Abstract
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.
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