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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 …
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) …
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 …
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 …
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 …
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.
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 …
Atomistic Simulations Of End-Linked Poly (Dimethylsiloxane) Networks: Structure And Relaxation, Mesfin Tsige
Atomistic Simulations Of End-Linked Poly (Dimethylsiloxane) Networks: Structure And Relaxation, Mesfin Tsige
Mesfin Tsige
The structure and elastic moduli of end-cross-linked poly(dimethylsiloxane) (PDMS) networks are studied using molecular dynamics (MD). The systems consist of 2000 PDMS chains of length 20 monomers and 1000 chains of length 40 monomers with varying amounts of cross-linker molecules. The networks are formed dynamically within the MD simulations. Starting from an equilibrated melt, tetrakis(dimethylsiloxy)silane cross-linkers are attached randomly to a fraction of the chain ends. When a free end comes within a short capture distance from an unsaturated cross-linker, a bond is formed between the chain and the cross-linker. The kinetics of the cross-linking process are studied as a …
Morphology Of Evaporated Multiblock Copolymer Membranes Studied By Molecular Dynamics Simulations, Mesfin Tsige
Morphology Of Evaporated Multiblock Copolymer Membranes Studied By Molecular Dynamics Simulations, Mesfin Tsige
Mesfin Tsige
No abstract provided.