A detailed computational study of the potential of mean force between a pair of spherical particles dissolved in a homopolymer melt has been performed using microscopic liquid state theory. The role of particle-to-monomer diameter ratio, degree of polymerization, strength and spatial range of monomer-particle attractions, and direct interfiller attractions has been established. Beyond the small particle regime, the potential of mean force scales linearly with the particle-to-monomer diameter ratio. This simple scaling allows the construction of master curves and the quantification of material specific aspects independent of the filler-to-monomer diameter ratio. For hard-sphere fillers, four general categories of polymer-mediated organization are found: contact aggregation due to depletion attraction, segment level tight particle bridging, steric stabilization due to thermodynamically stable "bound polymer layers", and "tele-bridging" where distinct adsorbed layers coexist with longer range bridging. The conditions on the strength and spatial range of monomer-particle attractive interactions that define these different modes of organization have been established. Direct interparticle van der Waals attractions favor contact aggregation and thus compete with the rich polymer-mediated behavior. As the direct attractions increase in strength, the globally stable noncontact bridging configuration is gradually destabilized and replaced by contact aggregation as the most favored state of packing. However, bridging states often remain as metastable local minima. Steric stabilization systems are much less affected by direct interfiller attractions due to the thermodynamic stability of distinct bound polymer layers. This suggests design rules for achieving good particle dispersion. In addition, the interesting possibility is raised that sterically stabilized nanofillers may crystallize in a homopolymer matrix at relatively low volume fractions. Our results have implications for nonequilibrium phenomena such as gelation or filler network formation and kinetic stabilization via large repulsive barriers, which are qualitatively discussed.

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(CH 2 ) 7 CB 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(CH 2 ) 7 CB 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°a nd 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. R ecently there has been growing interest in the liquid crystal (LC) phase behavior of achiral dimer molecules, such as cyanobiphenyl-(CH 2 ) n -cyanobiphenyl (CBnCB), shown for n = 7 in Fig. 1 A (1, 2). This arises from the observation of a transition in these mesogens from a typical nematic (N) to a lower-temperature (NX) phase, also apparently nematic, which exhibits a variety of unusual characteristics (3-10). These include: (i) textural features in depolarized transmission light microscopy (DTLM) similar to those found in fluid, lamellar smectic phases but with no X-ray scattering to indicate lamellar ordering of molecules (8); (ii) a variety of other completely unfamiliar DTLM textures (6), including the spontaneous appearance of director field deformation and evidence for small Frank elastic constants (3); (iii) evidence for the chiral molecular organization on the NMR timescale (4), and in macroscopic conglomerate domains in electrooptic experiments on monodomain textures (9); (iv) distinctive odd/even effects in the linker length n, including, in particular, that i-iii are found only in the n-odd homologs (6).These observations, combined with the fact that the all-trans conformations of the n-odd homologous dimers are distinctly bent (Fig. 1B), have led to the notion that the NX is a "twistbend" (TB) phase, sketched in Fig. 1C, a nematic having a conically helixed ground state of the sort originally proposed by Meyer as the result of the spontaneous appearance of bend flexoelectric polarization (11). More recently Dozov proposed such a ground state as a spontaneously chiral conglomerate domain stabilized by molecular bend (12), and Memmer obtained such structures in computer simulations of systems of bent GayBerne dimers (13). This ground-state helix can be written for CB7CB in terms of a half-molecular director n(z), ...

The microscopic polymer reference interaction site model theory of polymer nanocomposites composed of flexible chains and spherical nanoparticles has been employed to study second virial coefficients and spinodal demixing over a wide range of interfacial chemistry, chain length, and particle size conditions. For hard fillers, two distinct phase separation behaviors, separated by a miscibility window, are generically predicted. One demixing curve occurs at relatively low monomer-particle attraction strength and corresponds to a very abrupt transition from an entropic depletion attraction-induced phase separated state to an enthalpically stabilized miscible fluid. The homogeneous mixture arises via a steric stabilization mechanism associated with the formation of thin, thermodynamically stable bound polymer layers around fillers. The second demixing transition occurs at relatively high monomer-particle adsorption energy and is inferred to involve the formation of an equilibrium physical network phase with local bridging of particles by polymers. This spinodal is sensitive to both particlemonomer diameter ratio and the spatial range of the interfacial attraction. The miscibility window narrows, and can ultimately disappear, with increasing polymer chain length, direct van der Waals attractions between fillers, and/or particle-monomer size asymmetry ratio. The implications of our results for the design of well-dispersed thermodynamically stable polymer nanocomposites, and the formation of nonequilibrium gels, are discussed.

The Polymer Reference Interaction Site Model (PRISM) theory is employed to investigate structure, effective forces, and thermodynamics in dense polymer-particle mixtures in the one and two particle limit. The influence of particle size, degree of polymerization, and polymer reduced density is established. In the athermal limit, the surface excess is negative implying an entropic dewetting interface. Polymer induced depletion interactions are quantified via the particle-particle pair correlation function and potential of mean force. A transition from (nearly) monotonic decaying, attractive depletion interactions to much stronger repulsive-attractive oscillatory depletion forces occurs at roughly the semidilute-concentrated solution boundary. Under melt conditions, the depletion force is extremely large and attractive at contact, but is proceeded by a high repulsive barrier. For particle diameters larger than roughly five monomer diameters, division of the force by the particle radius results in a nearly universal collapse of the depletion force for all interparticle separations. Molecular dynamics simulations have been employed to determine the depletion force for nanoparticles of a diameter five times the monomer size over a wide range of polymer densities spanning the semidilute, concentrated, and melt regimes. PRISM calculations based on the spatially nonlocal hypernetted chain closure for particle-particle direct correlations capture all the rich features found in the simulations, with quantitative errors for the amplitude of the depletion forces at the level of a factor of 2 or less. The consequences of monomer-particle attractions are briefly explored. Modification of the polymer-particle pair correlations is relatively small, but much larger effects are found for the surface excess including an energetic driven transition to a wetting polymer-particle interface. The particle-particle potential of mean force exhibits multiple qualitatively different behaviors (contact aggregation, steric stabilization, local bridging attraction) depending on the strength and spatial range of the polymer-particle attraction.

The one-orbital model for manganites with cooperative phonons and superexchange coupling JAF is investigated via large-scale Monte Carlo simulations. The results for two orbitals are also briefly discussed. Focusing on the electron density n=0.75, a regime of competition between ferromagnetic metallic and charge-ordered (CO) insulating states is identified. In the vicinity of the associated bicritical point, colossal magnetoresistance (CMR) effects are observed. The CMR is associated with the development of short-distance correlations among polarons, above the spin ordering temperatures, resembling the charge arrangement of the low-temperature CO state.

A fundamental understanding of solid electrolyte interphase (SEI) properties is critical for enabling further improvement of lithium batteries and stabilizing the anode–electrolyte interface. Mechanical and transport properties of two model SEI components were investigated using molecular dynamics (MD) simulations and a hybrid MD-Monte Carlo (MC) scheme. A many-body polarizable force field (APPLE&P) was employed in all simulations. Elastic moduli and conductivity of model SEIs comprised of dilithium ethylene dicarbonate (Li2EDC) were compared with those comprised of dilithium butylene dicarbonate (Li2BDC) over a wide temperature range. Both ordered and disordered materials were examined with the ordered materials showing higher conductivity in the conducting plane compared to conductivity of the disordered analogues. Li2BDC was found to exhibit softening and onset of anion mobility at lower temperatures compared to Li2EDC. At 120 °C and below, both SEI model compounds showed single ion conductor behavior. Ordered Li2EDC and Li2BDC phases had highly anisotropic mechanical properties, with the shear modulus of Li2BDC being systematically smaller than that for Li2EDC.

Microscopic polymer liquid state theory is employed to study the real space pair correlation functions and collective scattering structure factors of melt polymer nanocomposites composed of hard spheres and adsorbing homopolymers over length scales ranging from monomeric to macroscopic. Increasing filler volume fraction has a profound effect on the polymer matrix, inducing oscillatory reorganization on a length scale commensurate with the nanoparticle diameter. The near contact interfacial monomer-filler pair correlations are suppressed by nanoparticle addition, but on larger scales power law correlations emerge with filler imprinted oscillatory features. Increased nanoparticle volume fraction also substantially changes interfiller packing, inducing a transition from a highly bridged or sterically stabilized type of organization in the infinite dilution limit to more diffuse liquidlike packing characterized by many-particle clustering. Distinctive modifications of the collective polymer structure factor include an increase in the osmotic compressibility, the emergence of a large bound layer or microphaseseparation-like scattering peak on a length scale controlled by filler size indicative of distinct bound polymer layers, and a local rarefaction and suppression of the coherence of the monomer cage scale packing. All the real and Fourier space correlations depend in distinctive manners on the physical and chemical variables (filler size, volume fraction, strength and spatial range of the interfacial cohesion) and proximity to the contact aggregation and bridging phase separation boundaries. The bulk modulus of the nanocomposite generically softens with the addition of fillers corresponding to enhanced total density fluctuations.

Hydroxylammonium nitrate (HAN) is a promising candidate to replace highly toxic hydrazine in monopropellant thruster space applications. The reactivity of HAN aerosols on heated copper and iridium targets was investigated using tunable vacuum ultraviolet photoionization time-of-flight aerosol mass spectrometry. The reaction products were identified by their mass-to-charge ratios and their ionization energies. Products include NH 3 , H 2 O, NO, hydroxylamine (HA), HNO 3 , and a small amount of NO 2 at high temperature. No N 2 O was detected under these experimental conditions, despite the fact that N 2 O is one of the expected products according to the generally accepted thermal decomposition mechanism of HAN. Upon introduction of iridium catalyst, a significant enhancement of the NO/HA ratio was observed. This observation indicates that the formation of NO via decomposition of HA is an important pathway in the catalytic decomposition of HAN.