Radiation Physics and Chemistry 62 (2001) 283–291

An ESR study on g-irradiated poly(vinyl alcohol)

Zainuddina,*,1, David J.T. Hillb, Tri T. Leb

aCenter for Research and Development of Isotopes and Radiation Technology, National Nuclear Energy Agency, Jl. Cinere Pasar Jumat,

P.O.Box 7002 JKSKL, Jakarta 12070 IndonesiabDepartment of Chemistry, University of Queensland, Brisbane, Qld 4702, Australia

Received 5 May 2000; accepted 23 December 2000

Abstract

The formation of radicals in poly(vinyl alcohol), PVA, powder irradiated at 77K by g-rays and the transformationsof these radicals during photolysis with visible wavelengths and on thermal annealing have been studied. Afterirradiation a four-line ESR spectrum was observed. It was assigned to a triplet of the Ca-radical (38%), with a splitting

of 3.27mT, superimposed on a doublet (62%) with a splitting of 2.7mT. The doublet appears to be composed of tworadicals, one of which is photo-bleachable (58%) and the other which is not photo-bleachable (42%). This suggests thatthe latter radical is a neutral radical. The photo-bleachable component of the doublet has been assigned to a carbonyl

anion radical, but the second doublet due to a neutral radical is unassigned. The total G-value for formation of radicalsat 77K was found to be 2.41� 0.03. Upon illumination with visible light, the anion radicals were removed and thedoublet components of the spectrum diminished in intensity, while the three-line spectrum of the Ca-radical became

more clearly visible. This transition was due to the photo-detachment of electrons from traps which were proposed to belocated on carbonyl groups in the polymer resulting from incomplete hydrolysis of the vinyl acetate. The photo-decayof the anion radicals could be satisfactorily described by a two-stage process. The first stage comprised the decay ofapproximately 80% of the anion radicals present, while the second stage was associated with the decay of the remaining

20%. Subsequent thermal annealing of a photolysed sample to 290K led to a change in the shape of the spectrum toform a more clearly defined triplet. As the doublet of the neutral radical decays on thermal annealing between 150 and250K, the Ca-radical is formed. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords: PVA; g-radiation; Radical; ESR; G-value; Photo-bleaching; Annealing

1. Introduction

Since the first successful preparation of poly(vinylalcohol) (PVA) by Herman and Haehnel in 1924 (Roff

et al., 1971) the research and development of PVA hasattracted much attention of researchers. This is becausePVA has been found to have a wide range of

applications in the industrial sector, as well as in thepharmaceutical and medical fields (Pritchard, 1970;

Meltzer, 1976; Blomstrom, 1989, 1990). Crosslinked

PVA has found many applications in the form of ahydrogel, particularly for medical applications. This isnot surprising since the hydrogel resembles living tissue

in that it is comprised of a polymer matrix swollen bywater. PVA gels have a high water content, they are softand flexible, and they have a low interfacial tension.

Accordingly, these gels have a better biocompatibilitytowards blood cells and proteins than some otherhydrogels (Ratner and Hoffman, 1976; Peppas, 1987;

Kaetsu, 1992; Hongiger et al., 1995; Ambrosio et al.,1996).Recently, it has been shown that ionizing radiation is

a suitable and simple tool to use for the preparation of

polymer hydrogels (Rosiak, 1991; Merril et al., 1993;

1 IAEA fellow (1 July–17 December1999) at the Department

of Chemistry, University of Queensland, Australia.

*Corresponding author. Tel.:+62-21-769-0709; fax:+62-21-

769-1607.

E-mail address: zainuddin27@hotmail.com (Zainuddin).

0969-806X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 9 - 8 0 6 X ( 0 1 ) 0 0 1 8 8 - 8

Nagaoka et al., 1993; Kaetsu, 1993; Yoshii et al., 1999).This is due to the fact that the radiation can induce the

formation of crosslinks in suitable polymers, eventuallyforming a three-dimensional network, without the needfor any crosslinking agent, which in some cases may be

toxic. There are two main methods used to producehydrogels by this technique; namely (i) irradiation of thepolymer in the solid state or in an aqueous solution or(ii) irradiation of the monomer in bulk or in an aqueous

solution to induce polymerization as well as cross-linking. The details of each of these methods can befound in a recent paper by Rosiak and Ulanski (1999).

It has long been known that radicals are formedduring the irradiation of a polymer. These primaryradicals then may undergo further transformations to

secondary radicals depending on the structure of thepolymer and its environment. Some radicals are verystable at room temperature, e.g. the polymer chain

radicals formed on radiolysis of poly(methyl methacry-late) below the Tg of the polymer (377K), and someothers are not, e.g. the methyl radicals formed on theradiolysis of poly(methyl methacrylate) at room tem-

perature (O’Donnell and Pomery, 1976). It is wellknown that electron spin resonance spectroscopy(ESR) can provide a sensitive method for detecting

unpaired electrons (radicals) in polymeric materials.In this paper, we have used the ESR technique to

study the formation and transformations of the radicals

formed in solid PVA when it is subjected to g-irradiation. In the study, radical formation at 77Kwas investigated, as well as the reactions of theseradicals on illumination with visible light with a long

wavelength and on thermal annealing to temperaturesabove the polymer Tg. The kinetics of the decayreactions of the anion radicals on photolysis and the

simulation of radical spectra are also reported anddiscussed in the paper.

2. Experimental

2.1. Material

Poly(vinyl alcohol) (PVA) powder with a degree ofpolymerization of 1700–2400 and degree of saponifica-

tion of 98mol% was obtained from Kuraray Poval Co.Ltd., Japan. It was used as received without furtherpurification. This commercial polymer may contain

small amounts of sodium acetate as an impurity.

2.2. Measurement of glass transition and melting

temperature

The glass transition temperature, Tg, and the melting

temperature, Tm, of the polymer was measured using aPerkin-Elmer DSC-7 with a heating rate of 208C/min.

The instrument was calibrated with high-purity indiumand zinc prior to the measurements on the polymer. For

DSC measurements approximately 10mg of sample wasplaced in an aluminum pan and the pan crimped. Thefirst scan was used to eliminate any thermal history of

the sample, and the Tg and Tm were obtained from thesecond scan.The onset temperature for crystalline melting was

found to be 478K and the crystallinity was calculated to

be approximately 35% using the equation:

% crystallinity ¼ 100� DHf=DHf ;100%;

where DHf is the enthalpy change for the measured

transition and DHf ;100% is the enthalpy change formelting of a pure PVA crystal.

2.3. Preparation and irradiation of the PVA sample

For the ESR studies described herein, 0.12–0.13 g ofPVA powder was packed into ESR tubes with an inner

diameter of 3mm (the lengths of samples in the tubewere in the range 2.5–2.8 cm). The tubes were thenevacuated (510�2 Pa) for approximately 24 h to removeall oxygen, then the ESR tubes were sealed under

vacuum. The samples were then irradiated at 77K by60Co g-rays (Nordion Gammacell-220) at a dose rate of5.6 kGyh�1to the doses range up to 10 kGy.

2.4. ESR measurement

A Bruker ER200D X-band ESR spectrometer inter-

faced to a PC computer was used for the acquisition ofthe spectra, which were obtained as the first derivative.The spectra were doubly integrated by means of ‘‘in-house’’ computer software to give peak areas and hence

radical yields. A Varian strong pitch (1% pitch in KCl)standard reference sample with a radical concentrationof 3� 1015 spins/cm was used to convert the peak areas

to radical concentrations. Spectral simulations werecarried out on a desk-top computer using ‘‘in-house’’computer simulation software.

Spectra were measured at 77K and over thetemperature range of 110–500K using a Varian variabletemperature facility. Photo-bleaching experiments were

carried out at 77K using an Oriel 1000W Hg/Xe lamp inconjunction with a filter to remove light with wave-lengths below 495 nm. No neutral radicals were gener-ated in the polymer under these photolysis conditions,

but electrons trapped as anion radicals were removedfrom the sample by electron photo-excitation and de-trapping. The photo-bleaching process was continued

until no further change in the radical concentrationcould be detected.The G-values for formation of radicals are expressed

as the number of radicals formed for 16 aJ of energyabsorbed.

Zainuddin et al. / Radiation Physics and Chemistry 62 (2001) 283–291284

3. Results and discussion

3.1. Characterization of the radical spectra

The ESR spectra observed at 77K for the PVA

powder irradiated to various doses at 77K are presentedin Fig. 1. These spectra are similar to that foundpreviously in our laboratory (Winzor, 1984) for acarefully purified sample of PVA on g-irradiation at

77K, and they displayed no evidence for the presence ofthe sharp quartet characteristic of methyl radicals,which would be expected to be formed from the

decomposition of sodium acetate if it was present as asignificant impurity in the PVA. The spectra arecharacterized by four lines, with the intensity of the

lines increasing monotonically with dose up to a doseof 10.28 kGy.Photo-bleaching of the irradiated sample with visible

light at 77K led to a change in the shape of the observedspectrum. This change was due to the photo-detachmentof electrons from ionic radicals (Hase and Yamaoka,1973; Mikhailik et al., 1981). On photo-bleaching, a

doublet component was lost from the spectrum, and theoriginal spectrum was gradually transformed into atriplet spectrum with a superimposed smaller doublet

component, as shown in Fig. 2. The spectrum of thecomponent which was lost on photo-bleaching is shownin Fig. 3A and it comprised 36% of the original

spectrum. However, clear features of a doublet compo-nent remained even after prolonged photolysis of thesample. The principal component of the spectrumobtained subsequent to photo-bleaching was that of a

triplet (splitting=3.27mT located at g ¼ 2:0034). Thus,the spectrum at 77K is comprised of contributions fromthree types of radicals but there was no evidence for the

presence of a quintet, as reported previously by Haseand Yamaoka (1973).

An irradiated sample of PVA was thermally annealed,along with a sample which had undergone photo-bleaching at 77K to remove the anion radicals. The

results of these annealing experiments are shown inFig. 4. Annealing of the photo-bleached sample from110 to 270K resulted only in a small change in theradical concentration over the temperature range.

However, thermal annealing of an unphotobleachedsample from 77 to 150K resulted in a rapid decrease inthe radical concentration, due to the decay of the anion

radicals. The spectral characteristics of the radicalswhich are lost over this temperature range are presentedin Fig. 3B. The spectrum is a doublet similar to that

observed to be lost during photo-bleaching.At 150K the spectra and the radical concentrations of

the two samples were close to identical, as demonstrated

in Fig. 4. Thus, the observed change in the spectrum andin the radical concentration on photo-bleaching at 77Kon the one hand, and on thermal annealing from 77 to150K on the other confirm that the anion radicals decay

thermally over this temperature range without theformation of new radical products.On photo-bleaching, or on annealing sample to 150K,

essentially all of the anion radicals decayed, but thereare still distinct features which indicated the presence ofa doublet in the spectra of the samples, as shown in

Fig. 5 for the spectrum at 170K. As the samples wereannealed from 170 to 250K, the radicals responsible forthe doublet converted into the radicals which areresponsible for the triplet, as demonstrated in Fig. 5.

However, over this temperature range the radicalconcentration remains almost constant. The character-istics of the doublet which was lost were revealed by

Fig. 1. ESR spectra at 77K of PVA powder irradiated to

various doses under vacuum at 77K.

Fig. 2. ESR spectrum of PVA powder irradiated under vacuum

at 77K followed by photo-bleaching at 77K.

Zainuddin et al. / Radiation Physics and Chemistry 62 (2001) 283–291 285

subtraction of the spectrum observed at 250K from that

observed at 170K. The difference spectrum is shown inFig. 3C. It is very similar to the spectrum of the anionradical, but, unlike the anion radical, the radical

associated with this doublet decays to a more stableradical on thermal annealing and it is not photo-bleachable. For these reasons we have associated thisdoublet with a neutral radical. The splitting of the

doublet was � 2.7mT.Further thermal annealing of the two samples to

290K led to identical triplet spectra (see Fig. 6) with no

features of the doublet component remaining. Thetriplet was characterized by a splitting of 3.27mT atg ¼ 2:0034, and was identical to the triplet observed

following radiolysis of a carefully purified sample ofPVA at room temperature (Winzor, 1984). The ratio ofthe intensities of the three peaks of the triplet were1:3.2:1, which compared well with ratios of 1:3.5:1

reported by Winzor (1984) and 1:3:1 by Wong (1978)following radiolysis of PVA at 298K. These values differfrom the expected ratio of 1:2:1 for a triplet, which led

Wong (1978) to suggest that a singlet was also present,which he assigned to the presence of a polyene radical.Annealing of the samples to temperature range of

290–343K led to a drastic reduction in the concentrationof radicals. Over this range, all of the peaks decaysimultaneously with the spectrum maintaining a peak

ratio of approximately 1:3:1. During the decay process,spectral subtractions showed no evidence for thepresence of a singlet, as proposed by Wong (1978).Most of the radicals were lost at temperatures signifi-

cantly below the glass transition temperature of 352K.Similar observations have been made for other poly-mers, which confirms the significant mobility of chain

segments in the vicinity of the radical sites at tempera-tures below Tg. However, some radicals still remained at

Fig. 3. ESR spectra of the PVA radicals lost from the spectrum

observed at 77K following radiolysis: (A) on photo-bleaching

at 77K; (B) on thermal annealing from 77 to 130K without

prior photolysis; and (C) on thermal annealing from 170 to

250K without prior photolysis.

Fig. 4. Yield of radicals versus annealing temperature for the

radiolysis of PVA under vacuum at 77K. (O) Following

photolysis and (&) without prior photolysis.

Zainuddin et al. / Radiation Physics and Chemistry 62 (2001) 283–291286

temperatures well above Tg, but heating to 423K led tothe decomposition of all radicals. The radicals thatdecomposed over the latter temperature range were

supposed to be localized in the crystalline inter-phaseregions. The crystalline melting temperature of the PVAwas � 478K.

3.2. Assignment of the radical spectra

PVA is prepared by hydrolysis of poly(vinyl acetate).

Poly(vinyl acetate) is prepared by free radical polymer-ization and it contains some long and short chainbranches formed by chain transfer to acetate groups and

back biting reactions on methylene units, respectively,during the polymerization (McMillan, 1990). The long

branches are cleaved during hydrolysis of the poly(vinylacetate), producing carboxylate chain ends, but theshort branches remain and are converted to short vinyl

alcohol branches during the reaction. Commercialpoly(vinyl acetate) also contains a small proportion ofhead–head and tail–tail links, each of which generally

amount to about 2% of the polymer units. In addition,on hydrolysis of the poly(vinyl acetate) to PVA someacetate groups remain unhydrolysed, for example thepolymer used in the present study contains � 2% of

unhydrolysed units. These defect structures in the PVAare considered to be weak links in PVA and are likely toplay some role in its radiation chemistry.

Pulse radiolysis studies by Samskog et al. (1981) haveidentified the alkoxy radical as the primary species

Fig. 5. ESR spectra observed on thermal annealing of the samples from 170 to 250K.

Fig. 6. ESR spectrum of PVA powder irradiated under vacuum at 77K and subsequently annealed to 290K.

Zainuddin et al. / Radiation Physics and Chemistry 62 (2001) 283–291 287

produced on radiolysis of low molecular weight second-ary alcohols. McMillan (1990) has also reviewed the

photolytic oxidation of low molecular weight alcohols inthe presence of Ce(IV) in which case the alkoxy radical isfound to be the primary species produced. These low

molecular weight secondary alcohols may be consideredto be models for PVA.Based on these pulse radiolysis and photolysis studies,

we propose that alkoxy radicals are formed in PVA and

that they decay by one of two processes; one which leadsto formation of a-carbon radicals and the other whichleads to formation of alkyl radicals and an aldehyde, as

shown below.

ð1Þ

ð2Þ

The chain end alkyl radical formed in reaction (2) would

readily undergo an abstraction reaction, and may formthe a-carbon radical, as has been observed in lowmolecular weight alcohols. The aldehyde protons are

also known to be readily abstracted by alkyl radicalsforming acyl radicals, which can subsequently undergo aproton transfer, as demonstrated below. This latter

transition has been reported by McMillan (1990) tooccur at around 170K in some low molecular weightalcohols.

ð3Þ

Sevilla and coworkers have reported extensively onelectron attachment reactions in aqueous glasses con-taining organic solutes. In particular, Sevilla et al. (1982)

studied the attachment of electrons to carbonyl groups,including those of aldehydes, esters and carboxylic acids,which they reported were characterized by a doubletspectrum with a splitting of � 2.6mT.

Thus, we have assigned the spectrum observedfollowing thermal annealing to 290K of PVA irradiatedat 77K to the Ca-radical (R1). This radical also forms a

major component of the spectrum observed at 77Kprior to thermal annealing. The mechanism for forma-tion of these radicals may follow that outlined in

reactions (1)–(3).

ðR1Þ

The spectrum of this radical would be characterizedby a triplet spectrum if one of the two protons on themethylene groups adjacent to the radical site lie in a

plane which leads to a small, unresolved hyperfinesplitting. This radical assignment is in agreement withthose of Wong (1978), Winzor (1984), Rao and Murthy

(1987), Kriminskaya et al. (1991), and has been gen-erally accepted as being the main precursor radical for

the crosslinking reactions in PVA during radiolysis atroom temperature.

The photo-bleachable doublet spectrum has beenassigned to the formation of anion radicals (R2)associated with electron attachment to the carbonyl

groups present in the polymer. Kriminskaya et al. (1991)also assigned the doublet to this type of anion radicallocated at unhydrolysed vinyl acetate groups, and isshown below. This assignment is consistent with the

observations of Winzor (1984) on the vacuum radiolysisat 77K of a well-purified PVA which was 99.5%hydrolysed, for which the 77K spectrum contained a

somewhat smaller doublet contribution. However, thecarbonyl group which traps the electron could also arisefrom other species, such as carboxyl groups formed

during hydrolysis of the poly(vinyl acetate) or aldehydegroups formed during radiolysis.

ðR2Þ

The origin of the neutral doublet, R3, is not knownunequivocally. It could be associated with an impurity inthe polymer, but this is unlikely because it has also been

observed in well-purified PVA (Winzor, 1984). Theradical could be associated with the formation of an acylradical at a scissioned chain end, such as that shown in

Eq. (3), which has been observed in model compounds,or it could result from the formation of a protonatedcarbonyl anion, such as those examined by Sevilla et al.

(1982), many of which have been reported to formdoublets. The latter radicals are well known to abstractprotons from isopropanol readily, so they would beexpected to decay on thermal annealing by hydrogen

abstraction to form the more stable radical R1.

3.3. Radical yields and photo-decay kinetics

Fig. 7 shows the yield of radicals formed duringirradiation of PVA powder at 77K. It is evident that the

concentration of radicals increases monotonically withincreasing absorbed radiation dose. From the slope ofthis linear relationship, the G-value for radical forma-

tion has been determined and found to be 2.41� 0.03.This value is in good agreement with correspondingvalues reported in the literature and that obtained in ourlaboratory for well-purified PVA samples (e.g. 2.5� 0.5,

Winzor, 1984).A simulation of the spectrum observed at 77K is

shown in Fig. 8, which is based on the presence of three

radical components; one radical characterized by atriplet with a splitting of 3.27mT and a line width of1.7mT comprising 38% of the radical species present,

and the other two by identical doublets with a splittingof 2.7mT and a line width of 1.5mT comprising 62% of

Zainuddin et al. / Radiation Physics and Chemistry 62 (2001) 283–291288

the radicals present. The simulation provides anacceptable representation of the observed spectrum,

except in the central region, as demonstrated in Fig. 8.The small difference in this region could arise throughincomplete matching of line shapes and widths and the

values of g for the component radicals.Fig. 9 shows the decay of the radicals formed on

radiolysis at 77K during subsequent photolysis at 77K.It is clearly seen that at the beginning of the photolysis

the concentration of the radicals decreased steeply andthen more slowly at longer photolysis times. Eventuallythe decrease in the concentrations reached a plateau

when approximately 36% of the total number of radicals

present have been lost. To highlight these processes thekinetics of the decay was examined.It was noted that neither classical first-order nor

second-order kinetics could account for the overallphotolytic decay process. Therefore, it was consideredthat the decay process may consist of two stages: in the

first stage, the decay is fast and occurs over the timeperiod 0–5min, and in the second stage, the decayoccurs at a slow rate until a plateau in the radicalconcentration is reached. To highlight the difference

between these two stages, the data have been plotted inFig. 10 in terms of the simplest kinetic model, a pseudo-first-order model.

For a first-order decay process, a plot of the logarithmof the radical concentration, [R], versus time would belinear. In fact, as indicated above, the radical decay

process will most probably be complex, and this isevident from an expansion of the data obtained over thefirst 5min of the decay, as shown in Fig. 10, but there

appears to be two decay regions. An initial fast rate ofdecay, which contributes about 80% of the anion decay,and a slow rate of decay, which contributes about 20%of the decay. From the slopes of the two straight lines

drawn in Fig. 10, the pseudo-first-order rate constantsfor the detrapping of the electrons would be5.8� 10�2min�1 (for the fast process) and

9.1� 10�4min�1 (for the slow process). Thus therate constant for the fast process is � 60 times greaterthan that for the slow process. These two decay

processes may be associated with two different regionsin the polymer, e.g. the amorphous and crystallineregions.

Fig. 7. Plot of radical yield versus absorbed dose for radiolysis

of PVA under vacuum at 77K.

Fig. 8. Simulation of the ESR spectra observed following

radiolysis of PVA at 77K: � � � Experimental spectrum; }

Simulated spectrum.

Fig. 9. Decay in radical concentration on photo-bleaching of

PVA following radiolysis of PVA under vacuum at 77K.

Zainuddin et al. / Radiation Physics and Chemistry 62 (2001) 283–291 289

The percentage of the radicals which were found to be

photo-bleachable at 77K was � 36%. Since the G-valuefor total radical production at 77K was 2.41, the G-value for anion radical formation, R2, must be � 0.86.

This value is in good agreement with the value cited byHase and Yamaoka (1973). Since the simulation of thespectrum at 77K was based upon a total contribution

from the two doublets of � 62%, approximate G-valuesfor formation of radicals R1 and R2 must be 0.92 and0.63, respectively.

4. Conclusions

The g-radiolysis of PVA powder at 77K resulted inthe observation of four-line ESR spectrum at 77K,

which was assigned to the presence of three radicals; anCa-radical (a triplet), a superimposed neutral radical (adoublet) and an anion radical (a doublet) formed bytrapped electrons probably located on carbonyl groups

in the unhydrolysed vinyl acetate segments. Uponillumination with visible light, the four-line spectrumwas transformed to a three-line spectrum (a triplet), with

the neutral radical doublet appearing as a shoulder onthe inner peak of the triplet. Subsequent thermalannealing of the sample to 290K resulted in the loss of

the doublet of the neutral radicals, yielding the triplet ofthe Ca-radical.

The total G-value for formation of radicals at 77K

was found to be 2.41� 0.03. Simulation of the spectrumobtained following radiolysis at 77K revealed that thespectrum was comprised of � 38% of the triplet and

� 62% of the doublet components.The photo-decay of radicals at 77K was due to the

detachment of trapped electrons. It was demonstrated

that the process was characterized by a fast decay overthe first 5min of the photolysis, followed by a muchslower decay of the remaining anion radicals.

Acknowledgements

One of the authors (Z) would like to thank Interna-tional Atomic Energy Agency (IAEA) Vienna, for thefellowship award under Technical Assistance Project

No. INS/8/021. The technical support of Mr. DavidHunter is also gratefully acknowledged.

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