Colorless transparent and thermally stable terphenyl polyimides with various small side groups for substrate application

Advances in flexible display technology, especially, under-screen camera and on-board display, have created an urgent need for high-temperature stable and colorless transparent polyimide (CPI), but meeting both properties at the same time is a huge challenge in molecular design. In this work, a series of rigid p-terphenylenediamines with different small side groups were synthesized, and the corresponding polyimides (PI-1 ∼ PI-15) were prepared by polymerization with 5,5′-(Perfluoropropane-2,2-diyl) bis(isobenzofuran-1,3-dione) (6FDA) and 9,9-Bis(3,4-dicarboxyphenyl)fluorene dianhydride (BPAF) as dianhydrides. The conformation and steric hindrance of terphenyl-based polyimide chains were affected by the type (-H, -F, -CF3, -CH3), number and substitution position of the small side groups. This feature could be used to regulate transparency and heat resistance of polyimides. Consequently, the rigid terphenyl polyimides with an average transmittance ranging from 79 to 89% in the visible region and the glass transition temperature (Tg) ranging from 339 to 458 °C were successfully obtained. PI-11 and PI-15 containing both -CH3 side groups and -CF3 side groups exhibited excellent comprehensive performance, with a yellowness index (YI) of 1.53 and 5.95, a Tg of 420 and 436 °C, a tensile strength of 148 and 164 MPa, respectively. The strategy of side-group-regulated terphenyl polyimides provides a way for designing multi-aromatic CPIs with high thermal stability and good transparency for photoelectric engineering.