Effect of physical state during the electron irradiation of hydrocarbon polymers. Part II. Additional experiments and discussion pertaining to trapped radicals in hydrocarbon polymers

Abstract

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.

Radiation effects in polyethylene and n-alkanes

“…Already in 1958 it had been proposed by Lawton et al [85] that cross-linking in irradiated PE takes place preferentially in the amorphous phase. Salovey and Keller [86] reported the surprising observation that the gel dose for solution-grown single crystals was ten times higher than that for bulk material.…”

“…As early as the late fifties Lawton et al [85] and Koritskii et al [62] noted that the radicals decay considerably more quickly in branched PE than in linear PE. Comparing different types of PE, Charlesby et al [63] also found that alkyl radical decay at 293 K was accelerated by the increasing degree of branching, i.e.…”

“…The rate of vinylene formation is almost independent of temperature down to 77K [85,96]. The mechanism and the products of decay have not yet been fully clarified.…”

The effects of small doses of radiation on the flow and other properties of ziegler polyethylenes

“…Such radicals have been shown by Lawton and co-workers to survive for considerable periods at room temperature, during which time they can cause the polymer to oxidize and become brittle. 2 We evaluated the flow properties, physical properties, thermal stability, and light stability of the irradiated products.…”

On the Structural Analysis ofγ-Ray Induced Primary Free Radicals in UHMWPE and Vitamin E Stabilized UHMWPE by ESR Spectroscopy

“…Although there are some established methods to study the secondary products generated as a result of free radical reactions but to have clear insight on the radiation chemistry of UHMWPE, it is very important to study primary products of radiation treatment, that is, free radicals themselves. In this regard, radical analysis technique, known as electron paramagnetic resonance (EPR) or electron spin resonance (ESR), is the only and most suitable technique for the direct detection of free radicals [2][3][4].…”