Vishnyakov Aleksey, Neimark Alexander V
Department of Chemical and Biochemical Engineering Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854, USA.
J Phys Chem B. 2008 Nov 27;112(47):14905-10. doi: 10.1021/jp802256q.
The interest in a better understanding of the specific interactions of phosphor-organic compounds and water with sulfonated polystyrene (sPS) is motivated by the use of block copolymers as 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 approximately 20 A 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 mixtures was found to differ substantially from Nafion, where DMMP forms a layer between the hydropholic and hydrophobic subphases. In sPS with divalent Ca(2+) counterions, DMMP and water compete for the solvation of the sulfonate group. At high water and DMMP contents, the diffusion of DMMP turned out to be rather fast with a 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 although sPS is permeable for both solvents, water and DMMP are partially segregated on the scale of 1-2 nm and have different pathways through the system. The nonuniform nanoscale distribution of water and DMMP in sPS is confirmed by analyses of different pair correlation functions. This feature may significantly affect the perm-selective properties of sPS-contained block copolymer membranes.
对更好地理解磷有机化合物和水与磺化聚苯乙烯（sPS）的特定相互作用的兴趣源于将嵌段共聚物用作对抗化学战剂的保护膜。通过经典分子动力学模拟，我们研究了在不同磺化和水合水平下用钙抗衡离子中和的sPS中，水和神经毒气模拟物甲基膦酸二甲酯（DMMP）的纳米级分离和扩散。水含量从干聚合物重量的15%变化到54%，DMMP含量从0变化到100 wt%。我们发现，在40%磺化的聚苯乙烯中，水形成了明确的聚集体，随着水合作用增加其尺寸增大，在最大水含量时达到约20 Å。在100%磺化的聚苯乙烯中，水合聚合物的整体结构更均匀，水聚集体更小。在每个磺酸根基团水分子数相同的情况下，较低磺化水平时水的扩散更快。发现sPS在水-DMMP二元混合物中的溶剂化与Nafion有很大不同，在Nafion中DMMP在亲水和疏水亚相之间形成一层。在带有二价Ca(2+)抗衡离子的sPS中，DMMP和水竞争磺酸根基团的溶剂化。在高水和DMMP含量下，DMMP的扩散相当快，扩散系数约为水的30%。同时，随着DMMP浓度增加水的扩散减慢。这一观察结果表明，尽管sPS对两种溶剂都可渗透，但水和DMMP在1-2 nm尺度上部分分离且在系统中有不同的通道。通过对不同对关联函数的分析证实了sPS中水和DMMP的非均匀纳米级分布。这一特征可能会显著影响含sPS的嵌段共聚物膜的渗透选择性性能。
