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Molecular dynamics (MD) simulations of melt films of poly(alkyl methacrylate)s (PAMAs) with methyl, ethyl, and n‐butyl substituents, respectively, have been performed using an all‐atom model to investigate their surface and thin film properties. The applied all‐atom force fields predict the bulk densities of PAMAs in good agreement with experiments. Moreover, predictions of the surface tensions of PMMA, PEMA, and Pn‐BMA melts are in reasonably good agreement with experiments. The density profiles and orientational‐order parameters of chain segments show atomic‐scale characteristics in the air/polymer interfacial region. In the surface region, the backbone segments of PAMAs form a well‐defined layer structure with the chain vectors oriented parallel to the surface, while the ester side‐chains strongly segregate to the surface region and show perpendicular orientation to the surface, with the most pronounced surface segregation noted for Pn‐BMA. Such surface segregations of chain segments make it difficult to apply a simple relationship between the cohesive energy density and the surface tension of polymers, for example, and should be taken into account in relating the surface/thin film characteristics to the bulk properties of polymers in general. [ABSTRACT FROM AUTHOR]

Interface characteristics of polystyrene (PS) melts in free-standing thin films and on a graphite surface were investigated by molecular dynamics simulations employing an explicit all-atom force field. The calculated surface tension is in good agreement with experiment, which provides good support for the force field parameters employed. In the polymer/vacuum free-surface region, the density profile exhibits an enrichment of phenyl groups relative to the backbone alkyl groups at the outermost low-density free surface, but this free surface is followed by a layer of relatively depleted phenyls and enriched alkyls of ca. 7 Å thickness. In the free surface, the phenyl-ring normal vectors and backbone chain vectors are both preferentially oriented along the film surface, in agreement with available experiments. At the polymer/graphite interface, the backbone chain vectors are strongly oriented along the graphite surface whereas the orientation distribution of phenyl-ring normal vectors exhibits two maxima along the nearly parallel (20°) and the perpendicular direction to the graphite-surface normal. A densely packed structure is formed at the PS-graphite interface, which strongly decreases the segmental chain mobility, in contrast to the enhanced segmental mobility in the free-surface region. [ABSTRACT FROM AUTHOR]

Using non-polarizable and polarizable molecular dynamics simulations, binary mixtures of propylene carbonate + dimethyl carbonate and propylene carbonate + ethylene carbonate with various compositions were investigated. The polarizable model produces more reasonable estimation of dielectric constants than the non-polarizable model; however, combining the electronic continuum model with the non-polarizable MD improves the comparison between the two models. Fair agreement was found between the results from these simulations and available experimental data. In addition, for a better understanding of the mixing behaviour, the excess dielectric constants over the entire composition were calculated. By comparison of the two mixtures in various mole fractions, distinctive mixing behaviours of propylene carbonate + dimethyl carbonate (poorly symmetric mixture) and propylene carbonate + ethylene carbonate (highly symmetric mixture) were observed. [ABSTRACT FROM PUBLISHER]

Abstract: Molecular dynamics simulations of binary mixtures of propylene carbonate (PC)+dimethyl carbonate (DMC) and PC+ethylene carbonate (EC) with various compositions were performed to study thermodynamic and dynamic properties such as density, dielectric constant and self-diffusion coefficient. For a better understanding of the thermodynamic behavior of the mixtures, the excess molar volume and excess dielectric constant with the various mole fractions were calculated. In addition, a computational approach of the Molecular Dynamics with Electronic Continuum (MDEC) model combined with density functional theory was used to determine the dielectric constants. Fair agreement was found between the results from our simulations and available experimental data. By comparison of the two mixtures observed in various mole fractions, the thermodynamic and dynamic behaviors of PC+DMC were highly asymmetrical whereas those of PC+EC were nearly symmetrical. [Copyright &y& Elsevier]

Abstract: A combination of quantum chemical calculation and molecular dynamics simulation is applied to compute refractive indices of liquid-forming silicon oils. The densities of these species are obtained from molecular dynamics simulations based on the NPT ensemble while the molecular polarizabilities are evaluated by density functional theory. This procedure is shown to yield results well compatible with available experimental data, suggesting that it represents a robust and economic route for determining the refractive indices of liquid-forming organic complexes containing silicon. [Copyright &y& Elsevier]

Abstract: Refractive indices of typical liquid-forming organic materials are calculated by computation from first principles in conjunction with molecular dynamics simulation. The molecular polarizability is obtained by density functional theory using the B3LYP potential, while the equilibrium density in the liquid phase is calculated utilizing the COMPASS force field. This combination of procedures is shown to provide an economical methodology for estimating the refractive indices of a wide range of organic species. By the standard of experiment, both the trends and the magnitudes of the available data are reproduced with good accuracy. [Copyright &y& Elsevier]

Molecular dynamics simulations of free-standing thin films of neat melts of polyethylene (PE) chains up to C150H302 and their binary mixtures with n-C13H28 are performed employing a united atom model. We estimate the surface tension values of PE melts from the atomic virial tensor over a range of temperatures, which are in good agreement with experimental results. Compared with short n-alkane systems, there is an enhanced surface segregation of methyl chain ends in longer PE chains. Moreover, the methyl groups become more segregated in the surface region with decreasing temperature, leading to the conclusion that the surface-segregation of methyl chain ends mainly arises from the enthalpic origin attributed to the lower cohesive energy density of terminal methyl groups. In the mixtures of two different chain lengths, the shorter chains are more likely to be found in the surface region, and this molecular segregation in moderately asymmetric mixtures in the chain length (C13H28 + C44H90) is dominated by the enthalpic effect of methyl chain ends. Such molecular segregation is further enhanced and dominated by the entropic effect of conformational constraints in the surface for the highly asymmetric mixtures containing long polymer chains (C13H28 + C150H3020). The estimated surface tension values of the mixtures are consistent with the observed molecular segregation characteristics. Despite this molecular segregation, the normalized density of methyl chain ends of the longer chain is more strongly enhanced, as compared with the all-segment density of the longer chain itself, in the surface region of melt mixtures. In addition, the molecular segregation results in higher order parameter of the shorter-chain segments at the surface and deeper persistence of surface-induced segmental order into the film for the longer chains, as compared with those in neat melt films. [ABSTRACT FROM AUTHOR]