Polyethylene radiation-induced structural

A BC NMR Study of Radiation-Induced Structural Changes in Polyethylene... 

The problems involved in observing resonances from gelled polyethylene were also encountered in this study. We decided, therefore, to examine polyethylene samples irradiated with absorbed doses less than the gel dose. By so doing, some of the radiation induced structural changes produced prior to gelation could be detected. 

Radiation induced structural changes in polyethylene have been investigated for over 30 years. These studies have shown that the principal chemical changes which occur during irradiation are ... 

Polyethylene is known as one of the typical polymers which crosslink under the influence of ionizing radiation. The radiation-induced crosslinking (the formation of C-C covalent bonds between polymer chains) of polyethylene has long attracted the interest of a large number of research workers, because this polymer has the most simple chemical structure for the fundamental study of radiation effects on polymers, and also because the irradiation with ionizing radiation is a practically important means of modifying the mechanical and thermal properties of polyethylene. 

The direct detection of radiation induced crosslinks in polyethylene has been a major goal of radiation chemists for many years. It was recognized as early as 1967 that solution 13c nuclear magnetic resonance (NMR) spectroscopy could be used to detect structures produced in polymers from ionizing radiation. Fischer and Langbein(l) reported the first direct detection of radiation induced crosslinks (H-links) in polyoxymethylene using 13c NMR. Bennett et al.(2) used 13c NMR to detect radiation induced crosslinks in n-alkanes irradiated in vacuum in the molten state. Bovey et al.(3) used this technique to identify both radiation induced H-links and long chain branches (Y-links) in n-alkanes... 

The concentrations of structures produced in irradiated polyethylene are on the order of 1 per 10,000 carbon atoms for absorbed doses of approximately 2.0 Mrad. Although the approach of examining polyethylenes irradiated with absorbed doses less than the gel dose placed a premium on sensitivity, we were able to detect the first direct radiation induced long chain branches in high density polyethylene (4). 

Nasef, M.M. and Saidi, H. 2004. Structure of polyethylene-graft-polystyrene sulfonic add membranes prepared by radiation-induced grafting. Int J Polym Mater. 53 1027-1043. 

Zhang, L., Zhang, W., Zhang, Z., et al. (1992) Radiation effects on crystalline polymers - I. Gamma-radiation-induced crosshnking and structural characterization of polyethylene oxide. International Journal of Radiation Applications and Instrumentation. Part C. Radiation Physics and Chemistry, 40 (6), 501-505. 

The electron irradiation of linear and branched polyethylenes causes several molecular rearrangements in the polymer structure [59]. In addition to the significant changes in the type and distribution of unsaturated groups, IR comparison of the radiation-induced chemical changes that occur in air and in a vacuum showed that oxygen has a marked influence on the structural rearrangements that occur on irradiation. 

A related study has been the elucidation of the crosslink structures induced within polyethylene by high energy radiation. The secondary carbon radicals thus produced by C—H bond scission may diffuse by hydrogen atom abstraction. They have been shown to combine in pairs to form H type junctions, and to create Y type junctions by reactions with the vinyl end groups of the chains and with primary carbon radicals produced by main chain scission. In each case the shifts characteristic of the new structure were identified [32], The shifts of the H junctions are distinct, being 41.1, 31.9 and 28.7 ppm respectively at the (CH) junction and the first and second linked carbons, as is shown in Scheme 1, but the shifts of the Y junctions coincide with those at the roots of long branches, and their formation is recognised only when a careful comparison has been made of the areas of these shifts before and after irradiation. 

FT-IR analysis of film blown thermoplastic zein plasticized with polyethylene glycol highlighted the interdependence of structure and processing. Different batches of zein powder contained different ratios of p-sheets to a-helices and the best blown films were prepared from those with largest relative a-helical content. In turn, processing increased the a-helical content and decreased the presence of ordered p-sheet regions. Cross-links induced by y-radiation in whey, casein and soya films appeared to have reduced p-sheet regions." ... 

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