Relationship between Molecular Aggregation Structures and Optical Properties of Polyimide Films Analyzed by Synchrotron Wide-Angle X-ray Diffraction, Infrared Absorption, and UV/Visible Absorption Spectroscopy at Very High Pressure

The relationship between the molecular aggregation structures and the optical properties of fully aromatic and semialiphatic polyimide (PI) films were analyzed by synchrotron wide-angle X-ray diffraction (WAXD), infrared absorption, and UV/visible absorption spectroscopy at very high pressures up to 8 GPa. The PIs showed significant reduction in the interchain distances in the first stage of compression up to 1 GPa, which resulted in an appreciable decrease in the interchain free volume. In addition, reduction in the C−C bond lengths of aromatic rings by ca. 0.7% was confirmed by the pressure-induced high wavenumber shift of the infrared stretching vibration of the PIs. Furthermore, pressure-induced bathochromic shifts were observed in the locally excited (LE) absorption band of PIs, which are related to the enhanced van der Waals interaction caused by the reduced interchain distances. The intensity of the charge transfer (CT) absorption band of s-BPDA/PDA poly(p-phenylene biphenyltetracarboximide) PI was reduced up to 0.3 GPa, indicating that conformational changes affect the intramolecular CT interactions. In contrast, the CT absorptions of PMDA/ODA (poly(4,4′-oxidiphenylene pyromelliticimide) and PMDA/DCHM (poly(4,4′-diaminocyclohexylmethane pyromelliticimide) PIs were enhanced by increasing the pressure, which was caused by an enhancement of intermolecular CT interactions. The significant variations observed in the LE and intermolecular CT bands below 1 GPa accord with the significant decrease in the interchain distance, as indicated by synchrotron WAXD.