Journal of Nuclear and Radiochemical Sciences, Vol. 1, No. 2, pp. 77−80, 2000

77

Radiation Effects on the Property of 238U(VI) or 239Pu(IV) Doped Polyvinyl Alcohol Films
Investigated by EPR and PAS
Mithlesh Kumar, R. M. Kadam, A. R. Dhobale, and M. D. Sastry*Radiochemistry Division, BARC, Trombay, Mumbai-400 085, India
Received: May 10, 2000; In Final Form: September 1, 2000
Electron paramagnetic resonance (EPR) and photoacoustic spectral (PAS) studies were conducted on pure
polyvinyl alcohol (PVA) film, 238U(VI) doped PVA film after gamma irradiation and 239Pu(IV) doped PVA film
after in-situ alpha irradiation. Studies on gamma irradiated U(VI) doped PVA film revealed the formation of
U(V) complex and free radical -CH2-C˙(OH)CH2-. The later was also formed in pure PVA film. The in-situ
alpha-irradiation resulted in a broad line at g = 1.998 ± 0.001 in addition to weak lines due to -C(˙O)=CH2- and CH2-C˙(OH)CH2-. The broad line appears to be due to high local concentration of these radicals which increased
with longer in-situ irradiation. With long storage, a doublet, presumably due to interaction of -C(˙O)=CH2- with I= 1/2 of 239Pu got evolved.

1. Introduction
Polyvinyl alcohol (PVA) is reported to get grafted to vinyl
monomers forming (graft) co-polymer when radical sites are
introduced on PVA back bone. It was made possible due to anelectron transfer reaction1 using oxidant such as Ce4+. The
importance of such grafted co-polymers arises due to theirhydrophilic characteristics. Furthermore, electron transfer
reactions are known to play an important role in nonlinear
optical properties in organic polymers. The electron transfer
reaction involving photoexcited extrinsic metal ions is an
important research area with wide range of interesting applications. It is well known 2 that U(VI) ion, UO 2 2+ , is a strong
photo oxidant reacting with alcohol to produce aldehyde/
ketone. It was reported2-4 that in PVA solutions, reaction withthe excited U(VI) ion, UO22+*, normally occurs via abstractionof alpha hydrogen atom. This work suggests that UO22+ can bea good candidate to photo-initiate radical formation on PVA
chain. So far this photoinduced reaction between U(VI) and
PVA was reported only in aqueous solutions in which theexcited state reaction was found to be highly pH dependent. In
the present investigation we are interested in radiation chemical/ photo-physical properties of solid PVA films containing
different metal ions. In view of above mentioned interesting
reactions between UO22+ and PVA, we have initiated electron
paramagnetic resonance (EPR) and photoacoustic spectral(PAS) studies of PVA film doped with actinide ions, U(VI)and 239Pu(IV). These elements, analogous to Ce, exhibit multivalence and furthermore provide opportunity for continuous
self irradiation particularly in the case of 239Pu. The alpha and/or external gamma irradiation producing both electronic excitation and ionisation should create conditions conducive forelectron transfer reaction between these ions and PVA matrix.
Such a study in solid state has not been reported.
EPR technique has been extensively used for identifying
paramagnetic metal ions in different valence states, neutral
radicals, radical cations/ anions formed when the organic
system are subjected to ionising radiations and high energy
photons like UV light. In aliphatic alcohols the hydrogen atom
is removed by interaction of radiation yielding α-hydroxy
alkyl and hydrogen atom. Polyvinyl alcohol on irradiation
yields α-hydroxy alkyl radical which has been investigatedpreviously.5-7*Corresponding author. E-mail: mdsastry@magnum.barc.ernet.in.
FAX: +91-22-5505151.

In this work we present evidence for the formation of U(V)on gamma irradiation and associated free radical formation.
Results of the in-situ alpha-irradiation in the 239Pu doped film
will also be presented.
2. Experimetal
The solution of pure PVA was prepared by swelling it inquartz double distilled water. The solution was then warmed
under infrared lamp at about 60˚C and stirred thoroughly until
the polymer became completely soluble. 2 3 9 PuO 2 was
dissolved in HNO 3 under infrared lamp and later it wasconverted into water soluble chloride. The maximum amountof Pu is expected to be in Pu(IV) state in freshly preparedsample due to addition of a few drops of peroxide and was
immediately added to PVA. Its disproportionation to a small
extent yielding Pu(III) and Pu(VI) is not expected to be significant under these condition. The solutions were mixed together
and stirred with a glass rod. Finally, the completely miscible
solutions were poured on a level glass plate and dried in a
glove box at room temperature for 48 hours to obtain homogeneous and bubble free 239Pu(IV)-doped films.8 A small portionof film was cut and sealed in the quartz glass tube. The outersurface of the quartz glass tube was decontaminated using
teepol and dil. HNO3. 238U(VI)-doped PVA and pure PVA
films were also made in a similar way as above. The isotopic
composition of uranium is that of natural abundance and of Pu
is 94.5% Pu-239 and 5.3% Pu-240. The EPR spectra wereobtained at room temperature on an X-band spectrometer
between 9-10 GHz. The films of UO22+ doped PVA and pure
PVA were irradiated using a 60 Co source with dose rate 2
kGy/h. PAS spectra between 350-800 nm were recorded using
home-built PA-spectrometer.9
3. Results
3.1. EPR Investigations. (i) Pure PVA film: The nonirradiated film did not show any EPR signal and also no absorption
in PAS. On gamma-irradiation (dose 4 kGy), EPR spectrum
showed presence of a triplet signal as shown in Figure 1. The
spectrum was symmetrical with hyperfine coupling of 35 ± 2
G. The g value of the triplet is g = 2.003 ± 0.001. The intensity of the lines was in the ratio of 1:3:1.
(ii) U(VI)-doped PVA films: Virgin and stored films (over a
period of one year) did not show any EPR signal. The self
irradiation effects in this are not expected to be significant due© 2000 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2000

Kumar

78 J. Nucl. Radiochem. Sci., Vol. 1, No. 2, 2000
238

to low specific radioactivity of U. EPR spectra of gamma
irradiated U(VI)-doped PVA film showed a broad and asymmetric line at g = 2.037 ± 0.001 and a weak triplet at g = 2.001± 0.001 (aH ≈ 33 ± 2 G) as shown in Figure 2.
(iii) 239Pu(IV) doped PVA film: Figure 3 shows the EPR
spectra for 239Pu(IV)-doped PVA, stored for different times. In
freshly prepared sample, a sharp line g = 2.000 ± 0.001 alongwith an intense and broad line appeared at g = 1.998 ± 0.001,
associated with satellite lines. In samples stored for nearly one
year, these features got broadened and disappeared. However,a clear doublet with a separation of 120 G got developed. It
was observed that 239 Pu(IV)-doped PVA film had becomecompletely black and brittle on long storage for about three
years.
3.2. Photoacoustic Spectral Investigations. Photoacoustic
spectra have the potential to give electronic absorption spectral
information on thin film and opaque samples. Photoacoustic
spectrum (PAS) of nonirradiated U(VI)-doped PVA film
showed intense optical absorption band at 420 nm and weakbroad band at 620-650 nm. PAS spectrum of gamma irradiated U(VI)-doped PVA showed built-up in the intensity ofbroad band at 620-650 nm at the expense of 420 nm band.
These spectra are shown in Figure 4.

Figure 1. EPR spectrum of gamma irradiated (dose 4 kGy) PVA filmat room temperature.

Figure 2. EPR spectrum of gamma irradiated (dose 4 kGy) U(VI)doped PVA film. The weak triplet marked by 'a' at g = 2.001 ± 0.001.

4. Discussion
4.1. Radiation Effect in PVA. In aliphatic alcohols thehydrogen atom was reported to be removed by interaction ofradiation yielding α-hydroxy alkyl radical and hydrogen atom.
Polyvinyl alcohol on irradiation yields α-hydroxy alkyl radical
which has been investigated. 5 - 7 The triplet EPR signal
observed in gamma irradiated sample with intensity distribution of 1:3:1 is similar to that reported earlier.5 Since it does
not follow the intensity distribution of 1:2:1, it is clear that thesignal is a composite one arising from at least two radicals of a
1:2:1 triplet and a singlet overlapping with central line. These
results are similar to those reported by Ogawa5 assigning thetriplet to -CH2-C˙(OH)-CH2-. For the signal with binomial
intensity distribution 1:2:1 interaction of unpaired electron was
attributed to two equivalent beta protons. The observation of
triplet in this case instead of quintet was explained as due tostreaic configuration with the original hydrogen bonds reverse
almost completely so as to minimize the repulsion between
unpaired spin and C-H bond. This makes one proton of eachadjacent CH group predominantly interacts with unpaired spin
density on p orbitals. The singlet at g = 2.003 ± 0.001 is
assigned to oxygen center radical. Since singlet signal overlaps central part of triplet, this contributes to additional intensi-

Figure 3. EPR spectra of 239 Pu(IV)-doped PVA film, stored for
different times.

Figure 4. PA spectra of U(VI)-doped PVA film (a) nonirr-adiated (b)gamma irradiated at 4 kGy.

Radiation Effects on the Property

ty in the central region giving intensity distribution 1:3:1.
Pure PVA film irradiated at 77˚K showed a broad unresolved quintet as expected due to CH(OH)-C˙H-CH(OH) a
percursor for a alpha hydroxy alkyl radical10 -CH2-C˙(OH)CH2-.
4.2. U(VI)-Doped PVA. The EPR spectrum of gamma irradiated U(VI)-doped PVA film showed the signal at g = 2.037 ±0.001, and weak triplet (marked by 'a' in Figure 2) with intensity distribution of 1:3:1 at g = 2.001 ± 0.001 (aH ≈ 33 ± 2G).
It may also be noticed that g-value of this triplet is different
from that discussed in 4.1. The paramagnetic species ofuranium that can be generated by electron transfer is pentavalent uranium U(V). The pentavalent uranium, U(V), has one
electron in 5f orbital and the crystal field interaction is givenby the Hamiltonian.11
Hcrystal field = HOh + θ (O20 + γO22 ),The parameters θ and γ represent axial and orthorhombic
components in the crystal field, respectively, and O20 and O22
are the appropriate operator equivalents.12
Miyake 1 1 reviewed the magnetochemistry of U(V)complexes and compounds. U (V) was also reported to bestabilized at Nb sites in LiNbO3 (Ref. 13) and at Ca site in CaO(Ref. 14) having octahedral coordination in both cases. An
important aspect of U(V) in octahedral symmetry is its relatively long spin-lattice relaxation time facilitating the observation of its EPR at relatively higher temperature. Underoctahedral symmetry, the lowest crystal field state of 5f 1 is adoublet, Γ2, which takes the form of Γ7 under spin-orbit interaction. Miyake11 discussed the effect of different extents ofaxial (C4v) and orthorhombic (C2v) distortions on the g value ofU(V). In spite of low symmetry perturbations on octahedral
crystalline field, the g value was reported to be nearly isotropic
for small distortions. The absolute g value for overall octahedral symmetry is expected to be around 1.1 which remains
constant up to a distortion (θ) of 800 cm−1 (for a fixed value of
γ = -0.2464) beyond which the absolute value of g decreases.13
In C4v symmetry, the g value smoothly increases with increasein θ from the octahedral value of 1.1. Therefore g = 1.1 can betaken as a finger-print for near octahedral symmetry at the siteof U(V). Deviation from this suggests a strong distortion.
High value of θ and γ yield absolute value of g greater than
1.1. In the earlier report, 15 U(V) of 5f 1 configuration in
LiNbO 3 with uranium at Nb site having perfect octahedral
coordination showed EPR signal at g ⁄ ⁄ = 0.70, g⊥ = 0.724. The
EPR signal of U(V) was reported 16 to be at g ⊥ = 2.5 andg ⁄ ⁄ ≈ 0.0 on photo- and electrolytic-reduction of uranyl(dimethylformamide)5(ClO4)2 in dimethylformamide solution.
Miyake et al16 interpreted the results of U(V) in dimethylformamide on the basis of unpaired electron on U(V) complex
with stronger bonding in equatorial plane. These authors veryclearly rejected the possibility of formation of UO2+. These
results suggest that there can be a large spread in the g-values
expected for U(V) species. In the case of U(V) in LiNbO3, theunpaired electron has more metal like character i.e. 5f character, giving large deviation in g from free electron value andis around -1.0. On the other hand in uranyl dimethylformamide perchlorate the U(V) in 5f has yielded a signal aroundg = 2.5. Therefore, it is clear that in U(V) complexes [Notenot uranyl complexes], the |g⊥| value can vary from 1.0 to 2.5
depending upon the strength of bonding in the equatorial
plane. The g-value (absolute) obtained in the present study
viz. 2.037 ± 0.001 suggests that uranium after electron transfer
bridges the vinyl chains in such a way that it bonds in a direction perpendicular to O-U-O bond direction. This is taken as
evidence for the role of uranium complex in crosslinking thepolymer. It may be mentioned that it is the first EPR evidence
for this phenomenon on trapping an electron.

J. Nucl. Radiochem. Sci., Vol. 1, No. 2, 2000 79
In gamma irradiated U(VI)-doped PVA, in addition to U(V)signal, a weak triplet was also observed. In comparision withgamma irradiated pure PVA film, the intensity of triplet wasless and also the g and A values are about 10% less. The
reduction in g and A values suggests that it may be coupled toU(V) complex.
In view of these results following scheme is proposed forthe radical formation in U(VI)-doped PVA film by gamma
irradiation[UO22+]:PVA--->[UO22+]*:PVA--->(UO2H)+ + -(CH2-C˙(OH)-CH2)-

U(V) complex crosslinking
the polymer chain
The U(V) complex may have the following structure-CH2-CH--CH-CH2O

OH

OH
U

OH

OH
O

-CH2-CH--CH-CH2-

The U(V) complex will be analogous to [UO6]7− complex
except for stronger bonding in the equatorial plane. The
formation of U(V) was further confirmed by PAS experimentin which built up of 620-650 nm band was noticed due toconversion of U(VI) to U(V) on gamma irradiation.
4.3. 2 3 9 Pu(IV)-Doped PVA. The EPR spectrum of
239
Pu(IV)-doped PVA contained a rather broad feature due tothe in-situ alpha irradiation, in addition to less intense otherspecies containing hyperfine patterns. The predominantfeatures of the EPR spectra are:(i) a broad line at g = 1.998 ±0.001, (ii) a sharp line at g = 2.000 ± 0.001 along with satellites, and (iii) a doublet on long storage film with associateddisappearance (or overlap with broad line) of sharp line. The
second feature, however, gets buried in the broad spectrum
over a long period of storage.
From a careful comparision of the spectra obtained in
239
Pu(IV)-doped PVA with that of gamma irradiated PVA, it isrevealed that the line position of relatively sharper line alongwith satellites coincides with the triplet obtained at room
temperature after gamma irradiation in pure PVA as discussed
earlier: these radical ions are -C˙(=O)-CH2- and -CH(˙O)-CH2.
The latter radical contributes only to the intensity at the central
line. The broad line, which keeps building on prolonged irradiation, is probably due to clustering of these radical ions inclose proximity due to higher LET effect.
Over a prolonged alpha irradiation, it is quite probable thatsome of these centers with unpaired electron on oxygen getstabilized closer to the impurity 239Pu(IV), interacting with thenuclear spin I=1/2 of 239Pu(IV). This would result in the observation of a doublet. The hyperfine coupling constant value
suggests significant covalency between 239 Pu(IV) and theradical ion -CH(O˙)-CH2-. It is difficult, however, to unambiguously assign this to hyperfine interaction with 239 Pu.
Electron nuclear double resonance studies would have showed
this but could not be conducted due to low intensity of thesignal.
5. Conclusions
The EPR investigations of gamma irradiated U(VI)-doped
PVA film revealed the formation of U(V) complex and radical.

80 J. Nucl. Radiochem. Sci., Vol. 1, No. 2, 2000
Concomitantly, radical of the type -CH2-C˙(OH)-CH2- is also
formed in PVA.
Alpha irradiation effects in 239 Pu(IV)-doped PVA film
produced different kind of radical compared to those by
gamma irradiation. Furthermore, one of the radical centersprobably got stabilized near 239Pu(IV) (I = 1/2) resulting indoublet structure. The hyperfine coupling constant value
suggests covalency between 239Pu(IV) and the radical.
Acknowledgment. The authors are grateful to Dr. S. B.
Manohar, Head, Radiochemistry Division, BARC, for encouragement during the course of this work.
References(1) H. D. Burrows, S. J. Formosinho, and P. M. Saraiva, J.
Photochem. Photobiol. A: Chem. 63, 67 (1992).
(2) H. D. Burrows and T. J. Kemp, Chem. Soc. Rev. 3, 139(1974).
(3) D. Greatorex, R. J. Hill, T. J. Kemp, and T.J.Stone, J.
Chem. Soc. Faraday Trans. 1 68, 2059 (1972).
(4) M. E. D. G. Azenha, H. D. Burrows, S. J. Formosinho,and M. G. M. Miguel, J. Chem. Soc. Faraday Trans. 1 85,
2625 (1989).

Kumar(5) S. Ogawa, J. Phys. Japan 16, 1488 (1960).
(6) B. Sanjeeva Rao and M. R. Murthy, J. Polymer sciencepart B: Polymer Physics 25, 1897 (1987).
(7) R. J. Abraham and D. H. Whiffen, Trans. Faraday Soc. 54,
1291 (1958).
(8) Mithlesh Kumar, A. R. Dhobale, Mukesh Kumar, and M.
D. Sastry, J. Polymer science part B: Polymer Physics 35,
187 (1997).
(9) A. R. Dhobale, T. P. Chaturvedy, S. Venkiteswaran, andM. D. Sastry, Solid State Physics (India) 38C, 455 (1995).
(10) H. Hase and H. Yamaoka, Rad. Effects 19, 195 (1973).
(11) C. Miyake, in Handbook on the Physics and Chemistry of
Actinides edited by A. J. Freeman and C. Keller (Elsevier
Science Publishers, Amsterdam, 1991), Ch.7, p.337.
(12) A. F. Leung and Y. Poon, Can. J. Phys. 55, 937 (1977).
(13) W. Burton Lewis, H. G. Hecht, and M. P. Eastman, Inorg.
Chem. 12, 1634 (1973).
(14) V. Natarajan, T. K. Seshagiri, and M. D. Sastry, PramanaJournal of Physics 47, 325 (1996).
(15) N. K. Porwal, Mithlesh Kumar, and M. D. Sastry,
Pramana-Journal of Physics 47, 481 (1996).
(16) C. Miyake, Y. Yamana, S. Imoto, and H. O. Nishiguchi,
Inorganica Chimica Acta 95, 17 (1984).