Twisted Fluorene Cardo Structures for Synergistic Dielectric, Thermal, and Mechanical Optimization in Ternary Polyimides

This study aims to develop high-performance polyimide (PI) dielectrics with enhanced intrinsic dielectric properties while retaining excellent thermal and mechanical stability. To achieve this, a series of ternary copolyimide films were designed by incorporating 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF), which contains a twisted fluorene cardo structure, into the rigid polyimide backbone derived from pyromellitic dianhydride (PMDA) and 4,4′-oxidianiline (ODA). The BPAF/PMDA molar ratio was systematically varied to regulate chain packing, free volume, and segmental mobility. Molecular simulations combined with experimental characterizations, including Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), atomic force microscopy (AFM), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), tensile testing, and dielectric measurements, were employed to elucidate structure–property relationships. Results showed that the twisted fluorene unit effectively disrupted regular chain packing, increased free volume fraction, and enhanced dipolar accessibility. Among all samples, PI-2 (BPAF:PMDA = 70:30) exhibited a well-balanced performance, featuring high temperatures at 5% and 10% weight loss (Td5 = 531 °C, Td10 = 566 °C, respectively), excellent storage-modulus retention, a relatively high tensile strength (101 MPa), and favorable dielectric properties (dielectric constant 6.5 at 1 kHz with a low loss of 0.0032). The study demonstrates that rational molecular design via dianhydride composition tuning can achieve synergistic optimization of thermal, mechanical, and dielectric performance without inorganic fillers. This work provides both experimental evidence and theoretical guidance for the molecular design of intrinsic high-performance PI dielectrics, providing a molecular-design basis for further investigation of polyimide dielectrics for high-temperature and flexible electronic applications.