Molecular engineering of polyphosphazene for low toxicity, flame retardant and low dielectric semi-aromatic polyimide composites

The application of semi-aromatic polyimide (PI) is limited by its relatively high dielectric constant (D_k) and flame retardancy. To address this problem, a novel fluorophenoxy-based linear polyphosphazene (FB-PDCP) was synthesized as a multifunctional additive and incorporated into a PI matrix to fabricate a series of high-performance FB-PDCP/PI composite films. The introduction of FB-PDCP establishes strong interfacial interactions within PI matrix, significantly increasing the glass transition temperature (T_g) of the composite films from 299 °C to 345 °C. The unique structural effects of FB-PDCP concurrently optimized the overall performance of the composites. At 1 kHz, the FB-PDCP-7/PI film exhibited an ultralow D_k of 2.24, a 33.92% reduction compared to neat PI. In terms of flame retardancy, the composite films achieved a high limiting oxygen index (LOI) of 30.0% and passed the UL-94 through V-0 rating, demonstrating exceptional fire safety. Meanwhile, Thermogravimetry-infrared spectroscopy (TG-IR) revealed the incorporation of FB-PDCP suppressed the release of fluorine-containing substances from PI composites, indicating superior toxic gas suppression. Furthermore, the FB-PDCP-7/PI composite films obtain a tensile strength of 103 MPa, with a storage modulus of 3.93 GPa and an elongation at break of 13.1%. This study provides an integrated solution to the conflict between achieving low D_k and high flame retardancy in PI materials, and the developed FB-PDCP/PI composite films show great potential for applications in electronic packaging.