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.

Using molecular dynamics simulations and model graphene layers in an organic matrix we demonstrate that interfacial thermal resistance determined via "thermal relaxation method" is up to an order of magnitude larger than that determined from "direct simulation method" of heat transfer across the matrix-graphene-matrix interface. We provide an explanation of this difference based on the spectral analysis of the frequency dependent vibrational temperature. The importance of our finding lies in the fact that the relaxation method mimics experimental laser based pump-probe measurements of the interfacial thermal resistance, while the direct simulation method provides information relevant to predicting and understanding thermal conductivity of nanocomposites.

Molecular dynamics simulations on the Kremer-Grest bead-spring model of polymer melts are used to study the effect of spherical nanoparticles on chain diffusion. We find that chain diffusivity is enhanced relative to its bulk value when polymer-particle interactions are repulsive and is reduced when polymer-particle interactions are strongly attractive. In both cases chain diffusivity assumes its bulk value when the chain center of mass is about one radius of gyration R(g) away from the particle surface. This behavior echoes the behavior of polymer melts confined between two flat surfaces, except in the limit of severe confinement where the surface influence on polymer mobility is more pronounced for flat surfaces. A particularly interesting fact is that, even though chain motion is strongly speeded up in the presence of repulsive boundaries, this effect can be reversed by pinning one isolated monomer onto the surface. This result strongly stresses the importance of properly specifying boundary conditions when the near surface dynamics of chains are studied.

The distribution and atomic structure of grain boundaries has been investigated in UO2. Our scanning electron microscopic/electron backscatter diffraction experiments on a depleted UO2 sample showed real nuclear fuels contain a combination of special coincident site lattice (CSL) and general boundaries. The experimental data indicated that ∼16% of the boundaries were CSL boundaries and the CSL distribution was dominated by low Σ boundaries; namely Σ9, Σ3, and Σ5 Based on our experimental observations, the structures of select low Σ (Σ5 tilt, Σ5 twist, Σ3 tilt) and a random boundary were analyzed in greater detail using empirical potential atomic‐scale calculations. Our calculations indicate that the boundaries have very different structures and each CSL boundary had multiple minima on the γ‐surface. The presence of a significant fraction of CSL boundaries and the differences in their structures are expected to have important consequences on fuel properties.

Using molecular dynamics simulations with a reactive force field (ReaxFF), we generate models of amorphous carbon (a-C) at a wide range of densities (from 0.5 g/cc to 3.2 g/cc) via the "liquid-quench" method. A systematic study is undertaken to characterize the structural features of the resulting a-C models as a function of carbon density and liquid quench simulation conditions: quench rate, type of quench (linear or exponential), annealing time and size of simulation box. The structural features of the models are investigated in terms of pair correlation functions, bond-angles, pore-size distribution and carbon hybridization content. Further, the influence of quench conditions on hybridization/graphitization is investigated for different stages of the simulation. We observe that the structural features of generated a-carbon models agree well with similar models reported in literature. We find that in the low-density regime, 2 effects play an important role in determining the pore size distribution and the structures are predominantly anisotropic. Whereas, at densities larger than 1.0 g/cc, the structures are spacefilling and differences exist only in terms of carbon hybridization. The rate of structural evolution (pore size and hybridization) during the quench process is observed to be dependent on the quench type, rate and the annealing time. IntroductionCarbon shows remarkable versatility since it exists in various chemical and structural forms. On one hand, crystalline and ordered phases such as graphene, diamond, carbon nanotubes, etc., confer an extraordinary range of properties. On the other, equally important is the plethora of amorphous structures of carbon (denoted as a-C and alternately referred to as disordered carbon) existing in a wide range of densities ranging from low-density, char-like carbon to high-density diamond-like and tetrahedral amorphous carbon (denoted as ta-C [1]), with varied structural and chemical features. Correspondingly, this confers a-C with a wide variety of properties and applications, ranging from low-conductivity heat-shield ablators [2,3] in the low-density regime, to high-hardness, chemically inert and optically transparent coatings [4,5], magnetic storage applications [6] among others [5] for diamond-like amorphous carbon films.The term "amorphous carbon" can be attributed as an umbrella term to carbon at a large range of densities ranging from char-like carbon (~ 0.2 to 0.5 g/cc [3]) to high-density, diamond-3 like carbon (~3.2 g/cc [5]). Since there is no well-defined order for amorphous carbon, it has been a challenge to characterize and fully understand their structure [7]. It is in this regard that models of a-C structure generated by computer simulation techniques become very useful in understanding complex structure-property relations and optimize desired properties.

We performed molecular dynamics (MD) simulations on amorphous polyethylene (PE) and polystyrene (PS) in order to elucidate the effect of crosslinks between polymer chains on heat conduction. In each polymer system, thermal conductivities were measured for a range of crosslink concentration by using nonequilibrium MD techniques. PE comprised of 50 carbon atom long chains exhibited slightly higher conductivity than that of 250 carbon atom long chains at the standard state. In both cases for PE, crosslinking significantly increased conductivity and the increase was more or less proportional to the crosslink density. On the other hand, in the PS case, although the thermal conductivity increased with the crosslinking, the magnitude of change in thermal conductivity was relatively small. We attribute this difference to highly heterogeneous PS based network including phenyl side groups. In order to elucidate the mechanism for the increase of thermal conductivity with the crosslink concentration, we decomposed energy transfer into modes associated with various bonded and non-bonded interactions. V

A systematic comparison of atomistic modeling methods including density functional theory (DFT), the self-consistent charge density-functional tight-binding (SCC-DFTB), and ReaxFF is presented for simulating the initial stages of phenolic polymer pyrolysis. A phenolic polymer system is simulated for several hundred picoseconds within a temperature range of 2500 to 3500 K. The time evolution of major pyrolysis products including small-molecule species and char is examined. Two temperature zones are observed which demark cross-linking versus fragmentation. The dominant chemical products for all methods are similar, but the yields for each product differ. At 3500 K, DFTB overestimates CO production (300-400%) and underestimates free H (~30%) and small C(m)H(n)O molecules (~70%) compared with DFT. At 3500 K, ReaxFF underestimates free H (~60%) and fused carbon rings (~70%) relative to DFT. Heterocyclic oxygen-containing five- and six-membered carbon rings are observed at 2500 K. Formation mechanisms for H2O, CO, and char are discussed. Additional calculations using a semiclassical method for incorporating quantum nuclear energies of molecules were also performed. These results suggest that chemical equilibrium can be affected by quantum nuclear effects at temperatures of 2500 K and below. Pyrolysis reaction mechanisms and energetics are examined in detail in a companion manuscript.

Using molecular dynamics simulations, we study model graphene nanoplatelets and carbon nanotubes in an organic matrix. We demonstrate that, despite relatively high interfacial thermal resistance between the filler and the matrix, the thermal conductivity enhancement of the nanocomposite can be very significant. Our results suggest that agglomeration and low aspect ratio of the conductive nanofiller additive are primarily responsible for the limited conductivity enhancement reported to date. Mapping of the simulation results on the homogenization model, accounting for interfacial resistance, allows us to predict the full potential of the nanocarbon filler addition for thermal conductivity enhancement.