Radiation grafting polyfunctional monomer additives

With radiation grafting, there is also an additional mechanism for enhancement unique to acid and not applicable to the polyfunctional monomer additives. This process is particularly relevant to irradiations performed in air and involves the acid induced decomposition of peroxy species formed radiolytically in the backbone polymer, thus generating further sites where copolymerisation may occur (Equation 3). Current evidence (17) indicates that the contribution... 

In preliminary work (19), divinylbenzene (DVB) has been reported to be a useful additive for enhancing the above grafting reactions. These early data (19) indicate that there are possible common mechanistic pathways between the acid effect and the DVB process. More detailed DVB studies are discussed in this paper for enhancing the radiation grafting yields of styrene in methanol to films of polyethylene and polypropylene. The work has been extended to include the use of other polyfunctional monomers such as tri-methylol propane triacrylate (TMPTA) as additives. The possibility of being able to use these additives for copolymerisation of monomers to naturally occurring trunk polymers such as cellulose will also be considered. 

The mechanismsof the acid effect has been extensively investigated (12-15, 21) whereas the current use of the polyfunctional monomers as enhancement additives in grafting is novel. The role of acid in these radiation grafting reactions is complicated and there is evidence that a number of pathways contribute to the overall enhancement effect. Thus mineral acid, at the levels used, should not affect the physical properties of the system such as swelling of the trunk polymer or precipitation of the grafted polystyrene chains. Instead evidence (12) indicates that the acid effect is due to a radiolytic increase in G(H) yields in the monomer-solvent system due to reactions similar to those depicted in Equations 1 and 2 for styrene-methanol. 

Comparison of Acid with Polyfunctional Monomers as Additives in Polyethylene Grafting. New additives, as well as acid, have been found to accelerate grafting in the presence of ionizing radiation. Much of this recent work has been performed with polyethylene. Only very preliminary studies of these new additives have been carried out with cellulose, however these early results suggest that these latest additives will also be very valuable for copolymerization reactions with cellulose. The more comprehensive polyethylene data will therefore be presented here to act as a guide for what might be predicted when cellulose is used as backbone polymer. 

Table VIII. Synergistic Effect of Acid and Polyfunctional Monomers as Additives for Enhancing UV and Ionizing Radiation Grafting of Styrene in Methanol to Polyethylene Film...

Although considerable work has been reported using preirradiation grafting, the present treatment will be confined to the mutual or simultaneous procedure since by this latter technique, much lower doses are needed to accomplish a particular percentage graft The simultaneous method is also amenable to the use of additives to accelerate copolymerization. The additives to be discussed in this paper include solvent, mineral acid and polyfunctional monomers for the grafting of styrene monomer to polyethylene and polypropylene films in the presence of gamma radiation ... 

In previous work, polyfunctional monomers such as divinyl-benzene (DVB) and trimethylol propane triacrylate (TMPTA) have been used to enhance the rates of radiation induced homopolymerisation of monomers such as methyl methacrylate. Since common intermediates may well exist in both radiation grafting and polymerisation processes, it is conceivable that these polyfunctional monomers when used in additive amounts ( 1%) would also accelerate radiation grafting. 

More recent preliminary studies show that the grafting results for DVB and TMPTA can be extended to other polyfunctional monomers, including tetrafunctional compounds. The enhancement observed with these additives thus appears to be a general phenomenon in radiation copolymerisation and is of value in a preparative context since the radiation dose required to achieve a particular percentage graft can be significantly reduced with the consequence that potentially less radiation damage occurs in the backbone polymer. 

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