Configurations of the free radicals in irradiated polypropylene and the relation of their decay reactions to the molecular motion

Free radicals produced in irradiated polypropylene were studied by the electron spin resonance method. Two temperature regions in which the free radicals decay rapidly were found at around 170°K. and 260°K. The first temperature region corresponds to the γ-dispersion of polypropylene and the second to the β-dispersion. Steric configurations of the free radicals were investigated, and it was concluded that the free radicals trapped in polymer, conformation of which is appreciably twisted from the stable 3₁-helical structure, decay with small-scale motion of the matrix polymer. The decay of free radicals trapped in polymer of less twisted conformation is associated with the large-scale motion of the matrix polymer. Activation energies of decay were found to be 11 kcal./mole at the lower temperature and 48 kcal./mole at the higher temperature. Time constants of the decay reactions were compared with those for molecular motion of the matrix, with results reflecting the relations of the decay of the polymer radicals to molecular motion in the matrix.     

Influence of pressure of free radical decay in irradiated polyethylene

Information is given on the effect of high pressure on the decay of free radicals in irradiated polyethylene. The dependence of the rate constant for decay on pressure (1–13,000 atm) was determined at 80, 90, 100 and 110°. The mechanism of the effect of pressure on the rate constant and activation energy for free radical decay is discussed.     

ESR study of free radicals produced in polyethylene irradiated by ultraviolet light

ESR studies of ultraviolet-irradiated polyethylene (PE) were carried out. Irradiation effects different from those of high-energy radiation are observed. Ultraviolet radiation is absorbed selectively, and especially in carbonyl groups in PE produced by oxidation. Radicals produced were identified as \documentclass{article}\pagestyle{empty}\begin{document}$ \hbox{---} {\rm CH}_2 \hbox{---} {\dot {\rm C}} {\rm H} \hbox{---}{\rm CHO}$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \hbox{---} {\rm CH}_2 \hbox{---} {\dot {\rm C}} {\rm H} \hbox{---}{\rm CH}_2 \hbox{---}$\end{document}. Some radicals giving a quintet signal stable at room temperature were also observed but remained unidentified. The radical \documentclass{article}\pagestyle{empty}\begin{document}$ \hbox{---} {\rm CH}_2 \hbox{---} {\dot {\rm C}} {\rm H} \hbox{---}{\rm CHO}$\end{document} undergoes a mutual conversion with the acyl radical:      

Generation and decay of peroxy radicals in deformed polyethylene

The rate of peroxy radical accumulation as a function of strain at various temperatures in AC1220 high molecular weight polyethylene has been determined by EPR spectroscopy. The results of isothermal radical decay experiments are used, where appropriate, to correct the apparent accumulation rate to the actual rate. An exponential dependence of radical concentration [R], on true strain is observed at all temperatures investigated in the range from 160 to 294°K. For constant effective strain, measured from the approximate strain at which radical accumulation initiates, it is found that d[R]/de exhibits two sharp transitions as a function of temperature. One of these, at low temperature, is believed to be associated with the glass transition of the amorphous phase of the material; the other, at higher temperature, is believed to occur as a result of a change in the rate-controlling mechanism of deformation.     

Light-induced decay of free radicals in wool keratin

The kinetics of free radicals created by blue light in wool keratin are interpreted in terms of two decay processes: a first-order decay occurring spontaneously even in the dark, and another first-order decay directly proportional to the light intensity. This interpretation is supported by an analysis of previous experimental results and by more direct new experimental evidence.     

Cage effects in recombination of allyl radicals in irradiated polyethylene

A cage model has been presented to describe the kinetics of recombination of radicals in solid polymer. The theory includes Torrey's treatment for jump diffusion and radiative boundary condition in the diffusion equation to account for the hindrance to the diffusion of macroradicals and the finite cage process of recombination reaction, respectively. The result has been applied to the interpretation of data on the decay of allyl radical in irradiated polyethylene.     

Cage effects in recombination of allyl radicals in irradiated polyethylene. II. Cumulative reaction rates and decay kinetic processes

Hwang and Cheng have studied the recombination reaction of allyl radicals in irradiated polyethylene by including the effects of (i) diffusion of macroradicals by jumps of finite size in the crystalline phase and (ii) a caging reaction with a finite rate in the disordered region. In this work their results are used to analyze cumulative reaction rate data on the decay of allyl radicals in extended-chain and Marlex film polyethylene. The kinetic parameters obtained show the effects of reaction temperature, irradiation dose, and morphological differences.     

Kinetics and mechanisms of free radical decay reactions in irradiated solids

A new second order kinetic equation is given which takes into account the fraction of free radicals which cannot react in solids at the observation temperature. This corresponds to the situation when solids irradiated at liquid nitrogen temperature are then heated to a temperature where only some of the free radicals disappear by recombination. This equation is applied to data for isotactic polypropylene, polybutadiene, linear low density polyethylene, etc.     

Pressure dependence of free radical decay in irradiated isotactic polypropylene

Free radicals were generated in isotactic polypropylene by gamma-irradiation. The samples were annealed at pressures between 1 and 8000 atm and temperatures between 60 and 110°. The concentration of free radicals was estimated by the ESR method. The rate constants of free radical decay were determined for various pressures and temperatures. The rate constant of free radical decay decreases with increasing pressure while the activation energy increases. The relationship between the kinetics of molecular motion and the kinetics of free radical decay is discussed.     

Radical kinetics and characterization of the free radicals in gamma irradiated red pepper

Kinetic behaviors and characterization of the natural and γ-induced radicals in irradiated red pepper have been investigated by electron spin resonance (ESR) spectroscopy to explore the possibility of using this technique in detecting irradiated red pepper and to evaluate the eventual dosimetric features of this widely used food. Unirradiated samples exhibited a single resonance line centered at g=2.0050±0.0005. Photo-exposure of the samples was found to increase the signal intensity. An increase in temperature created a drastic decrease in the concentration of natural radicals responsible for the single resonance line. Irradiation was observed to induce increases in the intensity of single resonance line (signal I) and a radiation specific doublet and/or triplet (signal II) also centered at g=2.0050 but detectable only at high spectrometer gains. The intensities of both signals increased with increasing radiation dose. The signals I and II were found to decay with different rates depending on the temperature. The results of a fitting procedure applied to the experimental signal decay curves and those obtained from room temperature spectra simulation calculations were used together to determine radical species and their spectral characteristics giving rise to the observed experimental spectra. Four radical species, three carbohydrate and one semiquinone radical assigned as radicals A, B, C and D, respectively, were found to best explain the experimental results. All the radicals show large g and hyperfine splitting anisotropies varying between g=2.0028–2.0062 and 1.07–2.58 mT, respectively. The half lives of the radicals were found to depend strongly on temperature. The activation energy calculated using temperature dependent half-life data were the highest for radical A (33.68 kcal/mol) and smallest for radical C (11.83 kcal/mol).     

Lifetime of alkyl macroradicals in irradiated ultra-high molecular weight polyethylene

The interaction of UHMWPE with an electron beam in vacuum and in the presence of oxygen has been investigated. UHMWPE irradiated to various doses was examined with Electron Paramagnetic Resonance (EPR) and Fourier Transform infra-red (FTIR) spectroscopies. EPR was used to explore the nature of radicals produced in UHMWPE upon irradiation and to follow their decay as a function of time. Hydroperoxides formation and distribution throughout the samples were studied with FTIR spectroscopy.A correlation between the rate of decay of macroradicals and that of hydroperoxide formation has been proposed. Accordingly, the lifetime of secondary alkyl macroradicals in the amorphous phase of the polymer was found to range approximately from 3 to 10 h.