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

The subsurface structure of many technically applied materials is characterised by alterations of the composition (swelling or surface reactions on polymers, leaching of glasses etc.). The same situation holds for coated materials where graded layers are more or less typical with the gradient strongly influencing the materials properties. The attenuated total reflection (ATR) spectroscopy is a powerful tool for analysing such surface regions. Because of problems with optical contact the ATR method is difficult to handle in practise. 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 sample, (ii) normalising the ATR absorbance spectra by using an appropriate band of the substrate as an internal standard, (iii) making difference spectra of the normali sed 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. Clearly, the method is only applicable to samples and ATR configurations where the penetration depth for both angles of incidence is higher than layer thickness. Furtheron, the substrate band used as an internal standard must be within the linear absorbance range and should exhibit a low structural sensitivity. The method has been tested at polyamid -12 foils: (a) coated by methacrylate layers, (b) plasma treated, and (c ) leached by a surface treatment with solvents. More detailed results are presented. 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 and the two pola risations. By applying steps (ii) to (iii) the experimental results are confirmed. In conclusion, the test method show some significant advantages in comparison with common ATR handling techniques: The method can be applied on one test specimen, there is no need for any reference material. The measurements are carried out by only changing the angle of incidence, without any additional sample preparation or optical re-adjustments