Preparation and characterization of low dielectric cross-linked polyimide containing fluorene group via post-polymerization cross-linking

Abstract

Fluorene-containing polyimides with pendant hydroxyl groups were synthesized via azeotropic imidization from BPADA, mTB, and APFD in varying feed ratios. Post-polymerization cross-linking with terephthaloyl chloride (TPC) generates covalent junctions between polymer chains, disrupting dense packing and arresting expanded inter-chain free space. Wide-angle X-ray Scattering (WAXS) analysis confirmed that the formation of cross-linked networks increased free volume in the polymer matrix, leading to a reduction in dielectric constant (D_k) and dissipation factor (D_f). The CBMAD73(1.0) film exhibited the advanced low dielectric properties with D_k = 2.66 and D_f = 0.0056 at 10 GHz, compared to 3.13 and 0.0246 at 10 GHz for the polymeric precursor, BMAD73. Thermal analysis demonstrated that the cross-linked polymer network retained thermal stability, which is comparable to that of the neat polyimide precursor, demonstrating structural integrity of the cross-linked network. These results highlight post-polymerization cross-linking as an effective approach for modulating nanoscale free volume to achieve low dielectric performances while maintaining thermal stability required for high-frequency electronic applications.