AbstractA bending‐beam technique has been used to in situ monitor the diffusion of water in various polyimide films. The polyimides studied are pyromellitic dianhydride‐4.4′‐oxydianiline (PMDA–ODA), pyromellitic dianhydride‐p‐phenylenediamine (PMDA–PDA), and 3,3′,4,4′‐benzophenone tetracarboxylic dianhydride‐p‐phenylenediamine (BPDA–PDA), and their blends and random copolymers. The diffusion of water in these films obeys Fick's law. In PMDA–ODA, the mean diffusion constant is 5.2 ± 0.4 × 10−9(cm2/s) for thicknesses ranging from 6.7 to 27.3 μm. In PMDA–PDA, it is 2.0 ± 0.4 × 10−9(cm2/s) for thicknesses ranging from 7.3 to 20.0 μm, and in BPDA–PDA, 0.27 ± 0.02 × 10−9(cm2/s) for thicknesses ranging from 4.8 to 21.0 μm. In the blends and random copolymer with 50 wt % PMDA–ODA and 50 wt % PMDA–PDA, the diffusion constants are slightly smaller than those in the pure PMDA–ODA, but much larger than in the pure PMDA–PDA. On the contrary, in those with 50 wt % BPDA–PDA and 50 wt % PMDA–PDA, the diffusion constants are much smaller than those in the pure PMDA–PDA, but slightly larger than in the pure BPDA–PDA. These diffusion constants are primarily affected by the chemical structure of the imide molecule. The morphology, such as crystallinity, of the films has played a secondary factor. Hygroscopic stresses due to water uptake in all the studied films increase as the film thickness increases. It can be attributed to th
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