Vibrational Spectroscopic Analysis of Water Absorption in Polyimides and the Correlation with Dielectric Properties at 10 GHz

Unveiling the mechanism behind the relative humidity (RH) dependence of the dielectric properties of polyimides (PIs) in the high-frequency (GHz) range is a crucial challenge in the development of novel low-dielectric thermally stable polymers. Herein, the correlations among the dielectric constant (D_k), dissipation factor (D_f), water absorption content, and hydrogen-bonding structures of sorbed H₂O molecules were precisely analyzed for 15 types of aromatic and semiaromatic PIs based on variable-RH Fourier transform infrared (FT-IR) vibrational spectroscopy. The absorbance of the IR bands assignable to the O–H stretching (ν(OH)) and H–O–H bending (δ(HOH)) vibrations of H₂O molecules sorbed in PI films gradually increased with an increase in the RH of the atmosphere. Notably, the slopes of the increase in D_k and D_f against the integrated area of the δ(HOH) IR band (A_δ(HOH)), which can be used as a measure of the concentration of sorbed H₂O, were nearly constant and independent of the chemical structure of PIs. However, the slopes of the RH dependences of D_k and D_f, that is, h_Dk and h_Df, were strongly dependent on the structure of PIs and proportional to each other. This confirms that the amount of water absorbed is a crucial factor in the RH dependence of D_k and D_f. The D_k values of the PIs increased linearly with an increase in A_δ(HOH), whereas the D_f values increased linearly with A_δ(HOH) at a lower RH but gradually showed a concave-up increase at a higher RH. In addition, the relationship between A_δ(HOH) and D_f for all PIs was nearly aligned along a single master curve. This unified and marked increase in D_f could be induced by both the large D_f of water (∼0.5) and the plasticization effect of the H₂O molecules sorbed in the PI films. This water-induced plasticization activates the local relaxation motions of the moieties with large dipole moments, i.e., the imide and ester groups, in the PI chains, resonating at 10 GHz. Furthermore, the spectral decomposition of the IR ν(OH) band enables the classification and characterization of the structures of sorbed and hydrogen bonded H₂O molecules at the molecular level, namely 1) H₂O molecules directly form hydrogen bonds (HBs) with imide carbonyl (C═O) groups, called “bound water,” and 2) H₂O molecules form HBs with bound water, called “self-associated water.” Notably, with an increase in A_δ(HOH), the total amount of the spectral components of self-associated water (A_assoc) increased curvilinearly and simultaneously with D_f. The quasi-linear relationship between A_assoc and D_f suggests that a rapid increase in D_f with A_δ(HOH), which reflects the activation of the local motion of PI chains via water plasticization, is strongly correlated with the amount of self-associated water. To reduce the dependence of RH and water absorption on D_k and D_f of PIs at higher frequencies, effective strategies include the introduction of hydrophobic fluorine atoms and the suppression of self-associated water-induced local molecular motion resonating in the same frequency range.