Photoacoustic Infrared Spectroscopy of Thin Polyimide Layers on Glass Substrates

摘要

Three commercially-available polyamic acid solutions were deposited on smallglass substrates, cured in nitrogen or air by heating at 2 deg C/minute or 20 deg C/minute to 350 deg C, and cooled over 16 h. Most of the resultant polyimide layers were 8-13 cubic meters thick. The cured layers were examined on their substrates using photoacoustic spectroscopy (PAS) in step-scan, time-sliced mode, where each spectrum contains time-weighted contributions from different depths in the layer. The three polymides can easily be distinguished from one another using only the most intense spectrum from a set of timed-delayed spectra. Differences in polyimide layer thickness on the glass substrate are evident. The presence of bi-layers can be discerned. Effects of oxidation were observed. Two of the polyimides, characterized as semi-flexible and semi-rigid, tend to order physically when cured on a substrate. Birefringence measurements suggested that each polyimide had its own degree or order, but the amount of order did not change signficantly for the various specimens of a given polyimide. X-ray wide angle diffraction and photoacoustic spectra also suggested no difference in the order for a given polyimide. A correlation algorithm was used to quantify differences between sets of time-weighted spectra from different specimens. The algorithm used all the time-delayed spectra in a set except one or two of the weakest and indicates whether (but not how) two specimens differ over their 'thermal depths'. However, the complete set of time-delayed spectra provided no new information over that found in the most intense spectrum.