Formation of free radicals in photoirradiated cellulose. I. Effect of wavelength

Formation of free radicals in photoirradiated cellulose has been studied by means of ESR spectroscopy at 77°K. Three kinds of light sources with different wavelengths between 2500 and 4000 ÅR were employed. No radicals detectable by ESR were recorded when cellulose was irradiated with light of wavelength longer than 3300–3400 ÅR. Hydrogen atoms that generated a doublet spectrum (ΔH = 508 G) were observed when cellulose was irradiated with light longer than 2800 ÅR. Hydrogen atoms and formly radicals that generated doublet spectra with splitting constants of 508 and 129 G, respectively, were observed when irradiated with light shorter than 2800 ÅR. The scission of the polymer chain in cellulose is evident from decrease of the degree of polymerization, and the results of mass spectrometric analysis indicated H₂, CO, CO₂, and H₂O to be the main volatile products of cellulose upon photoirradiation.     

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.     

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.     

Photochemistry of fiber-forming polymers. II. Ketone-containing poly(ethylene terephthalates)

Poly(ethylene terephthalate) copolymers were prepared containing minor amounts of comonomers containing backbone and side-chain ketone groups. The photochemistry of these polymers was studied at 313 nm, both in film and fiber form. It was demonstrated that only the comonomer, dimethyl-γ-ketopimelate (DMKP), gave a copolymer with enhanced sensitivity to ultraviolet degradation in air. Since this sensitivity is reduced in a nitrogen atmosphere, it is suggested that the mechanism involves main-chain scission to form two free radicals by the Norrish type I mechanism, and that recombination of these radicals is reduced by reaction with oxygen. The use of copolymers of this type provides a method for the controlled photodegradation of polyester fibers.     

Free radical-induced oxidative degradation of polyacrylamide in aqueous solution

The oxidative main chain degradation of polyacrylamide initiated by ^·OH radicals attacking the polymer in aqueous solution was studied. ^·OH radicals were produced by irradiating dilute polymer solutions with high energy radiation. A bimolecular process (combination of PO₂ radicals) was found to be the rate determining step in the series of consecutive reactions leading to main-chain rupture. This was revealed from results obtained in pulse radiolysis studies using the light scattering detection method. Under the given experimental conditions, the number of radical sites per initial macromolecule exceeded unity with the consequence that intramolecular reactions of PO₂ radicals dominated intermolecular combinations. From both pulse radiolysis and continuous irradiations it was inferred that only a small fraction (about 1%) of the attacking ^·OH radicals initiated main-chain scission.     

ESR studies of photosensitized degradation of poly(L‐lactic acid) via photoionization of dopant

The photosensitized degradation of poly(L ‐lactic acid) (PLA) via an anionic reaction process was studied using spectrophotometry, electron spin resonance (ESR), and gel permeation chromatography (GPC) measurements. PLA film doped with N,N,N′,N′‐tetramethyl‐p‐phenylenediamine (TMPD) was irradiated at 77 K using UV light (λ_c = 356 nm) by which the PLA matrix itself cannot be directly excited. After photoirradiation, a new broad absorption band appeared over the original spectrum due to TMPD⁺ ·, which was produced by two‐photon ionization. The ESR spectrum of the irradiated sample indicated the presence of the TMPD⁺ · radical and main‐chain scission radical of PLA. During the thermal annealing at 0 °C, the latter radical changed to another radical species by dehydrogenation of the alpha hydrogen of the PLA main chain. TMPD⁺ · was extremely stable at room temperature for 7 d. However, by thermal annealing at 40 °C, all the radicals decayed due to the enhanced molecular motions near T_g of PLA (58.7 °C). Spectral simulation for the obtained ESR spectra revealed the relative amounts of four radicals: TMPD⁺ ·, a main‐chain scission radical, a main‐chain tertiary radical, and an unknown radical. The last one was tentatively assigned to the PLA radical anion because of its short decay time. GPC measurements clearly indicated a decrease in the molecular weight of PLA after irradiation. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 706–714, 2001     

On the kinetics of polymer degradation in solution

Poly(2,2,2-trichloroethyl methacrylate), PTCMA, was irradiated in dilute dichloromethane solution with 16 MeV electron pulses of^{6 0}Co γ-rays. Both in the presence and absence of O₂, G(S)=9–10 was found at [PTCMA]<9 g/dm³. Indirect and direct radiation effects contributed to main-chain scission at a ratio of 7∶3. In contrast to earlier findings with dioxane solutions, at concentrations of up to 20 g/dm³ indications of crosslinking were not obtained. With CH₂Cl₂ solutions two modes of light scattering intensity (LSI) decrease were detected indicating the existence of two different macroradicals, one generated by the direct and the other by the indirect action of radiation. These radicals decayed with rate constants of 1.0·10³ s^?1 and 1.4·10¹ s^?1 and also reacted with C₂H₅SH with rate constants of 8·10⁴M^?1·s^?1 and 1·10³M^?1·s^?1, respectively. In O₂-saturated solutions both radicals formed peroxyl radicals, PO₂, wich combined generating products whose decomposition involved main-chain scission. The combination of PO₂-radicals was rate-determining in the consecutive series of reactions as inferred from the 2nd order decrease of the LSI.     

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.     

Formation of free radicals in photoirradiated cellulose. III. Effect of photosensitizers

The effect of several photosensitizers on the formation of free radicals in cellulose has been investigated. Radicals were generated with ultraviolet light, and examined by means of ESR techniques; irradiations were carried out at 77°K. On irradiation of the sensitized celluloses with light of λ > 3400 Å, samples treated with benzoyl peroxide, hydrogen peroxide, benzophenone, riboflavin, azobisisobutylonitrile, and metallic ions Cr²⁺ and Ni²⁺ exhibited single-line spectra with line widths of 16 to 26 gauss. Samples treated with Pb²⁺ and Fe³⁺ exhibited three-line and five-line spectra, respectively. On irradiation with light of λ > 2537 Å, most of the sensitized samples exhibited five-line spectra with different relative signal intensities. Among these, samples treated with Fe³⁺ manifested the most prominent spectra with the strongest relative signal intensities.     

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.     

Formation of free radicals in photoirradiated cellulose. V. Effect of temperature

ESR spectra of purified and ferric ion-sensitized celluloses irradiated with ultraviolet light in vacuo at 45, 20, ?80, and ?196°C were recoreded and compared. Generally, several kinds of spectra, viz., singlet, three-line, five-line, and seven-line spectra, were observed. At higher temperatures, only singlet and three-line spectra of stable free-radical species were detected, whereas at lower temperatures such as at ?196°C, two doubled spectra of formyl radicals and hydrogen atoms were also detected in addition to cellulose radicals. It is believed that the intricate spectra observed at low temperatures are superimposed upon spectra generated by free radicals which may or may not be stable at high temperatures. During reirradiation at ?196°C with an alternative light sources, i.e., λ > 2537 Å and λ > 3400 Å, of samples which were irradiated at 20°C or at ?196°C, phenomena indicative of radical transformation and formation of new radicals or of decay of radicals in terms of ultraviolet bleaching were observed on studying the changes of line-shapes and relative signal intensities of the spectra.     

Photoinduced recombination reaction of trapped electrons and macroradicals in radiolyzed cellulose

Light-induced reaction of photorecombination of trapped electrons and free radicals (mainly, the C₁-centered radical) has been detected in cellulose γ-irradiated at 77 K. The dark process of annealing of trapped electrons in γ-irradiated cellulose occurs in the range of 77–170 K without participation of alkyl radicals and includes only the recombination of electrons with cations. The action of light on the radiolyzed cellulose increases the yield of carbon dioxide (upon heating up photo-bleached samples) by a factor of 2–2.5 as compared to the dark process. It is supposed that the reaction of photorecombination of trapped electrons and free radicals follows the chain mechanism: at doses of preliminary irradiation up to 100 kGy, the decay of one electron accompanied by disappearance of 4 to 20 alkyl radicals and evolution of up to 25 CO₂ molecules.     

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.     

Efficiency of radiation-induced main-chain scission of poly (methyl methacrylate) depends on the irradiation temperature because of coexisting monomer

Effect of irradiation temperature on the main-chain scission of poly (methyl methacrylate) (PMMA) caused by γ-irradiation was studied by means of gel permeation chromatography and ESR spectroscopy. Although no temperature dependency was observed on the scission efficiency for purified PMMA, the efficiency for crude or monomer-doped purified PMMA was decreased by decreasing the temperature below ca. 200 K. Above 200 K the efficiency was constant and did not depend on the purity of PMMA. ESR study of the irradiated PMMA revealed that the suppression of the scission below 200 K is induced by the addition of methyl methacrylate monomer to primary radical species, which otherwise cause the main-chain scission by warming the polymer above 200 K. The primary radical generated above 200 K immediately converts to the scission-type ? CH₂ ? ?(CH₃) COOCH₃ radical through the β-scission of the polymer main chain, so that the efficiency of the scission does not depend on both the impurity and the irradiation temperature. © 1994 John Wiley & Sons, Inc.     

Photochemistry of ring substituted polystyrenes—Part III: Photolysis of poly(para-methoxystyrene)

The photodegradation of films (4 × 10^?4 cm thick) of poly(p-methoxystyrene) with 254 nm radiation under high vacuum at 25°C has been studied. The principal gaseous product is hydrogen, but smaller quantities (in decreasing yield) of methane, methyl alcohol and ethane are also formed, indicating that fission of bonds in the para methoxy group is also involved. Ultraviolet and visible spectra of degraded films indicate the presence of unsaturated groups and of coloured species. Solubility data indicate that crosslinking and chain scission occur simultaneously. Rates of chain scission of a number of p-substituted styrenes are compared and a reasonable correlation between these and the electron donating character of the para group is observed, explainable in terms of the stabilising effect of such groups on the radicals formed during chain scission. Rates of crosslinking are greater than those for polystyrene and this is attributable to the participation of the substituted phenoxy radicals (formed by CH₃O fission) in addition reactions. Quantum yields for the gaseous products and for chain scission and crosslinking have been determined and a mechanism has been advanced to account for the experimental data.     

Neutron powder profile studies of the gamma uranium trioxide phases

Neutron powder profile studies show the existence of three phases in gamma uranium trioxide between 373°K and 77°K. The three phases are closely related and the transitions smooth and displacive. At 373°K, γ-UO₃ is tetragonal, with a = 6.9013 (5) and c = 19.9754 (18) Å, and space group $\text{I4}{}_1\text{amd(}\text{D}{}^{19}{}_{\mathrm{4h}}\text{)}$. At 323°K, γ-UO₃ becomes orthorhombic, space group F_ddd(D²⁴_2h), with the cell dimensions (293°K) a = 9.787 (3), b = 19.932 (4) and c = 9.705 (3) Å. There is a further transition between 293°K and 77°K, and, at 77°K, the orthorhombic dimensions of the pseudocell are a = 9.8225 (7), b = 19.8487 (15), and c = 9.6318 (7) Å. The neutron diffraction studies show that, in all three phases, the coordination polyhedra of the two crystallographically distinct uranium atoms are octahedral and (dodecahedral-2) respectively. At 293°K, the shortest UO distance is 1.796 (6) Å, and thus there are no pure uranyl bonds, in agreement with the infrared spectrum. The UO distances are precise to about ± 0.006 Å, about ten times the precision of an earlier X-ray single-crystal study, in which the conclusions were in conflict with the infrared spectrum. The structure is made up of parallel chains of edge-fused U(2) octahedra, cross-linked by U(1) dodecahedra. The atomic shifts are not great in going from 373°K to 77°K; at 293°K the data will refine in the pseudotetragonal cell as well as the true orthorhombic cell, and the 77°K data will refine in the F_ddd cell.     

Poly(vinyl acetate) paints in works of art: A photochemical approach. Part 1

The photochemistry of poly(vinyl acetate), PVAc, homopolymer, of PVAc mixtures with selected pigments and fillers, and of accurate historic reproductions based on the colours prepared by Portuguese artist Joaquim Rodrigo and on those supplied by the company A Favrel was studied. The systems, applied as films, were irradiated at λ ≥ 300 nm and the changes followed by size exclusion chromatography, infrared spectroscopy, and colorimetry.PVAc as homopolymer or as an emulsion paint proved to be very stable to light, and after 5000 h of irradiation no gel fraction was observed. Φ_R (λ_irr = 313 nm) for chain scission for PVAc was determined to be 7 × 10⁻⁸. This value indicates that the mechanism/s operating when irradiating at λ ≥ 300 nm are different from those previously published with irradiation involving 254 nm. No side-group scission was observed, and main chain scission is the foremost photodegradation mechanism. Also, the metal ions present in the pigments do not affect the photochemical stability of the polymer.     

Mechanism of radical reactions in cyclic and linear methylsiloxanes

The radical reactions of linear and cyclic dimethylsiloxanes were studied. It was found that methyl radicals decayed in linear and branched polymethylsiloxanes and dimethylsilanediol at 77 K as a result of radical center transfer. It was established that distonic radical cations were formed and the siloxane ring was opened in cyclic siloxanes D₃ and D₄ upon irradiation in the bulk at 77 K.     

Effect of photostabilizers on the rate of photodegradation of polymethylmethacrylate

Study of the action of photostabilizers on the rate of macromolecule scission for PMMA films irradiated with 2537 nm light in air or in inert gas demonstrated that the effectiveness of photostabilizers is determined solely by their absorbing capacity. Photoconversion of PMMA is accelerated by addition of monomer or of some antioxidants. Simultaneous acceleration of the accumulation of free radicals, photodegradation and scission of CH₂ groups is also observed. The initiating additives possess a lower photochemical activity relative to PMMA. The acceleration is due to stronger light absorption by these additives, compared to the polymer.     

Wavelength sensitivity of the photoinduced reaction in polycarbonate

Bisphenol A polycarbonate (PC) was irradiated with monochromatic light of wavelengths 260, 280, 300, 320, 340, 400, and 500 nm by use of the Okazaki large spectrograph (OLS). The quantum yield of main-chain scission (?_cs), efficiency of photo-Fries rearrangement (E_r), and effects of wavelength on ?_cs and E_r were investigated. It was found that photodegradation and photo-Fries rearrangement of PC took place by the irradiation of 260–300 nm light, but did not by the irradiation at λ ≧ 320 nm. The ?_cs has a maximum value in the case of the irradiation with 260 nm light, while E_r was found to have a maximum value by the irradiation of 280 nm light. © 1993 John Wiley & Sons, Inc.