Role of cationic groups on structural and dynamical correlations in hydrated quaternary ammonium-functionalized poly(p-phenylene oxide)-based anion exchange membranes.

Extensive atomistic molecular dynamics (MD) simulations employing a polarizable force field have been conducted to study hydrated anion exchange membranes comprised of a poly(p-phenylene oxide) (PPO) homopolymer functionalized with quaternary ammonium cationic side groups and hydroxide anions. Representative membranes with different cationic structures have been investigated to study correlations between polymer architecture, morphology and transport properties of hydrated membranes. Specifically, hydrated polymers with five different quaternary ammonium cationic groups (R1: -CH3, R2: -C2H5, R3: -C3H7, R4: -C6H13 and R5: -C4H8OCH3) and degree of functionalization of 50% were investigated at three hydration levels (λ = Nwater/Ncation = 5, 10 and 17). Effects of the polymer structure on the distribution of water-rich domains and dynamic relaxations were systematically investigated to uncover the complex interplay between the degree of hydrophobicity/hydrophilicity of the cationic groups, morphology, connectivity of water domains, and the hydroxide transport mechanisms. Structural and dynamical analysis indicates that the bottlenecks, formed between the water-rich domains, create a substantial free energy barrier for hydroxide transport associated with the partial loss of anion hydration structure. The energy penalty associated with the loss of the hydration structure hinders the vehicular transport of the hydroxide anion. The optimal structure of functionalized homopolymer chains should be sufficiently hydrophobic to create nanophase segregation and form an interconnected network of water channels with a minimal amount of narrow bottlenecks that inhibit the vehicular motion of hydrated anions. We demonstrate that utilization of asymmetrically modified cationic groups is a promising route to achieve the desired water channel morphology at low hydration levels.

Photoinduced and Thermal Relaxation in Surface-Grafted Azobenzene-Based Monolayers: A Molecular Dynamics Simulation Study.

Extensive atomistic molecular dynamics simulations have been employed to study the structure and molecular orientational relaxation of azobenzene-based monolayers grafted to a solid substrate. Systems with surface coverage of 0.6 nm(2)/molecule were investigated over a wide temperature range ranging from 298 K, where the mesogens show local ordering and the monolayer dynamics was found to be glassy, up to 700 K, where the azobenzene groups have a nearly isotropic orientational distribution, with a subnanosecond characteristic orientational relaxation time scale. Biased simulations that model single-molecule thermal excitation and conformational isomerization have been conducted to obtain insight into the mechanisms for photoinduced athermal fluidization and monolayer reorganization observed experimentally in this system. Our simulations clearly indicate that trans-cis conformational isomerization transitions of azobenzene units can lead to reorientation of mesogens and to the formation of a monolayer with strong macroscopic in-plane nematic order. While local heating created by excitation process can facilitate this process, thermal excitation alone is not sufficient to induce ordering in the monolayer. Instead, the work done by a molecule undergoing cis-trans isomerization on the cage of neighboring molecules is the key mechanism for photofluidization and orientational ordering in dMR monolayers exposed to linearly polarized light leading to relaxation dynamics that can be described in terms of higher effective temperature. The obtained simulation results are discussed in light of recent experimental data reported for these systems.

Effect of water on the structure of a prototype ionic liquid.

The influence of water on the structure of a prototype ionic liquid (IL) 1-octyl-3-methylimidazolium tetrafluoroborate (C8mimBF4) is examined in the IL-rich regime using high-energy X-ray diffraction (HEXRD) and molecular dynamics (MD) simulations. A many-body polarizable force field APPLE&P was developed for C8mimBF4-water mixture. It predicts structure factors of pure IL and IL-water mixture in excellent agreement with the HEXRD experiments. The MD results provide detailed insights into the structural changes from the partial structure factors, 2-D projections of the simulation box and 3-D distribution functions. Water partitioning with IL and its competition with BF4(-) for complexing the imidazolium rings was examined. The added water molecules occupy a diffuse coordination shell around the imidazolium ring but are not present around the alkyl tail. The strong coordination of the fluorine atoms of the BF4(-) anions to the imidazolium ring is not significantly changed by the addition of water. A complementary packing of water and imidazolium around BF4(-) was found. These results are consistent with the very small differences in the average structure between the pure IL and the mixture.

Flow-Tube Investigations of Hypergolic Reactions of a Dicyanamide Ionic Liquid Via Tunable Vacuum Ultraviolet Aerosol Mass Spectrometry.

The unusually high heats of vaporization of room-temperature ionic liquids (RTILs) complicate the utilization of thermal evaporation to study ionic liquid reactivity. Although effusion of RTILs into a reaction flow-tube or mass spectrometer is possible, competition between vaporization and thermal decomposition of the RTIL can greatly increase the complexity of the observed reaction products. In order to investigate the reaction kinetics of a hypergolic RTIL, 1-butyl-3-methylimidazolium dicyanamide (BMIM+DCA-) was aerosolized and reacted with gaseous nitric acid, and the products were monitored via tunable vacuum ultraviolet photoionization time-of-flight mass spectrometry at the Chemical Dynamics Beamline 9.0.2 at the Advanced Light Source. Reaction product formation at m/z 42, 43, 44, 67, 85, 126, and higher masses was observed as a function of HNO3 exposure. The identities of the product species were assigned to the masses on the basis of their ionization energies. The observed exposure profile of the m/z 67 signal suggests that the excess gaseous HNO3 initiates rapid reactions near the surface of the RTIL aerosol. Nonreactive molecular dynamics simulations support this observation, suggesting that diffusion within the particle may be a limiting step. The mechanism is consistent with previous reports that nitric acid forms protonated dicyanamide species in the first step of the reaction.

Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of achiral molecular dimers.

Freeze-fracture transmission electron microscopy study of the nanoscale structure of the so-called "twist-bend" nematic phase of the cyanobiphenyl (CB) dimer molecule CB(CH2)7CB reveals stripe-textured fracture planes that indicate fluid layers periodically arrayed in the bulk with a spacing of d ~ 8.3 nm. Fluidity and a rigorously maintained spacing result in long-range-ordered 3D focal conic domains. Absence of a lamellar X-ray reflection at wavevector q ~ 2π/d or its harmonics in synchrotron-based scattering experiments indicates that this periodic structure is achieved with no detectable associated modulation of the electron density, and thus has nematic rather than smectic molecular ordering. A search for periodic ordering with d ~ in CB(CH2)7CB using atomistic molecular dynamic computer simulation yields an equilibrium heliconical ground state, exhibiting nematic twist and bend, of the sort first proposed by Meyer, and envisioned in systems of bent molecules by Dozov and Memmer. We measure the director cone angle to be θ(TB) ~ 25° and the full pitch of the director helix to be p(TB) ~ 8.3 nm, a very small value indicating the strong coupling of molecular bend to director bend.

Thermophysical properties of energetic ionic liquids/nitric acid mixtures: insights from molecular dynamics simulations.

Molecular dynamics (MD) simulations of mixtures of the room temperature ionic liquids (ILs) 1-butyl-4-methyl imidazolium [BMIM]/dicyanoamide [DCA] and [BMIM][NO3(-)] with HNO3 have been performed utilizing the polarizable, quantum chemistry based APPLE&P(®) potential. Experimentally it has been observed that [BMIM][DCA] exhibits hypergolic behavior when mixed with HNO3 while [BMIM][NO3(-)] does not. The structural, thermodynamic, and transport properties of the IL/HNO3 mixtures have been determined from equilibrium MD simulations over the entire composition range (pure IL to pure HNO3) based on bulk simulations. Additional (non-equilibrium) simulations of the composition profile for IL/HNO3 interfaces as a function of time have been utilized to estimate the composition dependent mutual diffusion coefficients for the mixtures. The latter have been employed in continuum-level simulations in order to examine the nature (composition and width) of the IL/HNO3 interfaces on the millisecond time scale.

Molecular dynamics simulation studies of the influence of imidazolium structure on the properties of imidazolium/azide ionic liquids.

Atomistic molecular dynamics simulations were performed on 1-butyl-3-methyl-imidazolium azide [bmim][N(3)], 1-butyl-2,3-dimethylimidazolium azide [bmmim][N(3)], and 1-butynyl-3-methyl-imidazolium azide [bumim][N(3)] ionic liquids. The many-body polarizable APPLE&P force field was augmented with parameters for the azide anion and the bumim cation. Good agreement between the experimentally determined and simulated crystal structure of [bumim][N(3)] as well as the liquid-state density and ionic conductivity of [bmmim][N(3)] were found. Methylation of bmim (yielding bmmim) resulted in dramatic changes in ion structuring in the liquid and slowing of ion motion. Conversely, replacing the butyl group of bmim with the smaller 2-butynyl group resulted in an increase of ion dynamics.

Molecular dynamics simulations of N,N,N,N-tetramethylammonium dicyanamide plastic crystal and liquid using a polarizable force field.

A quantum chemistry based, dipole polarizable force field has been used to simulate the N,N,N,N-tetramethylammonium (TMA) dicyanamide (DCA) ionic salt, in both plastic crystalline and liquid phases. Simulations predicted the [TMA][DCA] crystal structure and dimensions in good agreement with experiment. Ion-counterion spatial distributions are used to understand the local environment and ion pairing of both ions in crystalline and liquid phases. The rotational dynamics of ions in the crystalline system are thoroughly explored. Arrest of the DCA rotational degrees of freedom was associated with the experimentally observed solid-solid phase transitions. The self-diffusion coefficient and conductivity were calculated for the liquid state; however no net ion diffusion is noted in the pristine crystalline state. Introduction of ion vacancy at 0.3% concentration is found to be sufficient to enable ion diffusive behavior and conduction at 425 K in the crystalline state, with good agreement found between the experimental and simulated conductivity.

Shock-induced transformations in crystalline RDX: a uniaxial constant-stress Hugoniostat molecular dynamics simulation study.

Molecular dynamics (MD) simulations of uniaxial shock compression along the [100] and [001] directions in the alpha polymorph of hexahydro-1,3,5-trinitro-1,3,5-triazine (alpha-RDX) have been conducted over a wide range of shock pressures using the uniaxial constant stress Hugoniostat method [Ravelo et al., Phys. Rev. B 70, 014103 (2004)]. We demonstrate that the Hugoniostat method is suitable for studying shock compression in atomic-scale models of energetic materials without the necessity to consider the extremely large simulation cells required for an explicit shock wave simulation. Specifically, direct comparison of results obtained using the Hugoniostat approach to those reported by Thompson and co-workers [Phys. Rev. B 78, 014107 (2008)] based on large-scale MD simulations of shocks using the shock front absorbing boundary condition (SFABC) approach indicates that Hugoniostat simulations of systems containing several thousand molecules reproduced the salient features observed in the SFABC simulations involving roughly a quarter-million molecules, namely, nucleation and growth of nanoscale shear bands for shocks propagating along the [100] direction and the polymorphic alpha-gamma phase transition for shocks directed along the [001] direction. The Hugoniostat simulations yielded predictions of the Hugoniot elastic limit for the [100] shock direction consistent with SFABC simulation results.

A molecular dynamics simulation study of the pressure-volume-temperature behavior of polymers under high pressure.

Isothermal compression of poly (dimethylsiloxane), 1,4-poly(butadiene), and a model Estane (in both pure form and a nitroplasticized composition similar to PBX-9501 binder) at pressures up to 100 kbars has been studied using atomistic molecular dynamics (MD) simulations. Comparison of predicted compression, bulk modulus, and U(s)-u(p) behavior with experimental static and dynamic compression data available in the literature reveals good agreement between experiment and simulation, indicating that MD simulations utilizing simple quantum-chemistry-based potentials can be used to accurately predict the behavior of polymers at relatively high pressure. Despite their very different zero-pressure bulk moduli, the compression, modulus, and U(s)-u(p) behavior (including low-pressure curvature) for the three polymers could be reasonably described by the Tait equation of state (EOS) utilizing the universal C parameter. The Tait EOS was found to provide an excellent description of simulation PVT data when the C parameter was optimized for each polymer. The Tait EOS parameters, namely, the zero-pressure bulk modulus and the C parameter, were found to correlate well with free volume for these polymers as measured in simulations by a simple probe insertion algorithm. Of the polymers studied, PDMS was found to have the most free volume at low pressure, consistent with its lower ambient pressure bulk modulus and greater increase in modulus with increasing pressure (i.e., crush-up behavior).

Structural and Dynamical Properties of Tetraalkylammonium Bromide Aqueous Solutions: A Molecular Dynamics Simulation Study Using a Polarizable Force Field.

Understanding the behavior of aqueous solutions containing tetraalkylammonium (TAA) cations is of great significance in a number of applications, including polymer membranes for fuel cells. In this work, a polarizable force field has been used to perform atomistic molecular dynamics (MD) simulations of aqueous solutions containing tetramethylammonium (TMA) or tetrabutylammonium (TBA) cations and Br counterions. Extensive MD simulations of TMA-Br/water and TBA-Br/water systems were conducted as a function of solution composition (ion pair:water molar ratios of 1:10, 1:20, 1:30, 1:63, and 1:500) at atmospheric pressure and 298 K. Our simulations demonstrate excellent agreement with available experimental data for solution densities and diffusion coefficients of different species as a function of solution composition, providing us confidence in analyzed structural and dynamic correlations. Various ion-ion and ion-water spatial distributions and the extent of cation aggregation are discussed in light of changes in the structure of cations hydration shells. The delicate balance between cation ionic core interactions with water and the hydrophobic interactions of alkyl tails leads to nontrivial self-assembly of TAA cations and the formation of an interpenetrating cationic network at higher concentrations. The ions and water dynamics are strongly coupled with the observed structural correlations and are analyzed in terms of various residence time, diffusion coefficients, and ionic conductivity.

Insight into hydrazinium nitrates, azides, dicyanamide, and 5-azidotetrazolate ionic materials from simulations and experiments.

A transferrable, polarizable, quantum chemistry (QC) based force field has been developed for hydrazinium (N(2)H(5)(+)), monomethylhydrazinium ((CH(3))N(2)H(4)(+)), and dimethylhydrazinium ((CH(3))(2)N(2)H(3)(+)) cations in combination with the nitrate (NO(3)(-)), azide (N(3)(-)), dicyanamide (N(CN)(2)(-)), and 5-azidotetrazolate (CN(7)(-)) anions. Inclusion of the off-atom charge center to represent a lone pair on the hydrazinium-based cations significantly improved the electrostatic potential description around cations and led to overall a more accurate prediction of ionic crystal cell parameters in molecular dynamics (MD) simulations. Seven different ionic systems have been investigated: [N(2)H(5)][NO(3)], [(CH(3))N(2)H(4)][NO(3)], [(CH(3))(2)N(2)H(3)][NO(3)], [N(2)H(5)][CN(7)], [(CH(3))N(2)H(4)][N(3)], [(CH(3))(2)N(2)H(3)][N(3)], [N(2)H(5)][N(CN)(2)]. For all but [(CH(3))(2)N(2)H(3)][NO(3)] and [N(2)H(5)][N(CN)(2)], QC calculations of a single, gas-phase ion pair predicts spontaneous deprotonation of the cation. Crystal lattice parameters obtained from MD simulations for these seven ionic crystals were compared with the previously published experimental data as well as the crystal structure of [N(2)H(5)][N(CN)(2)] determined in this work from X-ray data. In general, MD simulations predicted crystal lattice vectors/angles (volumes) within a 5% (3%) absolute margin of error from experiments, with outlying volume deviations of 5-6.6% for three crystals [(CH(3))N(2)H(4)][N(3)], [N(2)H(5)][NO(3)], and [(CH(3))N(2)H(4)][NO(3)] with combinations of particularly small anions and/or cations. Structural comparisons between ionic materials in the liquid and crystalline states are made, including the observation of two crystalline systems where the crystalline state induces conformational changes in the methylated hydrazinium cations between the gas-phase and liquid states. Elastic constants and estimated shear and bulk moduli were extracted from MD simulations for all seven ionic crystals and correlated with the structural motifs of ion interactions in the crystals.

The influence of polymer architecture on the assembly of poly(ethylene oxide) grafted C60 fullerene clusters in aqueous solution: a molecular dynamics simulation study.

The effect of polymer architecture on the aggregation behavior of C60 fullerenes tethered with a single chain of poly(ethylene oxide) (PEO) in aqueous solution has been investigated using coarse-grained, implicit solvent molecular dynamics simulations. The PEO-grafted fullerenes were comprised of a single tether of 60 repeat units represented as a linear polymer, a three-arm star (20 repeat units/arm) or a six-arm star (10 repeat units/arm). Additionally, the influence of arm length on self-assembly of the PEO-fullerene conjugates was investigated for the three-arm stars. Self-assembly is driven by favorable fullerene-fullerene and fullerene-PEO interactions. Our simulations reveal that it should be possible to control the size and geometry of the self-assembled fullerene aggregates in water through variation of PEO architecture and PEO molecular weight. We found that aggregate size and shape could be understood qualitatively in terms of the packing parameter concept that has been employed for diblock polymer and surfactant self-assembly. Higher molecular weight PEO (longer arms) and more compact PEO (more arms for the same molecular weight) resulted in greater steric repulsion between fullerenes, engendering greater aggregate surface curvature and hence the formation of smaller, more spherically shaped aggregates. Finally, weak attractive interactions between PEO and the fullerenes were found to play an important role in determining aggregate shape, size and the dynamics of self-assembly.

Supramolecular self-organization in PEO-modified C60 fullerene/water solutions: influence of polymer molecular weight and nanoparticle concentration.

Utilizing a first-principles-based coarse-grained implicit solvent model, we have investigated the self-association of C(60) fullerenes that have been symmetrically modified with six grafted poly(ethylene oxide) (PEO) chains in aqueous solution. Despite the highly symmetric nature of the pair interactions between PEO-grafted fullerenes, their supramolecular assemblies are highly anisotropic and resemble the linear clusters formed in Stockmayer fluids. The dipole-like interaction between these symmetrically modified fullerenes results from the shielding of the C(60) fullerenes by PEO, favoring the addition of more PEO-grafted fullerenes to the linear clusters at the relatively unprotected ends. At low nanoparticle concentrations, self-association is dominated by the formation of stable dimers and trimers resulting from fullerene-fullerene contact and favorable PEO-fullerene interactions. With increasing nanoparticle concentration, larger clusters become increasingly probable. The molecular weight of the PEO tethers can be treated as a temperature-like analogue, with a reduction in average cluster size with increasing chain length due to increased steric repulsion, which is qualitatively similar to effects observed in Stockmayer fluids with increasing temperature. The role of PEO in supramolecular self-organization in PEO-modified C(60) fullerene/water solutions is complex, contributing not only to steric stabilization but also to favorable energetic interactions, nanoparticle shielding, and depletion-driven aggregation.

Chain conformations and bound-layer correlations in polymer nanocomposites.

Small angle neutron scattering studies on polystyrene loaded with spherical silica nanoparticles under contrast-matched conditions unequivocally show that chain conformations follow unperturbed Gaussian statistics independent of chain molecular weight and filler composition. Liquid state theory calculations are consistent with this conclusion and also predict filler-induced modification of interchain polymer correlations which have a distinctive scattering signature that is in nearly quantitative agreement with our observations.