Near-infrared spectroscopy of polymers

Although near–infrared (NIR) spectroscopy has been used over many decades for the – primarily quantitative – analysis of polymers containing OH–, NH– and CH–functionalities (e.g. determination of OH–number, water content and residual carbon–carbon double–bonds), it has never been established as a wide–spread analytical and physical tool comparable to other spectroscopic techniques. In the late seventies, however, two new developments have initiated a renaissance of NIR spectroscopy in analytical chemistry. On the one hand, chemo–metric data evaluation techniques have – in combination with diffuse reflection measurements – opened up the possibility of non–destructive, rational multicomponent analysis and identity control of solid polymers with varying morphologies. On the other hand, the introduction of optical light fibres has contributed to an enormous expansion of conventional NIR spectroscopy in terms of remote control. Thus, specific fibre–optic probes allow a separation of the spectrometer and the location of sample measurement over several hundred meters and tremendously alleviate the analysis of toxic and hazardous materials including process and reaction control. Recently, further progress is made by the development of new, rapid-scan NIR monochromator systems without mechanically moved parts, such as acousto–optic tunable filters. Last, but not least, it should be mentioned, that – although not treated here – the new approach of Fourier–Transform Raman spectroscopy with Nd–YAG laser excitation at 1064 nm is principally also a near–infrared technique.