Achieving Low Dissipation Factors and Low Dielectric Constants via Thermally Stable Naphthalene-Based Poly(ester-imide)s with Fluorine Groups

In recent years, polyimides (PIs) and their functional derivatives (i.e., poly(ester-imide)s (PEIs), poly(amide-imide)s (PAIs), etc.) have gained more attention in the microelectronic and optoelectronic industries because of their excellent thermal-mechanical stabilities, heat resistance, dielectric properties, and solution processabilities. Nevertheless, developing PIs with low dielectric constants (D_k) and low dissipation factors (D_f) at high frequencies (≥10 GHz) is crucial for advanced mobile communications and high-frequency applications. Therefore, to achieve low dielectric properties in both D_k and D_f at high frequencies, using PEIs with strong electron-withdrawing fluorine groups is a good synthetic strategy. Herein, a series of PEIs with trifluoromethyl (−CF₃) groups are prepared from 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine (TFMB), 1,4-phenylene bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylate) (TAHQ), and naphthalene-2,6-diyl bis(1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylate) (NPDA) monomers. In addition, the molar ratio of TAHQ/NPDA modulates the comprehensive performances of the derived PEI films, and each performance of the PEI films is addressed in detail. From the theoretical calculations, when increasing the NPDA ratio from 0 to 100% in the TFMB/TAHQ system, the molar free volume increases and balances the polarizability and linearity, which is beneficial for achieving low dielectric properties. As a result, all the PEI films exhibit excellent thermal stabilities with thermal decomposition temperature at 5% weight loss (T_d5%) over 470 °C and glass transition temperature (T_g) at 215–250 °C. In addition, all the PEI films show decent coefficients of thermal expansion (CTE) in the range of 16–40 ppm/°C and good mechanical stabilities. Moreover, all PEI films exhibit low dielectric properties at high frequencies. When increasing the NPDA concentration from 0 to 100%, both D_k/D_f values gradually decrease from 3.19/0.00322 for PEI-NPDA-0 to 2.90/0.00174 for PEI-NPDA-100 at a 10 GHz frequency. Particularly, the D_f values are one of the lowest values under the 10 GHz frequency. The results demonstrate that preparing PEI backbones with fluorine groups is a good synthetic strategy for achieving low dielectric properties at higher frequencies.