Mixed Langmuir and Langmuir-Blodgett films of disperse red-13 dye-derivatized methacrylic homopolymer and cadmium stearate

Stable mixed monolayers of disperse red-13 dye (DR13)-derivatized methacrylic homopolymer (HPDR13) and cadmium stearate (CdSt) have been obtained and transferred as mixed multilayer Langmuir-Blodgett (LB) films. Langmuir monolayers with different mole percentages of HPDR13 were studied by surface pressure and surface potential isotherms. In contrast to the poor transferability of pure HPDR13 monolayers, mixed monolayers could be successfully transferred as Y-type LB films up to a large number of layers. Fourier transform infrared results on the mixed LB films confirmed the transfer of CdSt and HPDR13, while UV-vis spectra indicated a uniform transfer of HPDR13 during multilayer LB deposition and the possible J-aggregation of HPDR13 molecules. X-ray diffraction results were close to those of pure CdSt films, indicating the existence of CdSt domains in the mixed LB film. As expected from the presence of DR13 chromophores, nonlinear optical effects were observed. However, by using the Z-scan technique for measuring the refractive indices we were able to demonstrate that they were mainly caused by thermal effects, with no clear indication of an electronic origin. This calls for caution when analyzing nonlinear optical effects.