Kinetic behaviour of a structural phase transition in Langmuir-Blodgett multilayers studied by energy dispersive x-ray reflectivity

Abstract

Various multilayers of Cd stearate were prepared by means of the Langmuir-Blodgett technique and investigated at T = Ts - ΔT (Ts, melting point of LB film). The x-ray scattering spectrum was recorded in several time intervals using an energy dispersive set up. For ΔT = 3 ... 9 K the observed intensity reduction of Bragg peaks, which corresponds to the multilayer periodicity, is recorded as a function of time. In addition new peaks appear. This is interpreted in terms of a kinetically driven phase separation of the metastable LB-phase. The time needed for phase disorder and the nucleation of the new phase decreases for smaller ΔT. The measured layer spacing of the new phase is larger than the length of a molecule but smaller than a LB bilayer. We suggest the new phase to be interpreted as a two dimensional array of cylindrical structure elements as known from bulk material. Obviously this phase transition needs the existence of an intermediate disordered state and is kinetically restricted at lower temperature.