Polarization Analysis and Humidity Dependence of Dielectric Properties of Aromatic and Semialicyclic Polyimides Measured at 10 GHz

In this study, dielectric constants (Dk) and dissipation factors (Df) of 15 types of polyimides (PIs), including wholly aromatic, semialicyclic, partially fluorinated, and perfluorinated PIs, were measured in the transverse electric (TE011) mode at a frequency of 10 GHz and analyzed based on their chemical structures and the dominant factors of dielectric polarization per unit volume (Pt). The relationships among the dipolar orientational (Pd) and electronic (Pe) polarizations and dielectric properties (Dk and Df) were quantitatively investigated. Although a systematic correlation between the structure and Df of PIs has not yet been established, we found that the Df of PIs is anisotropic and linearly proportional to Pd in the in-plane direction, which can be estimated from the Dk and in-plane refractive index (nTE) at 1310 nm. This could be related to the bidirectional relationship between the real (Dk) and imaginary (Df) parts of the relative permittivity. Furthermore, the Dk and Df of all PIs increased linearly with respect to the relative humidity (RH) of the measurement environment, and their slopes (hDk and hDf), i.e., the RH sensitivities of Dk and Df, showed a proportional relationship with a high correlation coefficient (R2 = 0.986). The weight fraction of polar imide and ester groups (Polar %) and the hydrophobicity of PIs, which are closely related to the fluorine content (F %), are essential factors in determining the values of Dk, Df, hDk, and hDf. These relationships are crucial for developing low-dielectric PI materials exhibiting low Dk and Df values, as well as reduced sensitivity to humidity.