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

A series of aromatic polyimides (PI-IIa–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-Ia–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-IIa–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-IIa–d showed 5% weight loss at 472–476 °C, which was about 50 °C lower than those of their PI-Ia–d analogues. However, –CF3 group apparently improved the optical transparency and decreased the refractive indices of the PI films. PI-IId 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.