Effects of Processing Conditions on the Quality and Properties of Vapor-Deposited Polyimide Shells

The processing conditions for vapor-depositing polyimide shells were studied to improve the surface finish, tensile properties, and gas permeability for the inertial confinement fusion application. The vapor-deposited (VDP) polyimide possessed distinct properties from solution-cast Kapton, resulting perhaps from its being physically or chemically crosslinked. The VDP polyimide was characterized to be semicrystalline with molecular chains parallel to the shell's surface. Varying the imidization conditions, i.e., using different atmospheres, heating rates, and heating durations, increased the gas permeability while maintaining the Young's modulus. Plastically deforming the shells under biaxial stress increased the permeability by up to 1000-fold, which could be reversed when heated to 350°C. Analyses using x-ray diffraction, infrared spectroscopy, and solubility tests indicated that these modifications in properties may have arisen from changes in the crystallinity, crosslinking, and molecular weight. The low-mode (2 to 20) surface roughness was reduced when the shells were slightly inflated; the high-mode roughness (coating-induced bumps) was increased when the substrate was heated to a temperature of 90°C to 140°C.