# UV-Polymerized Vinylimidazol Ionic Liquids for Permselective Membranes
## 3 Results and Discussion  
### Formation of Polymeric Layers  
For the synthesis of membranes with thin layers functioning as a selective barrier between two phases [43], different methods are reported [44; 45; 46; 47], namely, phase inversion, sol-gel or extruding processes using organic polymers, ceramics or composite materials [43]. Membrane selectivity allows one or more substances to pass the barrier (permeate), while others are rejected (retentate or concentrate) [44].  
The synthesized membranes are flat sheets obtained by UV-induced polymerization of a casting solution containing the ionic liquid (IL), other monomers, a crosslinker (CL) and a photo initiator (PI) (Figure 1). Although benzoin ethyl ether (BEE) is reported as a suitable photo initiator [31; 32; 40; 48; 49], diphenyl(2,4,6-trimethylbenzoyl)phosphine (TPO) was nevertheless used as photo initiator in the following as it showed high reliability in reactivity when using different ILs.  
Acrylonitrile (CAN) and styrene were used because they are known to provide polymeric layers with high chemical stability and improved mechanical properties [31; 40; 48]. They also served as solvents for solid and viscous ILs to provide a homogeneous casting solution. A glass plate served as a base, though it was not part of the following membrane process. Compared to materials like Teflon or poly(oxymethylene), the casted monomer film did not change its shape for hydrophilicity reasons and contracts.  
All membranes were synthesized following the same procedure--only the IL was changed in the experiments. Experiments without the ionic liquids, using only the ratio of the other monomers, were not successful, as a solid layer was formed. Besides the IL, the photo initiator was also crucial to the reaction system.  
The ionic liquids used are shown in Figure 2. Contrary to the literature [31], a casting solution with A was possible by treating the solution eight hours in an ultrasonic bath. Hereby, the ultrasonic bath was heated up to 53 \({}^{\circ}\)C. However, the casted film did not react under UV irradiation and dissolved immediately and completely in the EtOH bath. No membrane was obtained.  
Figure 1: General polymerization reaction using diphenyl(2,4,6-trimethylbenzoyl)phosphine (TPO) as a UV photo initiator.  
More demanding imidazolium side chains, like in IL [VBInIm]Br (E) and [VPrylIm]Br (F), were challenging the reaction system. While F could not even be dissolved in the other monomers after eight hours of treatment in the ultrasonic bath and shaking, E always led to big holes in the resulting polymer layer.  
Using IL [VBInIm]Br (B), [VHexIm]Br (C) and [VDodecIm]Br (D), a homogeneous PIL layer was obtained. The thickness chosen, adjustable from 1 to 3000 \(\upmu\)m by the film applicator, was 300 \(\upmu\)m. Thicker layers decreased the flux too much, whereas thinner layers were too unstable for the implementation in the filtration cell. By longer irradiation of thinner casted layers, the mechanical stability could not be increased. Nevertheless, it should be mentioned that the actual thickness after the reaction and washing is somewhat lower than 300 \(\upmu\)m.