M Pliquet

M Rapeaux

F Delange

D Bourgogne

J L Gardette

S Therias

P O Bussiere

Multiscale analysis of thermal degradation of polyamide 6,6 -Influence of temperature on oxygen diffusion-limited oxidation profiles

Keywords: Polyamide 6, 6, Thermo-oxidation, profile, AFM (atomic force microscopy), diffusion-limited oxidation (DLO)

scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

1.

Introduction

Aliphatic polyamides are engineering polymers. They can be used in various fields, such as textiles, automotive, and electrical insulation. Certain applications require high-temperature exposure, which can reduce the performance of the material during its service life. The thermal oxidation of polyamides in the bulk is a widely studied process since polyamides are commonly used in applications that require long-term/high-temperature exposure. [START_REF] Shu | Study on the long-term thermal-oxidative aging behaviour of polyamide 6[END_REF] [START_REF] Grigg | Thermo-oxidative degradation of polyamide 6[END_REF] It is known that polyamide 6,6 oxidation is a chain radical mechanism that starts with the removal of the most labile hydrogen in the α-position to the amide nitrogen. Regarding the basic autoxidation scheme, hydroperoxides are the first oxidative products. [START_REF] Bolland | Kinetic studies in the chemistry of rubber and related materials. I. The thermal oxidation of ethyl linoleate[END_REF] [START_REF] Sagar | Autoxidation of n-alkyl-amides. Part II. N-alkyl-amide hydroperoxides and di-N-alkyl-amide peroxides[END_REF] The decomposition of α-amino hydroperoxides then leads to the formation of imides, carboxylic acids, amines and aldehydes. [START_REF] Sagar | Autoxidation of n-alkyl amides. Part III. Mechanism of thermal oxidation[END_REF] [START_REF] Gijsman | Differences and similarities in the thermooxidative degradation of polyamide 46 and 66[END_REF] Changes in the chemical structure can cause a degradation of the macromolecular architecture [START_REF] Okamba-Diogo | Investigation of polyamide 11 embrittlement during oxidative degradation[END_REF] [START_REF] Okamba-Diogo | Molecular and macromolecular structure changes in polyamide 11 during thermal oxidation[END_REF] and microstructure [START_REF] El-Mazry | Thermal oxidation kinetics of additive free polyamide 6-6[END_REF] [START_REF] Jain | Effect of thermal ageing on nylon 6,6 fibres[END_REF] and a loss of the functional properties. [START_REF] Pavlov | Physical transformations and change in mechanical properties on thermal ageing of aliphatic polyamides[END_REF] [START_REF] Forsstrom | Thermo oxidative stability of polyamide 6 films. I. Mechanical and chemical characterization[END_REF] Beyond chemical changes, oxygen permeation can also spatially affect the degradation of polymer materials. [START_REF] Celina | Oxygen diffusivity and permeation through polymers at elevated temperature[END_REF] [START_REF] Clough | Oxygen diffusion effect in thermally aged elastomers[END_REF] When the oxygen consumption rate is higher than the oxygen permeation, oxidation then becomes limited by oxygen diffusion (DLO). [START_REF] Mc | Review of polymer oxidation and its relationship with materials performance and lifetime prediction[END_REF] [START_REF] Quintana | Overview of DLO modelling and approaches to predict heterogeneous oxidative polymer degradation[END_REF] As a result, an oxidation profile across the material thickness develops and is indicative of heterogeneous oxidation. Oxygen is primarily consumed during surface oxidation and thus does not penetrate deeper layers in the material.

Various experimental techniques are employed to investigate the oxidation heterogeneity in polymers, including infrared microspectroscopy [START_REF] Celina | Correlation of chemical and mechanical property changes during oxidative degradation of neoprene[END_REF] [START_REF] Mailhot | Depth profiling by AFM-nanoindentations and micro-FTIR spectroscopy for the study of polymer ageing[END_REF], chemiluminescence [START_REF] Ablblad | Imaging chemiluminescence instrument for the study of heterogeneous oxidation effects in polymers[END_REF] [START_REF] Celina | Heterogeneous and homogeneous kinetic analyses of the thermal oxidation of polypropylene[END_REF] and oxygen absorption measurements. [START_REF] Billingham | Autoxidation of poly(4-methyl-pentene-1). I. The role of diffusion in autoxidation kinetics[END_REF] These techniques allow for the characterisation of chemical changes induced by oxidation. Novel analytical methods are required to characterise degradation at the upper levels. Atomic force microscopy (AFM) is an advanced technique that determines material properties with nanometric precision. In this study, AFM is used to measure thermal and mechanical properties. Recent studies using AFM to characterise the changes in material properties due to ageing have suggested that it can be an efficient tool. [START_REF] Larché | Photooxidation of polymers: Relating material properties to chemical changes[END_REF] [START_REF] Larché | Chemical structure evolution of acrylic-melamine thermoset upon photo-ageing[END_REF] In this paper, oxidation profiles of PA 6,6 thick samples were determined using different techniques that characterise the material changes from molecular to macroscopic scales. The aim was to determine pertinent degradation probes to validate the multiscale approach protocol.

2.

Experimental methods

Materials

The experimental raw material was commercial grade polyamide 6,6 supplied by DuPont™ (Zytel ® 101L). The samples were injection moulded directly into plates (80 x 80 mm) with a thickness of 1.5 mm. After moulding, the materials were kept in specific packs to prevent any moisture uptake before thermal ageing.

Oven ageing

The thermo-oxidative ageing was carried out in a forced air venting oven (Memmert UF30) at temperatures between 140 °C and 200 °C. The samples were collected after different time intervals up to 3000 h. A minimum of three samples were prepared and characterised per ageing time to ensure reproducibility of the measurements.

Characterisation techniques

FTIR microscopy

Infrared microspectroscopy was used to measure the oxidation profiles on the PA 6,6 plates.

The specimens were 20 µm-thick films from the cross section of oxidised PA 6,6 that were cut using a LEICA RM2165 microtome with tungsten carbide blades. The micro-FTIR analyses were performed on a Nicolet 6700 spectroscope coupled with a Nicolet continuµm FTIR microscope (Thermo Scientific). The dimensions of the infrared beam were a 30 µm length and 100 µm height. The infrared beam scanned the cross-sectional film every 20 µm.

The spectra were collected for 128 scans at a resolution of 4 cm -1 .

AFM nanoscale thermal analysis (VITA)

Changes in the melting temperature with ageing were followed by an AFM nanoscale thermal analysis system. Measurements were performed on the cross section of oxidised PA 6,6 to determine the melting temperature profile. The technique used a thermal probe to heat a small region on the sample slice and locally measure its thermal properties, such as the glass transition temperature or melting temperature. During the measurement, the probe was fixed at a specific thickness. When the probe temperature is increased, the sample heats and undergoes progressive thermal expansion. With the expansion, the probe pushes up, which leads to a positive cantilever deflection. At a transition temperature (such as T m or T g ), a material softens, allowing the probe to penetrate into the sample. The penetration causes a decrease in the deflection of the cantilever. To determine the transition temperature, the deflection signal is plotted as a function of the probe temperature. The maximum of the curve corresponds to this transition temperature.

The VITA system contained a probe (tip + cantilever) connected to an electric circuit. The probe was heated by the Joule effect by applying an electric current on the system. A calibration was mandatory to correlate the applied tension to the corresponding temperature.

The probe was calibrated with three standard polymers with known melting temperatures: polycaprolactone (T m = 55 °C), polyethylene (T m = 116 °C) and polyethylene terephthalate (T m = 235 °C). The spring constant of the VITA probe was between 0.5 N/m and 3 N/m, and its cantilever was 200 µm long.

Measurements were performed on the profiles of non-aged and aged PA samples to determine changes in the thermal properties with thermo-oxidative ageing. [START_REF] Claudé | Etude de la durabilité d'un système bicouche polymère utilisé pour le vitrage plastique[END_REF] After cross section cutting, a mapping of the melting temperature variation was performed between depths of 0 and 350 µm. For each depth, nine measurements were performed. A distance of 15 µm was fixed between two measurements points to prevent the thermal expansion effect. During these experiments, the probe was heated from 30 °C to 350 °C at a heating rate of 5 °C/s. AFM images of the zone under study were collected before and after testing to observe the shape and the dimensions of the hole left behind the probe during melting.

AFM nanoscale mechanical measurements

The PeakForce QNM mode in AFM was used to determine mechanical properties, such as the modulus, adhesion, roughness and deformation, with a nanometric resolution. The AFM measurements were performed with a Bruker Multimode 8 model equipped with a Nanoscope 9.20 in tapping mode. The sample was placed on a piezoelectric, which assured XYZ displacement. Topographic images were collected with a probe on silicon from Bruker. The RTESPA 300-30 model was used that had a curvature radius of 30 nm and a spring constant near 40 N/m. The topographic images were processed to determine the roughness (R q )

according to equation 1:

Eq. 1 where z t is the current height value and N is the number of points within the considered area.

While obtaining the topographic images, force curves were collected at every pixel of the image. The analysis of the force curve data with the Nanoscope 9.20 software provided the mechanical properties. For the study, the stiffness was tracked by the reduced Young's modulus E * obtained in the DMT modulus images. The model used to calculate E * was provided by Derjaguin et al. [START_REF] Derjaguin | Effect of contact deformation on the adhesion of particles[END_REF] Measurements were performed on cross-sections of PA 6,6 samples in VITA mode to determine the mechanical properties in the bulk of the material. Images with dimensions of 5 µm x 5 µm at a scan rate of 0,5 Hz with 512 scan lines were captured. The mapping was conducted by collecting images between depths of 0 and 325 µm. At each depth, five areas were measured, and 3 images were collected in each measurement area. For probe calibration, the sample must be clean and hard with a known mechanical modulus. The probe was calibrated with silicon (E = 130-185 GPa). A quantitative measurement of the reduced Young modulus also requires a calibration of the cantilever spring constant and piezoelectric scanner movement. As a polymer surface is rarely flat and the tip vertex geometry is not exactly spherical, this can induce calculation errors not accounted for in the Derjaguin-Muller-Toporov (DMT) model used to determine the mechanical module [START_REF] Young | The use of the PeakForce TM quantitative nanomechanical mapping AFM-based method for highresolution Young's modulus measurement of polymers[END_REF]. This is why the study was limited to the measurement of the relative variation of the stiffness. [START_REF] Smolyakov | AFM PeakForce QNM mode: Evidencing nanometre-scale mechanical properties of chitin-silica hybrid nanocomposites[END_REF][40]

3.

Results and discussion

Chemical structure

Infrared microspectroscopy was used to investigate the distribution of the chemical changes in the thickness of PA 6,6 plates induced by thermal degradation and determine the influence of temperature on this distribution. Samples of 1500 µm thickness were aged in thermo-oxidative conditions for different times and temperatures. After ageing, the distribution of the oxidation products through the thickness of samples aged at 140 °C, 160 °C, 180 °C and 200 °C were monitored. The distribution of the oxidation products was recorded at 1734 cm -1 which reflects the formation of imides. [START_REF] Okamba-Diogo | Molecular and macromolecular structure changes in polyamide 11 during thermal-oxidation -Kinetic modelling[END_REF] The oxidation profiles which were obtained are displayed in The curves in Fig. 1 show an oxidation depth profile that reflects a diffusion-controlled oxidation process (DLO). [START_REF] Gijsman | Influence of temperature and stabilization on oxygen diffusion limited oxidation profiles of polyamide 6[END_REF] The aged samples present a typical skin-core oxidation profile with the formation of oxidation products at the surface of the sample while the core remains unchanged. The profiles are symmetric with the formation of imides at the surface, which increases progressively over time. The profile is marked, and the oxidation becomes more superficial as the ageing temperature increases. The oxidation depth can be defined as the distance from the surface at which no oxidation products are formed. [START_REF] Ahlblad | Oxidation profiles of polyamide 6,6 studied by imaging chemiluminescence and FTIR[END_REF] In Table . 1, the oxidation depth is indicated for when the absorbance at the surface exceeds 1 for each exposure temperature. The data in Table . 1 clearly indicate that the thickness of the oxidised layer is temperature dependent. A decrease in oxidation depth is observed when the ageing temperature rises, which is in accordance with the results in the literature. [START_REF] Gijsman | Influence of temperature and stabilization on oxygen diffusion limited oxidation profiles of polyamide 6[END_REF][44] [START_REF] Langlois | Thermooxidative ageing of crosslinked linear polyethylene : Stabilizer consumption and lifetime prediction[END_REF] The increase in temperature and ageing time enhances the skin-core oxidation profile. The development of diffusion-limited oxidation profiles is the result of competition between the oxidation rate and oxygen diffusion. [START_REF] Gillen | Rigorous experimental confirmation of a theoretical model for diffusion-limited oxidation[END_REF][47] Consequently, as the oxidation rate increases faster than the oxygen diffusion with temperature, one can suggest that the total amount of oxygen is consumed at the surface of polymer plates and then cannot diffuse deeply in the bulk when the ageing temperature increases.

Crystallinity

Infrared spectroscopy has also been used to investigate the microstructure properties of a polymer, such as the crystallinity and crystalline morphology. [START_REF] Sandeman | Crystallinity studies of polyamides by infrared, specific volume and X-ray methods[END_REF] In the literature, various IR bands have been assigned to the crystalline and amorphous phases in PAs. [START_REF] Vasanthan | Crystallinity determination of nylon 66 by density measurement and Fourier transform infrared (FTIR) spectroscopy[END_REF][50] The bands at 906, 936 and 1200 cm -1 are attributed to the crystalline phase, while the bands at 924 and 1136 cm -1 are assigned to the amorphous phase. [START_REF] Arimoto | α-γ transition of nylon 6[END_REF][56] The bands at 936 and 924 cm -1 that correspond to the C-C=O stretching in the respective crystalline and amorphous phases overlap. A deconvolution is necessary to distinguish the absorbance of the two bands. For simplicity, the bands at 906 (am) and 1136 cm -1 (cry), which are directly accessible in IR spectra, were chosen to determine the amount of crystalline material. The crystallinity ratio determined by FTIR spectrophotometry was calculated according to the following equation:

Eq. 3

where A cry and A am are the absorbance of crystalline and amorphous bands, respectively, and α is the ratio between both molar extinction coefficients:

Eq. 4

According to the literature, the coefficient α is equal to 0,57 for the selected band ratio. [START_REF] El Mazry | A new kinetic model for predicting polyamide 6-6 hydrolysis and its mechanical embrittlement[END_REF] The crystallinity as a function of the thickness were determined before ageing and after ageing for 100 h at 180 °C. The resulting crystallinity depth profiles are displayed in Fig. 2.

Fig. 2 Crystallinity depth profiles determined by FTIR microspectroscopy for a sample before ageing (■) and for two samples aged at 180 °C for 100 h with (▲) and without oxygen (Δ).

Interestingly, it should be noted that the relative crystallinity determined by infrared spectroscopy in the bulk (≈ 27 %) for the sample before ageing is close to the degree of crystallinity measured by DSC (≈ 30 %) [START_REF] Pliquet | Multi-scales analysis of thermal degradation of polyamide 6,6: Correlating chemical structure to mechanical properties[END_REF], which confirms the relevance of infrared to determine the crystallinity.

From the curves presented in Fig. 2, we can notice that the crystallinity amount is lower (≈ 19 %) in the first 200 µm than in the core (≈ 27 %) for the PA 6,6 sample before ageing. This may result from the processing of the samples. During injection moulding, crystallisation occurs during cooling of the melt. The skin parts that are directly in contact with the cold mould surfaces are quickly cooled. Many studies have investigated the influence of thermal history on the skin/core morphology. [START_REF] Murthy | Effect of melt temperature and skin-core morphology on the mechanical performance of nylon 6[END_REF][64] Based on the literature, it is assumed that the core cooling rate ranges from approximately 400 to 600 K.s -1 and is slower than that in the skin. [START_REF] Spoerer | Process induced skin-core morphology in injection molded polyamide 66[END_REF][62] [START_REF] Drummer | Correlation of processing, inner structure, and part properties of injection moulded thin-wall parts on example of polyamide 66[END_REF] During slow cooling, the chains have sufficient time to arrange into crystals. In comparison, the rapid cooling in the skin inhibits the development of crystals and spherulites, which is reflected in a lower crystallinity ratio than that of the core. This result can directly explain why the mechanical properties of this kind of polymer present before aging a such strong difference from the surface to the bulk and why it is so difficult to directly compare directly hardness measurements (i.e. surface analysis) to tensile test.

After thermal ageing in air, a significant increase in the crystallinity can be seen over the entire thickness of the sample thermo-oxidised at 180 °C for 100 h. In addition, the crystallinity profile of the thermo-oxidised sample is heterogeneous; in other words, the crystallinity of the surface is significantly higher than that of the core. The crystallinity increase was previously observed in the bulk by DSC [START_REF] Forsström | Thermo-oxidative stability of polyamide 6 films I. Mechanical and chemical characterisation[END_REF] and ascribed to chain scissions that release chain fragments that are able to integrate into the crystalline phase by chemicrystallisation or to reorganise in new crystals. [START_REF] Rabello | Crystallization and melting behaviour of photo-degraded polypropylene -I. Chemi-crystallization[END_REF] The crystallinity profile evolves from the surface up to 200 µm towards the core and then stays at approximately 40 %. The profile shape and depth are comparable to the infrared profile obtained by monitoring the oxidation products. However, while no oxidation products are detected in the core of the material due to the DLO effect, an increase in the crystallinity from 27% to 40% occurs simultaneously. This significant increase in the crystallinity in few hours of aging can also explain why, in our previous paper [START_REF] Pliquet | Multi-scales analysis of thermal degradation of polyamide 6,6: Correlating chemical structure to mechanical properties[END_REF], we have seen a drop in the tan δ peak maximum, which indicated a loss in molecular mobility in the first hours of aging when the chainscissions mechanism was not the major phenomenon.

To explain this behaviour, we measured crystallinity after ageing for the same time (100 h) but in the absence of oxygen. The results obtained show a homogeneous crystallinity of approximately 37% over the entire thickness, which is higher than the crystallinity measured in the sample before ageing. The polymer is quenched during moulding as it is cooled rapidly, which results in the development of an unstable crystalline morphology. Submitting the sample to a temperature above its glass transition temperature (Tg = 65 °C) allows the macromolecular chains to rearrange and crystallise, which improves the order in the crystalline phase and increases the crystallinity. [START_REF] Spinu | Physical aging of nylon 66[END_REF] This also occurs when the sample is subjected to oxidation, which explains the increase in the crystallinity in the bulk, despite no oxidation occurring as a result of oxygen depletion.

The different semi-crystalline morphologies in the various regions of the moulded sample can have an impact on macroscopic properties, such as the mechanical or optical properties.

[69] [START_REF] Marcellan | Nano) structure, skin/core and tension behavior of polyamide fibres[END_REF] These results permit also to better understand the consequences of the oxidation and to definitely separate the consequences of an oxidation process versus an annealing process.

This crystallinity profiles in presence or in absence of oxygen also lead to a new view on the hardness evolution that we have presented in our previous paper. The important increase of this parameter and especially in the first aging time should also be linked to the annealing process that takes place at the polymer surface and not only to the chemicristallisation process.

AFM nanoscale thermal analysis

In addition to the spectroscopic changes presented above, macroscopic modifications also occur during thermo-oxidative ageing of samples. Thermal analysis, such as differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA), are commonly used techniques to characterise the transition temperatures of a polymeric material. However, their main drawback is that they give only an average result because these techniques are not able to conduct a local investigation (i.e., they conduct bulk measurements). In contrast, a mapping of the properties (thermal analysis and/or mechanical) at nanometric scale can be performed using AFM. This new technique allows determining the variation of a property not only from the surface to the core of the sample as many depth profiling techniques, but also in a particular area of interest. Before AFM measurements pictures of the area were recorded (SI Figure 10) in order to see if the mapping area presents cracks, roughness… that could completely modify the obtained results.

AFM thermal analysis (VITA  ) is a technology that allows a local determination of thermal properties, such as the melting point or glass transition temperature. [71][73] To collect the mapping results, the melting temperature was chosen to evaluate changes in the microstructure with thermo-oxidative ageing. First, thermal mapping was carried out on PA 6,6 before ageing. (Fig. 3) Fig. 3 Melting temperature mapping of non-aged PA 6,6.

From the map presented in Fig. 3, we notice that the melting temperature in the first 50 µm is lower than that in the core for the PA 6,6 sample before ageing. This result agrees with the crystallinity results presented in Fig. 2 and highlights the skin effect.

Mapping was also performed on two samples aged at 140 °C for 2000 h and 180 °C for 197 h, and the obtained results are presented in Fig. 4. These ageing times were selected because they correspond to the maximum thickness of the oxidised layer for each of the temperatures selected. Changes in the melting temperature are observed from the surface to the core (until 320 µm is reached). With thermal ageing, the melting temperature ranges from 125 °C to 275 °C. The hatched area corresponds to a region where no melting is observed. It is important to specify that the melting temperature measured by AFM corresponds to the onset of the melting peak measured by DSC. Indeed, the AFM probe deflects as soon as the material starts to melt. In Fig. 4a, we observe that the melting temperature of the sample aged at 140 °C gradually evolves across the whole profile. The melting temperature decreases as one approaches the surface. In the first 80 µm, the melting temperature is approximately 125 °C and 145 °C.

These findings are relevant to our previous results measured by DSC [START_REF] Pliquet | Multi-scales analysis of thermal degradation of polyamide 6,6: Correlating chemical structure to mechanical properties[END_REF], where we observed a large distribution of the melting peak towards lower temperatures with thermal ageing. For the sample aged at 180 °C presented in Fig. 4b, the melting temperature gradient extends from the surface to a depth of 170 µm. From there, the melting temperature remains at 265 °C and does not change. The thermal property profile at 140 °C is deeper and more progressive than the profile determined at 180 °C, which is consistent with the results previously obtained by infrared microspectrometry. The melting temperature at the surface and near the surface could not be determined despite heating up to 350 °C (hatched area). The lack of melting in this particularly degraded area may be caused by crosslinking and/or carbonisation of the surface with thermal ageing. Authors that characterised ageing by SEC or rheology have shown that thermo-oxidation mainly causes chain scissions [START_REF] Okamba-Diogo | Molecular and macromolecular structure changes in polyamide 11 during thermal oxidation[END_REF][13], but a crosslinking process cannot be excluded and has been observed for samples that underwent advanced ageing. [START_REF] Shu | Study on the long-term thermal-oxidative ageing behavior of polyamide 6[END_REF]

AFM nanoscale mechanical properties

AFM in Peak Force QNM mode was employed to characterise the mechanical properties at the surface and in the bulk of the polymeric materials. [75][78] This AFM mode gives access to the DMT modulus, which can be likened to the reduced Young's modulus. [START_REF] Young | The use of the PeakForce TM quantitative nanomechanical mapping AFM-based method for highresolution Young's modulus measurement of polymers[END_REF] Mapping was carried out on the same two samples as for those analysed with AFM thermal analysis but in a different area because this analysis is destructive. The results of the DMT modulus mapping are presented in Fig. 5. A large distribution of the DMT modulus from the surface to the core can be seen in both samples. The DMT modulus varies between 1,5 and 2,5 GPa in the core and can increase to 10 GPa at the surface. The effect of thermal oxidation on the mechanical properties can be seen up to a depth of 200 µm in the sample aged at 140 °C, while it remains very superficial in the sample aged at 180 °C. One can also notice that the modulus mapping does not show like in the case of mapping of melting temperature a blank zone in the first micrometers (surface). This zone corresponds to a degraded area with an important stiffness that can be linked to a crosslinking of the surface with thermal ageing. Authors that characterised ageing

DMT modulus (GPa)

by SEC or rheology have shown that thermo-oxidation can also lead to a crosslinking process and this has been observed for samples that underwent advanced ageing. [START_REF] Shu | Study on the long-term thermal-oxidative ageing behavior of polyamide 6[END_REF] Thus, this experiment allows us not only to specifically separate the crosslinking reactions from the chain scission mechanism but also to identify areas where one of the reactions is predominant over the other.

Multi-scales correlations

In a previous paper, we proposed a multiscale approach to characterise the PA 6,6 thermal degradation. [START_REF] Pliquet | Multi-scales analysis of thermal degradation of polyamide 6,6: Correlating chemical structure to mechanical properties[END_REF] We showed that following the variations in the intensity of the band at 3080 cm -1 , assigned to the overtone of the amide II band, was a relevant chemical probe to correlate changes in the chemical structure with the resulting modifications of the microstructure and loss of functional properties. The variations in absorbance at 3080 cm -1 fit well with the changes in the crystallinity, molar mass and microhardness. In the current study, the variation in the absorbance at 3080 cm -1 as a function of the distance from the outer surface was also investigated. Fig. 6 shows the variations in absorbance at 3080 cm -1 through the thickness of thick samples aged for various times at 140 °C. Fig. 6 shows a similar skin-core profile as those reported above, with a decrease in the absorbance from the core to the surface. The drop in the amide absorbance reflects that oxidation occurs.

The absorbance at 3080 cm -1 may be correlated to the modifications of other properties of the polymer to establish a multiscale characterisation of the profiles after ageing. Fig. 7 displays the thermo-oxidation profiles determined by AFM thermal mode and micro-FTIR in the first 400 µm of a thick sample (1.5 mm) aged at 180 °C for 197 h. As shown above in Fig. 7, the loss of the amide bond is only observable in the first 200 microns and is more apparent as the surface is reached. Correspondingly, in the first 200 microns, the melting temperature measured using AFM nanoscale thermal analysis is decreasing. According to the oxidation mechanism, chain scissions occur at the C-N bond of the amide, resulting in the formation of aldehydes, carboxylic acids and primary amides as oxidation products. [START_REF] Sharkey | Mechanism of the photooxidation of amides[END_REF][80] The resulting crystallites composed of chain fragments are small and thus tend to melt at a low temperatures, which can be seen in the nanothermal profile measured by AFM. The absorbance at 3080 cm -1 is also plotted with the DMT modulus in Fig. 8 to compare changes in the chemical structure with those of the mechanical properties during thermal ageing.

Fig. 8 Profile measured by micro-FTIR (■) and AFM peak force (▲) of a sample thermoaged at 180 °C for 197 h Fig. 8 shows that the DMT modulus increases from the core to the surface, contrary to the absorption band that decreases at the same depth. The change in slopes occurs at a depth of approximately 125 µm. According to the thermo-oxidation mechanism, chain scissions occur at the C-N bond of the amide group and thus lead to a decrease in this absorbance band.

Previously, we found that chain scissions caused an increase in the crystallinity. As crystalline regions have higher mechanical properties than amorphous regions, this increase results in a rise in the stiffness, represented in this study by the DMT modulus.

In this article, the crystalline fraction was chosen as a simple factor to characterise the microstructure degradation, while the DMT modulus represents the mechanical properties.

Conclusion

Oxidation depth profiles were obtained for PA 6,6 samples thermally aged from 140 °C to 200 °C at different times by measuring the distribution of imides. We show that PA 6,6

degradation is controlled by oxygen diffusion, resulting in a higher oxidation degree at the surface than in the bulk. The heterogeneity of the degradation is temperature dependent, which means that oxidation becomes more superficial when the temperature increases. FTIR results also show the existence of a crystallinity profile where the surface has a higher crystallinity than the core. Additionally, we have shown that changes in the thermal and mechanical properties as a function of sample thickness can be measured by AFM. The comparison of the chemical structure changes with the thermal and mechanical properties shows a good correlation. Finally, the results presented in this study clearly show that AFM is a convenient and complementary method for determining the oxidation depth profiles of polyamide 6,6.

Fig. 1 Fig. 1

 11 Fig. 1 and represented in differential values according to the following equation:

Fig. 4 .

 4 Fig. 4. Melting temperature mapping of samples thermo-oxidised at (a) 140 °C for 2000h and

Fig. 5

 5 Fig. 5 Stiffness mapping of samples thermo-oxidised at (a) 140 °C for 2000 h and (b) 180 °C

Fig. 6 Δ

 6 Fig. 6 Δ A 3080 cm-1 depth profiles of the samples aged with thermo-oxidation for different times

Fig. 7

 7 Fig. 7 Thermo-oxidation profile of a PA 6,6 sample measured by micro-FTIR (■) and AFM -

Fig. 9 Fig. 9

 99 Fig. 9 compares the infrared and nanomechanical AFM profiles of a sample oxidised for 197

Figure 11 :

 11 Figure 11: Pictures of samples: (left) before aging and (right) thermo-oxidised at 140 °C for 2000h.

Table . 1

 . Total oxidation depth for each ageing temperature

	a.	1,4			non aged			b.	1,4				non aged
					24 h								5 h
		1,2			98 h 550 h Ageing temperature (°C)	1,2	80,5 h 199 h Total oxidation depth (µm)
					2000 h								508 h
		1,0							1,0
	cm -1 DA 1734				140 °C	140 160		-1 cm DA 1734	0,8 0,6			560 360	160 °C
						180			0,4			270
						200			0,2			200
									0,0
										0	250	500	750	1000	1250	1500
				Thickness (µm)						Thickness (µm)
	c.	1,4			non aged		d.	1,4				non aged
					5 h								5 h
		1,2			24 h 100 h				1,2				24 h 48 h
					197 h								280 h
		1,0							1,0
	cm -1	0,8						cm -1	0,8
	DA 1734	0,6			180 °C			DA 1734	0,6				200 °C
		0,4							0,4
		0,2							0,2
		0,0							0,0
		0	250	500	750	1000	1250	1500		0	250	500	750	1000	1250	1500
					Thickness (µm)						Thickness (µm)

Acknowledgements

The authors wish to thank Schneider Electric and ANRT for financial support, and Dupont ™ and Philippe Miniou for the sample preparation.


Yoann Quesnel
email: y.quesnel@cerege.fr

Pascal Sailhac

Johanna Lofi

Philippe Lambert

Pierre Rochette

Minoru Uehara

Christian Camerlynck

Multiscale Geoelectrical Properties of the Rochechouart Impact Structure, France

Key points

-All impactite formations of the Rochechouart impact structure have a clear electrical signature -Large-scale electrical investigations unveil a rough transition between impactites and unbrecciated basement, implying possible megablocks -Fracturing/brecciation in the Rochechouart basement locally reaches depths larger than 200 m

Introduction

Terrestrial mid-size (apparent diameter D = 10-60 km) complex impact structures are usually heavily eroded, and often buried below a post-impact sediment cover or filled by a lake, thus precluding a clear estimate of the crater size [START_REF] Melosh | Impact cratering: A geologic process[END_REF]. The presence of a central structural uplift is systematic and characterizes this category of craters [START_REF] Osinski | Impact cratering: Processes and products[END_REF]. [START_REF] Osinski | The effect of target lithology on the products of impact melting[END_REF] mentioned that massive impact melt rock (IMR) formations are usually observed for crystalline targets while for sedimentary targets, the melt appears more scattered in breccia. The distinction between clast-rich/clast-poor IMR, impact melt-rich/melt-poor breccia, fractured/brecciated basement, and unaffected basement is however difficult for these highly eroded (or buried/underwater/covered by vegetation) structures. Thus the only way to distinguish between these types of units requires drilling and geophysics where impacts are buried. In those cases, geophysics provides a means of investigating the lateral and vertical extent of the impactite formations and of the brecciation/fracturing in the basement. Still numerical models of the sources of geophysical anomalies are limited by non-uniqueness and require additional geological data from drillings or trenches. This is particularly true for models built using potential-field data, which are the most widely used geophysical data for investigating impact structures, since the associated signatures are usually significant (e.g., a circular low gravity anomaly). Other geophysical methods (e.g., seismic reflection and refraction) suffer less from this issue. However, the few examples of electrical investigations over impact structures also reveal the necessity for additional constraints, from drilling to other geophysical data (see Henkel, 1992;[START_REF] Pilkington | The geophysical signature of terrestrial impact craters[END_REF], in order to reduce the ambiguity in the geological interpretations of the electrical contrasts. Among the 27 known terrestrial mid-size complex impact structures in pure crystalline targets (Kenkmann, 2021), Rochechouart is accessible for surface investigations (i.e., not filled by a lake or buried below younger sediments) of the deposits within the crater and the target below. However, the outcrops are sparse due to vegetation and crop fields, and do not allow for a clear 3D representation of the impactite deposits (thickness, lateral extension), the stratigraphic relationships between these impactites and the distribution of the fracturing/brecciation in the basement. In addition, this impact structure has no crater rim preserved, but only shows a relatively flat-lying crater floor (i.e., no central uplift, or it is not in the impactite deposits' area; [START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF]. Therefore, it has prevented a robust estimate of the diameter of the final crater. Its morphology and the possible regional consequences associated with the impact remain unclear. For instance, concerning its size, the gravity survey described in [START_REF] Pohl | Spatial U-Pb age distribution in shock-recrystallized zircon-A case study from the Rochechouart impact structure, France[END_REF] revealed a 20-25 km wide -9 mGal Bouguer gravity anomaly. The same data further reprocessed by [START_REF] Schmidt | Bearbeitung und Auswertung von Schweremessungen im Gebiet der Impakt-Struktur von Rochechouart (Massif Central, Frankreich)[END_REF] and [START_REF] Pohl | Modelling the gravity anomaly of the Rochechouart impact structure[END_REF] led to the estimation of a crater in the range of 25-35 km in diameter, while the geological observations of impactites, shocked basement and damages are only mapped over a 10-12 km wide area (Chèvremont & Floc'h, 1996;Lambert, 1977;[START_REF] Lambert | Rochechouart 2017-Drilling campaign: First results[END_REF]. In addition, most Rochechouart studies only focused on the impact event dating and/or on the projectile with geochemical observations [START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF]. Therefore there is no real data and scientific agreements about the processes leading to each impactite type, about the vertical extension and size of the impact crater. Again, only modeling of the gravity data helped to suggest a central 6 km-deep shocked zone (in the basement target) with most damage affecting the first kilometer in the center [START_REF] Pohl | Modelling the gravity anomaly of the Rochechouart impact structure[END_REF], while no significant magnetic field anomalies were reported until now in this area. Such uncertainties reveal the need for more geophysical investigations, at all scales. In the fall 2017, in parallel to a drilling campaign performed by the Center for International Research on Impacts and on Rochechouart (CIRIR) in Rochechouart [START_REF] Lambert | Rochechouart 2017-Drilling campaign: First results[END_REF], several geophysical surveys started at the same time to better understand this impact structure. This paper reports on the results of electrical investigations. It aims at linking the properties (lithology, porosity, electrical resistivity) of the new core samples with the kilometer scale electrical resistivity properties of the Rochechouart impact structure. This allows updating our knowledge of the impactite deposit composition and geometry, and of the fracturing/brecciation extent within the crystalline target for a medium-size impact crater. We first detail the geological context of the Rochechouart impact structure, before introducing the used electrical methods at different scales. The results are then described from sample to field scale, which allows to discuss the electrical properties and the geometry of the geological formations within the impact structure. This work is part of the CIRIR initiative mobilizing the scientific community and the local authorities and institutions on site for valorizing the Rochechouart impact geoheritage [START_REF] Lambert | Rochechouart 2017-Drilling campaign: First results[END_REF].

Geological Context

The Rochechouart impact structure was created around 205 Ma ago (Cohen et al., 2017;Rasmussen et al., 2020;[START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF] on the crystalline basement of the western Massif Central, a part of the Hercynian Belt. The crater is now eroded down to the bottom of the crater fill deposits that appear in outcrop over a 12 km diameter area (Figure 1; Chèvremont & Floc'h, 1996;Lambert, 1977[START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF]. The river drainage locally erodes the impactites and exposes the target rocks underneath. The latter is composed by Variscan granitoids and gneiss formations with some isolated mafic units. Despite erosion, the complete sequence of impactites and shock effects expected due to impact in crystalline target are present [START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF][START_REF] Sapers | Revisiting the Rochechouart impact structure, France[END_REF]. Out of the brecciated basement, four typical units of impactites were so far described from surface observations: yellow IMR (such as observed on outcrops at Babaudus), red IMR with clastic/glassy matrix (such as observed on outcrops at Montoume), polymict melt-bearing breccia (such as observed on outcrops at Chassenon) and polymict melt-poor breccia (such as observed on outcrops at Rochechouart). In fall 2017, 18 holes (SC1 to SC18 on Figure 1) were drilled at eight sites approximately aligned along two transects across the impact structure. They intercept the major types of impactite lithologies down to the underlying basement (Figure 1). About 540 m of core were recovered, and the observed lithologies are described in [START_REF] Lambert | Rochechouart 2017-Drilling campaign: First results[END_REF][START_REF] Lambert | The Rochechouart 2017-Cores rescaled: Major features[END_REF]. The first observations on core samples are in agreement with the various mineralogy and textures exposed over the study area (Chèvremont & Floc'h, 1996;Kraut & French, 1971;Lambert, 1977[START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF]. They allow refining the thicknesses of each unit forming the impactite deposits and provide more details about the stratigraphic relationships among these units. The deepest (121 m) drill hole was performed in the Chassenon Butte through the melt-bearing breccia formations in the northern part of the studied area (Figure 1). [START_REF] Lambert | The Rochechouart 2017-Cores rescaled: Major features[END_REF] identify four units with distinctive lithological signature from top to bottom: about 40 m of Chassenon-like melt-rich polymict breccia, 48 m of Rochechouart-like melt-poor polymict breccia, and 26 m of essentially monomict breccia overlying about 7 m of quasi-unbrecciated basement (more detailed geological logs are described in [START_REF] Lambert | The Rochechouart 2017-Cores rescaled: Major features[END_REF]; some photographs of samples are also shown in Figure S1). Seven other sites distributed along the transects have been drilled down to 65 m for 6 of them, supplemented by 10 drill holes with maximum depth ranging from 1 to 10 m, all within the breccia deposits [START_REF] Lambert | Rochechouart 2017-Drilling campaign: First results[END_REF][START_REF] Lambert | The Rochechouart 2017-Cores rescaled: Major features[END_REF]. The 65 m deep hole at the center of the structure near Valette (Figure 1) did not reach the basement. All the other 65 m deep holes did. The cores from this drilling campaign are made available to the impact science worldwide community through the CIRIR repository settled in Rochechouart, and will serve a variety of research projects aiming at better understanding the impact processes at the bottom of impact structures on crystalline targets [START_REF] Lambert | Rochechouart 2017-Drilling campaign: First results[END_REF][START_REF] Lambert | The Rochechouart 2017-Cores rescaled: Major features[END_REF]. Chèvremont &Floc'h, 1996, and[START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF], with location of the drilling sites (SC1 to SC18) of the 2017 campaign. Coordinates are in metric units with Lambert 93 projection and RGF93 datum. Geology is draped over a shaded digital elevation model grid with 5 m pixel size (RGE Alti, product from Institut National de l'information Géographique et forestière). The cross section is modified from [START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF] with 2017 drilling updated information.

Methods

In parallel to the initial analyses of the core samples, four methods were used to unveil the electrical resistivity properties and contrasts of the Rochechouart impact structure: downhole logging tools, petrophysical measurements on core samples, electrical resistivity tomography (ERT) and controlled-source audio-magnetotelluric (CS-AMT) measurements (Figure 2; see also Figure S2 for field and instruments photographs). The downhole logging measurements were acquired in the deepest holes in the fall 2018 (Figure 2). The majority of measurements were performed in open borehole conditions (no casing). Among them, electrical resistivity of the formation (composed by its solid matrix plus interstitial fluids) was measured every 5 cm with a galvanic focused resistivity tool (Geovista DLL3 slimline tool) that provides both deep (>1 m) and shallow (<0.5 m) readings and with an electromagnetic induction resistivity probe (EM51 slimline tool). The EM51 was used for air-filled part of the borehole, while the DLL3 system was used below the water table. The EM51 resistivity data were subsequently corrected in order to make the EM51 resistivity curve matching the DLL3 curve by using an exponential correlation derived from intervals where both logs were overlapping. This correction provides good DLL3-equivalent values for the electrical measurements performed in the upper air-filled part of the holes. The porosity of water-saturated core samples was measured using a Quantachrome Helium stereopycnometer after drying each sample at 40°C during 24 h. The envelope volume of each sample was determined by scanning a paper with all surfaces drawn and measuring them using a GIS software, while each height was measured using a caliper. We also measured the electrical conductivity of the same samples, which were immersed 24 h in water with 400 µS/cm of conductivity, like in the SC2 borehole, by using a 3532-50 LCR Hi Tester (Hioki E.E. Corp.) equipped with a test fixture (Figure S2d). A cylindrical sample was installed between the top and bottom stainless steel electrodes (diameter = 4 cm). To reduce the polarization between the rock and the steel, acetate filter papers saturated with the same water were placed between the electrodes and the samples. The measured apparent resistance was converted in the resistivity of a cylinder having the same volume with the sample. ERT profiles were performed across each drilling site in order to better map the spatial extent of the drilled lithologies (Figure 2). A 4-channel ABEM Terrameter SAS 4000 Lund imaging system with a Wenner-Schlumberger protocol was used, with 64 electrodes. Spacing of 2, 5, and 10 m between each electrode allowed profile lengths of 126, 315, and 630 m, respectively. A 32 electrode roll-up protocol was used with 5 m of electrode spacing for the profile in Valette over SC7-10, which allowed to reach 480 m of length. Depending on the number of rejected measurements during the acquisition, the raw data sets contain 832 to 920 electrical resistivity observations along these profiles, except for the roll-up profile with 1,520 observations. The raw data of each profile were processed using the RES2DINV resistivity imaging software of Geotomo Software. A L1-norm "robust" inversion method was used to converge on final models. CS-AMT measurements were also performed to investigate the fracturing and brecciation at larger penetration depth than the ERT, and mainly to image the electrical resistivity of the brecciated/unbrecciated crystalline basement. AMT is strongly sensitive to ambient electromagnetic fields, mainly those generated by the 50 Hz local electrical network, but this sensitivity was reduced by using the CS-AMT technique, which increases the signal-to-noise ratio. The GPS-synchronized Phoenix RXU-TMR was used with a T3 transmitter controlling signals at frequencies between 512 Hz up to 9,600 Hz. A N-S profile of 17 measurement stations was acquired running from the north of the Chassenon area to the south of the Champonger area (Figure 2). The distance between the (line of) stations and the transmitter is 2.2 ± 0.5 km, while the mean spacing between each station is 230 ± 90 m. Such small distance between AMT stations allowed us to measure the magnetic field signal (with Phoenix AMTC-30 magnetic induction coils, which have noise below 0.01 nT/(Hz) 3/2 in AMT band) every three stations only, while the electrical field was measured (with standard copper electrodes) at each station. Significant data processing and inversion were done to make a cross section of stitched 1D electrical resistivity logs of the earth beneath each observation point. The investigation depth is limited by the conductivity of the shallow lithologies, the distance and angle from the transmitter, and the electromagnetic signal frequencies. Therefore, the 1D inversion of these data led to different vertical extent beneath each observation point.

Results

Figure 3 shows the vertical variations of the downhole electrical resistivity for the drilling site of Chassenon (hole SC2). It reveals that each layer has its own specific electrical signature. For instance, on average, the conductive melt-rich (50-200 Ohm.m) and the more resistive (100-500 Ohm.m) melt-poor impact breccia layers are contrasted, but both layers remain less resistive than the underlying brecciated gneiss (>200 Ohm.m). Table 1 shows the ranges of electrical resistivity values for the various geological units measured by downhole logging in the deepest holes, and by additional laboratory measurements on core samples from 3 drilling sites (Figure 4). The latter confirm the electrical resistivity contrasts between melt-rich and melt-poor breccias. Porosities larger than 10% are mainly observed for the Chassenon polymict breccias, up to 17%-22% for some samples (Figure 4). Concerning IMR samples, those of the Puy Chiraud site show electrical resistivities between 80 and 100 Ohm.m, whereas the ones from the Valette site range between 100 and 800 Ohm.m. In the drill holes, the IMR layers of Montoume, Recoudert, Valette, and Puy Chiraud sites show relatively low electrical resistivity values, on the order of 20-200 Ohm.m (Table 1 and Figure 3). In Chassenon and several other sites, the brecciated basement (here gneiss) yielded downhole logging electrical resistivities between 200 and 2,000 Ohm.m. These values are partially in the same order of magnitude of the ones observed in the polymict melt-poor breccia, but are much lower than the high resistivities (2,000-4,000 Ohm.m) characterizing the unbrecciated gneiss which has only been drilled at the bottom of the Chassenon and Champagnac drilling sites. The transition between the brecciated and unbrecciated gneiss units in Chassenon is associated with a thin cataclasite zone at 112-115 m of depth in hole SC2, which is clearly marked by a reduction of electrical resistivity observed by downhole logging (Figure 3), due to the intense brecciation. Core samples and downhole logs locally permit the identification of the depth at which the transition between the fractured/brecciated basement and the unbrecciated basement of the Rochechouart impact structure occurs. The ERT profiles allow spatial variations of this transition to be imaged. Figure 5 shows four resulting N-S sections, centered on the Chassenon (ERT01 profile), Champonger (ERT02), Recoudert (ERT03), and Valette (ERT04) drilling sites. The other ERT profiles, with similar ranges of electrical resistivities for the same lithologies, are shown in Figure S3. Percentages of root mean square residuals range between 4.2% and 7.4% after the inversion of the apparent electrical resistivity data along these selected four profiles. As expected from the downhole logs, the vertical variations of sub-surface electrical resistivity nearby the drilling sites are well correlated with the stratigraphy. On the ERT01 profile, the conductive (<100 Ohm.m) melt-rich breccia are clearly identified above the more resistive (100-1,000 Ohm.m) meltpoor breccia. Moreover, the melt-rich/melt-poor transition is not flat: about 40 m relief can be observed in the northern part of ERT01. The local resistive dome at the bottom of the ERT01 profile ties with the proximity of the basement (brecciated and unbrecciated gneiss). The ERT02 profile clearly images some >10 m-wide and thick resistive (800-3,000 Ohm.m) "blocks" overlying the conductive polymict breccia in the northern and central part of the profile, and the resistive (>2,000 Ohm.m) gneissic basement, cropping out to the South. The transition between the breccia and this basement is not flat nor homogeneous, but seems to dip toward the North with sparse basement blocks consolidated by impact breccia, resembling a mega-breccia formation. In the ERT03 and ERT04 profiles acquired in Recoudert and Valette (Figures 5e and5f), the IMR deposits are visible and show electrical resistivities of 50-300 Ohm.m, which are coherent with the downhole log measurements (Table 1). As for Chassenon, the presence of the water table may have slightly reduced the apparent electrical resistivity. Still those values are here close to ones of the melt-bearing breccia (perhaps due to the alteration in clays). In those locations, the impactite deposits are not uniform in thickness, nor lying horizontally, and the bottom interface with the basement is not obvious (with "blocks" rather than "layers"). The CS-AMT data inversion yields to a wide cross section across the Chassenon butte (Figure 6a). At shallow depths, it is possible to observe the melt-rich (resistivities less than 100 Ohm.m; in blue)/melt-poor (resistivities between 100 and 500 Ohm.m; in green) impactite transition (Figures 6b and6c). The contrast between the melt-poor polymict impact breccia formation and the brecciated gneiss (200-2,000 Ohm.m; green to orange colors) is less obvious (Figure 6b) and tentatively interpreted as shown in Figure 6c. The relatively "conductive" top layers prevent imaging deeper in the Chassenon butte area, and in particular the transition between the brecciated gneiss and the strongly resistive unbrecciated gneiss. The resistivity values for the brecciated basement also appear to be not greater than about 1,000 Ohm.m when it is covered by other impactites, but its thickness is larger than 200 m. However, southward, where the brecciated gneiss outcrops, the investigation depth increases and reaches about 300 m in the crystalline basement. Resistivities larger than 2,000 Ohm.m corresponding to the unbrecciated basement are well observed in the valley south of Champonger. Despite these lateral differences of investigation depths, the measurements clearly confirm that there is a significant topography formed by each lithological transition, with sometimes a relief greater than 100 m (i.e., greater than the variations of the present-day surface topography). On average, taking the vertical exaggeration into account, this CS-AMT section reveals that these transitions dip between 10° and 30° northward (i.e., outward from the expected crater center).

location of controlled-source audio-magnetotelluric (CS-AMT) measurement sites (red disks) and the associated projected profile (dashed red line), the transmitter site (dashed blue line on the east), the electrical resistivity tomography (ERT) sections (green line) and drilling sites; (b) resulting electrical resistivity cross section after inversion of CS-AMT data from north of Chassenon to south of Champonger, with locations of ERT01 and 02 sections (shaded polygons), and SC2 and SC6 drilling sites; (c) same as (b)

with our interpretations (dashed lines). The vertical exaggeration for these sections is 2.5.

Discussion

Electrical Properties of Impactites

Our results suggest that the electrical resistivity decreases when the melt content (and its associated alteration product(s)) in breccia increases. It is confirmed by the downhole logging, laboratory measurements and ERT soundings performed at other drilling sites, including those where IMR crops out. This trend is linked to the porosity of these formations. Indeed the impact melt-rich breccia have significant porosities (>10% on average; Figure 4). This behavior between the porosity/melt content and the electrical resistivity is slightly observed in the IMR too. In details, downhole logging over the Babaudus-like and Montoume-like IMR of the Rochechouart impact structure displays medium resistivity values, ranging between 100 and 1,000 Ohm.m. However, higher resistivities would have been expected for such type of massive deposits (Figure 4 showing porosities <10% on average), even taking into account the presence of the water table. Indeed, several terrestrial impact structures show that some melt sheets are associated with electrical resistivities larger than those of the brecciated gneiss (i.e., >1,000 Ohm.m; Henkel, 1992). In the central uplift of the Araguainha impact structure (Brazil) with a mixed target, using ERT soundings only, [START_REF] Tong | Geoelectric evidence for centripetal resurge of impact melt and breccias over central uplift of Araguainha impact structure[END_REF] observed low resistivities (<100 Ohm.m) in porous polymict breccia, and relatively high resistivity anomalies in IMRs (300-1,000 Ohm.m vs. 50-300 Ohm.m in Rochechouart). These resistive zones within Araguainha are interpreted as IMR "blocks" nearby the central uplift of the structure, and their crystalline/glassy matrix induces a lower porosity than the heterogeneous matrix of the polymict breccia (with microfractures). On the other hand, the IMR from the M0077 drilling site of the IODP-ICDP Exp 364 within the peak ring of the Chicxulub impact structure (Mexico; [START_REF] Morgan | The formation of peak rings in large impact craters[END_REF] show extremely low electrical resistivity values considering either multiscanner core logging data (0.4-3.4 Ohm.m) or downhole logging data using EM51 induction tool (3-6 Ohm.m;[START_REF] Morgan | Site M0077: Introduction[END_REF][START_REF] Lofi | Drilling-induced and logging-related features illustrated from IODP-ICDP Expedition 364 downhole logs and borehole imaging tools[END_REF][START_REF] Morgan | Site M0077: Introduction[END_REF]. The large porosities (about 20%) of the Chicxulub IMR units and their saturation in saline water largely decrease their initial electrical resistivity. Despite fresh water saturation, the moderate resistivities observed in the IMR, and more largely in the melt-bearing breccia, of the Rochechouart impact structure should be explained by another process. A central negative Bouguer gravity anomaly of -9 mGal is observed over the whole area of impact deposits, with a minimum where the IMR outcrop in Valette and Babaudus [START_REF] Pohl | Modelling the gravity anomaly of the Rochechouart impact structure[END_REF]. It means that the whole column of rocks in these areas, including the IMR (Figures 345), is porous via fracturing and/or brecciation at all scales. Following the impact, for instance in the formations of the present-day Chassenon butte, fluid circulation may have been favored by fractures and connected porosity (see Figure S4 for an example in the brecciated gneiss of SC2 borehole). Associated with the heat stored in the impacted area, post-impact hydrothermalism led to an intense alteration of the melt into secondary phases, likely clays (Kukkonen et al., 1992;[START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF][START_REF] Sapers | Revisiting the Rochechouart impact structure, France[END_REF]. The latter minerals were also observed in the core samples from the peak-ring of Chicxulub, where the hydrothermal system seems to have been deeper, longer and in some places, more intense than expected (Kring et al., 2020). Impact melt glasses of the Chicxulub IMR were altered into clay minerals, in the form of smectites for example, and other minerals (Kring et al., 2020). On average, electrical resistivities associated to these IMR units seem to be slightly lower than the ones of the suevite (mean value of about 5 Ohm.m), as well as of the underlying units of granitoids (mean value of about 7 Ohm.m; [START_REF] Morgan | Site M0077: Introduction[END_REF][START_REF] Morgan | Site M0077: Introduction[END_REF]. Therefore the role of post-impact alteration of the melt could be significant to reduce the electrical resistivities of IMR. Preliminary petrographical analyses of some thin slices (work in progress) of the recent core samples from the Chassenon drilling site already reveal a significant amount of minerals typical of post-impact hydrothermal alteration, which was also previously described on field samples by Kukkonen et al. (1992), [START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF][START_REF] Sapers | Revisiting the Rochechouart impact structure, France[END_REF]. We thus suggest that this weathering is the reason of the high conductivities observed within the melt-rich breccia layer of Chassenon, even above the water table (clays remain wet). The first analyses of the core samples of IMR of other sites also revealed their heterogeneity and the presence of isolated vesicular textures which, combined with the general alteration, may explain their moderate electrical resistivity signal [START_REF] Lambert | Rochechouart 2017-Drilling campaign: First results[END_REF][START_REF] Lambert | The Rochechouart 2017-Cores rescaled: Major features[END_REF]. In the melt-poor (Rochechouart-like) breccia layer, electrical resistivities are larger, due to either a deeper burial, or less intense weathering, or just because the melt was less abundant. To date, no other electrical studies of impact structure formations were able to unveil such detailed electrical contrasts between melt-bearing and melt-poor impact breccia formations. It argues in favor of a more widespread use of the electrical methods over Rochechouart (perhaps outside the impactite area too) as well as over any (eroded) impact structures, for instance to map the melt content or alteration. However, more detailed petrographical and/or geochemical data are needed to better conclude on the amount and role of the clay (and perhaps other) minerals to influence the electrical resistivity properties. Interestingly, the melt-rich breccia layer of Chassenon is also strongly magnetized compared to the underlying melt-poor breccia layer. Magnetic anomaly mapping in the vicinity of the Chassenon drilling site reveals strong gradients, as well as in Champonger at the border of the Chassenon butte (Figure S5) where the impactites and the underlying gneiss are fractured and mixed as blocks. This confirms the lateral heterogeneities unveiled by electrical resistivity investigations. The CS-AMT data also image the fracturing/brecciation in the basement with electrical resistivity values of 1,000-3,000 Ohm.m, lower than expected for crystalline rocks ( 5,000-10,000 Ohm.m). Bobée ∼ et al. ( 2010) mention electrical resistivities on the order of 600-800 Ohm.m for the brecciated basement beneath Chassenon, which is lower than the values revealed by our ERT soundings (1,000-3,000 Ohm.m). Henkel (1992) however reported higher resistivities (between 2,000 and 8,000 Ohm.m) for brecciated crystalline rocks of the impacted Fennoscandian shield in Sweden, and resistivities exceeding 10,000 Ohm.m for the unaffected basement. It is also interesting to compare the electrical resistivity of the basement in the Chassenon area with the models given for Araguainha and Serra da Cangalha impact structures in Brazil (Adepelumi et al., 2005;[START_REF] Masero | Electrical conductivity and crustal deformation from magnetotelluric results in the region of the Araguainha impact, Brazil[END_REF][START_REF] Tong | Geoelectric evidence for centripetal resurge of impact melt and breccias over central uplift of Araguainha impact structure[END_REF]. For Araguainha (with a mixed target), the unaffected upper crust seems to have electrical resistivities on the order of 5,000 Ohm.m, while the depth of fracturing/brecciation may reach about 5 km. For Serra da Cangalha (with a sedimentary target), Adepelumi et al. (2005) reported a weathered basement layer with resistivities on the order of 500 Ohm.m while the crystalline basement resistivity values are larger than 2,000 Ohm.m. Such values are observed at depth in the valley between Chassenon and Valette impactite areas (southern part of Figures 6b and6c), which could be linked to the presence of an unbrecciated gneissic basement too.

Geometry of Impactite Deposits

As already mentioned, the bottom of the Rochechouart impact crater, which today is exposed in outcrops, should not be considered as a regular and flat interface. Indeed, we characterized and quantified local relief within the impactite layers (maybe due to lateral non-uniform mixing processes), between impactites and gneiss, as well as between the brecciated/fractured and unaffected basement (Figures 567). Bobée et al. ( 2010) already reported some of these features using interpretations of shallow EM data over a smaller area of the Chassenon butte. These rough topographies derived from the electrical resistivity stratigraphy reflect the creation (by fracturing and brecciation), the sliding/collapsing along faults, and the final emplacement of, megablocks during the modification stage of the impact event (Kenkmann et al., 2014;Lambert, 1977;[START_REF] Osinski | Impact cratering: Processes and products[END_REF]. This phase may be associated with a collapsing central uplift or with a terrace zone (Figure 7; [START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF]. The former is more likely than the latter since we mainly observed interfaces (or faults) dipping outwardly (see below). In this case, the observed amplitudes, with 40-50 m high reliefs on the ERT sections and >100 m in the CS-AMT section could reflect the presence of such megablocks and would thus provide an estimate of their minimum size. The presence of megablocks and the complex stratigraphical relationships between the impactites and the basement in the Chassenon area may also be consistent with the existence of an eventual general tilting of the whole structure. Indeed [START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF] pointed the fact that the bottom of the final crater may have been inclined by 0.6° toward North (potentially due to pre-existing reliefs), resulting in preferential erosion of the impactite deposits in the south. Interestingly, the CS-AMT electrical resistivity logs show that the surface separating the impactites and brecciated basement is dipping northward at an angle of about 10° (Figure 6), or maybe less, as seen in the ERT cross sections (Figure 5). Henkel (1992) mentioned that for "large" complex craters, the outer edge of the brecciation is associated to significant electrical resistivity variations and seems to be buried under (mega-)breccia ring (see also Kenkmann et al., 2014). Our electrical resistivity observations of uneven reliefs in the impactite deposits and basement are consistent with the presence of mega-breccia formations, which may have moved laterally, perhaps through the preexisting topography and/or northward-dipping fractures/faults. These movements may have occurred during a preferentially outwarding collapse of a temporary central uplift, which is not remaining in Rochechouart (Figure 7), while systematically observed for medium-size structures in crystalline target. It may then be an intermediate structure with a "(partially)collapsed" central uplift, which may explain megablock displacements (see Baker et al., 2011, for some examples of transitional craters). This aspect could be investigated by 2D/3D modeling of the structure using constraints from potential-field data and/or from seismic cross sections, but also by performing further specific numerical simulations of impacts on crystalline targets with pre-existing fractures and/or topography.

Conclusions

This study aims at using electrical resistivity observations at different scales to reveal lithological and geometrical variations within the Rochechouart impact structure, the latter showing the bottom of a medium-size impact crater in a crystalline target. Logging investigations within recent boreholes, associated to core sample measurements, first described that each impactite formation has their own specific electrical resistivity properties where more melt, and/or its alteration products like clays increases the conductivity. We also observed that the electrical resistivity also decreases with porosity. These first observations helped to extrapolate the lithologies observed within the boreholes using ERT. The latter image up to 50 m local relief of the interfaces between each impactite formations (in particular between the melt-rich and melt-poor impact breccia), revealing geometries compatible with megablocks emplacements. This is confirmed by CS-AMT observations which show northward-dipping electrical resistivity contrasts between the meltbearing breccia layer and the brecciated and unbrecciated basement. All these observations support an impact structure briefly possessing a central uplift which may have collapsed through megablocks sliding.

Figure 1 .

 1 Figure1. Simplified geological map and associated N-S cross section of the Rochechouart impact structure (modified fromChèvremont & Floc'h, 1996, and[START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF], with location of the drilling sites (SC1 to SC18) of the 2017 campaign. Coordinates are in metric units with Lambert 93 projection and RGF93 datum. Geology is draped over a shaded digital elevation model grid with 5 m pixel size (RGE Alti, product from Institut National de l'information Géographique et forestière). The cross section is modified from[START_REF] Schmieder | A Rhaetian 40 Ar/ 39 Ar age for the Rochechouart impact structure (France) and implications for the latest Triassic sedimentary record[END_REF] with 2017 drilling updated information.

Figure 2 .

 2 Figure 2. Location of the electrical measurements performed via downhole logging, electrical resistivity tomography (ERT) soundings, and controlled-source audio-magnetotelluric (CS-AMT) data. Background is shaded and colored digital elevation model grid with 5 m pixel size (RGE Alti, product from Institut National de l'information Géographique et forestière).

Figure 3 .

 3 Figure 3. (Left) Synthetic geological log of the drilling sites (from Lambert et al., 2019) and (right) associated downhole logging electrical resistivity of the formations, for (a) SC2 hole in Chassenon, (b) SC7 hole in Valette, (c) SC11 hole in Puy Chiraud, (d) SC14 hole in Rochechouart, (e) SC16 hole in Montoume, and (f) SC17 hole in Recoudert. For resistivity logging, the top part, above the water table, corresponds to rescaled-EM51 data (see text for details).

Figure 4 .

 4 Figure 4. Electrical resistivity versus porosity of core samples from SC2 (Chassenon), SC7 (Valette), and SC11 (Puy Chiraud) drilling sites.

Figure 5 .

 5 Figure 5. Location maps (a and b; see Figure 1 for legend) and electrical resistivity cross sections after inversion of ERT01 (c), ERT02 (d), ERT03 (e), and ERT04 (f) profiles' data (top), and associated geological interpretations (bottom) considering the lithologies observed in the corresponding drilling. See Figure S3 for other electrical resistivity tomography (ERT) profiles.

Figure 6 .

 6 Figure 6. (a) Geological map (see legend in Figure 1) showing the location of controlled-source audio-magnetotelluric (CS-AMT) measurement sites (red disks) and the associated projected profile (dashed red line), the transmitter site (dashed blue line on the east), the electrical resistivity tomography (ERT) sections (green line) and drilling sites; (b) resulting electrical resistivity cross section after inversion of CS-AMT data from north of Chassenon to south of Champonger, with locations of ERT01 and 02 sections (shaded polygons), and SC2 and SC6 drilling sites; (c) same as (b)with our interpretations (dashed lines). The vertical exaggeration for these sections is 2.5.

Figure 7 .

 7 Figure 7. (Top) Geological cross section of Figure 1, with constraints from this study, (middle) interpreted schematic 2D model of the Chassenon area (showing megablocks and tilting), and (bottom) its eventual position within the whole Rochechouart impact structure about 200 Ma ago.

Table 1 .

 1 Ranges of mean electrical resistivity properties of the impactite and basement units

			Electrical Resistivity (Ohm.m)
	Geological units	From downhole	From water-	From ERT	From Bobée et al.
		logging	saturated samples a	profiles	(2010)
	Impact Melt Rocks	20-200	80-700	50-300	-
	Melt-rich breccia	50-200	20-60	10-100	180-240
	Melt-poor breccia	100-500	80-200	100-1000	100-600
	Brecciated basement	200-2000	100-400	1000-3000	> 600-700
	Unbrecciated basement	2000-4000	1100-6000	-	-
	ERT: Electrical Resistivity Tomography.
	a water conductivity = 400 µS/cm, which is equivalent to the one measured in SC2 borehole

Acknowledgments

This work was supported by the Programme National de Planétologie (PNP) of CNRS/INSU, cofunded by CNES, as well as by A*Midex foundation from Aix-Marseille University. It is part of the CIRIR program and benefited from the field expertise and the logistical support of the CIRIR on site. The authors thank the Reserve Naturelle Nationale de l'Astroblème de Rochechouart Chassenon, for allowing the field work carried out on its territories. The authors also acknowledge ANR project EQUIPEX CRITEX (grant # ANR-11-EQPX-0011) for its financial support to the CS-AMT equipment. B. Célérier, G. Henry, L. Brun, and J. Paris are thanked for the downhole logs acquisition, while L. Hervieu, O. Cherait, G. Telansamou, who were students in training internships at the time of the surveys, F. Demory and J. Gattacceca, are warmly acknowledged too. Lastly, we thank the editor J. Feinberg for his support and suggestions, and the two anonymous reviewers who greatly contributed to improve the manuscript.

Data Availability Statement

The data supporting the conclusions of this study can be downloaded here: https://doi.org/10.34930/be0549d1-e876-49c5-b07f-bf04d398b25e (electrical resistivity logging data and laboratory measurements on samples, apparent electrical resistivity data of ERT cross sections, and CS-AMT inverted data).

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