Radiation Chemistry of Polymers

Encyclopedia of Polymer Sceince and TechnologyCopyright c© 2005 John Wiley & Sons, Inc. All rights reserved.

RADIATION CHEMISTRY OF POLYMERS

Introduction

The fundamental property of polymeric materials which sets them apart fromother materials is their very large molecular weight. The arrangement of a largenumber of structurally similar chemical units into linear or branched chains pro-vides polymers with their unique viscoelastic properties. This large molecularweight necessarily leads to both transient and permanent molecular entangle-ments, which determine the response of the polymer to applied loads. It is essen-tially this property which is altered, or deliberately manipulated, when a polymeris subjected to ionizing radiation.

High energy radiation leads to a succession of chemical reactions which ul-timately lead to either an increase or a decrease in the molecular weight of apolymer. As will be discussed below, either result may be desirable. The initialprocesses which occur when a high energy photon interacts with an organic poly-mer are reasonably well established and do not depend essentially on the chemicalstructure of the material. However, these primary processes lead to a cascade offurther reactions, the nature of which depends sensitively on the nature of thepolymeric material; for example, the ability to dissipate excess energy, the rela-tive bond energies within various structures, and the presence of oxygen, radicalscavengers, and other moderating factors.

This article approaches the field of radiation chemistry of polymers from amechanistic viewpoint, and does not describe in detail the growing list of applica-tions of high energy radiation in polymer science. The basic principles of radiationchemistry are described, and subsequently the interaction of radiation with spe-cific classes of polymers. The mechanisms of reaction are described by referenceto specific materials. Furthermore, the means by which these mechanisms havebeen determined, ie, the methods of analysis, are described in detail. Finally, anattempt is made to draw overall conclusions about the radiation sensitivity of theconstituent units of synthetic polymers.

Interaction of High Energy Radiation with Organic Matter. The pas-sage of a high energy photon or particle through matter leads to a complex cascadeof processes which results in the dissipation of the initial energy, ultimately asthermal energy and chemical reactions (1–5). If the source of radiation consistsof high energy photons, the energy is deposited in a highly nonuniform mannervia a number of processes. In organic polymers the most important mechanismof energy loss is Compton scattering, in which a photon interacts with an elec-tron resulting in ejection of the electron and deflection of the photon with reduced

1

2 RADIATION CHEMISTRY OF POLYMERS

energy. The probability of the photon undergoing Compton scattering and the re-sultant energy of the ejected electron and scattered photon depend on the incidentenergy of the photon and the electron density of the material through which it istraversing.

Of less importance for high energy photons is the photoelectron effect, inwhich the entire energy of the photon is transferred to a single electron, whichis ejected with energy equal to the incident photon energy minus the bindingenergy of the electron. Also of less importance for most commercial sources ofionization radiation is the mechanism of pair production. This also involves com-plete absorption of the incident photon energy, and results in the formation of anelectron–positron pair. The combination of the two particles results in emission oftwo 0.51-MeV γ -rays, which may undergo further interaction with the material asdescribed above. Pair production is only of importance when the incident photonshave energies greater than 1.02 MeV.

If on the other hand the source of ionization is high energy electrons, theinitial steps toward loss of the energy to the material involves inelastic collisionsresulting in ionization and excitation of the material and loss of energy of theincident particle. On ejection of the secondary electrons, the resultant processesoccurring are identical to those occurring on irradiation with high energy photons.These can be summarized as follows.

Initial interaction with photons:

R∼∼∼∼∼∼→R+ + e− (1)

Interaction of primary electrons with other structures:

R + e− → R+ + 2 e− (2)

As the energy of the electrons is reduced, there is increased probability ofrecombination of the cations and secondary electrons to form excited states:

R+ + e− → R∗ (3)

The excited-state molecules may return to the ground state through radiationlessdecay, or undergo homolytic dissociation reactions to form free radicals, whichare believed to be the main protagonists in subsequent radiochemical reactions.Heterolytic bond cleavage may in addition result in the formation of chargedspecies.

Linear Energy Transfer and Recombination Reactions. The vast ma-jority of studies on the radiation stability of polymeric systems have been con-cerned with either incident γ photons or electrons. This clearly reflects the greateravailability of γ sources or electron accelerators. However, there has been a smallerbut increasing interest in the effects of heavier charged particles on polymers, ei-ther because of interest in ion implantation or concern for the effects of heavyions on materials in, for example, the space environment, or indeed processingof polymers using ion beams. Of most interest is the effect of the increased lin-ear energy transfer (LET) of heavier charged particles on the mechanism of the

RADIATION CHEMISTRY OF POLYMERS 3

radiation chemistry. The results presented so far are conflicting. For example,While Parkinson and co-workers (6) and Boyett and Becker (7) did report differ-ences between neutron and γ irradiation, others have indicated identical changesin properties (8). The effect of proton irradiation was found to be identical to elec-tron irradiation for aliphatic polymers; however, some differences were observedfor aromatic materials (9,10). However, Hill and Hopewell (11) found no differencebetween the mechanical properties of polyimides irradiated with γ rays or pho-tons. A survey of several studies of the effects of ion beam irradiation on variouspolymers (12–17) again does not present a clear picture. This is clearly an areafor further investigation.

Energy Transfer and Dissipation. From early times it was recognizedthat the presence of small concentrations of certain structures in a material canlead to a pronounced decrease in the radiation sensitivity of the material as awhole. The earliest observations of this type were of the evolution of gases frommixtures of small aromatic and aliphatic molecules. For example, Manion andBurton (18) reported in 1952 that the yields of reactions in irradiated mixturesof cyclohexane and benzene were reduced significantly compared to the yield ex-pected from the linear addition of the yields of the pure individual molecules.This led to the concept of protection effect, in which the introduction of aromaticgroups results in significant protection of the surrounding aliphatic structures.The mechanism of “protection” afforded by the aromatic molecules has been asubject of considerable discussion.

Since these early studies of mixtures of aromatic and aliphatic liquids (18,19),the phenomenon of radiation protection has been observed in copolymers ofstyrene with other monomers (20–25), in a range of alkylbenzenes (26), and inmiscible polymer blends (27). The description of the mechanism of radiation pro-tection has often in the past relied upon the assumption that the amount of energydeposited in each molecule or molecular segment within a polymer is proportionalto its electron fraction. As a consequence the decrease in radiation sensitivity ofone component must be due to either transfer of energy or charge prior to disso-ciation, or scavenging of intermediates in systems where significant mobility ispresent. It has been reported, however, that the assumption that the cross sectionis proportional to electron density is not at all times obeyed (28). Therefore theassignment of protection effects wholly to transfer processes must be made withcaution, although the relative stabililizing effect of aromatic structures is not inquestion.

Yields and Units. The unit of radiation dose used in this text is the Gray(Gy); 1 Gy corresponds to an energy absorption of 1 J/kg of material. Typical ap-plied doses in the field of radiation modification of polymers are in the range oftens of kGy; however, in some cases irradiation up to 10 MGy has been requiredto induce sufficiently large yields of new molecular structures. The yield of radio-chemical events is expressed as the G value [written, for example, G(X) for the Gvalue of cross-linking]. The G-value is defined as the number of products formedfor every 100 eV, or 16 aJ, of energy absorbed by the material. Thus, 1 kGy ofradiation will produce G × 1.036 × 10− 7 moles of product per gram of irradiatedpolymer (4). The SI unit for radiation yield is µmol/J, which is equivalent to 10Gy (29).


Summary of Radiation-Induced Reactions in Polymers

The preceding section has described briefly the primary reactions occurring oninteraction of high energy radiation with organic molecules in general. Followingthe succession of reactions detailed above, a number of subsequent combinationand bond-breaking reactions lead to permanent changes in the properties of thematerial. Much experimental evidence exists to support the contention that mostof these reactions are initiated by free radicals. The observation, by electron spinresonance (ESR) spectroscopy, of relatively large concentrations of radicals in poly-mers irradiated at low temperatures is one example of this.

In the vast majority of experimental studies of the radiation chemistry ofpolymers, we observe usually the final products of the passage of the high en-ergy photons. Attempts to trap radical and charged species at low temperaturesusually succeed in revealing part of the radiochemical history of the material.An indication of this can be obtained by consideration of the relative yields ofproducts on radiolysis of simple alkane molecules. The respective bond energiesof C H and C C bonds in linear alkanes are 4.3 and 3.4 eV (415 and 328 kJ/mol)(1). On consideration of these numbers alone, one would expect that the predom-inant products of radiolysis of linear alkanes would be due to rupture of C Cbonds. However, the main product of the radiolysis of linear alkanes, and indeedof polyethylene, is hydrogen gas. Furthermore, there is some debate as to whetherpermanent cleavage of the main-chain C C bond actually occurs at all on irradi-ation of polyethylene. The origin of these results is of course the “cage” effect. Thelarger radical fragments formed on cleavage of C C bonds are unlikely to escapefrom the reaction cage, and recombination reactions tend to occur. The smaller,perhaps thermally excited, hydrogen radicals formed by cleavage of C H bondshave a much greater probability of escaping and undergoing recombination andother reactions. This simple and important observation also helps us understandthe differences in the radiation chemistry of hydrocarbon polymers and perfluo-rocarbon polymers, as discussed below. In addition the unusual temperature de-pendence of the radiochemical yields of a number of materials can be explainedby the cage effect. For example, the radiochemical G-values for formation of stablefree radicals in γ -irradiated fluorinated ethylene–propylene copolymers (FEP) are0.22 at 77 K, and 2.0 at 298 K (30), since at low temperatures cage recombinationof the primary radicals is very effective, while at higher temperatures sufficientthermal energy is available to remove the radical fragments from the reactivecage.

Additional evidence of the very large number of nonpermanent bond cleav-ages initiated by ionizing radiation is the observation of large changes in thestereochemistry of initially highly stereoregular polymers, such as isotacticpoly(methyl methacrylate) (iPMMA) and related polymers (31–34), as well asisotactic polypropylene (iPP) (35–38). The loss of stereoregularity arises from aracemization reaction occurring as a result of temporary scission of either themain chain or the side chain. The latter mechanism is favored for syndiotacticPMMA. The yield of temporary chain scissions is approximately 10 times greaterthan the yield of permanent chain scissions determined by measurement of gelfraction or molecular weight of the soluble polymers.


Cross-Linking and Chain Scission. The most important reactions oc-curring during radiolysis of polymers are those that lead to permanent changes intheir molecular weight. The reactions leading to either increases or decreases inmolecular weight are referred to as cross-linking and chain scission, respectively.End-linking, ie, reaction between a polymer main-chain species and a polymerchain end, is regarded as a cross-linking reaction of lower functionality. In generalcross-linking and scission processes can occur simultaneously in any irradiatedmaterial; however, it is often observed that one tends to dominate over the other,and thus polymers can be broadly placed into the categories cross-linking or de-grading. Over the past 50 years a solid understanding of the relationship betweenpolymer structure and the relative yields of cross-linking and chain scission hasbeen acquired. Thus, for linear polyolefins, the proportion of chain scission re-actions increases as the number of tertiary and especially quaternary carbonsalong the backbone is increased. Polyethylene undergoes primarily cross-linkingreactions, polypropylene undergoes approximately equal amounts of scission andcross-linking, which ultimately leads to an increase in molecular weight, and fi-nally polyisobutylene undergoes chain-scission reactions exclusively. However, itmust be clearly stated here that respective and absolute yields of the two typesof reactions depend also on a number of extrinsic factors, such as the presenceof a crystalline phase, the temperature, the presence of chain-end unsaturation,and perhaps most profoundly the presence of oxygen gas. In Table 1 below, weattempt to classify the various polymer types as either cross-linking or degradingon exposure to ionizing radiation in the absence of oxygen. Specific examples arediscussed later in this article.

Evolution of Gases. When polymers are exposed to high energy radi-ation, the reactions induced will lead to the formation of low molecular weightgaseous molecules. A study of the structure and yields of the gaseous products

Table 1. Cross-Linking vs Chain Scission for the Polymer Classes onExposure to Ionizing Radiation in Vacuum

Cross-linking dominates Chain scission dominates

Polyethylene PolyisobutylenePolypropylenePolydienesPolystyrene Poly(α-methyl styrene)Polyacrylates PolymethacrylatesPolyacrylamide PolymethacrylamideFully fluorinated elastomers Fully fluorinated thermoplasticsPoly(vinyl chloride) Poly(vinylidene chloride)

PolychlorotrifluoroethylenePolyamides CellulosicsPolyestersPolysiloxanesPoly(vinyl alcohol)Poly(N-vinylpyrollidinone)Main-chain aromatic polymers


can provide great insight into the mechanisms of the reactions occurring on radi-olysis. In general, gaseous products can arise either from primary cleavage of themain-chain bonds or from cleavage at the side chains.

Polymers that fall into the class of degrading polymers can, on irradiation atelevated temperatures (more specifically at temperatures approaching their ceil-ing temperature), yield high concentrations of monomers among the gaseous prod-ucts. The ceiling temperature is the temperature at which the reversible additionof monomers to a radical chain end is balanced by the tendency of the monomers todepolymerize. Thus, it was noted as early as 1953 (39) that poly(methyl methacry-late) foams on irradiation due to the formation of gaseous products, includingmethyl methacrylate monomer. Subsequently, it was reported that the yield ofmethyl methacrylate monomer increased substantially to a limiting yield around180◦C (40,41). Related to this is the pronounced tendency of these materials toundergo depolymerization after exposure to high energy radiation.

The most important commercial exploitation of this phenomenon has beenthe development of positive radiation resists for use in the semiconductor industry.A number of methacrylates and butene sulfones have been developed for theseapplications (42–44). Another important class of materials which undergo large-scale degradation of the main chain are the aliphatic polysulfones. For example,in 1981 Bowmer and O’Donnell (45) examined the yields of a number of aliphaticpolysulfones as a function of temperature and discussed these results in terms ofthe change in the equilibrium between polymerization and depolymerization asthe ceiling temperature is approached. This aspect of radiation chemistry has hadfar-reaching consequences for our modern society.

The second major mechanism leading to the formation of volatile products onirradiation of polymers is cleavage of the side-chain groups. In the case of linearpolymers, such as polyethylene, scission of the C H bond leads to the formation ofthermally activated hydrogen atoms. As discussed above, these small entities areable to escape the reactive cage and undergo subsequent reaction. These includeaddition reactions to double bonds, and also abstraction of hydrogen atoms andrecombination with hydrogen atoms to form molecular hydrogen gas. Thus, themain gaseous product formed on radiolysis of hydrocarbons is hydrogen gas. Theyield of hydrogen gas in polymers of the cross-linking type is often in excess of thatexpected from consideration of the yield of alkyl radicals leading to the formationof cross-links. As early as 1954, Dole and co-workers (46) determined that up to80% of the hydrogen gas measured after irradiation of polyethylene arose fromthe formation of vinylene unsaturation, with cross-linking reactions accountingfor the remainder. The presence of hydrogen gas has an important influence on thekinetics of radical reactions, and in principle on the overall product distributionin irradiated polymers. It has been suggested (47,48) that hydrogen gas is capableof enhancing the rate of diffusion of free radical species via reaction with alkylradicals as depicted below:

R′ • + H H + H R → R

′ · · · H · · · H · · · H · · · R → R′H + H H + •R (4)

The importance of the presence of hydrogen gas was first demonstrated by Doleand Cracco in 1961 (49–51) by measuring the rate of hydrogen exchange fromirradiated high density polyethylene to gaseous D2, and explored more completely


by Dole and co-workers over the next decade (47). Clough (48) much later provideda detailed analysis of the mechanism of migration of radicals through the solidstate in aliphatic hydrocarbon polymers, including an analysis of the numberof these “hydrogen-hopping” reactions expected on the production of main-chainradicals. The calculations accounted for localized hydrogen–deuterium exchangereactions in linear alkanes irradiated in the presence of D2 gas. It should alsobe noted that in aromatic polymers, hydrogen atom addition to aromatic ringsresults in the formation of cyclohexadienyl radicals and, potentially, initiation ofcross-linking reactions (22,24,52–54).

The great importance of the hydrogen-hopping reaction can be appreciatedby considering the differences in the radiation chemistry of polyhydrocarbonsand polyfluorocarbons. In the latter materials, reaction 4 is of course not oper-ative, and the corresponding reaction involving fluorine radicals is believed to beunimportant after consideration of the respective C F and F F bond energies(approximately 510 and 155 kJ/mol, respectively). Thus, the reactivity of fluo-rocarbon radicals in irradiated fluoropolymers is much more dependent on theinherent mobility of the polymer chains than in hydrogenated polymers. Thus,cross-linking reactions, which require two alkyl radicals to meet each other, areunlikely to occur in fluoropolymers irradiated at normal temperatures. Thus, poly-tetrafluoroethylene (PTFE) undergoes chain scission at all temperatures belowthe melting temperature (55,56). On the other hand the fluoroelastomer Kalrez(DuPont), a copolymer of tetrafluoroethylene and perfluorinated methyl vinylether, can be readily cross-linked by exposure to γ radiation, because of the mo-bility of the polymer chains and related higher probability of radical recombina-tion reactions (57). A corollary to this is the marked long-term stability of freeradicals in irradiated fluoropolymers. Indeed, Judeikis and co-workers (58) andothers have proposed the use of PTFE as a radiation dosimeter. Of more prac-tical concern is the propensity of these stable radicals to react slowly with dis-solved oxygen, a phenomenon leading to the well-known poor radiation stability offluoropolymers.

Finally, examination of the volatile products of irradiation of branched poly-mers reveals the importance of loss of the side chains. For example, irradiation ofbranched copolymers of ethylene and α-olefins results in volatile products charac-teristic of both the frequency and structure of the side chain (59,60). In this workthe authors also highlight the influence of radiation temperature and morphologyof the irradiated semicrystalline polymer.

Formation of Unsaturated Groups. As indicated above, an importantresult of the radiolysis of polymer is the formation of unsaturated groups. In1954, Dole and co-workers (46) reported the formation of main-chain unsatura-tion in irradiated polyethylene and demonstrated that the yield of these groupswas proportional to the yield of hydrogen gas. In these materials the mechanismof formation of unsaturation was thus established at an early time. In this anda subsequent article (46,61), Dole and co-workers also indicated that other typesof unsaturated groups, for example vinyl and vinylidene groups, are rather con-sumed in the radiation-induced reactions. Later, this same group deduced frommeasurements of gel fractions that the terminal unsaturation was reacting viaan end-linking reaction, a result confirmed much later by Randall and co-workers(62–64) and Horii and co-workers (65,66).


Perhaps the most well-known system in which large amounts of unsaturatedgroups are formed is poly(vinyl chloride). It is well established that irradiationleads to the formation of alkyl radicals, which are the locus for subsequent dehy-drochlorination reactions. This results in characteristic changes in the infraredand visible absorption spectra (67–69), as well as the formation of stable polyenylradicals (67,68).

Color Centers. An obvious corollary of the formation of polyenyl radi-cals and conjugated double bonds in irradiated poly(vinyl chloride) is a markedchanged in the visible appearance of the polymer (67–70). Color changes on irra-diation of polymers may have important implications for a number of commercialapplications, for example when the appearance of the material is of importance,as in medical applications, or when the optical properties of the material mustbe maintained in a high dose environment, for example in polymeric fiber opticsin particle physics detectors. On the other hand, color development for radiationdosimeters has been exploited by a number of workers, most notably the group ofMcLaughlin (71–74).

Until recently the phenomenon of color changes in irradiated polymers hasnot received detailed attention. Clough and co-workers (75–77), however, haverecently reviewed studies of color changes in a number of important syntheticpolymers. They report that there are two classes of color centers in polymers,annealable and permanent. The former class of color center appears to be as-sociated with free radicals trapped within the polymer matrix, whereas per-manent color centers are stable chromophores formed as a result of radio-chemical reactions. The free radicals can be destroyed by the application ofheat, or often through reaction with atmospheric oxygen. Clough and co-workers (77) also reported that whereas the incorporation of aromatic groupsprovides a degree of protection against radiochemical damage to polymers,it does not necessarily protect against color formation, because presumablythe products of reaction in aromatic polymers tend to have high extinctioncoefficients.

Changes in Molecular Weight of Polymers on Exposure to Radiation

The effects of cross-linking and main-chain scission on the molecular weight andthe molecular weight distribution of polymers have been examined in great detailby a number of authors (78–89). The exact method of treatment of the problemsdepends on the molecular weight distribution of the polymer prior to irradiation,and on whether it can be described by a most-probable distribution or recoursemust be made to a higher-order distribution. For simplicity, in this article we shallconsider the effects of cross-linking and scission on polymer having an initial most-probable distribution; however, reference will be made to more complete analyseswhere appropriate.

Effects of Chain Scission. In the early 1950s, Charlesby and co-workerswere considering theoretical approaches to the description of the changes inmolecular weight of polymers during irradiation. In 1954 they reported thatthe molecular weight of PMMA, measured by solution viscometry, was in-versely proportional to the radiation dose (80,90). Since that time a number of


refinements to these initial observation and theoretical expression have beenmade (81,91–94).

In summary, the number- and weight-average molecular weights change asdescribed below:

1Mn(D)

= 1Mn(0)

+ 2k1G(S)D (5)

1Mw(D)

= 1Mw(0)

+ k1G(S)D (6)

In these expressions the constant k1 is equal to 5.18 × 10− 8 if the dose D isexpressed in units of kGy. Most importantly, equations 5 and 6 are applica-ble to any initial molecular weight distribution, because in the case of degrad-ing polymers, the molecular weight distribution will approach the most-probabledistribution.

Effects of Simultaneous Cross-Linking. Charlesby (78) was also thefirst to describe the relationship between the molecular weight of a polymerand the radiation-induced chain cross-linking. He reported in 1954 that thedose required to produce incipient gel formation, known as the gel dose Dg,was inversely proportional to the initial molecular weight of the irradiatedsiloxane.

1Mn(D)

= 1Mn(0)

+ 2k1[G(S) − G(X)]D (7)

1Mw(D)

= 1Mw(0)

+ k1[G(S) − 4G(X)]D (8)

In this case, equation 7 applies to all initial molecular weight distributions,whereas equation 8 applies only to polymers having an initial most-probable dis-tribution undergoing H-linking reactions. Expressions for changes in molecularweight in polymers having other initial molecular weight distributions, and alsofor the case of cross-linking via a Y-linking mechanism, have been developed, andthese have been summarized by various authors (86–88,93).

The above expressions have been derived for the first two moments of themolecular weight distribution, Mn and Mw. However, the third moment, the z-average molecular weight, Mz, is experimentally accessible through measurementof sedimentation velocity of polymer chains. O’Donnell and co-workers (85,86)have reported the respective relationships between Mz and the radiochemicalyields for scission and cross-linking. The results of analysis of the changes inMz and Mw of irradiated poly(acrylic acid) and poly(methacrylic acid) (85) andpolystyrene have also been reported (86).

Soluble Fractions. The above descriptions of the change in molecularweight on irradiation presuppose the ability to accurately measure the molecularweight distribution of the polymer after irradiation. A number of authors havepointed out that for polymers for which cross-linking dominates chain scission,measurement of molecular weight is limited to doses well below 50% of the gel


dose. More conveniently measured parameters are therefore the gel dose, thatis, the dose to achieve incipient gel formation, and the residual soluble fractionbeyond the gel dose.

The most widely used expression for analysis of the soluble fraction is alsodue to Charlesby and Pinner (95).

S + S0.5 = G(S)2G(X)

+ k2/[G(X)Mn(0)D] (9)

The constant k2 is equal to 4.82 × 106 if the dose is expressed in kGy. Equation9 assumes a post-probable initial molecular weight distribution, random scissionand cross-linking reactions, and cross-linking by an H-linking mechanism. If cross-linking occurs by a Y-linking mechanism, the relationship between soluble fractionand dose is given by (96)

S + S0.5 = 2G(S)G(X)

+ k3/[G(X)Mn(0)D] (10)

The constant k3 is equal to 1.93 × 104 if the dose is expressed in kGy. Clearly,the choice of application of either equation 9 or equation 10 must be directedby additional evidence of the mechanism of cross-linking. Finally, a number ofworkers have derived expressions for the relationship between the soluble fractionand radiation dose for more general initial molecular weight distributions (81,88,91–93,97).

The Radiation Chemistry of Specific Polymers

The radiation chemistry of polymers has attracted enormous interest since theinitial studies by Charlesby, Dole, and Chapiro in the early 1950s. This interesthas been a result of the recognition of the tremendous commercial importance ofthe radiation chemistry of polymers, and of the importance of an understanding ofradiation chemistry to other fields. A number of excellent reviews have appearedover the past 45 years, including the seminal texts by Charlesby (1) and Chapiro(2), and the comprehensive review texts edited by Dole (98,99). Since then, a num-ber of broad reviews of the effects of radiation have been published. For example,the Polymer Handbook has previously tabulated lists of the radiochemical yieldsreported elsewhere (100). The American Chemical Society has published a num-ber of collections of papers presented at radiation chemistry meetings (101,102),and the two journals Radiation Physics and Chemistry and Polymer Degradationand Stability are of much interest. The 11 international meetings on radiationprocessing, which have been published in Radiation Physics and Chemistry (103–113), and the text by Singh and Silverman (114), provide an excellent overview ofthe field.

A number of more specific reviews have also been published. For example,Reichmanis has written about the use of radiation chemistry for electronics appli-cations (43,115). Clough (116,117) has written a number of excellent reviews with


an emphasis on commercial applications of radiation chemistry, and the effects ofoxygen on radiochemical events. The radiation chemistry of fluoropolymers hasbeen reviewed recently by Lyons (118) and Forsythe and Hill (119). Burillo and co-workers (120) have addressed the use of radiation in the polymer recycling indus-try. Rosiak (121,122) has reviewed the emerging field of the radiation processing ofbiomedical polymers. More recently, Pruitt (123) has reviewed the major changesinduced in medical polymers on irradiation, and given specific case studies of anumber of materials. Hill and Whittaker (124) have detailed the importance ofNMR spectroscopy in this field.

Polyethylene. It has been known for many years that polyethylene un-dergoes cross-linking on exposure to high energy radiation. In 1952 Charlesby(125) reported that exposure of polyethylene to pile radiation resulted in chemicalchanges consistent with cross-linking reactions. This initial observation promptedextensive work by him and others, most notably Dole, which has laid the founda-tions for the modern field of radiation chemistry of materials. The main find-ings of these and subsequent workers are summarized below (see EthylenePolymers).

Free-Radical Intermediates. From an early time the participation of freeradicals in the radiation chemistry of polymers has been well understood.Charlesby (125) in 1952 invoked a free-radical mechanism of cross-linking ofpolyethylene, although for other materials the participation of ionic species hasalso been suggested (126–137). The primary radicals observed at 77 K, at whichtemperature a significant proportion of the radicals are assumed to be trapped andprevented from further reaction, are the secondary alkyl radicals I (49,138–148).

Dole and co-workers have reported yields of alkyl free radicals in polyethy-lene irradiated at 77 K ranging from 2.7 to 3.7 (141,145,149). Furthermore,Cracco, Arvia, and Dole (49) reported that on warming, alkyl radicals decay bya first-order process, and they attributed this to reactions between alkyl radi-cals within isolated spurs. The persistent free radicals on warming to room tem-perature are the allyl radicals II. The impact of long-term stability of radicalspecies on the stability of polyethylene has been underlined by studies of Jahanand co-workers (150–157) of ultrahigh molecular weight polymer used in medicalimplants.

Cross-Linking. In addition to conversion of alkyl radicals to more thermallystable allyl radicals, a significant proportion can undergo radical–radical recom-bination reactions (eq. 11).

(11)

The product shown on the right of equation 11 is called an H-type cross-link. A number of alternative mechanisms of cross-linking have been proposed,


the most important being the formation of Y-type cross-links through eitherradical recombination reactions (eq. 12) or end-linking to terminal vinyl groups(eq. 13).

(12)

(13)

In the case of polyethylene the initial cleavage of carbon–carbon main-chainbonds is believed to be relatively unimportant compared with end-linking reac-tions. Evidence for this includes the absence of a contribution from chain-end freeradicals in the ESR spectra of polyethylene irradiated at low temperatures, andcareful analysis of the residual soluble fractions formed on irradiation at highertemperatures. Dole and Katsuura (158) suggested that if main-chain scission doesoccur, the cage effect is responsible for an overwhelming proportion undergoingrecombination reactions. Not long after this, Kang, Saito, and Dole (159) measuredthe yields of cross-linking and main-chain scission in high density polyethyleneat a range of temperatures by analyzing the residual soluble fractions with aCharlesby–Pinner equation modified for a Wesslau molecular weight distribu-tion. They found that at each temperature over 308–393 K, the ratio of scissionto cross-linking reactions was significant (ca 0.2) and decreased slightly at higherdoses, an effect they ascribed to the reactions of vinylene double bonds to formcross-links. Similar results were reported by Dole and co-workers (149,160). Incontrast to these articles, Lyons and Fox (161) reported negligible dose-dependenceof G(X) and 0.03 as the ratio of the yield of scission to that of cross-linkingreactions.

The yields of cross-linking reactions in polyethylene have been reported ex-tensively (97,149,159,160,162) and generally fall in the range G(X) = 0.8–2.5 forsamples irradiated at ambient temperatures. The principle reason for this widerange of values of cross-link yields is the variation in crystallinity in the mate-rials studied. It is reasonably well accepted that cross-linking reactions do notoccur within the crystalline lamellae of polyethylene; Patel and Keller (163) con-firmed this by analyzing the residual molecular weights of the chains within thecrystalline regions of irradiated high density polyethylene after the amorphousregions had been removed by exposure to ozone. There was no evidence for cross-linking reactions between the chains previously within the crystalline regions. Tofurther underline this relationship, O’Donnell and Whittaker (164) have reportedthat the value of G(X) decreases progressively with increasing crystalline contentfor a range of materials having a similar initial molecular weight distribution.

In this same article, O’Donnell and Whittaker (164) compared the yields ofcross-linking and scission, and other radiochemical products, in fully amorphousethylene–propylene rubber (EPR) with the yields measured by them and reportedby other workers for semicrystalline polyethylene and polypropylene. The yieldsof cross-linking and scission were significantly lower than those expected fromextrapolation of the G-values for the homopolymers to zero crystalline content. In


other words, the yields for the fully amorphous materials were much lower thanexpected from considering yields within the amorphous regions of the semicrys-talline polymers. It was concluded that the radiation-induced reactions must befavored at the crystalline–amorphous interface and that this might be a result ofthe stabilization of free-radical intermediates at the interface.

The relative importance of reactions 11–13 in the radiation cross-linking ofpolyethylene has been resolved to a large extent through a number of elegant stud-ies of the structures of polymer irradiated to doses below the gelation dose, usinghigh resolution solution-state 13C NMR spectroscopy. The initial use of 13C NMRto study radiochemical reactions was by Bennett and co-workers (165), who re-ported structural changes in irradiated liquid model compounds of polyethylene,n-hexadecane, and n-eicosane. These authors reported the formation of H-linkstructures apparently via recombination of two secondary main-chain alkyl rad-icals. Not long after this, Bovey and co-workers (166) reported the identificationof both H- and Y-type links in n-C44H90 irradiated in the melt using solution-state 13C NMR. Irradiation in the crystalline state produced only linear dimers,apparently through the end-linking of molecules at the crystal surfaces.

Several years after this, Randall and co-workers (62–64) studied irradiatedhigh density polyethylenes using 13C NMR, and identified a number of new struc-tures, including internal double bonds and Y-type cross-links. The materials underconsideration initially contained a high concentration of vinyl end groups, whichreact readily with secondary alkyl radicals (reaction 12 above) to form Y-links. Theauthors were unable to identify H-links in their irradiated HDPE samples, andsuggested that for these materials irradiated to low doses, the H-linking mecha-nism is relatively unimportant.

More recently, Horii and co-workers (65,66) examined changes in thesolution-state 13C NMR spectra of relatively low molecular weight PE samplesirradiated under a range of experimental conditions. The lower viscosity of thesolutions as a result of the low molecular weight was manifested in narrow linewidths in the NMR spectra, and thus the authors were able to observe resonancesin the spectra due to H-type cross-links in all samples. The yields of H-links andY-links were measured as a function of temperature (66), and it was found thatH-links were the dominant product when PE was irradiated in the molten state,whereas Y-links were more commonly formed on irradiation at ambient temper-atures. It was suggested that at higher temperatures the probability of reactionof the primary radicals (in reaction 12 above) with hydrogen radicals is higherin the molten state, and thus recombination of the longer-lived secondary rad-icals (reaction 11) dominates. These conclusions are supported by the study ofO’Donnell and Whittaker (167) in which the formation of H-type cross-links infully saturated ethylene–propylene rubber (EPR) irradiated well above Tg wasconfirmed. In addition, O’Donnell and Whittaker (168) have reported the obser-vation of resonances due to H-type cross-links in LLDPE irradiated to very highdoses.

Volatile Products. As discussed above, Dole and co-workers were the firstto observe that hydrogen gas was a major product formed on the irradiation ofpolyethylene (46). Gaseous hydrogen was formed by recombination of hydrogenatoms formed by cleavage of C H bonds, as illustrated in reaction 4 above, orby abstraction of hydrogen atom from the polymer main-chain by this ejected,


perhaps thermally activated, hydrogen atom itself. The role of the hydrogen gasso formed in radical migration was fully described by Dole and Cracco in 1961 and1962 (50,51), and later through observation of the catalytic effect of hydrogen gason the kinetics of decay of alkyl free radicals by Waterman and Dole (169) andJohnson and co-workers (145).

The yield of hydrogen gas in irradiated polyethylene has been reported bya number of workers to fall in the range of G(H2) = 3–4, and shown by Kangand co-workers (159) to increase with increasing irradiation temperature. Thereis also evidence that the yield does depend to some extent on radiation dose (159)and polymer crystallinity (164).

In addition to hydrogen gas, small quantities of hydrocarbon gas reflectiveof the concentration and identity of side chains are produced on irradiation ofpolyethylene containing short chain branches. Bowmer and co-workers in 1983(59) reported that the primary scission process leading to the formation of thesesmall alkanes must occur at the tertiary carbon on the main chain. They alsoreported that the relative yields of volatile saturated products were essentiallyconstant with increasing temperature, but that the total overall yield of gasesdid increase. On the other hand the formation of unsaturated products, princi-pally ethylene and butylene, attributed to thermal depolymerization, increasedmarkedly at elevated temperatures.

Unsaturation. The formation and reaction of unsaturated groups inpolyethylene has been extensively studied by Dole and co-workers over a numberof years, with a particular emphasis on the relationship between the rate of for-mation and decay of unsaturation with other radiochemical reactions. The mainunsaturated product formed on irradiation of polyethylene is internal vinyleneunsaturation. Dole and co-workers (46) suggested that up to 80% of the hydrogengas evolved on irradiation resulted from processes leading to formation of viny-lene groups. Dole and co-workers (61) further reported that the rate of formationof vinylene unsaturation was scarcely affected by irradiation at 77 K, at whichtemperature migration reactions are presumably retarded. This led them to sug-gest that molecular detachment of hydrogen molecules may be involved. Later,this group (159) reported that the G-value for the formation of vinylene groupswas close to 2.4 at all temperatures in the solid state.

In addition to the formation of vinylene unsaturation, evidence for the for-mation of conjugated double bonds can be found in the appearance of new ab-sorption bands in the UV spectrum of irradiated polyethylene (170–172). The rateof formation of dienes is significantly higher than that expected from considera-tion of the statistics of formation of vinylene unsaturation; Fallgatter and Dole(170) speculate on possible cooperative mechanisms of formation. The formationof more highly conjugated species is suggested to result from secondary reaction(171,172).

The kinetics of decay of double bonds initially present in the material havealso been studied in detail. Dole and co-workers (61) reported that the decay ofterminal vinyl or vinylene groups follows first-order kinetics until the formationof vinylene groups, mentioned immediately above, becomes significant. The exactmechanism of reaction of the vinyl end groups remains unclear; however, the par-ticipation of these groups in end-linking reactions (reaction 12) has been confirmedby Randall and co-workers (62–64) and Horii and co-workers (65,66).


Polypropylene. Polypropylene undergoes net cross-linking when exposedto ionizing radiation in the absence of oxygen. However, the radiation chemistry ofpolypropylene differs markedly from that of polyethylene as a consequence of thepresence of tertiary carbons along the polymer backbone. Therefore, polypropyleneundergoes significant levels of main-chain scission on irradiation (see PROPYLENE

POLYMERS (PP)).Initial studies of this polymer were mainly concerned with understanding

the changes in molecular weight and the rate of gel formation on irradiation. Muchof this work was concerned with the development of fundamental relationshipsbetween the radiochemical yields and molecular weights. The reported yields ofchain scission and cross-linking fall in the ranges G(S) = 0.198–0.62 and G(X) =0.0645–0.272, respectively (81,82,95,173–176). The apparent large variation invalues of G(S) and G(X) are in part due to the difficulty in analysis of molecularweight or solubility changes in materials having broad molecular weight distribu-tions, and also possibly due to a dose dependence of the G-value for chain scission(81,175,176).

Despite the increased probability of main-chain scission in polypropylenecompared with polyethylene, there is no evidence of the formation of stable scissionradicals on irradiation at low temperatures. Rather extensive ESR evidence hasbeen accumulated that the stable free-radical intermediates are those listed below:

On irradiation at low temperatures, alkyl radicals (III and IV) are formedwith a radiochemical yield G(R) of 2.4–2.6, depending on the crystallinity of thematerial (99,164).

A particularly informative set of experiments which reveal the extent ofinitial bond cleavage has been conducted by Busfield and co-workers (35–38). Theyhave studied the radiation-induced changes in the stereochemistry of isotacticpolypropylene (iPP). The changes in the stereochemistry are a result of initialchain scission, isomerization, and rehealing of the broken bond, as shown belowin Figure 1.

Studies of irradiated iPP (35–38,177) have provided evidence that the racem-ization reactions do not occur as isolated events, but rather that they may occurin clusters. The methyl region of the solution-state 13C NMR spectrum of iPPcan be resolved to the pentad level. The changes in the pentad distribution onirradiation are not consistent with random reactions occurring along the polymerbackbone (35–38). A number of mechanisms have been suggested to account forthe observed changes in tacticity, involving for example an increased probabilityof reaction after initial reaction, but all schemes have in common the observa-tion that the proportion of syndiotactic sequences formed on irradiation is highand that the rate of equilibration of these sequences is low. It was also found that


Fig. 1. The racemization of an initially isotactic polymer through chain scission, isomer-ization, and bond reforming.

electron-beam irradiation at high dose rates was much more effective in promotingchanges in tacticity than γ irradiation to comparable doses (37).

Polyisobutylene. In 1954 Charlesby (80) noted that the molecular weightof polyisobutylene (PIB) decreased rapidly on exposure to high energy radiation.It has since been well understood that the presence of quaternary carbons in themain chain of polymers predisposes such a material to degradation (see BUTYL

RUBBER). In the case of PIB, the primary reactions during radiolysis have beensuggested by Alexander and co-workers (178) and Chapiro (179) to be those de-scribed in Figure 2. On the other hand Miller and co-workers (180) suggested thatthe initial chemical changes involve loss of a methyl hydrogen to form radical VII,followed by spontaneous β-cleavage (Fig. 3). In addition, Il’icheva and Slovokho-tova (181) have suggested that the energetically favoured radical VIII can alsoundergo β-cleavage, as in Figure 4.

The ESR spectra of PIB irradiated at low temperature consists of a broaddoublet with a hyperfine coupling constant of 2 mT, attributed to either radicalVIII (182) or a combination of contributions from radicals VII and VIII (183,184). The primary radicals V and VI (Fig. 2) have not been observed by ESRspectroscopy. Despite extensive studies of the radiochemical changes in PIB byESR, volatile product analysis, and UV and IR spectroscopy, the mechanism ofdegradation had been uncertain until the advent of high resolution 13C NMRmethods. In a comprehensive study, Bremner and co-workers (185) and Hill and

Fig. 2. Proposed mechanism of degradation of PIB via initial main-chain cleavage.


Fig. 3. Proposed mechanism of degradation of PIB via β scission of the side-chain methy-lene radical.

co-workers (186) identified the major products of degradation by NMR and othermethods. They concluded that the schemes shown in Figures 2–4 are all possibledegradation pathways, and that the major products are structures formed throughthe reaction scheme shown in Figure 2.

Polyethers. The radiation chemistry of the polyethers has received rel-atively little attention. It is known, however, that all of this class of materialsundergoes net cross-linking reactions. A comprehensive study of the radiationchemistry of polyoxymethylene (POM) has been reported by Fischer and Lang-bein (187). These authors reported that irradiation at the high dose rates of 150kGy/min with 1-MeV electrons at 287 K resulted in a reduction in the total massof the sample of approximately 1% per 100 kGy absorbed dose. There was a cor-responding increase in the yield of new acetate chain ends determined by IRspectroscopy. The evolved gases consisted of many products, however the mainproducts were hydrogen (G = 1.7), methane (G = 0.14), and CO (G = 0.013). Thegases also included formaldehyde and higher molecular weight molecules. Theseresults were combined with 1H NMR analysis of the polymer after irradiation.Peaks due to new acetate and methoxy chain ends, and H-type cross-links, wereobserved. The yields of products obtained from the NMR spectra appear to behigh, perhaps reflecting the difficulty in obtaining quantitative NMR spectra ofirradiated materials. Despite this, they were able to confirm that this polymerdoes crosslink on irradiation.

The effects of irradiation on the properties of poly(ethylene oxide) (PEO)have been examined by a number of workers (see ETHYLENE OXIDE POLYMERS).A number of these studies have been concerned with irradiation cross-linking ofPEO in solution (188–201). When irradiated in the solid state, PEO undergoes netcross-linking with the reported gel doses of 3.5 and 1.1 kGy, respectively, for highlycrystalline, lower molecular weight and more highly disordered, high molecularweight materials (202). King reported earlier changes in solution viscosity of PEOirradiated in air and in vacuum (203). Other studies have been concerned withchanges in the crystalline structure on irradiation; the crystalline order is reducedat moderate radiation doses consistent with cross-linking reactions (202,204,205).

Fig. 4. Proposed mechanism of degradation of PIB via β scission of the main-chain me-thine radical.


Finally, the effects of radiation cross-linking on the chain dynamics have beenexamined by Schilling and co-workers (206) using solid-state 13C NMR techniques.

Poly(propylene oxide) (PPO) has also been observed to form a gel on irradi-ation (207). Roberts and co-workers (207) reported G(S) values of 0.22 and 0.55and G(X) values of 0.15 and 0.31 for atactic and isotactic PPO, respectively. In thisarticle the products of degradation are compared with previous reports of productsof irradiation of PP and POM. It was found that the radiation chemistry is broadlyconsistent with that expected from previous studies.

Poly(vinyl alcohol). The radiation chemistry of poly(vinyl alcohol) (PVA)is similar in many respects to the degradation of PVC, described below (see VINYL

ALCOHOL POLYMERS). PVA undergoes a progressive darkening on irradiation atelevated temperatures or gets heated subsequent to irradiation due to the forma-tion of sequences of conjugated double bonds (208). The mechanism of degradationproposed by Zhao and co-workers (208), and shown in Figure 5 below, involves theparticipation of radical cations. Previous studies have indicated that hydrogengas is a primary product of the degradation of PVA (209,210), leading to the for-mation of the α-carbon radicals, as shown in Figure 5. More recently, using ESRspectroscopy, Zainuddin and co-workers (211) have examined in detail the radi-cal species formed on irradiation of PVA. They identified contributions for threeradical species at low temperatures: the α-carbon radical (triplet ESR spectrum),a radical anion (doublet spectrum), and an unidentified neutral species (doubletradical). The latter two species contribute 62% of the total radical concentrationat 77 K. These results are broadly consistent with the mechanism proposed inFigure 5; however, neither article comments on the expected changes in molecu-lar weight on irradiation. Lu and co-workers (212) have shown that the molecularweight of PVA decreases on irradiation, with the polymer remaining soluble up

Fig. 5. Proposed mechanism of formation of conjugated structures during the radiolysisof poly(vinyl alcohol).


to doses greater than 1000 kGy. It is apparent that chain scission reactions mustdominate over cross-linking reactions.

Poly(vinyl chloride). The mechanism of radiation chemistry of poly(vinylchloride) (PVC) in many ways resembles the well-known thermal degradation ofthis polymer (see VINYL CHLORIDE POLYMERS). The principal results of exposure ofPVC to ionizing radiation are evolution of hydrochloric acid and development ofintense absorption bands in the UV and visible regions of the electromagnetic spec-trum (67–69,213). The result of dehydrochlorination is the formation of polyeneradicals. For example, absorption peaks at 252, 291, and 330 nm were assignedto allyl, dienyl, and trienyl radicals, respectively. The dehydrochlorination reac-tions dominate the radiation chemistry; the yield of cross-linking reactions is verylow, as reported by Chapiro (69). Salovey and Gebauer (214) later reported fromthe changes in the polydispersity of irradiated PVC that chain scission and cross-linking are occuring simultaneously on irradiation in air.

Aliphatic Polyesters. Over the past 10 years the linear aliphaticpolyesters, the polyalkanoates, and the copolymers of lactic and glycolic acids haveattracted enormous interest as a result of their widespread use as biomedical ma-terials. The radiation stability of these materials is of importance because of thepotential to effect sterilization through exposure to ionizing radiation (215–225).

Polyhydroxybutyrate. Copolymers of hydroxybutyrate and hydroxyvaler-ate possess many mechanical properties in common with synthetic polyolefins,and so have attracted much attention as replacements for these materials inenvironments where biodegradability is an important parameter (see POLY(3-HYDROXYALKANOATES)). Carswell-Pomerantz and co-workers (217,218) have re-ported a detailed study of the radicals formed on irradiation of such materials.They found that the yield of radicals at 77 K was G(R) = 1.7 ± 0.2, independent ofcopolymer composition, but that on irradiation at 300 K, the yield of radicals wasreduced for the copolymers because of their lower glass-transition temperaturescompared with the homopolymer. At low temperatures a significant contributionto the ESR spectra from radical anions was noted. These radicals were observedto decay on warming to produce scission radicals. At still higher temperatures,radicals produced by abstraction of a methylene proton adjacent to the carbonylgroup were detected.

The predominant reaction on radiolysis of polyalkanoates is chain scission.Carswell-Pomerantz and co-workers (218) have shown that the main volatile prod-ucts of radiolysis are CO, CO2, and H2. Scission at the ester unit results in areduction in the molecular weight of these materials, and the G-value for scis-sion G(S) for PHB was calculated to be 1.3. Using solution NMR, a very similaryield of reactions was determined from measurement of the yield of new chainends. Cross-linking is not important in the radiation chemistry of PHB; however,analysis of the changes in molecular weights of irradiated PHBV copolymers pro-vided evidence of some cross-linking, with the ratio of scission to cross-linkingbeing 9 and 6.8 for materials having valerate mole fractions of 0.184 and 0.263,respectively.

Poly(lactic acid-co-glycolic acid). The degradation of the homopolymersand copolymers of lactic and glycolic acids (PLGA) has been reported by a num-ber of authors (220–223). Babanalbandi and co-workers (220) have measured theG-radical values at 77 K for poly(L-lactic acid) (L-PLA) and poly(D,L-lactic acid)


(D,L-PLA) and reported these to be 2.0 and 2.4, respectively. They also reportedG(S) = 2.3 and G(X) = 0.0 for D,L-PLA and G(S) = 2.4 and G(X) = 0.28 for L-PLA.These results are in contrast to the values of Collett and co-workers, who sug-gested that rather than chain scission dominating the radiation chemistry, thesepolymers form a gel on irradiation.

The results of Babanalbandi and co-workers (221), in which new aliphaticchains ends formed by cleavage of the main chain at the ester unit are observed, arein support of a mechanism in which chain scission dominates cross-linking. Theseauthors reported G-values for the formation of chain end structures comparablewith earlier study. Furthermore, the main volatile products of radiolysis of PLAand poly(glycolic acid) (GPA) are CO2 and CO, consistent with chain scission beingthe most important reaction. In addition, small amounts of hydrogen and ethanegas were observed on the radiolysis of PLA. Finally, Montanari and co-workers(226) have examined the effects of radiation sterilization on the stability of PLGAmicroparticles used for drug delivery.

Aliphatic Polysulfones. The class of copolymers of olefins and sulfurdioxide are well known for undergoing extensive degradation on exposure to highenergy radiation. In 1965 Ayscough and co-workers (227) reported an ESR studyof the degradation products of sulfones and poly(olefin sulfone)s. They were able toconfirm that the relatively weak C S bonds are more susceptible to rupture com-pared with the C H bonds. Later, Brown and O’Donnell (228,229) confirmed thatthis initial chain breakage leads to loss of SO2 and permanent chain scission andreduction of molecular weight. The yield of scission reactions was found to increaseprogressively up to the ceiling temperature for the propagation–depropagationequilibrium, at which temperature rapid depropagation occurred. The yield ofolefin gas evolved on radiolysis was observed to be lower than the yield of sulfurdioxide, because of the cationic homopolymerization of the olefins initiated by apolymeric carbonium ion (45,230). Isomerization of the olefin via a carbonium ionintermediate was also confirmed. The mechanisms of these reactions are summa-rized in Figure 6.

Polymethacrylates and Polyacrylates. The high energy radiationchemistries of polyacrylates and polymethacrylates were some of the first studiedin detail for any polymer (see ACRYLIC ESTER POLYMERS; METHACRYLIC ESTER POLY-MERS). For example, in the 1950s, Charlesby and co-workers reported extensivelyon the breakdown of poly(methyl methacrylate) (PMMA) on exposure to high en-ergy radiation, and also on studies of coloration and gas formation associated withthe loss of the side chains (20,39,80). The molecular weight of PMMA was shownto decrease on irradiation (80,231), with the decrease inversely proportional tothe irradiation dose. The latter observation was later rationalized through thefamous Charlesby–Pinner relationship (95), which allowed G-values of chain scis-sion and cross-linking to be evaluated. Shultz and co-workers (232) reported thatfor 1-MeV electron irradiation of PMMA under vacuum, G(S) = 0 and G(X) = 1.65,and Wall and Brown (231) examined the yields for the principal gases formed onγ radiolysis under vacuum: H2, CH4, CO, and CO2, for which the G-values were0.21, 0.54, 0.45, and 0.32, respectively [G(gas) = 1.6].

ESR studies by Ovenall and co-workers (233–236) during the same periodidentified the natures of the spectra of the radicals formed in PMMA on radiolysisover a temperature range from ambient to 77 K. They identified the multiline


Fig. 6. Proposed mechanism of degradation of poly(olefin sulfone)s.

spectra observed at room temperature as being principally associated with chain-end radicals, which were characterized by a nine-line spectrum at room tempera-ture, with G(R) = 2.5. Later, O’Donnell and co-workers (237) were able to accountfor the nine lines in terms of two overlapping spectra, one consisting of five andthe other of four lines, arising from two conformations of the chain-end radicals.Bowden and O’Donnell (238) later showed that the relative contributions of thetwo conformations varied with the observation temperature.

A consideration of the molecular weight changes and the natures and yieldsof the gaseous and radical products formed on radiolysis at room temperatureallowed a mechanism for the radiolysis of PMMA to be formulated. The princi-pal process associated with the degradation was scission of the ester side chain,forming the principal gaseous products and a main-chain radical which can fur-ther undergo β-scission to form the propagating radical and a chain-end doublebond. It was also thought that some direct main-chain scission may occur at theα-carbon. The coloration of PMMA on radiolysis was believed to arise from theformation of oxygen and temperature-sensitive radical and charged species.


David and co-workers (32) confirmed the occurrence of direct main-chainscission and reported that racemization also occurred during radiolysis of syndio-tactic PMMA. In the presence of a radical scavenger, no racemization was foundto take place, suggesting that radical processes are involved in the racemizationprocess (33). These results therefore indicate that irradiation must induce sometemporary main-chain scission, which is followed by recombination of the scissionradicals within the cage leading to a concomitant change in the stereochemistryof the polymer. The extent of the racemization observed for a γ -ray dose of 8 MGywas reported to be approximately 24%.

Because PMMA has a low ceiling temperature, the formation of propagatingradicals on radiolysis at 453 K, with G(S) = 10, Charlesby and Moore (40) foundspontaneous depolymerization of the polymer to monomer at this temperature.In a later article, David and co-workers (239) showed that depolymerization alsooccurred when PMMA was irradiated at ambient temperature and then heated to433 K, and that the yield of monomer was independent of the absorbed dose andthe initial molecular weight of the PMMA.

By contrast with PMMA, poly(methyl acrylate), PMA, and several otheraliphatic polyacrylates were found by Shultz and Bovey (240) to undergo cross-linking and gel formation on irradiation with 1-MeV electron beams. They re-ported G(S) = 0.15 and G(X) = 0.52 for PMA. Graham (241,242) reported that thephenyl, benzyl, and 2-phenyl ethyl acrylate polymers also undergo cross-linkingon γ radiolysis under vacuum. There was evidence of side-chain scission also, andaccording to Fox and co-workers (243,244) the major volatile products were similarto those observed for PMMA.

In these early years, the high energy radiation induced reactions of PMAand PMMA were not the only polyacrylates and polymethacrylates investigated,but they were found to be representative of the radiation chemistries of the twopolymer families with different ester side-chains.

With improvements in the sensitivities of NMR and ESR spectrometers dur-ing the 1970s and the growing importance of microlithography in the production ofsolid-state devices, extra interest was directed to studies of the radiation-inducedreactions of both polymethacrylates and polyacrylates. Because the methacrylatepolymers almost entirely undergo scission on exposure to radiation under vac-uum and the acrylates undergo cross-linking, they can be utilized as the basisfor positive and negative resists, respectively. Thompson and co-workers (42) andBowden (245) reported studies of the applications of PMMA in resist development,and examined the effect of incorporation of heavy atoms to enhance the sensitivityof PMMA to X-rays. Bowden (245) found that the sensitivity of a resist dependsnot only on the nature of the polymer but also on its molecular weight and molec-ular weight distribution, and that it increases with deceasing polydispersity. Thedevelopment dose was found to decrease with increasing G(S) for positive resistssuch as the polymethacrylates.

In a series of articles, Busfield, O’Donnell and co-workers reexamined theradical yields, gaseous products, molecular weight, and material property changeson γ radiolysis of PMMA and PMA (22,23,27,246–248).

O’Donnell and co-workers (22,248) have reevaluated radical formation onirradiation of PMMA at 77 K and room temperature. At 77 K the spectrum wasanalyzed in terms of the presence of CHO•, CH3•, •COOCH3, and COOCH2•,


as well as the propagation radical. At room temperature, only the propagatingradical spectrum was observed. The G-value for overall radical formation at 77 Kwas 0.58, which is lower than that found at room temperature, 2.2, indicating theimportance of radical recombination within the cage at 77 K. On annealing of theradicals produced at 77 K to room temperature and above, the radical decay is notlinear, but reflects the onset of thermal transitions in PMMA (249).

A wide range of gaseous products have been identified by Busfield and co-workers (246) on γ radiolysis of PMMA at room temperature, which arise prin-cipally from a breakdown of the ester side-chains. The principal products andtheir yields G are respectively H2, 0.34; CH4, 0.66; CO, 1.08; CO2 0.68; CH3OH,0.36; HCO2CH3, 0.69; dimethoxymethane, 0.11; and CH3Ac, 0.02. Kudoh and co-workers (250) have measured the yields of H2, CH4, CO, and CO2 on radiolysisat 77 K and report that whereas the hydrogen yields are the same at 77 K androom temperature, those for the other gases are lower at 77 K. This is in accordwith the temperature dependence of the observed radical yields (248) and withthe mechanical properties (251).

Busfield and co-workers (246) have examined the molecular weight depen-dence for γ radiolysis of PMMA in vacuum at room temperature, and Kudoh andco-workers (250) have also reported the dependence at 77 K. The values of G(S)at room temperature were 1.3 (246) and 1.7 (250) and at 77 K, 0.24 (250). Thesereflect the reported radical yields at the two temperatures. The variation of themolecular weight distribution with dose for e-beam irradiation of PMMA has beenstudied by Viswanathan (83). He has modeled the dependence of the distributionsfor use in predicting the sensitivities of positive resists in terms of product solu-bilities for development following exposure.

Busfield and O’Donnell (23) have examined the correlations between thechanges in the molecular weight and the tensile properties of PMMA followingradiolysis at room temperature. Both the flexural and compressive strengths de-crease with increasing dose, reflecting the falling molecular weight of the polymer.Kudoh and co-workers have drawn similar conclusions for radiolysis at 77 K andambient temperature (251).

The racemization of PMMA on radiolysis has been studied by Luo and co-workers (34), Dong and co-workers (31), and Thominette and Verdu (252) by useof NMR. Luo and co-workers (34) reported that for radiolysis of isotactic PMMAin solution, the yield of scission was greater than that for racemization, but theorder was reversed for radiolysis in the solid state, indicating an important role formolecular motion of the scissioned chains. Dong and co-workers (31) investigatedthe γ radiolysis of both highly isotactic and highly syndiotactic PMMA. They foundthat their results were consistent with a temporary chain scission G-value of 18.6at 353 K, and the polymers tended to approach a common average tacticity withincreasing absorbed dose, which was the same as that found for PMMA preparedby free-radical polymerization.

By comparison with PMMA, the radiolysis of PMA has received much lessrecent attention. Busfield and co-workers (247) have measured the G-values forgaseous product formation at room temperature and 423 K, and they have alsomeasured the dose to gel. The major volatile products were the same as thoseobserved for the radiolysis of PMMA, but the total gas yield was much smaller:G(gas) = 1.6 compared with 3.9 for PMMA. However, the gas yield increased with


temperature, reaching 3.9 at 423 K. At 195 K the dose to gel was approximately0.25 MGy, and decreased with increasing temperature.

The sensitivities of a wide range of other acrylate and methacrylate polymersto γ radiation have also been investigated, including poly(acrylic acid) (85,253),poly(methacrylic acid) (85,254–263), poly(hydroxyethyl methacrylate) (264,265),polymethacrylates with longer ester side-chains (266,267), and poly(ethylene gly-col dimethacrylate) (268). In general, the behaviors of these polymers mirror thebehaviors of the related methyl ester polymers, but some observations are worthyof special mention.

Hill and co-workers (85) have reported a study of the scission and cross-linking yields on γ radiolysis of poly(acrylic acid) and poly(methacrylic acid) usingsedimentation equilibrium to obtain the weight- and Z-average molecular weightsas a function of absorbed dose. The values G(S) = 0, G(X) = 0.44 and G(S) =6.0, G(X) = 0, respectively, were obtained for the two polymers, highlighting thesignificant role of the substituent methyl group in determining the outcome of theradiolysis. Kudoh and co-workers (259) have investigated LET effects for electronradiolysis of PMMA. A comparison of the scission probabilities for γ radiation (0.5Gy/s) and high energy electron beams (up to 4 × 1010 Gy/s) showed no evidencefor dose rate effects; thus they found no evidence for spur overlapping at the highelectron beam dose rates. Dong and co-workers (267) studied the role of the lengthof the aliphatic side chains in a series of highly syndiotactic polymethacrylates andfound that poly(heptyl methacrylate) indeed undergoes gelation on γ radiolysis,via the heptyl side chain.

Heavy ion irradiation of PMMA has also received some attention, particu-larly by researchers at the Japan Atomic Energy Research Institute. Kudoh andco-workers (269–273) have compared the effects of γ radiation and electron beamswith those for 30 and 45 MeV protons and heavier ion beams on the tensile andmolecular weight properties of PMMA. They have found no difference betweenthe sensitivities of the tensile properties of PMMA to γ -rays and electron andproton beams as a function of dose. However, the molecular weight dependencefor PMMA showed a clear LET dependence for heavy ions above a critical LET,indicating overlapping between spurs. For low LET radiation the scission prob-ability remains constant, but the scission probability decreases with increasingLET for high LET radiation. The critical LET level for PMMA is a few hundredMeV · cm2/g.

Esters of Other Acids. Poly(tert-butyl crotonate) (PtBC) is an analogueof poly(tert-butyl methacrylate) (PtBMA) but it does not have the disubstitutedmain-chain carbon atom. Thus, on these grounds, PtBC might potentially undergocross-linking, but this process may potentially be hampered by the severe sterichindrance involved in the formation of a cross-link at the main-chain ester–carbonatom. O’Donnell and co-workers (274,275) have reported studies of the yields forradical formation, cross-linking and scission, and the gaseous products on radiol-ysis of PtBC. Three major radicals were observed on radiolysis at 77 K: tertiarybutyl radicals, oxygen centred radicals formed by loss of the tertiary butyl groups,and a third unidentified radical. The value of G(R) was 1.28. The major gaseousradiolysis products were the same as those observed by Ungar and co-workers(276) for PtBMA, and the yield of CO2 (G = 1.0) was greater than that for CO (G= 0.26), in contrast to the observations for PMMA and PMA. This is consistent


with the preferential loss of the labile t-butyl ester group. The values of G(S) =0.59 and G(X) = 0.66 for PtBC can be compared with those for PtBMA, G(S) =0.21 and G(X) = 0.17, reported by Shultz and Bovey (240). These values indicatethat PtBC indeed undergoes cross-linking, while PtBMA undergoes net scission.However, the nature of the cross-links formed in PtBC have not been identified.

Polysiloxanes. Poly(dimethyl siloxane) (PDMS) undergoes cross-linkingon exposure to ionizing radiation. As early as 1954, Charlesby (78,277,278) hadreported that PDMS can be cross-linked to form a transparent rubbery material,and that the dose to achieve a cross-linked network was inversely proportionalto the initial polymer molecular weight (see SILICONES). Early work also reportedthat the main volatile products of irradiation are hydrogen, methane, and ethanegases (277,279,280). Other chemical changes observed with IR spectroscopy wereSi H structures and new, unidentified Si-O structures (280,281). ESR studieshave revealed that on irradiation at 77 K CH3·, CH2·, Si·, and O· radicals areproduced, and that, as expected, the radicals react on warming above the glass-transition temperature (127,282,283). G-values between 2.3 and 2.7 for cross-linking have been reported (279,284,285).

Whereas some evidence for chain-scission structures has been obtained fromthese previous studies, not until recently have conclusive measurements of theyields of scission reactions been reported (286–288). Hill and co-workers (286,287)have identified new structures formed in PDMS using solid-state 29Si and 13CNMR. They were able to measure the rate of formation of a number of chain-end,side-chain, and cross-link structures. The mechanism of cross-linking is domi-nated by Y-linking, although two minor H-linking products were also observed.On irradiation at 303 K, the radiochemical yields were G(S) = 1.3, G(H) = 0.34,and G(Y) = 1.7.

Fluorinated Polymers.Fully Fluorinated Polymers. The radiation chemistry of fully fluorinated

polymers shows remarkable temperature dependence, with all of the fluorinatedthermoplastics undergoing degradation, ie, chain scission, at ambient tempera-tures, but with an increasing yield of cross-linking reactions at elevated temper-atures. Over the past 10 years, this has led to renewed interest in the radiationchemistry and applications of these materials (see PERFLUORINATED POLYMERS,POLYTETRAFLUOROETHYLENE).

As discussed in a number of reviews (1,2,289), the parent of the fluoropoly-mer family, polytetrafluoroethylene (PTFE), undergoes degradation on irradia-tion. Over a number of years, this process has been exploited commercially toreduce the molecular weight of PTFE resins. Despite this recognition, the detailsof the mechanism of reaction have remained unclear, in part due to the paucity ofexperimental techniques available to study these materials. The inherent insolu-bility of PTFE has until recently restricted studies of radiation chemistry of PTFEto measurements of mechanical and thermal properties, and melt viscosity. For ex-ample, Bro and co-workers (290) reported in 1963 that the melt viscosity of PTFEdecreased on irradiation followed by heating. This effect was more pronounced inthe presence of air. In another example, Hedvig (291) in 1969 showed that themechanical strength and in particular the elongation at break of irradiated PTFEdropped dramatically on irradiation, but that these properties disappeared onirradiation in air.


As mentioned above, the mechanism of degradation of PTFE is unclear,particularly the location of radiation-induced chemical reactions. However, ESRspectroscopy (290–293) has confirmed that the stable free-radical species at lowtemperatures are the chain-end radicals IX (shown below) and that at highertemperatures the secondary radicals X dominate the spectrum. Hedvig (291)has suggested that chain-end radicals terminate by abstraction to form themore stable secondary radical. These radicals persist for very long times, andthis leads some authors (58) to suggest the suitability of PTFE as a radiationdosimeter.

The effect of irradiation at elevated temperatures was initially investigatedby Lovejoy and co-workers (294), who found that irradiation of PTFE above theglass-transition temperature resulted in a decrease in the molecular weight of thematerial, as evidenced by a decrease in melt viscosity. In a later series of articlesTutiya (295–299) studied the effect of various irradiation conditions on the broad-line 19F NMR spectra of PTFE. He reported in 1972 (295) that on irradiation abovethe melting temperature there was a significant decrease in the crystalline con-tent, ascribed in part to the increased melt viscosity of the irradiated polymer. Itwas not until about 20 years later that these observations were pursued. In 1994and the early 1990s, Sun and co-workers (55,300) and Oshima and co-workers (56)reported the radiation-cross-linking of PTFE above its melting temperature. Sincethen, Oshima and co-workers (56,301–310) have examined in detail the mecha-nism of formation and properties of radiation-cross-linked PTFE. It was foundthat cross-linked PTFE had significantly enhanced radiation stability comparedwith the uncross-linked resin despite the radical yield at low temperatures beingsignificantly higher (304,308,309).

PTFE undergoes cross-linking via a Y-type mechanism (reaction 12 above).This has been confirmed by solid-state 19F NMR measurements both by Katoh andco-workers (307,310) and Fuchs and co-workers (311,312). Initially, the Y-linkingmechanism was favored over the H-linking mechanism after considering stericeffects. However, it has been conclusively shown that branch points are formedand that the yield of branch points is superior to the yield of new chain ends(312), whereas Oshima (310) has suggested that double bond structures may alsoparticipate in the branching reactions.

In an early article Lovejoy and co-workers (294,313) reported that the copoly-mer of tetrafluoroethylene and hexafluoropropylene (FEP) would undergo degra-dation on irradiation at room temperature, but that it cross-links at higher tem-peratures. In 1985 Seguchi and co-workers (8) confirmed that FEP indeed un-dergoes cross-linking reactions through measurements of the soluble fractionsafter irradiation. The primary free radicals have been identified by Hill andco-workers (30) to be radicals IX and X above. In addition the yield of freeradicals is significantly enhanced at room temperature compared with irradia-tion at 77 K. This is a consequence of the lower probability of cage recombina-tion of the primary species at higher temperatures. In a later series of articles


(314–316) these authors have measured the yields of new structures by solid-state 19F NMR spectroscopy and found that, as with PTFE, irradiation belowthe melting temperature resulted in predominately new chain ends due to chainscission. On the other hand, upon irradiation at 523 K, just above the melt-ing temperature, new branch or cross-link structures were formed in significantquantities.

Similar conclusions have been reached for the third commercially importantfully fluorinated polymer, namely the copolymer of tetrafluoroethylene and per-fluoropropyl vinyl ether (317–320). This is not unexpected, because this materialtypically contains approximately 2 mol% of the vinyl ether. Irradiation at ele-vated temperatures by electron beam or with γ -rays resulted in increased yieldsof cross-links. Finally, Forsythe and co-workers (321) have also reported the ra-diation chemistry of members of the Teflon AF DuPont family of thermoplastics,and found that degradation proceeded through loss of hexafluoroacetone of thedioxole ring, leading to chain-scission reactions.

Fluoroelastomers. The above discussion has highlighted the importance ofmolecular motion on the ultimate radio-chemical products formed in fluorinatedpolymers. This is a consequence of the limited number of mechanisms of termi-nation of radicals in fluorinated polymers when compared with protonated poly-mers. Specifically, in fluoropolymers there is no mechanism of radical migrationequivalent to the hydrogen-hopping mechanism at work in irradiated hydrogen-containing polymers. The three-body intermediate suggested to participate in thehydrogen-hopping mechanism (eq. 4) is unlikely to form on radiolysis of fluorocar-bons, on purely energetic grounds. Thus, the radicals produced in fluoropolymersare particularly long-lived, and when they do react it is usually through com-bination reactions. This means that the radicals must have sufficient mobilityto be able encounter one another; hence the probability of chain cross-linkingincreases with increasing temperature, and in the semicrystalline materials dis-cussed above, sufficient mobility is only achieved above the crystalline meltingtemperature.

The situation is somewhat similar for fluorinated elastomers (see FLUORO-CARBON ELASTOMERS). The radiation chemistry of the copolymer of tetrafluoroethy-lene (TFE) and perfluoro-methyl vinyl ether (PFMVE), known commercially asKalrez, has been studied in detail by Hill and co-workers (57,322–327) This ma-terial is a random copolymer of these two monomers with a TFE to PFMVE molarratio of 2:1. It is a fully amorphous polymer with a glass-transition temperatureclose to 273 K. On irradiation at room temperature it undergoes cross-linking, asconfirmed by measurement of structural changes by NMR spectroscopy (57), andchanges in mechanical and solubility properties (327). When this material is ir-radiated below its glass transition temperature, chain scission tends to dominateover cross-linking reactions (326).

Partially Fluorinated Polymers. The radiation chemistry of fluoropolymersis very sensitive to the presence of hydrogen in the chemical structure. The in-troduction of hydrogen to the backbone generally results in a marked increase inthe probability of cross-linking reactions occurring. Presumably, this is a resultof the increased mobility of radicals as discussed immediately above. In addition,as pointed out by Wall and co-workers (328), the introduction of hydrogen leadsto the formation of HF on irradiation, with concomitant formation of main-chain


Table 2. Radiochemical Yields of Cross-Linking andScission for Several Partially Fluorinated Polymers

Polymera G(X) G(S) p0/q0 Ref.

PVDF 1.0 0.3 0.15 319PVDF 0.95–1.05 0.2–0.4 0.18–0.21 317PVF 3–8 1.8–4.0 0.28–0.35 317PTrFE 1.1 0.4 0.18 319PTrFE 1.1–1.2 0.3–0.5 0.16–0.21 317ECTFE 0.3 322aPVDF, poly(vinylidene fluoride); PVF, poly(vinyl fluoride); PtrFE,poly(trifluoroethylene); ECTFE, poly(ethylene-co-trifluoroethylene).

double bonds. These double bonds act as efficient sites for subsequent cross-linkingreactions.

Of this class of polymers, the radiation chemistry of poly(vinylidene fluoride)(PVDF) has been examined in most detail. In 1962, Timmerman and Greyson (329)compared the mechanical properties of PVDF with PTFE and found that overallPVDF was more resilient to exposure to radiation. Not long after this, Yoshidaand co-workers (330) reported the yields of cross-linking and scission reactionsin irradiated PVDF, and that cross-linking dominated. The radiochemical yieldsof scission and cross-linking of PVDF and a number of other materials are listedin Table 2. The identity of the radical species formed on irradiation has beenconfirmed by Seguchi and co-workers (331). They found that the chain-end radicalXI dominated the ESR spectrum at low temperatures, but that the main-chainradical XII became more important on warming. On long standing the polyenylradical XIII came to dominate the ESR spectrum, highlighting the importanceof loss of HF, and the increased radical mobility compared with fully fluorinatedpolymers.

As mentioned above, all of this class of polymers undergoes cross-linking onirradiation in vacuum. The yields of cross-linking and scission for these polymersare listed in Table 2. Of note also are the reports of cross-linking predominatingin the copolymers of ethylene with TFE (332) and of ethylene with chlorotrifluo-roethylene (333).

Polychlorotrifluoroethylene. In contrast to the above materials, the ho-mopolymer of polychlorotrifluoroethylene (PCTFE) undergoes radiation degra-dation on exposure to ionizing radiation. Florin and Wall reported in 1961 thatPCTFE undergoes chain scission, and calculated a G-value of 0.67 for scissionfrom the dose to achieve zero strength. A relatively low yield of volatile productswas reported: G(volatiles) = 0.11. Very recently, Hill and co-workers reexaminedthe radiation chemistry of this polymer (334,335). They measured the yields offree radicals to be 1.55 at 77 K decreasing to 0.32 at room temperature (335). The


ESR spectra were difficult to identify because of the large anisotropic hyperfinecoupling to the fluorine nuclei. These authors also examined the radio-chemicalproducts using solid-state 19F NMR and FTIR spectroscopies (334). On irradia-tion below the melting temperature, there was no evidence of branch/cross-linkformation. However, the yields of chain ends, double bonds, and branch pointson irradiation at 493 K were 3.6, 0.2, and 0.5, respectively. The structure of thebranches could not be confirmed; however, the respective yields of chain ends andbranches indicate that scission dominates for this polymer.

Polydienes. The polydienes undergo cross-linking on radiolysis in vac-uum. The principle reactions occurring are cross-linking, consumption of doublebonds, isomerization of double bonds, and evolution of hydrogen gas. There is con-siderable variation in the reported G-values for these reactions, arising primarilyfrom differences in the experimental techniques used in their determination. Forexample, physical measurements, usually of soluble fraction or swelling ratio,used to determine yields of cross-linking and scission reactions are particularlysusceptible to errors in the case of polydienes. These techniques assume a ran-dom distribution of cross-linking and scission reactions; as will be seen below, thisassumption may break down in the case of radiolysis of polydienes.

Polybutadiene. The primary products of irradiation of polybutadiene (PBD)are allyl free radicals. The yield of radicals has been reported to range from 0.19to 0.44 (336,337). On heating the radicals decay in a monotonic fashion to lowconcentrations just above the glass-transition temperature of the polymer (337)(see BUTADIENE POLYMERS).

As mentioned above, the net result of irradiation of PBD is cross-linking. G-values for cross-linking, determined as described immediately above, range from3.2 to 6.7 (338–343). In one case a G(S) value of 0.52 has been reported, althoughit is more generally accepted that the G-value for scission in irradiated PBD ismuch smaller than this (343).

As will be discussed below, these G-values contrast with the very high yieldof reactions which consume double bonds in these polymers. The reason for this isthe highly nonuniform cross-linking reactions in polydienes. The advent of solid-state 13C NMR allowed direct observation and quantitation of the cross-link struc-tures in irradiated polymers. In the case of poly(1,4-butadiene), O’Donnell andWhittaker (344,345) have reported very high G-values for consumption of dou-ble bonds, and formation of cross-link structures. The authors have suggested achain reaction through successive double bonds to form a highly nonuniform net-work structure. As a result, the effective cross-link density is much lower than themeasured chemical cross-link density. Similar results were reported for materialshaving different initial microstructure.

A number of workers have reported a very high rate of disappearance of dou-ble bonds on irradiation of PBD. Using IR spectroscopy, Parkinson and Sears (346)determined G(cis) to be 15, G(-trans) 11–22, and G(cis–trans) 7. The last reactionis the isomerization of a cis double bond to a trans double bond via an allylicradical intermediate. The rate of consumption of 1,2-vinyl groups was found todepend on the initial vinyl content; values from 0.2 to 40 were reported. Similarly,Golub (347,348) reported a G-value of 7.9 for consumption of double bonds, and7.2 for isomerization. He suggested the formation of cyclic structures. O’Donnelland Whittaker (344) have reported the yields of these reactions for a number


of polybutadienes. The calculated G-value for loss of double bonds was initially41.8 over a dose range of 0–1 MGy and decreased to 6.3 at higher doses (4–10MGy). For poly(1,2-butadiene) the G-value for loss of double bonds was 237 (0–0.5MGy), because of, it was suggested, an intramolecular chain reaction of the vinylgroups.

Polyisoprene. As with polybutadiene, polyisoprene (PIP) and natural rub-ber are cross-linking polymers. The radical intermediates have been identified asallyl radicals (336,337,342). Radical yields at 77 K ranging from 0.28 to 0.73 havebeen reported. It is also clear that the mechanism of reaction of these radicalsis similar to that discussed above for polybutadiene. A G-value of 6.7 for the de-crease in unsaturation has been reported by Golub and Danon (347). Much later,using solid-state NMR, Whittaker (349) obtained a G(d.b.) value of 45.8 (0–1 MGy).Significantly lower yields of cross-links have been reported; Boehm (350) has com-piled a list of G(X) values ranging from 0.4 to 3.5 as determined by measurementof physical properties.

Polychloroprene. In contrast to PBD and PIP, the radiation cross-linkingof polychloroprene does not appear to proceed by a chain-reaction mechanism(see CHLOROPRENE POLYMERS). The yields of cross-links [G(X)] were measured by anumber of methods and determined to be 3.9 (NMR), 4.8 (swelling ratio), and 3.2(sol. fraction) (351). This close correlation, and the observation of a narrow reso-nance in the 13C NMR spectrum assigned to cross-link structures, indicated thatcross-linking in this material occurs randomly throughout the material. Peaksare observed in the NMR spectra because of new chain structures formed onirradiation.

Aromatic Polymers.Polystyrene. Polystyrene undergoes net cross-linking on irradiation (see

STYRENE POLYMERS). In 1953 Charlesby (352) reported that the polymer undergoesgelation on irradiation, and that the yield of permanent chain scission reactionsis small or negligible. Furthermore, he reported that despite cross-link reactionsdominating the radiation chemistry, the overall yield of cross-linking is very smallcompared with nonaromatic polymers, because of the presence of the benzenering. A major study of the changes in molecular weight (viscosity) and formationof volatile products was subsequently undertaken by Wall and Brown in 1957(231). The major volatile product, hydrogen gas, is produced in low yields [G(H2)= 0.022–0.026] compared with nonaromatic polymers.

Since these early reports, a number of further studies have appeared(6,231,339) and these have been summarized in 1973 by Parkinson and Keyser(353). Since that date, efforts have been made to understand the relatively widediscrepancies in reported yields of radiochemical reactions, and also the effect oftemperature on the radiolysis products. For example, values of the yield of rad-icals, G(R), measured at 77 K after irradiation at that temperature range from0.04 to 0.50 (22,100,354–357). O’Donnell and co-workers (358) have proposed thatthe variation in reported yields of radical formation, and of chain scission andcross-linking, may be in part due to the presence of trace amounts of residualoxygen.

Using ESR spectroscopy, the mechanism of radiation-induced cross-linkingof polystyrene has been revealed to a large extent by careful studies of radi-cal intermediates. The major free-radical products formed on irradiation are the


α-carbon radical XIV and the cyclohexadienyl radical XV shown below. The con-tribution of radical anions to the ESR spectra was confirmed by Garrett (357) andHill and co-workers (24). On heating these samples toward the glass-transitiontemperature (ca. 373 K), free-radical reactions occur until at room temperaturethe ESR spectrum resembles that observed for a sample irradiated at the ele-vated temperature. The radical concentration has decayed to negligible values atthe glass-transition temperature. The mechanism of cross-linking has not beenconfirmed, but is suggested to involve recombination reactions between radicalsX and XI. Measurements of gel doses for materials having different initial molec-ular weight distributions have suggested that H-linking rather than end-linkingis the major mechanism of cross-linking (359).

The effect of irradiation temperature has been examined in some detail.A number of workers have reported that the yield of cross-links decreases pro-gressively above room temperature with a corresponding increase in the yieldof chain-scission reactions (302,360–362). Burlant and co-workers (363) initiallysuggested that as the temperature increases, an increasing proportion of main-chain radicals XIV undergo a β-cleavage reaction to form terminal unsaturationand chain-end radical. On the other hand, from analysis of changes in molecularweights, Bowmer and co-workers (360) reported that the ratio of yields of scis-sion to cross-linking reactions, G(S)/G(X), increases from 0.02 at 303 K to 2.8 at423 K. These authors suggest that competition between combination and dispro-portionation reactions between temporary scission radicals is responsible for thistrend, with disproportionation, and hence permanent chain scission, becomingmore likely at higher temperatures. A comparison was made with the mecha-nism of termination in the free-radical polymerization of styrene. Seguchi andco-workers (302,362) have also examined in detail the effects of temperature andtacticity on the relative yields of cross-linking and scission reactions in irradiatedpolystyrene, and have arrived at similar conclusions.

Poly(α-methyl styrene). Poly(α-methyl styrene) (PαMSTY) is one of theclass of degrading polymers, having along its backbone a quaternary carbon, atwhich a stable scission radical can be formed. In addition to this, PαMSTY has alow polymerization ceiling temperature of 334 K (364). At and above this temper-ature, the polymer will undergo depolymerization to its monomers. Garrett andco-workers (365) measured the yield of chain scission G(S) and the rate of depoly-merization at temperatures of 298, 353, and 536 K. At the two lower temperaturesthe weight loss due to depropagation was negligible and the authors were able tocalculate yields of chain scission from changes in the number-average molecular


weights (see eq. 5 above). The values of G(S) were 0.29 ± 0.01 and 0.48 ± 0.06 at298 K and 353 K, respectively. However, at the highest temperature, extensive andrapid first-order weight loss occurred. The G-value for depolymerization [G(−M)]was found to be 320 ± 45. The results of a Monte Carlo simulation of this processresulted in an estimate of the yield of chain scission [G(S)] of 1.8 and the averageunzipping length of 400 (365).

This same group has also reported an ESR study of the radicals formed onirradiation at 77 K and room temperature (54). The authors identified a numberof radical species including a propagation radical formed by direct chain scission,and radicals associated with the side chain and the aromatic ring.

Main-Chain Aromatic Polymers. The above discussion highlights the rel-ative stability of polymers containing aromatic rings, due to the ability of thesestructures to dissipate excess energy through resonance effects (1). For examplethe yields of cross-links and free radicals in irradiated polystyrene are approxi-mately 50 times lower in polyethylene. It has also been noted from comparisonsof the radiation sensitivity of polymers having similar structures that those inwhich the structures are incorporated into the main chain are less sensitive todegradation than those with similar side-chain structures. This is partly due tothe more effective cage effect in main-chain polymers, which tend to have higherglass-transition temperatures than those of side-chain polymers, but also due tomore effective resonance coupling to the aromatic rings. It follows therefore thatmain-chain aromatic polymers represent some of the most radiation-resistant or-ganic materials discovered so far.

Aromatic Polysulfones. The presence of the relatively labile sulfone link-age in Polysulfones (qv) results in these materials having a tendency to undergodegradation reactions. Under many conditions, however, aromatic sulfones arerelatively stable and undergo net cross-linking. Brown and O’Donnell (366) ini-tially reported the radiation chemistry of copolymers of bisphenol A and 4,4′-dichlorodiphenyl (Table 3). They found that on irradiation in vacuum at ambienttemperatures, this material undergoes cross-linking with relative yields of cross-linking and scission reactions being respectively 0.051 and 0.012. These studieslead to a large body of work considering the effect of chemical structure on the ra-diation sensitivity of aromatic polymers. In general for all aromatic polysulfones,on irradiation at ambient temperature, cross-linking predominates. In addition,as aliphatic groups are introduced into the main chain, the radiation sensitivityis increased.

Lewis and co-workers (367) have reported a detailed study of the effect ofpolymer structure on the yields of chemical reactions for a series of polysulfoneterpolymers containing varying amounts of bisphenol A and biphenyl units. Theyfound a close to linear relationship between the yields of radicals and volatilesulfur dioxide with the ratio of the bisphenol A and biphenyl units. They reportthat the biphenyl unit imparts significantly higher radiation stability. As withother polymers, the details of the radiation chemistry are affected by the irradia-tion temperature, with Hill and co-workers (368) providing evidence of enhancedchain scission on irradiation of polysulfone copolymers (Table 3) at temperaturesapproaching the glass-transition temperature.

The ESR spectra of irradiated aromatic polysulfones, and aromatic polymersin general, are often poorly resolved, because of the effect of delocalization of


Tab

le3.

Rad

ioch

emic

alY

ield

sao

fF

orm

atio

no

fF

ree

Rad

ical

sin

Aro

mat

icM

ain

-Ch

ain

Po

lym

ers

Un

it1

Un

it2

Nam

eR

adic

alY

ield

1020

spin

s/kg

Ref

.

PE

EK

1.0

379

PE

EK

K1.

137

9

Kap

ton

1.7

371

Ult

em2.

237

1


Tab

le3.

(Co

nti

nu

ed)

Un

it1

Un

it2

Nam

eR

adic

alY

ield

1020

spin

s/kg

Ref

.

Bis

-AP

EP

O2.

537

1

Bis

-AP

SO

2.8

371

Kev

lar

5.6

371

Nom

ex5.

637

1

PP

O5.

737

1

aT

he

yiel

dof

free

radi

cals

isex

pres

sed

inth

eu

nit

sof

spin

s/kg

afte

rir

radi

atio

nto

ado

seof

10kG

y,ra

ther

than

aG

valu

e,si

nce

the

incr

ease

inra

dica

lco

nce

ntr

atio

nw

ith

dose

ish

igh

lyn

onli

nea

rw

ith

dose

inth

ein

itia

lst

ages

.To

obta

ina

sin

gle-

poin

tG

valu

eat

10kG

y,di

vide

the

radi

caly

ield

by6.

24×

1020

.


the electron spin across aromatic structures, and the contribution from radicalanions to the spectra at low temperatures. For example, Lewis and co-workers(367) reported that up to 75% of the total area of the ESR spectra of bisphenol Apolysulfone irradiated to 10 kGy arose from radical anions, which were susceptibleto photobleaching with UV radiation. These relatively uninformative spectra offerlittle information to assign reaction mechanisms. On the other hand, Hill and co-workers (96) have deduced, from measurements of changes in molecular weightsand the formation of new structures detected on irradiation using solution-state13C NMR, that the main cross-linking mechanism involves Y-linking.

Polyimides. The two most important commercial Polyimides (qv) are Kap-ton and Ultem (structures shown in Table 3). Whereas Kapton is insoluble inmost convenient solvents, Ultem is soluble and hence amenable to detailed studyby standard methods. In 1985 Basheer and Dole (369) reported the yields of cross-linking and scission in irradiated Ultem to be respectively 0.014 and 0.005 onirradiation at room temperature. The gel dose of commercial Ultem film is of theorder of 10 MGy. Devasahayam and co-workers (370) have reported a more de-tailed analysis of the changes in molecular weights on irradiation, and calculatedG-values for scission and cross-linking for either H- or Y-linking mechanisms.

Basheer and Dole (369) reported that the ESR spectrum of irradiated Ultemwas composed of contributions from radicals from scission at the isopropylideneunit and aromatic ether group. Recently, Devasahayam and co-workers (370) re-ported a detailed study of the free-radical yields and radical reactions in irradiatedUltem. At 77 K, the spectra contain significant contributions from radical anions,which were susceptible to thermal or photolytic annealing. Potential scission siteswere identified from the radical products. Similarly, the formation of methyleneradicals through loss of hydrogen atoms from the isopropylidene unit was sug-gested as a precursor to cross-linking reactions. Associated NMR analysis (371)of the radiolysis products identified a large number of products, the majority ofwhich were identified with chain-scission products.

Kapton polyimide is an intractable material under usual conditions, andhence studies of its radiation chemistry are confined to measurements of volatileand free-radical products. The ESR spectrum of irradiated Kapton is a featurelesssinglet, and the yield of free radicals (1.7 × 1020 spins/kg after 10 kGy irradiation)was the one of the lowest of a large number of aromatic polymers compared in thisstudy (372). Earlier, Hegazy and colleagues (373,374) confirmed the very highradiation stability of Kapton by measurement of the yields of volatile products ofthis polymer and other aromatic materials, including several aromatic polyimides.For Kapton, the evolved gases, in decreasing yields, were CO2, CO, nitrogen, andhydrogen. Again Kapton was the most stable material studied.

A number of other polyimides have been studied by Hill and co-workersDevasahayam and co-workers (375) reported changes in the optical properties,and formation of free radicals in a series of fluorinated polyimides, and foundthat all materials were less stable than Kapton film. The fluorinated groups areincorporated in the polyimides to reduce coloration, and to provide oxygen atomresistance for space applications. Alexander and co-workers (376) have measuredthe radical yields for two fluorinated polyimide containing phosphine oxide groupsattached to the aromatic main chain. The phosphine oxide unit also provides pro-tection for the polymer against degradation from atomic oxygen. The relatively


high yields of radicals at 77 K, G(R) = 0.50 and 0.42 indicate, however, that thisprotection has been gained at the expense of radiation stability. Prior to this,Hopewell and co-workers (129) measured the radical yields and the solution vis-cosity of a number of phosphine oxide containing polyimides, and found that forthese materials chain scission and cross-linking occur simultaneously on irradia-tion, but that scission is predominant on irradiation at room temperature.

Polyarylene Ether Ketones. The polyarylene ether ketone, known commer-cially as PEEK, is a semicrystalline thermoplastic of high melting temperature,and is insoluble in common organic solvent. The fully aromatic structure of PEEKimparts very high radiation resistance to this material. In a series of articles,Sasuga and co-workers (377,378) and Hegazy and co-workers (373,374,379) havemeasured a number of properties of PEEK and compared its radiation stability toa number of materials. The main volatile products, in decreasing yields at 15 MGy,are methane, hydrogen, CO, and CO2. The source of the methane is not discussed.However, the overall yield of gases is very low compared to other aromatic poly-mers. The effect of crystallinity on the radiation chemistry is also demonstratedthrough the significantly higher yields of gases obtained from irradiation of a fullyamorphous polymer.

In a related study, Hegazy and co-workers (379) examined the changes inthe thermal properties of irradiated PEEK. An increase in glass-transition tem-perature and a small decrease in the heat of crystallization are both evidence ofcross-linking formation. Similarly, the decrease and broadening of the recrystal-lization transition is due to cross-links.

Heiland and co-workers (380) have examined the radiation sensitivity of a se-ries of polyarylene ether ketones using ESR spectroscopy and gas chromatography.The yields of radicals formed in all materials at 77 K were very low (approximately1–2 × 1020 spins/kg after 10 kGy irradiation). Introduction of methyl groups on themain-chain aromatic rings decreased the radiation stability, and further substi-tution of isopropylidene units into the main chain reduced the radiation stabilityeven further.

Polycarbonates. The most commercially important Polycarbonates (qv) isbisphenol A polycarbonate (Bis-A PC). The effects of radiation on this materialhave been studied for a number of years, with a particular interest in the identity ofthe species responsible for a pronounced color change in Bis-A PC after irradiation.The radiation-induced green coloration has been assigned variously to trappedelectrons (381) or a combination of trapped electron and radical ions (382,383).Golden and co-workers (384–386) have also in some detail examined the evolutionof gases on irradiation (386), and found that the major volatile products are carbondioxide and oxygen, with small amounts of hydrogen, methane, and benzene.

Bis-A PC undergoes net chain scission during irradiation in vacuum, presum-ably because of the high sensitivity of the carbonate group to chain scission. Evi-dence for this is the decrease in limiting viscosity after irradiation (384,386,387).More recently, further insight into the radiation degradation of Bis-A PC has beengained in a study by Babanalbandi and co-workers (388). In this the authors wereable to confirm that the G-value for radical formation at 77 K is 0.5 ± 0.02, andthat approximately 50% of the observed radicals are able to be photobleached withvisible light, and hence are radical anions. NMR analysis of the soluble polymer af-ter irradiation revealed new peaks, assigned to phenol-type chain end structures,


with a G-value for formation of 0.7. This observation confirmed that the main siteof radiation degradation of Bis-A PC is the carbonate unit.

Aromatic Polyesters. The degradation of aromatic polyester has been in-vestigated by a number of authors (7,53,389–398). The most commercially impor-tant material of this class is poly(ethylene terephthalate), a semicrystalline ma-terial of high melting temperature (Tm ∼ 538 K) and moderate glass-transitiontemperature (Tg ∼ 343 K) (see POLYESTERS, THERMOPLASTIC). On irradiation atlow temperatures the observed ESR signal is primarily attributed to the sec-ondary alkyl radicals formed by loss of a hydrogen atom from ethylene segments(53,391,394). The yield of radicals at room temperature was 0.025 (394). Sincethen, Choi and co-workers (132,398–401) have reported the radical yields of anumber of allied main-chain aromatic polyesters. They found that for series ofpolymers containing fluorinated isopropylidene units the radiation stability wasenhanced with higher content of the fluorinated residues (400). The yields of freeradicals at 77 K were found to fall in the range 0.38–0.46. Later, this same groupreported the radical yields for a number of linear halogenated aromatic polyesters[G(R) = 0.41–0.81] (401), terephthalate-based polyesters [G(R) = 0.19–0.41] (398),and naphthalene-containing polyesters [G(R) = 0.08–0.12] (132,399). In the laststudy the yield of radicals was found to increase, as expected, with the length ofthe aliphatic chain in the polyester, with, for example, G(R) at 77 K increasingfrom 0.19 to 0.27 to 0.41 for terephthalate polyesters with the increasing lengthof the alkyl chain (n) of 2, 4, and 10, respectively (394).

Limited data is available on the respective rates of cross-linking and scis-sion of the aromatic polyesters under radiolysis. Turner (394) has collected thedata available from measurements of residual soluble fractions of irradiatedpoly(ethylene terephthalate), and reported values of G(S) and G(X) ranging from0.07 to 0.17 and 0.07 to 0.14, respectively. The large range reflects the experimen-tal difficulties and the range of irradiation conditions employed.

Babanalbandi and co-workers (388) have used ESR and NMR to determinethe yields of radical intermediates and final products in irradiated U-polymer, apolyester of bisphenol A and terephthalic acid. The radical yield G(R) = 0.5 at 77K was consistent with previous studies of similar polyesters. The NMR spectra ofpolymer irradiated to very high doses at room temperature revealed that phenol-type chain ends were the main stable products. The results are consistent with theobservation of CO and CO2 being the main gaseous products of the degradationof polyesters. For this material no evidence of cross-linking reactions was found.

Copolymers. The properties of polymers can be enhanced extensivelythrough Copolymerization (qv) or blending of homopolymers. The effects of ra-diation on copolymers and blends might be expected to show behaviors that arerelated to the behaviors of the homopolymer constituents, modified by the poly-mer mole fraction composition and the electron densities of the components. Forsimple polymers, eg, those composed of carbon, hydrogen, and oxygen, randomcopolymers show a linear relationship between the properties of the homopoly-mers and the weight fraction of the components, eg, for copolymers of aliphaticesters (402). Although this behavior has been found to apply for many copolymerand blend systems, major differences from linear relationships are also common.

Aromatic components in mixtures of low molecular weight materials havelong been known (18) to exhibit a protective effect on other components. This


protective effect is also found in random copolymers, such as the copolymersof styrene and methyl methacrylate, which have been studied by Busfield andco-workers (246), where the presence of styrene lowers the yields for radicalintermediates, volatile products, and chain scission below those expected fromthe linear additivity rule. Similar observations have been made for radical for-mation in copolymers of glutamic acid and tyrosine (403), and several othersystems.

On the other hand, positive deviations from the additivity rule for copolymershave also been reported. For example, Fox and co-workers (404) have reported thatthe radical yields at 77 K for γ radiolysis of the copolymers of methacrylic acidand acrylonitrile exhibit a positive deviation from the rule. They have attributedthis to the breakup of the hydrogen bonding between methacrylic acid units bythe acrylonitrile units in the copolymer.

Polymer Blends (qv) can be classified as being either compatible or incom-patible. Most blends fall into the latter category. If this is the case, the additivityrule might be expected to apply, even for blends of homopolymers such as PSand PMMA, because the two components are phase separated. Garrett (357) hasshown that this is indeed the case when homopolymers of comparable molecu-lar weight are blended. However, if PS of low molecular weight is blended withPMMA of high molecular weight, a level of compatibility is achieved, and the PSthen provides protection of the PMMA against degradation (357).

Many other instances of deviations from the additive rule and of protectiveeffects in copolymers and blends have been reported in some of the studies dis-cussed elsewhere in this article. The use of aromatic groups to provide protectionagainst degradation has been utilized in some commercial materials. One exam-ple of this has been the incorporation of aromatic groups into polyester medicalsutures (405) in order to protect the sutures against damage during radiationsterilization.

The Influence of Oxygen

Oxygen is a diradical, and so it can readily react at diffusion-controlled rateswith free radicals formed in a polymer during radiolysis. This process has beenwell researched because it also occurs as a result of the thermal degradation ofpolymers, and consequently occurs during polymer processing. The mechanisms ofthese oxidation processes have been well documented in the literature and involvepropagation, termination, and branching steps as part of a chain reaction. Thesemechanisms are summarized in Figure 7 (P = polymer).

Methods for stabilizing polymers by the addition of antioxidants and hy-droperoxide decomposers are also well known, and are generally used with poly-mers which undergo radiolysis. For example, so called antirads are added topolypropylene to allow medical goods manufactured from it to be radiation-sterilized without discoloration or degradation.

The mechanism and kinetics of the reactions of oxygen with polymer radicalsproduced during radiolysis have been studied and extensively reviewed by Gillen,Clough, and co-workers (117,406–409), who have contributed greatly to this field.The roles of added polymer antioxidants and stabilizers have also been reviewed


Fig. 7. A summary of the reaction pathways in the oxidative degradation of polymers.

in a treatise edited by Clough, Billingham, and Gillen (410). Therefore, a detailedreview will not be presented herein, and only some recent developments will bediscussed.

In many polymers, radicals can have very long lifetimes, particularly if theyare located in crystalline regions where they are protected from any added sta-bilizer. Jahan and co-workers (153), for example, have reported that long-livedradicals formed during sterilization of PE artificial joint components are at leastpartially responsible for their long-term deterioration. In addition, Young and


Slemp (411) have found that polymers exposed to space irradiation can exhibit aslow deterioration of properties on their return to earth atmosphere. These pro-cesses are just two examples of slow polymer aging due to polymer oxidation inair.

Clough and co-workers (409,412,413) have recently studied the structuralchanges induced in polymers and model compounds by oxidation through the useof 17O-labeled oxygen and by the analysis of the polymer using 17O NMR. Thisapproach has provided a new insight of the oxidation and subsequent aging ofirradiated polymers.

The Influence of Irradiation Temperature

The chemical changes that occur when polymers are irradiated include the for-mation of free-radical intermediates, the scission of main-chain bonds, the lossor modification of side-chains, the cross-linking of chains, the formation of newpolymer structures, and the production low molecular weight products. One ofthe most important factors determining the extent to which these processes canoccur is the prevailing temperature. For example, the crystalline melting and theα-transition temperatures, along with the temperatures of other secondary tran-sitions in a polymer, play an important role in controlling chain motions, andhence the rates of reactions. In addition, the ceiling temperature determines theoverall thermodynamic stability of a polymer chain when propagating radicalsare present.

The temperature dependences of the rates of radiolysis reactions are deter-mined by their activation energies, and can be predicted through the Arrheniusequation. In polymers, the activation energies for radiolysis reactions are positive,so that the rates of these reactions show a positive dependence on temperature.However, the activation energies for radiolysis reactions can show a considerablevariation depending on both chemical and physical factors. For example, the ac-tivation energies for reactions can be quite different in the crystalline and amor-phous regions of a polymer. Thus, the overall rates for formation of radiolysisproducts depend on the morphology of the polymer, which can vary from sampleto sample. In addition, the rates of radiolysis reactions are likely to exhibit discon-tinuities at any first- and second-order polymer transition temperatures, whichare characteristic of a polymer.

Thermodynamic considerations must also be considered. Negative en-thalpies and negative entropies of reaction characterize free-radical polymeriza-tion reactions. Thus, whereas the free energy change for polymerization of a vinylmonomer may be negative at a low temperature, it will become zero at a highertemperature, the ceiling temperature. Above this temperature, spontaneous de-polymerization of the polymer is thus thermodynamically possible.

Some representative examples of the effects of temperature drawn from re-ported work are presented below.

Secondary Transitions. The secondary transitions in polymers charac-terize the onset chiefly of side-chain motions and the motions of chain ends. Thesecondary transitions for PMMA and PS are given in Table 4. In these polymers,the β transitions occur below room temperature, and are associated with the


Table 4. Transition Temperatures forPMMA and PS

Transition PMMA, K PS, K

Tα 378 K 370 KTβ 270 K 300 KTγ 103 K 138 KTδ 4 K 48 K

large-scale motions of the side chains. The γ and δ transitions occur at muchlower temperatures.

At 77 K the polymer chain motions in PMMA and PS are very restricted,but as the temperature is increased to room temperature, the side chains becomemobile. Thus, the polymer radicals formed in PMMA and PS on radiolysis at 77K are relatively stable, but they decompose on annealing to higher temperatures.

Garrett (357) has reported a careful study of the reactions of the radicalsformed on irradiation of PMMA and PS at 77 K when they are annealed to highertemperatures. For PS a small decrease in the radical concentration was observedat approximately 140 K and a further decrease occurs at about 270 K. The temper-atures at which these changes occur coincide closely with the δ- and β-transitiontemperatures, respectively, for PS.

The radical concentration for PMMA decreased by approximately 45% atabout 190 K and is associated chiefly with the loss of the anion formed on radiolysis(357). The temperature of 190 K corresponds to the temperature at which a changein the spin-lattice relaxation time has been reported to occur in s-PMMA (414),which is believed to be associated with the superposition of the motions of theα-methyl group and rapid fluctuations of the adjacent main chain.

Wundrich and co-workers (415) has examined the variation in G(S) forPMMA following radiolysis from 77 K to room temperature. From 77 K to ap-proximately 140 K, G(S) was found to be constant, but above 140 K the valuesincreased with temperature. He observed a second discontinuity at approximately268 K. His reported values of G(S) in these regions are as follows: 0.3, 77–140 K;0.3–0.9, 140–268 K; and 0.9–1.15, 268–293 K. Although there is no reported chaintransition at 140 K, 268 K corresponds to the β transition in PMMA.

It has been proposed that the loss of the ester side group is the precursorfor main-chain scission in PMMA, and so the lower G-values for scission belowthe β-transition temperature should be associated with lower G-values of volatileside-chain products. Kudoh and co-workers (250) found that the yield for hydrogenfor PMMA was not sensitive to the radiolysis temperature. However, the yields ofthe products of ester side-chain scission and decomposition, principally CO andCO2, were depressed by a factor of about 5 to 10 times when compared with thosefound for room temperature radiolysis. Kudoh and co-workers (260) also found alarge difference in the tensile properties consistent with less chain scission at 77 K.

α Transition. The α transition in polymers, Tg, is associated with the onsetof significant main-chain motion spread over several adjacent carbon atoms. AboveTg the free volume of the polymer increases, and so the rates of radical and otherreactions may increase significantly. The effect of the glass-transition temperature


on radical reactions was clearly demonstrated by the work of Hill and co-workers(186) on polyisobutene (PIB). Annealing of PIB beyond its glass-transition tem-perature, 203 K, results in a rapid decrease in the radical concentration. At roomtemperature the radical concentration is very small and cannot be measured ef-fectively by ESR spectroscopy. Many other polymers have been reported to behavesimilarly. However, Garrett and co-workers (416) found no evidence for a disconti-nuity in G(S) of PMMA at its α transition, 378 K. They reported that the value ofG(S) increases continuously with temperature between 268 and 400 K and risessmoothly from 1.3 at 303 K to 3.9 at 413 K.

A similar observation has been reported by Bowmer and co-workers (360)for PS, which undergoes cross-linking on radiolysis at room temperature, G(S) =0.0086 and G(X) = 0.043. However, as the temperature increases, G(S) increasesand G(X) decreases. As a result, although no significant discontinuity was observedin the scission and cross-linking yields, in the neighborhood of the α transition, thebehavior of PS changes, and it becomes a polymer which undergoes net scissionon radiolysis [at 423 K G(S) = 0.074, G(X) = 0.027].

So, although the α transition does not appear to have a significant influenceon the scission and cross-linking in PMMA or PS, it does play an important rolein PIB and several other polymers. For example, the temperature dependence ofG(S) has been found to increase significantly above this transition temperature forPIB (186), and for Kalrez, Forsythe and co-workers (57,383) have demonstratedthat cross-linking only occurs on radiolysis above Tg.

Although no discontinuity has been found for G(S) at the α-transition inPMMA, Dong and co-workers (31) have shown conclusively that tacticity changesdo occur in i-PMMA and s-PMMA with high yield above Tg. Similar changes inthe tacticity of isotactic polypropylene have also been observed by Busfield andco-workers (35) for radiolysis at room temperature, which is above its α-transitiontemperature. The G-value for the loss of isotactic triads was estimated to be 64 at298 K.

Crystalline Melting. Many of the radiolysis reactions of polymers are be-lieved to occur principally in the amorphous regions or interphase regions at thecrystalline boundaries. A strong influence of morphology has been reported byBusfield and Hanna (38) for the radiolysis of i-PP. They found that above the crys-talline melting temperature the values of G(S) and G(X) at low dose are 11.6 and3.5, respectively, whereas below the crystalline melting temperature the valuesare approximately equal and fall in the range 0.1–0.9. They also report that inthe molten state the G-values for inversion of isotactic sequences in i-PP are alsomuch higher: G(inversion) = 106.

Another recently reported difference in radiation chemistry above the crys-talline melting temperature has been for polytetrafluoroethylene (PTFE). In thesolid state PTFE is known to undergo scission readily. However, Oshima and co-workers (56) have found that just above the melting temperature of 600 K, PTFEundergoes cross-linking on e-beam irradiation, with a concomitant enhancementin the mechanical and chemical properties of the polymer.

Ceiling Temperature. For some polymers, for example PTFE, the ceilingtemperature, Tc, is very high (693 K), whereas for others it may be below roomtemperature. Polymers with quaternary carbon atoms in the main chain, suchas PMMA, PIB, and poly(α-methyl styrene), PαMS, are particularly susceptible


Table 5. The Temperature Dependence forG(−M) for Poly(α-methyl styrene)

T, K G(−M)

303 0.3373 95383 153393 182453 529

to radiation-induced depolymerization at elevated temperatures, because theyhave relatively low ceiling temperatures. For example, the ceiling temperaturefor PαMS is only slightly above room temperature, 339 K (for the pure liquidmonomer standard state). Hill and co-workers (417) have reported the G-valuesfor loss of monomer, G(−M), on radiolysis of PαMS over a range of temperatures(see Table 5). Clearly, G(−M) increases significantly for radiolysis above Tc wherespontaneous depolymerization can occur.

Property discontinuities which can be associated with temperature transi-tions in many polymers have been observed, and the cases discussed above serveonly as a few examples. Attention has been drawn to many other cases elsewherein the article.

Summary and Major Conclusions

The radiation chemistry of polymers is an extremely rich field of study, both froman intellectual and a practical viewpoint. The very large body of research devotedto this field is testament to this fact. The overall outcome of this work is a clearunderstanding of the effect of radiation on this class of materials, and the factorswhich influence the rates of reaction and the product distribution. These includefactors intrinsic to the polymers, and extrinsic factors more readily under thecontrol of the experimental scientist or engineer. These are summarized below.

Intrinsic Factors.Aliphatic Polymers. The main reactions occurring on radiolysis of aliphatic

polymers are cross-linking, chain scission, evolution of gaseous molecules, forma-tion of unsaturation, and reaction of unsaturated groups. Changes in physicalproperties of polymers on irradiation depend on the balance and overall yieldsof cross-linking and chain-scission reactions. For polyolefins, the presence of sec-ondary, tertiary, and quaternary carbons in the main-chain leads to a progres-sively higher probability of chain scission. The presence of main-chain units suchas sulfone, ester, and amide groups results in a higher probability of main-chainscission. Cross-linking can proceed through H- or Y-type linking. The rate of gelformation depends on the mechanism of cross-linking. The introduction of doublebonds, either as terminal double bonds, or main-chain double bonds in polydienes,results in an increased probability of cross-linking reactions. Fully fluorinatedthermoplastics tend to undergo chain scission reactions on irradiation at ambienttemperature.


Aromatic Polymers. The introduction of aromatic groups results in signif-icantly higher stability to radiation due to the resonant structure of the aromaticrings. Main-chain aromatic groups provide the highest stability to degradation.

Morphology. In semicrystalline polymers, chemical reaction is largely con-fined to the disordered, amorphous regions. Chemical reactions appear to be en-hanced by the presence of an interface between crystalline and amorphous phases.

Extrinsic Factors. The temperature of irradiation can dramatically af-fect the radiation stability of a polymer. Irradiation of degrading polymers abovetheir ceiling temperatures leads to extensive depolymerization. Unsaturated poly-olefins tend to undergo increasing amounts of H-linking as opposed to Y-linkingas the temperature is raised. Irradiation of fluoroplastics immediately above theirmelting temperature, or of fluoroelastomers above Tg, increases the probability ofcross-linking reactions. Irradiation in the presence of oxygen increases the prob-ability of long-term degradation reactions.

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DAVID J. T. HILL

ANDREW K. WHITTAKER

The University of Queensland

Radiation Chemistry of Polymers

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