Five OH form membrane samples were immersed into 2, 4, 6, 8,10 mol L1 NaOH solutions at 60 C for 120 h, respectively.
The sixth sample was immersed into 1 mol L1 NaOH solution for 120 h as a comparison. All the membrane samples were thoroughly washed by deionized water until the free NaOH inside the samples was completely removed. The ionic conductivities of six samples were tested at 30 C to investigate the changes