Methyl Groups Pendant on Triphenylmethane Toward Modulating Thermal Stability and Dielectric Properties of the Crosslinkable Fluorinated Polyimide Films with High Transparency

It is urgent to develop high-performance polyimide (PI) films that simultaneously exhibit high transparency, exceptional thermal stability, mechanical robustness, and low dielectric to fulfil the requirements of flexible display technologies. Herein, a series of fluorinated polyimide films (FPIs) were fabricated by the condensation of 5,5′-(perfluoropropane-2,2-diyl) bis(isobenzofuran-1,3-dione) (6FDA) and the fluorinated triphenylmethane diamine monomer (EDA, MEDA and DMEDA) with heat-crosslinkable tetrafluorostyrene side groups, which was incorporated by different numbers of methyl groups pendant in the ortho position of amino groups. Subsequently, the FPI films underwent heating to produce crosslinking FPIs (C-FPIs) through the self-crosslinking of double bonds in the tetrafluorostyrene. The transparency, solvent resistance, thermal stability, mechanical robustness and dielectric properties of FPI and C-FPI films can be tuned by the number of methyl groups and crosslinking, which were deeply investigated by virtue of molecular dynamics (MD) simulations and density functional theory (DFT). As a result, all the films exhibited exceptional optically colorless and transparent, with transmittance in the visible region of 450–700 nm exceeding 79.9%, and the cut-off wavelengths (λ_off) were nearly 350 nm. The thermal decomposition temperatures at 5% weight loss (T_d5%) for all samples exceeded 504 °C. These films exhibited a wide range of tunable tensile strength (46.5–75.1 MPa). Significantly, they showed exceptional dielectric properties with the dielectric constant of 2.3–2.5 at full frequency (10⁷–20 Hz). This study not only highlights the relationship between the polymer molecular structure and properties, but offer insights for balancing optical transparency, heat resistance and low dielectric constant in PI films.