Determination of molecular orientations in Langmuir-Blodgett films by polarized Fourier transform IR attenuated total reflection and transmission spectroscopy

Abstract

New methods for evaluating the molecular orientations using polarized Fourier transform IR spectroscopy are presented. When used together, the attenuated total reflection and the transmission modes can provide detailed orientation information of simple and complex Langmuir-Blodgett (LB) systems. The methods were applied to IR dichroic ratio data of cadmium stearate, atactic poly(methyl methacrylate) and isotactic (PMMA), and rigid-rod oligoimide LB multilayer films. The chain axes of the cadmium stearate films were uniaxially oriented at 37° on average. Orientations of PMMA LB films depended on the tacticity. Whereas uniaxial orientation was found for the methylene and the carbonyl stretching transition moments in the atactic PMMA films, biaxial orientation was indicated in the isotactic PMMA films. A least-squares analysis of the rigid-rod oligoimide data suggests that biaxial orientation is more probable than the single-crystal-type orientation. Orientation polydispersity, as characterized by a gaussian standard deviation, affects the dichroic ratio significantly, and hence affects the average orientation angle deduced from the data. The average orientation angles estimated with the conventional assumption of a single tilt angle for a uniaxial system are valid when the standard deviation is less than 0.2 rad (11.5°).