The effects of morphology and hygrothermal aging on water sorption and transport in Kapton® polyimide

A comparison of water sorption and diffusion behavior in 2 mil, as‐received, Kapton = Trademark of E.I. Dupont de Nemours Inc. film, and in otherwise identical but hygrothermally aged samples, suggests chemical differences between the samples. These differences can be related to known polyimide chemistry and the specific history encountered by the samples. Although the sorption isotherms are similar for an as‐received 0.3 mil sample studied earlier and for the as‐received 2 mil samples studied here, the diffusion coefficient of water in the as‐received 2 mil sample is approximately 300% larger than in the as‐received 0.3 mil sample. This large effect is believed to be related to the presence of small, paracrystalline aggregates with large aspect ratios. Differences in orientation measured by birefringence for the two samples suggest that the barrier properties of Kapton® are strongly affected by the detailed morphological organization of the ordered aggregates. Comparison of the sorption isotherms for the as‐received and hygrothermally aged 2‐mil films suggests a significant increase in the hydrophillic nature of the aged film. This change is consistent with an apparent chemical reaction between water and uncyclized amic acid residues in the film. The tendency for water to cluster in as‐received Kapton® is essentially eliminated by the chemical modification associated with hygrothermal aging. The hygrothermal aging produces a dramatic reduction in the water vapor diffusion coefficient at low vapor activities but a rather modest change at high activities.