Combining Gravimetric and Vibrational Spectroscopy Measurements to Quantify First- and Second-shell Hydration Layers in Polyimides with Different Levels of Fluorination

In-situ FTIR measurements have been carried out at different relative pressures of water vapour to study the H2O diffusion in three polyimides differing for their molecular structure and fluorine substitution. Spectral data have been analysed by difference spectroscopy, least-squares curve fitting and 2-D correlation spectroscopy, which provided molecular level information on the diffusion mechanism. In particular, two distinct water species were identified corresponding, respectively, to the first and second-shell hydration layers. The spectroscopic analysis demonstrated that the relative population of these species is a function of the total water content in the system. A method has been devised to quantify the water concentration in the two hydration layers, based on a combination of spectroscopic and gravimetric data. The results have been compared with those from an earlier spectroscopic approach reported in the literature and based on the analysis of the carbonyl region.