Radiation Mechanisms in Polymers

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1 Radiation Mechanisms in Polymers ARTHUR CHARLESBY

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Physics Department, Royal Military College of Science, Shrivenham, Swindon, Wiltshire, England

Investigating polymers irradiated under various conditions provides information on many of the processes occurring between initial energy absorption and final physical and chemical change. Among factors which can modify these reactions are the type of radiation, presence of oxygen and of additives, degree of crystallinity, and presence of solvent. Reactions may also be affected by nonlocalization of ab­ sorbed energy, energy transfer, electron trapping and recap­ ture, and abstraction by high and low energy hydrogen atoms. Several new techniques are available to study these intermediate states. Radiation treatment of polymers can be used to investigate crystal morphology, reinforcement, and electrical properties. Many of these results are applicable to radiobiology. Τ η the early 1950s, considerable interest was shown in the potential use of high energy radiation to initiate polymerization or to modify poly­ mers by such processes as crosslinking and degradation (3, 4, 5). The ability to initiate useful chemical reactions in the solid state by a readily controllable source was only one of the factors which encouraged indus­ trial studies. I cannot help feeling that the prevailing atmosphere of awe engendered by the revelation of tremendous energy concealed in the nucleus must have influenced the decision of some research directors to enter this apparently virgin field. This resulted in a mass of parallel research programs, often in fields where it could be shown, even with the limited knowledge then available, that applications at an economic cost were extremely unlikely. Nevertheless some radiation processes have been developed successfully on a commercial basis but usually only in specific areas such as solid state reactions where no simple chemical alternative is readily available. It is to be hoped that further successful applications will emerge from the present intensive investigations into A

1 Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

2

IRRADIATION OF POLYMERS

graft polymerization and solid state polymerization; these are not dis­ cussed further in this review. By far the most interesting application to date is the ability to cross­ link polymers in the solid state, and much research has been devoted to studying the reactions involved. The initial physical process of energy absorption and the final chemical change—formation of crosslinks—can be readily determined. However, there is still considerable doubt as to the intermediate reactions, and this problem offers an appropriate start of this review.

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Ionization

and

Excitation

By analogy with the reactions in gases, it is usually assumed that the first reactions following absorption of energy from the incident beam are ionization and excitation and that these occur at random throughout the specimen. (Account may have to be taken of the varied binding energies of electrons in different shells, but this is usually ignored. ) How far can we assume that ionization and excitation occur at random? The average energy lost by an incident fast electron per collision is roughly 100 e.v. The uncertainty in time corresponding to this energy change is immediately deduced from the uncertainty principle: At—h/ΔΕ*—4 Χ 10" sec. 17

For an electron moving close to the speed of light, this corresponds to an uncertainty in position of about 120 Α., far greater than the indi­ vidual electron orbit. We may therefore envisage the energy as being deposited over a relatively large volume and only later being directed toward a specific electron within it. If the nature of the chemical bonds within this volume is heterogeneous, some selectivity can appear sub­ sequently. For example, if radicals are present, owing to prior irradiation, these may be the preferred site. We might then expect to see a change in reaction processes when the radical concentration is one per vol­ ume ofy^(120) A . (equals 1.4 χ 10 per c c ) . If (Δ*)(ΔΕ)^Λ, the 3

3

17

corresponding concentration is 2 Χ 10 per cc. In fact, the radical con­ centration in many irradiated polymers, as determined by electron spin resonance, does reach a maximum in this range. This agreement may, however, be fortuitous, and alternative suggestions need to be examined. Next we must consider the precise meaning to be attached to the term "ionization" in the condensed phase. Unlike the situation in an irradiated gas, the electron liberated by ionization of a molecule loses energy rapidly by colliding with other molecules and may have insuffi­ cient kinetic energy to escape the field of its parent ion. In this case we may justifiably speak of a superexcited state not to be found in gases. 19

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

1.

CHARLESBY

3

Radiation Mechanics

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The critical escape distance, a, beyond which the electron can be con­ sidered free—i.e., outside the influence of its parent ion—can be esti­ mated: when Ί is the effective dielectric constant, allowing for a lower value in the immediate vicinity of the ion. In the case of water a is only a few Angstroms, but in organics, with a bulk dielectric of about 2, the critical distance becomes 50A. One can therefore expect that only a small frac­ tion of the ejected electrons can truly escape and eventually produce ionic reactions. The rest will be recaptured by the parent ions to give highly excited molecules and eventually lead to radical reactions. In conformity with this view, there is considerable evidence that in many organic systems, G values for radical production (resulting from excitation) run to about 3 to 5 whereas the G values for ion formation are down by a factor of 10 or more (11, 13). In radiation-induced ionic polymerization conducted in the presence of fine powders (ZnO, SiOo, etc.) much higher G values for initiation have been reported, but it now appears that they are caused by chain transfer reactions (8,10, 25), giving a higher number of molecular chains than were initiated by radiation. Again the presence of N 0 acting as an electron scavenger is reported (21) to yield apparently high values of G (ion) up to 3. This would imply a high ionic contribution to many radia­ tion-induced reactions, for which there is little evidence. However, it can be argued that the concentration of the scavenger is in fact sufficiently high to capture some of these super-excited electrons which would other­ wise return to their parent ions after a lengthy journey in the neighbor­ hood, to give excited states and eventually radical reactions (1, 24). However, this cannot be the complete story since we find with paraffins irradiated in the presence of N 0 an increase in radical production and crosslinking density which can be determined by the reaction of such radicals with iodine (Table I). 2

2

Mechanisms

of

Crosslinking

Considerable disagreement still prevails as to the mechanism of crosslinking in polymers. Is it an ionic or a radical process? While the author admits the existence of some ionic species ( as revealed by ESR at low temperature and by radiation-induced conductivity), his present view is that the ionic contribution to crosslinking in solids and liquids is only minor. This attitude is based inter alia on the following evidence. Crosslinking can occur readily in dilute aqueous polymer solutions when there is little likelihood that two polymer ion molecules will be in the correct position for linking during the short life of the ion.

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

4

IRRADIATION OF POLYMERS

Table I.

Irradiation of Cyclohexane in the Presence of N 0 and Iodine 0

2

AG(I)

AG(R )

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2

mM 0 15-130

mM 0 0

1.63 2.48

0 100

0.59 0.59

0.3 0.3

0

0 100

0.89 0.89

0.3 0.3

0

0 100

6.22 6.22

0.3 0.3

0

0.85

—

5.1 6.9 5.1 6.8 5.5 7.3

—

1.8

1.7 1.8

In the absence of iodine, N2O increases crosslinking; ^(^2^22) = 0.85. Iodine can scavenge the cyclohexyl radicals, leading to dimer formation (except a residual 0.3 dimer) and produce the iodide; G(R2> decreases from 1.63 to 0.3, and iodine is lost. In the presence of N2O there is a greater loss of iodine, corresponding to the larger amount of dimer which would otherwise be formed; 2 X 0.85 —1.7 to 1.8. β

Polymer radicals can be observed in irradiated solid polymer (ESR) and in aqueous solutions (pulse radiolysis), in concentrations comparable with the eventual crosslinking density. Low concentrations of radical scavengers can greatly reduce the density of crosslinking of liquid polymers although most radicals will be formed some distance from a scavenger. For a solid polymer much higher concentrations are needed, but crosslink densities can still be approxi­ mately halved. If we assume that crosslinking is primarily caused by the reaction between two polymer radicals, we must still explain how they come sufficiently close together to react. Here we must choose between two alternatives: • The two radicals are formed in close proximity; at the low doses required for gelation this implies that each pair is the consequence of a single ionization or excitation. • Radicals are formed at random and migrate until they find a partner. Neither of these explanations is entirely satisfactory. In particular the former would not allow for significant radiation protection by small concentrations of additive; in fact, the gelation dose can be doubled by only 1% of additive in solid polymer and considerably less in liquid polymer. The second explanation would lead to almost complete pro­ tection since the additive concentration vastly exceeds the polymer radical concentration, and radicals would react with the additive before meeting another radical. In practice partial protection is observed. The explanation which seems at present most feasible is as follows. Following each excitation-ionization, a polymer radical and a hydrogen

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

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1.

CHARLESBY

Radiation Mechanics

5

atom are formed. Some of these hydrogen atoms (which are released with considerable kinetic energy) abstract in the immediate vicinity, yielding secondary polymer radicals. Pairs of adjacent radicals (formed one by radiation, the other by abstraction) can then crosslink readily, with the minimum chance of interference by other molecules. If hydro­ gen does not abstract in the first few collisions, it becomes thermalized and can then travel long distances and suffer numerous collisions before abstracting a second hydrogen, to give a second polymer radical. Pro­ tection against such dispersed radicals (but not initially adjacent pairs) is readily achieved at low additive concentration—e.g., by reaction of additive with a polymer radical or by trapping of thermalized hydrogen before it abstracts. This explanation for crosslinking, though attractive, is still far from proved—for example, additives should influence hydrogen yield as should radicals trapped in the solid phase. One no longer expects the density of crosslinking to remain proportional to dose. Neither of these predic­ tions is adequately confirmed. We also must account for the mobility of polymer radicals, even in solid polymer. One method of transfer is hydrogen addition to radicals, and hydrogen abstraction from polymer, this process being accelerated by the presence of hydrogen gas in the system. ESR results (18) show that the radical concentration in poly­ ethylene decays more rapidly in the presence of hydrogen. Crosslinking occurs most readily in the amorphous regions between polymer crystal­ lites (where the chains can move into a suitable spatial arrangement), so we may visualize a slow migration of trapped radicals from within the crystallite into the amorphous regions. This is revealed by a reduc­ tion with time after radiation in the asymmetry of ESR signals from radicals in stretched polyethylene. The larger the crystal, the slower should be the rate of radical arrival at the crystal surface; this is con­ firmed by the more rapid decay rate of radical concentration in the low density polyethylenes. Influence

of Type

of

Radiation

Further insight into radiation mechanisms can be obtained by com­ paring different types of radiation, in particular alpha radiation, gamma (or fast electron) radiation, and ultraviolet light. Their effects are also important in such varied fields as space materials and radiological pro­ tection. Alpha radiation differs from gamma or electron radiation primarily in the much denser distribution of reactive entities along the alpha particle track; with ultraviolet light at 254m^, often requiring the pres­ ence of a sensitizer, one is dealing primarily with excitation and not ionization.

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

6

IRRADIATION

OF POLYMERS

In this short review it is perhaps sufficient to indicate that in organic materials the primary processes and products of alpha and gamma radi­ ation are usually fairly similar. However, major differences occur in the presence of so-called protecting additives, which are far less effective in the case of alphas. This is to be expected from the high concentration of radicals built up in the dense alpha track, far higher than in gamma or electron spurs. From the data on the effect of radical scavengers on the final product, the diameter of the alpha track can be estimated. Both alpha and gamma radiation destroy crystallinity in polyeth­ ylene but only at high doses (2). This is surprising since the energy released within the narrow track of each alpha particle as it passes through a polymer corresponds to a rise in temperature of over 1 0 C , even after allowing for the energy lost to ionization and excitation. This local heating dissipates outwards from the initial track with a velocity equal to that of sound in the medium so that the high temperature is retained for an appreciable time. Yet there is little evidence of local melting and loss of crystallinity in polyethylene within this time. Lengthy heating to temperatures only a few degrees above the melting point of the irradiated material is far more effective in causing loss of crystal­ linity. This observation throws a surprising light on the thermal spike concept used in investigating irradiated materials.

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4o

trans-Vinylene

Unsaturation

A second important reaction observed in irradiated polyethylene and certain other polymers as well as paraffins is the formation of transvinylene. This product is far less influenced by such factors as tempera­ ture or radical scavengers than is crosslinking, and it is therefore often assumed to be formed directly by a "molecular" process—Le., molecular detachment of hydrogen. Even this simple explanation cannot be fully sustained. Long after radiation has ceased, the frans-vinylene concen­ tration continues to rise or fall, whether the specimen is exposed to oxygen or not. This behavior may possibly be correlated with the reac­ tion of trapped alkyl and allylic radicals, which, being slightly mobile, can add to frans-vinylene or each other over a period of days. Apart from this behavior, the concentration of frans-vinylene in­ creases with dose, at first linearly but then tends to a limiting value which depends on the type of radiation (7). This limit is possibly caused by the capture by frans-vinylene of the thermalized hydrogen atoms released elsewhere by radiation. From the limiting concentration of unsaturation the number of collisions made by each thermalized hydrogen atom be­ fore it abstracts can be deduced; this lies in the range 10 to 10 . Furthermore the limiting trans-vinylene concentration is greatly af­ fected if oxygen is present during—but not after—irradiation. The con3

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

4

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1.

CHARLESBY

Radiation Mechanics

7

centration of oxygen is inadequate to ensure that one molecule is likely to be immediately present at most potential sites of molecular detach­ ment and so would not be able to influence a simple reaction of this type. Hence, we are forced to seek alternative explanations—for ex­ ample, oxygen can be expected to act as an effective electron trap and may therefore interfere even with simple reactions by hindering the recapture of superexcited electrons. So far I have confined my remarks to some aspects of radiation mechanisms in polymers. However, the use of radiation as a scientific tool has grown and is yielding valuable information in various fields, including polymer structure and radiation physics of polymers, and the results of this work are impinging on the vastly complex problems of radiobiology. It therefore seems appropriate to give a few selected samples of these techniques. Crystal

Morphology

Our views on the arrangement of molecules in a partially crystalline polymer have been greatly affected by the studies of Keller and others, largely based on electron microscopy. Under suitable conditions, large single crystals of polyethylene can be grown, and it appears that each polymer molecule is folded back and forth to give a series of parallel chains, connected top and bottom alternatively by loops, somewhat like a firecracker. This model can be conveniently analyzed by subjecting such crystals to radiation, which produces crosslinking and eventually insolubility. It can be shown theoretically that for a random distribution of crosslinks, the radiation dose required for incipient gelation corre­ sponds to one crosslink unit (0.5 crosslink) per weight average molecule. Internal links—i.e., links between different units in the same molecule —are ineffective for gelation and must be ignored. With large single crystals of polyethylene, the gelation dose is some 10 times greater than for the same polymer grown under conventional conditions (14). This is not caused by any inherent difference in the effect of radiation since both the radical concentration (deduced from ESR measurements) and hydrogen production are similar. We must therefore assume that most of the links produced in the single crystal are internal links which do not influence solubility. This is understand­ able in a crystal where each molecule folds backward on itself. As high doses these internal links would be expected to produce looped structures, with a marked effect on viscosity. No such effect is observed. To explain this, we must assume that the radicals, trapped in the crystalline regions, migrate to the surface where the molecular loops are found (corresponding to some extent to the old amorphous

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

8

IRRADIATION OF P O L Y M E R S

regions), where they form internal links enclosing only a few units; these would have only a small effect on polymer viscosity. If, during growth, the single crystals are under some pressure, the gelation dose can be greatly decreased. This means that a higher per­ centage of the links are formed between separate molecules. Our model allows for this since growth under pressure would bring the surfaces of adjacent crystals closer and result in an interleaving of the surface loops.

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Reinforcement

Increases in such mechanical properties as elastic modulus and tensile strength are obtained if fine particles—e.g., carbon black—are incor­ porated in elastomers before vulcanizing or crosslinking. The nature of this reinforcement is not yet adequately understood. Radiation offers a particularly suitable way to investigate the cause of this reinforcement since it obviates many of the difficulties of chemical cure involving the incorporation of chemical agents and thermal treatment. At the same time the degree of the reaction can be readily controlled in a reproducible and quantitative way, merely by altering the radiation dose. Youngs modulus in an irradiated nonfilled elastomer increases uni­ formly with dose—i.e., with density of crosslinks. If a fine powder filler is incorporated, a much higher modulus is obtained at the same dose (16). From this we may infer that the density of crosslinks is increased by the formation of additional links between elastomer and filler. How­ ever, this can be shown not to be the case. Using mixtures of low mo­ lecular weight silicones or squalene (as a model for rubber), with carbon black or silica powder, and irradiating to the same dose as for high molecular elastomers, does not give any evidence for major attachment between the two constituents. (This method avoids the difficulty of extracting filler from a polymer network, where extraction is hindered by the network as well as by the hypothetical filler-elastomer bond.) Again it is claimed that ESR shows evidence for radicals in carbon blacks and that these radicals link with those formed on elastomer mole­ cules to provide a strong chemical bond. We also reject this interpre­ tation. In our view the increase in modulus is caused by the physical presence, within each network loop, of small particles which reduce the maximum extension of which each network chain is capable merely by their physical presence within the available volume. A lengthy series of measurements with silicone gums, incorporating varying concentrations of a series of fillers, and exposed to a range of doses, shows that Young's modulus Ε is approximately proportional to filler concentration c and to radiation dose r. The effectiveness of a filler as a reinforcing agent can therefore be represented at least approximately by the ratio E/cr. Our results show that E/cr is determined primarily

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

1.

CHARLESBY

9

Radiation Mechanics

by filler size and only to a small extent by its nature (carbon black, rein­ forcing or nonreinforcing; silica, surface-treated or not). Of course, it is important to ensure good dispersion of the filler (Figure 1). Other factors which play a role are filler-filler interactions (mainly at low ex-

OSl

2491 (NT)

PHLlBLACK^E, T) I e# EP93 1· Ψ*. J (Nt(T.

'

(TAEROSIL) I

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(Mi

/

'

03

^

Ι

t

***!

ΙΡΛ4^ — AfHOSlL)

f

,

02i

t AfROSIL

W

/ /

S

y

(NR)

// /

/

O!

/ I / φ γιοχιοε I / 1/

CARBON BLACK

•f-

P-331 2

1 1 3 4

1 1 1—8 5 6 7

X 10 cm (PARTICLE DIAMETER)

9

H

Figure 1. Reinforcement of silicone gums by carbon bfocks (including non-reinforcing NR), silica powder (both surface-treated Τ and untreated NT), and titanium dioxide. The increase in modulus per unit dose and filler concentration depends on filler particle size. tensions) and chemical reactions between filler and reactive end groups in the elastomer, but these are usually of minor importance. The presence of filler does not greatly affect the maximum elongation at break of the elastomer; hence, the increase in tensile strength is largely caused by the higher Youngs modulus at the same extension.

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

IRRADIATION OF P O L Y M E R S

10 Tbermoluminescence

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Many polymers, after irradiation at low temperature, give off light when allowed to warm. This phenomenon of thermoluminescence de­ pends not only on the chemical structure but also on crystal mor­ phology. In polyethylene, for example, peaks in the thermoluminescence glow curve correspond, respectively, to the crystalline and the amorphous regions (9, 19, 22) (Figure 2).

Figure 2. Glow eûmes of various polyethylenes with different ratios of crystattinity-amorphous components. Temperature rises from left to right. Peaks «, β, y, where a is caused hy crystalline component, and y by amorphous component. We are in fact dealing with traps for the electron ejected during ionization, these traps being related to the polymer itself. On warming, these electrons are released and then captured by luminescent centers, which must be some impurity or structural modification associated with the polymer. Thus the temperature at which the electron is released relates to crystal morphology while the spectrum of thermoluminescence (which is closely related to the fluorescence and phosphorescence spec­ tra) provides information on these impurities or polymer chemical modi­ fications. The method is extremely sensitive to even minute concentra­ tions of these luminescent impurities, some of which can be washed out and reappear in the solvent. Our tentative analysis indicates rather surprisingly that they contain aromatic residues (Figures 3 and 4). Fur­ ther work is proceeding, but it is already shown that in several ways

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

1.

CHARLESBY

Radiation Mechanics

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F

5SOO

SOOO

4SOO

400U

1500

3000

χίΛ)

Figure 3. Thermoluminescence in polyethylene. The spectrum com­ prises bothfluorescent(F) and phosphorescent (?) components. Contri­ bution of Ρ falls as the temperature rises owing to competitive, nonradiative processes. Polyethylene alkathene 20, no 0 , dose 0.8 Mrad, heating rate 2.7°/min. Temperature in °K. Intensities not to same scale. 6

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

12

IRRADIATION O F P O L Y M E R S

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A

6OOO

5SOO

SOOO

4SOO

4000

χω Figure 4. Effect of oxidation on phosphorescent spectrum. A: Polyethylene milled for 30 min. at 120°C. B: Initial polyethylene. Note several new peaks owing to oxidation. Other peaks are unaffected. radiation can be used as a sensitive technique for studying polymer structure and morphology (17, 20). Polymers

in

Solution

Radioprotection. The processes of crosslinking and degradation ob­ served in polymers irradiated in the pure state can also be observed in polymers irradiated in solution. The presence of a solvent can inter­ vene in the reaction in several ways; thus it allows increased polymer mobility, and some of the radiolytic products of the solvent may react with the polymer or with the polymer radicals, etc. The polymer-water system is of particular interest in that it provides a simple model for some radiobiological systems and can be analyzed far more readily. In certain water-soluble polymers such as polyvinylpyrrolidone, the minimum dose required for gel formation shows a curious depend­ ence on concentration. As this is reduced to about 1%, the gelation dose decreases in spite of the fact that the polymer molecules are further apart. This is ascribed to the higher contribution made to polymer radi-

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

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1.

CHARLESBY

Radiation Mechanics

13

cal formation by the radiolytic products of water (indirect effect). A similar effect is observed in many biological systems where increased lethality is observed as the concentration of the biological component is decreased. At lower concentrations the gelation dose increases sharply until no network is formed, even at the highest doses. This competing effect is ascribed to internal linking—links occur between different monomers in the same chain but are ineffective for network formation. Systems such as these are extremely valuable for measuring radia­ tion protection by various additives. The increased gelation dose is a direct measure of this protection and can be used to compare the effects of polymer concentration, additive concentration, oxygen, p H , etc. In this way a table of protecting effectiveness of additives can be prepared. It is highly significant that over a wide range of compounds for which such tests have been carried out, the most effective are those containing the sulfhydryl group. These compounds are similar to, or closely related to, those most effective for radiobiological protection (6, 15). To follow up this line of research we have recourse to two new techniques. The first—pulse radiolysis—measures the change in optical absorption over a range of visible and ultraviolet wavelengths of poly­ meric or biological solution subjected to short, intense pulses of radiation from a linear accelerator (Figure 5). The short-lived spectrum of the polymer radical can be readily traced as can its subsequent decay as these radicals react and disappear (12). This gives a direct measure of the reaction rate. An extension of this work relates to the influence on these spectra of the presence of traces of a protecting agent within the solution (Figure 6). Here we see most directly and vividly the manner in which these additives intervene in these radiation-induced reactions. The second new technique envisages the use of ESR to detect these radicals. This technique has been used to study a number of polymers irradiated in the solid state. Applying it to polymers irradiated in water is difficult because of the extremely high absorption of electromagnetic radiation at these frequencies. We have therefore designed a new type of spectrometer which is capable of detecting radicals in the presence of large excess of water and obtained spectra from such irradiated sys­ tems. This technique shows considerable promise for investigating radia­ tion effects in polymeric and biological aqueous solutions. Extension

to Radiobiological

Systems

Although the precise mechanisms of radiation-induced changes in polymers are not yet fully agreed upon, much of the information already gathered is in a form which can be readily analyzed quantitatively. We can therefore ask whether any of the main conclusions already reached

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

14

IRRADIATION OF P O L Y M E R S

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C MEASUREDCMvAT24.0003 SUT IES

Figure 5. Pulse radiolysis of deoxygenated solutions of polyvinylpyrrolidone showing peaks at 250 and 400 ηΐμ, and a broad absorption ascribed to electrons up to 700 ηΐμ in low concentration solution are applicable to the far more complex systems studied in radiobiology. The answer to this important question is very much in the affirmative. Thus we can usefully extend our ideas into the more difficult problems of cell structure and radiation damage and analyze, on the basis of our knowledge of polymer radiation, some aspects of the behavior of model biological systems exposed to radiation. In particular we can differen­ tiate between the radiation physical and chemical events and those which are specifically biological in nature. In this way we can relieve the radiobiologist of many of his problems and set up a sound framework within which he can further develop his specifically biological ideas. The following reactions are among those which seem to show the same relationship as between polymeric and biological macromolecules. The sensitivity of enzymes at various temperatures follows closely that measured in some polymers. Thus we need not propound abstruse biological explanations for this behavior, which can be readily explained on purely physical lines.

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

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1.

CHARLESBY

15

Radiation Mechanics

Many biological systems show an increased sensitivity to radiation in the presence of oxygen. This again is observed in polymers, where the chemical reactions can be more readily followed. Radiation protectors can increase the dose needed to produce a cer­ tain degree of damage in a biological system. The same additives also protect polymers—e.g., by reducing crosslinking—and this to an extent which canfeereadily determined. The mechanisms of protection by these compounds have been evalu­ ated for irradiated polymers. Many so-called protectors contain sulfhydryl groups and appear to operate by replacing or capturing the hydrogen atom lost by irradiating the macromolecule. It appears most reasonable to assume that the same mechanism often occurs in biological systems. Only a limited degree of protection can be achieved, even with high concentrations of protecting additive. The results obtained with polymers account readily for this limit. With most solid polymers the G values for radical production, as determined by ESR, lie in the range 2 to 5. With biological macromolecules irradiated in the solid state comparable G values are obtained. Following irradiation at low temperatures, the ESR spectrum of polymers shows changes in radical structure, ascribed to the movement of electrons and radicals to other and more stable sites. The behavior of many dried biological macromolecules is similar although a detailed interpretation may be more difficult. 0 0-5^ h W.V. RE.0.CMn2C000)+50m9/{TJ«0U REAtyffrSnlOf

10

F.E.aCfln20,00(D tyk«l*9«IO

f

/ / y /s

9ft

so

0

y

SO

100

ISO

200

2S0

TINE μ stc.

Figure 6. Effect of thiourea on decay of radicals in aqueous solutions of polyethylene oxide. Time after pulse in μ6βο. Broken line is decay in PEO radicals. Solid line is decay when thiourea is present. Dose about 5000 rads.

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

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16

IRRADIATION OF P O L Y M E R S

In the presence of water, polymers reveal a greater sensitivity to radiation. This increase has been shown to be caused by the indirect effect of radiation—i.e., the radiolytic products of water react with and form new radicals in the dissolved polymer. This indirect effect is also present in aqueous biological systems. The usual ESR techniques do not enable such radicals to be readily revealed because of the high absorp­ tion of water, but other techniques using radical scavengers reveal their presence. These similarities encourage us to assume that the initial physical and chemical effects following exposure to radiation are inherently similar in simple macromolecules such as polymers and in the far more complex biological systems. The pathological changes are then the consequence of these physical and chemical events, whose nature we can already sur­ mise, at least in general terms. However, there appear to be several serious objections to this basic assumption; two major difficulties relate to the great radiation sensitivity of many biological systems and to the effect of different forms of radiation. Radiation

Sensitivity

To produce marked changes in most polymeric materials requires roughly one chemical change per molecule. Since a dose of r rads de­ posits an average of 0.625 Χ 10 rM/6.02 χ 10 = 1.04 Χ 10" r M e.v. per molecule of molecular weight M , and 100 e.v. produces G chem­ ical changes (with G between 1 and 5 in all but exceptional cases), the dose needed to achieve such changes (with M ~ 10 , G ^ 2) is: 14

23

10

6

r = ( 100/G ) ( 0.96 X 10 /M ) ~ 500,000 rads 10

This is roughly the order of dose needed to modify most polymeric ma­ terials. At much lower doses, say 10 rads, most molecules will be un­ affected. In biological systems similar G values are in fact observed by ESR techniques, and the molecular weights usually reported are no greater. Yet effective doses involved in radiobiology are frequently much smaller and can be as low as a few hundred rads. We are therefore driven to seek an alternative explanation. Among others, the following possibilities may be advanced. • The mechanism of radiation damage is quite different from the simple type of radical reaction envisaged—e.g., a chain reaction via hydrogen bonds. • Major damage results from the radiolytic products of water; experiments with aqueous solutions hardly increase the damage by much more than a factor of about 10. • One modified chemical group in any one of a number of macromolecules is sufficient to cause serious pathological damage to the whole cell. • The molecular weight of the macromolecule or structure involved is in fact larger than the values previously quoted for such molecules. r>

Irradiation of Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1967.

1.

Radiation Mechanics

CHARLESBY

17

It is satisfying to find recent reports that unfractured D N A molecules from E. colt have molecular weights of about 2 X 10°.

Downloaded by 80.82.77.83 on April 13, 2017 | http://pubs.acs.org Publication Date: June 1, 1967 | doi: 10.1021/ba-1967-0066.ch001

Effect

of Radiation

Type

The second difficulty arises from the greater sensitivity of biological systems to radiation of high L E T (high ionization density along the particle track). This is notably the case for alpha and neutron radiations (via ejected protons), where the relative biological effect (RBE) may be higher by a factor of up to 10 than for the same dose of gamma or fast electron radiation. For simple polymers the RBE of alphas is usually equal to 1 or less. To explain this difference, it is usually assumed that several adjacent "hits" are needed to inactivate a biological molecule. The dense alpha track, of radius about 15 Α., and with about one ionization per Angstrom along the track, would be far more effective in scoring multiple hits on a molecule than would the same number of ionizations (or excitations) scattered at random throughout the sample, as results from gamma or fast electron radiation. Evidence for this multihit process is provided by the shape of survival-dose curves, which show a shoulder at low doses. This conclusion militates against the possibility that the vulnerable mole­ cule is of molecular weight 10 or less since the chance of multiple hits with randomly distributed ionizations becomes extremely small at low doses. The argument can be pursued along more quantitative lines. A dose of r rads of gamma or electron radiation achieves a random "hit" density of 0.625 Χ 10 Gr per ce. ( G hits per 100 e.v., density of material ρ ~ 1). A target volume ν ce. therefore receives an average number of hits: 6

12

P

m = 0.625 χ 10 Gr( v ) 12

P

For a random distribution, the fractions of target volumes hit 0 or 1 times are exp (—m) and m exp( —m), and for 50% damaged by mul­ tiple hits following exposure to r rads; 5 0

(1+m) exp(— m) =0.5;

m = 1.7

The mass of the target volume, and its molecular weight, M , are p V

= (1.7/0.625 X 1 0 ) ( l / G r ) 12

50

M — 6.02 Χ 10 v — 1.6 X 10 /Gr 23

12

P

50

For G ^ 2 and r>