NEUTRON REFLECTIVITY STUDY OF STRUCTURAL-CHANGES IN BARIUM STEARATE LANGMUIR-BLODGETT-FILMS DURING ANNEALING

Neutron and X-ray reflectivity measurements are used to characterize L angmuir-Blodgett films composed of 20 bilayers of barium stearate. To facilitate contrast for neutron measurements, molecules with deuterate d aliphatic tails were used for the depostion of every odd-numbered bi layer. Model fitting of both data sets yields values of the following: the bilayer thickness, the area per molecule, the ratio of undissocia ted carboxylic acid groups to Ba salts, and the degree of mixing betwe en neighboring bilayers (30%) ocurring on deposition. Structural chang es of the film during temperature annealing at 66 degrees C were studi ed. Interlayer diffusion, evaporation of matter, and distortion were o bserved, Quantitative parameters of these processes were calculated us ing two complementary approaches - modeling and Bragg peak analysis ba sed on general ideas about film structure. In particular, interdiffusi on reduces the intensities of different Bragg peaks differently, depen ding on the operative mode of interdiffusion. After 90 min of annealin g, complete mixing of deuterated and hydrogenated aliphatic chains was achieved. At that time, 35% of the film had evaporated. This amount i s very close to the content of undissociated acid groups.