Exploration of anion transport in a composite membrane via experimental and theoretical methods

Exploration of Ion Transport in Blends of an Ionic Liquid and a Polymerized Ionic Liquid Graft Copolymer

In this study, we prepared a composite membrane consisting of a poly(1-butyl-3-vinylimidazolium-tetrafluoroborate) (poly([BVIM]-[BF₄])) polymerized ionic liquid graft copolymer (PILGC) and a blend of PILGC and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]-[BF₄]) ionic liquid (IL) to explore techniques for improving the conductivity of PILGCs, which is normally three orders of magnitude lower than that of ILs. PILGCs, which are environmentally friendly, have attracted much interest. To gain a better understanding of ion transport in composites, the mechanisms of ion transport in composite components should be explored. We investigated anion transport in ILs and PILGCs and were able to obtain the correct ion transport mechanisms in IL-PILGC blends based on a previous work. We performed molecular dynamics (MD) simulations, which are commonly used to investigate molecular mechanisms. According to the MD simulation results, in most IL-PILGC blends of various compositions, the contributions of cations are greater than those of anions. This is one reason that blends have higher conductivities than their component PILGCs. To the best of our knowledge, we are the first to identify ion transport mechanisms in PILGCs and their blends with ILs by exploring subdiffusive ion motion regimes. The ratio of the number of cages with more than three cationic branch chains in the blend with 50 wt % PILGC, the blend with 80 wt % PILGC, and the PILGC was 0.26:0.39:0.65. Therefore, the ratio of firm cages gets a promotion as the PILGC content increases. Because the ratio of fast ions decreases as the ratio of firm cages increases, the blend with 80 wt % PILGC has lower anion diffusivities than the blend with 50 wt % PILGC. It was inappropriate to probe ion transport in PILGCs (or IL-PILGC blends) solely via analyzing ion association interactions. Analysis of only ion association interactions led to the incorrect conclusion that the time scales of ion transport in PILGCs are given by the continuous ion association time, which is the time when the ion association remains paired rather than the time when an ion is caught inside a cage. Proper methods should be used to obtain more accurate theories.

Molecular Modeling Investigations of Sorption and Diffusion of Small Molecules in Glassy Polymers

With a wide range of applications, from energy and environmental engineering, such as in gas separations and water purification, to biomedical engineering and packaging, glassy polymeric materials remain in the core of novel membrane and state-of the art barrier technologies. This review focuses on molecular simulation methodologies implemented for the study of sorption and diffusion of small molecules in dense glassy polymeric systems. Basic concepts are introduced and systematic methods for the generation of realistic polymer configurations are briefly presented. Challenges related to the long length and time scale phenomena that govern the permeation process in the glassy polymer matrix are described and molecular simulation approaches developed to address the multiscale problem at hand are discussed.