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Present and future developments in polymer radiation
Nuclear Instruments and Methods in Physics Research B 105 (19951217-224 __ Beam Interactions with Materials & Atoms
@ ELSEVIER
Present and future developments
in polymer radiation
Arthur Charlesby Silver Spring, Watchfield, Swindon SN6 BTF, UK
1. Introduction The study of high energy radiation and its effects covers an immense range of scientific and technological interests, from the basic nature of radiation in all its forms on to its interaction with matter and the physical, chemical and biological changes it can produce. One can then consider potential applications over a wide range of subjects, many still to be explored, understood and evaluated. Here one can only give a short outline of a number of these possibilities, knowing well that many more, some as yet unsuspected, will be studied and evaluated in the next decades. The earlier applications such as X-rays for determination of structure and medical purposes could only use the low doses available at the time and are best considered as limited to research and examination purposes only. It is only with the advent of far more powerful sources, either nuclear or high-voltage electrical, that one could seriously consider the practical applications of high energy radiation to the large-scale production or modification of materials by exposure to radiation. The earliest large-scale irradiation processes were the modification of polymers, usually by crosslinking, and the sterilisation of medical products. It will be noted that both are concerned with high molecular weight materials in which a very small change in chemical structure can lead to major changes in physical, chemical, or biological behaviour. For over about forty years these very practical processes have grown throughout the world and now amount to many billions of dollars yearly. Considering the immense scientific and technical effort which has been devoted to many aspects of the subject it is perhaps surprising that more major new radiation applications have not entered the industrial and medical worlds in the last four decades. This does not mean that considerable advances have not been made both on the more academic side or in technical improvements. But why no more entirely new applications into other major fields? Is it because academic studies and technical progress have diverged? Or is there some simpler explanation such as the economic climate and high capital cost of radiation sources. 0168-583X/95/$09.50
It will be noted that the greatest successes have arisen when two apparently very different fields have been allowed to overlap; as with nuclear energy and the physical properties of polymers; similarly for medical sterilisation. In the latest period much excellent research and technical work has been devoted to understanding and improvement within these well-established fields. Considerable efforts have also been devoted to the more scientific aspects of the subject. Examples are the determination of reaction constants in chemical kinetics and the data for energy transfer from energetic particles. Such basic information is of the greatest value in academic research, in the production of theses and papers for scientific journals; resulting in improvement rather than innovations in these respective fields. Many of these researches have involved the application of the formulae of quantum theory, although its fundamental understanding in physical rather than mathematical terms still remains obscure. We still need to derive a physically acceptable model; most of us are not satisfied with a series of mathematical relations based on incompatible scientific models. Another line of great interest lies in the development of new methods of analysis, based on the study and measurement of a series of samples of known characteristics determined by the radiation doses received. Such new techniques can then be applied in many other situations or specimens where no radiation need be involved. One notable example is in the use of pulsed NMR T, spin-spin relaxation curves to determine many of the fundamental and largely physical characteristics of macromolecules. For some considerable time Nuclear Magnetic Resonance has been used to trace chemical changes in structure. Such changes could have occurred in many ways, including exposure to high energy radiation. A more recent set of discoveries, based on a series of measurements of irradiated polymeric samples, has led to the conclusion that pulsed NMR T, curves can be used to trace many of the more physical aspects of the samples examined including such important features as average molecular weight, viscosity and chain mobility but also other aspects of its morphology such as the presence of a partially crosslinked
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network, the average chain distance between successive links I&, the presence of chain entanglements which on a temporary scale can play the same role as permanent crosslinks, the existence of a partially crystalline or even a less regular semi-solid component, and other factors such as temperature, dilution and orientation which influence the physical properties of an irradiated structure. It was concluded that pulsed NMR could be used to measure in a simple way many fundamental features of a macromolecular system. Since it is largely for these characteristics that these materials are produced, one can hope to see this simple, rapid and non-destructive technique greatly extended and utilised in fields remote from radiation studies. One possible line involves changes in morphology in irradiated biological systems at low doses. Studies in one field i.e. radiation effects can be applied in scientific or technical fields where radiation itself is not involved. For example this NMR technique might well prove a great value in the determination of important physical characteristics of polymeric systems in routine industrial production. One could then extend this method to study the effect of polymer orientation and tensile strength, a long way from high energy radiation but we can then double back to discover whether the irradiation of polymers with some degree of asymmetry may serve to enhance their physical behaviour in one direction even if this reduces mechanical properties in other directions. The effect of radiation on electrical properties is very well known but this refers primarily to the immediate effects. Far less is known about the longer-term effects in some selected structures and here one of the major advantages of radiation treatment might prove of special value. I refer to the ability it confers to vary the effective dose with depth as well as over the exposed area. How far might electron irradiation treatment over a depth of perhaps a mm or two from the surface provide a thin surface layer of irradiated product with very different electrical properties to the overall system. To carry out exploratory studies on such systems would require the closest collaboration of an electronic physicist of great imagination with a radiation physicist and an experienced materials scientist. Overlap of interests could result in major advances. Radiation chemistry has grown into a major field of chemistry and a most valuable and well-recognised method of determining reaction constants (even when they are not really constants but can be determined over a range of other variables). Many of these constants can be utilised in other branches of chemistry and this method has acquired full respectability e.g. for the reactions of liquids. The number of such determinations is legion and also serves to fill scientific journals and student theses. The difficulty I find is to know which of these determinations has proved of value in other studies. (Of course I speak now as a physicist with many far-sighted chemical friends.) However there remain further chemical fields to explore as in
mixtures, comparison of such constants in the solid and liquid states and the effect of very low concentration of selected additives and possibly of orientation. A major subject of interest concerns the mechanism of radiation energy absorption from an incident high energy particle; its transfer as one or more quantities of energy from the initial site of absorption to neighbouring atomic or molecular structures; alternatively the immediate reaction to give radicals, excited species or ions plus charges which are then distributed through neighhouring portions of the irradiated species. How far is this initial charge transfer dependent on the chemical rather than physical structure (e.g. is the loss of perhaps 100 eV from the primary irradiation species dependent on a primarily chemical difference between saturated and unsaturated bond and how far does this initially absorbed energy move as energy before initiating the primary chemical reaction leading to radical formation or ionisation along a molecular chain, or between such chains? My own estimates, long out-of-date, led me to a distance of some 30 C-C spacings. Can we have better values? There are many other aspects of this early history between the initial energy absorption from an incident high energy particle and its almost ultimate appearance in a more conventional chemical state as a trapped radical or as an ion plus an electron. This is certainly a field which has received much academic study and deserves more reviews and summaries written primarily for those unfamiliar with the subject (or is it subjects) and wishing to know more. At this stage it may be worth bringing up the question of the different types of radiation. We can accept (for the time being at least) the true unity (i.e. single nature) of electrons, protons, etc. which in large numbers constitute a beam of high energy particles. In passing through a thin layer of a specimen they do so as individuals and react also as individuals; they are too far apart to react with each other at normal intensities, and this is not always appreciated. Any intensity effect must then arise as a reaction between longer-lived products of the primary irradiation process. But what are we to make of such particles as the photon for example, with a spin of 1, zero rest mass and considerable extent, i.e. covering many wavelengths? How can this extensive wave packet be concentrated instantaneously into atomic dimensions to enable it to react with one specific bond? One possibility is to assume a composite nature of a photon, as comprising an electron (-charge, + mass) and an antielectron (+ charge, -mass) created from a composite nothing by external energy to separate the charges and masses The creation and nature of such a composite particle, its velocity c in vacuum, the electromagnetic wave it produces in the vicinity during its passage, its unit spin, etc. all argue for a composite structure. This composite photon (such as e-m+ chasing e fm-) moves freely in space at velocity c, and when it eventually reacts with any
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structure, the leading antielectron is annihilated by collision with an electron of the structure, releasing its following electron as a photoelectron! Another type of radiation which is receiving much attention for the effects it produces is the laser; this is sometimes described as a series of photons of light waves in phase, so that their amplitudes A add simply without loss due to phase differences. The difficulty here is that for ten photons in phase in a laser beam, the total amplitude in phase is tenfold greater than that of a single one but the intensity which is the square of the amplitude is one hundred times greater. Since the total energy is proportional to intensity and not amplitude, one must then inquire where the extra energy (10-100) comes from. Studies in the effects of laser effects might well elucidate such questions as the true nature of the photon. Does laser light really consist of waves rather than separate particles which can only appear at the site of interaction. There are many other fundamental questions on the basic nature of radiation which remain unsolved although the mathematics involved are frequently used with excellent numerical results. The difficulty of reconciling both extensive waves and atomic dimensions for the same particle remains. To discuss this aspect more closely would lead us into a different direction from that usually covered in such conferences, rather more in the direction of basic physics than of possible radiation applications. We are still very far from having an adequate understanding of the fundamental nature of radiation and here we may expect and hope for important advances in the next generation, not in terms of better mathematical relations but rather of their interpretation in physical terms. It cannot yet be said that we understand the physical meaning of the wave function P yet we use it most frequently and successfully in so many problems of quantum mechanics. An outline of a possible newer and simpler interpretation of this fundamental scientific problem is outlined below. A mathematical expression (or rather two) cannot replace an acceptable physical explanation as a form of scientific understanding, and this is best shown by the remarks of leading experts in the field.
2. Energy deposition in polymers During the passage of high energy radiation through a medium it loses energy by one or more physical processes and possibly only after its redistribution can this result in atomic displacement, ionisation and excitation followed by chemical reactions in organic systems resulting from the formation of radicals and of ionised species together with free or trapped electrons and displaced atoms. These can then react in ways similar to those resulting from other chemical processes. However their concentration and distribution as a result of irradiation may be very different.
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The subsequent chemical processes in polymeric structures are not inherently different to those produced in related low molecular compounds but because of the high molecular weight involved each chemical change produced in this way can greatly affect the subsequent behaviour of a large molecule. One reaction in simple linear hydrocarbons often results in linking them together and this can require a dose (energy per g) far greater with low molecular weight material as compared with high molecular weight polymer simply because there are far more molecules in I g of irradiated material, and each molecule must on the average be affected once in this way. In biological molecules of far greater molecular weight the doses required could be further reduced. Of course differences in chemical structure are also involved. Thus for radiation modification of polymers the dose requirements can be so low as to render radiation treatment economically feasible, even without a chain reaction. On the other hand, largescale radiation processing of lower molecular weight systems is only justified if the modified material is very valuable or if a chain reaction is involved whereby one radiation-induced reaction can lead to many further chemical reactions. The use of radiation to promote such chain reactions is demonstrated in grafting reactions, discussed in a number of papers at this meeting. A further means of enhancing the radiation-induced reaction is with a polymer dissolved e.g. in water. The radiation energy absorbed in the water provides reactive species which then react further with the polymer. Although a desired effect is produced at a lower dose this does not necessarily represent an improved yield since only the smaller amount of polymer contained in the solution is thereby modified. However the product itself can then be of special value, such as a swollen system in which the degree of swelling is determined in an accurately controlled manner. It is even possible to produce swollen polymeric structures whose degree of swellability varies at different parts within the system. I do not know how such a system can be utilised but it is useful to know that it could be manufactured readily if required for some technical, biological or medical purpose. It will be realised that the cost of radiation plays the most important role in deciding whether a large-scale industrial process can be justified. Since the cost of radiation is primarily that of the capital installation, especially for particle sources such as electron beams, and the latter is greatly dependent on building and shielding costs, the scale of industrial production is the most important factor. Advances into new fields with a smaller production rate, at least in the initial stages, may be greatly influenced by the production of lower cost irradiation equipment for development purposes. To avoid these initial costs advantage can be taken of existing research installations in universities and other research institutes holding appropriate radiation equipment, albeit on a small scale. Radiation doses are usually expressed in kGy or in III. ADVANCED
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Mrad, (1 Mrad = 10 kGy). 1 kW h = 3.6 X lo6 J or if absorbed 1 Mrad = 10 kGy to 360 kg of irradiated material. For sterilisation purposes a dose of 2.5 Mrad or 25 kGy is generally accepted as effective so that 1 kW h can produce a maximum of 140 kg of sterile material if fully absorbed. For many polymers undergoing crosslinking without any chain reaction higher doses are required due in part to their smaller molecular weight. Depending on the changes desired a dose several times greater is usually needed, amounting to about lo-25 Mrad or loo-250 kGy with a corresponding increase in radiation cost. Another manner of considering dose requirements is to express the average energy input to promote a certain reaction in eV; or more frequently as the number G of such reactions resulting from an energy absorption of 100 eV. Typical G values for many radiation-induced reactions in organic systems are in the range 1 to 3. Thus a total energy absorbed of 1 kW h or 2.25 X 10” eV can produce G X 2.25 x 10z3 reactions of that type. If G = 3 this amounts to 1.1 moles of radicals, ions or any other species to which the G value refers. The main advantages of irradiation of polymers of certain types notably polyethylene, silicones, etc. are to promote crosslinking and to form a network structure. The properties of such networks are well known and need not be outlined further here. Further research on similar reactions in other polymers, copolymers and mixtures are still proceeding apace, and are often described in the appropriate scientific and technical journals. Other changed properties including electrical, luminescent and crystalline state have also been reported. Modifications such as irradiation in solution or in the presence of reactive additives have also been investigated and we may expect such exploratory investigations to serve in opening new lines of parallel studies of additive behaviour in biologically active structures. Irradiation of oriented or stressed polymeric systems may be found of scientific and technical value. Interest must also be paid to the physical behaviour of polymeric systems with differential dose treatment and degrees of crosslinking, swelling, etc. For such studies the reactions induced by high energy radiation throughout a solid specimen are uniquely qualified. Many of us who have long been concerned with radiation of various types are very excited by its past and promises of wide possibilities of the future. It is highly likely that there exist needs which could well be met by some form of radiation modified reaction or treatment but of which we as radiation specialists are unaware. At the same time there are scientists, engineers, medical practitioners and others working in very different fields fully acquainted with these unfulfilled requirements but who are not aware of the new possibilities opened up by some radiation treatment. Perhaps at some informally organised encounters between two such very different groups, new targets could be set and further progress achieved in new and unexpected directions.
3. Polymers and biopolymers There is one example of the value of overlap between adjacent branches of science and especially of radiation science where it seems to me that great steps may be expected in the near future. Some may indeed already have been made without being fully recognised. I refer to the considerable amount of information we have already gained from the irradiation of simple macromolecules and the potential expansion of such studies into the field of biomolecules and eventually into radiobiology. In particular how far have the radiation processes studied and evaluated quantitatively in simple macromolecules been assessed critically in irradiated biomolecules? It need hardly be said that an immense amount of fine work has been devoted to the chemical changes which occur e.g. in irradiated DNA. But is this approach taken by radiobiologist looking for a change in chemical structure, the most suitable line to follow? Here is one argument which I believe indicates that the major effects are not simple changes in chemical structure but more likely changes in molecular configuration. Taking the lethal dose for a human being as about 500 rad or its equivalent of 0.3 X 10lh eV/g, the number of changes resulting from this dose is G X 0.3 X lO”/g where G is the average number of chemical changes per 100 eV energy absorbed, typically about l-3. This lethal dose would result in only about one radiation-induced chemical change in a molecular weight of 10’” (this figure would be modified to a limited extent by the presence of water and changes in chemical structure but is certainly of the correct order of magnitude). Thus a lethal dose need involve only about one chemical change within each molecule such as one DNA. Although there may be some degree of selectivity as there is in simpler organic systems, this would not greatly detract from the need to explain this very remarkable degree of selectivity in chemical reaction, probably impossible to attain in any organic structure, and indeed to detect in such systems. To achieve this very high degree of response would involve a high concentration of the very sensitive reacting group but there is no such evidence. Another form of reaction would seem to be needed and one assumption is a major change in shape or structure of the DNA molecules. In irradiated polymers, drastic changes do in fact occur at low doses, corresponding to about one change per molecule. These can take the form of linking molecules together to form ultimately a network structure. This behaviour following a relatively low dose has been fully investigated and is used on a large scale commercially to change drastically the physical properties and behaviour of simple polymers. Another possible radiationinduced reaction at very low doses involves scission in the main chain of the macromolecule. Some modification of the doses required may indeed occur owing to the presence of water and to some forms of chemical bonding but the overall picture remains valid. A lethal dose of 500 rad (5
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Gy) can result in a drastic change in configuration and other behaviour of a very high molecular weight molecule. For example the linking of two DNA molecules together by radiation would prevent their separation and their ability to initiate the growth of further molecules. Scissioned DNA molecules would be likewise unable to replicate and the system may be considered as effectively dead. The doses involved for this fate would appear to agree with those deduced from simple radiation chemistry of macromolecules. Methods to follow up this suggestion are outlined below. Another line of research in biological systems and models can also be derived based on studies with simpler macromolecules. 1 refer to the influence of very low concentrations of additives which are found to modify radiation-induced reactions in macromolecules. By reacting with radicals and other reactive species notably oxygen such additives can greatly affect the course of a radiationinduced reaction, and since in an aqueous medium they can have great mobility, their influence on the less mobile macromolecules can be very considerable. The study of the effect they can have on irradiated simpler macromolecules and its extension to biomolecules therefore offers an exciting area for research. combining as it does the simpler radiation-chemical with the somewhat analogous radiation biology fields.
4. Pulsed NMR and radiation-induced changes
morphological
Much effort has been spent in devising methods of analysing the effects of exposure of polymers to radiation. Apart from conventional chemical methods to determine changes in chemical structure, methods particularly suitable for high molecules deal with changes in molecular weight, degree of crystallinity, viscosity, solubility and swelling. The use of pulsed T1 NMR offers a different type of information, more closely related to the morphology of these molecules. Thus a closely wound molecule will offer a very different NMR pattern than it would show in an extended form, even if the chemical structures are identical. For many purposes this distinction could be of the greatest importance when more physical properties such as elasticity or chain flexibility are involved, possibly with little or no change in chemical structure. A simple example is the range of different chemical macromolecules which can provide very similar rubberlike properties. Such materials can be analysed very readily from the pulsed NMR spectra even without any detailed knowledge of their chemical structure. This method is especially valuable when it is the change in certain aspects of morphology following irradiation rather than chemical structure whose behaviour is being investigated. In such NMR measurements it is the Tz spin-spin relaxation curves which are studied rather than the more usual T,. T, provides infor-
2’1
mation on changes in morphology (molecular configuration) of macromolecules resulting from irradiation even if the chemical changes are minute due to the low doses needed. For simple flexible macromolecules below a certain molecular weight (about lo4 typically) the pattern follows a simple exponential decay exp( - r/7’,) where T2 is simply related to average molecular weight. T2 = C.M”.‘. The value of C depends on the nature of the macromolecule being studied, as well as the temperature and environment which influence molecular mobility. In polyisobutylene for example this relation applies down to a very low molecular weight (less than 100). Once the value of C has been measured for a characterised macromolecule it can be used not only for determining the effect of radiation but also more generally for large-scale and rapid testing of industrial products. r, may be considered as a measure of molecular flexibility and of course it varies with temperature, molecular weight and concentration if in solution. In a partially crosslinked system (whether by chemical crosslinking agents or by radiation) the NMR spin-spin relaxation curve becomes the superposition of two exponential T2 curves; f.exp( -t/r,,) + (1 -f).exp(-r/T,,) and the values of f and 1 -f represent the relative importance of the non-network and network fractions. T,, represents the length of chains not incorporated into the network structure or at one end only, while T2\ measures the length of chains between successive links, the quantity usually denoted by M, in measurements of chain length within the network; it is T2, which relates to elastic behaviour in rubberlike systems and this permits us to deal directly with the physical behaviour of rubberlike or flexible systems whose physical/mechanical properties depend directly on the length of these flexible chains M,. When dealing with macromolecules of much higher molecular weight the double components of the NMR signal strength may well appear even when there is no permanent crosslinked network, as can be readily verified by complete solubility in a solvent. The explanation is that the specimen under study has in effect two components. free chains and temporary networks due to entanglements of adjacent chains. At the temperature envisaged these disappear and reform elsewhere to give a temporary network, one which lasts long enough to be detected by the NMR technique. Such mobile sets of entanglements can greatly influence the physical behaviour of polymeric systems and can be in addition to any permanent crosslinks. Further information can be derived concerning the presence of partially crystalline or other more rigid structures within the specimen. Measurement of such T, pulsed NMR spectra can thus provide considerable amounts of quantitative information on the physical/mechanical behaviour of macromolecular systems, information which would be difficult to obtain in other ways, one which is nondestructive and requires only small amounts for examination. Here radiation appears as a
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vital provider of well-characterised macromolecular systems which can then be used to study and interpret more fully the NMR patterns; these can then serve to trace the morphology of a wide range of materials under a range of conditions, and not necessarily involved in radiation effects. This is one more example showing how a series of experiments with irradiated samples can lead to a powerful new technique with very wide applications in many other fields. Although this method of analysis has been evolved for simpler macromolecular systems such as polymers, one would hope to see it extended in more complex systems and especially to biological macromolecules. For example can the considerable biological effects seen at very small doses be ascribed to changes in morphology and entanglements in biological macromolecules rather than to chemical changes? This can be traced by pulsed NMR T, measurements. Could the great sensitivity of DNA to extremely small radiation doses be traced from TZ measurements? If the major effect of radiation on DNA associated in pairs is to link them together by only one permanent crosslink, this would prevent their separation into single DNA molecules so that they are effectively dead from the point of view of further duplication. Could this difference in mobility under different conditions be traceable from the T, NMR spectra of unirradiated and irradiated specimens? Another possibility is radiation-induced chain scission as the major effect of radiation, assuming always that immediate repair cannot take place. Here pulsed NMR could provide valuable evidence. In any case it appears that the expanding use of pulsed NMR spin-spin relaxation curves with their present or more advanced interpretation offers the most promising new line of research, ranging from physical properties and behaviour of the mechanical properties of simple polymeric systems to a better understanding of the behaviour of biological macromolecules following exposure to very low doses of radiation, together with minute chemical changes.
5. The nature matter
of radiation
and its interaction
with
During most of the twentieth century scientists concerned with radiation and its interaction with matter have been faced with a very fundamental question which even now has not been settled, the nature of radiation itself. In certain experiments radiation has to be considered as a flow of well-spaced and independent particles each of subatomic size and each reacting with atoms or molecules in its immediate vicinity. In other experiments radiation behaves as a wave, but this is not due to a flood of individual particles (electrons, alphas, etc.); each single particle still behaves independently but still shows this extensive spread over a considerable area. This dual be-
haviour represents a complete and fundamental contradiction, shown in many of our radiation experiments. When should we ascribe an effect as caused by a wave extending over an extensive structure and interacting over complete macromolecules or more, and when by a particle now reacting on a far smaller atomic scale? Vastly different effects might be expected from these two quite valid but contradictory models. Far example when a visible or UV photon with a wavelength of many thousand atomic spacings is absorbed and reacts, does it do so with its extensive wave structure covering a large molecular array still intact or does it change and cover only a single atom or chemical bond? Obviously the whole subject of radiation would be entirely different in these two scenarios especially when dealing with macromolecules. How do we decide? In practice we choose the most convenient for our purpose. Numerous attempts have been made by our leading scientists to resolve the problem but despite extraordinary assumptions none has been fully accepted. One involves probability concepts leading to a mathematical expression based on the simultaneous passage of each single particle through all the slits of an interference lattice. The mathematics provides an accurate answer for its distribution as shown in the interference pattern, but achieves this only by ignoring the physical conditions; each unit particle cannot be split between all these slits far apart. What happens to our Radiation Physics and Chemistry where a particle reacts with atoms in its close vicinity? Another attempted solution is purely linguistic and replaces the word “contradictory” by “complementary”. A third is more philosophical; objects only become real when they have been observed. No explanation need be provided for the wave aspect if it is not observed! if no universe existed until some gifted real observer was present (but who observed him?). A fourth is to postulate a quasi infinite number of Universes of which we only inhabit one (very expensive on Universes to solve a mathematical difficulty). It is clear from these and many other attempted explanations, none widely accepted, that this very fundamental problem remains to be solved. In macromolecular studies waves and particles may both be involved. Which of these contradictory models do we accept? The solution I wish to outline is one which reexamines our basic concepts of time and space. We have always assumed these to be continuous. However the mass of our unit particle at rest (m, of an electron, proton, etc.) is certainly quantised (no half electron) and so must be the frequency v0 since there is a direct relation between energy, mass and frequency E, = mOcZ = hv, (and hence its reciprocal a unit time to = l/u, = h/m,c’). All these change with velocity (relativity) but there is another unit so, a combination of time and distance which does not vary in this way; s0 is our true fundamental unit. For an electron s0 = 0.809 X lo-21 s and for a proton s0 = 0.44077 X 10pz3 s. With these very small units certain
A. Charlesby/Nucl. Ins@. and Meth. in Phys. Res. B 105 (1995) 217-224 features of the quantum theory begin to appear, such as the Uncertainty Principle. This simply means that the interval s of space-time between two events cannot be determined to closer than one unit su. The corresponding units of time t,, and distance r(, depend only on sa and the relative velocity I’. t,, = h/me’ = h/E and r. = h/mrs. Thus the minimum unit distance r,, turns out to be nothing other than A. the de Broglie wavelength which now acquires a real physical meaning and distances cannot be measured to closer than A, anymore than can we have fractions of an electron mass or charge. The resolved components of s,, i.e. t,, and rr, represent basic time and distance units for our measurements including those involved in radiation physics and chemistry; t,, = h/me’ = h/E and r. = h/me,. Here s,, = h/m,,c’ is the fundamental unit, dependent only on rest mass m,,. However in an interaction with another particle of rest mass M,, this unit for the pair decreases to the much smaller value s * = h/cm,, + M,)c’. This could account for differences in reactions with very different rest masses since these fundamental units differ. This could explain differences in radiation effect, depending on masses of radiation particle and irradiated sample. One of the great unsolved mysteries of this behaviour of particles/waves is in interference. Each electron etc. passing through a structure with regular lattice can produce an interference pattern (wave behaviourl. However, if detected immediately after this passage by some detecting device, it appears to have passed through one slit only (particle behaviourl; and the interference pattern disappears. It therefore appears as if the electron knows beforehand that it will be observed immediately after passage and therefore chooses passage as a single particle or wave accordingly; this represents the most extraordinary inversion of cause and effect. Although the location of detection follows the passage of radiation, it determines its prior behaviour as particle or wave. This recognition argues most strongly in favour of one of the most fundamental concepts debated in philosophy namely predestination. In fact this complete reversal of all our customary scientific/philosophical beliefs is not necessary with the umttsatton mto .s,) units. The incident particle passing through the lattice is spread very widely over a single unit of distance r,) = A, corresponding to sa (= h/m,c’) but when it encounters the detector of rest mass Ma this unit shrinks to the much smaller value sty = h/(m,, + M&z and this may correspond to a single interatomic spacing. It is not due to the presence of an observer that this abrupt shrinkage occurs, but merely to its interaction with a much heavier particle. The same pattern of shrinkage behaviour is to be expected in Radiation Chemistry, when each incident particle can traverse a specimen without interaction and covering a large transverse area (sol but when interacting this lateral dimension shrinks to a much smaller dimension s(;. This transition in pattern of interaction must be quite fundamental in our basic studies in radiation interactions in different environments and masses and will
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no doubt emerge from further studies on the behaviour of radiation passing through matter. It may also intervene when investigating such subjects as the frequency of interaction of each electron penetrating a system of diverse atoms and molecules with additives, radiation protectors, etc. This would open up a very exciting and fundamental new field for Radiation Physics, Chemistry and 1 hope Radiobiology. It may also intervene in energy transfer. What are the conditions promoting this very effective interaction and resultant collapse from extensive wave to atomic particle and what is its effective range? This seems to me to be one of the most important problems we have to face in radiation effects. I speak as one who spent years of research with extensive electron waves, and many more with electrons as subatomic particles but found no difference between these electrons. This is a problem we cannot avoid by holding different conferences. one for the electron as a wave, the other as a particle.
6. Ionic tracks and reactions At this symposium there have been an exceptionally large interest and number of papers concerning the irradiation effects of heavier ions. One of the most interesting aspects has been the observation and analysis of the structure of the individual tracks and even of possible links with some fundamental biological studies. It is from such unexpected overlaps between very different fields of science that we can anticipate further major new advances. One question which was not settled was the cause of the track dimensions of an individual heavy ion progressing into a solid structure. For such an ion the primary lateral dimensions should involve the uncertainty principle almost on an experimental scale, since we can measure its position or track and know its energy or momentum. It this a promising new method of investigating the meaning of the uncertainty principle? Surrounding this internal core there is a much wider cylinder arising from secondary effects due to low energy deltas. Other aspects that might be followed up are the mechanisms by which deposited energy is conveyed to its track by a high energy ion i.e. the repartition of the energy it distributes. Is this spread in a purely physical form and almost at random and only later redistributed to selected chemical bonds according to their chemical nature and reactions, or is the initial distribution of the energy transfer already selected by the chemical nature; an example would be the differences in energy transfer between linear and aromatic structures. In any case we still have to account for the difference between the two fundamental wave and particle models for the individual heavy ion traversing an irradiated solid, where electrical and chemical changes result. It was noted that the presentations at this symposium considered the heavy ion as a particle; its complementary wave structure is largely ignored. This alternative wave model should
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appear in some form of interference; one of the points raised was its occurrence with electrons and neutrons passing through a regular molecular network but not with a similar passage by a high energy proton. This should certainly be verified; if confirmed the more generally accepted wave mechanics would have to be reconsidered.
References [l] A. Charlesby, Radiat. Phys, Chem. 1 (1992) 45. [2] A. Charlesby, Radiat. Phys. Chem. 45 (1995) 175
@ ELSEVIER
Present and future developments
in polymer radiation
Arthur Charlesby Silver Spring, Watchfield, Swindon SN6 BTF, UK
1. Introduction The study of high energy radiation and its effects covers an immense range of scientific and technological interests, from the basic nature of radiation in all its forms on to its interaction with matter and the physical, chemical and biological changes it can produce. One can then consider potential applications over a wide range of subjects, many still to be explored, understood and evaluated. Here one can only give a short outline of a number of these possibilities, knowing well that many more, some as yet unsuspected, will be studied and evaluated in the next decades. The earlier applications such as X-rays for determination of structure and medical purposes could only use the low doses available at the time and are best considered as limited to research and examination purposes only. It is only with the advent of far more powerful sources, either nuclear or high-voltage electrical, that one could seriously consider the practical applications of high energy radiation to the large-scale production or modification of materials by exposure to radiation. The earliest large-scale irradiation processes were the modification of polymers, usually by crosslinking, and the sterilisation of medical products. It will be noted that both are concerned with high molecular weight materials in which a very small change in chemical structure can lead to major changes in physical, chemical, or biological behaviour. For over about forty years these very practical processes have grown throughout the world and now amount to many billions of dollars yearly. Considering the immense scientific and technical effort which has been devoted to many aspects of the subject it is perhaps surprising that more major new radiation applications have not entered the industrial and medical worlds in the last four decades. This does not mean that considerable advances have not been made both on the more academic side or in technical improvements. But why no more entirely new applications into other major fields? Is it because academic studies and technical progress have diverged? Or is there some simpler explanation such as the economic climate and high capital cost of radiation sources. 0168-583X/95/$09.50
It will be noted that the greatest successes have arisen when two apparently very different fields have been allowed to overlap; as with nuclear energy and the physical properties of polymers; similarly for medical sterilisation. In the latest period much excellent research and technical work has been devoted to understanding and improvement within these well-established fields. Considerable efforts have also been devoted to the more scientific aspects of the subject. Examples are the determination of reaction constants in chemical kinetics and the data for energy transfer from energetic particles. Such basic information is of the greatest value in academic research, in the production of theses and papers for scientific journals; resulting in improvement rather than innovations in these respective fields. Many of these researches have involved the application of the formulae of quantum theory, although its fundamental understanding in physical rather than mathematical terms still remains obscure. We still need to derive a physically acceptable model; most of us are not satisfied with a series of mathematical relations based on incompatible scientific models. Another line of great interest lies in the development of new methods of analysis, based on the study and measurement of a series of samples of known characteristics determined by the radiation doses received. Such new techniques can then be applied in many other situations or specimens where no radiation need be involved. One notable example is in the use of pulsed NMR T, spin-spin relaxation curves to determine many of the fundamental and largely physical characteristics of macromolecules. For some considerable time Nuclear Magnetic Resonance has been used to trace chemical changes in structure. Such changes could have occurred in many ways, including exposure to high energy radiation. A more recent set of discoveries, based on a series of measurements of irradiated polymeric samples, has led to the conclusion that pulsed NMR T, curves can be used to trace many of the more physical aspects of the samples examined including such important features as average molecular weight, viscosity and chain mobility but also other aspects of its morphology such as the presence of a partially crosslinked
0 1995 Elsevier Science B.V. All rights reserved
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network, the average chain distance between successive links I&, the presence of chain entanglements which on a temporary scale can play the same role as permanent crosslinks, the existence of a partially crystalline or even a less regular semi-solid component, and other factors such as temperature, dilution and orientation which influence the physical properties of an irradiated structure. It was concluded that pulsed NMR could be used to measure in a simple way many fundamental features of a macromolecular system. Since it is largely for these characteristics that these materials are produced, one can hope to see this simple, rapid and non-destructive technique greatly extended and utilised in fields remote from radiation studies. One possible line involves changes in morphology in irradiated biological systems at low doses. Studies in one field i.e. radiation effects can be applied in scientific or technical fields where radiation itself is not involved. For example this NMR technique might well prove a great value in the determination of important physical characteristics of polymeric systems in routine industrial production. One could then extend this method to study the effect of polymer orientation and tensile strength, a long way from high energy radiation but we can then double back to discover whether the irradiation of polymers with some degree of asymmetry may serve to enhance their physical behaviour in one direction even if this reduces mechanical properties in other directions. The effect of radiation on electrical properties is very well known but this refers primarily to the immediate effects. Far less is known about the longer-term effects in some selected structures and here one of the major advantages of radiation treatment might prove of special value. I refer to the ability it confers to vary the effective dose with depth as well as over the exposed area. How far might electron irradiation treatment over a depth of perhaps a mm or two from the surface provide a thin surface layer of irradiated product with very different electrical properties to the overall system. To carry out exploratory studies on such systems would require the closest collaboration of an electronic physicist of great imagination with a radiation physicist and an experienced materials scientist. Overlap of interests could result in major advances. Radiation chemistry has grown into a major field of chemistry and a most valuable and well-recognised method of determining reaction constants (even when they are not really constants but can be determined over a range of other variables). Many of these constants can be utilised in other branches of chemistry and this method has acquired full respectability e.g. for the reactions of liquids. The number of such determinations is legion and also serves to fill scientific journals and student theses. The difficulty I find is to know which of these determinations has proved of value in other studies. (Of course I speak now as a physicist with many far-sighted chemical friends.) However there remain further chemical fields to explore as in
mixtures, comparison of such constants in the solid and liquid states and the effect of very low concentration of selected additives and possibly of orientation. A major subject of interest concerns the mechanism of radiation energy absorption from an incident high energy particle; its transfer as one or more quantities of energy from the initial site of absorption to neighbouring atomic or molecular structures; alternatively the immediate reaction to give radicals, excited species or ions plus charges which are then distributed through neighhouring portions of the irradiated species. How far is this initial charge transfer dependent on the chemical rather than physical structure (e.g. is the loss of perhaps 100 eV from the primary irradiation species dependent on a primarily chemical difference between saturated and unsaturated bond and how far does this initially absorbed energy move as energy before initiating the primary chemical reaction leading to radical formation or ionisation along a molecular chain, or between such chains? My own estimates, long out-of-date, led me to a distance of some 30 C-C spacings. Can we have better values? There are many other aspects of this early history between the initial energy absorption from an incident high energy particle and its almost ultimate appearance in a more conventional chemical state as a trapped radical or as an ion plus an electron. This is certainly a field which has received much academic study and deserves more reviews and summaries written primarily for those unfamiliar with the subject (or is it subjects) and wishing to know more. At this stage it may be worth bringing up the question of the different types of radiation. We can accept (for the time being at least) the true unity (i.e. single nature) of electrons, protons, etc. which in large numbers constitute a beam of high energy particles. In passing through a thin layer of a specimen they do so as individuals and react also as individuals; they are too far apart to react with each other at normal intensities, and this is not always appreciated. Any intensity effect must then arise as a reaction between longer-lived products of the primary irradiation process. But what are we to make of such particles as the photon for example, with a spin of 1, zero rest mass and considerable extent, i.e. covering many wavelengths? How can this extensive wave packet be concentrated instantaneously into atomic dimensions to enable it to react with one specific bond? One possibility is to assume a composite nature of a photon, as comprising an electron (-charge, + mass) and an antielectron (+ charge, -mass) created from a composite nothing by external energy to separate the charges and masses The creation and nature of such a composite particle, its velocity c in vacuum, the electromagnetic wave it produces in the vicinity during its passage, its unit spin, etc. all argue for a composite structure. This composite photon (such as e-m+ chasing e fm-) moves freely in space at velocity c, and when it eventually reacts with any
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structure, the leading antielectron is annihilated by collision with an electron of the structure, releasing its following electron as a photoelectron! Another type of radiation which is receiving much attention for the effects it produces is the laser; this is sometimes described as a series of photons of light waves in phase, so that their amplitudes A add simply without loss due to phase differences. The difficulty here is that for ten photons in phase in a laser beam, the total amplitude in phase is tenfold greater than that of a single one but the intensity which is the square of the amplitude is one hundred times greater. Since the total energy is proportional to intensity and not amplitude, one must then inquire where the extra energy (10-100) comes from. Studies in the effects of laser effects might well elucidate such questions as the true nature of the photon. Does laser light really consist of waves rather than separate particles which can only appear at the site of interaction. There are many other fundamental questions on the basic nature of radiation which remain unsolved although the mathematics involved are frequently used with excellent numerical results. The difficulty of reconciling both extensive waves and atomic dimensions for the same particle remains. To discuss this aspect more closely would lead us into a different direction from that usually covered in such conferences, rather more in the direction of basic physics than of possible radiation applications. We are still very far from having an adequate understanding of the fundamental nature of radiation and here we may expect and hope for important advances in the next generation, not in terms of better mathematical relations but rather of their interpretation in physical terms. It cannot yet be said that we understand the physical meaning of the wave function P yet we use it most frequently and successfully in so many problems of quantum mechanics. An outline of a possible newer and simpler interpretation of this fundamental scientific problem is outlined below. A mathematical expression (or rather two) cannot replace an acceptable physical explanation as a form of scientific understanding, and this is best shown by the remarks of leading experts in the field.
2. Energy deposition in polymers During the passage of high energy radiation through a medium it loses energy by one or more physical processes and possibly only after its redistribution can this result in atomic displacement, ionisation and excitation followed by chemical reactions in organic systems resulting from the formation of radicals and of ionised species together with free or trapped electrons and displaced atoms. These can then react in ways similar to those resulting from other chemical processes. However their concentration and distribution as a result of irradiation may be very different.
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The subsequent chemical processes in polymeric structures are not inherently different to those produced in related low molecular compounds but because of the high molecular weight involved each chemical change produced in this way can greatly affect the subsequent behaviour of a large molecule. One reaction in simple linear hydrocarbons often results in linking them together and this can require a dose (energy per g) far greater with low molecular weight material as compared with high molecular weight polymer simply because there are far more molecules in I g of irradiated material, and each molecule must on the average be affected once in this way. In biological molecules of far greater molecular weight the doses required could be further reduced. Of course differences in chemical structure are also involved. Thus for radiation modification of polymers the dose requirements can be so low as to render radiation treatment economically feasible, even without a chain reaction. On the other hand, largescale radiation processing of lower molecular weight systems is only justified if the modified material is very valuable or if a chain reaction is involved whereby one radiation-induced reaction can lead to many further chemical reactions. The use of radiation to promote such chain reactions is demonstrated in grafting reactions, discussed in a number of papers at this meeting. A further means of enhancing the radiation-induced reaction is with a polymer dissolved e.g. in water. The radiation energy absorbed in the water provides reactive species which then react further with the polymer. Although a desired effect is produced at a lower dose this does not necessarily represent an improved yield since only the smaller amount of polymer contained in the solution is thereby modified. However the product itself can then be of special value, such as a swollen system in which the degree of swelling is determined in an accurately controlled manner. It is even possible to produce swollen polymeric structures whose degree of swellability varies at different parts within the system. I do not know how such a system can be utilised but it is useful to know that it could be manufactured readily if required for some technical, biological or medical purpose. It will be realised that the cost of radiation plays the most important role in deciding whether a large-scale industrial process can be justified. Since the cost of radiation is primarily that of the capital installation, especially for particle sources such as electron beams, and the latter is greatly dependent on building and shielding costs, the scale of industrial production is the most important factor. Advances into new fields with a smaller production rate, at least in the initial stages, may be greatly influenced by the production of lower cost irradiation equipment for development purposes. To avoid these initial costs advantage can be taken of existing research installations in universities and other research institutes holding appropriate radiation equipment, albeit on a small scale. Radiation doses are usually expressed in kGy or in III. ADVANCED
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Mrad, (1 Mrad = 10 kGy). 1 kW h = 3.6 X lo6 J or if absorbed 1 Mrad = 10 kGy to 360 kg of irradiated material. For sterilisation purposes a dose of 2.5 Mrad or 25 kGy is generally accepted as effective so that 1 kW h can produce a maximum of 140 kg of sterile material if fully absorbed. For many polymers undergoing crosslinking without any chain reaction higher doses are required due in part to their smaller molecular weight. Depending on the changes desired a dose several times greater is usually needed, amounting to about lo-25 Mrad or loo-250 kGy with a corresponding increase in radiation cost. Another manner of considering dose requirements is to express the average energy input to promote a certain reaction in eV; or more frequently as the number G of such reactions resulting from an energy absorption of 100 eV. Typical G values for many radiation-induced reactions in organic systems are in the range 1 to 3. Thus a total energy absorbed of 1 kW h or 2.25 X 10” eV can produce G X 2.25 x 10z3 reactions of that type. If G = 3 this amounts to 1.1 moles of radicals, ions or any other species to which the G value refers. The main advantages of irradiation of polymers of certain types notably polyethylene, silicones, etc. are to promote crosslinking and to form a network structure. The properties of such networks are well known and need not be outlined further here. Further research on similar reactions in other polymers, copolymers and mixtures are still proceeding apace, and are often described in the appropriate scientific and technical journals. Other changed properties including electrical, luminescent and crystalline state have also been reported. Modifications such as irradiation in solution or in the presence of reactive additives have also been investigated and we may expect such exploratory investigations to serve in opening new lines of parallel studies of additive behaviour in biologically active structures. Irradiation of oriented or stressed polymeric systems may be found of scientific and technical value. Interest must also be paid to the physical behaviour of polymeric systems with differential dose treatment and degrees of crosslinking, swelling, etc. For such studies the reactions induced by high energy radiation throughout a solid specimen are uniquely qualified. Many of us who have long been concerned with radiation of various types are very excited by its past and promises of wide possibilities of the future. It is highly likely that there exist needs which could well be met by some form of radiation modified reaction or treatment but of which we as radiation specialists are unaware. At the same time there are scientists, engineers, medical practitioners and others working in very different fields fully acquainted with these unfulfilled requirements but who are not aware of the new possibilities opened up by some radiation treatment. Perhaps at some informally organised encounters between two such very different groups, new targets could be set and further progress achieved in new and unexpected directions.
3. Polymers and biopolymers There is one example of the value of overlap between adjacent branches of science and especially of radiation science where it seems to me that great steps may be expected in the near future. Some may indeed already have been made without being fully recognised. I refer to the considerable amount of information we have already gained from the irradiation of simple macromolecules and the potential expansion of such studies into the field of biomolecules and eventually into radiobiology. In particular how far have the radiation processes studied and evaluated quantitatively in simple macromolecules been assessed critically in irradiated biomolecules? It need hardly be said that an immense amount of fine work has been devoted to the chemical changes which occur e.g. in irradiated DNA. But is this approach taken by radiobiologist looking for a change in chemical structure, the most suitable line to follow? Here is one argument which I believe indicates that the major effects are not simple changes in chemical structure but more likely changes in molecular configuration. Taking the lethal dose for a human being as about 500 rad or its equivalent of 0.3 X 10lh eV/g, the number of changes resulting from this dose is G X 0.3 X lO”/g where G is the average number of chemical changes per 100 eV energy absorbed, typically about l-3. This lethal dose would result in only about one radiation-induced chemical change in a molecular weight of 10’” (this figure would be modified to a limited extent by the presence of water and changes in chemical structure but is certainly of the correct order of magnitude). Thus a lethal dose need involve only about one chemical change within each molecule such as one DNA. Although there may be some degree of selectivity as there is in simpler organic systems, this would not greatly detract from the need to explain this very remarkable degree of selectivity in chemical reaction, probably impossible to attain in any organic structure, and indeed to detect in such systems. To achieve this very high degree of response would involve a high concentration of the very sensitive reacting group but there is no such evidence. Another form of reaction would seem to be needed and one assumption is a major change in shape or structure of the DNA molecules. In irradiated polymers, drastic changes do in fact occur at low doses, corresponding to about one change per molecule. These can take the form of linking molecules together to form ultimately a network structure. This behaviour following a relatively low dose has been fully investigated and is used on a large scale commercially to change drastically the physical properties and behaviour of simple polymers. Another possible radiationinduced reaction at very low doses involves scission in the main chain of the macromolecule. Some modification of the doses required may indeed occur owing to the presence of water and to some forms of chemical bonding but the overall picture remains valid. A lethal dose of 500 rad (5
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Gy) can result in a drastic change in configuration and other behaviour of a very high molecular weight molecule. For example the linking of two DNA molecules together by radiation would prevent their separation and their ability to initiate the growth of further molecules. Scissioned DNA molecules would be likewise unable to replicate and the system may be considered as effectively dead. The doses involved for this fate would appear to agree with those deduced from simple radiation chemistry of macromolecules. Methods to follow up this suggestion are outlined below. Another line of research in biological systems and models can also be derived based on studies with simpler macromolecules. 1 refer to the influence of very low concentrations of additives which are found to modify radiation-induced reactions in macromolecules. By reacting with radicals and other reactive species notably oxygen such additives can greatly affect the course of a radiationinduced reaction, and since in an aqueous medium they can have great mobility, their influence on the less mobile macromolecules can be very considerable. The study of the effect they can have on irradiated simpler macromolecules and its extension to biomolecules therefore offers an exciting area for research. combining as it does the simpler radiation-chemical with the somewhat analogous radiation biology fields.
4. Pulsed NMR and radiation-induced changes
morphological
Much effort has been spent in devising methods of analysing the effects of exposure of polymers to radiation. Apart from conventional chemical methods to determine changes in chemical structure, methods particularly suitable for high molecules deal with changes in molecular weight, degree of crystallinity, viscosity, solubility and swelling. The use of pulsed T1 NMR offers a different type of information, more closely related to the morphology of these molecules. Thus a closely wound molecule will offer a very different NMR pattern than it would show in an extended form, even if the chemical structures are identical. For many purposes this distinction could be of the greatest importance when more physical properties such as elasticity or chain flexibility are involved, possibly with little or no change in chemical structure. A simple example is the range of different chemical macromolecules which can provide very similar rubberlike properties. Such materials can be analysed very readily from the pulsed NMR spectra even without any detailed knowledge of their chemical structure. This method is especially valuable when it is the change in certain aspects of morphology following irradiation rather than chemical structure whose behaviour is being investigated. In such NMR measurements it is the Tz spin-spin relaxation curves which are studied rather than the more usual T,. T, provides infor-
2’1
mation on changes in morphology (molecular configuration) of macromolecules resulting from irradiation even if the chemical changes are minute due to the low doses needed. For simple flexible macromolecules below a certain molecular weight (about lo4 typically) the pattern follows a simple exponential decay exp( - r/7’,) where T2 is simply related to average molecular weight. T2 = C.M”.‘. The value of C depends on the nature of the macromolecule being studied, as well as the temperature and environment which influence molecular mobility. In polyisobutylene for example this relation applies down to a very low molecular weight (less than 100). Once the value of C has been measured for a characterised macromolecule it can be used not only for determining the effect of radiation but also more generally for large-scale and rapid testing of industrial products. r, may be considered as a measure of molecular flexibility and of course it varies with temperature, molecular weight and concentration if in solution. In a partially crosslinked system (whether by chemical crosslinking agents or by radiation) the NMR spin-spin relaxation curve becomes the superposition of two exponential T2 curves; f.exp( -t/r,,) + (1 -f).exp(-r/T,,) and the values of f and 1 -f represent the relative importance of the non-network and network fractions. T,, represents the length of chains not incorporated into the network structure or at one end only, while T2\ measures the length of chains between successive links, the quantity usually denoted by M, in measurements of chain length within the network; it is T2, which relates to elastic behaviour in rubberlike systems and this permits us to deal directly with the physical behaviour of rubberlike or flexible systems whose physical/mechanical properties depend directly on the length of these flexible chains M,. When dealing with macromolecules of much higher molecular weight the double components of the NMR signal strength may well appear even when there is no permanent crosslinked network, as can be readily verified by complete solubility in a solvent. The explanation is that the specimen under study has in effect two components. free chains and temporary networks due to entanglements of adjacent chains. At the temperature envisaged these disappear and reform elsewhere to give a temporary network, one which lasts long enough to be detected by the NMR technique. Such mobile sets of entanglements can greatly influence the physical behaviour of polymeric systems and can be in addition to any permanent crosslinks. Further information can be derived concerning the presence of partially crystalline or other more rigid structures within the specimen. Measurement of such T, pulsed NMR spectra can thus provide considerable amounts of quantitative information on the physical/mechanical behaviour of macromolecular systems, information which would be difficult to obtain in other ways, one which is nondestructive and requires only small amounts for examination. Here radiation appears as a
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vital provider of well-characterised macromolecular systems which can then be used to study and interpret more fully the NMR patterns; these can then serve to trace the morphology of a wide range of materials under a range of conditions, and not necessarily involved in radiation effects. This is one more example showing how a series of experiments with irradiated samples can lead to a powerful new technique with very wide applications in many other fields. Although this method of analysis has been evolved for simpler macromolecular systems such as polymers, one would hope to see it extended in more complex systems and especially to biological macromolecules. For example can the considerable biological effects seen at very small doses be ascribed to changes in morphology and entanglements in biological macromolecules rather than to chemical changes? This can be traced by pulsed NMR T, measurements. Could the great sensitivity of DNA to extremely small radiation doses be traced from TZ measurements? If the major effect of radiation on DNA associated in pairs is to link them together by only one permanent crosslink, this would prevent their separation into single DNA molecules so that they are effectively dead from the point of view of further duplication. Could this difference in mobility under different conditions be traceable from the T, NMR spectra of unirradiated and irradiated specimens? Another possibility is radiation-induced chain scission as the major effect of radiation, assuming always that immediate repair cannot take place. Here pulsed NMR could provide valuable evidence. In any case it appears that the expanding use of pulsed NMR spin-spin relaxation curves with their present or more advanced interpretation offers the most promising new line of research, ranging from physical properties and behaviour of the mechanical properties of simple polymeric systems to a better understanding of the behaviour of biological macromolecules following exposure to very low doses of radiation, together with minute chemical changes.
5. The nature matter
of radiation
and its interaction
with
During most of the twentieth century scientists concerned with radiation and its interaction with matter have been faced with a very fundamental question which even now has not been settled, the nature of radiation itself. In certain experiments radiation has to be considered as a flow of well-spaced and independent particles each of subatomic size and each reacting with atoms or molecules in its immediate vicinity. In other experiments radiation behaves as a wave, but this is not due to a flood of individual particles (electrons, alphas, etc.); each single particle still behaves independently but still shows this extensive spread over a considerable area. This dual be-
haviour represents a complete and fundamental contradiction, shown in many of our radiation experiments. When should we ascribe an effect as caused by a wave extending over an extensive structure and interacting over complete macromolecules or more, and when by a particle now reacting on a far smaller atomic scale? Vastly different effects might be expected from these two quite valid but contradictory models. Far example when a visible or UV photon with a wavelength of many thousand atomic spacings is absorbed and reacts, does it do so with its extensive wave structure covering a large molecular array still intact or does it change and cover only a single atom or chemical bond? Obviously the whole subject of radiation would be entirely different in these two scenarios especially when dealing with macromolecules. How do we decide? In practice we choose the most convenient for our purpose. Numerous attempts have been made by our leading scientists to resolve the problem but despite extraordinary assumptions none has been fully accepted. One involves probability concepts leading to a mathematical expression based on the simultaneous passage of each single particle through all the slits of an interference lattice. The mathematics provides an accurate answer for its distribution as shown in the interference pattern, but achieves this only by ignoring the physical conditions; each unit particle cannot be split between all these slits far apart. What happens to our Radiation Physics and Chemistry where a particle reacts with atoms in its close vicinity? Another attempted solution is purely linguistic and replaces the word “contradictory” by “complementary”. A third is more philosophical; objects only become real when they have been observed. No explanation need be provided for the wave aspect if it is not observed! if no universe existed until some gifted real observer was present (but who observed him?). A fourth is to postulate a quasi infinite number of Universes of which we only inhabit one (very expensive on Universes to solve a mathematical difficulty). It is clear from these and many other attempted explanations, none widely accepted, that this very fundamental problem remains to be solved. In macromolecular studies waves and particles may both be involved. Which of these contradictory models do we accept? The solution I wish to outline is one which reexamines our basic concepts of time and space. We have always assumed these to be continuous. However the mass of our unit particle at rest (m, of an electron, proton, etc.) is certainly quantised (no half electron) and so must be the frequency v0 since there is a direct relation between energy, mass and frequency E, = mOcZ = hv, (and hence its reciprocal a unit time to = l/u, = h/m,c’). All these change with velocity (relativity) but there is another unit so, a combination of time and distance which does not vary in this way; s0 is our true fundamental unit. For an electron s0 = 0.809 X lo-21 s and for a proton s0 = 0.44077 X 10pz3 s. With these very small units certain
A. Charlesby/Nucl. Ins@. and Meth. in Phys. Res. B 105 (1995) 217-224 features of the quantum theory begin to appear, such as the Uncertainty Principle. This simply means that the interval s of space-time between two events cannot be determined to closer than one unit su. The corresponding units of time t,, and distance r(, depend only on sa and the relative velocity I’. t,, = h/me’ = h/E and r. = h/mrs. Thus the minimum unit distance r,, turns out to be nothing other than A. the de Broglie wavelength which now acquires a real physical meaning and distances cannot be measured to closer than A, anymore than can we have fractions of an electron mass or charge. The resolved components of s,, i.e. t,, and rr, represent basic time and distance units for our measurements including those involved in radiation physics and chemistry; t,, = h/me’ = h/E and r. = h/me,. Here s,, = h/m,,c’ is the fundamental unit, dependent only on rest mass m,,. However in an interaction with another particle of rest mass M,, this unit for the pair decreases to the much smaller value s * = h/cm,, + M,)c’. This could account for differences in reactions with very different rest masses since these fundamental units differ. This could explain differences in radiation effect, depending on masses of radiation particle and irradiated sample. One of the great unsolved mysteries of this behaviour of particles/waves is in interference. Each electron etc. passing through a structure with regular lattice can produce an interference pattern (wave behaviourl. However, if detected immediately after this passage by some detecting device, it appears to have passed through one slit only (particle behaviourl; and the interference pattern disappears. It therefore appears as if the electron knows beforehand that it will be observed immediately after passage and therefore chooses passage as a single particle or wave accordingly; this represents the most extraordinary inversion of cause and effect. Although the location of detection follows the passage of radiation, it determines its prior behaviour as particle or wave. This recognition argues most strongly in favour of one of the most fundamental concepts debated in philosophy namely predestination. In fact this complete reversal of all our customary scientific/philosophical beliefs is not necessary with the umttsatton mto .s,) units. The incident particle passing through the lattice is spread very widely over a single unit of distance r,) = A, corresponding to sa (= h/m,c’) but when it encounters the detector of rest mass Ma this unit shrinks to the much smaller value sty = h/(m,, + M&z and this may correspond to a single interatomic spacing. It is not due to the presence of an observer that this abrupt shrinkage occurs, but merely to its interaction with a much heavier particle. The same pattern of shrinkage behaviour is to be expected in Radiation Chemistry, when each incident particle can traverse a specimen without interaction and covering a large transverse area (sol but when interacting this lateral dimension shrinks to a much smaller dimension s(;. This transition in pattern of interaction must be quite fundamental in our basic studies in radiation interactions in different environments and masses and will
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no doubt emerge from further studies on the behaviour of radiation passing through matter. It may also intervene when investigating such subjects as the frequency of interaction of each electron penetrating a system of diverse atoms and molecules with additives, radiation protectors, etc. This would open up a very exciting and fundamental new field for Radiation Physics, Chemistry and 1 hope Radiobiology. It may also intervene in energy transfer. What are the conditions promoting this very effective interaction and resultant collapse from extensive wave to atomic particle and what is its effective range? This seems to me to be one of the most important problems we have to face in radiation effects. I speak as one who spent years of research with extensive electron waves, and many more with electrons as subatomic particles but found no difference between these electrons. This is a problem we cannot avoid by holding different conferences. one for the electron as a wave, the other as a particle.
6. Ionic tracks and reactions At this symposium there have been an exceptionally large interest and number of papers concerning the irradiation effects of heavier ions. One of the most interesting aspects has been the observation and analysis of the structure of the individual tracks and even of possible links with some fundamental biological studies. It is from such unexpected overlaps between very different fields of science that we can anticipate further major new advances. One question which was not settled was the cause of the track dimensions of an individual heavy ion progressing into a solid structure. For such an ion the primary lateral dimensions should involve the uncertainty principle almost on an experimental scale, since we can measure its position or track and know its energy or momentum. It this a promising new method of investigating the meaning of the uncertainty principle? Surrounding this internal core there is a much wider cylinder arising from secondary effects due to low energy deltas. Other aspects that might be followed up are the mechanisms by which deposited energy is conveyed to its track by a high energy ion i.e. the repartition of the energy it distributes. Is this spread in a purely physical form and almost at random and only later redistributed to selected chemical bonds according to their chemical nature and reactions, or is the initial distribution of the energy transfer already selected by the chemical nature; an example would be the differences in energy transfer between linear and aromatic structures. In any case we still have to account for the difference between the two fundamental wave and particle models for the individual heavy ion traversing an irradiated solid, where electrical and chemical changes result. It was noted that the presentations at this symposium considered the heavy ion as a particle; its complementary wave structure is largely ignored. This alternative wave model should
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appear in some form of interference; one of the points raised was its occurrence with electrons and neutrons passing through a regular molecular network but not with a similar passage by a high energy proton. This should certainly be verified; if confirmed the more generally accepted wave mechanics would have to be reconsidered.
References [l] A. Charlesby, Radiat. Phys, Chem. 1 (1992) 45. [2] A. Charlesby, Radiat. Phys. Chem. 45 (1995) 175