Cross-Linkable Spirobisindane-Based Polyimides with Enhanced Low-Dielectric Properties 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.