(255av) Structural Evolution of a Polymeric Porous Medium Induced By Cross Flow or Tangential Flow of Solvent

Polymeric soft materials are ubiquitous in nature and in man-made matter. They comprise a variety of components and encompass a range of physical states with characteristic time and length scales over several orders of magnitude. Many of them are highly porous with solvents, solutes, and natural or artificial additives physically and/or chemically accommodated in the pore space. The resultant polymeric porous media and pore structures could be sensitive, by nature or by design, to physical and chemical changes that may arise externally or internally, and could exhibit significant structure-property relationships in the physicochemical, functional, and processing aspects of their properties. By filling the pore space and interacting with the skeleton polymers and other solute species, solvent molecules are involved during the synthesis and applications of polymeric porous media and capable of playing a very important role in determining the structures, properties, and functions. In fact, recent molecular modeling and simulation studies have shown that stable pore structures of polymeric porous media should be considered as being achieved by a balance between the polymer-polymer interactions, the solvent-polymer interactions, as well as the solvent-solvent and the solvent-solute interactions. This work is aimed at investigating the dynamical effects instigated by solvent flows on the pore structure and properties of a polymeric porous membrane. For this purpose, a biocompatible polymeric porous membrane was constructed by coarse-grained molecular dynamics (MD) modeling and simulation, and water was considered as the solvent to not only fill the pores but also form an exterior aqueous phase that submerge the model membrane. The whole model system was equilibrated for the membrane to develop a stable pore structure based on a balance between the polymer chains' flexibility, mutual steric support, and interactions with the solvent molecules under static condition. The pore structure was rigorously characterized by a molecular approach in order to establish a reference for evaluating the dynamical effects caused by different solvent flows. Tangential flows of solvent were imparted by moving the topmost layers of the water molecules in the exterior phase at different speeds in a direction parallel to the water-polymer interface. Cross flows of solvent were induced by applying different pressures on the topmost layers of water molecules in the exterior phase in a direction perpendicular to the water-polymer interface. Through the water-water and water-polymer interactions, the polymer molecules individually and the pore structure collectively experience the effects of the imposed momentum transfer and mass transfer and exhibit interesting and important changes including the membrane thickness, pore-size distribution, and the interaction energetics of water molecules in the pores. When the flows were stopped, the pore structures, however, were found not to be able to return back to the original structure. In general, the cross flows of solvent were found to cause greater pore structural reduction than the tangential flows and the resultant reduced pore structures were more dissimilar from the original one during and after the flow conditions. Other aspects of structural and energetic characterizations will also be compared and presented for a more complete elucidation.