# Gamma-ray-induced modifications of the chemical structure of an ion exchange resin

Micheline Draye, Bilal Nsouli, Hakim Allali, Jean-Paul Thomas

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Micheline Draye, Bilal Nsouli, Hakim Allali, Jean-Paul Thomas. Gamma-ray-induced modifications of the chemical structure of an ion exchange resin. Polymer Degradation and Stability, 1997, 56, pp.157-167. 10.1016/S0141-3910(96)00176-0. hal-00006561

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# Gamma-ray-induced modifications of the chemical structure of an ion exchange resin

M. Draye,ª B. Nsouli,ᵇ H. Allali,ᵇ M. Lemaireª & J.-P. Thomasᵇ*

Gamma-ray irradiation of ion exchange resins used for processing nuclear effluent may lead to considerable modifications of the molecular structure and macroscopic properties. In the present study, crosslinked poly(4-vinylpyridine) (P4VP), irradiated by a ¹³⁷Cs γ-source up to an integrated dose of 14 400 kGy, has been investigated. The particle-induced desorption-mass spectrometry (PDMS) technique is able to detect the radiation-induced structural degradation of such an infusible and insoluble material. From the results of PDMS, a coherent description of the production of specific radical fragments and modified chains is proposed. Both the nature of these fragments as well as their formation, as a function of the dose of irradiation, correlate with results of electron paramagnetic resonance spectrometry and Fourier transform infrared spectroscopy, and with thermogravimetric and dynamical mechanical analysis. In addition, analysis of the soluble and insoluble fractions helps to discriminate between different possible processes. In particular, it is shown that the loss of pyridine fragments correlates well with chemical measurements like the loss of protonatable sites.

## 1 INTRODUCTION

The acid-base and loading properties of its pyridine group have stimulated the use of poly(4-vinylpyridine) (P4VP) resin in various applications such as a chemical reagent,¹ a catalyst² or catalyst support,³ an extractant for organic pollutants⁴ and for the selective isolation of a host of dissolved metals.⁵ Among these applications, there is particular interest in its adsorbing properties towards radionucleides from liquid wastes of low or medium activity.⁶,⁷ The on-going effects of radiation-induced ageing on the properties and performances of such materials have been a topic of considerable interest. It has been shown that, among the synthetic resins, those of the vinylpyridine type are more resistant towards ionizing radiation.⁸ A chemical evaluation of the stability of such crosslinked materials has already been under-

taken,⁹,¹⁰ but a detailed investigation of the structural modifications induced by gamma radiation has still to be performed before any industrial technique aimed at nuclear waste reprocessing can be considered.

As part of such a programme is the study of the radiochemical stability of P4VP irradiated with a dose of up to 14 400 kGy, and this work is aimed at obtaining an insight into the modification of both molecular structure and macroscopic properties. For such an infusible and insoluble material most of the commonly used techniques for polymer characterization [infrared spectroscopy (IR), ultraviolet spectrophotometry (UV), nuclear magnetic resonance spectroscopy (NMR), electron paramagnetic resonance spectrometry (EPR), electron spectroscopy for chemical analysis (ESCA), etc.] give limited or uncorrelated results. In a forthcoming paper¹¹ we demonstrate that negative ion emission under ion beam impact can be sensitive to the chemical modifications induced in such a material by

<footnote>ªInstitut de Recherches sur la Catalyse, Laboratoire de Catalyse et Synthèse Organique, CPE-ESCIL,
Université Claude Bernard Lyon I, 43 bd du 11 Novembre 1918, 69622 Villeurbanne Cédex, France</footnote>

<footnote>ᵇInstitut de Physique Nucléaire (et IN2P3), Université Claude Bernard Lyon I, 43 bd du 11 Novembre 1918,
69622 Villeurbanne Cédex, France</footnote>

<footnote>*To whom correspondence should be addressed.</footnote>
γ-irradiation, provided that the primary ions are used in the MeV instead of the keV regime. Thus particle-induced desorption-mass spectrometry (PDMS)—or, adopting the terminology proposed by Sundqvist,¹² HSF-SIMS—has been widely used in this study to give a coherent description of these modifications that is in agreement with results of other techniques such as EPR, IR, thermogravimetric analysis (TGA) and dynamical mechanical analysis (DMA).

# 2 EXPERIMENTAL

## 2.1 Materials

The crosslinked (2% with divinylbenzene) P4VP resin used in this study was supplied by Reilly Industries Inc., Indianapolis, IN, USA. The material was a fine, white, insoluble and infusible powder, with a specific area of ~0.5 m²g⁻¹. It has the chemical structure sketched in Fig. 1.

## 2.2 Gamma irradiation

Plugged glass containers filled with 1.8 g of P4VP were exposed to various irradiation doses of a ¹³⁷Cs γ-source. The doses absorbed were determined by calibration to be 148, 480, 3360, 7200 and 14 400 kGy at a dose rate of 20 kGy h⁻¹.

## 2.3 Sample preparation

### 2.3.1 General procedure

Pellets of thickness 1 mm were made by compressing (at 2 tonne cm⁻²) 0.1 g of the powder into a stainless steel cylinder of 8 mm internal diameter.

### 2.3.2 Extraction of the soluble part

The soluble part of the irradiated samples was obtained by shaking 0.15 g of the P4VP powder

<figure><img src="image_2.png" /><figcaption>Fig. 1. Molecular structure of the P4VP resin crosslinked with 2% divinylbenzene.</figcaption></figure>

with 2 cm⁻³ ethanol for 15 min. After filtration, the resin in the filter was dried and weighed. The value obtained was then compared with that resulting from evaporation to dryness of the organic phase. For the most soluble samples (high irradiation doses) good agreement was found from such a comparison. The percentage of soluble part was expressed according to the following expression:

$$ \% = \frac{M_i - M_f}{M_i} \times 100 $$

where Mᵢ and Mᵣ are the weights of the dried resin before and after extraction.

### 2.3.3 Spin-coating deposition

In order to determine the polymer amount of these solutions of different concentrations, thin films were prepared by spin-coating deposition on aluminized Mylar backings. However, as explained later in the text, such solutions have to be subjected to a given number of dilutions. Thus the films were made from assays of 10 µl. For a typical area of deposit around 2.5 cm² the largest thickness value never exceeded 5 µg cm⁻².

## 2.4 Instrumentation

### 2.4.1 FTIR spectra

Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer 1720X spectrometer from P4VP pellets (1% in KBr).

### 2.4.2 EPR spectra

The EPR spectra were recorded, at room temperature, on a Varian E Line electron paramagnetic resonance spectrometer, at a magnetic modulation frequency of 100 kHz. The quartz EPR cells contained 30 mg of resin powder. The spin concentrations were obtained by comparison of the experimental spectra with a calibrated Varian pitch sample EPR spectrum containing a definite number of spins.

### 2.4.3 Thermal analysis

Thermogravimetric (TG) measurements were performed on a DuPont Instruments TGA 2950 apparatus, under helium atmosphere. Amounts
of 10–15 mg of P4VP were weighed into the thermobalance and the weight loss was determined as a function of temperature from 30 to 600°C with a heating rate of 10°C min⁻¹.

The dynamical mechanical (DMA) experiments were carried out with a DMA 7 analysis system from Perkin-Elmer, on samples in pellet form (diameter = 0.8 cm) between two parallel plates, using the compression mode (surface area submitted to the forces = surface area of the analysed pellet). The frequency was 3.00 Hz and the dynamic and static forces applied were 900 and 1200 mN, respectively. The scanned temperature range was between 230 and 40°C at a rate of 1°C min⁻¹.

### 2.4.4 HSF-SIMS measurements

Since HSF-SIMS analysis provides information from the surface layer, great care must be taken to ensure that this information can be correlated with the bulk results of the preceding techniques. Among the possible sources of artefacts are, of course, accidental contamination and analysis-induced damage. In both cases, there are some decisive advantages in using argon ions accelerated up to 9 MeV by the 4 MV Van de Graaff machine of the Institut de Physique Nucléaire, instead of the more conventional ²⁵²Cf source. First, the special configuration of our set-up uses a sharp and well-aligned primary beam¹³ which offers the possibility of lateral homogeneity probing, even at the micrometre level of resolution.¹⁴ As for the second point, since the intensity of the continuous beam is around 1000 ions s⁻¹, the analysis is essentially non-destructive and no charge compensation is required. This does not mean that the detection is always free from secondary effects (background or peak broadening) but, with a typical integrated dose of the order of 5 × 10⁹ ions cm⁻², no surface evolution can be observed for reasonable counting statistics and secondary ion experimental yields are still high (as determined from reference materials such as alkali halides).¹³ This has been made possible because of the optics of our time-of-flight (TOF) system: with a flight path of 125 mm only the transmission is around 85%. However, the mass resolution is poor (typically 190 FWHM at mass 133).

The price to pay for processing samples in the simple pellet form for time-of-flight spectrometry is to set a reproducible electric field between the sample surface and the extraction electrode,

which are 2 mm apart.¹³ Once correctly applied and controlled, the 5 kV biasing applied to the target allows the secondary ions to be accelerated over the flight distance of 125 mm towards the microchannel plates assembly (MCP). Based on ion-by-ion detection conditions, such spectrometry implies that the time of arrival of each primary ion must be recorded (start signal). This is easily done for the thin films, which can transmit fast projectiles and let them start at a surface barrier detector (SBD). For analysis of the thick samples and since we are only interested in negative ion emission,¹¹ we take full advantage of the so-called ‘start electron’ technique.¹³ As sketched in Fig. 2, each primary ion impact will give rise to an electron shower preceding the emitted ions, if any. Then, once split, the same MCP signal can be used for generation of the start signal from the electron pulse and, after an appropriate delay, for the stop signals (the same electron pulse plus ion pulses, if any). The analysis is performed in conventional vacuum conditions (10⁻⁷ mbar).

# 3 RESULTS AND DISCUSSION

## 3.1 Bulk sample characterization

The consequences of the γ-irradiation of P4VP are visible in the negative ion spectra of Fig. 3. Under the present experimental and material preparation conditions, C$_{n}$H$^-$ fragments are emitted with quite high intensity. For odd values of n (49, 73, 97, 121) surface contamination resulting from the vacuum conditions is observed along with inorganics,¹⁵ while C$_{n}$H$^-$ fragments

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** TOF spectrometer experimental set-up using the ‘start electron’ technique. Constant fraction discriminators (CFD) are used for digitization of the time signals.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Negative ion mass spectra: (a) from an unirradiated P4VP pellet sample; (b) from an irradiated sample with an integrated dose of 148 kGy. Characteristic ions are underlined, new ions following γ-ray irradiation are circled, contaminants or C<sub>n</sub>H<sub>m-n</sub> fragments bear a * label.</figcaption></figure>

with even values of *n* come from multifragmenta-
tion of the polymer backbone and also do not
reflect its chemical structure. Four peaks have
been selected and will be considered as
characteristic of the material. From the unir-
radiated material they correspond to masses at
41, 78, 146 and 158 Da. Of course, the ion
emission itself is just a consequence of a localized
energy deposition inducing scissions along the
chains, as-formed or modified fragments being
susceptible to escape in a more or less stable
charged form. Thus the increase in intensity of
the characteristic peaks observed for the smallest
γ dose [148 kGy, Fig. 3(b)] is a strong indication
that the pre-formation of some fragments
facilitates ion emission. Finally, the emission of
new peaks at higher masses (183–184, 197 and
211 Da) has to be explained by the production
under γ-irradiation of fragments not formed
under ion bombardment. As sketched in Fig. 4,
the randomly distributed scissions induced by the
interaction with γ-rays could lead to fragments or
modified chains from which the ion emission
under primary ion interaction is easier, since one
of the simultaneous processes needed is no

longer required. As an extreme example, mass
158, which is produced from two pyridinic
scissions (A + A) plus a benzylic scission (B), is
more easily emitted from A + B or A + A
scissions from a modified chain or a fragment. As
for the ions appearing only after γ-irradiation, as
sketched in Fig. 5, their production from a
modified chain only requires a B scission instead
of the B + B scissions that are never observed on
the unirradiated material. On the other hand, the
fragment of mass 211 when formed under ion
bombardment is preferentially ionised and leads
to [M + H]<sup>+</sup> ion emission.¹¹ Formed under
γ-irradiation, it can be responsible for production
of the associated ions. More details on these
processes are given in Ref. 11.

All these tentative interpretations require the
formation of stable free radicals as well as the
formation of double bonds, single or conjugated.
In addition, this production should correlate with
yield variation of the associated ions as a function
of the integrated dose. For the characteristic ions,
such a variation is shown in Fig. 6.

Concerning the radicals, EPR spectrometry
gives unambiguous results concerning the exis-
tence of paramagnetic centres. Since we are
dealing with an amorphous polymer, the absence
of hyperfine structure cannot be attributed to the
delocalized single electrons of ethylenic and/or
aromatic free radicals.¹⁶ Quantitatively, the
variation of the number of spins per g of resin as
a function of the integrated dose is shown in Fig.
7. Such a curve, which averages the contribution
of the different free radicals, is comparable to the
variation of the related individual ions (Fig. 6).
This is a first support of our interpretation of the
formation of various entities.

As far as double bond formation is concerned,
some lack of sensitivity (already evoked for such
type of samples) has prevented the detection of
any significant modification of IR spectra below
the irradiation dose of 3360 kGy. As shown in
Fig. 8, the presence of well-defined peaks at 1690
and 1654 cm⁻¹ can be attributed to the stretching
vibration mode of aliphatic, double and conjug-
ated double bonds, respectively.¹⁷ The increase in
intensity of these two bands with integrated dose
is qualitatively observed, also in agreement with
our interpretation. Particularly interesting is the
seemingly steeper increase of intensity of the
band corresponding to conjugated C=C bonds,
when increasing the integrated dose. This is in
agreement with the yield variation of fragments
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Proposed mechanisms for emission of the characteristic negative fragments from A and B scissions induced by γ-irradiation.</figcaption></figure>

of mass 158 and 146 (Fig. 6) and would favour, among their mechanisms of formation proposed in Fig. 5, the one involving A scissions from γ-irradiation.¹¹ Finally, although the samples were not irradiated in vacuum, no oxidation has been detected from these spectra (absence of CO, RCO₂H and RCO₂R₁ bands). It must be

noted, however, that the containers were closed during irradiation, and that the pyridine groups of the crosslinked P4VP are highly resistant to oxidizing and reducing agents.¹⁸

The evolution of these γ-induced modifications, either observed from the yield of specific ions or from the concentration of paramagnetic
<figure><img src="image_6.png" /><figcaption>Fig. 5. Proposed mechanisms for emission of new negative fragments resulting from modifications to B scissions induced by γ-irradiation.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. Evolution, as a function of the integrated dose, of the yield of fragments observed on the unirradiated sample (m/z 41, 78, 146 and 158).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** Evolution, as a function of the integrated dose, of the number of spins per g of P4VP resin.</figcaption></figure>

centres, is characterized by an almost linear increase in the low-dose region (<500 kGy), followed by a more or less marked saturation regime. The occurrence of recombination processes taking place once a given concentration of fragments or modified chains is reached, is suspected. This would explain why the emission of new ions following low-dose irradiation

exhibits a marked decrease and then disappears (Fig. 9) while the specific ion emission tends to slow down. A good approach to investigating such bulk modifications is to perform TGA in an inert atmosphere. The variation of weight loss as a function of temperature is shown for various integrated doses in Fig. 10. No significant deviation from the unirradiated case is found until the 3360 kGy integrated dose, when a noticeable amount of dry tar and a decrease of the degradation temperature are observed. Since this last parameter is related to early desorption of the fragments formed under irradiation, it is not surprising to observe a further evolution

<figure><img src="image_9.png" /><figcaption>**Fig. 9.** Evolution, as a function of the integrated dose, of the yield of fragments resulting from modifications induced by γ-irradiation (m/z 183–184, 197 and 211).</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** FTIR spectra of the P4VP resin: (a) unirradiated; (b) irradiated with 3360 kGy integrated dose; (c) irradiated with 14 400 kGy integrated dose.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Relative residual weight as a function of temperature for P4VP samples at different integrated doses.</figcaption></figure>
limited to some change of the slope. Moreover, it
is particularly interesting to note that the amount
of dry tar reaches a value of 50% for a 25%
crosslinked unirradiated resin. Thus its con-
tinuous increase up to a value of 18% is typical of
a continuous increase of crosslinking processes as
a function of integrated dose.

Another powerful technique for assessing the
existence of crosslinking under γ-irradiation and
its evolution as a function of integrated dose is
dynamical mechanical analysis, by which the glass
transition temperature (T<sub>g</sub>) can be deduced from
the variation of tan δ, δ being the loss angle. As
can be seen from Fig. 11, the data have been
obtained with decreasing temperatures; this
removes the possibility of artefacts due the initial
presence of a significant amount of water in the
samples, as observed in the low-temperature
region of the previous thermograms (Fig. 10).
The main result is an important (around 40°C)
increase in T<sub>g</sub> for the largest dose together with a
significant decrease of the chain mobility from
the decrease of tan δ. For the lowest dose
(148 kGy) the observed decrease is not as
significant because of the reproducibility of the
measurements for such a sintered material, and
one has to be cautious when observing an
increase of T<sub>g</sub> since undesirable crosslinking
processes can develop when starting at such a
high temperature (230°C).

**3.2 Analysis of the soluble fraction**

The previous results obtained from the different
techniques clearly establish the formation of
stable free radicals within the irradiated material,

<figure><img src="image_12.png" /><figcaption>**Fig. 11.** Dynamical mechanical analysis: variation of tan δ as a function of annealing temperature.</figcaption></figure>

as well as the development of crosslinking
processes, as a function of the integrated dose.
Starting with such a resin initially crosslinked
with divinylbenzene (DVB) implies the forma-
tion of two types of radical. Summarized in Fig.
12 are the most illustrative cases. While the
radicals incorporating one DVB bridge are
denoted R<sub>c</sub>, the others can be intact oligomers
resulting from a benzylic scission (denoted R<sub>1</sub>),
or modified by the loss of a pyridine and the
creation of double bonds (denoted R<sub>2</sub>) or of
simple fragments like the pyridine unit (denoted
R<sub>3</sub>). Then, recombinations leading to R<sub>c</sub>–R<sub>c</sub> or
R<sub>c</sub>–R<sub>1</sub> fragments will be responsible for a higher
density of crosslinking, while R<sub>1</sub>–R<sub>1</sub> fragments
lead to intact or modified oligomers of longer
chains. The free radicals, as well as this last
category of fragments, must be soluble in organic
polar solvents.

Indeed, the variation of the quantity of soluble
material extracted by ethanol is remarkably
comparable to the EPR data variation, as shown
in Fig. 13. From such solutions where the
concentration increases as a function of inte-
grated dose, HSF-SIMS analysis can help in
discriminating the relative abundance of the
individual fragments. This requires the prepara-
tion of spin-coating targets for which the amount
of deposited material is proportional to the
concentration. Then, if the thickness of the
samples is maintained below the depth of origin
of the secondary ion emission and if a linear
correspondence between ion yield and sample
thickness can be established, the variation of the
amount of a given species can be obtained. Such
a procedure has been described in detail in Ref.
11. As observed in Fig. 14, from the yield
variation of the ions of mass 41 and 78, the
corresponding fragments are extracted in the
same proportions as free radicals are formed.
The difference with what is observed from the
bulk, is that the ion yield of the mass 41 is
proportionally smaller. This gives some credit to
the various origins (free radicals or modified
chains) of these smallest fragments. Moreover,
from the HSF-SIMS analysis of the insoluble
residue one can determine what has been lost.
For the highest dose (14 400 kGy), where the
effect is expected to be the most important, the
yield is reduced by a factor of 5.0 and 2.6 for
masses 78 and 41, respectively. By comparison,
the emission of ions of mass 146 and 158 is only
reduced by a factor of 1.6 and 1.2, respectively.
<figure><img src="image_13.png" /><figcaption>**Fig. 12.** Summary of fragment formation under γ-irradiation and subsequent ion emission under primary beam impact.</figcaption></figure>

From this result it can be concluded that the negative ions are a direct signature of the pyridinic scissions and that the double bond formation is associated with the crosslinked material.

Conversely, the positive ion emission is unambiguously a consequence of benzylic scissions¹¹ and analysis of the same spin-coating targets indicates that production of the related fragments (Fig. 12) follows the same previous trend. The dimer of mass 211 is not the only one leading to the [M + H]⁺ ion of the highest yield,

but the detection in solutions of the negative ions of mass 197 and 211 also validates their formation from it, as proposed in Fig. 5. Moreover, as the yield of these ions remains almost constant with dose, the cessation of their emission from the bulk shown in Fig. 9 is related to some kind of ion trapping, instead of recombination processes, of these larger fragments.

As a final comparison, we present in Fig. 15 the evolution of the number of spins g⁻¹, the
<figure><img src="image_14.png" /><figcaption>**Fig. 13.** Variation of the weight fraction of the P4VP material soluble in ethanol, as a function of integrated dose.</figcaption></figure>

evolution of the yield of the pyridine ion ($m/z = 78$) from the soluble fraction and the evolution of the relative loss of protonatable sites (%, as obtained from acid-base titration¹⁰). It can be observed that the largest discrepancies are observed at the lowest doses. However, in terms of palladium efficiency of extraction (in which two pyridine sites are required for the extraction of one atom of palladium), no significant loss is observed before 3360 kGy.¹⁰ Then, for the low-dose region where the recombination processes are not yet observed, the production of free pyridine radicals does not seriously affect the extraction efficiency because free pyridines can still participate in the extraction process. From

<figure><img src="image_15.png" /><figcaption>**Fig. 14.** Evolution, as a function of the integrated dose, of the yield of ions of mass 41 and 78 from thin films corresponding to the resulting concentration of soluble material.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 15.** Comparative evolution of number of spins g ¹, yield of pyridine ion from the soluble fraction and loss of protonatable pyridine sites (%), as a function of the integrated dose. Data have been normalized on the value at 14 400 kGy.</figcaption></figure>

the 3360 kGy irradiation dose, the recombination is responsible for a decrease in the amount of free pyridine radicals and new crosslinks make the extraction sites less accessible, resulting in a decrease in the efficiency of palladium extraction.

## 4 CONCLUSION

From the negative ion emission observed under HSF-SIMS conditions, chemical modifications induced by γ-irradiation of P4VP resin are shown to involve the production of small fragments including the pyridine radical, easily extracted in the soluble fraction. On the other hand, double bonds or conjugated double bonds are essentially formed on insoluble modified chains. Such a picture is in good agreement with EPR and IR data. As a function of absorbed dose, a fast and linear increase of this production is observed in the low-dose range (<150 kGy), followed by a much slower variation. Larger fragments (dimers or trimers) are also found in the soluble fraction but the corresponding ion emission from the bulk disappears above 150 kGy. The recombination processes responsible for these changes are supported by TGA and DMA measurements, where irradiation-induced crosslinking processes are qualitatively shown to increase up to a saturation level. Finally, as the production variation of the pyridine fragments is comparable to that of the loss of protonatable sites, one may
interpret the excellent stability of the extraction efficiency of the material as being due to the presence of pyridine fragments left undestroyed.

## ACKNOWLEDGEMENTS

The dedicated assistance of the accelerator crew of the Institut de Physique Nucléaire, M. Ferrari and A. Gardon, and of M. Espagnan from COGEMA-Marcoule who performed the γ-irradiations, are gratefully acknowledged. M. D. and M. L. are grateful to R. Chomel from COGEMA-Marcoule for his support to the work.

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