Neutron reflectivity study of structural changes in barium stearate Langmuir-Blodgett films during annealing

Neutron and X-ray reflectivity measurements are used to characterize Langmuir-Blodgett films composed of 20 bilayers of barium stearate. To facilitate contrast for neutron measurements, molecules with deuterated aliphatic tails were used for the depostion of every odd-numbered bilayer. Model fitting of both data sets yields values of the following: the bilayer thickness, the area per molecule, the ratio of undissociated carboxylic acid groups to Ba salts, and the degree of mixing between neighboring bilayers (30%) ocurring on deposition. Structural changes of the film during temperature annealing at 66 degrees C were studied. Interlayer diffusion, evaporation of matter, and distortion were observed, Quantitative parameters of these processes were calculated using two complementary approaches - modeling and Bragg peak analysis based on general ideas about film structure. In particular, interdiffusion reduces the intensities of different Bragg peaks differently, depending on the operative mode of interdiffusion. After 90 min of annealing, complete mixing of deuterated and hydrogenated aliphatic chains was achieved. At that time, 35% of the film had evaporated. This amount is very close to the content of undissociated acid groups.