(703g) Molecular Simulation of Nanoscale Distribution and Mobility of Water and Dimethylmethylphosphonate in Sulfonated Polystyrene
The interest in a better understanding of the specific interactions of phosphor-organic compounds and water with sulfonated polystyrene (sPS) ? polyolefin block copolymers is motivated by the prospective use of these materials in protective membranes against chemical warfare agents. Using classical molecular dynamics simulations, we explored the nanoscale segregation and diffusion of water and nerve gas simulant dimethylmethylphosphonate (DMMP) in sPS neutralized with calcium counterions at different sulfonation and hydration levels. The water content was varied from 15% to 54% of dry polymer weight, and the DMMP content was varied from 0 to 100 wt %. We found that in the 40% sulfonated polystyrene, water forms well defined aggregates, which grow in size as the hydration increases, reaching 20Å at the maximum water content. In the 100% sulfonated polystyrene, the overall structure of hydrated polymer is more uniform with smaller water aggregates. Diffusion of water at the same number of water molecules per sulfonate group is faster at a lower sulfonation level. The solvation of sPS in water-DMMP binary mixture was found to differ substantially from Nafion, where DMMP formed a layer between the hydropholic and hydrophobic subphases. In sPS with divalent Ca2+ counterion, DMMP and water compete for the solvation of the sulfonate group. At high water and DMMP content, the diffusion of DMMP turned out to be rather fast with the diffusion coefficient of ca. 30% of that of water. At the same time, water diffusion slows down as the DMMP concentration increases. This observation suggests that the two solvents have different pathways through the system
Using the results of our MD simulations and experimental data on sorption of water and DMMP in sPS and polybutadiene (PB), we compose a coarse-grained model for dissipative particle dynamics simulations and predicted the morphology of certain sPS-PB-sPS triblock copolymers. At low water activity, these materials show hexagonal symmetry, which undergoes a transition to lamellae structure as hydration increases. Our predictions on the symmetry and segregation scale in sPS-PB-sPS triblocks agree well with the available experimental data.
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