Determination of Pore Size Distribution of Nafion/Sulfated β-Cyclodextrin Composite Membranes as Studied by 1H Solid-State NMR Cryoporometry

Nafion/sb-CD membranes were prepared by mixing 5 wt% Nafion solution with H-form sulfated β-cyclodextrin (sb-CD), and their water uptake, ion exchange capacity (IEC), and ionic cluster size distribution were measured. The water uptake and IEC of the membrane increased with an increase in the sb-CD content. The SAXS experiments confirmed that an increase in the sb-CD content of the membranes shifted the maximum SAXS peaks to lower angles, indicating an increase in the cluster correlation peak. NMR cryoporometry is based on the theory of the melting point depression of a liquid confined within a pore, which is dependent on the pore diameter. The intensity-temperature (IT) curves showed that the cluster size distribution gradually became broader with an increase in the sb-CD content due to the increased water content, indicating an increase in the ionic cluster size. This result indicates that the presence of sb-CD results in increases in the cluster size as well as in the water uptake and the IEC.