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, H2, D2, O2 and N2. 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 N2), 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.
The Young's modulus and helium gas permeability of vapor-deposited poly͑4,4Ј-oxydiphenylenepyromellitimide͒ were measured at cryogenic and elevated temperatures ͑10-573 K͒. The Young's modulus decreased with increasing temperature from 5.5 GPa at 10 K to 1.8 GPa at 573 K. The temperature dependency of the permeability followed the Arrhenius' relationship, with different activation energy for permeation for samples imidized under different conditions. The effect of the imidization conditions on the permeation properties could be explained in terms of morphology/crystallinity as determined by x-ray diffraction techniques. Imidizing in air instead of nitrogen increased the permeability while lowering the activation energy for permeation and crystallinity. Imidizing at higher heating rates ͑in nitrogen͒ resulted in higher permeability, lower activation energy for permeation, and larger and fewer crystallites with better-aligned lattice planes.
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