An investigation of the spot profiles in transmission electron diffraction from Langmuir-Blodgett films of aliphatic chain compounds

Abstract

The profiles of the transmission diffraction spots from a Langmuir-Blodgett film of aliphatic chain compound are derived from three plausible models of molecular organisation : a polycrystalline phase with long-range orientational order; a paracrystal possessing a density of point defects which do not interrupt the lattice rows ; and a Nelson and Halperin hexatic smectic phase in the «Debye-Hückel» limit of weakly-interacting dislocations. The three resulting predictions are distinctly different. Experimental results are presented for the room-temperature diffraction patterns from Langmuir-Blodgett films of a lipid, DMPE, and the cadmium soap of a fatty acid, 22-tricosenoic. Only the Nelson and Halperin theory gives a satisfactory fit, although there are deviations attributable to paracrystal-type defects or to non-ergodic behaviour of the layer. Dislocation densities of between 10-4 and 10 -5 per molecule are deduced