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.

The effects of interphase topology, entanglements, and chain dynamics on the mechanical response of semicrystalline polyethylene have been examined using atomistic simulations. In particular, the prevalence of the cavitation and melting/recrystallization mechanisms for yield and plastic flow were found to depend on both topological and dynamical properties of the molecular segments in the semicrystalline interphase. First, two different protocols were used during preparation of the interphase ensemble to modulate the distribution of (i) loops, bridges, and tails and (ii) entanglements within the noncrystalline domain. A protocol denoted "step-wise cooling" produced structures having a large fraction of long, entangled segments that yielded by the melting/recrystallization mechanism about 50% of the time. By contrast, the protocol denoted "instantaneous quench" produced structures that yielded by melting/recrystallization about 73% of the time. Second, two different united atom force fields, PYS and TraPPE-UA, that exhibit nearly identical topological characteristics of the noncrystalline domain but different mobilities were used to study the effect of chain dynamics on yield mechanisms. At the slower strain rate used in this work, yield and plastic flow proceeded exclusively via cavitation for the model using the TraPPE-UA force field, whereas both cavitation and melting/recrystallization were observed for the model using the PYS force field. The greater prevalence of melting/recrystallization in the latter case is attributed to faster chain-sliding dynamics in the crystalline domain. The dependences of the yield mechanism on topology and dynamics are found to be related.

Titanium diboride (TiB2), a layered ceramic material, comprised of titanium atoms sandwiched in between honeycomb planes of boron atoms, exhibits a promising structure to utilize the rich chemistry offered by the synergy of titanium and boron.

The electronic structure of polymers contains signatures that correlate with their short-term and long-term integrity when subjected to large electric stresses. A detailed picture of the electronic structure of realistic models of polymers has been difficult to obtain, mainly due to the chemical and morphological complexity encountered in polymers. In this work, we have undertaken a comprehensive analysis of the electronic structure of six model polymers displaying chemical and morphological diversity, namely, polyethylene (PE), polypropylene (PP), polystyrene (PS), poly(methyl methacrylate) (PMMA), polyethylene terephthalate (PET), and polybutylene terephthalate (PBT), using first-principles density functional theory computations and classical molecular dynamics simulations. In particular, we have studied the role of monomer chemistry, tacticity, and large-scale morphological disorders in shaping the electronic structure of these polymers. We find that monomer chemistry and morphological disorder cooperate to create localized energy states and the formation of shallow/deep trap depths near the band edges, but tacticity has little effect on the band structure. Appropriate connections and comparisons between the computed results (e.g., band gap and trap depths) and the available experimental data have also been provided. Critical insights on physicochemical and electronic structure relationships are revealed, providing a pathway for understanding the factors that control electrical conduction and degradation of polymers (i.e., charge transport mechanisms, ionization processes, and carrier injections from electrodes).