Orientational changes of side chain nonlinear chromophore in polyimide Langmuir-Blodgett films

The orientational changes in optical nonlinear active side chains of polyamic acid alkyl amine salt monolayers before and after the imidization process were studied by optical surface second-harmonic generation (SHG). The polymeric Langmuir-Blodgett (LB) monolayer composed of the triphenylamino-(p-nitrophenyl)-ethylene group as a side chain of polyamic acid was prepared by the precursor method. The analysis of the experimental results showed that the symmetry of the side chain unit in the monolayer remains C_∞ν, after imidization process. Moreover, the side chain molecular orientational distribution functions (ODFs) of the polymeric monolayer before and after imidization were obtained using the modified maximum entropy method. From the obtained ODFs, it was found that the polar angle of the side chain increases by imidization and this causes the decrement of the nonlinear optical components. However the half-width (Δθ) of the major molecular tilt angle decreases by imidization, suggesting that the distribution of the chromophore is more uniformly packed through the imidization process.