Processing vapour-deposited polyimide

Freestanding polyimide (PMDA-ODA) thin films ( thickness = 1 to 30 µm) and spherical shells ( diameter = 950 to 1100 µm, wall thickness = 0.7 to 11.0 µm) have been fabricated by vapour deposition polymerization (VDP). The deposition process was optimized to obtain smooth and uniform deposition composed of equal-molar PMDA and ODA. The tensile properties, including elastic modulus, tensile strength, and elongation at break, were determined by buckling and bursting the shells and using a nanoindentation test on the films. The gas permeability was measured for He, H 2 , D 2 , O 2 and N 2 . The VDP polyimide possessed distinct properties from solution-cast PMDA-ODA polyimide (Kapton ® ), including better tensile properties and lower permeability, and was insoluble in concentrated sulfuric acid. The differences in the properties were attributed to the presence of physical or chemical cross-linking in the VDP polyimide. The effects of thermal imidization conditions on the properties were studied, including different heating rates (0.1 to 1.0 °C min -1 ), atmospheres (air and N 2 ), and imidization durations (1 to 6 h). Varying the imidization conditions effectively modified the tensile strength, elongation at break and permeability but had little effect on the elastic modulus. Infrared spectroscopy and solubility tests indicated that the changes in the properties resulted from changes in the molecular weight or degree of cross-linking.