Electron Spin Resonance: Senior Reporter (7) - Rilegato

Rilegato

ISBN 10: 0851868118 ISBN 13: 9780851868110

Casa editrice: Royal Society of Chemistry, 1982

Riassunto

Specialist Periodical Reports provide systematic and detailed review coverage of progress in the major areas of chemical research. Written by experts in their specialist fields the series creates a unique service for the active research chemist, supplying regular critical in-depth accounts of progress in particular areas of chemistry. For over 80 years the Royal Society of Chemistry and its predecessor, the Chemical Society, have been publishing reports charting developments in chemistry, which originally took the form of Annual Reports. However, by 1967 the whole spectrum of chemistry could no longer be contained within one volume and the series Specialist Periodical Reports was born. The Annual Reports themselves still existed but were divided into two, and subsequently three, volumes covering Inorganic, Organic and Physical Chemistry. For more general coverage of the highlights in chemistry they remain a 'must'. Since that time the SPR series has altered according to the fluctuating degree of activity in various fields of chemistry. Some titles have remained unchanged, while others have altered their emphasis along with their titles; some have been combined under a new name whereas others have had to be discontinued. The current list of Specialist Periodical Reports can be seen on the inside flap of this volume.

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Electron Spin Resonance Volume 7

A Review of the Literature Published between December 1979 and May 1981

By P. B. Ayscough

The Royal Society of Chemistry

Copyright © 1982 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85186-811-0

Contents

Chapter 1 Applications of E.S. R. in Polymer Chemistry By D. J. T. Hill, J. H. O'Donnell, and P. J. Pomery, 1, 
Chapter 2 Theoretical Aspects of E.S. R. By A. Hudson, 41, 
Chapter 3 Triplets and Biradicals By A. Hudson, 56, 
Chapter 4 Transition-metal ions By A. L. Porte, 69, 
Chapter 5 Inorganic and Organometallic Radicals By M. C. R. Symons, 124, 
Chapter 6 Organic Radicals: Structure By B. C. Gilbert, 174, 
Chapter 7 Organic Radicals: Kinetics and Mechanisms of their Reactions By P. B. Ayscough, 216, 
Chapter 8 Organic Radicals in Solids By T. J. Kemp, 252, 
Chapter 9 Spin Labels: Synthetic Macromolecules By A. T. Bullock, 280, 
Chapter 10 Spin Labels: Biopolymers By B. H. Robinson, 293, 
Chapter 11 Spin Labels: Biomembranes By Ching-San Lai, 313, 
Chapter 12 Metalloproteins By N. J. Blackburn, 340, 
Chapter 13 Applications of E.S.R. in Medicine By N. J. F. Dodd, 382, 

CHAPTER 1

Applications of E.S.R in Polymer Chemistry

BY D. J. T. HILL, J. H. O'DONNE LL, AND P. J. POMER Y

1 General Introduction

E.s.r. has a long history of applications in polymer chemistry since the pioneering experiments of Schneider on %-irradiated poly(methyl methacrylate) in 1951, at the time when e.s.r. was first being utilized in chemist ry. For most of this period , e.s.r. studies in polymer chemistry have been confined mainly to solids in which radicals were produced by u.v. or ionizing radiation, the latter giving a more uniform distribution throughout the sample rather than at the surface. The elegant single-crystal studies used for small molecules could not be performed with polymers owing to the small size of the crystalline regions, but limited measurements were made on oriented radicals; this was achieved by tensile elongation of fibres or films.

These e.s.r. investigations of irradiated polymers had practical importance since they were relevant to the modification of the properties of polymer materials by crosslinking and scission. Industrial processes have been developed in this area and the selection of polymers for use in radiation environments is also now better understood.

The identification of trapped radicals, especially after irradiation at low temperatures, and their transformations and decay on warming were used as a basis for the proposal of mechanisms for radiation degradation. Two assumptions were usually implicit: (i) non-radical reactions were unimportant and (ii) the sequence of radical reactions during warming was the same as that occurring on a much shorter time-scale during (and following) irradiation at a higher temperature. These are undoubtedly both gross oversimplifications in many systems and continue to present great difficulties in the relationship between e.s.r. measurements and radiation chemistry of polymers. E.s.r. evidence for the production of radicals must always be taken as only one piece of evidence contributing towards the overall picture. In particular, quantitative measurements of the concentrations of individual radical species must be compared with the yields of small molecular products, chain scissions, and crosslinks, etc. More use of selective ionic scavengers could also be combined with e.s.r., despite dispersion and reactivity problems.

Solid-state polymerization is an area in which e.s.r. has been widely used to show the presence of radicals and to follow details of initiation, propagation, and termination reactions, inaccessible by other methods. The original interest in producing stereospecific polymers has been largely unrewarded, but recent work on diacetylenes has provided a new stimulus in this field.

E.s.r. investigations of irradiated polymers and solid monomers suffer from the poor resolution of the spectra due to the large linewidths resulting from anisotropic spin-lattice interactions. The difficulty with liquid systems is that the steady-state radical concentrations are normally below the detection limit. However, very successful liquid-phase work has been done by flowing initiator and monomer solutions, usually in water and with titanous ion-hydrogen peroxide as initiator, into a mixing chamber immediately before entry into a flow cell in the spectrometer. A wide variety of propagating radicals have been observed in this way, with narrow linewidths permitting accurate measurements of α, β and γ hyperfine splittings.

Many studies have been made of the decay rates of radicals in polymers, but the measurements have been difficult to interpret by any consistent mechanism. Initial fast decays are usually attributed to radicals produced in close proximity to one another, i.e. in spurs or blobs. The actual mechanism of radical decay may involve chain movement, as at glass transition (Tg) or melting (T) temperatures, when decay is usually rapid and often complete, or hydrogen hopping, which has been proposed at lower temperatures. Certainly, greater mobility of radical sites, to form crosslinks for example, than of polymer chains seems to occur.

There is currently an upsurge of interest in the utilization of e.s.r. in polymer chemistry. This can be partly attributed to the availability of a new generation of e.s.r. spectrometers, which have greatly improved sensitivity, field stability, and variation of microwave power into the cavity - this last feature is very useful for selective saturation in polymers, enabling identification of the component radicals in a sample. Digital data output and interfacing to computer facilities are now standard. The advantages over the limitation to chart output are immense, particularly for immediate double integration of first-derivative spectra to measure radical concentrations, storage of spectra, and comparisons of experimental and simulated spectra. Rapid-scanning facilities enable kinetic studies using the entire spectrum and immediate observation of spectra on the oscilloscope screen to be made so that instrumental and sample conditions can be rapidly adjusted to obtain optimum spectra.

The use of spin traps for e.s.r. studies in polymer chemistry is a field which is certain to expand rapidly in the immediate future. The advantages include well resolved spectra, high concentrations of radicals from the liquid phase, and information on the early steps in polymerization or degradation reactions, depending on the reactivity of the chosen spin trap and the radicals in the system.

This is the first occasion when e .s.r. studies in polymer chemistry have appeared as a separate section in these Specialist Periodical Reports. Previously, relevant papers have appeared in different sections. The book by Ranby and Rabek entitled 'E.S.R. Spectroscopy in Polymer Research' provides a comprehensive account of the field until early 1975. This present review attempts to summarize briefly the developments since then with particular emphasis on the period January 1980 to June 1981, as do the other reports in this Volume. Ranby, Roth, and Kobelia have provided recent brief reviews.

The applications of e.s.r. in polymer chemistry have been divided into two main areas: (i) polymer degradation and (ii) polymerization. The related area of spin labels and spin probes, which has developed rapidly in the last few years, gives information primarily about the degree of molecular motion in polymers, including libration and rotation of substituent groups, segmental motion, and diffusion. This topic is dealt with specifically in Chapter 9 of this Report.

2 Polymer Degradation

Ionizing Radiation.- Introduction. There has been considerable interest for many years in the possibilities for modifying the properties of polymer materials by crosslinking the polymer molecules using high-energy, or ionizing, radiation. The availability of high-intensity radiation sources utilizing spent nuclear-reactor-fuel elements, or radioactive cobalt-60, produced by exposure of normal cobalt-59 in a nuclear reactor, from 1945, stimulated this interest. Over the last decade, the increasing availability of electron accelerators, producing radiation beams on demand , has led to further interest in this field . Several processes utilizing radiation crosslinking of polymers are currently in use on an industrial scale. Thus, crosslinked polyethylene used as an insulator for high-current conductors permits higher operating temperatures and hence greater power transmission. The irradiation of pure polymers has been extended to include additives, such as acetylene and acrylic monomers, which enhance the crosslinking many-fold and correspondingly reduce the radiation doses required.

There is also increasing interest in the scission of polymer molecules by high-energy radiation. This leads to reduction in molecular weight and an associated increase in solubility. An application of particular interest is the use of poly(olefin sulphone)s as electron-beam resists for the manufacture of silicon-chip micro-electronic devices. Further developments in this field, including exposure to soft X-rays, are under investigation. In resists, the use of high-energy radiation complements irradiation by u.v. light , the main difference being that the shorter associated wavelength of the radiation enables sharper definition to be achieved.

There is obviously a great practical need for the mechanisms of these radiation degradation reactions to be understood. The effects of variable parameters, such as polymer structure and morphology, irradiation dose rate, dose and temperature, additives, and environment (vacuum, inert gas, such as nit rogen, helium, or argon, or a reactive gas, e.g., chlorine, and the pressure), need to be quantitatively assessed. The fundamental chemistry of the radiation-induced changes also offers an attractive challenge to the scientist. The role of e.s.r. in this field of the interaction between ionizing radiation and polymers is most important, as it is one of the few techniques which can be used to observe the free-radical intermediates in solid polymers and to follow their reactions without perturbing the system. The opportunities, and indeed necessity, for identification of multiple radical components of spectra and for quantitative measurements of radical conformational changes, chemical reactions, and decays require all the sensitivity and versatility which are available from modern e.s.r. spectrometers and their associated computerized data-handling and spectral-simulation facilities.

Radiation Type and Radical Distribution. A variety of radiation sources, producing γ-rays, β-particles, electron beams, a-particles, X-rays, neutrons, fission fragments, etc., may be used to irradiate polymers. The spatial distribution of radicals resulting from exposure to these different types of radiation will differ greatly, depending on their linear energy-transfer (LET) values. Low LET radiation, such as γ-rays, will produce well separated regions of ionization and radical formation, but local blobs and spurs of radicals may be found, whereas high LET radiation, such as a-particles, will produce a continuous track of ions and radicals. There is considerable interest in the spatial distribution of the radicals from the increasing diversity of radiation sources available and the effects on the subsequent chemical reactions.

Pasalskii et al. have obtained average concentrations of 7.4 x 1018 and 5.1 x 1019 radicals cm-3 in the tracks of γ-rays and β-particles, respectively, in polyisobutene. The selective saturation of radicals in close proximity to one another has been utilized by Katsumura et al. to show that different spatial distributions of radicals are produced in poly(methyl methacrylate) (PMMA) and polyethylene (PE) by neutron and γ-irradiation, although the spectra are quite similar. They deduce that the average, local concentration of radicals prod uced by neutron irradiation is 3-7 times higher by γ-irradiation. Ishigure et al. have calculated the anticipated LET effects fromneutron and γ-irradiation of PMMA and polystyrene (PS) and compared these with the significant differences in G(scission) and G(crosslink) values and the changes in molecular-weight distributions observed experimentally.

Radical production in pairs, presumably by homolytic bond scission without geminate recombination or loss of one member of the pair by diffusion or secondary reaction, can be detected by an e.s.r. resonance at g= 4(ΔMs= 2). Hesse and Heusinger have found ca. 2% of radicals observed in poly(ethyl acrylate) and atactic 1,2-polybutadiene to be in pairs.

It has frequently been speculated that crosslinking of polymers by radiation proceeds particularly through radical pairs, but they found no evidence that this was an important reaction.

Electrical discharges also constitute a form of high-energy radiation: these include Tesla coil discharges, microwave generators, and electrical 'tree' failure under high-voltage loading. Surface phenomena will be more important under these conditions. A short Tesla coil discharge has been reported as equivalent to a high dose of γ-radiation.

Polyolefins. Polyethylene, polypropylene - the industrially important polyolefins - and the other polyolefins are normally partially crystalline, the degree and perfection of the crystallinity depending on the polymerization conditions and subsequent thermal treatment. There has been considerable interest in the extent to which crosslinking and scission reactions occur in the amorphous and crystalline regions and also in the distribution of radicals between the two phases. Significantly higher radical yields have been obtained in quenched than in annealed samples of linear PE; annealing results in more perfect and higher crystallinity. Thermal treatment before irradiation reduced the radical yield in PE and PP.

Fujimura has distinguished intra- and inter-chain radical pairs in PE and in the model hyd rocarbon n-eicosane. He suggests that these two types of radical pairs are the precursors of double bonds and crosslinks, respectively, in contrast to Hesse and Heusinger. More rapid decay of radical pairs was observed than of isolated radicals, but the relative number of radical pairs was very small (ca. 0.3%). The angular dependence of the .6.Ms= 2 transition from radical pairs in irradiated oriented PE has been examined.

Alkyl radicals -CH2-CH-CH2-, formed by C-H scission (and probably also by abstraction of hydrogen atoms, possibly by 'hot' H atoms), are observed in PE at 77 K, but these radicals react on warming, apparently with C=C double bonds, to form the more stable allyl radicals -CH2 -CH-CH=CH-CH2-. Fujimura et al. claim that these are formed in both crystalline and non-crystalline regions, based on the anisotropy of the spectra from single-crystal mats of PE. Nitric oxide has been used to 'titrate' the radicals produced in atactic and isotactic PP by γ-irradiation by means of a comparison of the relative rates of the reations R · + R · and R ·+ NO with temperature. The allyl radicals in high-density, or linear, polyethylene (HDPE) have been found to react more rapidly than the alkyl radicals with chlorine, the radicals being produced by γ-irradiation.

Radical Decay. It is custormay to irradiate polymers at low temperatures, usually 77K in liquid nit rogen, and then to observe the reactions of the radicals during progressive warming. This temperature profile of the radicals is commonly held to be equivalent to the time dependence, over a much shorter time interval, for irradiation at higher temperatures. This assumption is frequently considered to be confirmed by irradiating the polymer at ambient temperature and showing an equivalence between the e.s.r. spectrum obtained then and by warming up to that temperature after irradiation at 77 K.

Further radical reactions, and decay, occur on heating polymers above ambient temperatures. Increasing mobility in the polymer matrix usually leads to radical decay. However, chain-hopping mechanisms, involving the movement of H a toms, or the equivalent 'holes', have been suggested , and these should not be dependent on molecular mobility. This may explain partial radical decay which frequently occurs around 150 K, although the non-random distribution of radical sites may also be involved. The glass transition temperature, or the melting point, usually marks the complete disappearance of radicals trapped in amorphous or crystalline regions, respectively. In conformity with these ideas, Fujimuri et al. have observed three steps in the decay of radicals trapped in irradiated PE during warming from 77 K, which they identify with the successive decay of radicals in pairs, amorphous and crystalline regions. Pressures up to 13000 atm have been found to affect the decay rates of radicals in irradiated PE.

The interpretation of the e.s.r. spectrum of irradiated PP has been the subject of controversy for many years. The principal radical formed on irradiation is believed to be -CH2-CMe-CH2-, and the 9- or 17-line spectrum (depending on the temperature) from isotactic PP and the 6-line spectrum from atactic PP are attributed to different conformations of this radical. At room temperature the spectrum changes to a singlet, attributed to the polyenyl radical. This is considered to be the most stable radical at higher temperatures in many irradiated polymers. A thermodynamic model has been proposed to relate the changes in the radical concentrations at Tg and Tm.

(Continues...)

Excerpted from Electron Spin Resonance Volume 7 by P. B. Ayscough. Copyright © 1982 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Dettagli dell?edizione

• Editore: Royal Society of Chemistry
• Data di pubblicazione: 1982
• ISBN 10: 0851868118
• ISBN 13: 9780851868110
• Rilegatura: Copertina rigida
• Lingua: Inglese
• Numero di pagine: 447
• Redattore: Ayscough P. B.
• Contatto del produttore: non disponibile