Relative thermophysical properties of some polyimides

A comparative thermogravimetric study of ten polyimide films of varying composition revealed that thermal and thermo-oxidative stability of the polymers increases directly with increasing imide content. Films degraded more rapidly in air than in vacuo; the insertion of connecting groups within the polymer repeat unit decreased stability in both environments. The effect of inserting “ladder-like” segments in the polymer structure did not enhance resistance to oxidation; but under vacuum, those segments probably operated beneficially in promoting the retention of the carbon-like residue. Infrared spectral measurements showed little loss of imide bands with progressive weight loss in air, indicating unit-by-unit degradation. Conversely, infrared spectra of films heated under vacuum conditions showed that progressive loss of imide bands occurred simultaneously with an increasing development of bands related to carbon–nitrogen double bonds. Dynamic heating under vacuum developed singularly uniform weight loss curves; however, concurrent mass-spectrographic analyses indicated two distinct reactions. Gaseous carbon monoxide and carbon dioxide species accounted for the major weight change, and their periods of maximum evolution occurred at different temperatures for each film. A subsequent minor weight loss period was associated with the liberation of hydrogen cyanide which maximized at the same temperature for all ten films.