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 degrees C for differential scanning calorimetry measurement, the 5% weight loss temperatures of 533-610 degrees C, and residual weights of 45.8-58.6% at 800 degrees 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 (n(av)) and in-plane/out-of-plane birefringence (n) 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 n(av), and low n are very suitable for optical applications.