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Molecular dynamic simulations of pressure-driven water transport through polyamide nanofiltration membranes at different membrane densities
In the real membrane manufacture of polyamide (PA) thin-film composite membranes, the membrane density is unevenly distributed in the PA active layer, and the membrane performance may be influenced by the membrane density. Due to the limitation of traditional experimental approaches, molecular dynamics (MD) simulations are carried out to investigate the effect of PA density on membrane structures and transport properties. The amorphous aromatic PA membranes studied in this work were modeled as nanofiltration (NF) membranes, and the water flows passing through the membranes were driven by 1.0 to 100 MPa pressure difference. Each membrane was a cube composed of several linear PA polymeric molecules, and the number of linear PA polymers in the cube was varied to give different densities. The simulation results show that the fractional free-volume and average free-volume pore size of the PA membrane both decrease with increasing membrane density, leading to a decrease in water permeability. When the membrane is denser than 1368 kg m ~(?3) , the membrane is nonporous and the water flow is blocked even at 100 MPa pressure difference. The structures and the transport properties of the modeled PA membranes are compared with the PA top layer of a commercial NF membrane; the FilmTec? NF 90 membrane. This comparison shows that the simulation results of the modeled PA membrane are similar to the experimental results of the real NF 90 membrane. The MD simulations are thus feasible as a means of studying pressure-driven water transport through an amorphous polymeric membrane on a molecular scale, and provide a direct way to reveal the relationship between the free-volume property of the polymeric membrane and the pressure-driven water transport.
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机译：在聚酰胺（PA）薄膜复合膜的真实膜制造中，膜密度在PA有源层中不均匀地分布，并且膜性能可能受到膜密度的影响。由于传统实验方法的局限性，进行了分子动力学（MD）模拟，以研究PA密度对膜结构和运输性能的影响。在该工作中研究的无定形芳族PA膜被建模为纳米滤膜（NF）膜，通过膜的水流被1.0至100MPa的压差驱动。每个膜是由几种线性Pa聚合物分子组成的立方体，并且多维数据集中的线性PA聚合物的数量变化以产生不同的密度。仿真结果表明，PA膜的分数自由体积和平均自由体积孔径随着膜密度的增加而降低，导致水渗透性降低。当膜密度小于1368 kg m〜（α3）时，膜是无孔的，并且即使在100mPa压力差的情况下也被阻断水流。将建模的PA膜的结构和运输性能与商业NF膜的PA顶层进行比较; filmtec？ NF 90膜。该比较表明，模拟的PA膜的模拟结果类似于真实NF 90膜的实验结果。因此，MD模拟是可行的，作为通过在分子尺度上通过无定形聚合物膜研究压力驱动的水运输的方法，并提供一种直接的方式来揭示聚合物膜的自由体积与压力驱动之间的关系之间的关系水运。
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