THERMAL-BEHAVIOR OF LANGMUIR-BLODGETT-FILMS .2. X-RAY AND POLARIZED REFLECTION MICROSCOPY STUDIES ON COEXISTING POLYMORPHISM, THERMAL ANNEALING AND EPITAXIAL LAYER GROWTH OF BEHENIC ACID MULTILAYERS

The structure and thermal behaviour of behenic (docosanoic) acid multilayers deposited onto silanized silicon surfaces with the Langmuir-Blodgett technique were investigated by x-ray diffraction and polarized light microscopy. Various multilayer samples were prepared from the L2, L2' and CS monolayer phase. Irrespective of the different phases of the precursor monolayers on water, all deposited multilayers showed similar Bragg peak triplets from three coexisting phases with 48.3 angstrom, 52.8 angstrom and 55.8 angstrom bilayer thickness, corresponding to tilt angles of almost-equal-to 36-degrees, almost-equal-to 28-degrees and almost-equal-to 21-degrees, respectively. The samples differed only in the relative peak intensities. The observed layer spacings exactly match the large repeat units of the three behenic acid bulk phases. None of the obtained multilayer phases equalled the structures of the precursor monolayers from which the samples were prepared. Once heated to almost-equal-to 65-degrees-C, all multilayer samples annealed irreversibly into the maximum tilted phase with 48.0 angstrom bilayer spacing; the Bragg peaks of the 52.8 angstrom and 55.8 angstrom bilayer spacings disappeared. Regardless of the annealing, the multilayers remained a film of uniform thickness, as proved by pronounced Kiessig fringes. Reflection polarized microscopy revealed a grainy morphology (grains almost-equal-to mum diameter) with locally varying brightnesses (birefringence) for the unannealed samples. In the case of the annealed samples, domains with diameters of several tens of micrometres and uniform birefringence were observed. Additional layers deposited on top of annealed (monophasic) multilayer templates were again triphasic and qualitatively not different from those directly deposited on silanized silicon wafers. Based on the experimental results, models of the multilayer architectures are presented. It is suggested that the layers consist of stacks of epitaxially grown domains of uniform bilayer spacings.