IR studies of polyimides. I. Effects of chemical and physical changes during cure

A detailed study has been made of the thermal cure of two polyimides using infrared analysis. It is shown that two of the IR bands commonly used to determine the amount of imidization are affected by both interference from other species and by polymer morphology. These bands, located near 1780 cm−1 and 730 cm−1, overlap bands attributed to anhydride, which is formed when the polymer is heated. A correction for this effect is described. However, the measured absorbances of these bands are also subject to a dichroic effect. Any anisotropy in the samples can therefore affect the results. A third band, that located near 1370 cm−1, does not overlap anhydride peaks and did not, using the approach described here, display any effect due to dichroism. Thus it appears to offer a reliable way of following imidization by IR.