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 (Dk), dissipation factor (Df), 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 Dk and Df against the integrated area of the δ(HOH) IR band (Aδ(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 Dk and Df, that is, hDk and hDf, 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 Dk and Df. The Dk values of the PIs increased linearly with an increase in Aδ(HOH), whereas the Df values increased linearly with Aδ(HOH) at a lower RH but gradually showed a concave-up increase at a higher RH. In addition, the relationship between Aδ(HOH) and Df for all PIs was nearly aligned along a single master curve. This unified and marked increase in Df could be induced by both the large Df 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 Aδ(HOH), the total amount of the spectral components of self-associated water (Aassoc) increased curvilinearly and simultaneously with Df. The quasi-linear relationship between Aassoc and Df suggests that a rapid increase in Df with Aδ(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 Dk and Df 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.