STRUCTURAL EFFECTS ON THE TRANSPORT OF WATER IN POLYIMIDES

The influence of molecular primary and secondary structure on the tran sport of water has been investigated for a series of chemically simila r polyimides using an infrared ATR method. The diffusivity of water ge nerally decreased with increased chain backbone stiffness (as determin ed from the mechanical beta-transition), and the activation energies f or diffusion generally increased with backbone stiffness. For a given backbone composition, the diffusivity was also shown to markedly incre ase as the density of the amorphous phase decreased. The vibrational s pectra show that the absorbed water partitions into a bimodal distribu tion of sites, presumably representing single and clustered water mole cules. This distribution changes markedly with saturation and is hence a sensitive probe of the local heterogeneity of the glassy structure.