J. Matthew D. Lane

Effective interactions between grafted nanoparticles in a polymer matrix

Molecular dynamics simulations were used to delineate the separation dependent forces between two polymer-grafted nanoparticles in a polymer melt, the associated potential of mean force (PMF), and the molecular origins of these forces. The nanoparticle radius (=5, in units of the size of the chain monomers) and grafted brush length (=10) were held constant, while the grafting density and the polymer matrix length were varied systematically in a series of simulations. We first show that simulations of a single nanoparticle do not reveal any signatures of the expected autophobic dewetting of the brush with increasing polymer matrix length. In fact, density distributions of the matrix and grafted chains around a single nanoparticle appear to only depend on the grafting density but not on the matrix chain length in the regime where autophobic dewetting is expected, i.e., when the matrix chain length is equal to or longer than the graft chain length. We thus conjecture that two nanoparticle simulations might be more illuminating in these situations. Indeed, the calculated forces between two nanoparticles in a melt show that increasing the matrix chain length from 10 to 70 causes the inter-nanoparticle potential of mean force (PMF) to go from purely repulsive to attractive with a well depth on the order of kBT. These results are purely entropic in origin and arise from a competition between brush-brush repulsion and an attractive inter-nanoparticle interaction caused by matrix depletion from the inter-nanoparticle zone. The matrix-induced Asakura-Oosawa type inter-nanoparticle attraction, which dominates at intermediate nanoparticle separations especially in the case of long matrix chains, is thus implicated as the essential player in the autophobic dewetting phenomenon, which drives phase separation in these situations.

Forces between functionalized silica nanoparticles in solution.

To prevent the flocculation and phase separation of nanoparticles in solution, nanoparticles are often functionalized with short chain surfactants. Here we present fully atomistic molecular dynamics simulations which characterize how these functional coatings affect the interactions between nanoparticles and with the surrounding solvent. For 5-nm-diameter silica nanoparticles coated with poly(ethylene oxide) (PEO) oligomers in water, we determined the hydrodynamic drag on two approaching nanoparticles moving through solvent and on a single nanoparticle as it approaches a planar surface. In most circumstances, macroscale fluid theory accurately predicts the drag on these nanoscale particles. Good agreement is seen with Brenners analytical solutions for wall separations larger than the soft nanoparticle radius. For two approaching coated nanoparticles, the solvent-mediated (velocity independent) and lubrication (velocity-dependent) forces are purely repulsive and do not exhibit force oscillations that are typical of uncoated rigid spheres.

Water in nanoconfinement between hydrophilic self-assembled monolayers.

Molecular dynamics (MD) simulations of water confined to subnanometer thicknesses between carboxyl-terminated alkanethiol self-assembled monolayers (SAMs) on gold were performed to address conflicts in the literature on the structure and response of water in confinement. The amount of water was varied to yield submonolayer to bilayer structures. The orientation of the water is affected by the confinement, especially in the submonolayer case. We find that the diffusion coefficient decreases as the film becomes thinner and at higher pressures. However, in all cases studied, liquid diffusion is always found. At maximal suppression, the diffusion constant is 2 orders of magnitude smaller than the bulk value.

Effects of functional groups and ionization on the structure of alkanethiol coated gold nanoparticles

We report classical atomistic molecular dynamics simulations of alkanethiol-coated gold nanoparticles solvated in water and decane, as well as at water/vapor interfaces. The structure of the coatings is analyzed as a function of various functional end groups, including amine and carboxyl groups in various ionization states. We study both neutral and charged end groups for two different chain lengths (9 and 17 carbons). For the charged end groups, we simulated both mono- and divalent counterions. For the longer alkanes, we find significant local bundling of chains on the nanoparticle surface, which results in highly asymmetric coatings. In general, the charged end groups attenuate this effect by enhancing the water solubility of the nanoparticles. On the basis of the coating structures and density profiles, we can qualitatively infer the overall solubility of the nanoparticles. This asymmetry in the alkanethiol coatings is likely to have a significant effect on aggregation behavior. Our simulations elucidate the mechanism by which modulating the end group charge state can be used to control coating structure and therefore nanoparticle solubility and aggregation behavior.

Fully atomistic simulations of the response of silica nanoparticle coatings to alkane solvents.

Molecular dynamics simulations are used to study the effect of passivating ligands of varying lengths grafted to a nanoparticle and placed in various alkane solvents. Average height and density profiles for methyl-terminated alkoxylsilane ligands (-O-Si(OH)(2)(CH(2))(n)CH(3), with n = 9, 17, and 35) attached to a 5-nm-diameter amorphous silica nanoparticle with coverages of between 1.0 and 3.0 chains/nm(2) are presented for explicitly modeled, short-chain hydrocarbon solvents and for implicit good and poor solvents. Three linear solvents, C(10)H(22) (decane), C(24)H(50), and C(48)H(96), and a branched solvent, squalene, were studied. An implicit poor solvent captured the effect of the longest chain length solvent at lower temperatures, while its temperature dependence was similar to that of the branched solvent squalene. In contrast, an implicit good solvent produced coating structures that were far more extended than those found in any of the explicit solvents tested and showed little dependence on temperature. Coatings equilibrated in explicit solvents were more compact in longer-chain solvents because of autophobic dewetting. Changes in the coating density profiles were more pronounced as the solvent chain length was increased from decane to C(24)H(50) than from C(24)H(50) to C(48)H(98) for all coatings. The response of coatings in squalene was not significantly different from that of the linear chain of equal mass. Significant interpenetration of the solvent chains with the brush coating was observed only for the shortest grafted chains in decane. In all cases, the methyl terminal group was not confined to the coating edge but was found throughout the entire coating volume, from the core to the outermost shell. Increasing the temperature from 300 to 500 K led to greater average brush heights, but the dependence was weak.

Shock compression of hydrocarbon foam to 200 GPa: Experiments, atomistic simulations, and mesoscale hydrodynamic modeling

Hydrocarbon foams are versatile materials extensively used in high energy-density physics (HEDP) experiments. However, little data exist above 100 GPa, where knowledge of the behavior is particularly important for designing, analyzing, and optimizing HEDP experiments. The complex internal structure and properties of foam call for a multi-scale modeling effort validated by experimental data. We present results from experiments, classical molecular dynamics simulations, and mesoscale hydrodynamic modeling of poly(4-methyl-1-pentene) (PMP) foams under strong shock compression. Experiments conducted using the Z-machine at Sandia National Laboratories shock compress ∼0.300 g/cm3 density PMP foams to 185 GPa. Molecular dynamics (MD) simulations model shock compressed PMP foam and elucidate behavior of the heterogeneous foams at high pressures. The MD results show quantitative agreement with the experimental data, while providing additional information about local temperature and dissociation. Three-dimensional ...

Shock compression of hydrocarbon polymer foam using molecular dynamics

Organic polymers and nanocomposites are increasingly being subjected to extreme environments. Molecular-scale modeling of these materials offers insight into failure mechanisms and response. In previously published work, we used classical molecular dynamics (MD) and density functional theory (DFT) simulations to determine the principal shock Hugoniot for two hydrocarbon polymers, polyethylene (PE) and poly(4-methyl-1-pentene) (PMP). DFT was in excellent agreement with experiment, and one of four classical MD potentials, ReaxFF, was found to be suitable for studies up to 50 GPa. Here, we extend these results to include low-density polymer foams using NEMD techniques. We find good quantitative agreement with both experiment and hydrocode simulations. Further, we have measured local temperatures to investigate the formation of hot spots and polymer dissociation near foam voids.

Nanotribology of water confined between hydrophilic alkylsilane self-assembled monolayers

We report the results of large-scale molecular dynamics simulations of water confined between alkylsilane Si(OH)3(CH2)10COOH self-assembled monolayers (SAMs) on an amorphous silica substrate. The structure and dynamics of the confined water are studied for applied pressures ranging from approximately 50 to 400 MPa. The viscosity and microscopic friction of the confined water are determined from steady-state shear simulations. We find that the viscosity of the water increases only slightly compared with bulk water under comparable pressures. There is no evidence of ice-like layers being formed near the COOH end groups of the SAMs. The microscopic friction coefficients could only be calculated at high shear rates due to the low viscosity of the water and are found to decrease with increasing amounts of water, similar to experiment.

Molecular Dynamics Simulations of Water Confined between Matched Pairs of Hydrophobic and Hydrophilic Self-Assembled Monolayers

We have conducted a molecular dynamics (MD) simulation study of water confined between methyl-terminated and carboxyl-terminated alkylsilane self-assembled monolayers (SAMs) on amorphous silica substrates. In doing so, we have investigated the dynamic and structural behavior of the water molecules when compressed to loads ranging from 20 to 950 MPa for two different amounts of water (27 and 58 water molecules/nm2). Within the studied range of loads, we observe that no water molecules penetrate the hydrophobic region of the carboxyl-terminated SAMs. However, we observe that at loads larger than 150 MPa water molecules penetrate the methyl-terminated SAMs and form hydrogen-bonded chains that connect to the bulk water. The diffusion coefficient of the water molecules decreases as the water film becomes thinner and pressure increases. When compared to bulk diffusion coefficients of water molecules at the various loads, we found that the diffusion coefficients for the systems with 27 water molecules/nm2 are reduced by a factor of 20 at low loads and by a factor of 40 at high loads, while the diffusion coefficients for the systems with 58 water molecules/nm2 are reduced by a factor of 25 at all loads.

Structure of Rigid Polymers Confined to Nanoparticles: Molecular Dynamics Simulations Insight

Nanoparticles (NPs) grafted with organic layers form hybrids able to retain their unique properties through integration into the mesoscopic scale. The organic layer structure and response often determine the functionality of the hybrids on the mesoscopic length scale. Using molecular dynamics (MD) simulations, we probe the conformation of luminescent rigid polymers, dialkyl poly(p-phenylene ethynylene)s (PPE), end-grafted onto a silica nanoparticle in different solvents as the molecular weights and polymer coverages are varied. We find that, in contrast to NP-grafted flexible polymers, the chains are fully extended independent of the solvent. In toluene and decane, which are good solvents, the grafted PPEs chains assume a similar conformation to that observed in dilute solutions. In water, which is a poor solvent for the PPEs, the polymer chains form one large cluster but remain extended. The radial distribution of the chains around the core of the nanoparticle is homogeneous in good solvents, whereas in poor solvents clusters are formed independent of molecular weights and coverages. The clustering is distinctively different from the response of grafted flexible and semiflexible polymers.