Radiation Physics and Chemistry of Polymeric Materials

Abstract

The material properties can be modified/tailored by either of the techniques available such as top-down method, bottom-up method, composite ratio variation, doping of a suitable dopant, ion beam-related methods and many others. The modifications by ion beam and radiation treatment are quite effective techniques to calibrate the physical, chemical, surface and structural properties of the materials. Polymeric materials are highly radiation sensitive and their properties can be modified by exposing the material to different ions and radiation such as gamma rays, electron and proton beams as well as swift heavy ions. The focus of the present discussion is pointed towards the radiation (mainly swift heavy ions and gamma rays) induced modification of polymeric materials and their physical and chemical aspects. The fundamental concepts of energy transfer of swift heavy ions and the post-irradiation effects such as cross-linking and chain scissoring of polymeric materials have been discussed in this chapter. The polymeric chain scissoring and cross-linking are related to the structural, chemical, surface, electrical and free volume properties of the polymers. The concept of free volume is further related to gas diffusion and separation properties of some of the polymers. The discussion is limited up to the radiation-sensitive polymers such as polymethyl methacrylate, polyethylene terephthalate and polyallyl diglycol carbonate polymers in the present chapter. The applications related to ion beam technology have been discussed in the last section of this chapter.

Appendix

The crystallite size (C.S.) can be calculated using Scherrer formula given below [126]:

$$L = \frac{k\lambda }{b\cos \theta }$$

Here, b is the full width at half maximum (FWHM) of the XRD peak (in radian), $\lambda$ is the wavelength of the X-rays used (1.54 Å in most of the cases for Cu K_α radiation), and θ is the angle which is calculated by taking $\frac{1}{2}$ of 2θ value in above equation. k is a constant of proportionality (called the Scherrer constant), and its value depends on how the width is determined and the shape of the crystal. The value of k is 0.9 for polymeric samples.

The optical band gap energy (E_g) can be calculated from the absorption spectra by extrapolating the linear portion of the plot of $\left( {\alpha h\nu } \right)^{n}$ against $\left( {h\nu } \right)$ to the energy axis. The value of $E_{\text{g}}$ is calculated using Tauc’s relation given below [127]:

$$\left( {\alpha h\nu } \right)^{n} = B \left( {h\nu - E_{\text{g}} } \right)$$

Here, $E_{\text{g}}$ is average band gap energy of the material. B in the above equation is the band tailing parameter that depends on the transition probability and can be assumed to be constant within the optical frequency range. The value of n characterizes the transition processes in K-space. Its value is 2, 3, $\frac{1}{2}$ and $\frac{3}{2}$ for direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions, respectively. Here, $\alpha$ is known as the optical absorption coefficient and its value is calculated from the absorbance (A), after correction for reflection losses using the equation: $\alpha \left( \nu \right) = \frac{2.303A}{l}$. Here, l is the sample thickness in centimetres.