γ-radiolysis of acetylated cotton cellulose: An ESR study

The nature and behavior of free radicals induced in acetylated cotton celluloses irradiated with γ-rays have been studied by electron spin resonance (ESR) spectroscopy. Dehydrogenation and deacetylation appear to be responsible for the free radicals observed from samples irradiated at 77°K. The degree of substitution enhanced the yield of acetyl radicals when the samples were irradiated at 77°K and adversely affected the overall radical concentration when irradiation was done at 300°K. In addition, the ESR spectra of samples irradiated under vacuum at 300°K were more intense than those obtained from samples irradiated in air. The nature, yield, and post-irradiation behavior of the primary radicals are discussed in the light of the ultimate chemical effects observed.     

Resonance raman and infrared spectra of ClO3 radicals in electron irradiated NaClO3

Resonance Raman and infrared absorption spectra of ClO₃ radicals have been observed from measurements of crystalline NaClO₃ irradiated at 77 K with 1.5 MeV electrons. The ClO₃ radicals occupy two sets of nonequivalent sites; radicals at one site are observed from resonance scattering using 6328 Å excitation while those at the other site are observed from resonance scattering with 5145 Å excitation. The red sensitive radicals decompose at 77 K with a half-life of ≈29 min while the green sensitive radicals are more stable at this temperature.     

Comparison of radiolysis of cyclopentane, 2,3-dimethylbutane,and neopentane at 4.2 K with that at 77 K as studied by analysis of dimer products

The γ-radiolysis of cyclopentane, 2,3-dimethylbutane, and neopentane at 4.2 K was compared with that at 77 K by analysis of dimer products with capillary gas chromatography. In the radiolysis of cyclopentane, the yield of bicyclopentyl dimer at 4.2 K is lower than that at 77 K. The difference of the yields at two temperatures was explained in terms of disproportionation reaction of cyclopentyl radicals at very low temperature. In the radiolysis of 2,3-dimethylbutane, the distribution of dimer products at 4.2 K is different from that at 77 K. The results is due to the effect of phase change between two temperatures. In the radiolysis of 2,3-dimethylbutane, the fraction of unsaturated dimers in all dimer products at 77 K is much larger than that at 4.2 K. The olefinic dimer-products at 77 K is related to favorable formation of olefinic cations at 77 K. In the radiolysis of solid neopentane, addition of helium gas promotes the formation of 2,2,3,3-tetramethylbutane by a non-radical process.     

Photodegradation of cellulose acetate fibers

The photodegradation of cellulose acetate fibers by ultraviolet light in vacuo at 77°K and at ambient temperature was studied. Three kinds of light sources with different wavelengths between 2353 and 6000 Å were employed. ESR studies at 77°K show that several kinds of free radicals are produced from cellulose diacetate (CDA) and cellulose triacetate (CTA) fibers when irradiated with light of wavelength shorter than 2800 Å. Among these methyl radicals formed decayed within 210 min at 77°K. When the temperature was raised above 77°K, radical transformation occurred at 87°K and most of the free radicals decayed at 193°K, whereas the cellulosic radicals were stable at this and even at higher temperatures. Ultraviolet spectroscopy studies revealed that the main chromophores are the carbonyl function of the acetyl group and acetal groups in the polymer. The photodegradation of the polymers at ambient temperature resulted in the formation of gaseous products (mainly CO, CO₂, and CH₄), together with the loss of bound acetic acid content and sample weight. Decreases in viscosity and reduction of tensile strength and elongation were also observed in the irradiated samples, revealing that the overt effects of ultraviolet light on cellulose acetate fibers are interpreted in terms of free-radical reactions ultimately leading to main-chain and side-group scissions, unsaturation, and the formation of small molecule fragments. Among these, main-chain scission took place predominantly in CDA fiber and side-group scission in CTA fiber. The mechanism of the fundamental photochemical degradation processes of cellulose acetate fibers is elucidated.     

辐照交联聚甲基乙烯基硅氧烷的ESR谱研究

本文研究了GY-131医用级聚甲基乙烯基硅氧烷(PMVS)在辐照交联中的自由基产生和衰减的ESR潜,以及SiO₂填料对交联的影响。     

Change with temperature of the ESR spectra of peroxy radicals trapped in irradiated polytetrafluoroethylene

The ESR spectra of peroxy radicals in irradiated powders and oriented samples of polytetrafluoroethylene (PTEE) have been measured with a K-band spectrometer, and the principal values and directions of the g tensor were determined both at room temperature and at 77°K. In contrast to the spectra of the usual peroxy radicals, those trapped in γ-irradiated PTFE exhibited an ESR spectrum apparently having a larger principal value for g⊥ than for g∥ when measured at room temperature, although the normal principal values were observed at 77°K. As for the directions of the principal axes, g∥ was directed along the chain axis at room temperature and was perpendicular to the chain axis at 77°K. From the temperature change of the g tensor and the line shapes in the oriented samples, it is shown that the observed temperature change of the spectra is due to rapid rotation at room temperature around the chain axis rather than around the C? O bond axis. Assuming this, the apparent principal values of the g tensor at room temperature were calculated from the g tensor obtained at 77°K. for the rigid state, and the results are in good agreement with observations at room temperature. A structure for the peroxy radicals is also proposed. In addition, the spectral line shape function for the uniaxially oriented samples has been derived.     

Spectral Characteristics and Transformations of Intermediates in Irradiated Freon 11, Freon 113, and Freon 113a

Optical absorption bands observable in Freon 11, Freon 113, and Freon 113a irradiated at 77 K were assigned to various intermediates (radical cations, radical ion pairs, and complexes of radicals with ions). The transformations of these species in thermal and photochemical reactions occurring at 77 K were studied. On the basis of experimental results, it was suggested that the radical anions of Freon 11 and Freon 113 are unstable at 77 K and the spatial distribution of the intermediates produced is inhomogeneous.     

Formation of free radicals in photoirradiated cellulose. VII. Radical decay

The ESR spectra of untreated and photosensitized celluloses irradiated with three different ultraviolet light sources, i.e., λ > 2537 Å, λ > 2800 Å, λ > 3400 Å, at 77°K under vacuum were studied. Based on the warm-up process, that is, warming the sample from 77°K to 273°K for a certain time and recorded at 77°K, the decay behavior of free radicals of celluloses was examined for changes of the pattern and the intensities of ESR spectra. For the untreated samples irradiated with light of λ > 2537 Å and λ > 2800 Å, beside the two doublet spectra originating from hydrogen atoms (508 gauss splitting) and formyl radicals (129 gauss splitting), the observed sevenline spectrum was resolved to be a superposition of a singlet (ΔH_msl = 16 gauss), a doublet (24 gauss splitting), a triplet (34 gauss splitting), and a quartet (overall width, 88 gauss) spectrum. For the photosensitized samples irradiated with light of λ > 3400 Å, the 1:1:1 three-line spectrum was resolved to be a superposition of a singlet (ΔH_msl = 27 gauss), a doublet (43 gauss splitting), and a triplet (34 gauss splitting) spectrum. The five-line spectra of the photosensitized samples irradiated with light of λ > 2537 Å and λ > 2800 Å were resolved to be a superposition of a singlet (ΔH_msl = 27 gauss), a doublet (43 gauss splitting), and a triplet (34 gauss splitting) spectrum. Based on these findings, the conclusion was drawn that at least six kinds of spectra, generated from six kinds of radical species, were formed in cellulose irradiated with ultraviolet light under appropriate experimental conditions.     

Effects of ionizing radiation on epoxy,graphite fiber,and epoxy/graphite fiber composites. Part II: Radical types and radical decay behavior

Electron spin resonance (ESR) investigations of line shapes and radical decay behavior have been made on an epoxy based on tetraglycidyl diaminodiphenyl methane (TGDDM)/diaminodiphenyl sulfone (DDS), T-300 graphite fiber, and T-300/5208 (graphite fiber/epoxy) composites after irradiation with Co₆₀ γ-radiation or 0.5 MeV electrons. Two kinetically distinct radical species are found in the irradiated epoxy as the temperature is raised beyond 120 K following irradiation of samples at 77 K with Co₆₀ γ. One has been termed a fast-decaying species and the other a slow-decaying species. The ratio of fast-decaying/slow-decaying radicals increases as the decay temperature rises. The fast-decaying radicals at room temperature are attributed largely to alkyl type radicals residing in regions of relatively low crosslink density, while the long-lived radicals are attributed to radicals residing in the highly crosslinked regions of the epoxy. A large concentration (ca. 10₂₀ to 10₂₁ spins/g) of unpaired electrons was found in unirradiated graphite fiber which masked the ESR spectral change in irradiated composites.     

Formation of free radicals in photoirradiated cellulose and related compounds

Free-radical formation in various modifications of celluloses, namely, wood cellulose I, II, III, and IV, rayon cellulose, amorphous cellulose, cotton linters, and absorbent cotton, irradiated with ultraviolet light, has been studied by means of ESR spectroscopy at 77°K. Various types of free radicals were generated from these samples. The line shapes and the signal intensities of the ESR spectra depended greatly upon the degree of crystallinity, the lattice type, and the arrangement of molecules in cellulose. The effect of degree of crystallinity and the amount of sensitizer (Fe³⁺) absorbed revealed that photochemical reactions in cellulose occurred exclusively in the amorphous regions of the polymer. Free radicals formed in these samples behaved distinctively upon a warm-up process. Phenomena of radical migration and formation of new radicals were observed from the sensitized samples of rayon cellulose and amorphous cellulose.     

Analysis of ESR spectra of fluorocarbon radicals in irradiated tetrafluorethylene

ESR spectra in irradiated tetrafluoroethylene were studied. It was found that during radiolysis at 77 K several types of radicals are formed. The spectra of two types of radicals were established and were preliminarily interpreted as the spectra of fluorine atoms and CF₃-CF₂ radicals.     

Radiation chemistry of linear low-density polyethylene. II. Kinetics of alkyl and allyl free-radical decay reactions

Quenched and annealed samples of linear low-density polyethylene (LLDPE) were γ irradiated in vacuo at 77 K; the kinetics of the alkyl free-radical decay reactions were studied at room temperature, and of the allyl free-radical reactions at 60, 70, and 80°C. The ESR signals saturate at a slightly higher microwave power in the LLDPE than in high-density polyethylene (HDPE), and the alkyl radicals start decaying at a lower temperature in the LLDPE than in the HDPE. As in the HDPE the decay of the alkyl free radicals at room temperature in the LLDPE follows the kinetic equation for two simultaneous first-order reactions with the fraction of the faster-decaying component being slightly greater in the quenched than in the annealed samples. In the case of the allyl free radicals the decay at 60°C follows the equation based on one fraction of the radicals decaying according to second-order kinetics in the presence of other nondecaying radicals. At higher temperatures the data are best understood in terms of a second-order rate equation with a continuously variable time-dependent rate constant as suggested by Hamill and Funabashi.     

Formation of free radicals in photoirradiated cellulose. II. Effect of moisture

The effect of moisture content upon free radical formation in celluloses irradiated with ultraviolet light has been studied by means of ESR spectroscopy at 77°K. As studied from the variation of the line-shape and the relative signal intensity of the spectra observed, it is clear that moisture content in the sample greatly influences the formation of free radicals when irradiated with ultraviolet light. The moisture content in the range of 5–7% leads to a significant decrease of radical formation; and alternatively, when moisture content is lower or higher than this range, the formation of free radicals is increased.     

Radical products of γ-radiolysis of 12-crown-4 at 77 K

Paramagnetic products of γ-radiolysis of 12-crown-4 and its solutions in CFCl₃ and CFCl₂CF₂Cl at 77 K were studied by ESR spectroscopy. It was found that the ESR spectra of 12-crown-4 irradiated with γ-rays at 77 K contained superimposing signals of at least two species, the radicals resulting from macrocycle opening -?H-C(H) = O and macrocyclic radicals -O-?H-CH₂-, which are formed with nearly equal yields. It was shown that -O-?H-CH₂-radicals rapidly decayed at temperatures above 140 K. However, the -?H-C(H)=O radicals are stable almost up to the matrix softening temperature. The radical cations of 12-crown-4 are not stabilized in the matrices of Freon 11 and Freon 113, since they undergo transformation to macrocyclic radicals -O-?H-CH₂-via the deprotonation reaction.     

The oxidation and thermal transformations of macrooradicals in gamma irradiated cellulose

The secondary reactions of the oxidation and thermal transformations of gamma irradiated (at 77 K) and plasticized (with water) cellulose radicals were studied by 3 cm-and 2 mm-band EPR spectroscopy. The radiolysis of cotton cellulose was found to produce the H-C*=O formyl radical, and heating the irradiated samples to 190–200 K resulted in the formation of the ROO* peroxide radical. The EPR spectra of microcrystalline cellulose recorded at room temperature contained an individual triplet (α _β ^H = 2.5–2.7 mT) with an additional quadruplet structure (splitting 0.5–0.7 mT) from three γ-hydrogens. This triplet was interpreted as a signal of the primary radical at C4. The main direction of thermal transformations of primary radicals was synchronous reactions of the dehydration of the polycarbohydrate complex accompanied by the dissociation of the C-H, C-OH, and C-C bonds and elimination of H₂O, H₂, CO, and CO₂ with successive formation of allyl and then polyene radicals, which were a source of the growth of polyconjugated systems in macromolecules.     

On the reactivity of cellulose free radicals in graft copolymerization reactions

The formation and behavior of photo-and mechanoinduced free radicals in cellulose were studied by ESR spectroscopy and the capability of these free radicals to initiate graft copolymerization reactions was demonstrated. Although an 11-line ESR signal was detected from cellulose irradiated with ultraviolet (UV) light, a higher-intensity ESR signal with a five-line pattern was detected from a sample mechanically milled at 77 K. The decay of photoinduced free radicals when heated took place monotonously, whereas mechanoradicals exhibited an anomalous behavior with an increased signal intensity at 150 K before decaying at a higher temperature. Mechanoradicals have been found to react more efficiently and rapidly with oxygen and methyl methacrylate (MMA) than photoinduced free radicals. The peroxy mechanoradicals, however, were mobile and decayed more rapidly than the peroxy photoinduced radicals. Simultaneous graft copolymerizations of MMA to cellulose demonstrated that mechano-and photoinduced free radicals are capable of initiating grafting reactions, but a higher degree of grafting efficiency was obtained from cellulose treated mechanically.     

LIGHT-INDUCED FREE RADICALS IN ORIENTED DNA-PROFLAVINE COMPLEXES

Abstract— Oriented wet DNA-proflavine complexes were illuminated with visible light, Λ > 395 nm, at 77 K, in the presence or absence of oxygen. The electron paramagnetic resonance spectra at 77 K and low microwave power (3 μ W) indicated formation of anionic free radicals in thymine and cationic free radicals, probably in guanine, identical to those induced by y rays at 77 K in similar samples of pure DNA.
The free-radical formation rate showed a quadratic dependence on light intensity, indicating a biphotonic mechanism. The proflavine triplet spectrum was observed during illumination. If the exciting light includes wavelengths below 390 nm, significant amounts of hydrogen addition radicals in thymine are found.     

ESR study on free radicals trapped in crosslinked polytetrafluoroethylene (PTFE)

Free radicals in crosslinked PTFE which formed by ⁶⁰Co γ-rays irradiaion at 77 K and at room temperature were studied by electron spin resonance (ESR) spectroscopy. The crosslinked PTFE specimens with different crosslinking density were prepared by electron beam irradiation in the molten state. The ESR spectra observed in the irradiated crosslinked PTFE are much different from those in non-crosslinked PTFE (virgin); a broad singlet component increases with increasing the crosslinking density, G-value of radicals is much higher in crosslinked PTFE than in non-crosslinked one. Free radicals related to the broad component are trapped in the non-crystalline region of crosslinked PTFE and rather stable at room temperature, whereas radicals trapped in amorphous non-crosslinked PTFE are unstable at room temperature. It is thought that most of free radicals trapped in the crosslinked PTFE are formed in the crosslinked amorphous region. The trapped radicals decays around 383 K (110°C) due to the molecular motion of -relaxation.     

NMR and ESR studies of the solid-state polymerization of vinyl monomers. I. Acrylamide

The kinetics of the solid-state polymerization of acrylamide, γ-irradiated at 77°K, has been studied by wide line NMR and by ESR at temperatures above 300°K. This reaction may be followed by the growth of a narrow line superimposed on the NMR spectrum of the monomer. The amplitude of this narrow line has been found to be proportional to the polymer yield. By long continued annealing, the conversion yield approaches a limiting value, generally lower than 50% under most of our experimental conditions. The activation energy of post-polymerization is 19 ± 1 kcal/mole. The radiochemical yield of radicals, determined by ESR, is 0.27 ± 0.03 at 77°K before warming. In the course of annealing above 300°K, the overall concentration of radicals is reduced to about one tenth of its initial value at 77°K. However, the local concentration of the remaining radicals is constant and equal to 1.2 × 10¹⁹ spins/g, i.e., more than 100 times their overall concentration. The recombination of radicals does not seem to intervene in the kinetics of post-polymerization, so that it may be assumed that only the radicals remaining after annealing, have actually contributed to the reaction.     

Initial mode of radical pair formation in n-hydrocarbons irradiated at 1.5 and 4.2 K

The ESR spectra of X-irradiated (at 1.5 and 4.2 K) single crystals of n-decane-d₂₂, n-decane-h₂₂ and their mixtures have been measured at 1.5 and 4.2 K. It is concluded that almost all alkyl radicals are inherently formed in pairs in a fairly random fashion and that the spacial distribution of the radicals depends on the irradiation temperatures. In the crystal of n-decane-d₂₂, a 1,3-carbon intrachain radical pair is identified together with a number of very close interchain pairs which are formed between almost all kinds of the first neighboring chains. The crystal of n-decane-h₂₂ gives considerably larger spacial distribution of radicals, indicating a remarkable mass effect on the radical pair formation.