For a wide range of modern soft functional materials, the selective transport of subnanometer-sized molecules (“penetrants”) through a stimuli-responsive polymeric membrane is key to the desired function. In this study, we investigate the diffusion properties of penetrants ranging from nonpolar to polar molecules and ions in a matrix of collapsed poly(N-isopropylacrylamide) (PNIPAM) polymers in water by means of extensive molecular dynamics simulations. We find that the water distributes heterogeneously in fractal-like cluster structures embedded in the nanometer-sized voids of the polymer matrix. The nanoclustered water acts as an important player in the penetrant diffusion, which proceeds via a hopping mechanism through “wet” transition states: the penetrants hop from one void to another via transient water channels opened by rare but decisive polymer fluctuations. The diffusivities of the studied penetrants extend over almost 5 orders of magnitude and thus enable a formulation of an analytical scaling relation with a clear non-Stokesian, exponential dependence of the diffusion coefficient on the penetrant’s radius for the uncharged penetrants. Charged penetrants (ions) behave differently as they get captured in large isolated water clusters. Finally, we find large energetic activation barriers for hopping, which significantly depend on the hydration state and thereby challenge available transport theories.

中文翻译：

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在热响应性聚合物膜中的选择性分子运输：纳米级水合作用和波动的作用。

对于各种各样的现代软功能材料，亚纳米大小的分子（“渗透剂”）通过刺激响应性聚合物膜的选择性转运是所需功能的关键。在这项研究中，我们研究了塌陷的聚（N）基质中从非极性分子到极性分子和离子的渗透剂的扩散特性。广泛的分子动力学模拟，可将水中的（-异丙基丙烯酰胺）（PNIPAM）聚合物。我们发现水在聚合物基质的纳米级空隙中嵌入的分形状簇结构中分布不均。纳米簇水是渗透剂扩散的重要参与者，渗透是通过“湿”过渡态通过跳变机制进行的：渗透剂通过由稀有但决定性的聚合物波动而打开的瞬态水通道从一个空隙跳到另一个空隙。所研究渗透剂的扩散率几乎扩展到5个数量级，因此能够建立解析比例关系，其中无电荷渗透剂的扩散系数对渗透剂半径的明确非斯托克斯式，指数依赖性。带电的渗透剂（离子）在大型孤立的水团中被捕获时，其行为会有所不同。最后，我们发现了跳跃的巨大能量激活障碍，这很大程度上取决于水合作用状态，从而挑战了可用的运输理论。