On the Molecular Mechanism of H2O Diffusion into Polyimides: A Vibrational Spectroscopy Investigation

The diffusion of water into polyimide films was studied by in situ FTIR spectroscopy using several methods of spectral data anal., namely, difference spectroscopy, least-squares curve fitting, 2D correlation spectroscopy, and normal coordinate anal. The results gave an insight into the mol. mechanism of diffusion in terms of no. and population of penetrant species present in the system and with respect to the nature of the mol. aggregates. In particular, two water species were identified and quantified, i.e., H2O mols. interacting with the carbonyl groups of the polyimide and self-assocd. water. An enthalpy of formation of -0.9 kcal mol-1 was estd. for the H2O-polyimide interaction, which points to a relatively weak H-bonding tendency of the imide carbonyls. Finally, the IR spectrum of the H2O-imide aggregate was calcd. by a quantum mechanic (QM) model chem. to rationalize the effects obsd. in the spectrum of the water satd. films. The results of the computation were in good agreement with the expt., confirming the predictive capabilities of the chosen QM method and supporting the proposed mol. structure of the H-bonding aggregate.