Scientific problems at issue

Radiat. Phys. Chem. Vol. 25. Nos. 1-3, pp. 287-289. 1985

0146-5724/85 $3.00 + .00 Pergamon Press Ltd

Printed in Great Britain.

SCIENTIFIC

PROBLEMS AT ISSUE A. Charlesby

Silver Spring, Watchfield Swindon SN6 8TF United Kingdom

The industrial use of high energy radiation as a large-scale processing technique reaches back about three decades, and was based on earlier academic investigations largely in two fields - the modification of plastics by crosslinking and scission, and the sterilization of pharmaceutical disposables. These two applications now greatly exceed an annual output of $! 09 , and are still growing rapidly. Moreover new fields of application are being promoted, so that we can foresee further increases, not only in well established fields, but also in many new industrial processes. One might therefore expect to see increasingly closer links between academic research, primarily in universities and industrial applications which in the long run require a fundamental understanding best sought in universities. It is therefore disappointing to see a tendency for these two to drift apart. Many industrial laboratories are largely concerned with the search for improvements in existing production techniques while university research is often directed towards the study of radiation kinetics in model liquid systems - these being a valuable method of deriving reaction rates of importance in chemical kinetics in liquids but of far lesser importance to radiation processing in solid systems. Nevertheless there are a few laboratories, well represented here, where a major effort is devoted to provide research and understanding of the most important and valuable reaction processes used in industry. It is to enable their leaders to present and exchange their views and opinions that this session is devoted. After three decades of rapid growth, there still remain unsolved a number of basic problems, offering a strong scientific challenge, and also of direct interest to existing and potential industrial processes. In introducing the discussions I will outline some of these problems, and then open the meeting to the views of a number of leading experts present, many of whom have been involved in such researches for a considerable time. It is not expected that any final or clear solution will emerge, but we can hope that the presentation of these very different, and possibly contradictory views will give a more detailed and fuller perspective, and lead us to a clearer understanding of the present situation of difficulties and of promising lines of research. The first problem concerns the mechanism of crosslink formation in long-chain polymers, and as polyethelene, polyisoprene and polydimethysiloxane. In spite of its fundamental importance we still lack a generally accepted answer: (a) Do such crosslinks occur by an ionic on by a radical mechanism (both are possible at the energies available)? (b) Presumably in the latter cases two radicals are needed in close proximity, an adjacent chain in suitable relative configuration. If this is accepted, are such radicals formed in pairs, or do they migrate together until they meet? (c) If the migration mechanism is accepted, what is the mechanism of such radical mobility? (d) In a partially crytalline polymer such as polyethylene, are radicals formed in the crytalline phase, and eventually crosslink there, or do they migrate to the crystal surface, and do they crosslink there? (e) Protection against radiation-induced changes is possible. Is this due to energy transfer (please elaborate on what this means in detail), to reaction with radicals, and especially H groups, or to some ionic interaction (e.g. with electrons)? (f) Row far do the reactions studied in simple long-chain polymers relate to those involved in the more complex biopolymers, and eventually to those in biological systems? There are a number of other existing questions to answer. I will s,~-~srize present views and hope subsequent comments will help to confirm or reject them.

some

of my

I would suggest that crosslinkin g involves two quite separate reactions, one i--,ediate, the other long-term. The immediate reaction, which may be ionic in character, cannot be avoided by additives (except those which absorb energy before a chemical change can occur). No chemical protection is possible. The second reaction requires two radicals, initially separate, to migrate together, and this takes time, and is likely to be very temperature dependent. A suitable additive can prevent this encounter (e.g. by reacting with a radical) and hence offer protection in the longer run. Evidence for this view is the long life of radicals, seen in ESR measurements, and especially in the effect of additives. Even a very low concentration greatly reduces crosslink formation, yet a far higher concentration has 287

288

A. CHARC~SB¥

little further effect. If this is correct, reduce crossllnking to a well-defined extent.

we

would

expect

Radical mobility can be explained by a hole-hopplng mechanism, abstraction reactions.

that

lower

temperatures

can

or by a series of H addition,

-CH 2- -~ -CH- 441 H + - C H 2 - ÷ - C H - +H 2 H 2 +-CH--~-C~ 2- +H etc. These reactions may be very improbable (but not impossible) thereby accounting for the slow radical decay. In the presence of higher H 2 pressure, decay is faster. When D 2 is present, some is incorporated in polyethylene after irradiation. Protection may be provided by additives capable of reacting with, and removing H or H 2. The low additive concentration which is sufficient for protection is due to its higher reactivity than that of a simple alkane (or alkyl) for H (or H2) , and their consequent long path between abstraction/additlon. Migration allows radicals to move to the surface of crystallites, where the correct configuration of adjacent chains is more likely. But results with low molecular weight crystalline alkanes show internal linking. This raises the whole question of internal strains and defects within crystalline parafflnic structures, and polyethylene. This approach may also relate to radiation effects in biological systems, where double strand breaks have been assumed to occur due to two events in close proximity, e.g. within the same spur. Other analogies with biological systems include the effect of temperature, of protecting additives, of weter as a solvent, of G(R.) values, etc. SCISSION Each scisslon leaves two chains ending in free radicals in close proximity, especially in a frozen system such as PMMA. Why does the scission not reheal? One suggestion is that bulky side groups impose a state of strain within the polymer, such that a fracture once formed, persists due to mutual repulsion. In such cases the effect of additives can only be protective prior to scission, e.g. by energy absorption. Can additives protect and reduce the radiation effect after scission has occurred? GRAFTING It has been reported that grafted polymers have lower strength than the original ungrafted backbone polymer. In many experiemnts involving a polymeric film placed in monomer solution for irradiation, grafted film is indeed formed, but also a considerable amount of homopolymer on its surface. This can be largely avoided by the addition of a small amount of a salt such as FeSO 4 to the monomer solution. Various explanations have been given, not all convincing, to explain why the monomer (and solvent) can penetrate within the film, but not the salt, to react there. PROTECTION AND ENERGY TRANSFER Some immediate transfer of energy absorbed in a solid system must occur to explain the selective chemical reactions which subsequently ensue. But this must rapidly disappear to provide other types of reaction. By transfer of energy, I personally mean this literally, the transfer of energy, rather of chemically reactive groups. More experimental evidence on energy transfer as a fundamental reaction is badly needed. We would also welcome a comprehensive and critical review of the various distinct mechanisms of radiation protection. DISTRIBUTION OF IONS AND EXCITED SPECIES WITHIN THE PARTICLE TRACK Considerable research has been devoted to the distribution of ionized and excited species within the particle track. These are based largely on calculations derived from energy loss data etc. Within the spur one can expect two or more ionizations in very close proximity indeed this is sometimes used to explain the relatively high frequency of double bond breaks in D.N.A. However, there is little direct evidence of some such behavior in simple polymeric systems. For example in radical polymerization one can require the encounter of two growing chains to terminate the reaction, and this has been confirmed quantitatively in numerous experiments, assuming that these chains are formed, grow and encounter one another as random processes. With the introduction of high local concentration of radicals within a spur, we might expect to find a bimolecular distribution of molecular weights - two growing chains initiated wlthin the same spur, and therefore giving a short terminated chain, and those form two separately initiated growing chains with much longer molecular weight on termination. I have not seen evidence to favor this type of molecular weight distribution, though its occurrence must be vital to Justify the double hit process of chain breaks often assumed in D.N.A.

Scientific Problems at issue

289

ELECTRON TRAPS AND LUMINESCENCE In recent years increasing interest has been shown in the role of electrons ejected during irradiation, their trapping and subsequent release. Associated with this are electrical breakdown patterns (e.g. in irradiated PMMA), induced conductivity in insulators, thermoluminescence and thermally stimulated currents, etc. The whole problem of radiation induced conductivity in polymeric insulators for example was examined in great detail in the 1960'8, but is still not fully understood in spite of its obvious importance to the electric industry. In thermoluminescence, the light spectrum emitted is due to the fluorescence or phosphorescence of an impurity or additive - such as an aromatic - but the tmeperature of emission corresponds to the base polymer. It would therefore appear that the electron is trapped in the polymer matrix, but a release migrates to the additive in a cationic state, giving an excited product. But other explanations are equally possible. Experiments to distinguish between these various possibilities have been carried out but are not entirely conclusive. Release of an electron from its trap may occur by thermal stimulation, by tunnelling, or more simply by disappearance of the trap due to molecular motion. Here is a vast field of great potential wealth in research and industrial interest, as yet inadequately explored. A simple question to which I have not received an answer acceptable to me is this. What is the velocity of an electron while tunnelling - not the probability of tunnelling, but the time taken during the tunnelling operation itself? I have my own answer, but this is not acceptable to others! LITHOGRAP~Y An unexpected extension of our knowledge of electron radiation effects in polymers is its application to lithography on silicon chips. The advantage over visible or UV light is of course its much shorter wavelength and hence better resolution, at least in theory. Both positive and negative techniques can be used, with sclsslon or crosslinklng as the basic technique. Attempts are being made to improve the method by searching for more radiation sensitive polymers. It is not at all clear why the same result cannot be achieved directly by enhancing the beam intensity. Is this already so high that dispersion occurs by mutual electron repulsion, or by localized heating? We now appear to be entering a field of considerable basic interest, but this time almost directly from the potential application side. The results obtained from these lithography experiments could well provide information on behavior within the track of the electron beam and on an atomic scale. Let me give an example. Assume that during one encounter along its path, the incident electron of perhaps 1 M e V energy loses i00 ev. The time range over which this loss occurs is limited by the uncertainty principle. An uncertainty in energy of I00 ev cortes-ponds to an uncertainty in time of h/ A E ~ 4.14 x 10 -15 , and at these electron velocities over i0 nm. Under such conditions can we say that energy can be transferred to a specific chemical bond, or electron, or even to a small molecule, without infringing the uncertainty principle. Some interesting and challenging questions arise from these and similar considerations, but one sees very little discussion of such fundamental aspects. These implications can nevertheless have direct relevance in radiation applications, from lithography to radiation resistant polymers and radiation protection in biological systems. CONCLUSION I would dearly like to extend this discussion into another important area; where do we envisage new breakthroughs to come? Not those already described at this conference, but those taking into the fullest account the unique advantages that high energy radiation can show, and which are certainly not fully exploited. But this would merit at the very least a separate discussion period, and even a separate, very intensive conference of far-sighted individuals who can divest themselves of lifetime habits of thought and experience, and enter new avenues. Too much detailed knowledge can be an inhibitor!

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