(531b) Asymmetric Cross-Linked Polymer Membranes Constructed and Characterized By Molecular Dynamics Modeling and Simulations

One of the most important properties in determining the performance and application of a membrane is the porous structure and more specifically the distributions in the membrane of the pore sizes and of the pore connectivity.  In polymer membranes, the porous structure is determined mainly by the mutual interactions between the conformationally and configurationally flexible polymer chains, and the membrane structural and dynamic properties can be further modified by physical and/or chemical means such as cross-linking which covalently connects the polymer chains together.  When the pore sizes are made to be characteristically dissimilar in different parts of a membrane, the result is an asymmetric membrane that, in contrast to conventional membranes with comparable pore sizes throughout, can provide additional advantages including fast and consistent molecular transport while requiring lower in magnitude gradients of externally applied forces.  From both fundamental and practical points of view, asymmetric polymer membranes can be prepared by systematically utilizing cross-linkers differing in sizes and/or different extents of cross-linking in different parts of the membrane.  While this appears to provide a plausible strategy for the molecular design of membranes, its practical feasibility still remains to be realized and requires detailed characterization.  We present in this work a proof-of-concept study that employs the molecular dynamics (MD) modeling and simulation methodology to construct and characterize composite asymmetric polymer membranes formed by dextran chains and cross-linked by polyols.  This selection of model membrane systems is based on the rationale that dextran and polyols provide desirable biocompatibility and good solubility in water, which have led to many bio-related applications.  The pore structures formed by the dextran chains are characterized directly in terms of the distributions of the number of pore openings, pore radii, and pore volumes by dividing each of the polymer thin films into two levels of cubic lattices at the angstrom level and utilizing thin disks of varying sizes at each lattice point.  The degree of pore connectivity can then be ascertained by the intersection between distribution curves that represent pores of different size ranges. The interaction energetics and effective mass transfer coefficients of water molecules and their distributions on the two different sides of the asymmetric membranes are evaluated and are used as an indication of the relative difficulty or ease associated with water sorption and desorption, which represent parts of the mechanisms underlying the functioning of polymer membranes. In general, the pores are larger in diameter and the polymer film is more flexible on the side of the asymmetric membrane having a lower dextran density.  Within a distance corresponding to approximately three water molecular diameters from the dextran chains and the polyol cross-linkers, water molecules, in particular those adjacent to the dextran chains and cross-linkers, are found to have greater interaction energetics and, thus, are more strongly bound, and their mass transfer coefficients are significantly reduced in magnitude.