Ionic Cluster Size Distributions of Swollen Nafion/Sulfated -Cyclodextrin Membranes Characterized by Nuclear Magnetic Resonance Cryoporometry

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    Author(s):

    ; Jae-Deok Jeon Hyperstructured Organic Materials Research

    Center (HOMRC)

    ; Seung-Yeop Kwak Department of Materials Science and

    Engineering, Seoul National University, San 56-1,

    Sillim-dong, G

    Abstract:

    Nafion/sb-CD membranes were prepared by mixing 5 wt% Nafion solution with H+-form sulfated -cyclodextrin (sb-CD), and their water uptakes, ion exchange capacities (IECs), and ionic cluster size distributions were measured. Gravimetric and thermogravimetric measurements showed that the water uptake of the membranes increased with increases in their sb-CD content. The IECs of the membrane were measured with acid-base titration and found to increase with increases in the sb-CD content, reaching 0.96 mequiv/g for NC5 ("NCx" denotes a Nafion/sb-CD composite membrane containing x wt% of sb-CD). The

    cluster-correlation peaks and ionic cluster size distributions of the water-swollen membranes were determined using small-angle X-ray scattering (SAXS) and 1H nuclear magnetic resonance (NMR) cryoporometry, respectively. The SAXS experiments confirmed that increases 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, Tm, of a liquid confined within a pore, which is dependent on the pore diameter. The melting point depression was determined by analyzing the variation of the NMR signal intensity with temperature. Our analysis of the

    intensity-temperature (IT) curves showed that the ionic cluster size distribution gradually became broader with increases in the membrane 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 with its many sulfonic acid sites in the Nafion membranes results in increases in the ionic cluster size as well as in the water uptake and the IEC. We conclude that NMR cryoporometry provides a method for determining the ionic cluster size on the nanometer

scale in an aqueous environment, which cannot be obtained using

other methods.

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