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IRS provides information on molecular structure based on specific frequencies associated with internal vibrations of groups of atoms in molecules using a laser in the infrared region to excite the target material and analyse the frequencies absorbed. It makes use of the fact that for polymeric materials the chemical bonds between the atoms in the polymer molecules can vibrate at frequencies in the IR range of the electromagnetic spectrum (i.e. at wave numbers from 100–4000 cm-1). The technique can be used to analyse gases, liquids and solids. Both qualitative and quantitative chemical analysis data can be obtained. It is a key tool for assessing polymer chemistry (e.g. monitoring state of cure and to determining environmental effects).
By directing IR light at a polymeric material and measuring the absorption (or transmission) of the various frequencies, it is possible to characterise, or “finger-print,” the polymeric material since it will have specific frequencies at which it is seen to absorb strongly. This “finger-print” of the various frequencies at which the material absorbs is referred to as the IR spectrum. By monitoring the IR spectrum (i.e. absorption intensities) of an adhesive during cure it is possible to determine the formation/transformation of chemical species. Change in intensities, the basis of cure monitoring techniques, gives direct information about the timescale within which the chemicals involved in the cure reaction have been used and the reaction is complete.
IRS is often used in conjunction with Raman spectroscopy as a complementary identification technique. Some quantitative analysis is possible by analysing the magnitude of the characteristic absorption peaks. It requires minimal sample preparation, but samples need to be transparent to infrared, the method is less useful for inorganic materials. Water is problematic as it obscures the spectrum. IRS is not inherently surface-specific but variants improve this aspect. One variant, attenuated total reflectance uses a prism in optical contact with the sample resulting in total internal reflection after penetrating a small way into the sample surface and is particularly useful for investigating adhesion problems and surface treatment for strong IR absorbers provided the surface is flat. Analysis is possible to within 0.5 to 3 mm of sample surface. Another variant, grazing incidence reflectance spectroscopy, uses a photon beam impinging on the surface at a high angle of incidence giving good surface sensitivity. However this technique is only applicable to metallic surfaces.
The evolution of the chemical curing process has been successfully followed using IR spectroscopy in laboratory conditions, but applying the technique into the factory environment has many potential problems. These include: the method of introducing the IR beam into the sample; calibration of the system to give stability and repeatable information; the cost of IR spectrometers and the personnel using the equipment. There are various ways of performing IR spectroscopy on a material, and probably the most widely used is known as Fourier transform infra-red or FTIR. Computerised databases of spectra for common polymeric materials are available to enable characterisation of molecular structure by observing spectral differences between known materials and the test sample.
Next: X-ray Fluorescence Spectroscopy (XRF)