Synthesis and properties of (A2+B4)-type hyperbranched polyimides with different terminal groups

To investigate the effect of terminal groups on the properties of hyperbranched polyimides (HBPIs) for potential optical applications, a series of HBPIs bearing different terminal groups have been synthesized through a novel aromatic CF3-containing tetra-amine, 4,4′-di[3,5-di(2-trifluoromethyl-4-aminophenoxy)phenoxy] sulfone, and a long chain dianhydride, 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]hexafluoropropane, via a two-step polycondensation procedure. The synthesized HBPIs could form flexible and tough polyimide (PI) films, which exhibit good thermal stability with the glass transition temperatures of 191–234°C for differential scanning calorimetry measurement, the 5% weight loss temperatures of 533–610°C, and residual weights of 45.8–58.6% at 800°C under nitrogen atmosphere. The PI films have favorable mechanical properties with tensile strengths of 77–92 MPa, elongation at break of 3.55–7.99%, and initial modulus of 2.05–2.86 GPa. The films also exhibit the good transmittance of higher than 80% at 800 nm. The average refractive indices (nav) and in-plane/out-of-plane birefringence (Δn) measured at 632.8 nm are in the range of 1.5539–1.6022 and 0.0065–0.0077, respectively. The good transmittance in the visible region, tunable nav, and low Δn are very suitable for optical applications.