IMIDIZATION PROCESS IN A DYE-LABELED POLY(AMIC ACID) LANGMUIR-BLODGETT-FILM STUDIED BY OPTICAL 2ND-HARMONIC GENERATION

We have studied the dye orientation in the imidization process in a La ngmuir-Blodgett (LB) film of dye-labeled poly(amic acid) (PAA) (long a lkyl)amine salt by surface second-harmonic generation (SHG). We found that the PAA monolayer at the air/water interface and on a glass subst rate is SH active and has C(infinitynu) symmetry. In situ observation of SH intensity was made during the chemical imidization process of th e PAA LB film in a mixed solution of acetic anhydride, pyridine, and b enzene. We found that the SH intensity scarcely changes during the imi dization, so that the dye in the polyimide (PI) LB film maintains the polar C(infinitynu) symmetry in the solution. However, after taking th e PI LB film out of the solution, the film loses the SH activity gradu ally as the residual benzene attached to the film evaporates. The ther mal imidization also makes the LB film SH inactive. These results lead to the conclusion that the polar dye orientation is supported by a (l ong 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 soluti on.