X-ray diffraction study of cadmium arachidate/poly(tert-butyl methacrylate) alternating Langmuir-Blodgett film

Abstract

The layer structure of alternating Langmuir-Blodgett (LB) films of cadmium arachidate (CdAA(2)) and poly(tert-butyl methacrylate) (PtBMA) was investigated by an X-ray reflection and in-plane diffraction study. The LB films were deposited on a silicon wafer surface pretreated hydrophobically by transferring a behenic acid monolayer, and two or four PtBMA layers are inserted between each CdAA(2) bilayer. While CdAA(2) homo-LB films are known to show gradual deterioration of the transfer ratio as the number of accumulated layers increases, the alternating LB films maintain the transfer ratio near unity up to 32 layers. The layer spacing of the alternating LB films determined from the position of the Bragg diffraction peaks increases linearly by 9.9 Angstrom per PtBMA layer. The lateral lattice structure of CdAA(2) in alternating LB films maintains its hexagonal packing structure, as found in CdAA(2) monolayers, while the multilayer CdAA(2) LB film is known to change its structure from hexagonal to orthorhombic lattice. The spacing of the hexagonally packed CdAA(2) was 4.15 Angstrom, and the mean molecular area of CdAA(2) was 19.9 Angstrom(2), which is in good agreement with the value found for CdAA(2) monolayers at the air/water interface. Therefore, it appears that the polymer layers inserted between CdAA(2) bilayers in the alternating LB films effectively protect the CdAA(2) film not only from the growth of defects but also from the formation of three-dimensional crystallites of CdAA(2), which degrade the optical clarity of the LB film. The thermal stability of the alternating LB film was investigated by X-ray reflectivity over the temperature range 34-106 degrees C. The layer structure of the alternating LB film starts to phase-segregate around 70 degrees C, which is over 60 degrees C lower than the glass transition temperature of the bulk polymer. At higher temperature, Bragg peaks of the CdAA(2) bilayer spacing appear, indicating possible formation of a CdAA(2) crystal domain due to dewetting of the inserted polymer layers.