Characterization of long-periodic layered structures by X-ray diffraction III: Structure of a Langmuir-Blodgett film of cadmium arachidate at elevated temperatures

Abstract

The thermal behavior of a Langmuir-Blodgett (LB) multilayer of cadmium arachidate (CdA) was investigated by small-angle X-ray diffraction. Before melting, the LB multilayer underwent a three-step structural change: (1) from room temperature to 60°C, rearrangement of the CdA layers; (2) between 60 and 80°C, onset of the pretransitional disordering; (3) above 80°C, conformational change from trans to gauche state in the hydrocarbon chain. The 001 reflection intensity, a measure of the short-range disorder of the multilayer structure, is sensitive to these changes. In contrast, the lattice distortion parameter tends to increase monotonously up to 100°C. This indicates that the long-range order of the multilayer structure is well preserved at temperatures close to the melting point. On cooling down to room temperature after melting, the crystallites of a periodic lamellar structure grow. Their lattice distortion and dimension in the [001] direction are much larger than those of the multilayer before melting.