3.3.5 X-ray fluorescence (XRF)
The samples were analyzed with Energy-dispersive X-ray fluorescence spectrometer SPECTRO XEPOS (SPECTRO Analytical Instruments, Germany), equipped with a silicon drift detector and excitation system with a 50 W Pd anode X-ray tube. The chamber was flushed with helium during the sample analyses. The Spectro Xepos software (TurboQuant method) was used for data analysis.
3.3.6 Small-angle X-ray scattering (SAXS)
Membranes were characterized by a pinhole camera Molecular Metrology SAXS System connected to a micro-focused X-ray beam generator (Osmic MicroMax 002) working at 45 kV and 0.66 mA (30 W). Multiwire, gas-filled area detector was utilized, while its active area diameter was 20 cm (Gabriel design). To cover the q range of 0.005–1.1 Å–1, two experimental setups (different sample-to-detector distances) were used (q = (4π/λ) sin Θ, where λ = 1.54 Å is the wavelength and 2Θ is the scattering angle). Glassy carbon standard was utilized to put the scattering intensities on absolute scale.
3.3.7 Thermogravimetric analysis (TGA)
Thermogravimetric analysis for copolymer VPA-co-AN was recorded with Perkin Elmer thermal analysis controller TAC 7/DX from 30 to 750 °C, heating rate of 10 °C min-1 under a nitrogen atmosphere.
Thermogravimetric analysis for IL was recorded with Pyris I (Perkin Elmer), using the following TGA procedure: 5-10 mg of sample was heated at constant heating rate 10 °C min-1 under nitrogen flow of 30 mL min-1 from 30 °C to 650 °C.
3.3.8 Differential scanning calorimetry (DSC)
Differential scanning calorimetry measurements were carried out on a DSC 8500 (Perkin Elmer) with nitrogen as a purge gas (20 mL min-1). The instrument was calibrated using indium as a standard. Samples of approximately 10 mg were encapsulated in aluminium pans.
The DSC runs were performed in a cycle of heating–cooling–heating from −90 °C to 100 °C at 10 °C min-1. Glass transition temperature (Tg) was determined from the heating run as the midpoint between the glassy and rubbery branches of the DSC trace.
3.3.9 Water uptake (WU)
All membranes were dried at 160˚C before measurement for a half-hour. Once the dried membranes were measured, were the membranes immersed into demineralized water at 25, 60, and 90˚C, respectively, until reaching constant weight (24 h). Before measuring of hydrated membranes, water drops from membrane surface were removed by paper wipes. Water uptake (WU) was calculated using Eq. (4):
𝑊𝑊𝑊𝑊 = 𝑚𝑚𝑤𝑤𝑤𝑤𝑤𝑤 𝑚𝑚− 𝑚𝑚𝑑𝑑𝑑𝑑𝑑𝑑
𝑑𝑑𝑑𝑑𝑑𝑑 𝑥𝑥 100% (4)
where mwet and mdry are the weights of wet and dried membranes, respectively.
3.3.10 Oxidative stability
Fenton reagent test was used for determination of oxidative stability of membranes. Fenton reagent consists of 3% aqueous solution of H2O2 containing 4 ppm anhydrous FeSO4. A membrane was thoroughly washed in water and then dried and weighed, after this was membrane again swelled in water (48 h), and then immersed into Fenton reagent (0.5 g of the membrane into 25 g of the Fenton reagent) for 1 h at 70˚C and again dried and weighed.
The weight loss (WL) was calculated using Eq. (5):
𝑊𝑊𝑊𝑊= 𝑚𝑚0𝑚𝑚− 𝑚𝑚𝑤𝑤
0 𝑥𝑥 100% (5)
where m0 and mt are the weights of original and treated membranes, respectively.
3.3.11 Contact angle (CA)
For measuring of CA was used the sessile drop method with an optical contact-measuring apparatus (OCA20 DataPhysics, version 2.03) equipped with CCD video camera with a resolution of 768 × 576 pixel and up to 50 images per second and integrated temperature measurement unit TC400 as well as a display in the range from - 40 to 400 °C. All samples were cleaned with compressed air before the measurements. From three to five drops of distilled water were transferred with a final volume of 3.0 μl to each sample (with a diameter of 1.8 cm). After the analysis, the contact angle was calculated according to the Young-Laplace fitting.
3.3.12 Transport number and permselectivity
The membranes were placed into a two-chamber measuring cell (each chamber had a 25 mL volume with an Ag/AgCl reference electrode facing the membrane surface from the two sides) and the chambers were filled with 0.5 M and 0.1 M KCl solution, respectively. The resulting potential difference between the reference electrodes (Em) was registered over time using a multimeter, until a stationary value. The constant temperature (25 °C) was kept during the measurements. The transport numbers were calculated from the Nernst equation Eq. (6):
𝑡𝑡+ = 𝑅𝑅𝑅𝑅𝐸𝐸𝑚𝑚
𝐹𝐹 ln�𝑎𝑎1𝑎𝑎2� (6)
where a1 and a2 are the effective concentrations of the KCl solution on the two sides of the membrane. The permselectivity was calculated by considering Em and the theoretical potential difference in a perfectly permselective membrane (Eth) as described elsewhere (43).
3.3.13 Ionic conductivity (IC) 3.3.13.1 Membrane testing IC
The in-plane IC of the membranes was determined in a tempered box in gas-tight cell in an environment of deionized water using a four-electrode arrangement for electrochemical impedance spectroscopy. A Solartron SI 1250 Frequency Response Analyzer and Solartron SI 1287 Electrochemical Interface were used. The IC (σ) was calculated using Eq. (7):
𝜎𝜎= 𝑅𝑅𝑅𝑅𝑅𝑅𝑙𝑙 (7)
where l is the distance between reference electrodes (m), R is ohmic resistance of membrane (Ω), b is thickness of membrane (m), and d is width of membrane (m).
3.3.13.2 Ionic liquid testing IC
Ionic conductivity was measured by AC impedance spectroscopy using an Autolab 302N (Metrohm). Measurements were conducted in a PFA Swagelok cell containing 2 platinum and 2 glassy carbon electrodes at room temperature. The volume of the cell was approx. 4 cm3. Intervals for the frequency range between 1 Hz and 1 MHz applying a voltage amplitude of 0.1 V. The cell constant was determined using a standard solution of KCl, with a concentration range between 5 × 10-4 and 1 × 10-1 mol dcm3 at 25 °C. The resistivity (Ω) was
determined from the value of the real axis touchdown of the Nyquist plot, from which the conductivity (mS cm-1) was calculated.
3.3.14 Ion exchange capacity (IEC)
Ion exchange capacity (IEC) was measured by potentiometry during the transition of the membrane sample from the OH- to the Cl- form. Keithley 6514 electrometer with high input impedance (200TX) was used for recording the potential response of a Ross combined glass electrode (Orion). The potential value of the glass electrode was converted into the concentration of displaced OH- ions in the solution by a calibration curve. Experiments were performed in 125 mL of 0.1 mol dm–3 NaCl solution in a gas-tight cell under argon atmosphere to avoid the effect of carbon dioxide. Decarbonized demineralized water was used.
3.3.15 Cyclic voltammetry
Cycling voltammetry (CV) measurements were performed using an Autolab 302N (Metrohm). Measurements were conducted in three electrodes system PFA Swagelok cells.
Glassy carbon (Sigradur K, HTW, A= 0.189 cm2) was used as a working and counter electrode.
Silver wire was used as a quasi-reference electrode. The volume of the cell was approx. 1 cm3. ESW was determined as a potential of working electrode, applied current density was +200 µA cm-2, resp. -200 µA cm-2. Correction of the evaluated data was done by uncompensated resistance (Ru) of electrolyte. Ru was measured in three electrode system using RLC bridge (Hameg Instruments).