Surface orientation of polypropylene. I. Theoretical considerations for the application of internal reflection spectroscopy

Abstract

AbstractA theoretical consideration of the orientation of the transverse magnetic and the transverse electric polarization vectors of the evanescent field in attenuated total reflection spectroscopy was made. It was demonstrated that near the critical angle of total reflection the transverse magnetic polarization vector approaches a purely normal orientation to the interface between the sample and internal reflection element surfaces, while the transverse electric vector is parallel to this interface. With an appropriate experimental design a set of three orthogonal electric vectors can be obtained for the analysis of surface orientations. A method to obtain high‐quality infrared spectra of polypropylene surfaces at a fraction of a degree above the critical angle was developed using a KRS‐5 internal reflection element specially cut with a 40° entrance aperture. The surface orientation of uniaxially extended polypropylene films was studied by using the proposed experimental design. It was shown that the orientation of the helix axis of the macromolecular chains of polypropylene could be qualitatively detected. A method employing the 841‐ and 809‐cm−1 bands of polypropylene was used to show that the ratio of absorbances of these bands reflects the surface orientation of the uniaxially extended polypropylene films for a set of films extended 0–500%.