Dynamics of various Polymer/Graphene Interfacial Systems through Atomistic Molecular Dynamics Simulations

The current work refers to a simulation study on hybrid polymer/graphene interfacial systems. We explore the effect of graphene on the mobility of polymers, by studying three well known and widely used polymers, polyethylene (PE), polystyrene (PS) and poly(methyl-methacrylate) (PMMA). Qualitative and quantitative differences in the dynamic properties of the polymer chains in particular at the polymer/graphene interface are detected. Results concerning both the segmental and the terminal dynamics render PE much faster than the other two polymers, PS follows, while PMMA is the slowest one. Clear spatial dynamic heterogeneity has been observed for all model systems, with different dynamical behavior of the adsorbed polymer segments. The segmental relaxation time of polymer (τseg) as a function of the distance from graphene shows an abrupt decrease beyond the first adsorption layer for PE, as a result of its the well-ordered layered structure close to graphene, though a more gradual decay for PS and PMMA. The distribution of the relaxation times of adsorbed segments was also found to be broader than the bulk ones for all three polymer/graphene systems

A Molecular Dynamics Study of Polymer/Graphene Nanocomposites

Graphene based polymer nanocomposites are hybrid materials with a very broad range of technological applications. In this work, we study three hybrid polymer/graphene interfacial systems (polystyrene/graphene, poly(methyl methacrylate)/graphene and polyethylene/graphene) through detailed atomistic molecular dynamics (MD) simulations. Density profiles, structural characteristics and mobility aspects are being examined at the molecular level for all model systems. In addition, we compare the properties of the hybrid systems to the properties of the corresponding bulk ones, as well as to theoretical predictions

Molecular Dynamics of Polyisoprene/Polystyrene Oligomer Blends: The role of self-concentration and fluctuations on blend dynamics

The effect of self-concentration and intermolecular packing on the dynamics of polyisoprene (PI)/polystyrene (PS) blends is examined by extensive atomistic simulations. Direct information on local structure of the blend system allows a quantitative calculation of self- and effective composition terms at various length scales that are introduced to proposed models of blend dynamics. Through a detailed statistical analysis, the full distribution of relaxation times associated with reorienation of carbon-hydrogen bonds was extracted and compared to literature experimental data. A direct relation between relaxation times and local effective composition is found. Following an implementation of a model involving local composition as well as concentration fluctuations the relevant length scales characterizing the segmental dynamics of both components were critically examined. For PI the distribution of times becomes narrower for the system with the lowest PS content and then broadens as more PS is added. This is in contrast to the slow component (PS), where an extreme breadth is found for relaxation times in the 25/75 system prior to narrowing as we increase PI concentration. The chain dynamics was directly quantified by diffusion coefficients as well as the terminal (maximum) relaxation time of each component in the mixed state. Strong coupling between the friction coefficients of the two components was predicted that leads to very similar chain dynamics for PI and PS, particularly for high concentrations of PI. We anticipate this finding to the rather short oligomers (below the Rouse regime) studied here as well as to the rather similar size of PI and PS chains. The ratio of the terminal to the segmental relaxation time, τterm/τseg,c, presents a clear qualitative difference for the constituents: for PS the above ratio is almost independent of blend composition and very similar to the pure state. In contrast, for PI this ratio depends strongly on the composition of the blend; i.e. the terminal relaxation time of PI increases more than its segmental relaxation time, as the concentration of PS increases, resulting into a larger terminal/segmental ratio. We explain this disparity, based on the different length scales characterizing dynamics. The relevant length for the segmental dynamics of PI is about 0.4-0.6 nm, smaller than chain dimensions which are expected to characterize terminal dynamics, whereas for PS associated length scales are similar (about 0.7-1.0 nm) rendering a uniform change with mixing

Structure and Dynamics of Poly(methyl-methacrylate)/Graphene systems through Atomistic Molecular Dynamics Simulations

The main goal of the present work is to examine the effect of graphene layers on the sructural and dynamical properties of polymer systems. We study hybrid poly(methyl methacrylate) (PMMA)/graphene interfacial systems, through detailed atomistic molecular dynamics (MD) simulations. In order to characterize the interface, various properties related to density, structure and dynamics of polymer chains are calculated, as a function of the distance from the substrate. A series of different hybrid systems, with width ranging between [2.60 – 13.35] nm, are being modeled. In addition, we compare the properties of the macromolecular chains to the properties of the orresponding bulk system at the same temperature. We observe a strong effect of graphene layers on both structure and dynamics of the PMMA chains. Furthermore the PMMA/graphene interface is characterized by different length scales, depending on the actual property we probe: Density of PMMA polymer chains is larger than the bulk value, for polymer chains close to graphene layers up to distances of about [1.0-1.5]nm. Chain conformations are perturbed for distances up to about 2-3 radius of gyration from graphene. Segmental dynamics of PMMA is much slower close to the solid layers up to about [2-3]nm. Finally terminal-chain dynamics is slower, compared to the bulk one, up to distances of about 5-7 radius of gyration

Properties of short polystyrene chains confined between two Gold surfaces through a combined Density Functional Theory and classical Molecular Dynamics approach

The properties of atactic short-chain polystyrene films confined between two parallel gold surfaces at a temperature of 503 K are investigated using a combination of density functional theory calculations and classical atomistic simulations. A classical Morse-type potential, used to describe the interaction between the polymer and the gold surface, was parameterized based on the results of density functional calculations. Several polystyrene films were studied, with thicknesses ranging from around 1-10 nm. The structural, conformational and dynamical properties of the films were analysed and compared to the properties of the bulk polystyrene systems. The dynamics of the polystyrene close to the surface was found to be significantly slower than in the bulk

Expanding an abridged life table

A question of interest in the demographic and actuarial fields is the estimation of the age-specific mortality pattern when data are given in age groups. Data can be provided in such a form usually because of systematic fluctuations caused by age heaping. This is a phenomenon usual to vital registrations related to age misstatements, usually preferences of ages ending in multiples five. Several techniques for expanding an abridged life table to a complete one are proposed in the literature. Although many of these techniques are considered accurate and are more or less extensively used, they have never been simultaneously evaluated. This work provides a critical presentation, an evaluation and a comparison of the performance of these techniques. For that purpose, we consider empirical data sets for several populations with reliable analytical documentation. Departing from the complete sets of qx-values, we form the abridged ones. Then we apply each one of the expanding techniques considered to these abridged data sets and finally we compare the results with the corresponding complete empirical values.abridged life table, age-specific mortality pattern, complete life table, expanding method, interpolation, life tables, parametric models, probability of dying, splines

Hierarchical multiscale modeling of polymer-solid interfaces: atomistic to coarse-grained description, and structural and conformational properties of polystyrene-gold systems

A hierarchical simulation approach was developed in order to study polystyrene films sandwiched between two parallel Au(111) surfaces. The coarse-grained potentials describing the interaction of polystyrene with the gold surface were developed systematically using constrained all-atom molecular simulations of a styrene trimer on the Au(111) surface. The model was validated by studying a 5 nm film of short (10mer) polystyrene chains using all-atom and coarse-grained molecular dynamics simulations. The density, structure and conformational properties of coarse-grained films were found to be in excellent agreement with all-atom ones. The coarse-grained model was then used to study the structural and conformational properties of roughly 10 nm and 20 nm thick films with 10, 50, 100 and 200mer chains. The width of the interphase region of the polymer films is property specific. The density profiles reached the bulk value around 1.5 nm from the interface, for all chain lengths. An estimate of the width of the interphase region based on the conformation tensor profile indicates that the interphase width is proportional to the square root of the chain length (number of monomers) and for 200mer chains the interphase width is approximately 6-9 nm

Geographical competition-complementarity relationships between Greek regional economies

This paper examines the nature of interregional competition and complementarity in Greece. Covering the 1975-2002 period, the analysis provides a picture of the interregional interactions by focusing on derived linkages between the regional economies. The adopted methodology combines previous approaches based on the Dendrinos-Sonis model (e.g. Nazara et al 2002, Bonet 2003) and a cointegration modeling framework (Marquez and Hewings, 2003). A sensitivity analysis of the model coefficients that designate competition or complementarity, with respect to time, is undertaken as well.