Imidization Process in a Dye-Labeled Poly(amic acid) Langmuir-Blodgett Film Studied by Optical Second-Harmonic Generation

We have studied the dye orientation in the imidization process in a Langmuir-Blodgett (LB) film of dye-labeled poly(amic acid) (PAA) (long alkyl)amine salt by surface second-harmonic generation (SHG). We found that the PAA monolayer at the air/water interface and on a glass substrate is SH active and has C_∞v symmetry. In situ observation of SH intensity was made during the chemical imidization process of the PAA LB film in a mixed solution of acetic anhydride, pyridine, and benzene. We found that the SH intensity scarcely changes during the imidization, so that the dye in the polyimide (PI) LB film maintains the polar C_∞v symmetry in the solution. However, after taking the PILB film out of the solution, the film loses the SH activity gradually as the residual benzene attached to the film evaporates. The thermal imidization also makes the LB film SH inactive. These results lead to the conclusion that the polar dye orientation is supported by a (long alkyl)amine, N-icosylpyrrolidine, in the PAA LB film in air and by the solubility of the dye in the PI LB film in the imidization solution.