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Introduction
Room-temperature ionic liquids (ILs) have been widely studied as alternative, benign solvents and catalysts for chemical reactions in the context of sustainable chemistry. The theoretical possibility of designing ionic liquids with a wide variety of physico-chemical properties by introducing suitable functional groups has made them more recently attractive candidates as supposedly selective materials for membrane separations, gas sequestration and purification.
Conventionally, membrane separations involving ionic liquids are based on the principle of supported ionic liquid membranes which involve the immobilisation of the ionic liquids in a porous membrane support structure. The concept is wide-spread because such membranes are facile in preparation; however, they bear strong disadvantages such as little stability during operation under pressure, limited control over actual ionic liquid content in the pores as well as instabilities during solute uptake, resulting from a decreased viscosity and surface tension of the ionic liquid.
Methods
We here report on stable non-porous mixed-matrix-membranes based on the blend of a block co-polymer with different ILs which aims at overcoming the aforementioned drawbacks[i]. Rather than immobilizing the ILs in a porous membrane structure that by itself does not bear any selectivity, we introduced ionic liquids as a plasticizer in dense polymeric membranes during casting. Owing to the high mobility of the ionic liquid phase incorporated, we aimed at improving membrane selectivity as well as membrane permeability for gas or vapour separations. As a case study, we chose polyether block amide as a base material owing to its tremendous interest in membrane processing. ILs were chosen according to the supposed different degrees of interactions with the block co-polymer in order to elucidate how membrane performance could be correlated to molecular interactions occurring in the polymer blend.
Results
The synthesized membranes were investigated with regard to the mechanical stability and permeability to sample vapours and gases. Considerable variations in terms of permeability and membrane processability were observed when different ILs were incorporated into the membrane, proving that the IL determined significantly the overall membrane properties. Common thermal characterization techniques such as DSC or TGA were applied in order to observe the stability of the synthesized membranes; 13C solid NMR and 1H-NMR as well as quantitative FTIR-ATR were employed to elucidate possible interactions taking place within the polymer/IL blend and during uptake of gases and vapours. According to the physico-chemical properties of the ILs, permeability values determined during permeation experiments demonstrated that the membrane permeability and selectivity could be modulated in accordance with the affinity of the ionic liquids incorporated and the respective sample vapours, thus confirming that the incorporation of the ionic liquids indeed allowed tuning the overall membrane performance.
Permeability measurements were complemented with a non-conventional technique for studying the sorption of gases or vapours in polymer films, namely quartz crystal microbalance with dissipation monitoring (QCM-D). For this method, a homogeneous thin layer of the respective polymer-IL blend was deposited onto a QCM-D sensor and its sorption behaviour monitored in real-time. The method proved to provide extremely fast response times when the polymer-IL blend was submitted to either gas or organic vapours, as well as an excellent reproducibility. It will be shown that the incorporation of ionic liquids in polymers is a promising alternative to supported ionic liquid membranes. Based on the results obtained, it will also be highlighted that a thorough understanding of the underlying thermodynamics and hence design of the nanocomposites is required in order to obtain membranes that are both mechanically stable as well as free from migration of the ionic liquid with time.