Profile structures of ultrathin periodic and nonperiodic multilayer films containing a disubstituted diacetylene by high-resolution x-ray diffraction

Abstract

High-resolution, meridional x-ray diffraction data were recorded from ultrathin Langmuir-Blodgett multilayer films containing five bilayers of either arachidic acid (AA), pentacosa-10,12-diynoic acid (a disubstituted diacetylene monomer, DAM), the blue polymeric form of DAM (DAPb), or the red polymeric form (DAPr). In addition, periodic and nonperiodic three-bilayer multilayers containing various sequences of AA and DAPb or DAPr bilayers were similarly investigated at slightly lower resolution. A Patterson function deconvolution technique provided uniquely the relative electron density profiles for the average, symmetric bilayer within the five-bilayer multilayers. The average bilayer profiles were of sufficiently high spatial resolution (≃6Å) that one could distinguish the chain carboxyl and methyl end groups and a significant difference in the relative electron densities of the polymethylene groups in the carboxyl end and methyl end side chains of the polymer. These profile features established the average configurations of the polymer side chains. A refinement technique provided the relative electron density profile of the entire multilayer such that each individual monolayer in the multilayer was distinguished. The five-bilayer multilayer lattice exhibits long-range order for AA, DAM, and DAPb: This long-range order was effectively destroyed upon conversion of DAPb to DAPr. The relative electron density profiles for the three-bilayer multilayers allowed one to distinguish the different bilayer compositions within the multilayer. It was found that the incorporation of DAPb bilayers into a host AA multilayer preserved the long-range order of the multilayer lattice, while the incorporation of a single bilayer of DAPr was sufficient to effectively destroy the long-range order.