Synthesis and properties of hyperbranched polyimides derived from tetra-amine and long-chain aromatic dianhydrides

To investigate the properties of hyperbranched polyimides (HBPIs) for potential optical applications, novel fluorinated tetra-amine monomers with ether and sulfonyl groups, 4,4'-di[3,5-di(2-trifluoromethyl-4-aminophenoxy) phenoxy] sulfone, and five kinds of long-chain dianhydrides, have been designed and synthesized. A series of anhydride-terminated hyperbranched polyimides have been prepared via a two-step method with chemical imidization. The HBPIs possess outstanding solubility and show excellent thermal and optical properties. The glass transition temperatures (T-g) of the HBPIs range from 230-242 degrees C determined by differential scanning calorimetry (DSC), and 232-263 degrees C by thermomechanical analysis (TMA), depending on the dianhydride used. The 5% weight loss temperatures are in the range of 480-533 degrees C, showing high intrinsic thermal-resistant characteristics of the HBPIs. The HBPI films show good optical transparency, higher than 80% at 800 nm. The cast polyimide films have favorable mechanical properties with tensile strengths of 91-112 MPa, elongation at break of 5.99-7.99%, and initial modulus of 2.14-2.57 GPa. The polyimides exhibit average refractive indices of 1.5780-1.6271, and birefringence of 0.0065-0.0079 because of the hyperbranched structure. The PI derived from 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]hexafluoropropane possesses the lowest refractive indices due to the high fluorine content in the polymer chain.