(670d) Atomistic Simulations of Transport of Gas Molecules in Polymer/Nanoporous Inorganic Layered Nanocomposite Membranes

Nanocomposite membranes constructed from polymeric and nanoporous inorganic layered materials are under development as a new class of separations devices whose thickness can be scaled to the sub-100 nm regime [1]. They are made by synthesis of inorganic layers containing nanopores within the layers, exfoliation of the layers, and fabrication of a polymer-inorganic layered nanocomposite film. The nanoporous inorganic layer serves as a selective barrier, whereas the polymer provides mechanical stability and ease of processing. These nanocomposite membranes have been hypothesized to overcome the selectivity-versus-permeability trade-offs imposed by polymeric membranes used in gas separations. Here we present the first computational investigation of molecular transport behavior in this newly emerging class of membranes.

Our methodology involves the initial construction of realistic nanoscale membrane models using atomistic energy minimization and molecular dynamics (MD) simulations. Polydimethylsiloxane (PDMS) is chosen as the model polymeric material, whereas the 3-dimensionally nanoporous layered silicate AMH-3 [2] and a 2-D nanoporous layered aluminophosphate (AlPO) [3] were chosen as the functional materials. We then used the equilibrated structures to model the transport of small gas molecules - H₂, He, N₂ and O₂ ? through the membranes as a function of the loading of the inorganic layered material.

The results from our simulations show that diffusivities of He and H₂ are a strong function of the loading of AMH-3, and pass through a maximum as a function of inorganic layer loading. There was no appreciable increase in the diffusivity of O₂ with increase in AMH-3 loading, while the diffusivity of N₂ was found to decrease in the composite membrane. These effects are firstly due to the ?molecular sieving' effect of layered AMH-3, whose eight-membered rings are selective for molecules of small kinetic diameter such as He and H₂. Further, the diffusion of the penetrant molecules in the plane of the porous layer is increased compared to the diffusion in Z-direction (except for N₂), because the penetrants spend significant time in lateral motion in the porous layer. In addition, the diffusion selectivity of H₂/N₂ and H₂/O₂ in the composite membrane increased over pure PDMS, while the selectivity for O₂/N₂ showed little change. Further, we show the increasing effect of confinement of the polymer between the inorganic layers on the transport phenomena. In particular, at high AMH-3 loadings in the membrane, the polymer chains become more rigid and inhibit penetrant diffusion. This effect is visualized through analysis of the polymer chain dynamics. We then compare the case of the 3-D slab-like AMH-3/PDMS nanocomposites to that of the 2-D sheet-like AlPO/PDMS nanocomposites. Finally, we discuss the applicability of the Cussler model of transport in bulk-composite membranes [4], to the case of nanocomposite membranes. It is shown that 3-dimensionally nanoporous layered silicates like AMH-3 retain vestiges of ?bulk? behavior that allow semi-quantitative application of the Cussler model; however, this may not be generalized to membranes containing 2-D layered materials. Overall, this investigation forms the first step in development of a computational and predictive basis for the design and analysis of polymer/nanoporous layered nanocomposite membranes for potential applications in technological areas such as separations and fuel cell technology.

References [1] Jeong, H.K.; Krych, W.; Ramanan, H.; Nair, S.; Marand, E.; Tsapatsis, M. Chem. Mater. 2004, 16, 3838. [2] Jeong, H.K.; Nair, S.; Vogt, T.; Dickinson, L.C.; Tsapatsis, M. Nature Materials 2003, 2, 53. [3] Gao, Q.M.; Li, B.Z.; Chen, J.S.; Li, S.G.; Xu, R.R.; Williams, I.; Zheng, J.Q.; Barber, D.J. J. Solid State Chem. 1997, 129, 37. [4] Cussler, E.L. J. Membr. Sci. 1990, 52, 275.