Processing vapour-deposited polyimide

Freestanding polyimide (PMDA-ODA) thin films (thickness = 1 to 30 mum) and spherical shells (diameter = 950 to 1100 mum, wall thickness = 0.7 to 11.0 mum) have been fabricated by vapour deposition polymerization (VDP). The de position process was optimized to obtain smooth and uniform deposition comp osed of equal-molar PMDA and ODA. The tensile properties, including elastic modulus, tensile strength, and elongation at break, were determined by buc kling 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 polyimi de (Kapton((R))), including better tensile properties and lower permeabilit y, and was insoluble in concentrated sulfuric acid. The differences in the properties were attributed to the presence of physical or chemical cross-li nking in the VDP polyimide. The effects of thermal imidization conditions on the properties were studie d, including different heating rates (0.1 to 1.0 degreesC min(-1)), atmosph eres (air and N-2), and imidization durations (1 to 6 h). Varying the imidi zation conditions effectively modified the tensile strength, elongation at break and permeability but had little effect on the elastic modulus. Infrar ed spectroscopy and solubility tests indicated that the changes in the prop erties resulted from changes in the molecular weight or degree of cross-lin king.