Polyethylene-graft-polyacrylonitrile

Oxygen free irradiation of solid polymers was also carried out in the absence of monomer in this case free radicals with relatively long life time are produced which are able to initiate the polymerization of a second monomer (56,113). Several irradiated polymers have been used as initiators of polymerization, and, evidently, if the polymerization of the second monomer is initiated only by these frozen-in radicals, the product will undoubtedly be hundred per cent grafted. Bevington using 14C-acrylonitrile was able to estimate the amount of acrylonitrile grafted on preirradiated polystyrene, polyacrylonitrile, polyethylene terephtha-Iate and nylon yams (36,37). 

Superabsorbent polymers are now commonly made from the polymerization of acrylic acid blended with sodium hydroxide in the presence of an initiator to form a polyacrylic acid, sodium salt (sometimes referred to as cross-linked sodium polyacrylate). Some of the polymers include polyacrylamide copolymer, ethylene maleic anhydride copolymer, cross-linked carboxy-methyl-cellulose, polyvinyl alcohol copolymers, cross-linked polyethylene oxide, and starch grafted copolymer of polyacrylonitrile to name a few. The latter is one of the oldest SAP forms created. 

It Is known that most radiation Initiated polymerization processes are Initiated by Che free radicals created by radlolysls of Che monomers. If a monomer or a mixture of monomers Is Irradiated In Che presence of a polymer, a graft copolymer Is formed which has different physical properties. For example. If a second polymer like polyacrylonitrile or polyvlnylldlne chloride, which possess superior barrier properties against permeation by oxygen, carbon dioxide, water vapor etc.. Is grafted to a polyolefin film like polyethylene, polypropylene, etc., the barrier properties of the composite film are greatly enhanced compared with Che polyolefin film. 

Still another type of a copolymer is one that possesses backbones composed of one individual polymer and the branches from another one. It is called a graft copolymer, because many such materials were formed by grafting the branch polymers to the polymer backbone. This, however, is not always the case and many graft copolymers were formed by polymerizing the branch copolymer from a different polymer backbone. (The subject of block and graft copolymers is discussed in Chap. 9) A graft copolymer of polyacrylonitrile on polyethylene can serve as an example ... 

Thiohydroxamic esters have also seen applicadon in grafting of polyacrylonitrile onto polyethylene, of polystyrene (PSt), polyacrylamide (PAM), and poly(N-isopropylacrylamide) onto cellulose,and of PS, PMMA, poly(vinyl pyridine), and polyacrylamide onto poly(arylene ether sulfone). The process involves derivatization of a parent carboxy-fimctional polymer to form the thiohydroxamic ester 59 with R= polymer, which then behaves as a polymeric transfer agent. 

A third variety is the graft copolymer. In this case, branches of one monomer are grown on a main stem of a previously formed polymer molecule. For example, polyethylene can be irradiated in air with gamma rays or accelerated electrons, which leave peroxides or free radicals trapped on the polymer backbone. Exposed to a reactive monomer such as acrylonitrile (CH2=CHCN), polymerization is initiated at the free-radical sites, and branches of polyacrylonitrile grow on the polyethylene crystal stem [57]. 

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