Structure and dynamics in a Langmuir-Blodgett film at elevated temperatures

By a combination of waveguide Raman scattering, scanning microellipsometry and interference contrast microscopy some new structural details of Langmuir-Blodgett (LB) multilayers of cadmium arachidate on glass substrates have been determined. Raman measurements gave orientation and order as a function of temperature. Spectral changes, similar to those in the bulk, could be attributed to a progressive increase in librations of the hydrocarbon chain as the temperature was increased. Scanning microellipsometry on bilayer steps gave an upper limit for the lateral diffusion (D≤10-10 cm2 s-1), indicative of a solid phase at a temperature as high as 100°C. The optical thickness of a given multilayer decreased considerably on a time scale of 10 min, after an instantaneous temperature rise to 100°C. The initial loss of material apparently reflects the creation of voids needed to provide the additional space for the hydrocarbon chains to disorder. It was followed by a slower constant thermodesorption on a time scale of hours. This picture was supported by observations with an interference contrast microscope. Although films remained homogeneous upon heating to about 95°C, cracks were formed upon cooling to room temperature, probably a consequence of some ablation of chains at higher temperatures. Interestingly, the cracks were oriented mainly perpendicularly to the dipping direction, indicating some induced lateral structure. © 1988.