PXLink: A simulation program of polymer crosslinking to study of polyamide membrane
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Crosslinked network polymers have numerous important applications in engineering, biomedicine, and the environment. Establishing a crosslinked polymer network is an essential initial step toward studying the structure and function of polymer membranes. In this study, we developed a Python script named PXLink that integrates with GROMACS software to simulate the crosslinked aromatic polyamide (PA) membrane, which is widely used in desalination and water treatment. PXLink can create chemical bonds between the neighboring carboxyl and amine groups and relax the system during the crosslinking, utilizing energy minimization and molecular dynamics simulations implemented in GROMACS. The accuracy of our protocol has been successfully validated through a benchmark study of water diffusivity and the structural properties of a solvated membrane, including pore size distribution, membrane density, and the stacking of aromatic benzene rings (density profile and ordering). Moreover, the comparison between the dry and wet polymer membranes reveals that the swelling of the membrane surface leads to the expansion of subnanopores and the increased ordering of local structures. After the swelling of the membrane, the number of pores with a radius around the main region of 0.2 nm decreases, while more pores in the range of 0.22–0.38 nm are observed. There is also an increase in the orderliness of local structures, as evidenced by the increasing order parameter of aromatic ring pairs with a distance ranging from 0.7 to 0.8 nm. Our developed script will be a valuable tool for designing and discovering crosslinked polymers.
交联网络聚合物在工程、生物医学及环境领域均拥有诸多重要应用价值。构建交联聚合物网络，是开展聚合物膜结构与功能研究的必要前置步骤。本研究开发了一款名为PXLink的Python脚本，该脚本可与GROMACS软件集成，用于模拟广泛应用于海水淡化与水处理领域的交联芳香聚酰胺（PA）膜。PXLink可在相邻羧基与氨基之间生成化学键，并借助GROMACS内置的能量最小化与分子动力学模拟功能，在交联过程中对体系进行弛豫。本研究提出的流程的准确性，已通过水扩散系数测试以及溶剂化膜的结构特性分析得到有效验证，其中结构特性包括孔径分布、膜密度以及芳香苯环的堆叠行为（密度分布与有序性）。此外，通过对比干态与湿态聚合物膜，研究发现膜表面溶胀会导致亚纳米孔隙扩张，同时局部结构的有序性提升。膜溶胀后，半径主要集中于0.2 nm区间的孔隙数量有所减少，而0.22~0.38 nm范围内的孔隙数量则有所增加。局部结构的有序性同样有所提升，这一现象可通过距离在0.7~0.8 nm区间内的芳香环对的有序参数升高得到证实。本研究开发的脚本将成为交联聚合物设计与发现的宝贵工具。
