CHARACTERIZATION OF ULTRATHIN ORGANIC MEMBRANES BY VIBRATIONAL SPECTROSCOPY

We first deal with a new method for quantitative evaluation of the molecular orientation in thin Langmuir-Blodgett (LB) films by Fourier transform infrared (FT-IR) transmission and reflection-absorption (RA) spectroscopy. The method was applied to the study of the 7-monolayer LB film of cadmium stearate. The orientation angle of the hydrocarbon chain axis around the surface normal was found to be 7-degrees, being in good agreement with the average of the previous data obtained by other investigators. Then, we applied the same method to the study of temperature dependence of the molecular orientation in alternate LB films consisting of a phenylpyrazine-containing long-chain fatty acid and deuterated stearic acid, and in the corresponding alternate LB films of their barium salts. At the same time, pyroelectricity of these films was measured on heating the sample, and the results are discussed in connection with the molecular structure and orientation. It was concluded that the pyroelectricity is ascribed to changes of spontaneous polarization due to a structure modification of the polar groups of the constituent molecules with the highly oriented all-trans hydrocarbon chains.