Polymer standards for testing Fourier transform infrared spectrometers

Abstract

An ideal sample for monitoring the photometric accuracy of Fourier transform infrared (FT-IR) spectrometers should be a self-supporting solid that is stable over a period of several years. Ideally, the spectrum of this sample should also have no interference fringes and the bands of interest should be stable over a wide temperature range. Two differently processed isotactic polypropylene samples, one stress-relieved and the other extruded, mounted between two BaF2̇ windows were investigated for this purpose. The 841-cm−1 band of polypropylene has been investigated as a possible wavenumber and photometric standard, because it is sharp, is not overlapped by neighboring bands and absorbs in an atmospheric window. When measured at 2 cm−1 resolution, the band centers (as calculated by their center of mass) are 841.68 cm−1 and 840.08 cm−1 for the stress-relieved and extruded polymer respectively. When the sample temperature changes from 10 °C to 50 °C, the band shifts by 0.03 cm−1 and 0.014 cm−1 for the stress-relieved and extruded polymer respectively. The full width at half height (FWHH) of the band increases by 0.33 cm−1 for both films when the temperature is increased from 10 °C to 50 °C. Conversely, the absorbance decreases by 0.085 and 0.058 for the stress-relieved and extruded film respectively when the temperature is increased from 10 °C to 50 °C. The percent deviation of the band area is 4.69 and 2.96 for the stress-relieved and extruded polymer respectively. Another possible issue to be concerned with when using a polymer as a standard is the effect of dichroism. The intensity of the 841-cm−1 band of the stress relieved polymer varies by 4.5% when the orientation of incident polarized radiation is varied by 90° about an axis normal to the plane of the polymer sample. When two layers of the polymer are overlaid such that the backbone chains of each layer are oriented orthogonally, the intensity of the analogous band for the extruded polymer changes by only 1.2%. The results of studies on the effect of calculating the band center using a center of mass or a curve fit algorithm and the effect of band center shift due to small changes of sample position are also reported.