Organo-soluble and transparent polyimides containing phenylphosphine oxide and trifluoromethyl moiety: Synthesis and characterization

A series of aromatic polyimides (PI-II_a-d) containing lateral phenylphosphine oxide (PPO) and trifluoromethyl (-CF₃) moiety were prepared from an aromatic diamine, 2,5-bis(4-amino-2-trifluoromethylphenoxy)phenyl]diphenyl-phosphine oxide (BATFDPO) and various aromatic dianhydrides via a two-step chemical imidization procedure. In parallel, for comparison, another series of polyimides (PI-I_a-d) without trifluoromethyl were synthesized from a diamine, 2,5-bis(4-aminophenoxy)-phenyl]diphenylphosphine oxide (BADPO) and the same dianhydrides. It was found that both of the two series of polyimides (PIs) were soluble in polar aprotic solvents, such as N-methyl-2-pyrrolidinone (NMP) and the solubility of PI-II_a-d was highly enhanced by the introduction of the bulky -CF₃ group. Flexible and tough PI films with tensile strengths higher than 70 MPa were cast from the PI solution. The introduction of -CF₃ moiety slightly sacrificed the thermal stability and mechanical properties of the PI films. For example, PI-II_a-d showed 5% weight loss at 472-476 ^°C, which was about 50 ^°C lower than those of their PI-I_a-d analogues. However, -CF₃ group apparently improved the optical transparency and decreased the refractive indices of the PI films. PI-II_d derived from BATFDPO and 4,4’-hexafluoroisopropylidenediphthalic anhydride (6FDA) exhibited the highest optical transparency with the transmittance of 90% at 400 nm and the refractive index as low as 1.5511 at 1310 nm.