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Full-Text Articles in Engineering
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 …
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 …
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 …
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 …
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 …
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 …
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 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.
Interfacial Properties Of Free-Standing Poly (3-Hexylthiophene) Films, Mesfin Tsige
Interfacial Properties Of Free-Standing Poly (3-Hexylthiophene) Films, Mesfin Tsige
Mesfin Tsige
Using full atomistic classical molecular dynamics simulations, the interfacial properties of free-standing poly(3-hexylthiophene) (P3HT) films have been investigated. The orientations of different parts of the P3HT chain and the surface tensions of the films were calculated in a temperature range of 540 K–600 K. At the liquid/vacuum interface, the P3HT chain shows ordering by exposing hexyl groups at the interface, while the chain backbone lays flat with the thiophene ring preferentially tilt toward the surface. At the interface, the terminal methyl groups of hexyl side chains are in excess compared to the methylene groups or thiophene rings. The surface tension …
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.
Characterization Of Cf4/Cf3br Binary Mixture Adsorption On Hydrophobic/Hydrophilic Surfaces Via Atomistic Md Simulation, Mesfin Tsige
Characterization Of Cf4/Cf3br Binary Mixture Adsorption On Hydrophobic/Hydrophilic Surfaces Via Atomistic Md Simulation, Mesfin Tsige
Mesfin Tsige
Molecular dynamics simulations of multilayer adsorption of binary mixtures of two tetrasubstituted halomethanes (CF(4) and CF(3)Br) on two very different substrates (graphite vs hydroxylated SiO(2)) were performed for three different bulk compositions (40%, 50%, and 60% CF(4)) and over a range of temperatures from 80 to 200 K. The goal of these simulations was to investigate in depth how these factors affect film structure, layer composition, lateral arrangement, and molecular orientation in the first adsorbed layer on each substrate. In line with a previous study of single-component adsorption on these surfaces, mixtures adsorbed on the hydroxylated SiO(2) surface show stable …
Interfacial Structure And Dynamics Of Siloxane Systems: Pdms- Vapor And Pdms- Water, Mesfin Tsige
Interfacial Structure And Dynamics Of Siloxane Systems: Pdms- Vapor And Pdms- Water, Mesfin Tsige
Mesfin Tsige
Using a fully atomistic force field for polydimethylsiloxane developed by Smith et al. [J. Phys. Chem. B 2004, 108, 20340], we study the interfacial properties of polydimethylsiloxane (PDMS) as well as its interactions with water. We determine the surface tension of methyl- and hydroxyl-terminated PDMS chains with lengths between 20 and 100 repeat units and find good agreement between simulation results and experimental observations. The width of the polymer liquid−vapor interface is shown to depend on both molecular weight and temperature. The surface tension and contact angle are determined for the PDMS−water binary system using several different geometries and calculation …
Effect Of Cross-Linker Functionality On The Adhesion Of Highly Cross-Linked Polymer Networks: A Molecular Dynamics Study Of Epoxies, Mesfin Tsige
Mesfin Tsige
The effect of cross-linker functionality and interfacial bond density on the fracture behavior of highly cross-linked polymer networks bonded to a solid surface is studied using large-scale molecular dynamics simulations. Three different cross-linker functionalities (f = 3, 4, and 6) are considered. The polymer networks are created between two solid surfaces with the number of bonds to the surfaces varying from zero to full bonding to the network. Stress−strain curves are determined for each system from tensile pull and shear deformations. At full interfacial bond density the failure mode is cohesive. The cohesive failure stress is almost identical for shear …
An Improved Forcefield For Molecular Modeling Oof Crystalline Poly (3-Hexyl Thiophene), Mesfin Tsige
An Improved Forcefield For Molecular Modeling Oof Crystalline Poly (3-Hexyl Thiophene), Mesfin Tsige
Mesfin Tsige
An ab initio-based improved force field is reported for poly(3-hexylthiophene) (P3HT) in the solid state, deriving torsional parameters and partial atomic charges from ab initio molecular structure calculations with explicit treatment of the hexyl side chains. The force field is validated by molecular dynamics (MD) simulations of solid P3HT with different molecular weights including calculation of structural parameters, mass density, melting temperature, glass transition temperature, and surface tension. At 300 K, the P3HT crystalline structure features planar backbones with non-interdigitated all-trans hexyl side chains twisted 90° from the plane of the backbone. For crystalline P3HT with infinitely long chains, the …
Structure Of Poly (Dialkylsiloxane) Melts: Comparisons Of Wide-Angle X-Ray Scattering, Molecular Dynamics Simulations, And Integral Equation Theory, Mesfin Tsige
Mesfin Tsige
No abstract provided.
Role Of Network Connectivity On The Mechanical Properties Of Highly Cross-Linked Polymers, Mesfin Tsige
Role Of Network Connectivity On The Mechanical Properties Of Highly Cross-Linked Polymers, Mesfin Tsige
Mesfin Tsige
The effects of mixed functionality and degree of curing on the stress−strain behavior of highly cross-linked polymer networks are studied using molecular dynamics simulations. The networks are made dynamically in a manner similar to epoxy network formation, and the average functionality of the cross-linker, fav, is systematically varied from 3 to 6 by mixing cross-linkers with functionalities f = 3, 4, and 6. Stress−strain curves are determined for each system from tensile pull simulations. The range of strain of the plateau region (RP) in the stress−strain curve, failure strain (εf), and failure stress (σf) for fully cured networks are found …
Liquid- Liquid Interfaces Of Semifluorinated Alkane Diblock Copolymers With Water, Alkanes, And Perfluorinated Alkanes, Mesfin Tsige
Liquid- Liquid Interfaces Of Semifluorinated Alkane Diblock Copolymers With Water, Alkanes, And Perfluorinated Alkanes, Mesfin Tsige
Mesfin Tsige
The liquid−liquid interface between semifluorinated alkane diblock copolymers of the form F3C(CF2)n−1(CH2)m−1CH3 and water, protonated alkanes, and perfluorinated alkanes are studied by fully atomistic molecular dynamics simulations. A modified version of the OPLS-AA (Optimized Parameter for Liquid Simulation All-Atom) force field of Jorgensen et al. has been used to study the interfacial behavior of semifluorinated diblocks. Aqueous interfaces are found to be sharp, with correspondingly large values of the interfacial tension. Due to the reduced hydrophobicity of the protonated block compared to the fluorinated block, hydrogen enhancement is observed at the interface. Water dipoles in the interfacial region are found …
Template-Induced Enhanced Ordering Under Confinement, Mesfin Tsige
Template-Induced Enhanced Ordering Under Confinement, Mesfin Tsige
Mesfin Tsige
We report a surprisingly strong ordering of Si-(CH(3))(2) groups upon confinement between two surfaces, an oxidized poly(dimethyl siloxane) (PDMS(ox)) elastomer and a methyl-terminated self-assembled monolayer (octadecyltrichlorosilane (OTS)) on sapphire substrates. This enhanced ordering is induced by the template of ordered methyl groups of OTS and is not observed for other surfaces (fluorinated monolayers and sapphire substrates). This strong ordering is reminiscent of layering observed for confined symmetric molecules between two mica surfaces but was expected to vanish between rough macroscopic surfaces. These results provide new insights on confined structure at the interface between two solids and are important in the …