The “fluorine impact” on dielectric constant of polyimides: A molecular simulation study

Fluorinated polyimides (PIs) with low dielectric constant (ε) have attracted increasing attention with the rapid development of advanced power electronics and 5G communication. In this work, the dielectric constants of three types of fluorinated PIs are firstly predicted by using two “Group contribution” methods, the Clausius-Mossotti method and the molecular dynamics (MD) simulation. The most reasonably good agreement is found between the MD simulated values and the previously reported experimental data. Secondly, the so-called “fluorine impact” from fluorine content (F%), molecular polarization (α/V), fractional free volume (FFV) and PI/water interaction energy (Einter) caused by the fluorine substituents on dielectric constant of the PIs are investigated. The substitution position and structure of the fluorine moieties demonstrate pronounced influence on dielectric constant when the PIs have similar fluorine content. Finally, the weight of each factor obtained by principal component analysis (PCA) is applied to assess an overall evaluation of the “fluorine impact”. α/V and F% are revealed to be the dominant factors to affect the dielectric constant. Based on the studied PI systems, a “negative” fluorine impact was attributed by a high α/V value. It can be speculated that the structure of PI-b1 provides an ideal pattern for designing new low-ε polyimides, i.e., increasing the amount of rigid di-substituted symmetric fluorine side chains with low molecular polarity in PI backbones.