The thermal evolution of the profile structures of one, three, and ten bilayers of arachidic acid nested between two bilayers of a disubstituted diacetylene polymer in Langmuir-Blodgett multilayer films was studied by high-resolution meridional x-ray diffraction over the temperature range of 30–80 °C. A direct correlation was found between the average, in-plane molecular chain density in an individual monolayer within an arachidic acid bilayer and the mean tilt angle for straight (all trans) chains from the monolayer normal at lower temperatures. The chains of the lower-density monolayers had a greater mean tilt angle (and a broader distribution about the mean) than the higher-density monolayers. The addition of thermal energy resulted in a two-stage process consisting of a slowly evolving, ‘‘continuous’’ untilting of the initially tilted straight (all trans) chains, extending the profile thickness of the monolayer prior to a slowly evolving, ‘‘continuous’’ decrease in the profile thickness due to chain melting via the formation of kinks and jogs. The two phenomena, the straight chain untilting and the chain shortening by formation of kinks and jogs, compete with increasing temperature over a broad range; the initially tilted, straight chains do not fully untilt before the kinks and jogs shorten the chain. Initially untilted, straight chains in the higher-density monolayers melt in a single-stage process more abruptly or ‘‘discontinuously’’ over a much narrower temperature range (via the formation of kinks and jogs). The magnitude of these thermally induced structural changes in an individual monolayer depending on the in-plane molecular chain density varies strongly with the number of melting bilayers (or dimensionality), being much greater for one bilayer versus three and ten bilayers, over the accessible temperature range for these nested multilayer samples.

• Received 19 December 1988

DOI:https://doi.org/10.1103/PhysRevB.39.12165