310203 Direct Simulations of Molecular Transport and Permeation of Gases in Polymeric Membrane Materials for Separation Processes
The microscopic characteristics of highly permeable polymers depend strongly on material processing conditions. Membranes made with these materials suffer from physical ageing, which causes their suitability for separation processes to rapidly deteriorate over time. In contrast, Polymers of Intrinsic Microporosity (PIM) owe their microporosity to a rigid and contorted molecular structure which provide them with increased and stable free volumes and high surface areas making them ideally suitable as membrane polymers [1, 2]. The permeability and selectivity of these polymer membrane is further influenced by the polymer's pore size distribution and pore connectivity . Molecular models of PIMs allow a detailed investigation of these properties by providing a direct insight into the polymer's microporous structure [4, 5]. A molecular understanding of the transport phenomena in polymeric membrane materials at the nanoscale is required for further advances in membrane separations.
Molecular dynamics studies of gas permeation for three industrially relevant gases, namely helium, methane and carbon dioxide, through PIMs are presented. Helium, being an inert and light penetrant, exhibits a mobility which is almost two orders of magnitude higher than that of large penetrants such as methane and strongly-adsorbing carbon dioxide. As the diffusion of large penetrant molecules is very slow , obtaining transport properties from equilibrium molecular dynamics simulations for these molecules is extremely difficult since the necessary time scales are hardly accessible for traditional approaches.
In response to these issues, the direct simulation of gas permeation through an atomistically-detailed ultra-thin PIM-1 membrane was performed with a highly efficient boundary-driven non-equilibrium molecular dynamics approach , mimicking the experimental procedure for determining gas diffusion in polymers. The methodology is not limited by model complexity, system size or other computational difficulties. The method employed here avoids these issues by applying an external perturbation to the system which accelerates the diffusion process, imposing a concentration gradient developing a steady-state response. Process relevant properties, namely permeation and solubility are obtained directly from the measured gas flux and density distribution. It uniquely allows for a direct comparison of permeation and solubility selectivity for pure gases with experimental studies on PIM-1 polymer membranes, as well as the simulation of mixed gas streams .
The steady-state nature of the methodology allows for an accumulation of simulation statistics, reducing the error in the results. The NEMD methodology ensures that the properties computed in non-equilibrium simulations are in the linear response regime and thus amenable to extrapolation in order to extract the underlying transport coefficients. Furthermore, the methodology is not limited to homogeneous systems. Discontinuities in the microscopic topology can be treated in a straight-forward manner. The results show that differences in the microscopic properties have a remarkable influence on separation characteristics of PIMs. This has decisive implications for gas separations, considering that, while high permeation is desirable, it often goes at the expense of reduced selectivity.
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