聚甲基丙烯酸三氟乙酯单体与分子链在金属有机框架中吸附及取向特性的理论研究

Theoretical study on adsorption and orientation properties of poly(2,2,2-trifluoroethyl methacrylate) and its molecular chain within metal-organic frameworks

本文结合分子动力学、密度泛函理论和蒙特卡洛方法,研究了光学损耗低的氟化聚合物单体分子甲基丙烯酸三氟乙酯(TFMA)及其分子链在[Zn2(BDC)2TED]n和[Zn2(BPDC)2TED]n两种结构有序的金属有机框架(MOFs)材料中的吸附特性,得到相应的吸附数量和取向程度,以探索提高聚合物材料双折射性的有效途径.通过分析结果得出影响TFMA单体分子在这两种MOF孔道中吸附及取向特性的三个因素:a)由于MOF孔道壁是由金属离子和有机配体组成的极性表面,MOF孔壁与极性分子TFMA之间的静电相互作用对TFMA单体分子在孔道内的吸附和取向有促进作用;b)在受限空间中的聚合物单体分子之间的静电相互作用使其分子间隙更加紧密,同样也对其分子取向有促进作用;c)在这个两种孔洞与聚合物单体分子相对大小不同的MOF中,单体分子和分子链的取向度基本一致,但单体分子在孔径较大的MOF中吸附数量更多.这三个因素的重要影响,为聚合物单体或MOF的选取、对聚合物链取向及双折射性的控制提供理论预测方法.

In this work,to increase the birefringence of fluorinated polymer materials having low optical loss, a theoretical study on the adsorption properties of 2,2,2-trifluoroethyl methacrylate(TFMA) and its molecular chain within structurally-ordered metal-organic frameworks(MOFs) has been conducted by employing a combination of molecular dynamics, density functional theory and Monte Carlo method, where two MOFs, [Zn2(1,4-benzenedicarboxylate)2triethylenediamine]n([Zn2(BDC)2TED]n) and [Zn2(4,4’-biphenyldicarboxylate)2triethylenediamine]n([Zn2(BDC)2TED]n), were chosen. The corresponding numbers and the orientation degrees of adsorbed molecules in these two MOFs were also obtained from these simulations. The analyses from these results revealed the three factors that impact the adsorption and orientation of TFMA monomers in MOF pore channels. First, as the walls of the MOF pores are polar surfaces and consist of metal ions and organic ligands, the electrostatic interaction between the MOF walls and polar TFMA molecules promotes the adsorption and orientation of the TFMA monomers within the MOF pore. Secondly, the electrostatic interactions between monomers can reduce the intermolecular gaps. This also assists in their orientation. Lastly, the relative sizes of the MOF pores and the monomer molecules are also relevant. While the degrees of orientation of TFMA molecules and PTFMA chains in these MOFs are basically the same, the number of the absorbed TFMA monomers is higher in [Zn2(BDC)2TED]n. The results regarding the impact of these factors are expected to provide predictive methods for the selection of polymeric monomers or MOFs that may be ideal for the control of the orientation and birefringence of polymer chains.