Edmund B. Webb

Dynamics of n-alkanes: Comparison to Rouse model

The crossover to Rouse-type behavior for the self-diffusion constant D, the viscosity η, and the equilibrium structural statistics of n-alkanes (6⩽n⩽66) is studied numerically. For small n the chains are non-Gaussian and the mean squared end-to-end distance 〈R2〉 is greater than 6〈RG2〉, where 〈RG2〉 is the mean squared radius of gyration. As n increases, 〈R2〉/〈RG2〉→6(1+a/n), where a depends on the interaction model. At constant density, the Rouse model is used to extract the monomeric friction coefficient ζ and the viscosity η independently from the diffusion constant D and the longest relaxation time τR. ζD extracted from D is nearly independent of chain length while ζτ obtained from τR is much larger than ζD for small n. The viscosity measured in a nonequilibrium molecular dynamics simulation is closely approximated by the value of η determined from τR while η inferred from D is smaller for small n. For n≳60, the two estimates for both ζ and η agree as predicted from the Rouse model. D calculated from thr...

Spreading dynamics of polymer nanodroplets.

The spreading of polymer droplets is studied using molecular dynamics simulations. To study the dynamics of both the precursor foot and the bulk droplet, large hemispherical drops of 200 000 monomers are simulated using a bead-spring model for polymers of chain length 10, 20, and 40 monomers per chain. We compare spreading on flat and atomistic surfaces, chain length effects, and different applications of the Langevin and dissipative particle dynamics thermostats. We find diffusive behavior for the precursor foot and good agreement with the molecular kinetic model of droplet spreading using both flat and atomistic surfaces. Despite the large system size and long simulation time relative to previous simulations, we find that even larger systems are required to observe hydrodynamic behavior in the hemispherical spreading droplet.

Reconsideration of Continuum Thermomechanical Quantities in Atomic Scale Simulations

As motivation builds to consider mechanics of nanometer scale objects, it is increasingly advantageous to implement models with finer resolution than standard continuum approaches. For such exercises to prove fruitful, these models must be able to quantify continuum thermomechanical quantities; furthermore, it may be necessary to do so on a sub-system level in order to assess gradients or distributions in a given property. Herein we review the calculation of stress, heat flux, and temperature in atomic scale numerical simulations such as the molecular dynamics method.

Influence of intracrystalline diffusion in shape selective catalytic test reactions

Molecular dynamics simulations are used to measure the self diffusion constant D of linear decane and n-methylnonanes (n = 2, 3, 4, and 5) at a catalytically relevant temperature in seven 10 member ring zeolites. Two general behaviors are observed in D as the branch position is moved towards the center of the alkane chain. For three of the zeolites (MEL, MFI, and MTT), D decreases monotonically as expected based on a consideration of the bulkiness of the different isomers. For the other four, alkane diffusion is considered anamolous as D is not a monotonic function of branch position. For n-methylnonanes in three zeolites D shows a minimum at either n = 2 (FER), 3 (EUO), or 4 (TON). In AEL, D has a local maximum for n = 3. Alkane diffusion is anamolous in these zeolites because they have structural features that provide a unique hindrance to molecular motion along the main diffusion channel. The ability of the zeolite to hinder motion depends on the molecular structure of the isoparaffin, resulting in the anamolous behavior observed. The 10 member ring zeolites selected for this study represent the entire group for which known structures exist and values of the modified constraint index have been published. The diffusion data presented indicates that product shape selectivity may play some part in determining the modified constraint index.

Interactions and structure of poly(dimethylsiloxane) at silicon dioxide surfaces: Electronic structure and molecular dynamics studies

Electronic structure studies are used to probe the interactions and molecular dynamics simulations are used to study the structure of thin poly(dimethylsiloxane) (PDMS) films near hydroxylated SiO2 substrates. Results of the electronic structure calculations show that the PDMS end groups, rather than atoms such as oxygen in the PDMS backbone structure, dominate interactions at the interface. Methyl–terminated PDMS binds weakly with the substrate via interactions between H atoms on PDMS methyl groups and O atoms on the substrate hydroxyl groups, while hydroxyl–terminated PDMS binds strongly with the substrate via hydrogen bonding between hydroxyl groups on PDMS and the substrate. To study the effect of temperature and type of substrate on the structural ordering of the PDMS liquid near the solid/liquid and liquid/air interfaces, molecular dynamics simulations for two temperatures (300 and 400 K) are carried out for three hydroxylated SiO2 substrates (α–quartz, β–cristobalite and amorphous SiO2). A direct c...

Spreading dynamics of polymer nanodroplets in cylindrical geometries

The spreading of one- and two-component polymer nanodroplets is studied using molecular dynamics simulation in a cylindrical geometry. The droplets consist of polymer chains of length 10, 40, and 100 monomers per chain described by the bead-spring model spreading on a flat surface with a surface-coupled Langevin thermostat. Each droplet contains approximately 350,000 monomers. The dynamics of the individual components of each droplet is analyzed and compared to the dynamics of single-component droplets for the spreading rates of the precursor foot and bulk droplet, the time evolution of the contact angle, and the velocity distribution inside the droplet. We derive spreading models for the cylindrical geometry analogous to the kinetic and hydrodynamic models previously developed for the spherical geometry, and show that hydrodynamic behavior is observed at earlier times for the cylindrical geometry. The contact radius is predicted to scale as r(t) approximately t1/5 from the kinetic model and r (t) approximately t1/7 for the hydrodynamic model in the cylindrical geometry.

Crossing the Mesoscale No-Man's Land via Parallel Kinetic Monte Carlo

The kinetic Monte Carlo method and its variants are powerful tools for modeling materials at the mesoscale, meaning at length and time scales in between the atomic and continuum. We have completed a 3 year LDRD project with the goal of developing a parallel kinetic Monte Carlo capability and applying it to materials modeling problems of interest to Sandia. In this report we give an overview of the methods and algorithms developed, and describe our new open-source code called SPPARKS, for Stochastic Parallel PARticle Kinetic Simulator. We also highlight the development of several Monte Carlo models in SPPARKS for specific materials modeling applications, including grain growth, bubble formation, diffusion in nanoporous materials, defect formation in erbium hydrides, and surface growth and evolution.

Dynamics of linear and branched alkane melts: Molecular dynamics test of theory for long time dynamics

Molecular dynamics (MD) simulations of united atom models for alkane melts are compared with a recently developed theory for calculating the memory functions of flexible polymers. The theory is based upon an approximate solution of the diffusion equation without hydrodynamic interactions. The polymer dynamics are described by using time correlation functions which are expressed in terms of a set of equilibrium averages and the approximate eigenvalues and eigenfunctions of the diffusion operator. For flexible enough chains with sufficiently high molecular weight, the hydrodynamic interactions are screened, and the simplified solvent model used by the theory is expected to be adequate. The only parameter not defined by the MD simulations is the bead friction coefficient ζ. In the limit of weak hydrodynamic interactions (Rouse dynamics), ζ can be determined from the molecular diffusion coefficient by applying the Rouse relation D=kT/NζR. Given this choice of ζR, the time correlation functions computed from t...

Atomic assembly of Cu/Ta multilayers: Surface roughness, grain structure, misfit dislocations, and amorphization

Molecular dynamics simulations and selected experiments have been carried out to study the growth of Cu films on (010) bcc Ta and the deposition of CuxTa1−x alloy films on (111) fcc Cu. They indicate that fcc Cu films with a (111) texture are always formed when Cu is deposited on Ta surfaces. These films are polycrystalline even when the Ta substrate is single crystalline. The grains have one of two different orientations and are separated by either orientational or misfit dislocations. Periodic misfit dislocations and stacking faults develop within these grains to release structure difference induced misfit strain energy. The Cu film surface roughness was found to decrease with increase in the adatom energy for deposition. When CuxTa1−x is deposited on Ta, the films always have a higher Cu composition than that of the vapor mixture. This arises from a surface segregation phenomenon. When the Cu and Ta fractions in the films are comparable, amorphous structures form. The fundamental origins for the segreg...

Comparisons between integral equation theory and molecular dynamics simulations for realistic models of polyethylene liquids

Previous applications of DF theory required a single chain Monte Carlo simulation to be performed within a self-consistent loop. In the current work, a methodology is developed which permits the simulation to be taken out of the iterative loop. Consequently, the calculation of the self-consistent, medium-induced-potential, or field, is decoupled from the simulation. This approach permits different densities, different forms of U{sub M}(r), and different wall-polymer interactions to be investigated from a single Monte Carlo simulation. The increase in computational efficiency is immense.