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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.

The previous paper discusses the effect of physical state during the irradiation on the fate of the polymer radicals produced. Additional experiments relating to trapped radicals in hydrocarbon polymers are discussed. Three different methods of detection were used; namely, infrared absorption at 5.8 μ, electron paramagnetic resonance, and gasuptake by the irradiated polymer. Conditions for producing trapped radicals at room temperature are that the polymer exist in the crystalline, glassy, or highly crosslinked form during irradiation. Radicals trapped in the crystalline or glassy areas can dissipate either as the result of heating the polymer above Tm or Tg or else by exposure to a radical scavenger, e.g., oxygen or ethylene. The lifetime of radicals trapped in Marlex‐50 can be many thousands of hours at room temperature. Radical decay will take place even with storage in vacuum if given enough time. The decay is arrested by holding the sample at liquid nitrogen temperature. Loss in radical activity in Marlex‐50 when stored in vacuum is due mainly to delayed crosslinking. Trans‐vinylene and vinyl do not appear to enter into the reaction responsible for the decay. A comparison of the decay of radicals trapped in unbranched Marlex‐50 with that in a branched low‐density polyethylene when stored in ethylene, oxygen, and vacuum was made. The results suggest that although radicals are trapped in the crystalline regions of both polymers, there are differences between the two crystals—the crystals in the low‐density polymer do not appear to be as tight as those in the Marlex‐50. Delayed oxidation reactions occurring in the crystalline regions of irradiated Marlex‐50 when stored in oxygen at room temperature require a number of intermediate steps before appearing as carbonyl. An average of 5 molecules of oxygen per polymer radical was used in forming carbonyl. Delayed main chain scissions occur during this process which cause severe embrittlement and loss in physical properties. Significant improvements in physical properties of Marlex‐50 can be achieved through irradiation, but radicals must not be left trapped in the polymer.