Radiation grafting acid enhancement

Comparison of Cellulose with Other Backbone Polymers (Wool, Polyethylene, Polypropylene and PVC) for Acid Enhanced Radiation Grafting of Styrene in Methanol... 

Consistent with the above acid and salt additive effects is the use of organic compounds, typified by urea, for enhancing both photografting and radiation grafting yields (Table III, XII and XIII). 

A theory for this acid effect has been developed essentially from the wool and cellulose work (3,4). Recently, in a brief communication, we reported analogous acid enhancement effects in the radiation grafting of monomers such as styrene in methanol to nonpolar synthetic backbone polymers like polypropylene and polyethylene (5). In the present work, detailed studies of this acid enhancement effect are discussed for the radiation grafting of styrene in various solvents to polyethylene. The results are fundamentally important since most of the experiments reported here have been performed in solvents such as the low molecular weight alcohols which, unlike cellulose and wool systems, do not swell polyethylene. 

The Acid Effect. The possible mechanistic role of hydrogen atoms in the current radiation grafting work becomes even more significant when acid is used as an additive to enhance the copolymerisation. At the concentrations utilised, acid should not affect essentially the physical properties of the system such as precipitation of the polystyrene grafted chains or the swelling of the polyethylene. Instead the acid effect may be attributed to the radiation chemical properties of the system. Thus Baxendale and Mellows (15) showed that the addition of acid to methanol increased G(H2) considerably. The precursors of this additional hydrogen were considered to be H atoms from thermalised electron capture reactions, typified in Equation 5. 

Acid enhancement in the radiation grafting of styrene in methanol to cellulose (4), wool (3) and in preliminary work with the polyolefins (5 6) has been proposed as being predominantly due to such reactions. 

The fact that acid enhances grafting also indicates the possibility that ionic processes may also contribute to the present grafting mechanism. In this context, acid may be considered to be a catalyst for the cationic process especially since ionising radiation is the initiator for the reaction and both free radicals and ions are known to be species formed from interaction between molecules and such radiation. However, the ionic mechanism for grafting is favoured by anhydrous conditions, thus, in the present system, acid enhancement via the ionic pathway would not appear to be a predominant process. 

Further work on the mechanism of the acid enhancement in both UV and gamma ray grafting is reported. The copolymerisation of styrene in methanol and dioxan to cellulose is used as model system. Acid enhances grafting and homopolymer formation in both radiation systems. Analysis of the homopolymer from the grafting runs indicates that acid reduces the chain length, but increases the numbers, of grafted chains. 

In the present paper, this acid effect will be compared with other recently discovered additives (19), in particular the polyfunctional acrylates, for the enhancement in radiation grafting. 

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. 

Inclusion of sulfuric acid (0.2M) in the styrene-methanol grafting solution gives an enhancement in radiation grafting with polyethylene film similar to that obtained with DVB (Table II). 

The optimum in the copolymerisation process occurs at 50% monomer concentration with both additives. The behaviour of sulfuric acid in these reactions is representative of the most reactive of the mineral acids (12). A comparison of TMPTA with H SO (0.2 M) also indicates that similar trends in enhancement in radiation grafting to polyethylene film are observed with both additives (Table III). Again, when polypropylene film is used as backbone polymer (Table IV), acid and DVB show similar increases in grafting yield with styrene in methanol, however in this system, it is interesting to note that the concentration of monomer at the position of the Trommsdorff peak does not vary for neutral and acidified grafting solutions but changes from 30% to 35% when DVB is additive. 

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. 

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... 

When MFAs such as TMPTA are Included In the monomer solution enhancement in grafting to polyethylene Is observed at certain styrene concentrations (Table IX). In the presence of both acid and TMPTA as additives, a synergistic effect In the same grafting reaction Is observed (Table IX), consistent with the UV data, thus these additive effects appear to be a general phenomenon In UV and radiation grafting processes. 

Effect of Acid as an Additive on UV and Radiation Grafting. Previous studies have shown that mineral acids can enhance the radiation grafting of styrene to cellulose under certain radiation conditions. W The results of analogous experiments in photografting are shown in Table III, and compared with the corresponding gamma ray data. In... 

Mechanistically the role of acid In enhancing gamma radiation copolymerization Involves a number of competing processes. In previous work In this field with cellulose (i6), it has been suggested that acid at the concentrations used does not markedly affect the precipitation of the grafted chains or the swelling of the backbone polymer, especially with cellulose. Instead It has been proposed that part of the acid enhancement can be attributed to a radiation chemistry phenomenon Involving increased G(H) yields and thermallzed electrons (Equations 7 to 9). These processes can lead... 

Table VII. Comparison of Acid with Polyfunctlonal Monomers as Additives for Enhancing UV and Ionizing Radiation Grafting of Styrene In Methanol to Polyethylene Film...

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