Effect of physical state during the electron irradiation of hydrocarbon polymers. Part I. The influence of physical state on reactions occurring in polyethylene during and following the irradiation

Abstract

Abstract Measurements of crosslinking, trans ‐vinylene formation, and hydrogen evolution have been made on high‐ and low‐density polyethylenes irradiated at temperatures from about −170 to +240°C. Crosslinking produced during irradiation is mainly in the amorphous areas of the polymer; whereas trans ‐vinylene unsaturation is produced as readily in the crystalline areas as in the amorphous areas. Radicals are trapped in the crystalline regions. These can react with oxygen to form carbonyl groups, rather than crosslinks. Thus, in Marlex‐50 irradiation in the crystalline state at 25°C. followed by a 7 day storage in air prior to the crosslinking measurements, was only about one‐sixth as effective in producing crosslinks as irradiation in the amorphous state at 150°C. In the low‐density polyethylene where less trapping occurs the factor is about one‐half for the same conditions of irradiation and storage. The lifetime of radicals trapped in Marlex‐50 when stored in oxygen can be thousands of hours compared with a few hours in low density polyethylene. Both the low‐density polyethylene and Marlex‐50 irradiated in the crystalline state at 25°C. (in nitrogen) could be further crosslinked by quickheating them to the amorphous state at 150°C.—delayed carbonyl formation does not occur in this case. Delayed crosslinking resulting from the quick‐heating of irradiated Marlex‐50 gave only about one‐half of the crosslinks for the same irradiation as when irradiated in the amorphous state. Hydrogen evolution as a function of irradiation temperature for Marlex‐50 is discussed; a satisfactory material balance was obtained in which the radiation yield ( G value) for total hydrogen produced nearly equaled the sum of G for crosslinking and G for trans ‐vinylene formation. Hydrogen yield was found to be nearly constant in the temperature range −170 to +34°C. where crystallinity is also nearly constant, indicating hydrogen production to be nearly independent of temperature. Therefore, the main change in crosslinking yield in the +34 to +150°C. range is considered to be due almost entirely to change in crystallinity. A comparison method for determining relative values of the degree of crosslinking fom the gel measurements is described.