Soluble Polyimides Derived from a Novel Aromatic Diamine Containing an Imidazole Unit and Trifluoromethyl Groups

A series of polyimide (PI) films were prepared through a two-step polycondensation process via chemical imidization using five commercial aromatic dianhydrides and a novel diamine containing an imidazole unit and trifluoromethyl groups, i.e., 4,4′-(((4,5-diphenyl-1H-imidazole-1,2-diyl)bis(4,1-phenylene))bis(oxy)) bis(3-(trifluoromethyl)aniline) (referred to as diamine 3). The number average molecular weights of the resulting PIs ranged from 1.57 × 104 to 3.17 × 104 g·mol−1 with polydispersity indices between 2.53 and 3.60, which were obtained through gel permeation chromatography measurements using a polystyrene standard. All the PIs prepared using diamine 3 were amorphous because of the large side group and kink structure of the diamine. They are soluble in common high-boiling point solvents, such as N-methyl pyrrolidone, dimethyl acetamide, and m-cresol, as well as several low-boiling point solvents, including chloroform and tetrahydrofuran. They exhibit the thermal stability with 10% weight loss temperatures in the range of 545 to 562 °C and 5% weight loss temperatures in the range of 500 to 541 °C in a nitrogen atmosphere. Their glass transition temperatures exceed 245 °C. All the resulting PI films show transmittances of over 74% in the visible light region ranging from 400 to 760 nm. The tensile strength of the PI films is in the range of 30.1 to 52.8 MPa, while the tensile modulus ranges from 1.0 to 1.7 GPa.