Temperature-Dependent Infrared Refractive Index of Polymers from a Calibrated Attenuated Total Reflection Infrared Measurement

Abstract

We demonstrate a straightforward method by which a commonly available reference sample such as water can be used to calibrate an attenuated total internal reflection infrared absorbance measurement in order to account for the polarization of the beam incident on the internal reflecting element, and the spread of angles about the nominal angle of incidence. This enables quantitative comparison of ATR-derived absorbance data with spectra calculated from optical constants. We then apply this calibration to the measurement of temperature-dependent absorption spectra of a polydimethylsiloxane sample. We illustrate that the extracted optical constants scale with the temperature-dependent changes in the polymer density better than the raw absorbance values on vibrational resonance.

Sophisticated Attenuated Total Reflection Correction Within Seconds for Unpolarized Incident Light at 45°

“…3) in combination with Eq. 4 (Calibration Method I), 15 we used the possibility provided by our accessory-instrument combination, which allows the employment of a polarizer, and determined the polarization angle directly for the sample by Eq. 4 (Calibration Method II).…”

“…Unfortunately, the optical constant functions of water are not available for different temperatures and most ATR accessories do not allow us to measure and keep constant temperatures. This means that to determine the polarization angle φ by the calibration procedure suggested by Azam et al, 15 the standard residual sum of squares should not be used since its use implies the absence of systematic errors. Otherwise, changes in the intensities in the ATR spectra of water due to temperature changes will lead to different polarization angles resulting from the calibration procedure.…”

“…(ii) The polarization angle is then determined with a fit based on Eq. 2, Fresnel's equations, and the water optical constants according to the method of Azam et al 15…”

“…Recently, Azam et al 15 developed a technique for determining the polarization angle by comparing an experimental spectrum of water to a calculated one. By utilizing known optical constants, Fresnel's equations, and Malus' law, the polarization angle can be computed.…”