Title

The characterization of states of water in Nafion and the relationship between states of water and Nafion properties

Date of Completion

January 2008

Keywords

Chemistry, Physical|Chemistry, Polymer|Engineering, Materials Science

Degree

Ph.D.

Abstract

The state of water in Nafion 117 was studied using Near-Infrared spectroscopy (NIR) and solid-state NMR, with the objective of investigating the physical interactions between the state of water and Nafion properties such as de-ionic clustering temperature and proton conductivity during water-vapor sorption process. The NIR spectra of absorbed water in the region of about 7000cm -1 provided the information on three water types such as S0, S1, and S2. NMR gave us different water species such as free water, loosely bound water and bound water. The decrease of de-ionic clustering temperature due to absorbed water in Nafion was believed to break intermolecular hydrogen bonds between sulfonic groups. It was found that lowering de-ionic clustering temperature was related to the fraction of Lb2 and bound water until 10 wt %. After that, bound water was closely associated with the decrease of de-ionic clustering temperature. ^ The proton conductivity of Nafion increased at about 5 wt % due to the threshold of percolation structure and at about 12 wt % of water content due to saturation of sulfonic acid group. The fraction of free water increased rapidly at about 5 wt % of water content where percolation structure was expected to form, while the fraction of bound water and loosely bound water showed gradual decrease. In NIR experiments, the fraction of non hydrogen-bonded water (S0) decreased until about 12 wt % but increase sharply after that. The fraction of singly hydrogen-bonded water (S1) also increased sharply at about 13 wt %, while the fraction of doubly hydrogen-bonded water (S2) went down rapidly. These results indicated that the increase of proton conductivity might be due to F1 and F2 types of water at about 5 wt %, and S0 and S1 types of water at about 13 wt %. ^