Molecular modelling of high-performance polymer membranes and nanocomposites

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Polymers at the molecular level

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O2/N2 and CO2/CH4 separations in glassy membranes studied at the molecular level

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Modélisation moléculaire de la perméabilité du CO2 dans les membranes polymères

Les études concernant la séparation du dioxyde de carbone ont pris une importance majeure au cours du 21ème siècle. Dans le cas de matrices polymères, il est important d'examiner les relations entre les propriétés de perméation du CO2 et les caractéristiques structurales et dynamiques des polymères. Nous avons sélectionné trois polyimides fluorés (6FDA-6FpDA, 6FDA-6FmDA et 6FDA-DAM) et nous avons utilisé les techniques de simulations par dynamique moléculaire (DM) pour étudier en détails leurs propriétés relatives à la perméation du CO2 à l'échelle moléculaire. Des modèles moléculaires pour les trois polyimides fluorés ont été construits en utilisant la technique d'échantillonnage hybride de type Pivot Monte Carlo (PMC)-DM et ont ensuite été simulés par DM seule. Tous les modèles sont en bonne adéquation avec les propriétés dérivées des caractérisations expérimentales telles que les densités, les volumes libres, les d-spacings, les paramètres de solubilité, les énergies et les spectres de diffraction aux rayons-X. Les structures et morphologies des volumes libres dans les matrices pures ont aussi été analysées. Une addition par palier de CO2 a été effectuée dans les matrices polymères pures et une procédure itérative a été utilisée pour calculer les isothermes modèles de sorption de CO2. De plus, le gonflement en volume induit par la sorption du CO2 et l'effet immédiat des concentrations élevées en CO2 sur les isothermes de désorption ont été également étudiés. La morphologie des espaces vides, les énergies potentielles et la formation d'agglomérats de CO2 ont été caractérisés et comparés avec les caractérisations expérimentales. La diffusion de CO2 en fonction de sa concentration à l'intérieur des matrices polymères a été analysée et des facteurs affectant la mobilité ont été identifiés.Carbon dioxide (CO2) separation studies have become a major interest for researchers in the 21st century. In the case of polymer matrices, it is important to examine the relationships between the permeation properties of CO2 and the structural and dynamical features of the polymers. We have selected three fluorinated polyimides (6FDA-6FpDA, 6FDA-6FmDA and 6FDA-DAM) and used molecular dynamics (MD) simulations to provide a detailed picture of their CO2 permeation properties at the molecular level. Atomistic models of the three fluorinated polyimides were generated using the well-established hybrid Pivot Monte Carlo (PMC)-MD single-chain sampling technique, and were subsequently simulated using MD on its own. All the models were found to compare well with experimentally-derived properties such as densities, fractional free volumes, d-spacing parameters, solubility parameters, energies and X-ray data. Structures and void-space distributions in the pure matrices were also analysed. A realistic stepwise addition of CO2 was carried out in the pure polymer matrices and an iterative procedure was used to calculate the CO2 model sorption isotherms. In addition, the volume swelling induced by CO2 sorption and the immediate effect of exposure to high concentrations of CO2 on the desorption isotherms were also studied. The void spaces, potential energies and the formation of CO2 clusters were characterized and compared with experimental characterizations. The diffusivity of CO2 as a function of CO2 concentration within the polymer matrices was analysed, and the factors affecting mobility were reported in detail.CHAMBERY -BU Bourget (730512101) / SudocSudocFranceF

An optimized fully-atomistic procedure to generate glassy polymer films for molecular dynamics simulations

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Nanosecond-time-scale reversibility of dilation induced by carbon dioxide sorption in glassy polymer membranes

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The effect of structural isomerism on carbon dioxide sorption and plasticization at the interface of a glassy polymer membrane

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Single- and mixed-gas sorption in large-scale molecular models of glassy bulk polymers. Competitive sorption of a binary CH4/N2 and a ternary CH4/N2/CO2 mixture in a polyimide membrane

International audienceThree molecular simulation techniques to predict the gas sorption isotherms in a glassy polymer membrane in contact with a single- or a mixed-gas reservoir have been tested on a large-scale ~50000 atom 6FDA-6FpDA polyimide bulk model over a wide range of pressures. Both single- and mixed-gas uptake curves were obtained for CH4, N2 and CO2 over the 0–60 bar range using either a Grand Canonical Monte Carlo (GCMC), an iterative test particle insertion - molecular dynamics (TPI-MD) or an iterative GCMC-MD method. Virtual TPI and actual GCMC insertions of gas molecules into the polymer matrices were performed using the excluded-volume map sampling approach (EVMS), which improved the sampling efficiencies by a factor of ~10–20 over random insertions.The simulation techniques were first used to obtain the single-gas sorption isotherms and the associated ideal gas sorption selectivities. The TPI-MD and GCMC-MD approaches gave consistent results in agreement with experiment. Further tests were made on a binary 2:1 CH4/N2 and a ternary 16:8:1 CH4/N2/CO2 gas mixture in equilibrium with the 6FDA-6FpDA model matrix. For such mixed-gas feeds, the uptake of each gas in the polymer depends on its gas phase concentration and on its solubility in both the gas mixture and the polymer phases. Solubilities of penetrants in the polymer phase correlated well to the total penetrant concentration. In the binary mixture, the sorption of N2 was strongly hindered by that of CH4. In the ternary mixture, the introduction of the highly-soluble CO2 at a relatively low partial pressure significantly reduced the sorption of both CH4 and N2, although its concentration was insufficient to plasticize the polymer. As such, the mixed-gas CH4/N2, CO2/CH4 and CO2/N2 sorption selectivities were found to differ from their ideal values. Interference effects were characterized by a novel technique which estimates the proportions of molecules of each type of penetrant excluded by competitive sorption for the mixtures under study.The main asset of such iterative molecular simulations is their ability to take implicitly into account the interdependence of the different gas concentrations and solubilities as well as the associated changes in the matrices over a large range of pressures and temperatures. In addition, the iterative GCMC-MD method should be applicable to even more complex mixtures, which is obviously pertinent with respect to industrial gas separation applications