Strength of interaction and penetration of infrared radiation for polymer films in internal reflection spectroscopy

The strength of interaction of infrared radiation with relatively thick polymer films was systematically studied by internal reflection spectroscopy. The so‐called “effective thickness” was used as a measure of the strength of interaction. The experimental and theoretical effective thickness were compared, and it was found that the theoretically predicted effective thickness can be approached or achieved experimentally with film samples routinely encountered in the polymer industry when moderate attention is paid to surface preparation and clamping force on the film in the internal reflection sample holder. A series of curves for various types of film samples was developed and found to be useful in predicting the spectral contrast for KRS‐5 and germanium internal reflection elements. These data were collected for infrared radiation polarized parallel and perpendicular to the plane of incidence and for unpolarized radiation. The data for unpolarized radiation was checked for the effects of the inherent polarization of the Fourier‐Transform Infrared Spectrometer used by comparing it to the arithmetic mean of the data for parallel and perpendicular polarization. These data were in good agreement and this indicated that the inherent polarization of the instrument introduced only minor errors into the data collected with unpolarized radiation. The concept of the “sampling depth” was established where the sampling depth d s was defined as the depth normal to the surface sampled. The sampling depth was found to be on the order of three times the value of d p , the depth of penetration, at 45° for polypropylene and polystyrene on KRS‐5 and was observed to exceed the effective thickness values at the same conditions. The value of the electric field amplitude was observed to fall to 5% of its value at the surface at d s .