A molecular simulation study on gas diffusion in a dense poly(ether–ether–ketone) membrane (original) (raw)
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Diffusion of gases in PEEKs membranes: molecular dynamics simulations
Journal of Membrane Science, 2002
Results of molecular dynamics (MD) simulations on transport process of small molecules in amorphous cardo poly-etherether-ketone membranes, namely PEEK-WC, sulfonated PEEK-WC and nitrated PEEK-WC are discussed. Atomistic simulations techniques have proven to be a useful tool for the understanding of structure-property relationships of materials and in particular MD can be used for detailed descriptions of the complex morphologies and transport mechanisms associated with rigid glassy structures. The diffusion process results from jumps of penetrant molecules between adjacent holes in the polymer matrix. The occurring jump mechanism is characterised and visualised. Constants of diffusion and solubility coefficients have been calculated by the transition state Gusev-Suter Monte Carlo (MC) method.
Polymer, 1996
Molecular dynamics (MD) simulations were used to investigate the transport of different gases in a poly(amide imide) (PAI) and two polyimides (PI1 and PI2). The agreement between measured and simulated average diffusion coefficients (D) was acceptable. There was, however, a considerable scattering of the D values for the individual simulated gas molecules. While MD simulations are still not an ideal tool for the quantitative prediction of gas permeation properties of polymers, these methods can be used to obtain a better insight about the gas transport mechanism. Transport of small molecules occurs by jumps between individual sections of the free volume (holes) through temporarily open channels. The main difference between the PAI and the two polyimides is broader and slightly more permanent channels in the case of the PAI. Solubility investigations using the Widom method revealed a predominantly Henry sorption mechanism for hydrogen, a dual-mode sorption with a high degree of Langmuir-type immobilization for oxygen and nitrogen.
Journal of Physical Chemistry B, 2007
It is important for many industrial processes to design new materials with improved selective permeability properties. Besides diffusion, the molecule's solubility contributes largely to the overall permeation process. This study presents a method to calculate solubility coefficients of gases such as O 2 , H 2 O (vapor), N 2 , and CO 2 in polymeric matrices from simulation methods (Molecular Dynamics and Monte Carlo) using first principle predictions. The generation and equilibration (annealing) of five polymer models (polypropylene, polyvinyl alcohol, polyvinyl dichloride, polyvinyl chloride-trifluoroethylene, and polyethylene terephtalate) are extensively described. For each polymer, the average density and Hansen solubilities over a set of ten samples compare well with experimental data. For polyethylene terephtalate, the average properties between a small (n ) 10) and a large (n ) 100) set are compared. Boltzmann averages and probability density distributions of binding and strain energies indicate that the smaller set is biased in sampling configurations with higher energies. However, the sample with the lowest cohesive energy density from the smaller set is representative of the average of the larger set. Density-wise, low molecular weight polymers tend to have on average lower densities. Infinite molecular weight samples do however provide a very good representation of the experimental density. Solubility constants calculated with two ensembles (grand canonical and Henry's constant) are equivalent within 20%. For each polymer sample, the solubility constant is then calculated using the faster (10×) Henry's constant ensemble (HCE) from 150 ps of NPT dynamics of the polymer matrix. The influence of various factors (bad contact fraction, number of iterations) on the accuracy of Henry's constant is discussed. To validate the calculations against experimental results, the solubilities of nitrogen and carbon dioxide in polypropylene are examined over a range of temperatures between 250 and 650 K. The magnitudes of the calculated solubilities agree well with experimental results, and the trends with temperature are predicted correctly. The HCE method is used to predict the solubility constants at 298 K of water vapor and oxygen. The water vapor solubilities follow more closely the experimental trend of permeabilities, both ranging over 4 orders of magnitude. For oxygen, the calculated values do not follow entirely the experimental trend of permeabilities, most probably because at this temperature some of the polymers are in the glassy regime and thus are diffusion dominated. Our study also concludes large confidence limits are associated with the calculated Henry's constants. By investigating several factors (terminal ends of the polymer chains, void distribution, etc.), we conclude that the large confidence limits are intimately related to the polymer's conformational changes caused by thermal fluctuations and have to be regardedsat least at microscalesas a characteristic of each polymer and the nature of its interaction with the solute. Reducing the mobility of the polymer matrix as well as controlling the distribution of the free (occupiable) volume would act as mechanisms toward lowering both the gas solubility and the diffusion coefficients.
Journal of Membrane Science, 2008
An experimental and theoretical study has been used to investigate gas diffusion and solubility in PEBAX ® 2533 block copolymer membrane. Molecular simulations using COMPASS force field have been successful in predicting the gas-transport properties of a PEBAX ® 2533 block copolymer and of a pure PTMO homopolymer. Gusev-Suter transition state theory (TST) and Monte Carlo methods are used for simulating the transport of five permanent gases (He, H 2 , N 2 , O 2 , CO 2 and CH 4 ). Theoretical and experimental data have been compared. (E. Tocci).
Macromolecules, 2000
We compare molecular modeling results of two glassy polymer membranes and one rubbery polymer membrane with gas transport parameters and free-volume-related quantities from positronium annihilation. A simple geometric model reveals hole size distributions of asymmetric shape. Among glassy polymers, the distribution parameters show a good correlation with average hole sizes determined by positron annihilation lifetime spectroscopy. Higher permeability is measured in the glassy polymer with the higher mean value of the hole size distribution. The permselectivity of the membranes for permanent gases can be interpreted in terms of the distribution broadness via free-volume-controlled diffusion selectivity. A comparison with the rubbery polymer shows that the permeation behavior is not determined only by the free volume concentration. The thermal fluctuations of the polymer matrix play an important role for gas transport properties.
Molecular simulation of realistic membrane models of alkylated PEEK membranes
Molecular Simulation, 2006
Atomistic molecular modelling has proven to be a useful tool for the investigation of transport properties of small gas molecules in polymer membrane matrices. The quality of the predictions of these properties based on molecular simulation depends principally on the quality of the membrane model. The predicted gas transport properties of small gas molecules in the same glassy polymer membrane show often a large scatter in gas diffusion and solubility simulated values. In order to reduce the scatter in predicted gas transport properties in glassy polymer membranes, numerical analysis of structural features of the membrane model is used for pre-selecting only the realistic ones for further simulations using transition state theory (TST) approach. Simulation results of gas solubility and diffusion in alkylated poly-ether-ether-ketone (PEEK) membranes will illustrate the approach.
Molecular Modeling of Free Volume Distributions of Amorphous Membrane Polymers
MRS Proceedings, 2002
ABSTRACTThe paper deals with important differences in the distribution of free volume in high and low free volume polymers as indicated by a joint investigation utilizing molecular modeling and positron annihilation lifetime (PALS) studies. The main focus of this paper is on the molecular modeling approach. The first set of polymers in question are the ultra-high free volume polymer poly(1–(trimethylsilyl)-1-propyne) (PTMSP) and two polystyrene derivatives containing Si and F. Extended equilibration procedures were necessary to obtain reasonable packing models for the polymers. The transition state Gusev-Suter Monte Carlo method was utilized to prove a reasonable agreement between simulated and measured diffusivity and solubility values for the model structures. The free volume distribution was analyzed for the validated packing models and compared with respective PALS data. In both cases a bimodal distribution of free volume was observed for PTMSP while the polystyrene derivatives ...
Gas Transport in Glassy Polymers: Prediction of Diffusional Time Lag
Membranes
The transport of gases in glassy polymeric membranes has been analyzed by means of a fundamental approach based on the nonequilibrium thermodynamic model for glassy polymers (NET-GP) that considers the penetrant chemical potential gradient as the actual driving force of the diffusional process. The diffusivity of a penetrant is thus described as the product of a purely kinetic quantity, the penetrant mobility, and a thermodynamic factor, accounting for the chemical potential dependence on its concentration in the polymer. The NET-GP approach, and the nonequilibrium lattice fluid (NELF) model in particular, describes the thermodynamic behavior of penetrant/polymer mixtures in the glassy state, at each pressure or composition. Moreover, the mobility is considered to follow a simple exponential dependence on penetrant concentration, as typically observed experimentally, using only two adjustable parameters, the infinite dilution penetrant mobility L10 and the plasticization factor β, b...
Diffusion through rubbery and glassy polymer membranes
Makromolekulare Chemie. Macromolecular Symposia, 1991
Mass transport of a number of organic vapors through polydimethylsiloxane films (PDMS) and carbon dioxide through a variety of polyimides based on a hexafluorotetracarboxylic acid unit (6FDA) were investigated. Vapor diffusion through PDMS films strongly depends on the concentration of the penenant molecules in the network. For chloroform, increasing diffusivity at lower upstream activities occurs due to network plasticization, while a decreasing diffusion coefficient at larger concentration is supposed to stem from penetrant molecule clustering. The diffusion of carbon dioxide in 6FDA-based polyimides was modelled on a molecular basis. An exponential relation was found between Ac and the diffusion coefficient and the permeability, respectively. This relation hoks also for on-chain modifications.