Mechanical properties and gas permeability of polyimide shells fabricated by the vapor deposition method

Spherical polyimide (PMDA-ODA) shells with high aspect ratio (OD = 900 to 1 020 mu m and wall thickness = 0.7 to 8.0 mu m) were successfully fabricated by the vapor deposition method. These shells were characterized in terms o f gas permeability, Young's modulus, tensile strength, and ultimate elongat ion. The measured properties of the shells agreed with those of commercial films (Kapton(R)). Post-coating treatments of the shells, including thermal imidization in air and biaxial expansion, effectively increased gas permea bility. Air-curing resulted in shells that were more brittle and twice as p ermeable as those cured in nitrogen. Since no difference in chemical compos ition was found between the N-2- and air-cured shells, the effect of air-cu ring may be attributed to morphological or microstructural changes due to o xygen or water vapor in air. Shells that were expanded plastically showed o ver 300-fold increase in gas permeability, while retaining half of the orig inal Young's modulus. The effect of expansion on permeability may have aris en from localized plastic deformation. Both treatments will be useful for p reparing ICF targets that allow a rapid fill with DT fuel for cryogenic exp eriments.