Optical Transparency and Light Colour of Highly Soluble Fluorinated Polyimides Derived from a Novel Pyridine-Containing Diamine m, p-3FPAPP and Various Aromatic Dianhydrides

A novel pyridine-containing aromatic diamine monomer, 4-(4-trifluoromethylphenyl)-2,6-bis[3-(4-aminophenoxy)phenyl]pyridine (m,p-3FPAPP), was successfully synthesized by a modified Chichibabin reaction of 4-trifluoromethylbenzaldehyde and a substituted acetophenone, 3-(4-nitrophenoxy)acetophenone (m,p-NPAP), followed by a reduction of the resulting dinitro compound 4-(4-trifluoromethylphenyl)-2,6bis[3-(4-nitrophenoxy)phenyl] pyridine (m,p-3FPNPP) with Pd/C and hydrazine monohydrate. The aromatic diamine was employed to synthesize a series of pyridine-containing polyimides by polycondensation with various aromatic dianhydrides in N,N-dimethylformamide (DMF) via the conventional two-step method, and further thermal or chemical imidization forming polyimides. The inherent viscosities of the resulting poly(amic acid)s and polyimides were 0.62-0.89 and 0.58-0.69 dl/g, and most of the polyimides obtained by chemical imidization were readily soluble in common organic solvents such as DMF, N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP) and tetrahydrofuran (THF). Meanwhile, strong and flexible polyimide films were obtained, which had good thermal stability, with a glass transition temperature (T g ) of 234.4-246.6°C and the temperature at 10% weight loss of 561.4-600.6°C in nitrogen atmosphere, and good optical transparency with the cut-off wavelengths of 369-378 nm, as well as outstanding mechanical properties with tensile strengths of 89.5-96.4 MPa, a tensile modulus of 1.52-1.63 GPa and elongation-at-break of 7.2-8.7%. The polyimide films also were found to possess low water uptake (0.61-0.79%).