Humidity effect on temperature behavior of proton diffusion coefficient in sulfonated co-polynaphthoyleneimide membranes measured by 1H NMR diffusometry

Proton diffusion in the co-PNIS85/15 membrane was investigated in the temperature range from 200 to 363 K at different water contents (4 <= lambda <= 21) using H-1 SFG NMR diffusometry. At high hydration values, above the threshold value lambda(0) = 10.5, the ln(D-NMR(1/T)) dependences show two different activation modes, separated by a crossover point T-cr approximate to 250-260 K. At temperatures above T-cr, the activation energy is E-a similar to 0.20 eV, which is close to the value for bulk water (E-a similar to 0.17 eV). At temperatures below the crossover point, the ln(D-NMR(1/T)) dependences for different water contents merge together into one straight line characterized by a much higher E-a = 0.46 eV. At low hydration values lambda < lambda(0), the activation energies for the high-temperature and low-temperature modes converge, so that at lambda = 4, the dependence ln(D-NMR(1/T)) is described by one straight line throughout the studied temperature range with E-a = 0.38 eV. A model is proposed that phenomenologically describes the diffusion in the co-PNIS membrane at different moisture contents lambda.

Automated summary

Proton diffusion in the co-PNIS85/15 membrane was investigated at temperatures ranging from 200 to 363 K and different water contents using H-1 SFG NMR diffusometry. At high hydration values, two different activation modes were observed, with the crossover point occurring at approximately 250-260 K. Above the crossover point, the activation energy was similar to that of bulk water. Below the crossover point, the activation energy was higher. At low hydration values, the activation energies for the high-temperature and low-temperature modes converged.