Khawaja M, Sutton A P, Mostofi A A
Department of Physics and ‡Department of Materials, and the Thomas Young Centre for Theory and Simulation of Materials, Imperial College London , London SW7 2AZ, U.K.
J Phys Chem B. 2017 Jan 12;121(1):287-297. doi: 10.1021/acs.jpcb.6b09690. Epub 2016 Dec 20.
Molecular simulation is used to compute the solubility of small gases in nitrile butadiene rubber (NBR) with a Widom particle-insertion technique biased by local free volume. The convergence of the method is examined as a function of the number of snapshots upon which the insertions are performed and the number of insertions per snapshot and is compared to the convergence of the unbiased Widom insertion technique. The effect of varying the definition of local free volume is also investigated. The acrylonitrile content of the polymer is altered to examine its influence on the solubility of helium, CO, and HO, and the solubilities of polar gases are found to be enhanced relative to those of nonpolar gases, in qualitative agreement with experiment. To probe this phenomenon further, the solubilities are decomposed into contributions from the neighborhoods of different atoms, using a Voronoi cell construction, and a strong bias is found for CO and HO in particular to be situated near nitrogen sites in the elastomer. Temperature is shown to suppress the solubility of CO and HO but to increase that of helium. Increasing pressure is found to suppress the solubility of all gases but at different rates, according to a balance between their molecular sizes and electrostatic interactions with the polymer. These results are relevant to the use of NBR seals at elevated temperatures and pressures, such as in oil and gas wells.
采用分子模拟方法，利用基于局部自由体积的Widom粒子插入技术计算小分子气体在丁腈橡胶（NBR）中的溶解度。研究了该方法的收敛性与进行插入操作的快照数量以及每个快照的插入次数之间的函数关系，并与无偏Widom插入技术的收敛性进行了比较。还研究了改变局部自由体积定义的影响。改变聚合物的丙烯腈含量，以考察其对氦气、一氧化碳和水的溶解度的影响，发现极性气体的溶解度相对于非极性气体有所提高，这与实验结果定性一致。为了进一步探究这一现象，利用Voronoi元胞结构将溶解度分解为不同原子邻域的贡献，发现一氧化碳和水尤其强烈倾向于位于弹性体中的氮位点附近。结果表明，温度会抑制一氧化碳和水的溶解度，但会增加氦气的溶解度。发现压力升高会抑制所有气体的溶解度，但根据它们的分子大小以及与聚合物的静电相互作用之间的平衡，抑制速率不同。这些结果与在高温高压下使用丁腈橡胶密封件有关，例如在油气井中。
