(38h) Molecular Modeling and Simulation Studies of the Dehydration and Rehydration of Polymeric Porous Media

A biocompatible polymeric porous material was constructed and studied by coarse-grained molecular dynamics modeling and simulation that has a unique vectorial correspondence to the atomistic representation. The dehydration and rehydration phenomena of the polymeric porous material and those modified with a hydrophilic additive or cross-linkers were investigated using simulation approaches mimicking the actual processes. It was found that, in general, the strong interactions of water molecules with the polymer chains stabilize the porous structures of the polymeric materials and require the dehydration energy to be greater than that involved in pure water vaporization. The addition of the hydrophilic additive solutes and the incorporation of cross-linkers further enhance the water interaction energetics in the pore structures and increase the dehydration energy requirement. The water interaction energetics were found to be highly nonuniform in all three spatial dimensions, which has important implications in many fields. As dehydration proceeds, polymer conformational compaction and pore structural reduction occur due to downward stresses resulting from the surface tension of the descending water interface coupled with strong water-polymer and polymer-additive solute interactions. The stability of the porous structures was found to be enhanced by the presence of the hydrophilic additive because the additive molecules function as a filler in the pore space and interacts strongly with the polymer chains and water molecules so that they tend to be accumulated by the descending water level during dehydration at the vapor-liquid interface to form dense clusters of polymer chains at the interface. This phenomenon suggests that dehydration could be employed as a means to construct porous media with desirable spatial nonuniform density distributions of adsorption sites/ligands or catalyst sites/enzymes as the dissolved solutes. Cross-linkers provide additional steric support to the porous structures through the formation of three-dimensional networked structures that can sustain high structural stresses due to dehydration without minimal structural reduction. Extensively dehydrated porous structures were also found to have substantially improved stability so that the water molecules introduced during the rehydration process expand slightly the dehydrated porous structure of the systems examined here, but could not recover the original pore structures that existed before the commencement of dehydration. The approaches presented here could form a basis for the systematic evaluation of the physicochemical effects of additive components and cross-linkers on the polymeric porous structures and their stability. It could also provide a molecular level design method of chemical engineering science for mechanistically controlling the structure and morphology of polymeric porous media as well as the distribution of additive solutes in such media during dehydration and rehydration.