**2. Technique of infrared absorption spectroscopy**

Material testing by the technique of IR spectroscopy consists in making a spectrum of radiation energy absorbed by material molecules and interpreting the spectrum obtained. IR radiation within the wavelength range from 2.5 mm to 15 mm (the wave number from 4000 cm-1 to 666 cm-1) is selectively absorbed by material molecules and converted into their oscillatory energy. The oscillations of molecules are of various characters, connected with their chemical structure, and depend on the type of bonds (frequency increases with

© 2012 Urbaniak-Domagala, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

increasing bond energy), relative atomic weights (frequency decreases with increasing atomic weight), spatial position of atoms in a molecule, intra- and intermolecular interaction forces. During absorption, various vibration modes are generated that can be ranged with respect to energy in the following order: stretching vibration > bending vibration > oscillatory/torsional vibration. Vibration modes are active in IR only when the frequency of radiation coincides with the own frequency of molecule oscillation (resonance) and the dipole moments of molecules change in the same direction as the electric vector of IR radiation wave.

In the absorption spectroscopy techniques, IR radiation, after passing the material, where molecules selectively absorb radiation quanta, the absorption spectrum is recorded in the form of changes in the IR spectra radiation intensity as a function of radiation energy. The intensity of a beam after passing through sample (I), transmittance (T) or absorbance (A) is assumed as a measure of absorption. If the intensity of the primary incident beam on a sample is equal to I0, the relation between intensity, transmittance and absorbance is as follows: T = I/I0 , A = log (I0/I) = -log T. Energy is expressed in eV, but mostly practical parameters such as IR radiation wavelength (, nm), wave number (=1/ , cm-1) and radiation frequency (, Hz) are used to express energy.

Originally, tests and recording the IR radiation absorption spectra of samples were performed by means of two-beam diffraction spectrometers. Modern technical solutions of IR spectrometers consist in replacing the reticular monochromators with interferometers, which considerably increases the sensitivity of spectrometers (a high value of the signal to noise ratio is obtained), making it possible to shorten the spectrum recording and to obtain its good definition. Moreover, there occur the transformation and ordering of the interferogram obtained to the frequency domain by the use of Fourier Transform (FTIR). The high resolving power of spectrometer makes it possible to record complicated spectra of materials, spectra mixing, the distinction of band derived from crystalline and noncrystalline areas and performing static and dynamic tests.

The IR spectrometry technique can be used in two variants: transmission and reflection. The transmission version is used to test the effects of IR radiation absorption in the volume of sample. It is possible to test samples in any form: solid, liquid and gaseous with the use of an appropriate procedure. Gases and liquids are placed in special cuvettes with windows, made of transparent materials for IR radiation (e.g. ionic crystals: KBr, NaCl). The spectra of solids can be measured using previously prepared specimens on quartz plates, in a suspension in liquid paraffin or in the form of tablets made of KBr. If the object tested is sufficiently thin and transparent, its spectra are measured directly on a sample. The transmission technique cannot be used for materials that strongly absorb IR radiation and to test local areas of sample such as surface. In the sixties of the last century, the reflection variant was developed, so-called Attenuated Total Reflection (ATR), which makes it possible to test specific version of samples. The ATR-IR uses the phenomenon of a complete reflection during the transition of IR radiation from an optically denser medium (prism) to thinner medium (sample). A sample is placed on the IR-transparent prism surface with a refractive index being always higher than that of the sample (Figure 1). The radiation beam is directed by one of the prism wall to the prism-sample interface at angle higher than the limiting. Under these conditions, a complete reflection occurs at the internal prism side and the beam reflected comes out through the second prism wall, where the beam intensity and absorption spectrum are recorded.

86 Advanced Aspects of Spectroscopy

radiation wave.

increasing bond energy), relative atomic weights (frequency decreases with increasing atomic weight), spatial position of atoms in a molecule, intra- and intermolecular interaction forces. During absorption, various vibration modes are generated that can be ranged with respect to energy in the following order: stretching vibration > bending vibration > oscillatory/torsional vibration. Vibration modes are active in IR only when the frequency of radiation coincides with the own frequency of molecule oscillation (resonance) and the dipole moments of molecules change in the same direction as the electric vector of IR

In the absorption spectroscopy techniques, IR radiation, after passing the material, where molecules selectively absorb radiation quanta, the absorption spectrum is recorded in the form of changes in the IR spectra radiation intensity as a function of radiation energy. The intensity of a beam after passing through sample (I), transmittance (T) or absorbance (A) is assumed as a measure of absorption. If the intensity of the primary incident beam on a sample is equal to I0, the relation between intensity, transmittance and absorbance is as follows: T = I/I0 , A = log (I0/I) = -log T. Energy is expressed in eV, but mostly practical parameters such as IR radiation wavelength (, nm), wave number (=1/ , cm-1) and

Originally, tests and recording the IR radiation absorption spectra of samples were performed by means of two-beam diffraction spectrometers. Modern technical solutions of IR spectrometers consist in replacing the reticular monochromators with interferometers, which considerably increases the sensitivity of spectrometers (a high value of the signal to noise ratio is obtained), making it possible to shorten the spectrum recording and to obtain its good definition. Moreover, there occur the transformation and ordering of the interferogram obtained to the frequency domain by the use of Fourier Transform (FTIR). The high resolving power of spectrometer makes it possible to record complicated spectra of materials, spectra mixing, the distinction of band derived from crystalline and non-

The IR spectrometry technique can be used in two variants: transmission and reflection. The transmission version is used to test the effects of IR radiation absorption in the volume of sample. It is possible to test samples in any form: solid, liquid and gaseous with the use of an appropriate procedure. Gases and liquids are placed in special cuvettes with windows, made of transparent materials for IR radiation (e.g. ionic crystals: KBr, NaCl). The spectra of solids can be measured using previously prepared specimens on quartz plates, in a suspension in liquid paraffin or in the form of tablets made of KBr. If the object tested is sufficiently thin and transparent, its spectra are measured directly on a sample. The transmission technique cannot be used for materials that strongly absorb IR radiation and to test local areas of sample such as surface. In the sixties of the last century, the reflection variant was developed, so-called Attenuated Total Reflection (ATR), which makes it possible to test specific version of samples. The ATR-IR uses the phenomenon of a complete reflection during the transition of IR radiation from an optically denser medium (prism) to thinner medium (sample). A sample is placed on the IR-transparent prism surface with a refractive index being always higher than that of the sample (Figure 1). The radiation beam

radiation frequency (, Hz) are used to express energy.

crystalline areas and performing static and dynamic tests.

**Figure 1.** The schematic representation of infrared beam reflected on the crystal - sample interface in FTIR-ATR spectrometer. (on the base http://www.sprpages.nl/SprTheory/SprTheory.htm)

During the total internal reflection in the optically thinner medium (sample) is formed an electromagnetic wave, so-called evanescent wave that fulfills the condition of the continuity of electromagnetic field vectors at the interface of media with various wave refractive indices, n1 and n2 (Fornel, 2000). The IR evanescent wave has two wave vector components: parallel component to the interface of the contact between materials, under influence of which the wave propagates along surface resulting in the formation of so-called Goos-Hänchen's displacement (Goos&Hänchen, 1947), and perpendicular component, under the influence of which the wave propagates in the optically thinner medium in the direction perpendicular to the surface and exponentially disappears. The evanescent wave penetration depth, "dp", in sample depends on the IR radiation wavelength (), incident angle, (), prism refractive index, (n1), and sample refractive index in relation to the prism (n2,1) and is expressed by the following equation (Dechant, 1972):

$$d\_p = \frac{\lambda / m\_1}{2\pi \sqrt{\left(\sin^2 \theta - n\_{21}^2\right)}}\tag{1}$$

Along the path of IR evanescent wave the sample selectively absorbs energy to decrease the intensity of radiation. The weakened wave returns to the prism and then to an IR detector. There the system generates an FTIR-ATR absorption spectrum characteristic of the given sample. The FTIR-ATR absorption spectrum slightly differs from that obtained by the transmission method. The differences concern the intensity and frequency of absorption peaks characteristic of chemical groups in view of the phenomenon of reflection, e.g. Goos-Hänchen's displacement. Thus it is necessary to take corrective action that can be realized automatically. The penetration depth of IR beam can be controlled within some range by selecting an appropriate prism (selection of the refractive index) and the incident angle of beam. The commonly used prisms are made of diamond, germanium, silicon and ZnSe, whose refractive indices are equal to 2.4, 4.0, 3.4 and 2.4, respectively, and the beam penetration depths: 2.03 m, 0.67 m, 0.84 m and 2.03 m, respectively, at = 1000 cm–1 (Material Thermo Scientific Smart ITR). During testing sub-micrometric coating, the beam penetrates a higher depth than the coating depth and also passes to the substrate, on which the coating is deposited. The absorption spectrum then constitutes a superposition of the spectrum of coating material and substrate. In such cases, qualitative analysis is carried out, which takes into account the absorption spectrum of substrate.

The basic requirement for ATR technique is to place a sample in direct contact with the prism as only such conditions allow the IR evanescent wave to penetrate the sample surface layer. Moreover, there should be a considerable difference between the refractive indices of prism and sample to get the phenomenon of internal reflection occurred.

The drawback of ATR technique is a relatively low sensitivity and susceptibility to the effect of environmental conditions, which makes it necessary to calibrate the IR spectrum. Modern spectrometers have an option of automatic computer-aided spectrum correction. ATR technique has numerous advantages. FTIR-ATR shows the features of a routine method for testing the chemical and physical surface structure of materials such as polymers, films and membranes provided that these well adhere to the crystal. Tests with a modern instrumentation are characterized by a high reproducibility (better than 0.1%) (Urbanczyk, 1988). FTIR-ATR makes it possible to record spectra within a wider frequency range of IR radiation than transmission spectroscopy owing to the lack of limitations caused by the absorption of cuvette windows. An important advantage of this technique is the possibility of recording spectra *in situ* and *in vivo*, e.g. in testing biological objects and using it as a diagnostic tool in medicine.

In this work, the FTIR-ATR technique was used to analyze the surfaces of modified polymers and to test the polymeric layers deposited on substrates.
