Thermally induced order-disorder transitions in Langmuir-Blodgett films

Structural studies of Langmuir-Blodgett (LB) monolayers at elevated temperatures reveal the onset of structural disorder. A two-step melting process in LB monolayer assemblies of cadmium arachidate has been observed by transmission and grazing incidence reflection IR spectroscopy in conjunction with Raman spectroscopy and thermal measurements. Prior to the melting point, a pretransitional disordering of the hydrocarbon tails was observed, followed by a catastrophic break-up of the cadmium lattice at a temperature corresponding to the melting point as measured by differential scanning calorimetry. This thermal behavior has been modified by the incorporation of aromaticity into the molecular head group. Our IR studies of the thermal behavior of the cadmium salts of tetradecyl benzoic acid and 2-nitro-5(N-methyl, N-octadecyl) amino benzoic acid as LB films indicate that the introduction of aromaticity into the head group improves the high temperature stability of the films. We attribute this behavior to the enhanced intermolecular interactions between the head groups. © 1985.