Orientation Measurements from Polymer Surfaces Using Fourier Transform Infrared Photoacoustic Spectroscopy

Abstract

Fourier transform IR photoacoustic (FT-IR-PAS) dichroism studies have been applied to determine molecular orientation from the surface of one-way drawn poly(ethylene terephthalate) film. When compared with attenuated total internal reflectance (ATR) dichroic spectra obtained with a modified, rotatable sample holder on the same sample, photoacoustic (PA) spectra show saturation effects in strong absorption bands near 1730, 1250, and 1100 cm⁻¹, probably due to the greater penetration depth in the PA technique. The dichroic ratios ( k_x/k_y) on the plane of the sample film have been calculated from PA spectra, only on bands which do not show saturation and compared with the analogous ratios determined from ATR dichroic spectra. For three well-known parallel bands at 1335, 975, and 795 cm⁻¹ the dichroic ratio is greater than unity in both ATR and PAS, as expected from the drawing process. In general, somewhat lower dichroic ratios are observed by PA spectra than by ATR spectra with this sample. Possible reasons for this trend are discussed. This study demonstrates the potential of FT-IR-PAS dichroism as a complementary technique to ATR dichroism, especially on samples with rough or brittle surfaces.