ABSTRACT:· 4-Vinyl pyridine was grafted on polyethylene films of different thicknesses by the direct irradiation method using a Co-60 y-irradiation source. Addition of different amounts of solvents such as methyl ethyl ketone and cyclohexanone was found to decrease the swelling and the grafting extent on polyethylene. The dependence of the rate of graft polymerization on the thickness of the films was studied. The rate of grafting was found to decrease with increasing film thickness and a first order dependence was obtained. The effect of radiation dose on some of the mechanical properties of the ungrafted and grafted polyethylene was examined. It was found that low doses cause an increase in elongation due to a degradation process of polyethylene chains. At high doses, elongation tends to decrease again due to crosslinking, while the tensile load is not affected. Low grafting percents of 4-vinylpyridine on polyethylene lead to a marked decrease in elongation of the grafted polymer, with almost no change in the tensile load. Increasing the extent of grafting causes a significant increase in the tensile load and almost the same decrease in elongation as for the low grafted polymer.KEY WORDS Grafting I Solvent Effect I Thickness Effect I Tensile Load ICo-60 I Y-Irradiation I Radiation-induced graft polymerization of vinylpyridine1 on preirradiated polyethylene using yradiation and accelerated electrons has been proved to proceed by radical mechanism with a considerable rate. The grafting rate was dependent on the concentration of the polymer radicals (up to 5 Mrad dose) and on the concentration of the monomer. It was reported also that diffusion plays a certain role on the grafting rate, as the latter is dependent on the thickness of the polyethylene films. Several authors2 -6 studied the radiation-induced graft polymerization of 4-vinylpyridine onto different polymeric substrates to improve their properties: such as dyeability, ion exchange capability, and capacity to prevent blood coagulation.(diffusion of monomer into the polymer) on the extent and rate of graft polymerization.Graft polymerization of 4-vinylpyridine on polyethylene, or other polymeric substrates, is not a fully studied system. Little attention has been given to studying the effect of solvents and film thickness The aim of the present work is to study the effect of the addition of different solvents, such as methyl ethyl ketone and cyclohexanone, on the swelling and the grafting extent of 4-vinylpyridine to polyethylene. The dependence of the graft polymerization rate on the film thickness was also examined by kinetic methods. We also tried to characterize mechanically the behaviour of the grafted and ungrafted polymers by investigating the effect of dose on the elongation and the tensile load of the grafted and the ungrafted polyethylene samples. The apparent and true grafting were also determined.
ABSTRACT:The gel copolymer formed when monomer 4-vinylpyridine was grafted on polyethylene has been determined. The effect of radiation dose and film thickness on the amount of gel copolymer has been studied. Infrared and nitrogen analysis for the grafted films and the gel copolymer are given. Quaternium salts of the gel copolymer were prepared, and the melting point of the gel was determined. The insolubility of the gel copolymer was attributed to the probable formation of covalent-bond crosslinks between the grafted chains during irradiation.KEY WORDS Gel I Copolymers I Radiation I Grafting I 4-Vinylpyridine I Polyethylene IThe gel formation following the radiation grafting on polymers of acrylic monomers such as acrylic and methacrylic acids, acrylonitrile, 1 and acrylamide 2 has been reported in the literatures. Campbell and Charlesby 3 proposed that crosslinking of grafted polyethylene with acrylic acid occurs via a cationradical which forms on the polyethylene, while the graft copolymerization reaction is an ordinary free radical reaction; Singer et a/. 4 attributed the insolubility of the grafted poly(acrylic acid)-polyethylene copolymer to a high degree of intermolecular hydrogen-bonding followed by the formation of covalent anhydride bonds. They performed analytical measurements to determine the gel content by xylene extraction at 130°C and elasticity measurements at 150°C. The presence of these covalent bonds was confirmed by the infrared absorption spectrum.Joshi, Silverman, and Singer 1 studied the gel formation in the radiation grafting of acrylonitrile to polyethylene. The authors attributed the formation of covalent intermolecular links also to the postirradiation heat treatments used during coventional measurements of gel formation.In a previous publication, 5 we have studied the effect of grafting of 4-vinylpyridine on polyethylene on the mechanical properties. The results obtained show a significant increase in tensile load when the polyethylene films were highly grafted, and a drastic decrease in elongation when the same films were highly or slightly grafted.The aims of the present work are to determine the amount of the gel formed due to grafting and to study the effect of the initial film thickness on the gel formation. Quaternization of the pyridine's nitrogen in the gel copolymer was also studied. EXPERIMENTAL Materials4-Vinylpyridine produced by Fluka AG Chemische Fabric, Switzerland, of purity 95% was used with no further purification.
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