Cross-Linkable Spirobisindane-Based Polyimides with Enhanced Low-Dielectric Properties for High-Frequency Applications

Cross-linkable spirobisindane-based polyimides were synthesized and investigated as low-dielectric materials for high-frequency applications. The polymer composition was controlled by varying the molar ratio of diamines, and allyl groups were introduced through postpolymerization functionalization to enable subsequent thermal cross-linking. The successful introduction and cross-linking of the allyl groups were confirmed by 1H NMR and FT-IR spectroscopy, together with gel fraction measurements. The resulting films were readily fabricated by solution-based bar coating and exhibited enhanced hydrophobicity and thermal stability after cross-linking. Positron annihilation lifetime spectroscopy (PALS) revealed a composition-dependent reorganization of the local free-volume structure upon cross-linking, rather than a uniform increase in free volume. In particular, the 55 series exhibited a pronounced decrease in fractional free volume after cross-linking, whereas the 73 series showed relatively minor changes. Despite these different free-volume responses, the cross-linked X-BMDA films exhibited improved dielectric properties compared with their uncross-linked counterparts, indicating that the dielectric enhancement cannot be attributed solely to an increase in free volume. X-BMDA-55 exhibited the lowest dielectric constant (Dk = 2.20 at 28 GHz) and dissipation factor (Df = 0.0063 at 10 GHz). These results demonstrate that the combination of spirobisindane-based molecular design and post-cross-linking provides an effective strategy for controlling local molecular structure and achieving low dielectric properties in polyimide films.