Thermally Stable and Transparent Polyimide Derived from Side‐Group‐Regulated Spirobifluorene Unit for Substrate Application

Advancements in flexible electronic technology, especially the progress in foldable displays and under-display cameras (UDC), have created an urgent demand for high-performance colorless polyimide (CPI). However, current CPIs lack sufficient heat resistance for substrate applications. In this work, four kinds of rigid spirobifluorene diamines are designed, and the corresponding polyimides are prepared by their condensation with 5,5'-(perfluoropropane-2,2-diyl) bis(isobenzofuran-1,3-dione) (6FDA) or 9,9-bis(3,4-dicarboxyphenyl) fluorene dianhydride (BPAF). The rigid and conjugated spirobifluorene units endow the polyimides with higher glass transition temperature (T