Grafting radiation-induced techniques

The grafting of entirely different monomer on the polymeric backbone can be performed by different pathways of generation of the free radical sites (Fig. 3.3) on the preformed polymer. These techniques include chemical/conventional or radiation-induced techniques for grafting. 

The creation of active sites as well as the graft polymerization of monomers may be carried out by using radiation procedures or free-radical initiators. This review is not devoted to the consideration of polymerization mechanisms on the surfaces of porous solids. Such information is presented in a number of excellent reviews [66-68]. However, it is necessary to focus attention on those peculiarities of polymerization that result in the formation of chromatographic sorbents. In spite of numerous publications devoted to problems of composite materials produced by means of polymerization techniques, articles concerning chromatographic sorbents are scarce. As mentioned above, there are two principle processes of sorbent preparation by graft polymerization radiation-induced polymerization or polymerization by radical initiators. We will also pay attention to advantages and deficiencies of the methods. 

Radiation-induced grafting in its simplest form involves heferogeneous sysfems, with the substrate being film, fiber, or even powder and fhe monomer fo be graffed onto fhe subsfrafe a neaf liquid, vapor, or solufion. 4i Currently, three main radiation grafting techniques are known ... 

Armstrong, A. A., and H. A. Rutherford A vapor phase technique for radiation-induced grafting of vinyl monomers to fibers. Text Res. J. 33, 264... 

A radiation-induced grafting technique has been commercialized for the production of ion-exchange membranes as separators of batteries, hydrophilized hollow-fiber membranes for microfiltration of protein solutions, and ion-exchange nonwoven fabric for the removal of trace amounts of gases from ultraclean rooms [1,2]. 

Excitation sources for the production of radicals in grafting include chemicals, light, plasma, and radiation. Radiation-induced graft polymerization is superior to other grafting techniques because the high density of electron beams and gamma rays can create a large amount of radicals of arbitrary shapes of the p>olymer, such as a hollow fiber [2-41], nonwoven fabric [42] and film [43- ], and the quality of the polymer, such as polyethylene [2-41], polytetrafluoroethylene [42], and cellulose [55]. 

Radiation-induced graft polymerization can be classified into two techniques in terms of irradiation opportunity (1) simultaneous grafting, i.e., the mixture of a trunk polymer and a monomer is irradiated, and (2) preirradiation grafting, i.e., a trunk polymer is previously irradiated and then brought into contact with the monomer [56]. From a practical viewpoint, the preirradiation technique is preferable because of the negligible formation of homopolymers and easier control of the degree of polymerization. 

Another method of modifying dense membranes is by means of grafting (e.g. radiation-induced grafting, see chapter II). This method allows a number of different kinds of groups to be introduced into the polymer, resulting in membranes with completely different properties [14]. A representation of this technique is given in figure III -16. 

VF can be polymerized by a number of techniques, including suspension, bulk, and emulsion polymerizations in batch and continuous modes. Graft and radiation-induced polymerizations of VF have also been reported. 

In recent years, the radiation-induced graft copolymerization techniques have received considerable interest. The accelerated electrons have sufficient energy to induce cleavage of the chemical bonds in the polymeric structure to form macromolecule radicals which subsequently initiate graft copolymerization. 

UV and radiation induced polymerisation techniques are shown to be useful processes for enhancing the range of available renewable resource materials. In particular, grafting reactions have been examined as a means for modifying polymer structures. 

As the third technique for polymer modification, radiation-induced graft polymerization has been extensively used since the starting material polymer not only acquires required properties but also retains most of its inherent characteristics. 

Abstract Radiation-induced graft copolymerization is an attractive technique to prepare alternative proton conducting membranes (PCMs) for fuel cell applications. The purpose of this chapter is to review the latest progress made in the development of various radiation-grafted PCMs for fuel cells. The challenges facing the development of these membranes and their expected future research directions are also discussed. 

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