2.5. Water Uptake (WU), Ion-Exchange Capacity (IEC), and Linear Swelling Ratio (LSR)

IEC value was measured by acid-base titration. First, the membranes (0.2 g) were changed to acid form by being immersed in 1 M HCl aqueous solution for 24 h. Secondly, the membranes were washed with DI and weighed by analytical balance. Finally, the membranes were soaked in NaCl aqueous solution (1 M) and equilibrated for 24 h. Hydrogen ions were quantified by titration of 0.1 M NaOH aqueous solution using phenolphthalein as an indicator. The IEC value can be calculated by the mass of the dry membrane in H⁺ form (W_dry) and the amount of NaOH consumed in the titration (V_NaOH × C_NaOH) (Equation (1)).

WU and LSR of the membranes (10 × 30 mm) were calculated by following equations (Equations (2) and (3)) based on the measured data of weight and length of membranes after being immersed in water and equilibrated for 24 h, respectively.

where the weight and length of the wet membrane are marked as W_wet and L_wet, whereas the weight and length of the dry membrane are marked as W_dry and L_dry.