Radiochemical Ageing of Poly(Ether Ether Ketone) Emmanuel Richaud, Paulo Ferreira, Ludmila Audouin, Xavier Colin, Jacques Verdu, Carole Monchy-Leroy

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Radiochemical ageing of poly(ether ether ketone) Emmanuel Richaud, Paulo Ferreira, Ludmila Audouin, Xavier Colin, Jacques Verdu, Carole Monchy-Leroy

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Emmanuel Richaud, Paulo Ferreira, Ludmila Audouin, Xavier Colin, Jacques Verdu, et al.. Ra- diochemical ageing of poly(ether ether ketone). European Polymer Journal, Elsevier, 2010, 46 (4), pp.731-743. 10.1016/j.eurpolymj.2009.12.026. hal-02269369

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Emmanuel Richaud a,b,*, Paulo Ferreira a, Ludmila Audouin a, Xavier Colin a, Jacques Verdu a, Carole Monchy-Leroy b a Arts et Metiers ParisTech, CNRS, PIMM, 151 bd de l’Hôpital, 75013 Paris, France b EDF R&D, MMC, Site des Renardières, 77818 Moret Sur Loing, France

abstract

Thin (60 lm) and thick (250 lm) samples of poly(ether ether ketone) were subjected to radiochemical ageing at 24 kGy h1 dose rate for doses up to 30.7 MGy at 60 °C in air. FTIR spectrophotometry (hydroxyl and carbonyl build-up), ATR microscopy (oxidation proﬁles), ammonia gaseous treatment (determination of carbonyl nature), density, DSC (glass transition temperature, cold crystallization and melting point changes), and gel content measurements (crosslinking) were conducted for examination of polymer degradation. Thin Keywords: samples were shown to undergo principally chain scission process whereas thick ones Poly(ether ether ketone) undergo mainly crosslinking. This difference can be attributed to the kinetic control of oxi- Radiochemical ageing Oxidation dation by oxygen diffusion. A mechanistic scheme was proposed from radiochemical yields Chain scissions estimations. Crosslinking

1. Introduction conditions where oxidation effects are negligible because oxidation is diffusion limited and thus restricted to a very In the aim to lengthen the nuclear plant lifetime with- thin superficial layer. Since dose rates are considerably out replacing expensive materials, and guarantee they lower in practical applications, oxidation is expected to can safely operate beyond their 40 years license, many re- play a non negligible role. Hence, we were interested in searches were conducted on the radiochemical ageing of studying radiochemical ageing in conditions favouring oxi- cable insulation [1] made of rubbers, plasticized PVC or dation (lower dose rate, lower sample thickness), and polyethylene, or polymers of high thermomechanical per- appreciating the differences between anaerobic and oxida- formances such as poly(ether ether ketone) (PEEK). PEEK tive ageing as well from chemical (FTIR, derivation treat- is a semi-crystalline polymer relatively insensitive to comments) as from physical (DSC, density, sol–gel analysis) mon solvents including water. Its amorphous phase has a measurements. glass transition temperature (Tg) close to 140 °C. A relatively high stability in radiochemical ageing is expected due to its aromatic content. PEEK thermal ageing has been 2. Experimental widely studied [1–7] in comparison with radiochemical ageing. Sasuga et al. [8] have studied the effect of irradia- 2.1. Materials tion under vacuum. The changes of mechanical properties and glass transition temperature revealed thus the pre- PEEK (381G grade – VICTREXÒ) was supplied as pellets. dominance of crosslinking. Irradiation in the presence of These latter were extruded to obtain nearly amorphous oxygen was recently studied [9] at high dose rate i.e. in 60 lm thin films using the following procedure: pellets were first dried (3 h, 150 °C) [10] and then processed in a twin screws Brabender extruder (T = 350 °C, * Corresponding author. Address: Arts et Metiers ParisTech, CNRS, zone 1 1 PIMM, 151 bd de l’Hôpital, 75013 Paris, France. Tzone 2 = 380 °C, Tzone 3 = 390 °C at 10 rev min , and torque E-mail address: [email protected] (E. Richaud). between 72 and 75 N m). Extruded films were directly quenched in water so that to obtain quasi-amorphous 2.2.4. Sol–gel analysis samples. Gel content was estimated by immersing samples in hot PEEK was also supplied as 250 lm thick sheaths with an trichloroacetic acid (purity P 99.0%, titration grade sup- internal diameter close to 1000 lm wrapping a copper plied by Sigma Aldrich, melting point c.a. 60 °C) at 70 °C conductor. Their initial density about 1.290 corresponds in closed vessel. About 15 g of acid were used to swell to an initial crystallinity ratio close to 23%. Sheaths were polymer (m0 mass c.a. 20 mg). After 24 h, solution was fil- irradiated with the Cu conductor. It means that only one tered and sample was cautiously washed with distilled surface was in contact with air versus two for films. water. Insoluble fraction mass (mI) was weighted after dry- ing under vacuum (80 °C – 24 h). The gel fraction wI was 2.2. Characterization thus

wI ¼ mI=m0 ð3Þ 2.2.1. FTIR spectrophotometry 2.2.1.1. Transmission. FTIR spectra were recorded in transmission mode on an IFS 28 apparatus (Brucker). Free 2.3. Irradiation standing films spectra were obtained by the averaging of 60 32 scans at a 4 cm1 resolution. Irradiations were performed using a Co c-rays source at 60 °C under 1.5 bar air pressure (‘‘Brigitte” device, SCK CEN, Mol, Belgium) at 24 kGy h1 dose rate. Statistical 2.2.2.1. Attenuated Total Reflexion microscopy. Thickness of chain scission concentration in a Red Perspex sample oxidized layer was estimated by Micro-ATR Spectroscopy (PMMA) allows the total emitted dose to be measured with using a Spotlight 300 apparatus coupled with Spectrum accuracy better than 5%. 100 FTIR spectrophotometer (Perkin-Elmer) driven by Spectrum Image software. Sheaths were embedded in a 3. Results commercial epoxy-amine resin. After curing, samples were polished with 800 and 2400 granulometry disks with a 3.1. Thin samples cold water flow. The sample-resin interface was located using PEEK absorptions at 1485 and 926 cm1. The spectra 3.1.1. FTIR spectrophotometry were recorded at a 16 cm1 minimal resolution with IR spectra of virgin and irradiated samples are shown in 1.56 lm pixel size. Fig. 1. The main changes are:

2.2.3. Gaseous ammonia treatment – a wide band in the 3000–3500 cm1 range with a double NH3 derivatization was used to precise the nature of maxima at 3320 and 3260 cm1 ascribed to hydrogen oxidation products. Ammonia was generated from an equi- bonded (possibly phenolic) hydroxyl groups (denoted molar solution of NH4Cl (13.4 g) and NaOH (10 g) in 100 ml POH in the following), of water. 24 h treatments were performed in a closed ves- – a sharper one in the 1600–1800 cm1 range with a max- sel at room temperature before FTIR analysis. ima at 1725 cm1 attributed to various types of carbonyl groups (denoted by PC = O in the following). 2.2.4. Density measurements Density was measured at 22 °C using a Mohr balance To improve clarity, the spectrum of virgin sample was (Mettler Toledo). Samples ranging from 20 to 50 mg were subtracted to spectra of degraded ones and zooms of successively weighted in air (m1) and in acetone (m2) (sup- respectively hydroxyl and carbonyl regions are presented 3 plied by VWR, density qAcetone = 790 kg m ). PEEK density in Figs. 2a and 3a. Broad hydroxyl band displays a maxi- 1 (qPEEK) was thus assessed from mum at about 3320 cm indicating that OH groups are strongly linked to oxygen atoms and aromatic structures m1 qPEEK ¼ qAcetone ð1Þ m1 m2 4.0 Crystallinity ratio was then obtained using 3.5 qC q qA xC ¼ ð2Þ 3.0 q qC qA 2.5 with qA = 1.260 and qC = 1.400 [11]. 2.0 20.0 MGy 1.5 10.0 MGy 30.7 MGy

2.2.5. Differential Scanning Calorimetry Absorbance 1.0 DSC experiments were carried out on a TA Instrument 0.5 Q1000 apparatus driven by Q Series Explorer software. DSC was previously calibrated with Indium standard. 0.0 40080012001600200024002800320036004000 Approximatively 10 mg of sample in a closed aluminium Wavenumber (cm-1) pan was subjected to a 10 K min1 ramp from 50 to 400 °C (or from 400 to 50 °C) under nitrogen purge with Fig. 1. Full range FTIR spectra of 60 lm amorphous PEEK ﬁlms at 1 a 50 ml min ﬂow rate. different irradiation doses (up to 30.7 MGy). a 0.80 by hydrogen bonds. The negative peak in the 3000– 3050 cm1 indicates a partial consumption of aromatic 0.70 C–H groups. 0.60 A negative peak at 1660 cm1 corresponding to the con- -1 -1 0.50 3320 cm 3260 cm version of initial benzophenone unit into other species [12] 0.40 30.7 MGy is observed. Three carbonyl species absorbing at 1690, 20.0 MGy 1 0.30 1725 and 1775 cm appear with a clear predominance 1 Absorbanc 10.0 MGy of 1725 cm product. 0.20 A gaseous ammonia treatment induced an increase of 0.10 the band intensity in the 3200–3400 region (Fig. 4a) and 0.00 an absorbance decrease in the carbonyl region (Fig. 4b).

3800 3600 3400 3200 3000 As previously established, NH3 converts carboxylic acids -1 Wavenumber (cm ) into carboxylate salts (not observable here because the resulting signal overlaps with virgin PEEK absorptions) b 1.20 and esters to give amides [13,14]. The observed changes 1.00 correspond well to the conversion of initially present aromatic esters (absorbing at 1725 cm1) into amides ) -1 0.80 (stretching N–H vibrations at 3300 cm1). 1 0.60 The growth of hydroxyl groups (at 3320 cm ) and car- y = 0.0356x bonyl ones (making the assumption that these latter are 0.40 R2 = 0.9901 essentially esters) was quantiﬁed using Beer–Lambert law: [POH] (mol l DO 0.20 ½PC ¼ O¼ 1725 ð4Þ e:e1725 0.00

0 5 10 15 20 25 30 35 DO3320 dose (MGy) ½POH¼ ð5Þ e:e3320 Fig. 2. Subtraction IR spectra changes in the hydroxyl region (a) and where kinetic curve of hydroxyl build-up from 3320 cm1 signal growing-up (b). – e is the sample thickness (cm).

a 0.6 a 0.75 1725 cm-1 3475 cm-1 30.7 MGy 20.0 MGy 0.5 0.50 1775 cm-1 10.0 MGy 0.4 0.25 0.3

0.00 0.2 Absorbance Absorbance -1 -0.25 0.1 3060 cm

0.0 -0.50 3800 3700 3600 3500 3400 3300 3200 3100 3000 1850 1800 1750 1700 1650 Wavenumber (cm-1) Wavenumber (cm-1) b Wavenumber (cm-1) b 0.25 1850 1800 1750 1700 1650 1600 y = 0.0064x 0.0 0.20 R2 = 0.9610 ) -1 -0.5 -1 0.15 1725 cm -1.0 0.10 Absorbance

[PC=O] (mol l -1.5 0.05 -2.0 1660 cm-1 0.00 0 5 10 15 20 25 30 35 -2.5 dose (MGy)

Fig. 4. Effect of NH3 treatment on the IR spectrum of 30.7 MGy irradiated Fig. 3. Subtraction IR spectra changes in the carbonyl region (a) and sample (subtracted spectra between treated and untreated sample) in the kinetic curve of carbonyl build-up from 1725 cm1 signal growing-up (b). hydroxyl region (a) and in the carbonyl region (b). – e3320 and e1725 are the molar absorptivities equal to – The glass transition temperature at about 140 °C. 70 l mol1 cm1 [15] for hydroxyl groups and – The cold crystallization at about 170 °C. 600 l mol1 cm1 for carbonyl ones [16–18]. – The melting point at about 340 °C.

The corresponding kinetic plots (Fig. 2b for hydroxyls, 3.1.2.1. Glass transition temperature. Tg decreases almost Fig. 3b for carbonyls) are almost linear. The slope for hy- linearly with irradiation dose (Fig. 6). A linear regression droxyl groups is gave d½POH T ¼ 144:75 2:3 107 d ð12Þ ¼ 3:6 108 mol l1 Gy1 ð6Þ g dd d being in Gy, and Tg in °C. T dependance with molar mass is given by the Fox–Flo- d½POH g and ¼ 107 GðPOHÞI ð7Þ ry equation [19]: dt kFF Tg ¼ Tg1 ð13Þ – d being the dose (Gy) MN 1 – I being the dose rate (Gy s ) 1 kFF being the Fox–Flory constant (K mol kg ). – G being the yield of formation (groups/100 eV) According to Saito [20], the reciprocal average number molar mass varies with both chain scission (s) and cross- Since d ¼ I dt ð8Þ linking (x) concentration:

The radiochemical yield for POH formation is 1 1 ¼ s x ð14Þ M M d½POH N N0 GðPOHÞ¼107 ¼ 0:36 groups per 100 eV ð9Þ dd By neglecting the existence of crosslinking event in a first approach (which can be partially justified by the absence After a 30.7 MGy irradiation, the hydroxyl concentration is of gel content for irradiated films as mentioned below), close to 1.1 mol l1, i.e. 0.87 mol kg1 what corresponds we have thus nearly to one hydroxyl group every four monomer units. The esters grow-up almost linearly with dose: 1 1 Tg0 Tg ¼ kFF ¼ kFF s ð15Þ MN MN0 d½PC ¼ O ¼ 0:64 108 mol l1 Gy1 ð10Þ dd dTg Thus ¼kFF ð16Þ which gives ds

dT 7 d½PC ¼ O dT g GðPC ¼ OÞ¼10 Since g ¼ dd ð17Þ dd ds ds dd ¼ 0:06 groups per 100 eV ð11Þ 7 Carbonyl formation would be thus a relatively rare event and s ¼ 10 GðsÞð18Þ compared to hydroxyl formation. 107 dT Then GðsÞ¼ g ð19Þ 3.1.2. Differential Scanning Calorimetry kFF dd

DSC thermograms of irradiated amorphous ﬁlms are Any PEEK kFF value was reported in literature to our knowl- shown in Fig. 5. Three characteristic transitions are 2 3 edge. Assuming that kFF is proportional to Tg1, Tg1; Tg1 observable: (see ‘Appendix’), converging values ranging between 169

Temperature (°C) 147 100 150 200 250 300 350 400 146 4.0 145 144 2.0 y = -0.2323x + 144.75 143 T R2 = 0.8863 0.0 g 142 (°C) g

initial T -2.0 141 5.0 MGy 140 -4.0 10.0 MGy 139 Heat Flow (°C) Heat -6.0 138 20.0 MGy 137 -8.0 30.7 MGy 0 5 10 15 20 25 30 35 -10.0 dose (MGy)

Fig. 5. DSC thermograms of 60 lm thick amorphous ﬁlms for different Fig. 6. Decrease of glass transition temperature with irradiation dose for exposure doses. 60 lm thick amorphous ﬁlms. and 201 K kg mol1 were calculated. Taking 185 K 345 45 kg mol1, one obtains: GðsÞ¼0:012 340

40 ) 335 -1 (°C)

3.1.2.2. Cold crystallization characteristics. The cold crystal- g (J M M T lization temperature TCF and the corresponding enthalpy 330 H 35 Δ DHCF increase with irradiation dose (Fig. 7). TCF increases slowly from 173 °C to about 174.5 °C at 30.7 MGy. DHCF in- 325 y = -0.6124x + 340.62 creases markedly from nearly 20 J g1 for the virgin ﬁlm to R2 = 0.9766 1 about 34 J g for 30.7 MGy irradiated one. This is not sur- 320 30 prising because the polymer undergoes chain scission 010203040 facilitating the polymer crystallization [21]. dose (MGy)

Fig. 8. Melting point temperature () and melting enthalpy (j) against 3.1.2.3. Melting characteristics. Melting enthalpy under- irradiation dose for quenched 60 lm thick ﬁlms. goes small non-monotonic changes. In contrast, the melting point decreases almost linearly with dose rate (Fig. 8). The slope of straightline in Fig. 8 is 6 dT lC0 0:61 10 M ¼6:1 107 KGy1 ð20Þ i:e: ¼ 1 þ d ð24Þ dd lC TM1 TM0

TM decrease can be attributed to the limiting effect on crys- d being expressed here in MGy. Literature data [22–24] give: T = 663 ± 5 K. talline lamellae thickness lC of radiation induced structural M1 modiﬁcations. According to Gibbs–Thompson equation: l Thus C0 ¼ 1 þ 1:2 108 d ð25Þ 1 2r T lC l ¼ 1 M ð21Þ C q DH T C M1 M1 8 i:e: lC ¼ lC0:ð1 1:2 10 dÞð26Þ where A 30.7 MGy irradiation would induce a 35% decrease of – r is the surface energy between crystalline and amor- lamellae thickness: 2 phous phase (J m ), 8 6 lC ¼ lC0:ð1 1:2 10 30:7 10 Þ0:65:lC0 – TF1 and DHF1 are respectively the melting temperature and the melting enthalpy of an inﬁnite crystal, It means that the positive effect of chain scission on

– qC is the crystalline phase density. crystallinity [25] is compensated and even dominated by the negative effect of irradiation induced structural irregu-

Supposing that TF1, DHF1, qC, and r changes are negligible larities working as comonomers limiting the lamella during ageing, it gives growth. l T T C0 ¼ M1 M ð22Þ 3.1.3. Gel content measurements lC TM1 TM0 Thin films (60 lm) remained soluble in hot trichloroacetic acid even at high irradiation dose. It suggests that l T T i:e: C0 ¼ 1 þ M0 M ð23Þ chain scission is dominating which was already observed lC TM1 TM0 for many polymers when ageing is not controlled by oxygen diffusion (see ‘Section 4’). These results are confirmed by rheometry experiments (not presented here): the New- tonian viscosity decreases as soon as the beginning of 175.0 exposure. 34 174.8 174.6 32 3.2. Thick samples 174.4 30 ) 174.2 -1 3.2.1. FTIR analysis by Attenuated Total Reflection microscopy 28 (J g (J (°C) 174.0 The cross-section of the epoxy embedded irradiated CF CF T 173.8 26 H sheaths was analysed by FTIR in ATR mode. Absorbance Δ 1 173.6 24 at 3320 cm changes with depth gave the profile of 173.4 Fig. 9. Two interesting observations can be made: 173.2 22 173.0 20 1. The hydroxyl concentration profile displays a horizontal 0 5 10 15 20 25 30 35 plateau in the superficial layer. dose (MGy) 2. The thickness of oxidation layer assessed from the POH depth profile is on the order of 50–100 m, depending Fig. 7. Cold crystallization temperature TCF () and enthalpy DHCF against l irradiation dose (j). on the chosen criterion. 0.25 0:007 l ¼ 250 60—70 lm 0:027 0.20 in good agreement with the above micro-ATR results.

0.15 3.2.3. Differential Scanning Calorimetry 3320 Thermograms of 250 lm thick samples are shown in DO 0.10 Fig. 10. In these extruded samples, the initial crystallinity ratio is close to its maximum (25%) what explains the 0.05 quasi-absence of cold crystallization exotherm and the weakness of heat capacity change at T . T values were 0.00 g g 0 50 100 150 200 therefore hardly measurable. Even if a small increase was depth (µm) observed, data scattering prevents from a reliable theoretical exploitation. Fig. 9. Changes of oxidized layer profile for irradiated sheaths: j: The melting peak is displaced slightly towards low tem- 10 MGy, : 20 MGy, N: 30.7 MGy. peratures and tends to split into two components. How- ever, the melting enthalpy remained almost constant. We were interested in studying crystallization from the melt during cooling from 400 to 50 °Cat10°C min1 ramp. 3.2.2. Density measurements Comparison of 30.7 MGy irradiated sample with virgin Density measurements were monitored on irradiated sample (Fig. 11) calls for the following comments: sheaths. An increase from 1.290 for virgin sample to 1.297 for 30.7 MGy irradiated one was measured. 1. In irradiated samples, the first heating melting endo- Since density decrease can be attributed to destruction therm tends to split into two components. An explana- of crystals under irradiation [26,27], our results confirm tion is proposed in the ‘Section 4’ part. Nevertheless, the the stability of PEEK crystals under the employed dose rate. melting enthalpy remained nearly constant, which sug- Polymers being exposed in their glassy state, chains in gests that PEEK crystals are quite stable under the con- amorphous phase cannot integer the crystalline one be- sidered dose rate, in agreement with the absence of cause of restricted mobility. The observed density increase density decrease. cannot be caused by chemicrystallization but rather from 2. The ‘‘hot” crystallization peak of irradiated sample is oxidation since ‘‘heavy” oxygen atoms are grafted. Let us shifted towards lower temperatures by more than denote by M the average atomic mass: A 30 °C and its enthalpy is strongly decreased. This is con- MA ¼ mU=N ð27Þ firmed by the lower values of melting temperature for the second heating. It indicates that lamellae thickness where of crystallites formed from molten state is lower. Indeed, crosslinking prevents chain segments to integer – m is the molar mass of the representative structural U a crystal [28]. unit (RSU). – N is the number of atoms of RSU. 3.2.4. Gel content measurements Thick samples became partially insoluble at a dose d For non irradiated PEEK, one has G such as 5.0 MGy < dG < 10 MGy. From samples having 1 mU0 ¼ 288 g mol undergone sol–gel transition, the Charlesby–Pinner plot [29] was tried: N0 ¼ 34 1 7 MA0 ¼ 8:47 g mol GðsÞ 10 w þ w1=2 ¼ þ ð29Þ S S 2G x The 100% conversion of aromatic rings into phenolic by- ð Þ MW0:GðxÞd products would give

1 Temperature (°C) mU ¼ 304 g mol 0 100 200 300 400 N ¼ 35 0 0.0 1 MA0 ¼ 8:69 g mol -0.5 q q Assuming that 0 ¼ ð28Þ -1.0 M M A0 A -1.5 the total oxidation of amorphous phase would provoke a -2.0 initial density increase from 1.260 to 1.293. For a sheath having 5.0 MGy -2.5 a constant crystallinity ratio close to 25%, the density the- Heat Flow (mW) 10.0 MGy oretically increases from 1.290 to 1.317. The lower ob- -3.0 20.0 MGy served value (1.297) suggests that the degradation is -3.5 30.7 MGy confined in a superficial layer of which depth might be approximated by Fig. 10. DSC thermograms of sheaths at different irradiation doses. So that G(s) 0 a 10.0 298.0°C Let us recall that crosslinking predominates over chain 8.0 scission when 6.0 GðsÞ < 2 ð31Þ 4.0 2GðxÞ 2.0 cooling Thus crosslinking predominates undoubtedly for thick 1st heating samples such as sheaths, whereas chain scission predomi- Heat Flow (mW) Heat 0.0 341.1°C nates in 60 lm thin films. -2.0 2nd heating 342.1°C -4.0 2. The slope of the straightline is: 50 100 150 200 250 300 350 400 Temperature (°C) 1=2 7 dðwS þ wS Þ 10 1 ¼ ð32Þ dd MW0 GðxÞ b 10.0 So that MW0 GðxÞ0:61. 8.0 For determining G(x), MW0 is needed but is unfortu- 6.0 nately not available here. However, molar mass of PEEK general purpose grades is on the order of M 10– 4.0 265.2°C N 1 1 cooling 15 kg mol and MW 25–50 kg mol [2,9,30] so that 2.0 GðxÞ0:012—0:024

Heat Flow (mW) Heat 0.0 1st heating This result can be tentatively checked from the Saito’s the- -2.0 2nd heating ory [20]: 319.1°C 327.5°C -4.0 1 1 s 50 100 150 200 250 300 350 400 ¼ 2x ð33Þ Temperature (°C) MW MW0 2

Fig. 11. DSC thermograms of virgin (a) and 30.7 MGy irradiated (b) The crosslinking concentration at the gelation point sheaths during temperature cycles: heating (50–400 °C), cooling (400– (MW ? 1) in the absence of chain scission (s = 0) is equal 50 °C), heating (50–400 °C). to where 1 xG ¼ ð34Þ 2MW0 – wS =1 wI is the soluble fraction, 7 – G(s) and G(x) are the respective radiochemical yields for xG ¼ 10 GðxÞdG ð35Þ chain scission and crosslinking. From gelation point measurement, it is reasonable to con- The three points (Fig. 12) are close to a straightline sider that dG (7.5 ± 2) MGy, so that intercepting the origin with a slope close to 1.6 107 107 Gy1. One deduces: MW0 GðxÞ¼ ð36Þ 2 dG 0:53 6 M GðxÞ 6 0:91 1. From the ordinate value of intercept at the origin: W0 GðsÞ These values are in good agreement with the starting w þ w1=2 ¼ 0 ð30Þ S0 S0 2GðxÞ hypothesis. Although this can not be considered as rigor- ous evidence, it at least indicates a certain consistency in our results. 2.00 y = 1.660E+07x - 2.086E-02 R2 = 9.944E-01 4. Discussion 1.50 PEEK radiochemical ageing in air has been followed on 1/2 S 1.00 60 lm ﬁlms and 250 lm sheaths. Opposite trends in thin + w

S films (predominant chain scission) and in thick sheaths w (predominant crosslinking) were observed. Irradiation cre- 0.50 ates macroradicals reacting mainly by coupling giving thus crosslinking in the absence of oxygen (i.e. for irradiation 0.00 under vacuum or at high dose rate [31] at which thickness 0.0E+00 2.0E-08 4.0E-08 6.0E-08 8.0E-08 1.0E-07 1.2E-07 of oxidized layer is very low). When oxygen is present, it adds very fastly to macroradicals, inhibiting thus coupling 1/δ (Gy-1) and orientating the process towards chain scission [32]. Fig. 12. Radiochemical yields G(s) and G(x) estimation from Charlesby– The thickness of oxidized layer (denoted by l) can be Pinner approach. approximated by: rffiffiffiffi D H l ¼ 2 ð37Þ H k + H°

° where

– D is the oxygen diffusion coefficient in the polymer, + – k is the first-order (or pseudo first-order) rate constant of oxygen consumption (i.e. the ratio of oxygen con- ° sumption rate and oxygen equilibrium concentration). ° Additions lead also to crosslinking. The resulting cyclohexa- l is expected to decrease with this dose rate (k increase) dienyl radical is resonance stabilized and probably does not and crystallinity ratio (D decrease). Overall, our observa- propagate the reaction. tions suggest: 60 lm<2l < 250 lm. In other words, films would be almost fully oxygenated so that they would un- The simplest model for PEEK ageing under irradiation in dergo predominant chain scission. In contrast, thick sam- anaerobic conditions could be based on the following ples would be oxygenated only in a relatively thin mechanistic scheme: superficial layer (let us recall that only one wall of sheaths is in contact with atmospheric air, the other being in con- ðIÞ PHðpolymerÞþht ! P ðradicalsÞþ1=2H2 ri tact with copper conductor) and would undergo predomi- ¼ 107 GðP ÞI nant crosslinking in their bulk (Fig. 13). This interpretation is supported by ATR microscopy results. The heterogeneity 0 in thick samples can explain at least partially the splitting ðII Þ P1 þ P2H ! P1H þ P2 kp of melting peak (Fig. 11). The low temperature shoulder (Fig. 8) could be attributed to the oxidized layer where ðIIIÞ P þ P ! P—P þ x ðcouplingÞ k4 the melting point decreases with irradiation dose. In the above scheme, reaction II is only a transfer of the C–H bonds radio-cleavage is a likely reason of crosslinking: radical site without direct incidence on kinetics. However, it plays a major role in allowing the reactive site to diffuse + hν + H° by the so called valence migration process [33] favouring ° thus termination despite the relatively low segmental mobility 100 K below polymer glass transition temperature. Plot of radical concentration versus dose from ESR measurements at 77 K on irradiated PEEK samples [34] displays a negative concavity indicating the existence of a ter- + mination process. ° ° G(P°) can be estimated from: H° radicals abstract probably hydrogen atoms, leading to H 2 1. Kinetic analysis of Heiland’s result [34]. The rate law is: evolution.

+ H° + H d½P 2 2 ¼ r 2k ½P ð38Þ dt i 4 ° ri At steady state : ½P 1 ¼ ð39Þ The existence of additions cannot be totally excluded: 2k4

a bc ∞ ∞ MW MW MW

MW0 MW0 MW0

l l l

0 2L 0 2L 0 2L

Fig. 13. Schematic representation of the MW proﬁles in a thick PEEK sample submitted to irradiation in the presence of oxygen before (full line) and after irradiation (dashed line). (a) Case of the ﬁlms (two walls in contact with air). (b) General case (two walls in contact with air). (c) Case of sheaths investigated in this study (one wall in contact with air). 1 d½P Taking DHM1 = 37500 J mol [22], G(y) 0.012 is ob- So ¼ 2k ð½P 2 ½P 2Þð40Þ dt 4 1 tained. The fact that this value is lower than radical formation yield indicates that only a few part of radical affects the crystalline morphology. expð4k4½P 1tÞ1 The integration gives ½P ¼ When it is present, oxygen reacts fastly with primary expð4k4½P 1tÞþ1 aryl radicals to give peroxyl ones, preventing them from P 41 ½ 1 ð Þ crosslinking. Peroxy radicals are not very reactive, neither ! for hydrogen abstracting nor for adding to aromatic d½P nuclei. In contrast, they can participate to bimolecular Graphical measure gives : ¼ r dt i combinations giving relatively more reactive phenoxy t!0 radicals: 2:5 108 mol l1 s1 ð42Þ

So that G(P°) 0.25. It is noteworthy that this value is considerably lower than for aliphatic polymers, for instance + O2 G(P°) = 8 for PE [35] what illustrates the well-known stabi- ° lizing effect of aromatic groups [36–39]. OO°

2. In the absence of other termination (or radical rearrang- ing) reactions, the kinetic analysis of the scheme leads 2 O O O O to the following expression of the crosslinking rate: OO° dx r ¼ i ¼ k ½P 2 ð43Þ dt 2 4 2 + O2

G GðP Þ Thus GðxÞ¼ i ¼ ð44Þ O° 2 2

Since : GðxÞ¼0:012—0:024 Hydrogen abstraction by phenoxy radicals will give a phenolic group: so that GðP Þ0:024—0:048 The fact that its yield (0.024–0.048) is 5–10 times lower O + PH O than the radical yield (0.25) would indicate the existence of other termination than coupling, corresponding to 80– O° O H 90% of the whole termination events. The formation of (A) dibenzofuran derivates [7] could be a possible explanation but this question remains open. Irradiation is expected to create crystals defects O decreasing the melting point. After 30.7 MGy irradiation, C + PH C 15 K drop is effectively observed for the melting endo- O therm (Fig. 10). According to Flory: O° O H 1 1 R ¼ lnð1 yÞð45Þ (B) TM TM1 DHM1 OH groups are expected to be intramolecularly hydrogen where T is the equilibrium melting point, DH the M1 M1 bonded through a 5 or 6 members cycle. Orthohydroxy- melting enthalpy of inﬁnite crystal and y the molar frac- benzophenones of (B) type are strong UV absorbers [40] tion of crystalline defects. This relationship can be trans- what explains the observed yellowing (curves are not pre- posed into sented in this paper). 1 1 R 1 y Chains scission could be tentatively explained by rear- ¼ ln ð46Þ TM TM0 DHM1 1 y0 rangements of a small part of phenoxy radicals, for instance: TM0 and y0 corresponding to the initial state. Assimilating the initial molar fraction of defects to the initial molar frac- O O + ° tion of monomers units in the amorphous phase (y0 = 0.75), and derivating the above expression, one obtains O° O dy DH dT (C) ¼ M1 ð1 y Þ M ð47Þ dd 2 0 dd RTM0 Conjugated diketones (C) are also expected to be strongly absorbing in the near UV and at c.a. 1690 cm1 DH dT and GðyÞ¼107 M1 ð1 y Þ M ð48Þ (see FTIR results). Primary chain scissions can also be 2 0 dd RTM0 envisaged: The boundary conditions are: O O C + hν C° + ° –att =0: [P°]0 = [POO°]0 = 0, [POOH] = [POOH]0, [PH] = [PH]0 [45], –att = 0, every x:[O2]=[O2]S, –atx = 0, at every t:[O]=[O ] (sample surface). O + hν O° + ° 2 2 S

–DO2 is the oxygen diffusion coefﬁcient in PEEK amorphous phase. The compilation by Van Krevelen [46] 11 2 1 In the presence of oxygen, a new mechanistic scheme must suggests DO2 1 10 m s (assuming that PEEK be used. The simplest one is very close to the Basic amorphous phase behaves as bisphenol A polycarbonate Autooxidaiton Scheme extensively studied by Gillen and one). Clough [41–43]. It tries to take into account the above –[O2] the oxygen concentration in amorphous phase reactions: given by Henry’s law:

ðIÞ PHðpolymerÞþht ! P ðradicalsÞþcS sri ½O2C ¼ sO2 PO2 ð53Þ 7 ¼ 10 GðP ÞI – sO2 is the oxygen solubility in PEEK amorphous phase. The compilation for glassy amorphous polymers [39] 8 8 ðIIÞ P þ O2 ! POO k2 suggests that it ranges between 2.10 and 4.10 mol l1 Pa1. 0 ðII Þ P1 ! P2 þ cX xkP0 Two asymptotic kinetic regimes can be deﬁned: 1 ðIIIÞ POO þ PH ! POOH þ P k3 — At very low oxygen pressure; i:e: : ½O2S << b ð54Þ

2 ðIVÞ P þ P ! inactive products þ x ðcouplingÞ k4 @ ½O one has in steady state : r D 2 O 55 OX ¼ O2 2 ¼ a½ 2ðÞ @x ðVÞ P þ POO ! inactive productsþ x ðcouplingÞ k5 1 — Under oxygen excess : ½O2 >> b ð56Þ S ðVIÞ POO þ POO ! inactive products k6 @2½O a one has in steady state : r D 2 57 OX ¼ O2 2 ¼ ð Þ – The reaction (VI) is a kinetically equivalent to a more @x b complex mechanism of POO° + POO° termination The transition between these regimes is very progressive. involving, as mentioned above, PO° radicals and their One can define arbitrarily a ‘‘critical oxygen concentration” decomposition into ketones, alcohols, chain scissions in the polymer amorphous phase separating the two do- etc. [44]. 1 mains: [O2]C = qb with q 10. For [O2]S >[O2]C, it is con- 0 – The (II ) reaction corresponds to valence migration or sidered that oxygen is in excess and scavenges every P° cyclohexadienyl formation as previously proposed. radical to give a POO°. Termination results therefore only from POO° + POO° reaction [47]. In this regime, the oxida- Although this scheme is an oversimplification, it pro- tion rate rOX = a/b is independent of the oxygen concentra- vides a way to determine some kinetic parameters charac- tion. The hydroxyl concentration profile (Fig. 9) would thus terizing the PEEK oxidation. In a thick sample, oxidation is indicate that [O2]>[O2]C in the plateau length lP expressed kinetically controlled by oxygen diffusion. Hence, the var- by: iation of the oxygen concentration in a depth x elementary rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi layer would be given by: 2DO b l ¼ 2 ð½O ½O Þ ð58Þ P a 2 S 2 C 2 @½O @ ½O 2 ¼ D 2 k ½P ½O þk ½POO 2 ð49Þ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi @t O2 @x2 2 2 6 2DO2 b ½O2C lP ¼ ½O2S 1 ð59Þ The analytical solution of this scheme in steady state using a ½O2S 2 the classical assumption k5 =4k4k6 gives the expression of Except for a coincidence : O << O : 60 oxidation rate rOX: ½ 2C ½ 2S ð Þ

2 @ ½O a½O 2 r D 2 2 50 b l OX ¼ O2 2 ¼ ð Þ P @x 1 þ b½O2 so that ¼ ð61Þ a 2 DO2 sO2 PO2 rffiffiffiffiffiffiffiffi ri Using the above mentioned oxygen transport parameters, where a ¼ k2 ð51Þ 6 7 2k4 and lP 50 lm, one obtains b/a 10 –10 and the oxygen consumption rate in regime of oxygen excess: sffiffiffiffiffi rffiffiffiffiffi k2 k6 a ri 6 7 1 1 and b ¼ ð52Þ rS ¼ ¼ k3½PH 10 —10 mol l s ð62Þ k3½PH k4 b k6 where: This condition is here largely fulfilled. By supposing that crosslinking is totally inhibited in well-oxygenated sam-

– k3 is the rate constant for POO° +PH? POOH + P° ples, molar mass variations are given by: propagation, 1 1 7 – k is the rate constant for POO° + POO° ? inactive prod- ¼ s ¼ 10 GðsÞd ð67Þ 6 M M ucts termination, N N0 – [PH] is the oxidizable substrate concentration: As previously found: G(s) 0.012 1 [PH] 13 mol l , 1 1 9 ¼ þ 1:2 10 d ð68Þ – ri is the initiation rate depending on the radiochemical MN MN0 yield of radical formation G(P°): Amorphous PEEK embrittlement was observed at about 7 ri ¼ 10 GðP ÞI ð63Þ 8 MGy dose [52] so that G(P°) value can be estimated from the curves of alkyl 1 1 1 radical growth (G(P°) = 0.25), sol–gel analysis (G(P°)= s ¼ 0:01 mol kg M M 0.024–0.048), or the rate of formation of all stable oxida- N N0 tion products: since the radical yield of initiation cannot Since embrittlement is linked to the existence of a critical be lower than the radiochemical yield of product forma- 0 chain length (MN = MC), it comes tion, G(P°) P G(OH) + G(CO), i.e. G(P°) P 0.42. Let us note that a better agreement between this estimation and the 0 MN0 MC ¼ ð69Þ one from ESR measurement (0.25) could be obtained using 1 þ 0:01:MN0 a higher value of molar absorptivity for hydroxyl products. For example, 90 l mol1 cm1 was proposed by Carlsson As previously proposed, the following inequality seems [48]. According to our own experience on phenolic antiox- reasonable: idants in PP [49], a value close to 100–110 l mol1 cm1 10 kg mol1 6 M 6 25 kg mol1 could even be envisaged. N0 7 Keeping G(P°) = 0.25, one obtains: ri = 1.7 10 1 1 1=2 4 5 1/2 1/2 It comes : 8 kg mol1 6 M0 6 20 kg mol1 mol l s and k3/k6 =2 10 –2 10 l mol C 1/2 1=2 s . k3/k6 parameter characterizes the intrinsic substrate oxidizability independently of the initiation mode. The number of chain scissions per initial number average PP value (8 104 l1/2 mol1/2 s1/2 at 60 °C) has the same chain is order of magnitude than PEEK one. This relatively surpris- C s M 102 M 70 ing result can be explained as follows: ¼ N0 ¼ N0 ð Þ Propagation reaction POO° +PH? POOH + P° is proba- 0:1 6 C 6 0:25 bly not diffusion controlled because every POO° radicals can ﬁnd substrate molecules in its immediate vicinity. which means that embrittlement occurs at very low con-

Thus, k3 depends on the intrinsic reactivity of POO° to- version of the chain scission process, i.e. less than one scis- wards C–H bond i.e. on the dissociation energy of the bro- sion per 4 chains. ken C–H bonds [50]. Given C–H bond are considerably 1 stronger for aromatic (DHl 400–450 kJ mol ) [51] than 5. Conclusions 1 aliphatic C–H (DHl 380 kJ mol ) compounds [48], one expects PEEK radiochemical behaviour can be ﬁrst explained by its aromatic character which is doubly favourable: k3ðPEEKÞ << k3ðPPÞð64Þ

Termination reaction between two POO° involves rela- – Primary radiochemical events, presumably radiolytic C– tively rare species and is thus expected to be diffusion con- H bonds cleavage, have a radiochemical yield less than trolled, i.e. dependent on the polymer segmental mobility. 0.5, i.e. 5–50 times lower than for aliphatic polymers, Since the PP amorphous phase is in rubbery state whereas because aromatic groups are able to dissipate a great the PEEK one is in glassy state, and since termination is part of the absorbed energy into reversible processes minimised by the impossibility of radical disproportionna- (fluorescence, phosphorescence ...). As a consequence, tion, one expects PEEK belongs to the category of relatively stable polymers under irradiation. k6ðPEEKÞ << k6ðPPÞð65Þ – The propagation rate constant of hydrogen abstraction Thus, the difference in termination rates compensates the by peroxy radicals (k3) is a decreasing function of the difference in propagation ones. C–H bond dissociation energy, that takes a high value Let us now consider the consequences of polymer oxi- for aromatic hydrogen. dative degradation. The radiochemical yield G(s) of primary chain scission would be small enough to permit These positive effects are partially counterbalanced by predominant crosslinking in anaerobic conditions. Accord- the relatively low termination rate linked to the low seg- ing to Saito’s equations [20], crosslinking predominates if: mental mobility in glassy state. It is noteworthy that P° +P° termination (which leads to crosslinking when oxy- GðsÞ < 4 GðxÞð66Þ gen is lacking) is more efficient than expected because P° radicals can diffuse independently of segmental mobility – Vend is almost independent of the polymer structure. by the valence migration process (P1 +P2H ? P1H+P2). Chain scissions were found to predominate in well-oxy- Here, PEEK is compared to the PC. One can write genated samples (60 lm thickness). A Fox–Flory constant ðk Þ q ðT Þ value derived from theoretical considerations enabled FF PEEK ¼ PEEK g1 PEEK ð72Þ k T determination of the radiochemical yield of chain scission ð FFÞPC qPC ð g1ÞPC for PEEK: G(s) 0.012. Although this value is relatively where low, its consequences on glass transition decrease are dra- matic: T 7 K for a 30.7 MGy irradiation. 1 D g –(kFF)PC = 187 K mol kg [55], 3 In the conditions under study (oxygen pressur- – qPC is close to 1200 kg m , e = 3.0 104 Pa), oxidation kinetics is in the oxygen excess 3 – qPEEK (for the amorphous phase) is close to 1260 kg m , regime in the superficial layers. It means that P° radicals –(Tg1)PC is equal to 434 K [52], are fastly converted into POO° ones so that crosslinking –(Tg1)PEEK is unknown but is probably close to 420 K. resulting from P° +P° termination reactions is inhibited. 1 Results from ATR microscopy confirmed that oxidation which gives (kFF)PEEK = 190 K mol kg . is diffusion limited in thick samples (250 lm) where cross- 2. The second is derived from a classical copolymer law linking predominates in the sample core. Let us precise in which chain ends are considered as comonomers: that measurements of Thickness of Oxidized Layer for PEEK M M were not available in literature to our knowledge whereas N ¼ N þ 2b ð73Þ this feature is fundamental for understanding the ageing Tg Tg1 behaviour [53]. Here, it was used to assess some kinetic parameters characterizing the oxidation of PEEK using Tg1 i:e: Tg ¼ ð74Þ the yield of radical formation estimated from the radical 1 þ 2bTg1 M growth kinetics under irradiation at 77 K. N 1 b being the chain end contribution to MNTg . Classical Acknowledgements approximations give 2 2b Tg1 EDF R&D is gratefully acknowledged for financial Tg ¼ Tg1 ð75Þ M support. N Hans Ooms and Benoit Brichard from SCK CEN Lab (Mol, so that k ¼ 2b T2 ð76Þ Belgium) for having performed irradiations. FF g1 Serge Colombier, Thierry Jorand, Jean-Louis Pons (Pirelli Assuming that b is structure independent in a reasonably Prysmian, Sens, France) for having supplied PEEK pellets restricted polymer family, one can write and sheaths. ðk Þ ðT Þ 2 FF PEEK ¼ g1 PEEK ð77Þ ðkFFÞPC ðTg1ÞPC Appendix A. Fox–Flory constant 1 i:e: ðkFFÞPEEK ¼ 178 K mol kg Three ways were implemented to assess the PEEK Fox– 3. The third way is an empirical relationship proposed by Flory constant kFF: Bicerano [52]: 1. The first one is derived from the ‘‘free volume” the- 3 ory. If T is the T value for an hypothetical polymer of T g1 g T ¼ T 0:002715 g1 ð78Þ infinite molar mass, the volume excess of a finite molar g g1 MN mass polymer at Tg is proportional to the number of chain 1 1 ends, that gives The direct calculation gives: (kFF)PEEK = 201 K mol kg . 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