …
Multilayer Adsorption Of Methane And Chloromethane On The Molybdenum (100) Surface, Mesfin Tsige
Multilayer Adsorption Of Methane And Chloromethane On The Molybdenum (100) Surface, Mesfin Tsige
Mesfin Tsige
Using all-atom molecular dynamics simulations, the multilayer adsorption of two single-carbon alkane analogues—methane (CH4) and chloromethane (CH3Cl)—in the liquid phase on a metallic substrate (molybdenum (100) surface) has been studied to elucidate differences in the adsorbed film structure as a function of the overall film thickness, system temperature, substrate mobility, and pairwise substrate interaction potential (Lennard-Jones 12-6 versus embedded atom method versus Lennard-Jones 9-3 “wall” potential, used as a basis for comparison). Simulations suggest a clear predilection to well-ordered packing arrangements dictated by the adsorbate molecular geometry when adsorbed on the flat, featureless Lennard-Jones wall, but adsorption on the atomistic …
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 …
Conformations And Torsional Potentials Of Poly (3-Hexylthiophene) Oligomers: Density Functional Calculations Up To The Dodecamer, Mesfin Tsige
Mesfin Tsige
Fully optimized conformations of poly(3-hexylthiophene) (P3HT) oligomers up to the decamer and torsional potentials up to the dodecamer (302 atoms) are investigated by large-scale density functional calculations (B3LYP/6-31+G(d,p)). Fully relaxed P3HT oligomers were investigated at a variety of conformational minima, many with skeletal structures far from planarity. The lowest energy conformations found have each hexyl group ≈74° out of plane and each backbone twist angle out of plane by ≈47°. The energies of these non-planar conformations are lower than that of the planar reference geometry by ≈30 meV per monomer. Backbone torsional potentials and hexyl torsional potentials converge with oligomer …
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 …
Interfacial Properties Of Semifluorinated Alkane Diblock Copolymers, Mesfin Tsige
Interfacial Properties Of Semifluorinated Alkane Diblock Copolymers, Mesfin Tsige
Mesfin Tsige
The liquid-vapor interfacial properties of semifluorinated linear alkane diblock copolymers of the form F3C(CF2)n−1(CH2)m−1CH3 are studied by fully atomistic molecular dynamics simulations. The chemical composition and the conformation of the molecules at the interface are identified and correlated with the interfacial energies. A modified form of the Optimized Parameter for Liquid Simulation All-Atom (OPLS-AA) force field of Jorgensen and co-workers [J. Am. Chem. Soc.106, 6638 (1984);118, 11225 (1996);J. Phys. Chem. A105, 4118 (2001)], which includes specific dihedral terms for H–F blocks-and corrections to the H–F nonbonded interaction, is used together with a new version of the exp-6 force field developed …
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 …
An All-Atom Simulation Study Of The Ordering Of Liquid Squalane Near A Solid Surface, Mesfin Tsige
An All-Atom Simulation Study Of The Ordering Of Liquid Squalane Near A Solid Surface, Mesfin Tsige
Mesfin Tsige
An all-atom molecular dynamics study using the OPLS force field has been carried out to obtain new insights in to the orientation and ordering of liquid squalane near a solid surface. As observed in previous experiments, the squalane molecules closest to a SiO2 substrate are found to be tightly bound with their molecular axis preferentially parallel to the interface. Unlike linear alkanes, the squalane molecules are also found to lie preferentially parallel to the liquid/vapor interface. The simulation results predict that the molecular plane orientation of the squalane molecules changes from mainly parallel to perpendicular to the substrate in going …
Surface Tension Of Normal And Branched Alkanes, Mesfin Tsige
Surface Tension Of Normal And Branched Alkanes, Mesfin Tsige
Mesfin Tsige
We present results illustrating the effects of using explicit summation terms for the r − 6 dispersion term on the interfacial properties of normal and branched alkanes. We study two all-atom force fields, the OPLS force field of Jorgensen et al. and the exponential-6 force field of Smith and co-workers. We find that the OPLS force field offers excellent agreement with experimental data for surface tension at low temperatures, while the Smith force field agrees well at lower molecular weights. Cutting off the dispersion term at a finite distance r c can have pronounced effects on interfacial properties, with as …
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 …
Surface Tension And Surface Orientation Of Perfluorinated Alkanes, Mesfin Tsige
Surface Tension And Surface Orientation Of Perfluorinated Alkanes, Mesfin Tsige
Mesfin Tsige
Molecular dynamics simulations using two all-atom force fields, the OPLS force field of Jorgensen et al. and the recently developed force field of Borodin et al. (exp-6), have been used to study the equilibrium liquid−vapor interface properties of perfluorinated alkanes. Both force fields predict similar liquid densities and are in excellent agreement with existing experimental data for short chains. The OPLS force field offers better agreement with experimental data for surface tension at lower molecular weights than the exp-6 force field, which overpredicts it by as much as 9%. However, for longer chains the OPLS force field predicts higher surface …
Structural Properties Of Atactic Polystyrene Adsorbed Onto Solid Surfaces, Mesfin Tsige
Structural Properties Of Atactic Polystyrene Adsorbed Onto Solid Surfaces, Mesfin Tsige
Mesfin Tsige
In the present work, we are studying the local conformation of chains in a thin film of polystyrene adsorbed on a solid substrate by using atomistically detailed simulations. The simulations are carried out by using the readily available and massively parallel molecular dynamics code known as LAMMPS. In particular, a special emphasis is given to the density and orientation of side chains (which consist of phenyl groups and methylene units) at solid/polymer and polymer/vacuum interfaces. Three types of substrates were used in our study: α-quartz, graphite, and amorphous silica. Our investigation was restricted to atactic polystyrene. Our results show that …
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 …
Molecular Dynamics Study Of The Evaporation Process In Polymer Films, Mesfin Tsige
Molecular Dynamics Study Of The Evaporation Process In Polymer Films, Mesfin Tsige
Mesfin Tsige
No abstract provided.
Evaporation And Interdiffusion Of Solvent In Polymer Films, Mesfin Tsige
Evaporation And Interdiffusion Of Solvent In Polymer Films, Mesfin Tsige
Mesfin Tsige
Solvent evaporation from homopolymer and heteropolymer films along with the interdiffusion of solvent into these films are studied by molecular dynamics simulations. Due to the high viscosity of polymer melts, in many cases polymer films are made by first dissolving the polymer in a low viscosity solvent, spreading the solution on a substrate and subsequently evaporating the solvent. Here we study the last part of this process, namely the evaporation of solvent from a polymer film. As the solvent evaporates, the polymer density at the film/vapour interface is found to increase sharply, creating a polymer density gradient which acts as …
Molecular Modeling Of Thermosetting Polymers: Effects Of Degree Of Curing And Chain Length On Thermo-Mechanical Properties, Mesfin Tsige
Molecular Modeling Of Thermosetting Polymers: Effects Of Degree Of Curing And Chain Length On Thermo-Mechanical Properties, Mesfin Tsige
Mesfin Tsige
No abstract provided.