Comparison of Mid-IR with NIR in Polymer Analysis

Abstract

Both near-infrared (NIR) and mid-infrared (mid-IR) spectroscopies are extremely important techniques for polymer analysis because of their sensitivity and versatility. The basic difference between NIR and mid-IR spectroscopies is that bands in the mid-IR region, 400–4000 cm⁻¹, are primarily due to molecular fundamental vibrations, and absorptions in the NIR region, 800–2500 nm, are primarily due to overtone and combination bands of OH, NH, and CH functionalities. NIR lately has seen much wider application to chemical analysis due to recent advances. Mid-IR is also being more widely applied for applications such as at- or on-line analysis and quantitative analysis, due partly to the increased NIR applications and advances in mid-IR technique and computer analysis. Chemometric data evaluation techniques in combination with diffuse reflection spectroscopic measurements have allowed nondestructive, multicomponent analysis and identity control of solid polymers with varying morphologies. The introduction of optical light fibers has contributed to an enormous expansion of conventional NIR spectroscopy in terms of remote control, allowing a separation of the spectrometer and the sample measurement over several hundred meters, enabling analysis of process and reaction control. Further progress was made by the development of new, rapid-scan NIR monochromator systems without mechanically moved parts, such as acoustooptic tunable filters [l].