ESR Studies of Free Radical Decay in Irradiated Polyethylene

Abstract

A study has been made of the temperature and rates at which molecular hydrogen is evolved and free radical decay occurs when semicrystalline polyethylene, irradiated at liquid-nitrogen temperature, is warmed to room temperature. Hydrogen evolution and free radical decay roughly parallel each other. The temperature of half reaction is the same as the temperature of the gamma-maximum in the mechanical dispersion temperature curve. At constant temperature over the temperature range —71° to 41°C, the free radical decay is first order and the free radical concentration seems to decrease to a limiting value at each temperature. The first-order decay constants are approximately constant with temperature. The data are interpreted in terms of free radicals decaying in spurs. If a constant activation energy of 15 kcal mole—1 is assumed, the frequency factors decrease with rise of temperature. This decrease is interpreted on the basis of a decreasing probability of reaction for those free radicals that escape reaction at low temperatures, but react at higher and higher temperatures. An alternative model based upon the release of free radicals from traps as the temperature is raised is also discussed. The reaction rate constant for free radical decay calculated from D—H exchange experiments agrees with that deduced from the ESR measurements. It is also concluded that the persistent free radical in polyethylene irradiated to moderate doses is the allyl free radical.

ESR Spectroscopy in Polymer Research

“…Salovey and Yager 1891 ) observed the overlapping six-line and ten-line patterns of the ESR spectra from irradiated solution-grown crystals of polyethylene. When polyethylene containing free radicals was heated to room temperature, a substantial decrease in radical concentration was observed by several authors100, 104,486,568,569,1084,[1309][1310][1311]1191,1388,1574,1612,1638,1688,1936,1937,2045,2049,2391,2392), Free radicals present in polyethylene have different lifetimes. Sohma et al 1982Sohma et al , 1988 have recorded the variation of the alkyl radical (5.3) spectra at 77 K as a function of the angle (1/1) between the direction of magnetic field and the c-axis of polyethylene crystals (Fig.…”

Dihydrogen Catalysis: A Remarkable Avenue in the Reactivity of Molecular Hydrogen

“…An intermediary H2-RT-reaction mechanism involving a combined-stepwise relay transport of H-atoms (sH2-RT) has been hypothesized more than half a century ago to explain the transport of free valence in irradiated polymers (74)(75)(76)(77)(78)(79)(80)(81)(82)(83)(84)(85)(86)(88)(89)(90)(91)(92)(93)(94)(102)(103)(104)(105)(106)(108)(109)(110)(111)(112). It can be regarded as an alternative to the H2-RT-reaction (2.1.1) for solid state systems with rigid long-range order.…”

“…Downloaded by [George Mason University] at 02: 43 28 September 2014 The catalytic behavior of dihydrogen in the decay of free radicals in irradiated (or treated thermally) polyethylene (PE) has been recognized and experimentally confirmed in the early 1960 th (14,(77)(78)(79)(80)(81). It has been suggested that it involves a cascade of (relay) H-transfer acts (14,80,82,83). The migration of the free-valence (H-atoms) was explained to occur via a mechanism called "relay transfer of valence" (see, e.g., Emanuel and Buchachenko (14) and Ungar (81)).…”

The effect of irradiation, annealing temperature, and artificial aging on the oxidation, mechanical properties, and fracture mechanisms of UHMWPE

“…Because the annealing was performed below the melting temperature of the material, only the radicals present in the amorphous phase were annihilated, whereas those present in the crystalline were not. These free radicals could later on, during artificial aging, diffuse out of the crystalline phase via a proton exchange mechanism 5 and become available for oxidation to take place. As for the high temperature annealing, the two hours duration of the treatment may not have been long enough to melt all the crystals in the material, especially the ones in the core of the UHMWPE rods.…”

“…Annealing at high temperature allows the crystals to melt and with this procedure, any free radicals trapped into crystalline regions are released. 5 This treatment can be performed either in a vacuum or in an inert atmosphere, to suppress any oxidation reaction that can also be accelerated by the annealing. Because oxidation is dependent on oxygen, the diffusion rate of the latter is an important factor.…”