Mohamed Mohamady Ghobashy

By Mohamed Mohamady Ghobashy

Ionizing radiation can induce some kinds of reactions, other than polymerization, such as dimerization, oligomerization, curing, and grafting. These reactions occur through a regular radical chain causing growth of polymer by three steps, namely, initiation, propagation, and termination. To understand ionizing radiation-induced polymerization, the water radiolysis must be taken into consideration. This chapter explores the mechanism of water molecules radiolysis paying especial attention to the basic regularities of solvent radicals’ interaction with the polymer molecules for forming the crosslinked polymer. Water radiolysis is the main engine of the polymerization processes, especially the “free-radical polymerization.” The mechanisms of the free-radical polymerization and crosslinking will be discussed in detail later. Since different polymers respond differently to radiation, it is useful to quantify the response, namely in terms of crosslinking and chain scission. A parameter called the G-value is frequently used for this purpose. It represents the chemical yield of crosslinks, scissions and double bonds, etc. For the crosslinked polymer, the crosslinking density increases with increasing the radiation dose, this is reflected by the swelling degree of the polymer while being immersed in a compatible solvent. If crosslinking predominates, the crosslinking density increases and the extent of swelling decreases. If chain scission predominates, the opposite occurs. A further detailed discussion of these aspects is presented throughout this chapter.

Part of the book: Ionizing Radiation Effects and Applications

By Mohamed Mohamady Ghobashy

Superabsorbent hydrogel (SAH) is a cross-linked polyelectrolyte polymer that has the capability to absorb a lot of water by keeping it in a three-dimensional (3D) structure. The network’s structure of SAH has the high elasticity that gives the ability of pores to expand in an aqueous media into up to 150–1500 times their own size in a dry state. The size of pores is the major factor that controls the swelling degree of the hydrogel. In contrast, the swelling degree is related to cross-linked density and the number of polarizable functional groups that immobilize on the polymer backbone. The hydrogels could be made by radical-initiated polymerization of hydrophilic monomers, and/or linear polymers dissolve in an aqueous solution. Free radical polymerization of the hydrogel can be done physically or chemically. Advantages and disadvantages of each method will be elaborated in this chapter. The advances in radiation cross-linking methods for the hydrogel preparation are particularly addressed besides other different techniques, e.g., (freezing/thawing and chemical initiation). This chapter will review the preparation methods of superabsorbent hydrogels from synthetic and natural hydrophilic polymers with other new phases such as wax, gum, and rubber. Methods to characterize these hydrogels and their proposed applications (internal curing agent for cement, agricultural proposal, biomedical proposal, and environmental proposal) are also reviewed.

Part of the book: Hydrogels