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Calculation of structural, thermodynamic and hydrocarbon transport properties of silicon containing polymers via molecular simulation

By Βασίλειος Ράπτης

Abstract

The aim of this thesis is the development of a new realistic united-atom molecular model for poly(dimethylsilylenemethylene) or PDMSM, a novel rubbery organosilicon polymer, and the employment of this model to predict several structural, thermodynamic and mass transport properties of PDMSM. The motivation behind this work is the prediction of the permeability and permselectivity of PDMSM membranes used in light hydrocarbon separation processes. Extensive Density Functional Theory and MP/2 quantum-mechanical calculations with the 6-311G basis set were conducted for a model dimer in order to determine the energy variation with respect to the torsion of silicon-carbon bonds of the polymer chain backbone. An appropriate functional form including bonded and “local” non-bonded pair intramolecular interactions was fitted to the energy differences calculated. The “non-local” non-bonded pair interactions were optimized by means of Molecular Dynamics calculations, on the basis of the accurate prediction of monomer and dimer thermodynamic properties. Methyl and methylene groups were represented as single united-atom interaction centers. The parameters thus obtained were used in extensive MD simulations, for the prediction of important properties of the polymer melt at various conditions. These included the density, isothermal compressibility, thermal expansion coefficient and cohesive energy density, and structural properties, such as intra- and intermolecular distribution functions and the static structure factor. The MD simulations employed the Gear predictor-corrector integration scheme and most of them were performed at the isothermal-isobaric (NPT) ensemble with the aid of the Nose and Klein extended ensemble method. A constant bond length representation was used, based on the Edberg, Evans and Morriss method for holonomic constraints, and was compared to a model allowing for bond length fluctuations. Very good agreement was obtained by both models with experimental data whenever available. The dynamics of the polymer were also studied in terms of pendant bonds and chain backbone torsion angles time autocorrelation functions, at various temperatures. The free volume distribution and fluctuations with time were examined by means of the Greenfield and Theodorou method. The phase space trajectories generated by the above simulations were used for the prediction of the solubility coefficient S, of n-alkanes, from methane up to n-hexane, at the infinite dilution limit, and at various temperatures, using the Widom test particle insertion method. Again, very good agreement was obtained with experimental data. Configurations of polymer and alkane mixtures were then used as input to MD calculations in order to predict the diffusion coefficient D, of the alkanes through the polymer, at atmospheric pressure and 300K temperature, based on the Eistein relation connecting mean square displacement with time-scales. Although very limited experimental data exist in this area, fair agreement was observed in the case of methane and propane, and consistent values were obtained for the rest of the alkanes. An analysis of the diffusion at a microscopic level confirmed the findings of previous works in this area, such as the effect of the polymer matrix on the penetrant behaviour (anomalous and anisotropic diffusion at timescales extended up to several nanoseconds) and the jump-like molecular mechanism of the gas transport process. The product of the diffusivity times the solubility yielded the permeability of the polymer with respect to each of the alkanes examined. Permeselectivity of the polymer with respect to methane/alkane binary mixtures defined as the ratio of the permeabilities of the two gases, showed the PDMSM membranes to be capable of efficiently separating light hydrocarbon mixture compounds

Topics: Μοριακή προσομοίωση, Μοριακή δυναμική, Πολυμερικές μεμβράνες, Υδρογονάνθρακες, Διαχωρισμός, Πολυμερή, Οργανοπυριτικά, Πολυμερή, Δομικές ιδιοότητες, Πολυμερή, Θερμοδυναμικές ιδιότητες, Πολυμερή, Διαπερατότητα, Molecular simulation, Molecular dynamics, Polymer membranes, Hydrocarbon separation, Organosilicon polymers, Polymers, Structure properies of, Polymers, Thermodynamic properies of, Polymers, Permeability of
Publisher: National and Kapodistrian University of Athens
Year: 2004
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