Fourier-transform infrared spectroscopy FTIR is a powerful analytical technique used to characterize the chemical structure of polymers by measuring the absorption of infrared radiation by the sample. When a polymer is exposed to infrared radiation, specific functional groups within the polymer absorb the radiation at characteristic frequencies, causing molecular vibrations. The resulting absorption spectrum provides valuable information about the chemical structure, composition, and molecular interactions within the polymer.Let's consider the example of polyethylene terephthalate PET , a common polymer used in the production of plastic bottles and fibers for clothing. The chemical structure of PET contains ester functional groups, which are formed by the reaction between terephthalic acid and ethylene glycol. In an FTIR spectrum of PET, we would observe the following relevant spectral features:1. Carbonyl C=O stretching: The ester carbonyl group in PET absorbs infrared radiation at a frequency around 1710 cm-1. This strong and sharp peak is characteristic of the C=O stretching vibration in ester functional groups.2. C-O stretching: The C-O bond in the ester group exhibits a stretching vibration that appears as a peak around 1240 cm-1 in the FTIR spectrum. This peak is typically weaker than the carbonyl stretching peak.3. C-C stretching: The aromatic ring in the terephthalate moiety of PET contributes to a series of peaks in the region of 1600-1500 cm-1, which correspond to the C-C stretching vibrations in the aromatic ring.4. CH2 bending and stretching: The ethylene glycol component of PET contains CH2 groups, which exhibit bending vibrations around 1460 cm-1 and stretching vibrations around 2850-2920 cm-1. These peaks are characteristic of the aliphatic CH2 groups in the polymer.By analyzing these spectral features in the FTIR spectrum of PET, we can confirm the presence of ester functional groups, aromatic rings, and aliphatic CH2 groups, which are all essential components of the polymer's chemical structure. Additionally, FTIR can be used to monitor changes in the polymer's structure during processing, degradation, or chemical modification, providing valuable insights into the material's properties and performance.