Characterization of electronic transitions in polyimide films based on spectral variations induced by hydrostatic pressures up to 400 MPa.

To clarify the nature of optical absorptions in polyimides (PIs), the ultraviolet-visible optical absorption spectra of PI thin films were observed at high pressures up to 400 MPa using a custom-built hydrostatic pressure optical cell. A pressure-induced bathochromic shift with increasing bandwidth was observed in the locally excited (LE) absorption band for s-BPDA/DCHM PI (poly(4,4'-dicyclohexylmethylene biphenyltetracarboximide), which is related to increases in van der Waals interactions due to decreased intermolecular distances. A pressure-induced increase in absorbance was observed for the charge transfer (CT) absorption band of PMDA/ODA PI (poly(4,4'-oxidiphenylene pyromellitimide) and PMDA/TFDB PI (poly(2,2'-bis(trifluoromethyl)-4,4'-biphenylene pyromellitimide), which indicates that the bands are assignable to intermolecular CT transitions. On the other hand, a pressure-induced bathochromic shift without an increase in absorbance was observed for the CT band of s-BPDA/PDA PI (poly(p-phenylene biphenyltetracarboximide), which indicates that the band is assignable to intramolecular CT transitions. These characteristic changes observed in pressure-induced variations can be used to characterize absorption bands at longer wavelengths in PIs.