Preparation and characterization of low-κ co-polyimide films

In this study, five co-polyimide (co-PI) films were synthesized through copolymerization using 6FDA and PMDA as dianhydride monomers paired with ODA diamine. By systematically varying the 6FDA/PMDA molar ratio, we successfully regulated the microstructural characteristics of co-PI films, particularly their free volume and molecular chain rigidity. The structure–property relationships were comprehensively investigated using positron annihilation lifetime spectroscopy (PALS), with special emphasis on thermal, dielectric, and mechanical performance. Notably, the glass transition temperature (Tg) exhibited a progressive decrease from 384.9 ℃ for PI-1 to 307.8 ℃ for PI-5. This thermal behavior is related to the increase of free volume and the decrease of molecular chain rigidity caused by higher 6FDA content. Correspondingly, the dielectric constant (k) at 10⁴ Hz decreased from 3.55 to 2.46 across the series, demonstrating that enhanced free volume effectively reduces dielectric polarization. However, this microstructural modification came at the expense of mechanical integrity, as evidenced by the descending tensile strength trend with increasing 6FDA proportion. These findings demonstrate an effective strategy for developing high-performance polyimide films combining low dielectric constant with balanced thermal and mechanical properties.