Abstract In this study ab initio calculations, molecular dynamics (MD) and Monte Carlo (MC) simulation techniques are used to investigate the structural properties of triptycene based polymers of intrinsic microporosity (PIMs), consisting of polyimide branched with the side groups: C4H9, C3H7, CH3 and CF3, to evaluate their performance as polymeric membrane for separation of gases, O2, N2, CO2, CH4 and H2S, which are the constituents of natural gas and their separation is of high industrial interest. In the course of MD simulation, initially, the branched polyimide membranes are built to obtain the PIMs' model. Then the low-density membrane models undergo a consecutive simulation procedure of compression and relaxation to achieve the experimental density of equilibrated membrane. The structure of the constructed membranes is analyzed by calculating: dihedral angles, radius of gyration, fractional free volume, accessible free volume, cavity size distribution, and surface area. The behavior of the membranes against penetration and permeation of the studied gases is determined by evaluating the diffusion and solubility coefficients of the gases and by employing MD and MC simulation techniques, respectively. Comparison of the structural properties of the membranes shows that the PIM membranes with larger side branch groups in their polymeric chain structure are more rigid and therefore, due to restriction in chain packing and cavity formation between polymer chains, the free volume in the membrane's structure increases which as a result would promote the diffusion and permeation of gases into the membrane, where, the obtained results indicate that the membrane with C4H9, as the largest side branch in its polymer chain, has the greatest diffusivity and permeation. Also, the highest selectivity for all studied binary gas mixtures is manifested by the PIM membrane with C4H9 at its side branch, however, for (CO2/CH4) and (H2S/CH4) binary mixtures CF3 as the side branch of PIM membrane represents an acceptable selectivity. The obtained results illustrate that in addition to the membrane free volume, other parameters are influential in gas separation by these polymeric membranes which require further consideration. These parameters include gas adsorption, specific surface area of the membrane for adsorption, the size of gas molecules and their interaction with the PIM membranes which need to be investigated and discussed in the light of the obtained results. To the best of knowledge, based on a thorough investigation of the literature, no similar work can be cited which includes detailed properties of PIM membranes at the atomic scale by using quantum mechanical and simulation techniques in order to elucidate the behavior of PIMs for gas separation.

中文翻译：

---

侧支对三萜基PIM膜气体分离性能的影响：分子模拟研究

摘要 在这项研究中，从头计算、分子动力学 (MD) 和蒙特卡罗 (MC) 模拟技术用于研究具有固有微孔性 (PIM) 的三苯乙烯基聚合物的结构特性，该聚合物由带有侧基支化的聚酰亚胺组成：C4H9、 C3H7、CH3 和 CF3，以评估它们作为用于分离气体、O2、N2、CO2、CH4 和 H2S 的聚合物膜的性能，这些气体是天然气的成分，它们的分离具有很高的工业价值。在 MD 模拟过程中，首先构建支化聚酰亚胺膜以获得 PIM 模型。然后对低密度膜模型进行连续的压缩和松弛模拟程序，以达到平衡膜的实验密度。通过计算：二面角、回转半径、分数自由体积、可及自由体积、腔尺寸分布和表面积来分析构造膜的结构。通过评估气体的扩散和溶解系数以及分别采用 MD 和 MC 模拟技术来确定膜对研究气体的渗透和渗透的行为。膜结构特性的比较表明，在其聚合物链结构中具有较大侧支基团的 PIM 膜更刚性，因此，由于聚合物链之间的链堆积和空腔形成的限制，膜结构中的自由体积增加结果会促进气体扩散和渗透到膜中，其中，所得结果表明，以C4H9作为其聚合物链中最大侧支的膜具有最大的扩散性和渗透性。此外，对所有研究的二元气体混合物的最高选择性表现在 PIM 膜的侧支上有 C4H9，然而，对于（CO2/CH4）和（H2S/CH4）二元混合物，CF3 作为 PIM 膜的侧支代表可接受的选择性。所得结果表明，除了膜自由体积外，其他参数对这些聚合物膜的气体分离也有影响，需要进一步考虑。这些参数包括气体吸附、吸附膜的比表面积、气体分子的大小及其与 PIM 膜的相互作用，需要根据获得的结果进行研究和讨论。