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

The orientational changes in optical nonlinear active side chains of polyam ic acid alkyl amine salt monolayers before and after the imidization proces s were studied by optical surface second-harmonic generation (SHG). The pol ymeric Langmuir Blodgett (LB) monolayer composed of the triphenylamino-(p-n itrophenyl)-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-infinity v 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 t he obtained ODFs, it was found that the polar angle of the side chain incre ases by imidization and this causes the decrement of the nonlinear optical components, However the half-width (Delta theta) of the major molecular til t angle decreases by imidization, suggesting that the distribution of the c hromophore is more uniformly packed through the imidization process.