Nafion膜中に存在する束縛水の発見と水和構造の解明

Abstract

The strongly bound water’ about the sulfonic acid groups in Nafion is newly revealed by 1H NMR spectroscopy, and the hydration structure is investigated by using infrared spectroscopy (IR) with an aid of quantum chemical (QC) calculations. IR spectrum at 25℃ indicates that a Nafion membrane involves bulky water at ambient temperature, and the water is firstly dehydrated during a heated drying process at 60℃. After further dehydrations on heating from 60 ℃ to 140℃, the hydronium ion disappears, and some bands that have been assigned to the fully dehydrated species come up at 140℃. A spectral simulation based on QC reveals, however, that the spectrum at 140 ℃ comes from the SA group associated with a single-water molecule making two H-bondings. This implies that a thoroughly dried membrane is unavailable even at 140℃, and the involved water corresponds to ‘the strongly bound water.’ The QC-analytical results are experimentally confirmed by the evolved gas analysis mass spectrometry (EGA-MS). The result confirms that the last single-water molecule can remain on the SA group until the thermal decomposition at ca. 300℃, which is the temperature where the SA group is selectively decomposed. In this manner, the dehydration process of a Nafion membrane on heating is consistently revealed by IR spectroscopy, accompanying the fine band assignments by the QC calculation