Radiation-induced grafting processes

Radiation-induced grafting and curing processes have been discussed in a number of reviews. The process is widely used for surface modification. Recent applications are the modification of fuel cell membranes and improving 

The detailed chemistry of radiation grafting has, in most cases, not been rigorously established. Process characterization is complicated by the fact that often only surface layers are involved and, in other cases, by the substrates being cross-linked or intractable. 

Three main processes for radiation-induced grafting arc described  

These processes compete with radiation-induced crosslinking, scission and, for case (c), polymerization. 

The radiation sensitivity of polymers and monomers is characterized by a G value the number of radicals formed per 100 e.v. (16 aJ) absorbed. Radiation sensitive groups include -COOH, C-halogeii, -SOi-, -NHt and -C=C-. Radiation resistant groups are aromatic rings. It appears that the presence of aromatic moieties also offers some degree of radiation protection to the polymer chain as a whole. 

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Ionizing radiation is unselective and has its effect on the monomer, the polymer, the solvent, and any other substances present in the system. The radiation sensitivity of a substrate is measured in terms of its G value or free radical yield G(R). Since radiation-induced grafting proceeds by generation of free radicals on the polymer as well as on the monomer, the highest graft yield is obtained when the free radical yield for the polymer is much greater than that for the monomer. Hence, the free radical yield plays an important role in grafting process [85]. 

Another application in macromolecular chemistry is radiation-induced graft polymerization, by which favourable properties of two polymers can be combined. In this process, copolymers of A and B are produced by irradiation of the polymer A in the presence of the monomer B. Examples are graft polymers of polyethylene and acrylic acid or of polyvinyl chloride and styrene. The properties of textiles (cellulose, wool, natural silk, polyamides, polyesters) can also be modified by graft polymerization, for example for the production of weatherproof products. 

Processes of graft polymerization of vinyl monomers onto preirradiated films and nonwoven fabrics have been continuously operated on a commercial scale [1]. Radiation-induced grafting is still promising for the following reasons. 

Radiation-induced grafting is a process where, in a first step, an active site is created in the preexisting polymer. This site is usually a free radical, where the polymer chain behaves like a macroradical. This may subsequently initiate the polymerization of a monomer, leading to the formation of a graft copolymer structure where the backbone is represented by the polymer being modified, and the side chains are formed from the monomer (Fig. 1). This method offers the promise of polymerization of monomers that are difficult to polymerize by conventional methods without residues of initiators and catalysts. Moreover, polymerization can be carried out even at low temperatures, unlike polymerization with catalysts and initiators. Another interesting as-... 

Scherer et al. presented in 1994 a method for gamma radiation-induced graft copolymerization of styrene and acryhc acid monomers into Teflon-FEP (poly(tetra-fluoroethylene-co-hexafluoropropylene)) films with a view to develop proton exchange membranes for various applications [541]. This process oflbrs an easy control over the composition of a membrane by careful variation in radiation dose, dose rate, monomer concentration, and temperature of the grafting reaction. 

Song BJ, Ahn JW, Cho KK, Roh JS, Lee DY, Yang YS, Lee JB, Hwang DY, Kim HS (2013) Electrical and mechanical properties as a processing condition in polyvinylchloride multi walled carbon nanotube composites. J Nanosci Nanotechnol 13(ll) 7723-7727 Speltini A, Merli D, Dondi D, Milanese C, Galinetto P, Bozzetti C, Profumo A (2013) Radiation-induced grafting of carbon nanotubes on HPLC silica microspheres theoretical and practical aspects. Analyst 138(13) 3778-3785... 

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 modification or processing of a polymer is a relatively sophisticated method than conventional thermal and chemical processes. The radiation-induced changes in polymer materials such as plastics or elastomers provide some desirable combinations of physical and chemical properties in the end product. Radiation can be applied to various industrial processes involving polymerization, cross-linking, graft copolymerization, curing of paints and coatings, etc. 

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