The IEC was measured by a standard back titration method. A
piece of dry OH form membrane was soaked in 0.1 mol L1 HCl
solution (25 mL, VHCl, the volume for membranes soaking) at room
temperature for 48 h. The solution was titrated with 0.1 mol L1
NaOH solution (VNaOH, the volume for the back titration). After that,
the Cl form membrane was dried at 60 C under vacuum until the
weight remained constant and measured.