Extremely Low Humidity and Frequency Dependence of Dielectric Properties of Highly Fluorinated Polyimides in the 10–330 GHz Range
Ririka Sawada, Haonan Liu, Shana Yanagimoto, Yoshiyuki Yanagimoto, Shinji Ando
Abstract
High Resolution Image Download MS PowerPoint Slide The dielectric performances of six newly synthesized and four previously reported highly fluorinated polyimides (HFPIs) were evaluated under variable humidity and broadband frequency conditions (10–330 GHz). The HFPIs incorporated different fluorinated dianhydrides and diamines bearing −CF 3 or aromatic C–F groups, resulting in high fluorine weight contents ( F %) of 27–37 wt%. At 10 GHz, all HFPIs exhibited low dielectric constants ( D k = 2.6–2.9) and low dissipation factors ( D f < 0.005) at 23 °C and 30% relative humidity (RH). The coefficients of humidity-sensitivity of D k and D f, denoted as h Dk and h Df, respectively, monotonically decreased with an increase in F %, reflecting reduced water uptake by highly hydrophobic structures. Notably, h Df demonstrated a slight correlation with D f at 10% RH but strongly correlated with that at 60% RH, indicating that moisture-induced dielectric dissipation is primarily governed by absorbed water rather than by intrinsic dipolar polarization. In contrast, frequency-dependent measurements from 25 to 330 GHz at 45% RH revealed a systematic displacement in the dissipation mechanism. Although D k gradually decreased with frequency because it was mainly governed by electronic polarization, that is, the refractive index, D f unexpectedly increased for specific structures, including −CF 3 moieties. The coefficient of the frequency-sensitivity of D f ( k Df ) strongly depended on the backbone rigidity and the substituent effect of fluorine. In particular, 10F-pTFBZ exhibited the lowest D f (0.0089 at 330 GHz) and smallest h Df and k Df values, reflecting the efficient suppression of dipolar and vibrational relaxation processes as well as moisture absorption.