Polarization modulation infrared linear dichroism (PM-IRLD) is often used for measurements of molecular orientation with high sensitivity and good time resolution. However, PM-IRLD is unable to provide the structural absorbance spectrum because it does not measure separately the parallel and perpendicular spectra. Here we propose a new method, named polarization modulation infrared structural absorbance spectroscopy (PM-IRSAS), to overcome this limitation of PM-IRLD. PM-IRSAS measures the dichroic difference and structural absorbance spectra simultaneously and, therefore, allows quantitative analysis of molecular orientation and conformation with 200 ms time resolution. The PM-IRSAS method was first validated through comparison with conventional polarized FT-IR spectroscopy using drawn polymer films. Second, it was demonstrated that the PM-IRSAS method can provide a quantitative analysis of dynamic orientation and conformation changes in PET films during deformation and crystallization processes.
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