Infrared (“IR”) spectroscopy is useful in both qualitative and quantitative analysis of polymeric
materials. Modern IR spectroscopy is convenient and straightforward and can identify unknown
materials, determine the quality or consistency of a sample, monitor production and processing,
i.e., degree of crosslinking or crystallinaty, and determine the composition of a blended mixture.
The utility of IR spectroscopy comes primarily from the “fingerprint” spectral features that each
distinct compound provides. The fingerprint features are due to the various interrelated intramolecular
bending and stretching modes.
Modern IR spectrometers use broad band IR radiation, a beamsplitter, and moving mirror produce two beams with path lengths that vary periodically to give interference patterns. The Fourier transform is used in the data processing and the method is denominated as Fourier
transform infrared spectroscopy or FTIR.
Polymeric samples are usually analyzed either as a thin film or as a finely ground powder dispersed in a heavy hydrocarbon or fluorocarbon oil and placed between salt plates or mixed with potassium bromide and pressed into a pellet; both of these are known as a mull. Polymers
can also be analyzed by internal or attenuated total reflection methods.
Correlation charts are available from empirical observations of the frequency range that various
molecular components (functional groups) absorb. They are not exact, but a summary of where in the IR spectrum the molecular components will absorb and are a good starting point in identification of a polymer. A few important regions in the IR correlation chart are: hydrogen
stretching, 3700-2700cm-1; triple bond region, 2700-1850cm-1; and double bond region,
1950-1450cm-1.
Reference library spectra of thousands of common molecules/polymers are often used to obtain the “best fit” spectra. However, if the analyte polymer is not in the reference library, an exact fit will not occur. Nevertheless, it can give good information, and when added to some chemical intuition, a reasonable conclusion of the analyte can be made.
An example of a polymeric material (valve cover gasket from a diesel engine) by FTIR is shown.