Polymers are attractive membrane materials owing to their mechanical robustness and relatively inexpensive fabrication. An important dictator of membrane performance are free volume elements (FVEs), microporous void spaces created by the inefficient packing of bulky groups along the polymer chain. FVEs tend to degrade over time as polymer chains reorganize irreversibly; relating local chain dynamics to the distribution of FVEs can help control phenomena like plasticization and aging. In this work, we implement all-atom molecular dynamics (MD) simulations to study three polymers with different glass transition temperatures (Tg): polymethylpentene (PMP), polystyrene (PS), and HAB-6FDA thermally rearranged polymer (TRP). We observe that chain segments near FVEs have higher mobility compared to the atoms in the bulk; the extent of this difference is a function of Tg. To capture penetrant diffusion through the polymer matrix, hydrogen is inserted, and the mean squared displacement is calculated; penetrant mobility is dependent on both FVE distribution and polymer chain dynamics.

中文翻译：

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全原子分子动力学模拟中局部链动力学对玻璃态聚合物膜中自由体积元素结构的影响

聚合物由于其机械坚固性和相对便宜的制造而成为有吸引力的膜材料。膜性能的一个重要决定因素是自由体积元素 (FVE)，即由沿聚合物链的大体积基团的低效堆积产生的微孔空隙空间。随着聚合物链不可逆地重组，FVE 往往会随着时间的推移而降解；将局部链动力学与 FVE 的分布联系起来有助于控制塑化和老化等现象。在这项工作中，我们实施全原子分子动力学 (MD) 模拟来研究具有不同玻璃化转变温度 (Tg) 的三种聚合物：聚甲基戊烯 (PMP)、聚苯乙烯 (PS) 和 HAB-6FDA 热重排聚合物 (TRP)。我们观察到 FVE 附近的链段与本体中的原子相比具有更高的迁移率；这种差异的程度是 Tg 的函数。为了捕获通过聚合物基质的渗透扩散，插入氢气，计算均方位移；渗透流动性取决于 FVE 分布和聚合物链动力学。