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Full-Text Articles in Nanoscience and Nanotechnology

Molecular Dynamic Simulations Of The Effect On The Hydration Of Nafion In The Presence Of A Platinum Nanoparticle, Myvizhi Esai Selvan, Qianping He, Elisa Calvo-Muñoz, David Keffer Apr 2012

Molecular Dynamic Simulations Of The Effect On The Hydration Of Nafion In The Presence Of A Platinum Nanoparticle, Myvizhi Esai Selvan, Qianping He, Elisa Calvo-Muñoz, David Keffer

David Keffer

Platinum catalysts play a critical role in fuel cell technology. Current optimization efforts focus on reducing the amount of Pt in the system and optimizing the utilization of that which remains. The effect of the presence of Pt nanoparticles on the local structure and morphology of the polymer electrolyte membrane, water, and hydronium ions has been studied at molecular level in this work. Classical molecular dynamics simulation has been used to examine a system containing a 4 nm fcc cubic ({100} face) platinum nanoparticle at the center surrounded by Nafion polymer, water molecules, and hydronium ions at λ = 3, 6 ...


Multi-Scale Models For Sulfonated Cross-Linked Poly (1, 3-Cyclohexadiene) Polymer, Qifei Wang, David Keffer, Suxiang Deng, Jimmy Mays Feb 2012

Multi-Scale Models For Sulfonated Cross-Linked Poly (1, 3-Cyclohexadiene) Polymer, Qifei Wang, David Keffer, Suxiang Deng, Jimmy Mays

David Keffer

Atomistic and coarse-grained (CG) models of cross-linked sulfonated Poly (1, 3-cyclohexadiene) (xsPCHD) were developed and implemented in Molecular Dynamics (MD) simulations of PCHD chains with different architectures. In the atomistic model, PCHD chains are cross linked by a sulfur–sulfur bond. Sulfonic acid groups are evenly distributed along the chain. The architecture is specifically aimed for application as a proton exchange membrane used in fuel cells. An atomistic force field for this architecture was tested and applied in the atomistic MD simulation of xsPCHD for the first time. The atomistic simulations generate the density and cross-linker separation distribution. To further ...


A Coarse-Grained Model For Polyethylene Glycol (Peg) Polymer, Qifei Wang, David Keffer, Donald Nicholson Nov 2011

A Coarse-Grained Model For Polyethylene Glycol (Peg) Polymer, Qifei Wang, David Keffer, Donald Nicholson

David Keffer

A coarse-grained (CG) model of polyethylene glycol (PEG) was developed and implemented in CG molecular dynamics (MD) simulations of PEG chains with degree of polymerization (DP) 20 and 40. In the model, two repeat units of PEG are grouped as one CG bead. Atomistic MD simulation of PEG chains with DP = 20 was first conducted to obtain the bonded structural probability distribution functions (PDFs) and nonbonded pair correlation function (PCF) of the CG beads. The bonded CG potentials are obtained by simple inversion of the corresponding PDFs. The CG nonbonded potential is parameterized to the PCF using both an inversion ...


Reactive Molecular Dynamics Study Of Proton Transport In Polymer Electrolyte Membranes, Myvizhi Esai Selvan, David Keffer, Shengting Cui Jul 2011

Reactive Molecular Dynamics Study Of Proton Transport In Polymer Electrolyte Membranes, Myvizhi Esai Selvan, David Keffer, Shengting Cui

David Keffer

Dynamical properties of water and protons in Nafion with an equivalent weight of 1144 are studied using the recently developed reactive molecular dynamics (RMD) algorithm at various water contents. The structural diffusion of a proton along the aqueous domains is modeled via a mechanism similar to that observed in bulk aqueous systems. The algorithm implements reactivity in classical MD simulations by three steps: (i) satisfaction of the trigger, (ii) instantaneous reaction, and (iii) local equilibration. Two different schemes (Method 1 and Method 2) of execution of the algorithm are investigated, which differ in terms of the range of the local ...


Toward A Predictive Understanding Of Water And Charge Transport In Proton Exchange Membranes, Myvizhi Esai Selvan, Elisa Calvo-Muñoz, David Keffer Feb 2011

Toward A Predictive Understanding Of Water And Charge Transport In Proton Exchange Membranes, Myvizhi Esai Selvan, Elisa Calvo-Muñoz, David Keffer

David Keffer

An analytical model for water and charge transport in highly acidic and highly confined systems such as proton exchange membranes of fuel cells is developed and compared to available experimental data. The model is based on observations from both experiment and multiscale simulation. The model accounts for three factors in the system including acidity, confinement, and connectivity. This model has its basis in the molecular-level mechanisms of water transport but has been coarse-grained to the extent that it can be expressed in an analytical form. The model uses the concentration of H3O+ ion to characterize acidity, interfacial surface area per ...


Applications Of A General Random Walk Theory For Confined Diffusion, Elisa Calvo-Muñoz, Myvizhi Esai Selvan, Ruichang Xiong, Madhusudan Ojha, David Keffer, Donald Nicholson, Takeshi Egami Dec 2010

Applications Of A General Random Walk Theory For Confined Diffusion, Elisa Calvo-Muñoz, Myvizhi Esai Selvan, Ruichang Xiong, Madhusudan Ojha, David Keffer, Donald Nicholson, Takeshi Egami

David Keffer

A general random walk theory for diffusion in the presence of nanoscale confinement is developed and applied. The random-walk theory contains two parameters describing confinement: a cage size and a cage-to-cage hopping probability. The theory captures the correct nonlinear dependence of the mean square displacement (MSD) on observation time for intermediate times. Because of its simplicity, the theory also requires modest computational requirements and is thus able to simulate systems with very low diffusivities for sufficiently long time to reach the infinite-time-limit regime where the Einstein relation can be used to extract the self-diffusivity. The theory is applied to three ...


On The Relationship Between The Structure Of Metal-Organic Frameworks And The Adsorption And Diffusion Of Hydrogen, Nethika Suraweera, Ruichang Xiong, J. P. Luna, Donald Nicholson, David Keffer Dec 2010

On The Relationship Between The Structure Of Metal-Organic Frameworks And The Adsorption And Diffusion Of Hydrogen, Nethika Suraweera, Ruichang Xiong, J. P. Luna, Donald Nicholson, David Keffer

David Keffer

In this work, the adsorptive and diffusive behaviours of molecular hydrogen in 10 different isoreticular metal–organic frameworks (IRMOFs) are studied using molecular-level simulation. Hydrogen adsorption isotherms and heats of adsorption at 77 and 300 K were generated for 10 MOFs at low-pressure conditions (up to 10 bar) using Path Integral Grand Canonical Monte Carlo simulations. Self-diffusivities and activation energies for diffusion were generated using molecular dynamics simulation. Density distributions showing the location and the shape of the adsorption sites are also provided. Statistical correlations for all of the properties as a function of surface area (SA), accessible volume (AV ...


Molecular Dynamics Simulation Of Poly(Ethylene Terephthalate) Oligomers, David Keffer, Qifei Wang, Simioan Petrovan, J. Thomas Dec 2009

Molecular Dynamics Simulation Of Poly(Ethylene Terephthalate) Oligomers, David Keffer, Qifei Wang, Simioan Petrovan, J. Thomas

David Keffer

Molecular dynamics simulations of poly(ethylene terephthalate) (PET) oligomers are performed in the isobaric−isothermal (NpT) ensemble at a state point typical of a finishing reactor. The oligomer size ranges from 1 to 10 repeat units. We report thermodynamic properties (density, potential energy, enthalpy, heat capacity, isothermal compressibility, and thermal expansivity), transport properties (self-diffusivity, zero-shear-rate viscosity, thermal conductivity), and structural properties (pair correlation functions, hydrogen bonding network, chain radius of gyration, chain end-to-end distance) as a function of oligomer size. We compare the results with existing molecular-level theories and experimental data. Scaling exponents as a function of degree of polymerization ...


Molecular Simulations Of H2 Adsorption In Metal-Porphyrin Frameworks (Mpfs): A Potential New Material Evaluation, Ruichang Xiong, David Keffer Dec 2009

Molecular Simulations Of H2 Adsorption In Metal-Porphyrin Frameworks (Mpfs): A Potential New Material Evaluation, Ruichang Xiong, David Keffer

David Keffer

Path integral grand canonical Monte Carlo (PI-GCMC) simulations using standard force fields are carried out to calculate the adsorption of H2 in five metal-porphyrin frameworks (MPFs), a new class of metal organic framework (MOF)-type materials. These simulations are performed at 77 K and room temperature (300 K). The adsorption isotherms of H2 in IRMOF-1 and IRMOF-10 are also calculated as a comparison. All calculations indicate that all MPFs adsorbed a higher weight fraction of H2 than both IRMOF-1 and IRMOF-10, with one exception (MPF-2). The gravimetric hydrogen capacities are still well short of practical goals. The MPFs provide additional ...


Molecular Simulation Images, David Keffer Dec 2009

Molecular Simulation Images, David Keffer

David Keffer

These animations and interactive structures are created from various molecular dynamics simulations and quantum calculations. In order to view the interactive structures, you need the free "Chime" Plug-in. In order to view the movie files (in avi format), you will require the following codec: TSCC codec. This work has been supported by DOE BES, AFOSR, NSF and ACS PRF.


Effective Potentials Between Nanoparticles In Suspension, Gary Grest, Qifei Wang, Pieter In't Veld, David Keffer Dec 2009

Effective Potentials Between Nanoparticles In Suspension, Gary Grest, Qifei Wang, Pieter In't Veld, David Keffer

David Keffer

Results of molecular dynamics simulations are presented for the pair distribution function between nanoparticles in an explicit solvent as a function of nanoparticle diameter and interaction strength between the nanoparticle and solvent. The effect of including the solvent explicitly is demonstrated by comparing the pair distribution function of nanoparticles to that in an implicit solvent. The nanoparticles are modeled as a uniform distribution of Lennard-Jones particles, while the solvent is represented by standard Lennard-Jones particles. The diameter of the nanoparticle is varied from 10 to 25 times that of the solvent for a range of nanoparticle volume fractions. As the ...


Dynamics Of Individual Molecules Of Linear Polyethylene Liquids Under Shear: Atomistic Simulation And Comparison With A Free-Draining Bead-Rod Chain, David Keffer, J. Kim, B. Edwards, B. Khomami Dec 2009

Dynamics Of Individual Molecules Of Linear Polyethylene Liquids Under Shear: Atomistic Simulation And Comparison With A Free-Draining Bead-Rod Chain, David Keffer, J. Kim, B. Edwards, B. Khomami

David Keffer

Nonequilibrium molecular dynamics (NEMD) simulations of a dense liquid composed of linear polyethylene chains were performed to investigate the chain dynamics under shear. Brownian dynamics (BD) simulations of a freely jointed chain with equivalent contour length were also performed in the case of a dilute solution. This allowed for a close comparison of the chain dynamics of similar molecules for two very different types of liquids. Both simulations exhibited a distribution of the end-to-end vector, |Rete|, with Gaussian behavior at low Weissenberg number (Wi). At high Wi, the NEMD distribution was bimodal, with two peaks associated with rotation and ...


A Reactive Molecular Dynamics Study Of The Thermal Decomposition Of Perfluorodimethyl Ether, David Keffer, Bangwu Jiang, Myvizhi Selvan, Brian Edwards Mar 2009

A Reactive Molecular Dynamics Study Of The Thermal Decomposition Of Perfluorodimethyl Ether, David Keffer, Bangwu Jiang, Myvizhi Selvan, Brian Edwards

David Keffer

Classical reactive molecular dynamics (RMD) simulation is used to model the thermal decomposition of perfluorodimethyl ether (CF3OCF3), which is relevant as a simple molecule containing the necessary architectural elements to study the chemical stability of perfluoropolyether lubricants. The RMD algorithm employs nonreactive interaction potentials for the reactants and products. The reactivity is implemented through a coarse-grained simulation algorithm, incorporating elements from both the quantum and macroscopic descriptions of the reaction. The RMD scheme maps the quantum mechanically determined transition state onto a set of geometric triggers. When a configuration matching those triggers is found in the RMD simulation, the reaction ...


Self-Consistent Multiscale Modeling In The Presence Of Inhomogeneous Fields, David Keffer Dec 2008

Self-Consistent Multiscale Modeling In The Presence Of Inhomogeneous Fields, David Keffer

David Keffer

Molecular dynamics (MD) simulations of a Lennard–Jones fluid in an inhomogeneous external field generate steady-state profiles of density and pressure with nanoscopic heterogeneities. The continuum level of mass, momentum, and energy transport balances is capable of reproducing the MD profiles only when the equation of state for pressure as a function of density is extracted directly from the molecular level of description. We show that the density profile resulting from simulation is consistent with both a molecular-level theoretical prediction from statistical mechanics as well as the solution of the continuum-level set of differential equations describing the conservation of mass ...


Comparison Of The Hydration And Diffusion Of Protons In Perfluorosulfonic Acid Membranes With Molecular Dynamics Simulations, David Keffer, Shengting Cui, Junwu Liu, Myvizhi Selvan, Stephen Paddison, Brian Edwards Sep 2008

Comparison Of The Hydration And Diffusion Of Protons In Perfluorosulfonic Acid Membranes With Molecular Dynamics Simulations, David Keffer, Shengting Cui, Junwu Liu, Myvizhi Selvan, Stephen Paddison, Brian Edwards

David Keffer

Classical molecular dynamics (MD) simulations were performed to determine the hydrated morphology and hydronium ion diffusion coefficients in two different perfluorosulfonic acid (PFSA) membranes as functions of water content. The structural and transport properties of 1143 equivalent weight (EW) Nafion, with its relatively long perfluoroether side chains, are compared to the short-side-chain (SSC) PFSA ionomer at an EW of 977. The separation of the side chains was kept uniform in both ionomers consisting of −(CF2)15− units in the backbone, and the degree of hydration was varied from 5 to 20 weight % water. The MD simulations indicated that the ...


Energetic And Entropic Elasticity Of Nonisothermal Flowing Polymers: Experiment, Theory, And Simulation, David Keffer, T. C. Ionescu, B. J. Edwards, V. G. Mavrantzas Dec 2007

Energetic And Entropic Elasticity Of Nonisothermal Flowing Polymers: Experiment, Theory, And Simulation, David Keffer, T. C. Ionescu, B. J. Edwards, V. G. Mavrantzas

David Keffer

The thermodynamical aspects of polymeric liquids subjected to nonisothermal flow are examined from the complementary perspectives of theory, experiment, and simulation. In particular, attention is paid to the energetic effects, in addition to the entropic ones, that occur under conditions of extreme deformation. Comparisons of experimental measurements of the temperature rise generated under elongational flow at high strain rates with macroscopic finite element simulations offer clear evidence of the persistence and importance of energetic effects under severe deformation. The performance of various forms of the temperature equation are evaluated with regard to experiment, and it is concluded that the standard ...


Atomistic Simulation Of Energetic And Entropic Elasticity In Short-Chain Polyethylenes, David Keffer, T. C. Ionescu, V. G. Mavrantzas, B. J. Edwards Dec 2007

Atomistic Simulation Of Energetic And Entropic Elasticity In Short-Chain Polyethylenes, David Keffer, T. C. Ionescu, V. G. Mavrantzas, B. J. Edwards

David Keffer

The thermodynamical aspects of polymeric liquids subjected to uniaxial elongational flow are examined using atomistically detailed nonequilibrium Monte Carlo simulations. In particular, attention is paid to the energetic effects, in addition to the entropic ones, which occur under conditions of extreme deformation. Atomistic nonequilibrium Monte Carlo simulations of linear polyethylene systems, ranging in molecular length from C24 to C78 and for temperatures from 300 to 450 K, demonstrate clear contributions of energetic effects to the elasticity of the system. These are manifested in a conformationally dependent heat capacity, which is significant under large deformations. Violations of the hypothesis ...


Theoretical Calculation Of Thermodynamic Properties Of Naphthalene, Methylnaphthalenes, And Dimethylnaphthalenes, David Keffer, Mohamad Kassaee, William Steele Dec 2006

Theoretical Calculation Of Thermodynamic Properties Of Naphthalene, Methylnaphthalenes, And Dimethylnaphthalenes, David Keffer, Mohamad Kassaee, William Steele

David Keffer

For this work we performed quantum mechanical (QM) and statistical mechanical (SM) calculations to generate the entropy of 13 aromatic compoundsnaphthalene, 2 methylnaphthalene isomers, and 10 dimethylnaphthalene isomersin the ideal gas state. Density functional theory (DFT) was used to calculate the equilibrium structure and perform a full normal-mode analysis. The DFT level of theory used in this paper is B3LYP/6-31G(d,p). DFT has also been used to determine barriers for the internal rotation contribution to the entropy. For four compounds for which experimental data are available, the calculated entropies have been compared to the experimental values. The calculated ...


Surfactant And Electric Field Strength Effects On Surface Tension At Liquid/Liquid/Solid Interfaces, David Keffer, Johanna Santiago, Robert Counce Apr 2006

Surfactant And Electric Field Strength Effects On Surface Tension At Liquid/Liquid/Solid Interfaces, David Keffer, Johanna Santiago, Robert Counce

David Keffer

We performed a series of experiments designed to elucidate the effects of the presence of sodium dodecyl sulfate (SDS) surfactant and an applied electrical field on the wetting behavior in a system containing a sessile droplet of phenylmethyl polysiloxane (PMPS) oil on a polished stainless steel surface submersed in aqueous solution. The voltage difference ranged from −3 to +3 V, which is at least 3 orders of magnitude smaller than from comparable recent work. We report the measured equilibrium contact angle of the droplet as a function of surfactant concentration and field strength. We then modeled the system. We solved ...