THERMAL INDUCED HOMOLYTIC SCISSIONS OF LIGNIN INTERUNITARY BONDS IN THE HARDWOOD LIGNIN SOLUTION. AN ESR STUDY COMBINED WITH A SPIN TRAPPING METHOD

An electron spin resonance (ESR) method combined with a spin trapping reagent was successfully applied to trap and characterize unstable free radicals which were generated by heat-treatment of the dimethylsulfoxide (DMSO) solution of a hardwood, Japanese beech (Fagus crenata) lignin. It was found, consequently, that two unstable secondary carbon radicals, ～ CH? in the solution were created and the resulting radicals were trapped as the stable nitroxide spin adducts when the DMSO solution was heat-treated in the presence of a spin trapping reagent: 2,4,6-tri-tert-butylnitrosobenzene (BNB) at ca. 91°C. This means that so-called alkyl phenyl ether bonds, ～ CH-O- phenyl, known as important lignin interunitary bonds were homolytically scissoned by the heat-treatment of the lignin solution. Further the detailed analysis of the observed ESR spectrum revealed that two positions of alkyl phenyl ether bonds, i.e., β-O-4 and/or α-O-4 bonds as the interunitary linkages in the lignin are homolytically scissioned, although the phenoxy radical, Ph-O ? as the counter radical of the secondary carbon radicals was not trapped by the BNB spin trap. This suggests that fairly large steric hindrances operate between the syringyl with two methoxy moieties at the ortho positions and/or guaiacyl moieties with a methoxy moiety at the ortho position, and the BNB molecule bearing two bulky ortho tert-butyl groups in the phenyl ring.     

Relation between diffusion controlled decay of radicals and α-relaxation in polyethylene and polyoxymethylene

Diffusion controlled process theory was applied to the analysis of the data concerning decay reactions of free radicals in irradiated polyethylene and polyoxymethylene in detail. Diffusion constants of free radical sites were estimated from the data of the decay reactions and their time constants were calculated by using Stokes-Einstein relation. These time constants were plotted against the inverse temperature in the relaxation maps, which were arranged by Wada. In the case of polyethylene, the time constants of decay reactions of alkyl radicals trapped in the lamellar surface and the inner part of crystallites were in good agreement with the relaxation times of α′- and α-processes, respectively. However, it was found that the time constants of main chain radicals trapped in polyoxymethylene were closely related to α-relaxation processes. The activation energies of the decay reactions of free radicals were in good agreement with those of the corresponding relaxation processes in both cases. From these results, it was concluded that the decay reactions of the free radicals reflect the molecular motions associated with the respective relaxation processes. Concerning the decay reaction of free radicals in the inner crystallite of polyethylene, effects of lamellar thickness and crystal surface were also studied by using solution grown crystals, which were crystallized at various temperatures, and the fuming nitric acid treated materials. It was found that the diffusion constant of free radical sites and its activation energy were strongly dependent on the lamellar thickness and half-life of free radicals was a function of the amount of the fold surface; i.e., the reaction rate was much depressed as the lamellar thickness was increased and the amount of the fold surface was decreased.     

Thermally induced homolytic scissions of interunitary bonds in a softwood lignin solution: A spin‐trapping study

Unstable chemical species, that is, radicals generated by the thermal treatment of a dimethyl sulfoxide (DMSO) solution of the lignin of a softwood, Yezo spruce (Picea jezoensis Carr.), were studied in detail with an electron spin resonance (ESR) method combined with a spin‐trapping technique. An unstable secondary carbon radical (～CH ·) in the solution was trapped as a stable nitroxide spin adduct [R? (N? O ·)? CH～ (R = tert‐butyl benzene)] when the DMSO solution was heat‐treated in the presence of a spin‐trapping reagent [2,4,6‐tri‐tert‐butylnitrosobenzene (BNB)] at about 40°C. This meant that alkyl phenyl ether bonds (～CH? O‐phenyl), known as interunitary lignin bonds, were homolytically scissioned by the thermal treatment in the lignin solution. A detailed analysis of the ESR spectrum revealed that three kinds of radicals—primary (～CH₂ ·), secondary (～CH ·), and tertiary (～C ·) carbon radicals—were trapped as stable spin adducts at about 60°C, although the phenoxy radical (Ph? O ·) was not trapped by the BNB spin trap as the counter radical of the secondary carbon radical. This suggested that a fairly large steric hindrance existed between the so‐called guaiacoxy radical with a methoxy group in the ortho position and the BNB molecule bearing two butyl groups as bulky moieties in the ortho positions. However, the phenoxy radicals in the lignin solution were stable up to about 60°C. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2136–2141, 2004     

Theoretical aspects of free radical decay in polyethylene and comparison with experimental results

Results of the study of stability of free radicals using electron spin resonance suggest a close relationship between the glass transition temperature (T_g) and free radical decay. In a detailed study of radicals in polyethylene (PE) [J. Polym. Sci. A2 6 (1968) 1435] in the sixties, it was concluded that the decay of radicals in solid polymers is connected to molecular mobility. In this report on molecular mobility using a Monte Carlo method we show with the example of PE that it is the motion of molecular structures, which, depending on temperature, lead to a transfer of radical centers and thus to the approach of radicals and their decay. Theoretical and experimental decay curves for PE are compared and based on their close correspondence it is concluded how the individual types of motions affect the stability of free radicals.     

An e.s.r. study of the motion of peroxy radicals in isotactic and atactic polypropylenes

E.s.r. spectra of peroxy radicals trapped in isotactic and atactic polypropylenes have been observed at various temperatures. The spectrum observed at 77 K was shown to be composed of one component by comparing it with that at 4 K. The spectra observed at higher temperatures comprised of two spectra arising from the rigid peroxy radicals and the mobile peroxy radicals. All of the observed spectra have been reconstructed by means of computer simulation and the changes in anisotropic g-values with temperature were estimated. The motion of the mobile fraction was shown to be rotation or rotatory vibration of the radicals around the chain axis both in the isotactic polypropylene and in the atactic polypropylene as well as in polyethylene and poly(tetrafluoroethylene) although the structures of the polymers are different, that is helical (polypropylene) or planar zigzag (polyethylene, poly(tetrafluoroethylene)).     

Spin trapping of radicals produced by glow discharge on low-density polyethylene

An electron spin resonance study of radicals produced by glow discharge on low-density polyethylene powder shows three kinds of radicals, depending upon experimental conditions. The first formed, alkyl radicals, are unstable at room temperature and react rapidly with oxygen to form peroxide radicals. If a spin trapping compound, N-tert-butyl--phenyl nitrone, is added to the polyethylene, before discharge, nitroxide radicals are formed in place of peroxide. The interest in forming nitroxide radicals is their stability at room temperature. This simple method can be helpful for the observation and identification of radicals produced in polymers, for example by electrical treeing.     

ESR study on molecular motion of peroxy radicals of polytetrafluoroethylene

Temperature variations of the line shapes of e.s.r. spectra from the peroxy radicals of polytetrafluoroethylene ( ) were investigated in detail and the results were quantitatively analysed based on Kneubühl's theory on the line shape. It was found that there were two kinds of rapid molecular motions, of which the correlation times were about 10⁻⁹ s, for the peroxy radicals of ; one was the rotational motion related to the peroxy chain radical, the other was the three dimensional motion associated with the peroxy end radical. The activation energies of these molecular motions were estimated as 0·52 kcal/mol and 0·26 kcal/mol for the rotational motion and the three dimensional motion, respectively. Origins of such rapid motions were discussed in connection with the other molecular motions of polymers, which were found by other experimental methods.     

Stabilization of photo-induced radicals in hematoporphyrin-IX-doped chitosan film

Induction and stabilization of free radicals were investigated in hematoporphyrin-IX (Hp)-doped chitosan (Hp-Ch) film by electron spin resonance (ESR) following photo-irradiation. Induced radicals were more stable in chitosan and 6-O-carboxymethyl chitin films than in carboxymethyl cellulose and sodium alginate films. Hydroxyl groups on D-glucosamine residues in chitosan are suggested as participating in accepting radicals, since spin trapping was absorbed in ESR spectra of Hp-Ch film by the use of oxygen sensitive spin trapping reagent. An induced circular dichroism spectrum was observed only for Hp-doped chitosan film over a range 360–450 nm among seven pairs of polymers and dyes; it is suggested that Hp molecules are arranged parallel along the carbohydrate backbone of chitosan, resulting in the highest acceptance of photo-induced radicals in the polymer film.     

Radicals generated in autoxidized methyl linoleate by light irradiation

The structure of free radicals generated in the autoxidation of methyl linoleate (ML) was studied by the spin trapping technique using deuterated nitrosodurene, (CD₃)₄C₆HNO, as the spin trap. The secondary alkyl radicals were trapped after irradiation of ML with UV light. The formation rate of secondary alkyl radicals increased upon shortening the wavelength of irradiation light and was closely correlated with the peroxide value of autoxidized ML when a UV light longer than 250 nm was employed. When hydroperoxides separated from autoxidized ML were added to ML, the relationship between the formation rate of secondary alkyl radicals and the amounts of added hydroperoxides was nearly linear. These results suggest that secondary alkyl radicals are generated by proton abstraction of the active radicals, such as RO and HO, which are produced by the photolysis of hydroperoxides with UV light. The spin trapping technique can be applied to the study of lipid oxidation and/or photolysis of autoxidized lipid.     

Free radicals trapped in polyethylene matrix: 2. Decay in single crystals and diffusion

The kinetics of the decay of main chain free radicals trapped in polymer matrix in vacuum was investigated. The decay behaviour of alkyl radicals trapped in the lamellar surface of the crystallites of solution grown polyethylene should be different from that of the same radicals trapped in the inner part of the crystallite, and this difference was clearly observed. The analysis of the data of the decay reaction was made, based on three-dimensional diffusion-controlled reaction theory, and reasonable interpretation of the data was made. Alkyl radicals were also observed in an irradiated urea—polyethylene complex and the decay rate of these radicals were very slow in spite of the large mobility. According to these occurrences, it was concluded that the mechanism of three-dimensional diffusion of the free radicals across the chain may occur in the inner part of the crystallites via hydrogen abstraction reactions.     

E.s.r. studies of peroxy radicals in polyethylene: 1. Temperature dependence of spectra and molecular motion of radical sites

The temperature dependence of the e.s.r. spectrum of the peroxy radicals in polyethylene was successfully observed. Two kinds of radical site were found in the amorphous region of polyethylene, one bearing the radicals corresponding to the usual e.s.r. spectrum of the amorphous pattern as for the usual polymer peroxy radicals, and the other presenting the singlet-like pattern which has never actually been observed. Computer simulation of these spectra was carried out in order to determine the anisotropic g-values as well as the ratios of the amounts of the radicals located at respective radical sites. The temperature dependence of these values made it possible to discuss the difference of the radical sites and the molecular motion of these radical sites. The motion around the chain axis was found to be much more rapid than that around the CO bond axis in the peroxy raidcals.     

Fabrication of sintered porous polymeric materials: effect of chain interdiffusion time on mechanical properties

In this study, sintered porous polymeric materials made of high density polyethylene (HDPE) were fabricated through controlling the chain interdiffusion time at the transition temperature of semicrystalline and melt states. At this intermediate state, where both crystalline and amorphous phases coexist, the interfacial welding of HDPE particles is facilitated thanks to interdiffusion caused by chain relaxation phenomena. Then, by assuming a spherical shape and a cubic packing configuration of particles, a geometrical model was developed to predict porosity variations as sintering progresses. Moreover, the HDPE used, as a broad molecular weight distributed polymer, has different family chains with different specific molecular weight ranges. Accordingly, the melt coalescence rate of the particles was tracked using an optical microscope equipped with a hot stage, in order to determine the diffusion characteristic times for each family. During the characterization stage, SEM images proved the presence of porous structures in the sintered samples. In addition, mechanical properties were assessed through the shear punch test. It was shown that the mechanical properties are governed by the interdiffusion of long chains which occurs at relatively long sintering times. The results also demonstrated the role of reptation motion of long chains in the interfacial welding of polymeric particles. They revealed the compatibility of macroscopic properties of the samples and chain motions at microscopic levels. © 2017 Society of Chemical Industry     

Studies on the surface of as-grown and annealed polyethylene single crystals by the spin-probe method

Studies on molecular motions at the surface of as-grown and annealed polyethylene single crystals in relation to the surface structure have been made by dispersing electron paramagnetic probes on the surface of the crystals and detecting them by the electron spin resonance method. A relatively large scale molecular motion supposedly associated with the glass transition was detected as a change in hyperfine patterns at around −50°C. Rotational correlation frequency of the paramagnetic probe was found to rapidly increase above the crystallization temperature suggesting that the new mode of molecular motion takes place by thickening of crystals. For the annealed crystals the frequency was much depressed compared with that of the as-grown crystals indicating the decrease in mobility of the surface by annealing. Extensive study revealed that the mobility of the surface of annealed crystals is uniquely determined by the increased portion of the lamellar thickness by annealing.     

Deuteron n.m.r. study of chain motion in solid polyethylene

The chain motion in the amorphous regions in linear polyethylene is studied by exploiting various ²H n.m.r. pulse methods. The experimental techniques employed are described in considerable detail. In particular, the solid echo- and the spin alignment spectra are recorded over a wide range of temperatures (123–393 K) as a function of the pulse spacings. In addition the spin-lattice relaxation times of both polymerization and alignment are given for the crysralline as well as the amorphous regions. The data are analysed in terms of highly constrained conformational motions which generate an exchange of C-H bond directions between 2, 3, or all 4 tetrahedral sites on a diamond lattice. Thus in the temperature interval from the γ-relaxation up to ～200 K only 2 sites are accessible, whereas a third site is appreciably populated at room temperature. Between 330 K and the melting point the number of conformations accessible to the chain must be high enough to lead to an increasingly isotropic 4 site exchange. The motion remains highly constrained and localized, however, up to the melting region as shown by the analysis of the spin-alignment spectra. The intensity ratio of the deuteron n.m.r. signals from the rigid crystalline and the mobile amorphous regions, respectively, yields the crystallinity of the sample as a function of temperature in remarkable agreement with the results obtained by X-rays. Finally, frequency dependent relaxation of spin alignment due to torsional oscillations of the chains in the crystalline regions is observed and analysed.     

Conversion of alkyl radicals to allyl radicals in irradiated single crystal mats of polyethylene

The decay of alkyl radicals, the conversion of alkyl radicals to allyl radicals and the trapping of allyl radicals in irradiated single crystal mats of polyethylene have been studied by electron spin resonance (e.s.r.). It has been suggested that in the crystal core alkyl radicals react with trans-vinylene double bonds and are converted into trans-vinylene allyl radicals; at the crystal surface, alkyl radicals react with vinyl end groups and are converted into allyl radicals with vinyl end groups. The decay of radical pairs and the formation of trans-vinylene double bonds are discussed.     

E.s.r. studies of peroxy radicals in polyethylene: 2. Peroxy radicals in urea — polyethylene complexes

Temperature dependent e.s.r. spectra of peroxy radicals in urea—polyethylene complex materials (UPEC) were discussed. Peroxy radicals in UPEC were much more stable than in normal polyethylene. Change of anisotropic g-values of the radicals with temperature were investigated and it was found that the g₁-axis, which was assigned to be parallel to the chain axis, seemed to be almost unchanged throughout the temperature range in the present study, but the g₂- and g₃-axes changed drastically. Using these results, it was concluded that the main motion of the radical sites was rotation around the chain axis and this motion was found to be much faster than in normal polyethylene, which we have already studied and which we reported recently. This is a reflection of the properties of the materials studied, i.e. interaction between molecular chains in UPEC must be much weaker than in normal polyethylene because of the presence of the wall of urea molecules in the former.     

Free radicals and new end groups resulting from chain scission: 2. Mechanical degradation of polyethylene

The number of chain scissions accompanying mechanical degradation of polyethylene has been estimated from i.r. analysis of new end group concentrations. Polyethylene specimens fractured in tensile deformation and ground under liquid nitrogen were examined. The results are compared to the number of free radicals generated during mechanical degradation and measured by e.s.r. In comparison with previous results in the literature our results are lower by 1–2 orders of magnitude and in better agreement with estimates of the number of chain scissions from viscosity measurements. An ultra high molecular weight polyethylene was examined as a control specimen containing few end groups. The change in the number of vinyl groups resulting from grinding of this specimen were estimated to be at least an order of magnitude lower than that found for lower molecular weight polyethylenes. This finding suggests that large errors may be introduced into the determination of concentrations of end groups through subtraction of relatively intense absorption bands.     

Relaxation processes and mobility of spin labelled polyethylene

A high density polyethylene was spin-labelled by the reaction of the alkyl type free radical,

with 2,4,6-tri-t-butyl nitrosobenzene as a spin-trapping reagent. Extrema separations and line widths of e.s.r. spectrum of nitroxide free radicals were obtained and used in order to calculate correlation times of the motion of the label in a wide temperature range. Spin rotational frequencies of free radicals were compared with the correlation frequencies obtained from a relaxation map which revealed α-, β- and γ-relaxation processes in solid polyethylene. It was concluded that the motional narrowing of the line width in the high temperature region 283–343 K with an activation energy of 9.2 kcal/mol, was caused by the β-relaxation process, which is normally attributed to the large scale segmental motion over the whole molecular chain in the amorphous phase. The decrease of the extrema separation in the higher temperature region 353–383 K was conisdered due to the same mechanism of the relaxation observed in the 283–343 K region in the case of the line width study. These e.s.r. techniques gave lower activation energies of β-relaxation process than those obtained by dielectric and mechanical relaxation measurements. This discrepancy was interpreted in terms of the distribution of relaxation times. The relaxation spectrum for the β-process was discussed based on the Miyake equation.     

有机锡化合物光解活泼自由基的ESR研究

本文用自旋捕捉技术与ESR相结合的方法,研究了六种有机锡化合物的光解反应历程。结果表明:有机锡化合物紫外光照时,碳—锡键发生均裂,产生碳中心和锡中心自由基。其中碳中心自由基易被捕捉剂α-苯基-N-特丁基氮氧化物(PBN)或2,3,5,6-四甲基亚硝基苯(ND)所捕获;而锡中心自由基可与菲醌形成稳定的环状加合物。由于环状加合物中配体的不同,产生的空间效应也不同,从而导致加合物中的未偶电子云密度发生变化,给出相异的ESR信号。     

Fragmentation of Fluorinated Model Compounds Exposed to Oxygen Radicals: Spin Trapping ESR Experiments and Implications for the Behaviour of Proton Exchange Membranes Used in Fuel Cells

Low‐molecular weight model compounds (MCs) for Nafion membranes used in fuel cells were exposed at 300 K to ^·OH radicals produced by UV irradiation of aqueous H₂O₂ solutions. The MCs contained fluorinated and partially fluorinated groups terminated by sulphonic or carboxylic acid groups. The fragmentation process in the MCs was studied by spin trapping electron spin resonance (ESR) methods, using 5,5‐dimethylpyrroline‐N‐oxide (DMPO), N‐tert‐butyl‐α‐phenylnitrone (PBN) and 2‐methyl‐2‐nitrosopropane (MNP) as the spin traps. The objective of these experiments was to assess the effect of the type of ionic groups (sulphonic or carboxylic) and of fluorine substitution on the spin adducts detected. DMPO experiments led to the detection of spin adducts of ^·OH and of carbon‐centred radicals (CCRs), and allowed the determination of the ^·OH attack site on the ionic and/or on the protiated or fluorinated groups. CCR adducts were also detected when using PBN as a spin trap; a key point in the interpretation of the PBN results was, however, the realisation that MNP is formed during PBN exposure to UV irradiation and oxygen or other oxidants such as H₂O₂. Experiments with MNP as the spin trap were the most informative in terms of structural details for adducts obtained from each MC. The results allowed the identification of CCRs present as adducts, based on large hyperfine splittings (hfs) from, and the number of, interacting ¹⁹F nuclei; in addition, oxygen‐centred radicals (OCRs) as MNP adducts were also identified, with much lower hfs from ¹⁹F nuclei. Taken together, the results deduced by spin trapping suggest that both sulphonic acid and acetic acid groups can be attacked by ^·OH radicals and confirm two possible degradation mechanisms in Nafion membranes: initiated at the backbone and at the side chain.