A method for checking homogeneity of subsurface regions by variable angle ATR: Experiments on polymers vs. optical modelling

Abstract

The subsurface structure of many technically applied materials is characterized by alterations of the composition (swelling or surface reactions on polymers, leaching of glasses etc.). Attenuated total reflection (ATR) spectroscopy is a powerful tool for analyzing such surface regions. Because of problems with optical contact the ATR method is difficult to handle in practice. Based on variable angle ATR‐FTIR spectroscopy a straightforward method has been established which can detect compositional inhomogeneities in subsurface regions or across layers. The method works as follows: (i) making ATR measurements at two different angles of incidence without changing the sample, (ii) normalizing the ATR absorbance spectra by using an appropriate band of the substrate as an internal standard, (iii) calculating difference spectra of the normalized spectra. In the case of a homogeneous material the difference spectrum virtually vanishes whereas in the case of an inhomogeneous surface region the difference spectrum reflects the compositional gradient. The method has been tested at polyamide-12 foils: (a) chemically cleaned, (b) treated with initiator, and (c) treated with pluronic. The experimental findings have been supported by spectra modelling. Based on an optical multilayer model the ATR spectra of a homogeneous polymer and of the same material with a slightly altered surface layer have been calculated for different angles of incidence. By applying steps (ii) to (iii) the experimental results are confirmed.