Spectroscopy and Scattering of Radiation by Polymers

Spectroscopy and scattering methods have always been very important in polymer science and engineering, and they constitute wide-ranging groups consisting of a variety of individual methods, and the objectives and the outcomes of the studies based on these methods are also diversified. The main spectroscopic methods include vibrational spectroscopy (infrared (IR) and Raman spectroscopy), nuclear magnetic resonance (NMR) spectroscopy and a number of other techniques such as electron spin resonance (ESR) spectroscopy, ultraviolet (UV) spectroscopy, visible light (VIS) spectroscopy and X-ray photoelectron (XPS) spectroscopy. Spectroscopy is defined as the study of the interaction between electromagnetic radiation and matter over a defined spectral range. The absorption of radiation in two different spectral regions is shown in Fig. 3.3b and c. In the case of IR radiation, absorption occurs when the frequency of radiation is the same as the frequency of a vibration (cf. Sect. 3.2.1). Some of the components of visible light are absorbed every time you look at a coloured object. Raman spectroscopy is dependent on the exchange of radiation energy and vibrational energy resulting in inelastic scattering (Fig. 3.3c). NMR and ESR spectroscopy involve the simultaneous application of a magnetic field. The wavelength (λ) of the radiation used in the spectroscopy family varies by 11 orders of magnitude from 0.1 nm to 10 m (Fig. 3.1). This broad wavelength band corresponds also to a large variation in the energy per photon (U): U = hc/λ, where h is Planck’s constant and c is the velocity of the radiation. The relationship between photon energy and wavelength is U = 1.24⋅10–6/λ, where U is given in eV and λ in meter. In XPS, U ≈ 1240 eV/photon ≈ 120 000 kJ mol–1 (1 eV ≈ 96.5 kJ mol–1), which is much greater (240 times greater) than the dissociation energy of a covalent bond, which is typically 500 kJ mol–1. UV radiation covers a wide range of energy, from 400 to 12 000 kJ mol–1, whereas the energy of visible light with wavelengths from 380 to 740 nm ranges from 315 to 160 kJ mol–1. IR covers a range between 0.75 μm and 1000 μm, but conventional IR spectrophotometers cover the mid-IR range (2.5–50 μm), which corresponds to moderate of energies, 2.4–48 kJ mol–1. IR spectra can be recorded in the near-IR range (0.75–2.5 μm) using most UV-VIS spectrophotometers. ESR and NMR instruments operate with very low energies of radiation: 0.03 m (4⋅10–3 kJ mol–1) for ESR and 1–10 m (1⋅10–4–1⋅10–5 kJ mol–1) for NMR.