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In Silico Determination of Gas Permeabilities by Non-Equilibrium Molecular Dynamics: CO2 and He through PIM-1.

Frentrup H, Hart KE, Colina CM, Müller EA - Membranes (Basel) (2015)

Bottom Line: We study the permeation dynamics of helium and carbon dioxide through an atomistically detailed model of a polymer of intrinsic microporosity, PIM-1, via non-equilibrium molecular dynamics (NEMD) simulations.This work presents the first explicit molecular modeling of gas permeation through a high free-volume polymer sample, and it demonstrates how permeability and solubility can be obtained coherently from a single simulation.Solubilities in particular can be obtained to a very high degree of confidence and within experimental inaccuracies.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK. hendrik.frentrup10@imperial.ac.uk.

ABSTRACT
We study the permeation dynamics of helium and carbon dioxide through an atomistically detailed model of a polymer of intrinsic microporosity, PIM-1, via non-equilibrium molecular dynamics (NEMD) simulations. This work presents the first explicit molecular modeling of gas permeation through a high free-volume polymer sample, and it demonstrates how permeability and solubility can be obtained coherently from a single simulation. Solubilities in particular can be obtained to a very high degree of confidence and within experimental inaccuracies. Furthermore, the simulations make it possible to obtain very specific information on the diffusion dynamics of penetrant molecules and yield detailed maps of gas occupancy, which are akin to a digital tomographic scan of the polymer network. In addition to determining permeability and solubility directly from NEMD simulations, the results shed light on the permeation mechanism of the penetrant gases, suggesting that the relative openness of the microporous topology promotes the anomalous diffusion of penetrant gases, which entails a deviation from the pore hopping mechanism usually observed in gas diffusion in polymers.

No MeSH data available.


Related in: MedlinePlus

Schematic drawing of the simulation process. Initially, a simulation cell with two walls confining one specific dimension is populated with polybenzodioxane monomers (1). The sample is polymerized to create the PIM-1 2D sample (2). The box is elongated in the dimension of the two walls, which are made permeable for gases. While some selected carbon atoms on the polymer backbone are tethered to keep the structure from shifting inside the wall, the gases pass through the walls and adsorb to the polymer from the bulk regions during an equilibration run (3). Finally, an external field is imposed on the boundary, creating regions of higher and lower pressure on either side of the polymer. The steady-state flux and pressure difference are measured during this non-equilibrium molecular dynamics (NEMD) run (4).
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f4-membranes-05-00099: Schematic drawing of the simulation process. Initially, a simulation cell with two walls confining one specific dimension is populated with polybenzodioxane monomers (1). The sample is polymerized to create the PIM-1 2D sample (2). The box is elongated in the dimension of the two walls, which are made permeable for gases. While some selected carbon atoms on the polymer backbone are tethered to keep the structure from shifting inside the wall, the gases pass through the walls and adsorb to the polymer from the bulk regions during an equilibration run (3). Finally, an external field is imposed on the boundary, creating regions of higher and lower pressure on either side of the polymer. The steady-state flux and pressure difference are measured during this non-equilibrium molecular dynamics (NEMD) run (4).

Mentions: The walls confining the polymer in the z direction are made permeable to gas molecules, and two regions filled with gas molecules are constructed next to the polymer sample, creating an elongated simulation box in the z direction three-times the size of the original polymer sample (see Figure 4). Preliminary MD equilibration runs were performed to allow the gases to adsorb to the polymer. As gases saturate the polymer during equilibration, the gas uptake, which depends on the bulk pressure, was measured directly by integrating the gas density distribution:(2)c(P)=1VP∫ρ(z)dzwhere ρ denotes the gas density and VP denotes the volume of the polymer, which will specifically depend on the integration boundaries multiplied by the height and depth of the simulation box (in x and y directions, respectively). As expected, inert helium exhibits modest adsorption, while carbon dioxide adsorbed strongly. The gas regions initially contained 56 and 330 molecules for helium and carbon dioxide, respectively, such that the bulk gas pressure at equilibrium was in the order of 10 bar.


In Silico Determination of Gas Permeabilities by Non-Equilibrium Molecular Dynamics: CO2 and He through PIM-1.

Frentrup H, Hart KE, Colina CM, Müller EA - Membranes (Basel) (2015)

Schematic drawing of the simulation process. Initially, a simulation cell with two walls confining one specific dimension is populated with polybenzodioxane monomers (1). The sample is polymerized to create the PIM-1 2D sample (2). The box is elongated in the dimension of the two walls, which are made permeable for gases. While some selected carbon atoms on the polymer backbone are tethered to keep the structure from shifting inside the wall, the gases pass through the walls and adsorb to the polymer from the bulk regions during an equilibration run (3). Finally, an external field is imposed on the boundary, creating regions of higher and lower pressure on either side of the polymer. The steady-state flux and pressure difference are measured during this non-equilibrium molecular dynamics (NEMD) run (4).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4384093&req=5

f4-membranes-05-00099: Schematic drawing of the simulation process. Initially, a simulation cell with two walls confining one specific dimension is populated with polybenzodioxane monomers (1). The sample is polymerized to create the PIM-1 2D sample (2). The box is elongated in the dimension of the two walls, which are made permeable for gases. While some selected carbon atoms on the polymer backbone are tethered to keep the structure from shifting inside the wall, the gases pass through the walls and adsorb to the polymer from the bulk regions during an equilibration run (3). Finally, an external field is imposed on the boundary, creating regions of higher and lower pressure on either side of the polymer. The steady-state flux and pressure difference are measured during this non-equilibrium molecular dynamics (NEMD) run (4).
Mentions: The walls confining the polymer in the z direction are made permeable to gas molecules, and two regions filled with gas molecules are constructed next to the polymer sample, creating an elongated simulation box in the z direction three-times the size of the original polymer sample (see Figure 4). Preliminary MD equilibration runs were performed to allow the gases to adsorb to the polymer. As gases saturate the polymer during equilibration, the gas uptake, which depends on the bulk pressure, was measured directly by integrating the gas density distribution:(2)c(P)=1VP∫ρ(z)dzwhere ρ denotes the gas density and VP denotes the volume of the polymer, which will specifically depend on the integration boundaries multiplied by the height and depth of the simulation box (in x and y directions, respectively). As expected, inert helium exhibits modest adsorption, while carbon dioxide adsorbed strongly. The gas regions initially contained 56 and 330 molecules for helium and carbon dioxide, respectively, such that the bulk gas pressure at equilibrium was in the order of 10 bar.

Bottom Line: We study the permeation dynamics of helium and carbon dioxide through an atomistically detailed model of a polymer of intrinsic microporosity, PIM-1, via non-equilibrium molecular dynamics (NEMD) simulations.This work presents the first explicit molecular modeling of gas permeation through a high free-volume polymer sample, and it demonstrates how permeability and solubility can be obtained coherently from a single simulation.Solubilities in particular can be obtained to a very high degree of confidence and within experimental inaccuracies.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK. hendrik.frentrup10@imperial.ac.uk.

ABSTRACT
We study the permeation dynamics of helium and carbon dioxide through an atomistically detailed model of a polymer of intrinsic microporosity, PIM-1, via non-equilibrium molecular dynamics (NEMD) simulations. This work presents the first explicit molecular modeling of gas permeation through a high free-volume polymer sample, and it demonstrates how permeability and solubility can be obtained coherently from a single simulation. Solubilities in particular can be obtained to a very high degree of confidence and within experimental inaccuracies. Furthermore, the simulations make it possible to obtain very specific information on the diffusion dynamics of penetrant molecules and yield detailed maps of gas occupancy, which are akin to a digital tomographic scan of the polymer network. In addition to determining permeability and solubility directly from NEMD simulations, the results shed light on the permeation mechanism of the penetrant gases, suggesting that the relative openness of the microporous topology promotes the anomalous diffusion of penetrant gases, which entails a deviation from the pore hopping mechanism usually observed in gas diffusion in polymers.

No MeSH data available.


Related in: MedlinePlus