Theory of the Structure of lonomeric Membranes
Ionomers are two-phase materials which show considerable promise as ion-selective membranes. Nafion,† in particular, is being used on a limited commercial basis in chloralkali separation processes. Its chemical structure, shown in Figure 1, is typical of ionomers. Pendant side chains, each containing an ionizable group, are essentially spaced uniformly along a linear chain composed of relatively nonpolar groups. The number of nonpolar units is far greater than the number of polar-ionizable groups. This leads to the particular two-phase structure. Polar groups self-associate to form hydrophilic clusters in a sea of nonpolar material. In general, ionomeric structure has been studied by X-ray diffraction, dielectric and mechanical spectroscopies, and by electron microscopy.(1–3) The polar clusters of Nafion have also been investigated by infrared(4) and NMR(5) spectroscopies. Figure 2 shows a transmission electron micrograph of Nafion stained with silver ions. The clusters appear as dark dots.
KeywordsHydration Shell Decrease Water Content Polar Cluster Perfluorosulfonic Acid Elastic Deformation Energy
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