Effect of ATR sample holder on the FT-IR spectrum of polypropylene foil

Pressing by ATR (Attenuate Total Reflection) accessory on the sample can cause reversible or irreversible material structure changes in the plastics. In the successive ATR- FT-IR spectra of the PP (polypropylene) film under constant compression, the shift of the baseline and the changes in the peak intensities can be observed, respectively. To interpret the changes in IR absorbances of the PP foil sample, a semi-empirical model is proposed by us. To separate the wavenumber-dependent and independent factors influencing the spectra, a spectrum pre-processing method consisting of baseline correction (rubber band correction) and SNV1-Anti-SNV2 transformation (SAS; SNV = standard normal variate) has been developed. With this spectrum pretreatment, the wavenumber-independent effect of the ATR sample holder on the IR spectrum can be compensated. As a result, the wavenumber-dependent effect can be uncovered. The feature of our spectrum pretreatment strategy to eliminate the wavenumber-independent effect is also supported by a mathematical deduction. The significant absorbance change induced by continuous clamping to PP foil has been confirmed by regression analysis. Correlation analysis among the spectral peaks shows strong correlations separately for both the primary and for the transformed spectra. Any peak combination of the primary spectra results in positive correlations. For the transformed spectra reflecting back only the wavenumber-dependent factor effect, the correlations between the peak intensities of the stretching and the bending vibrations are all negative. These negative correlations suggest that the compression applied in the ATR sample holder is give a rise to steric hindrance in deformation vibrations and affects the polarities of C-H valence bonds.