Highly Transparent and Colorless Polyimide Film with Low Dielectric Constant by Introducing Meta-substituted Structure and Trifluoromethyl Groups

An effective design strategy for preparing highly transparent polyimide film with low dielectric constant is presented. The key to the strategy is to simultaneously introduce meta-substituted structure and trifluoromethyl in polymer chains. By using this design strategy, a highly transparent polyimide film with low-k was synthesized from 3,5-diaminobenzotrifluoride (m-TFPDA) and 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) through a two-step method. The obtained m-TFPDA/6FDA (CPI) film (∼30 µm) possesses high optical transparency (λcutoff=334 nm, T450nm=85.26%, Haze=0.31) and is close to colorless (L*=96.03, a*=−0.34, b*=2.12, yellow index=3.96). The intrinsic k and dielectric loss value of the film are 2.27 and 0.0013 at 10 kHz, respectively. More importantly, such low dielectric performance could remain stable up to 280 °C, and the film shows a low moisture rate (∼0.51%), which helps to maintain the low-k property stability in different humid environments. Meanwhile, the film also shows good thermal stability and mechanical properties, with a glass transition temperature (Tg) of 296 °C and the 5 wt% decomposition temperature (Td,5%) of 522 °C under N2. The tensile strength and tensile modulus of the film are 85.1 MPa and 1.96 GPa, respectively. In addition, the film is soluble in common solvents, which allows simple solution processing and low-cost, continuous roll-to-roll processes. This design strategy is beneficial to improving the transparency, lightening yellow color, lowering the dielectric constant and meanwhile maintaining the comprehensive properties of polyimide films, which is mainly due to the introduced meta-substituted and trifluoromethyl structures effectively inhibiting the transfer of charge transfer complex (CTC) effects and increasing the free volume of film. This design strategy could also be extended to other high-performance polymer systems.