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# Physical structure and architecture evolution of PEEK under proton irradiation: Comparison with electron irradiation

Lise Sujol, Thierry Paulmier, Éric Dantras

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Lise Sujol, Thierry Paulmier, Éric Dantras. Physical structure and architecture evolution of PEEK under proton irradiation: Comparison with electron irradiation. Polymer Degradation and Stability, 2025, 244, pp.111824. 10.1016/j.polymdegradstab.2025.111824. hal-05394634

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# Physical structure and architecture evolution of PEEK under proton irradiation: Comparison with electron irradiation

Lise Sujol<sup>a,b</sup>, Thierry Paulmier<sup>a</sup>, Éric Dantras<sup>b,●,*</sup>

## ARTICLE INFO

Keywords:

Poly(ether-ether-ketone) PEEK

Space

Ageing

Irradiation

Physical properties

Molecular mobility

## ABSTRACT

Orbiting satellites operate in a hostile environment characterized by a high vacuum of the order of $10^{-11}$ mbar, thermal cycling and particle irradiation. This study focusses on proton irradiation and their effects on the physical properties of poly(ether-ether-ketone) (PEEK). It is a high-performance thermoplastic, commonly used in space industry because of its low outgassing and good resistance to irradiations. 8 µm PEEK films were irradiated with 1 MeV protons to avoid any possible implantation and to obtain an homogeneous dose in the thickness. Four doses representative of the lifetime of a satellite in GEOstationary (GEO) orbit were carried out. Analysis of the physico-chemical structure revealed two main phenomena after irradiation: amorphization and crosslinking. The study of molecular mobility confirmed these evolutions. Correlation of the different analysis techniques highlighted a change in behaviour from 50 MGy: crosslinking prevails over amorphization and chain scission. Then, new pristine samples were irradiated with electrons at equivalent doses. The comparison showed no major difference between proton-irradiated and electron-irradiated PEEK, except crysallization. Indeed, proton irradiation leads to a higher decrease of crystallization temperature and crystallinity degree that can be due to the creation of a greater number of defects in the crystalline phase.

## 1. Introduction

In orbit, satellites are subject to the space environment which is characterized in particular by energetic particle flows, mainly electrons and protons. These particles can come from the solar wind in the form of radiation belts, or from more specific events such as solar flares or coronal mass ejections [1]. Depending on their energy, the particles affect the material in different ways. Low-energy particles (E < approx. 100 keV) tend to implant themselves in the material and can create surface charging that can lead to electrostatic discharges [2]. High-energy particles (E > approx. 100 keV) completely pass through the material and cause its ionization, that can modify its physical properties. The effect of electron irradiation on materials having been extensively studied in the literature [3,4] for space application, this work focuses on proton irradiation. As electrostatic discharges are much less present and intense with protons than with electrons [5], this study analyses the physico-chemical ageing of the material which may alter its good initial properties.

The research material is poly(ether-ether-ketone) (PEEK), a high-performance thermoplastic increasingly used in space industry because of its low outgassing and good resistance to irradiations. All these properties make it an ideal candidate for replacing certain metal parts

to lighten structures [6]. It is also of interest for long term storage of nuclear waste and for Polymer-Electrolyte-Membranes (PEMs) in fuel cells [7–9]. For these applications, it is necessary to study the evolution of PEEK structure when exposed to irradiations.

Since the 1970's, researchers began to analyse the effects of ionizing radiations on the structure of PEEK such as UV [10,11], gamma rays [12], electrons [13,14] or protons [15,16]. Polymers have been classified in two categories when exposed to irradiations: those who mostly crosslink and those who mostly degrade (i.e when chain scission is predominant) [17]. It is assumed that PEEK predominantly crosslinks [15]. However, the comparison between two sources of radiation is rarely made under the same conditions (absorbed dose, vacuum, etc.) for high dose levels. This study is important for finding out what governs the change in properties (type of particle, dose level, etc.). Since proton energy is limited (1 MeV), particles are rapidly implanted in matter. This leads to the use of very thin films (8 µm) in order to allow the proton beam to go through the overall bulk of the material and avoid any charge implantation during irradiation. Several adaptations have been necessary to obtain a good signal-to-noise ratio for each analysis technique.

<footnote><sup>a</sup> ONERA, Department of Physics, Instrumentation, Environment, Space, Toulouse, France</footnote>

<footnote><sup>b</sup> Université de Toulouse, CIRIMAT, Polymer Physics team, Toulouse, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: eric.dantras@utoulouse.fr (É. Dantras).</footnote>
The aim of this work is to determine the different phenomena involved with proton irradiations regarding the architecture and the physical structure of semi-crystalline PEEK. To do so, different tests were realized with pristine and irradiated samples. Four different doses were performed: 12, 34, 50 and 100 MGy. They are representative of what a satellite can receive in GEO orbit during its lifetime. Another focus of this study is the comparison with electron irradiation for the same absorbed doses (12 and 34 MGy). Only DSC tests were realized in order to compare the effects of both particles on the physico-chemical structure.

## 2. Material and methods

### 2.1. Study material : PolyEtherEtherKetone (PEEK)

The chosen study material is a commercial Victrex® APTIV™1000 PEEK film of 8 µm thick from GoodFellow. Its crystallinity degree has been evaluated at 32 ± 1% in DSC.

### 2.2. Simulations and irradiations

To simulate material ageing in orbit, PEEK samples have been irradiated with energetic protons in MIRAGE facility at ONERA Toulouse. Irradiations were carried out under vacuum at room temperature with protons of 1 MeV and a flux of 10 nA cm⁻². For the simulations, MULASSIS software was used. It is based on the Monte-Carlo method and was created as part of the SPENVIS project which aims to model the space environment and its effects [18,19]. Using the experimental conditions and knowing that the density of PEEK is 1.3 g cm⁻³, the dose profile shown in Fig. 1 can be obtained.

The maximum, at around 20 µm, corresponds to the Bragg peak. This is the thickness at which particles get implanted at the end of their trajectory in the material and can create charging problems. For this work, it is important to get rid of charging processes in order to study only the effects of ionization induced by protons. To be far from the Bragg peak and to have a homogeneous dose in the thickness, PEEK films of 8 µm thick have been selected.

To be as representative as possible of GEO orbit, OMERE 5.8.1 software has been used. It is developed by TRAD with CNES support to calculate the effects of radiation in terms of dose [20,21]. Various models are available to simulate particles flows and energy distribution, AE8-AP8 models have been chosen for their wide use [22,23]. Fig. 2 shows the spectrum for 15 years in GEO orbit (black solid line)

<figure><img src="image_4.png" /><figcaption>Fig. 1. Dose profile in the thickness for PEEK for 1 MeV protons.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 2. Dose for 15 years in GEO orbit in a worst case scenario for PEEK.</figcaption></figure>

with the contribution of electrons (blue dashed line) and protons (red dotted line) in a worst case scenario. Four doses were performed to represent different equivalent thicknesses of a satellite in this orbit during 15 years:

* 12 MGy which corresponds to an equivalent thickness of 40 µm.

* 34 MGy which corresponds to an equivalent thickness of 4 µm.

* 50 MGy which corresponds to an equivalent thickness of 3 µm.

* 100 MGy which represents the surface or more extreme conditions.

Electron irradiations were performed under vacuum at room temperature in SIRENE facility located at ONERA Toulouse. To obtain the highest dose degree, the following conditions were determined: electron energy of 150 keV and a flux of 35 nA cm⁻². Under these conditions, electrons pass completely through the sample, so no charging problem can occur. As irradiation times are much longer than with protons, only two doses were performed: 12 and 34 MGy.

### 2.3. Differential scanning calorimetry (DSC)

For this work, DSC Q2000 from TA Instruments was used. Heating and cooling runs of 10 °C min⁻¹ were performed under helium atmosphere with pristine and irradiated PEEK. Samples weights range from 5 to 8 mg. Concerning the temperature calibration, indium, tin and lead were used as references and the enthalpy calibration was realized using the melting peak of lead. Glass transition temperatures were determined using the tangent method, and the crystallinity degrees were calculated using Eq. (1):

$$ \chi_c = \frac{\Delta H_m - \Delta H_{cc}}{\Delta H_{\infty}} \qquad (1) $$

Where ΔH_m is the melting enthalpy (J g⁻¹), ΔH_{cc} is the cold crystallization enthalpy (J g⁻¹) and ΔH_{∞} is the theoretical enthalpy of 100% crystalline sample (J g⁻¹). For PEEK, ΔH_{∞} is evaluated at 130 J g⁻¹ [24].

### 2.4. Broadband dielectric spectroscopy (BDS)

Novocontrol spectrometer with Alpha-A module was used for the tests. Analysis were performed from -150 to 250 °C (240 °C for 100 MGy) in 5 °C steps and from 3 × 10⁶ Hz to 1.44 × 10⁻² Hz with a logarithmic factor of 1.3. The applied voltage was 1 V and the
aluminium electrode diameter was 35 mm. To obtain a better signal-to-noise ratio, two scans were realized in order to enhance the adhesion between the film and the electrodes. Relaxation modes corresponding to the glass transition and above were analysed in the first scan while the secondary modes were analysed in the second scan. The asymmetry of relaxation times distribution being negligible, Cole–Cole parametric equation (Eq. (2)) was used to analyse relaxation modes:

$$ \varepsilon^*(\omega) = \varepsilon_{\infty} + \frac{\varepsilon_s - \varepsilon_{\infty}}{1 + (i\omega\tau_{cc})^{\alpha_{cc}}} \quad (2) $$

Where $\varepsilon_{\infty}$ is the permittivity at high frequencies (F m⁻¹), $\varepsilon_s$ is the permittivity at low frequencies (F m⁻¹), $\tau_{cc}$ is the Cole–Cole mean relaxation time (s) and $\alpha_{cc}$ is the width parameter.

The Arrhenius diagram is then obtained by plotting Cole–Cole relaxation times as a function of temperature. The temperature dependence of these relaxation times can follow either an Arrhenius-type or a Vogel–Tammann–Fulcher (VTF)-type behaviour law. Arrhenius-type characterizes localized mobilities that are activated by temperature. The relaxation times follow the Eq. (3):

$$ \tau_{cc}(T) = \tau_{0,a} \exp \left( \frac{E_a}{RT} \right) \quad (3) $$

Where $\tau_{0,a}$ is the Arrhenius pre-exponential factor (s), $E_a$ is the activation energy (J mol⁻¹) and R is the gas constant ($R = 8.314$ J K⁻¹ mol⁻¹).

Relaxation modes associated with glass transition or above follow a VTF-type behaviour law. The molecular mobility is activated by free volume and concern larger entities. VTF-type equation is presented in Eq. (4).

$$ \tau_{cc}(T) = \tau_{0,v} \exp \left( \frac{1}{\alpha_f (T - T_{\infty})} \right) \quad (4) $$

Where $\tau_{0,v}$ is the VTF pre-exponential factor (s), $\alpha_f$ is the free volume expansion coefficient (°⁻¹) and $T_{\infty}$ is the free volume activation temperature (°C).

A Maxwell-Wagner-Sillars (MWS) contribution associated with interfacial polarization between amorphous and crystalline phases is present at high temperatures. It is difficult to analyse because of the conductivity front. Kramers–Kronig transform has been used to recalculate $\varepsilon''$ using $\varepsilon'$ which is less impacted by conductivity (Eq. (5)). Steeman Van Turnhout numerical algorithm of resolution has been performed [25].

$$ \varepsilon''(\omega_0) = \frac{2}{\pi} \int_{0}^{\infty} \varepsilon'(\omega) \frac{\omega_0}{\omega^2 - \omega_0^2} d\omega \quad (5) $$

BDS also allows the study of conductivity, more specifically the real part of the conductivity $\sigma'$ which is related to the continuous part $\sigma_{DC}$ by Jonscher's universal law (Eq. (6)) [26]:

$$ \sigma'(\omega) = \sigma_{DC} + A \times \omega^s \quad (6) $$

Where $\sigma_{DC}$ is the continuous conductivity (S m⁻¹), A and s are constants.

At low frequencies, the right term ($A \times \omega^s$) becomes negligible and the value of $\sigma_{DC}$ can be plotted as a function of temperature to obtain the Arrhenius diagram of conductivity.

## 2.5. Dynamic mechanical analysis (DMA)

Dynamic Mechanical Analysis (DMA) tests were carried out using an ARES G2 rheometer from TA Instruments in rectangular tension mode (elongation). Heating ramps were performed at 3 °C min⁻¹ from 25 to 230 °C and at a frequency of 5 Hz. A strain of 0.07% was applied. Loading gap was taken at 20 mm and the sample dimensions were 45 × 10 × 32(8 × 4) mm. Four films were thermo-welded to obtain a better signal-to-noise ratio. The complex elongation modulus is defined as the ratio of stress to strain (Eq. (7)):

$$ E^*(\omega, T) = \frac{\sigma^*(\omega, T)}{\gamma^*(\omega)} = E'(\omega, T) + iE''(\omega, T) \quad (7) $$

## 2.6. X-ray photoelectron spectroscopy (XPS)

Tests were performed in ALCHIMIE facility located at ONERA Toulouse. A SIGMA hemispherical analyzer combined with an Al Kα monochromatic ($h\nu = 1486.6$ eV) X-ray source was used for the XPS analysis. For this kind of measurements, an ultra-high vacuum level is required: the pressure in the chamber is then close to $10^{-9}$ mbar. The angle between the normal of the sample and the X-ray source is 60°. The one between the normal of the sample and the analyzer is 20°. For C1s and O1s spectra, 6 scans were realized with a resolution of 0.1 eV and a pass energy of 20 eV. For XPS Survey, a pass energy of 50 eV is used. This technique is based on the principle of energy conservation (Eq. (8)):

$$ E_b = h\nu - E_k - \phi \quad (8) $$

Where $E_b$ is the binding energy (eV), $h\nu$ is the energy of the incident photon (eV), $E_k$ is the kinetic energy measured by the analyzer (eV) and $\phi$ is the work function of the spectrophotometer (eV).

A Kimball Physics electron gun was used to equilibrate charges due to electron extraction. To compensate for residual charging effect, signals have been realigned on C–C bond (284.7 eV) for C1s spectrum and on C–O–C bond (533.1 eV) for O1s spectrum. Atomic percentages and areas were analysed using CasaXPS software.

# 3. Results and discussion

## 3.1. Evolution of the physico-chemical structure

Fig. 3 shows the DSC thermograms for pristine and irradiated PEEK obtained in the first (Fig. 3.a) and second (Fig. 3.b) heating run. Data extracted from these thermograms are reported in Table 1.

In the first scan, glass transition is masked by physical ageing for both pristine and 12 MGy irradiated samples, which is certainly due to the thermal history of the films during processing. In both scans, the glass transition temperature increases with the dose (+14 °C for 100 MGy in the second scan). In the literature, this phenomenon is generally associated with crosslinking [27]. Macromolecules mobility decreases because of crosslinking nodes, thus increasing the temperature of glass transition. Crystallization is also affected by the creation of crosslinks. It is delayed due to limited reorganization of macromolecules, resulting in a decrease in crystallization temperature ($T_c$) with the irradiations. Al Lafi et al. performed gel analyses on irradiated amorphous PEEK and observed an increase of the gel fraction with the irradiations [16,28, 29]. This result supports the hypothesis of the creation of crosslinking nodes in the amorphous phase of our semi-crystalline PEEK.

Concerning the melting temperature and the crystallinity degree, they both decrease with the irradiations. A broadening of the melting peaks is also observed, highlighting a wider distribution of crystallite size. Particles irradiation directly affect the crystalline regions by reducing their size and so their stability. Yoda showed a 15% reduction in crystalline (lamella) thickness by X-ray Scattering for a 50 MGy electron-irradiated PEEK [30]. The amorphization can also come from crosslinking. Indeed, some studies have revealed that amorphous PEEK is more resistant than semi-crystalline PEEK to irradiation, suggesting that crosslinking occur at the interfaces between the amorphous and crystalline phases, leading to amorphization [30–32]. The second run reveals very little remaining crystalline phase for 50 MGy and a total amorphization of the polymer for 100 MGy. This change in behaviour has already been observed by Vaughan et al. and has been associated with an inability of chain to reorganize due to crosslinking [33,34].

The first heating run revealed the presence of residual crystallites after the main melting peak. They are located at the same temperature as the melting peak of the pristine PEEK, and may therefore originate from crystalline regions that have resisted to irradiations. They cannot be reformed during the second scan due to crosslinking nodes that limit the mobility of macromolecules.
<table><caption>**Table 1**
Evolution of DSC parameters with proton irradiation.</caption><thead><tr><th rowspan="2"></th><th colspan="2">T<sub>g</sub><br>(°C)</th><th colspan="2">T<sub>m</sub><br>(°C)</th><th colspan="2">χ<sub>c</sub><br>(%)</th><th>T<sub>c</sub><br>(°C)</th></tr><tr><th>1st</th><th>2nd</th><th>1st</th><th>2nd</th><th>1st</th><th>2nd</th><th>1st</th></tr></thead><tbody><tr><td>Pristine PEEK</td><td>-</td><td>147</td><td>342</td><td>340</td><td>32</td><td>38</td><td>296</td></tr><tr><td>p<sup>+</sup>-Irradiated 12 MGy</td><td>-</td><td>152</td><td>327</td><td>327</td><td>31</td><td>36</td><td>279</td></tr><tr><td>p<sup>+</sup>-Irradiated 34 MGy</td><td>166</td><td>154</td><td>299</td><td>294</td><td>22</td><td>12</td><td>222</td></tr><tr><td>p<sup>+</sup>-Irradiated 50 MGy</td><td>171</td><td>154</td><td>287</td><td>286</td><td>16</td><td>3</td><td>191</td></tr><tr><td>p<sup>+</sup>-Irradiated 100 MGy</td><td>178</td><td>161</td><td>268</td><td>-</td><td>16</td><td>0</td><td>-</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** DSC thermograms for pristine and proton-irradiated PEEK during the first (a) and second heating run (b).</figcaption></figure>

DSC results show that amorphization and crosslinking occur at the same time for low doses, but above a certain dose (around 50 MGy) crosslinking takes over. However, the increase of glass transition temperature in the first scan is not as great as for lower doses (0.3 °C/MGy between 34 and 50 MGy and 0.1 °C/MGy between 50 and 100 MGy) suggesting a slowdown in the creation of crosslinking nodes which may be linked to the cessation of amorphization.

## 3.2. Study of molecular mobility

Fig. 4 shows the 3D dielectric map obtained by plotting the imaginary part of the permittivity as a function of temperature and frequency. Three relaxation modes and a dielectric contribution are noticeable. Their molecular origins have already been studied for PAEK [35].

* γ mode is associated with the interaction between ketone groups and absorbed water.

* β mode is associated with the mobility of aromatic groups around their axis.

* α mode is the dielectric manifestation of the glass transition.

* Maxwell-Wagner-Sillars (MWS) contribution, result of the accumulation of charges between the amorphous and crystalline phases [36-38].

At high temperatures and low frequencies, charge flow is activated, leading to the appearance of the conductivity wall.

After fitting these modes with the Cole-Cole parametric equation, relaxation times as a function of temperature can be plotted. Fig. 5 represents the Arrhenius diagram of pristine and irradiated PEEK. All the adjustment parameters are related in Table 2. T<sub>g</sub> and T<sub>α</sub> values obtained by DSC and DMA for pristine PEEK have been reported on the diagram, showing consistent results between these analysis techniques.

The primary relaxation modes γ and β can be fitted with an Arrhenius law, and therefore correspond to localized mobilities. Activation energies do not significantly vary with irradiation, being of the order

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** 3D map of ε'' as a function of temperature and frequency.</figcaption></figure>

of 55 kJ mol<sup>-1</sup> for γ and β modes. This can be explained by the fact that irradiation-induced changes occur on a larger scale. Since observation of β mode is difficult, especially for thin films, 12, 50 and 100 MGy are not represented. However, a slight shift is observed between the pristine sample and the one irradiated at 34 MGy. It is difficult to interpret due to this lack of information for all doses, but is consistent with the increasing scale of entities involved. The shift towards longer times may come from crosslinking nodes that stiffen macromolecular chains.

The α mode shifts toward higher temperatures with the dose, with the free volume activation temperature increasing by 22 °C for the
<table><caption>**Table 2**
Evolution of BDS adjustment parameters with proton irradiation.</caption><thead><tr><th rowspan="2"></th><th colspan="2">γ mode</th><th colspan="2">β mode</th><th colspan="3">α mode</th></tr><tr><th>τ<sub>0,a</sub><br>s</th><th>E<sub>a</sub><br>kJ mol<sup>-1</sup></th><th>τ<sub>0,a</sub><br>s</th><th>E<sub>a</sub><br>kJ mol<sup>-1</sup></th><th>τ<sub>0,u</sub><br>s</th><th>α<sub>f</sub><br>°<sup>-1</sup></th><th>T<sub>∞</sub><br>°C</th></tr></thead><tbody><tr><td>Pristine PEEK</td><td>3.6 × 10<sup>-16</sup></td><td>55</td><td>2.3 × 10<sup>-14</sup></td><td>57</td><td>2.9 × 10<sup>-14</sup></td><td>7.4 × 10<sup>-4</sup></td><td>110</td></tr><tr><td>p<sup>+</sup>-Irradiated 12 MGy</td><td>9.4 × 10<sup>-16</sup></td><td>54</td><td>–</td><td>–</td><td>1.5 × 10<sup>-14</sup></td><td>7.3 × 10<sup>-4</sup></td><td>112</td></tr><tr><td>p<sup>+</sup>-Irradiated 34 MGy</td><td>8.5 × 10<sup>-16</sup></td><td>53</td><td>5.1 × 10<sup>-13</sup></td><td>52</td><td>7.7 × 10<sup>-14</sup></td><td>7.7 × 10<sup>-4</sup></td><td>123</td></tr><tr><td>p<sup>+</sup>-Irradiated 50 MGy</td><td>6.7 × 10<sup>-16</sup></td><td>53</td><td>–</td><td>–</td><td>2.7 × 10<sup>-14</sup></td><td>6.9 × 10<sup>-4</sup></td><td>121</td></tr><tr><td>p<sup>+</sup>-Irradiated 100 MGy</td><td>1.9 × 10<sup>-16</sup></td><td>57</td><td>–</td><td>–</td><td>1.2 × 10<sup>-15</sup></td><td>5.1 × 10<sup>-4</sup></td><td>132</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Arrhenius diagram of pristine and proton-irradiated PEEK obtained by BDS.</figcaption></figure>

highest dose compared to the pristine one. It is consistent with the evolution of the glass transition temperature shown by DSC and therefore the hypothesis of crosslinking. Concerning the free volume expansion coefficient $\alpha_f$, it does not evolve for 12 and 34 MGy but starts decreasing at 50 MGy. DSC analysis highlight a stabilization of the crystallinity degree from 50 MGy, while glass transition temperature continues to increase, suggesting the creation of new crosslinking nodes. For 12 and 34 MGy, amorphization and crosslinking are well balanced resulting in no significant change. However, from 50 MGy crosslinking becomes dominant and $\alpha_f$ decreases. Fig. 6 shows a broadening of the distribution of relaxation times with the irradiations after normalization for the 190 °C isotherm. It highlights the presence of constraints around the relaxing entities that can be crosslinking nodes.

The MWS polarization shifts toward longer times, especially for the highest doses. Analysis of the static conductivity $\sigma_{DC}$ (Fig. 7) reveals that it decreases with irradiation across the entire temperature range, and this decrease is much more pronounced for 50 and 100 MGy. This is due to crosslinking nodes in the amorphous phase that create traps and prevent the charge flow. The number of crosslinkings increases with the dose, explaining the greater decrease of $\sigma_{DC}$. As the MWS contribution is directly linked to the difference of conductivity between the two phases [39], these results are consistent in explaining the evolution. Similar results have already been observed with electron irradiation on PEEK [40].

### 3.3. Mechanical properties and crosslinking evidence

DMA thermograms are presented in Fig. 8 for pristine and proton irradiated samples. Mechanical parameters are listed in Table 3.

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Normalization of the adjustment of α mode for the 190 °C isotherm for pristine and proton-irradiated PEEK.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Arrhenius diagram of $\sigma_{DC}$ for pristine and proton-irradiated PEEK.</figcaption></figure>

The glassy state is unaffected by irradiations, standard deviations overlap between the pristine and irradiated PEEK. The glassy plateau being characterized by limited molecular mobility and fixed macro-molecule chains, the scale is too localized to observe any evolution with irradiation. The glass modulus $E_g$ is around 2.7 GPa for both pristine and irradiated PEEK at 100 °C.
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** DMA thermograms of the first heating run for pristine and proton-irradiated PEEK.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Normalization of α mode for pristine and proton-irradiated PEEK.</figcaption></figure>

<table><caption>**Table 3**
Evolution of DMA parameters with proton irradiation.</caption><thead><tr><th></th><th>E<sub>s</sub> at 100 °C<br>(Pa)</th><th>E<sub>r</sub> at 230 °C<br>(Pa)</th><th>T<sub>α</sub><br>(°C)</th></tr></thead><tbody><tr><td>Pristine PEEK</td><td>2.6 × 10<sup>9</sup></td><td>1.2 × 10<sup>8</sup></td><td>153</td></tr><tr><td>p<sup>+</sup>-Irradiated 12 MGy</td><td>2.2 × 10<sup>9</sup></td><td>2.6 × 10<sup>8</sup></td><td>159</td></tr><tr><td>p<sup>+</sup>-Irradiated 34 MGy</td><td>2.9 × 10<sup>9</sup></td><td>1.7 × 10<sup>8</sup></td><td>166</td></tr><tr><td>p<sup>+</sup>-Irradiated 50 MGy</td><td>2.5 × 10<sup>9</sup></td><td>2.0 × 10<sup>8</sup></td><td>170</td></tr><tr><td>p<sup>+</sup>-Irradiated 100 MGy</td><td>3.2 × 10<sup>9</sup></td><td>6.5 × 10<sup>7</sup></td><td>177</td></tr></tbody></table>

The sharp drop in modulus represents the viscoelastic transition and thus the mechanical manifestation of the glass transition. The temperature associated with this mode $T_α$ increases with the irradiations as observed in BDS. This is again consistent with the increase in glass transition temperature seen in DSC. The normalization in temperature and modulus of the dissipative part of this mode is represented in Fig. 9. A broadening of the distribution with the irradiation is observed in the glassy state that testifies the presence of heterogeneities in the vicinity of the relaxing entities. In the rubbery state side, there is a shift compared to the pristine sample but no evolution is discernable between the different doses. One possible explanation is the difficulty to maintain the axial force in the rubbery state especially for thin films.

Rubbery plateau appears after the viscoelastic transition. Its study is interesting because it is influenced by the crystallinity degree, chain scission and crosslinking. DSC analysis showed a decrease in crystallinity with the irradiation that should imply a decrease in the rubbery plateau. However, thermograms revealed an increase in the rubbery plateau at 12, 34 and 50 MGy compared with pristine PEEK. The only explanation is the creation of crosslinking nodes with the irradiations. Chain scission probably occur at the same time but it is not predominant [15]. At 100 MGy, the decrease in the stability of the crystalline phase and/or chain scission is greater and exceeds the contribution of crosslinking. The rubbery plateau decreases compared to pristine PEEK, but this decrease could have been greater without the crosslinking nodes. Also, it is not straight but continuously decreasing, as it does for 50 MGy, certainly because of a too low crystallinity degree. A change in behaviour is once again observed from 50 MGy, resulting in the inability to form a rubbery plateau in DMA.

### 3.4. Scenarios for the evolution of chemical structure by XPS

Figs. 10 and 11 show the C1s and O1s XPS signals respectively. For this analysis, only the chemical bonds initially present in the material were taken into account [41], suggesting that no recombination with the environment is possible under vacuum during irradiation. Atomic percentages and deconvolution results are listed in Table 4.

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** C1s High-Resolution XPS spectrum for pristine and proton-irradiated PEEK.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** O1s High-Resolution XPS spectrum for pristine and proton-irradiated PEEK.</figcaption></figure>
<table><caption>**Table 4**
Evolution of XPS parameters with proton irradiation.</caption><thead><tr><th rowspan="2"></th><th colspan="2">XPS Survey</th><th colspan="4">Hi-Res C1s</th><th colspan="2">Hi-Res O1s</th></tr><tr><th>C<br>atom.%</th><th>O<br>atom.%</th><th>C-C<br>Area (%)</th><th>C-O-C<br>Area (%)</th><th>C=O<br>Area (%)</th><th>π-π*<br>Area (%)</th><th>C-O-C<br>Area (%)</th><th>C=O<br>Area (%)</th></tr></thead><tbody><tr><td>Pristine PEEK</td><td>86</td><td>14</td><td>74</td><td>18</td><td>4</td><td>4</td><td>70</td><td>30</td></tr><tr><td>p<sup>+</sup>-Irradiated 12 MGy</td><td>86</td><td>14</td><td>78</td><td>13</td><td>8</td><td>2</td><td>64</td><td>36</td></tr><tr><td>p<sup>+</sup>-Irradiated 34 MGy</td><td>83</td><td>17</td><td>81</td><td>11</td><td>7</td><td>1</td><td>61</td><td>39</td></tr><tr><td>p<sup>+</sup>-Irradiated 50 MGy</td><td>85</td><td>15</td><td>82</td><td>10</td><td>6</td><td>2</td><td>66</td><td>34</td></tr><tr><td>p<sup>+</sup>-Irradiated 100 MGy</td><td>88</td><td>12</td><td>83</td><td>13</td><td>3</td><td>1</td><td>67</td><td>33</td></tr></tbody></table>

Changes in atomic percentages are not significant, showing that the chemical structure is not drastically modified with the irradiations. However, the C1s and O1s XPS signals reveal several evolutions. The increase of C–C bonds associated with the diminution of π-π* concentration can be linked with crosslinking. This hypothesis is consistent with the scenario proposed by Patel et al. showing the creation of crosslinks between two aromatic cycles [42]. The increase in C–C concentration is less pronounced from 50 MGy, suggesting that most crosslinking nodes are already formed. This result is consistent with the DSC results, which show a slower increase in the glass transition temperature from 50 MGy.

Chain scission can explain the increase of C=O concentration and the decrease of C-O-C observed in the O1s signal between the initial state and 34 MGy. As ether bonds are less energetic than the ketone bonds, it can possibly break and recombine with a proton from irradiation to form an alcohol. As ratios are used, the increase in the relative area associated with C=O bond comes from the decrease of the C-O-C bond concentration. From 50 MGy, values start to stabilize indicating a change in behaviour that can be linked with DSC results. Assuming that chain scission occur in the crystalline phase or at the interface, the stabilization of the crystallinity degree may indicate a stabilization of chain scission and therefore of C-O and C=O concentrations. The binding energy of C-OH being very close to those of the other bonds taken into account, XPS cannot prove the formation of alcohols. Also, the quantity created is certainly very small and therefore difficult to discern. Some authors have also highlighted the formation of hydroxyl groups, but have associated this with the reduction of ketone groups [43]. Quantitative analysis being very delicate with XPS, the above interpretations are potential scenarios.

### 3.5. Proton vs. electron irradiations regarding the physico-chemical structure

The same doses of 12 and 34 MGy have been performed with electrons in order to compare the effect of protons and electrons on PEEK structure. Fig. 12 presents DSC thermograms for pristine and electron-irradiated samples during the first (Fig. 12.a) and second heating (Fig. 12.b). All parameters are listed in Table 5.

DSC parameters evolve in the same way between samples irradiated with electrons and protons, except crystallization temperature and crystallinity degree as heating ramps are performed. Their evolution is reported in Fig. 13 for the dose at 34 MGy. With proton irradiations, the crystallinity degree decreases with cycles of temperature, around 10% for the dose at 34 MGy after 1 cycle. It is the same for electron irradiations but the decrease is less significant, 4%. Concerning crystallization temperature, the decrease with proton irradiations is almost twice as pronounced as with electrons. These results highlight the hypothesis of the creation of a greater number of defects in the crystalline phase after proton irradiation. It can be linked with the Linear Energy Transfer (LET), which is the energy that a particle transfers to the material per unit distance. As protons are larger than electrons, they implant more quickly in matter and therefore have a higher LET. This difference in kinetics may explain the creation of a greater number of defects (crosslinking nodes or chain scission for example). T_c and χ_c continue to evolve after temperature cycles. This may be related to

the kinetics of crystallization, which slows down with irradiation. The crystalline phase does not have time to crystallize completely during the cooling ramps due to the formation of cross-linking nodes, which limit macromolecular mobility. As a result, the degree of crystallinity decreases with DSC scans, as does the crystallization temperature. This hypothesis is supported by the presence of a low-intensity exothermic phenomenon around 215 °C for 34 MGy in Fig. 3.b and Fig. 12.b, which could be cold crystallization.

## 4. Conclusion

This study focussed on the influence of proton irradiation on PEEK for space applications. Four representative doses of 15 years in GEO orbit were realized: 12, 34, 50 and 100 MGy. The correlation of different analysis techniques revealed two main phenomena after proton irradiation on PEEK: the amorphization of crystalline phases and crosslinking. The amorphization can come from defects created by irradiations or/and from crosslinking. The increase in glass transition temperature observed in DSC combined with the increase in the rubber plateau in DMA confirm the creation of crosslinking nodes during irradiations. XPS analysis allowed the proposition of two scenarios concerning crosslinking and chain scission.

From 50 MGy, a change in behaviour is observed in all the analysis techniques used. The results suggest that for low doses, amorphization, chain scission and crosslinking occur simultaneously but that from 50 MGy, amorphization and chain scission begin to stabilize while crosslinking continues.

Electron-proton comparison for the same absorbed doses showed few differences regarding the physico-chemical structure. However, the evolution of crystallinity ratio and crystallization temperature as heating ramps are performed is different between the two particles. It can be explained by the formation of a greater number of defects in the crystalline phases of proton-irradiated PEEK due to the larger size of the particle. For both particles, the evolution of T_c and χ_c with DSC scans may be linked with a decrease of the kinetics of crystallization with the irradiation due to crosslinking nodes that limit macromolecular mobility.

To conclude, the absorbed dose governs most of evolutions concerning the physico-chemical structure, but some differences start to appear after the melt.

### CRediT authorship contribution statement

**Lise Sujol:** Writing – review & editing, Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Thierry Paulmier:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition, Formal analysis, Conceptualization. **Éric Dantras:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Formal analysis, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
<table><caption>**Table 5**
Evolution of DSC parameters with electron irradiation.</caption><thead><tr><th rowspan="2"></th><th colspan="2">T<sub>s</sub><br>(°C)</th><th colspan="2">T<sub>m</sub><br>(°C)</th><th colspan="2">χ<sub>c</sub><br>(%)</th><th>T<sub>c</sub><br>(°C)</th></tr><tr><th>1st</th><th>2nd</th><th>1st</th><th>2nd</th><th>1st</th><th>2nd</th><th>1st</th></tr></thead><tbody><tr><td>Pristine PEEK</td><td>-</td><td>147</td><td>342</td><td>340</td><td>32</td><td>38</td><td>296</td></tr><tr><td>e<sup>-</sup>-Irradiated 12 MGy</td><td>-</td><td>152</td><td>326</td><td>325</td><td>30</td><td>30</td><td>275</td></tr><tr><td>e<sup>-</sup>-Irradiated 34 MGy</td><td>165</td><td>154</td><td>302</td><td>298</td><td>22</td><td>18</td><td>234</td></tr></tbody></table>

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** DSC thermograms for pristine and electron-irradiated PEEK during the first (a) and second heating run (b).</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 13.** Evolution of crystallinity degree (a) and crystallization temperature (b) as a function of DSC scans for PEEK irradiated at 34 MGy.</figcaption></figure>

## Acknowledgements

The authors would like to thank Romain Rey (ONERA) and Céline Michelin (ONERA) for the irradiations, Sarah Dadouch (ONERA) and Mohamed Belhaj (ONERA) for the XPS measurements and preliminary analysis. Thanks also to the Région Occitanie for financing this project.

## Data availability

Data will be made available on request.

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