P O Delange

S Bussiere

J L Therias

Gardette
email: luc.gardette@uca.fr

M Pliquet

M Rapeaux

F Delange

P O Bussiere

S Therias

J L Gardette

Multiscale analysis of the thermal degradation of polyamide 6,6: Correlating chemical structure to mechanical properties

Keywords: Polyamide 6, 6, thermo-oxidative degradation, multi-scales, crystallinity

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

1) Introduction

Polyamides are one of the first and most important families of engineering plastics because of their excellent properties, such as high resistance to mechanical stresses (tensile strength, fatigue behaviour, stiffness) and chemical substances (e.g., oils, greases, solvents), and good dielectric properties. These remarkable properties have led to the wide use of polyamides in various industrial fields, such as automotive, textile, electrical distribution and offshore fields. [START_REF] Mark | Encyclopedia of polymer science and technology[END_REF] Concerning the long-term applications of polyamides, their performance in ambient atmosphere is a critical factor. It is therefore of technical and commercial importance to predict the effective lifetime of the material for its applications. In circuit breakers, where polyamide 6,6 is used as an electric insulation component, the service temperature is typically within the range of 60 °C to 80 °C and may reach 110 °C in some harsh conditions. These elevated temperatures combined with the presence of oxygen can cause a thermo-oxidative ageing of the material which leads to the deterioration of its properties. Insulating materials should maintain their properties over a long period of time during service life. Rapid and easy test methods allowing for an early detection of material degradation are required to predict their lifetime.

The thermo-oxidative ageing of polyamides involves several chemical and physical processes followed by structural changes leading to a deterioration of the functional properties of the material. There are several functional properties that can be impacted: electrical [START_REF] Chailan | Effects of thermal degradation on the viscoelastic and dielectric properties of chlorosulfonated polyethylene (CSPE) compounds[END_REF][START_REF] Van Krevelen | Properties of polymers: their correlation with chemical structure; their numerical estimation and prediction from additive group contributions[END_REF][START_REF] Chisca | DIelectric behavior of some aromatic polyimide films[END_REF], mechanical [START_REF] Dong | Influence of temperature on the thermo-oxidative degradation of polyamide 6 films[END_REF], optical, aesthetic etc. Because there is not a single origin for the deterioration of the properties, clarifying the mechanism is of great significance both in theory and in practice. It is then important to characterize the degradation of the material entirely, from the molecular to the macroscopic scale. Many studies have been carried out focusing on characterization of the thermal oxidation of polyamides. Several analytical techniques were used, which have the capability to characterize the modifications of the material during the successive stages of degradation. Chemical changes in the polyamide matrix have been measured by infrared and UV/Vis spectroscopies [START_REF] Charles | FTIR and thermal studies on nylon-66 and 30% glass fibre reinforced nylon-66[END_REF][START_REF] Karstens | Thermo-oxidative degradation of polyamide 6 and 6,6 -Structure of UV/VIS-active chromophores[END_REF], photoacoustic [START_REF] Murty | Adaptation of photoacoustic Fourier transform infrared spectroscopy for studying the thermal oxidation of nylon 66 at 15O°C correlated to mechanical properties[END_REF], luminescence [START_REF] George | An oxyluminescence investigation of the auto-oxidation of nylon 66[END_REF][START_REF] Grigg | Thermo-oxidative degradation of polyamide 6[END_REF][START_REF] Cerruti | Chemiluminescence from oxidation of polyamide 6-6. I. The oxidation of pure polyamide[END_REF][START_REF] Frosstrom | Thermo-oxidative stability of polyamide 6 films. II. Chemiluminescence techniques[END_REF] and gas chromatography [START_REF] Gröning | Headspace solid-phase microextractiona tool for new insights into the long-term thermo-oxidation mechanism of polyamide 6.6[END_REF]. The consequences of the chemical changes have been investigated by tensile tests, rheology in the melting state, differential scanning calorimetry, X-ray diffraction etc. However, there is still a lack of knowledge on the cross-correlations between the various properties affected by thermo-oxidative ageing. The present article pays particular attention to correlations between the chemical changes in the PA matrix and the deterioration of its functional properties using relevant degradation indicators.

The present study has two main objectives: [START_REF] Mark | Encyclopedia of polymer science and technology[END_REF] to evaluate the impact of thermo-oxidation of PA 6,[START_REF] Charles | FTIR and thermal studies on nylon-66 and 30% glass fibre reinforced nylon-66[END_REF] on the modifications at the molecular level, the changes in the microstructure and macromolecular architecture as well as the loss in functional properties. [START_REF] Chailan | Effects of thermal degradation on the viscoelastic and dielectric properties of chlorosulfonated polyethylene (CSPE) compounds[END_REF] to correlate relevant chemical probes for measuring oxidation to the modification of the different properties from the molecular to the macroscopic scale.

2) Experimental

2.1) Materials

The experimental raw material is commercial grade polyamide 6,6 supplied by Dupont ™ (Zytel ® 101L). The samples were injection-moulded directly onto plates (80 x 80 mm) with a thickness of 1.5 mm and films with thicknesses ranging from 200 to 400 µm were obtained.

After moulding, materials were kept in special containers in order to prevent any moisture uptake before thermal ageing.

2.2) Oven ageing

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.

Ageing was also performed in the absence of oxygen in a vacuum chamber. The oven was coupled with a vacuum pump and a pressure of 10 -31 mbar was applied. The experiments were carried out in the same temperature range and time intervals as thermo-oxidative ageing.

2.3) Characterization techniques

2.3.a) FTIR spectroscopy

After thermal ageing, the samples were analysed by infrared spectroscopy in transmission mode using a Nicolet 6700 spectrophotometer and in attenuated total reflection mode (ATR) using a Nicolet 380 spectrophotometer equipped with a diamond crystal (refractive index = 2.5). Spectra were recorded with 64 scans at a resolution of 4 cm -1 . In the spectra obtained with the ATR mode, the angle of incidence is specified at 45° and the refractive index of the material is set at 1.5. In order to correct the wavelength-dependent penetration depth, we used the Omnic software produced by ThermoScientific. Moreover, the CH 2 scissor band at 1463 cm -1 was chosen as a reference band because of its insensitivity to the oxidation process. The absorbances at the various maxima considered were corrected using the absorbance of the reference band.

2.3.b) Differential scanning calorimetry

Differential scanning calorimetry was performed with a Mettler Toledo DSC 3+ device. The samples were cut on plates and heated from 25 °C to 300 °C at a heating rate of 10 °C/min under air flow. This analysis was carried out to determine changes in the semi-crystalline morphology of PA 6,6. A standard heat of melting (ΔH f ) of 196 J.g -1 for 100% crystalline PA 6,6 was used to calculate the crystallinity, as defined by equation (1):

Eq.1

2.3.c) Size exclusion chromatography

Size exclusion chromatography measurements were performed to follow the changes in average molar mass M n and M w during oxidative ageing. First, the samples were dissolved in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) at a concentration of approximately 2 g.L -1 . The dissolution was carried out at room temperature for 24 h. Then, the samples were filtered through a PTFE 0.2 mm membrane, and 50 µL was injected into the chromatograph equipped with two columns of the same dimension 8 mm x 300 mm. The columns were packed with 10 µm particles with 1000 Å and 100 Å pore sizes. Analyses were performed at 40 °C at a flow rate of 1 ml/min with a mixture of HFIP/0.1 mol.L -1 potassium trifluoroacetate the eluant. The calibration was performed using poly(methylmethacrylate) standards.

2.3.d) DMTA

A TA Instruments Q800 dynamic mechanical analyzer was used to determine the dynamic mechanical properties of unaged and aged PA 6,6. Rectangular samples of 40 x 10 x 1,5 mm were tested in single cantilever mode in a temperature range from 25 °C to 200 °C at a heating rate of 3 °C/min. The frequency was fixed at 1 Hz with an applied strain of 0,05%.

2.3.e) UV/Vis spectroscopy

The UV-Vis spectra of the polymer were recorded with a Shimadzu UV-2600 scanning spectrophotometer equipped with an integrating sphere, working with UVProbe software.

Measurements were performed with polymer plates (1500 µm). The spectra were recorded in transmission mode by analysis the 1.5mm plates, which were transparent enough to obtain absorbances in a convenient range (A<3). The degree of yellowing is determined by plotting the absorbance at 450 nm.

2.3.f) Vickers micro-hardness

The hardness of a sample was determined by applying a pyramidal tip under load on the surface of a PA 6,6 plate and measuring the penetration. The measurements were performed using a Shimadzu HMV hardness tester and indentations were created at a load of 2 N with a holding time of 10 s. The Vickers microhardness values were calculated by equation (2):

Eq.2

where P is the load applied (N), d is the average diagonal of the Vickers indentation (mm) and k is a geometrical factor equal to 1,854. [START_REF] Ania | Physical ageing and glass transition in amorphous polymers as revealed by microhardness[END_REF] 3) Results and discussion

3.1)

Modification of the chemical structure

3.1.a) FT-IR analysis

Infrared spectroscopy is one of the most common ways to investigate oxidative ageing, notably the formation of carbonyl species. [START_REF] Rivaton | Basic aspects of polymer degradation[END_REF][START_REF] Gardette | Infrared spectroscopy in the study of the weathering and degradation of polymers[END_REF][START_REF] Pearce | Fourier transform IR spectroscopy for the study of polymer degradation. Thermal and thermooxidative degradation of polyethylene terephthalate[END_REF] IR spectroscopy was applied to identify the characteristic bonds present in the chemical configuration of polyamide 6,6. The complete band assignment of PA 6,6 was studied by several authors [START_REF] Do | FT-IR spectroscopic study on the thermal and thermal oxidative degradation of nylons[END_REF][START_REF] Cannon | The infra-red spectra and molecular configuration of polyamides[END_REF][START_REF] Lim | Sorption and transport of water vapour in nylon 66 film[END_REF][START_REF] Roberts | Site-specific reversible scission of hydrogen bonds in polymers. An investigation of polyamides and their Lewis acid-base complexes by infrared spectroscopy[END_REF][START_REF] Tang | Photochemistry of polyundecanamides 1. Mechanisms of photooxidation at short and long wavelengths[END_REF][START_REF] Roger | Photochemistry of aliphatic polyamides 4. Mechanisms of photooxidation of polyamide 6, 11 and 12 at long wavelengths[END_REF][START_REF] Socrates | Infrared and Raman characteristic group frequencies[END_REF]. The spectrum of the polymer before ageing and the frequencies of the bands with their proposed assignments are shown in Fig. 1.

Wavenumber (cm -1 ) Assignment Ref

3488

"Free" N-H stretching [START_REF] Do | FT-IR spectroscopic study on the thermal and thermal oxidative degradation of nylons[END_REF] 3300 "Bonded" N-H stretching [START_REF] Do | FT-IR spectroscopic study on the thermal and thermal oxidative degradation of nylons[END_REF] 3100-3070 Overtone of Amide II band [START_REF] Cannon | The infra-red spectra and molecular configuration of polyamides[END_REF] 3080 Overtone of N-H deformation [START_REF] Socrates | Infrared and Raman characteristic group frequencies[END_REF] 2930 Asymmetric CH 2 stretching [START_REF] Lim | Sorption and transport of water vapour in nylon 66 film[END_REF] 2850 Symmetric CH 2 stretching [START_REF] Lim | Sorption and transport of water vapour in nylon 66 film[END_REF] 1638 Amide I (C=O stretching) [START_REF] Roberts | Site-specific reversible scission of hydrogen bonds in polymers. An investigation of polyamides and their Lewis acid-base complexes by infrared spectroscopy[END_REF] 1545 Amide II (N-H deformation and C-N stretching) [START_REF] Cannon | The infra-red spectra and molecular configuration of polyamides[END_REF] 1476 CH 2 deformation (α-N) [START_REF] Do | FT-IR spectroscopic study on the thermal and thermal oxidative degradation of nylons[END_REF] 1466 CH 2 deformation [START_REF] Do | FT-IR spectroscopic study on the thermal and thermal oxidative degradation of nylons[END_REF] 1440 N-H deformation [START_REF] Socrates | Infrared and Raman characteristic group frequencies[END_REF] Fig. 1 progressive increase in the absorbance between 1820 and 1680 cm -1 , and a notable shoulder was formed with various absorption maxima. Fig. 2a shows a magnified view of the carbonyl species region (1900-1500 cm -1 ) of the samples aged at 140 °C between 0 and 2000 h, and Fig. 2b shows the region at 3150-3000 cm -1 .

Fig. 2 ATR-FTIR spectra of the carbonyl region (2a. left) and the 3150-3000 cm -1 region (2b. right) of PA 6,6 thermo-oxidized at 140 °C During the first period, the main observed band showed an absorption maximum at 1734 cm -1 , and progressively the absorbance increased, and the maximum shifted to 1712 cm -1 . In addition, the formation of a shoulder band at 1756 cm -1 was observed. It should be noted that these bands overlapped after long periods of time. It is worth mentioning that in most many papers [START_REF] Tang | Photochemistry of polyundecanamides 1. Mechanisms of photooxidation at short and long wavelengths[END_REF][START_REF] Roger | Photochemistry of aliphatic polyamides 4. Mechanisms of photooxidation of polyamide 6, 11 and 12 at long wavelengths[END_REF][START_REF] Arnaud | Photochemical behaviour of semi-aromatic polyamides[END_REF][START_REF] Lemaire | Dual photo-chemistries in aliphatic polyamides, Bisphenol A polycarbonate and aromatic polyurethanes -A short review[END_REF], the absorbance at 1734 cm -1 was chosen to quantify the extent of oxidation. The absorption maximum at 1712 cm -1 has not been often selected because the extent of oxidation reached is too low. In the domain 3150 cm -1 -3000 cm -1 , the progressive disappearance of the band at 3080 cm -1 was observed, which was assigned to an overtone of the amide II band [START_REF] Do | FT-IR spectroscopic study on the thermal and thermal oxidative degradation of nylons[END_REF][START_REF] Cannon | The infra-red spectra and molecular configuration of polyamides[END_REF][START_REF] Lim | Sorption and transport of water vapour in nylon 66 film[END_REF] resulting from the combination of N-H bond deformation and C-N bond stretching of amide group.

The oxidation of polyamides provoked by exposure to UV-light or by increasing the temperature has been extensively studied in the past 50 years. The accepted mechanism is largely based on the pioneering work of Sagar [START_REF] Sagar | Autoxidation of n-alkyl amides. Part III. Mechanism of thermal oxidation[END_REF], who focused on the thermal oxidation of N-alkyl amides in the liquid phase. Since then, the photooxidation and thermo-oxidation of polyamides have received much attention in the literature, and mechanisms accounting for the modification of the chemical structure have been proposed. [START_REF] Tang | Photochemistry of polyundecanamides 1. Mechanisms of photooxidation at short and long wavelengths[END_REF][START_REF] Roger | Photochemistry of aliphatic polyamides 4. Mechanisms of photooxidation of polyamide 6, 11 and 12 at long wavelengths[END_REF][START_REF] Arnaud | Photochemical behaviour of semi-aromatic polyamides[END_REF][START_REF] Lemaire | Dual photo-chemistries in aliphatic polyamides, Bisphenol A polycarbonate and aromatic polyurethanes -A short review[END_REF] In the case of three peaks at 1756 cm -1 , 1734 cm -1 and 1711 cm -1 attributed to isolated carboxylic acids, imides and carboxylic acids respectively. [START_REF] Okamba-Diogo | Molecular and macromolecular structure changes in polyamide 11 during thermal-oxidation[END_REF] Because of these studies, the absorption maxima observed in Fig. 2a can be assigned to different products. (Table .1.) It is well-known that polyamide thermo-oxidation involves a radical chain mechanism with high selectivity. The initial stage of polymer oxidation is the thermal decomposition of hydroperoxide groups. [START_REF] Dong | Influence of temperature on the thermo-oxidative degradation of polyamide 6 films[END_REF] This decomposition can be unimolecular or bimolecular and leads to the formation of some very reactive species (hydroxyl and alkoxyl radicals). The oxidative attack is preferentially localized on the C-H bonds in the α position of nitrogen atoms (see Fig. 3). A radical is generated by removing the most labile hydrogen atom, which reacts and forms hydroperoxides. From the generation of these radicals, several reactions exist leading to the formation of oxidation products (the main routes of oxidation are illustrated by the mechanism shown in Fig. 3):

(a) The rearrangement of an alkoxy radical by β-scission causes the cleavage of C-N bonds. This β-scission results in the formation of primary amide and aldehyde.

[29]

(b) The abstraction of hydrogen induces β-scission leading to the production of a macroradical and a formylamide.

(c) The formation of imides results from the disproportionation process of secondary alkyl radicals. [START_REF] Gröning | Headspace solid-phase microextractiona tool for new insights into the long-term thermo-oxidation mechanism of polyamide 6.6[END_REF][START_REF] Richaud | Review: Auto-oxidation of aliphatic polyamides[END_REF] (d) Hydrogen abstraction by macroalkoxy radicals leads to the production of unstable alcohols, which decompose into primary amides and aldehydes. Such aldehydes can rapidly be converted into carboxylic acids in the presence of oxygen as a result of prolonged ageing. [START_REF] Chionna | Thermal oxidation products of nylon 6 determined by MALDI-TOF mass spectrometry[END_REF] Fig. 3 Mechanism of the thermo-oxidation of PA 6,[START_REF] Charles | FTIR and thermal studies on nylon-66 and 30% glass fibre reinforced nylon-66[END_REF] Furthermore, the mechanism shown in Fig. 3 indicates that hydroperoxides, identified as the primary oxidation products, involve abstraction of the most labile hydrogen in the α position of the C-N bond. Regardless of the predominant degradation mechanism, the C-N bond of the amide group is then affected, which explains the progressive disappearance of the band at 3080 cm -1 .

The kinetic curves of oxidation can be plotted following the absorbance at 1734 cm -1 , which reflects the formation of imides, mainly resulting from pathway (c) or 1712 cm -for the various temperatures of oxidation (Fig. 4.a and b). One can also follow the variations in the absorbance at 3080 cm -1 (Fig. 4c).

Fig. 4 Absorbance variations at 1734 cm -1 (a), 1712 cm -1 (b) and 3080 cm -1 (c) during thermal ageing of PA 6,[START_REF] Charles | FTIR and thermal studies on nylon-66 and 30% glass fibre reinforced nylon-66[END_REF] In Figs.4 a to c, no induction period was observed. This has already been observed with other polyamides and was attributed to the instability of hydroperoxides. [START_REF] Lánská | Chemiluminescence of polyamides: I. Luminescence accompanying autoxidation of lactams and thermolysis of lactam hydroperoxides[END_REF][START_REF] Lánská | Chemiluminescence of polyamides II. Luminescence accompanying thermooxidation of lactam-based polyamides related to the content of end-groups of molecules[END_REF]. Regarding Figs 4.a and b, the results reported for the four temperatures indicate that the absorbances at 1734 cm -1 and 1712 cm -1 increased with increasing ageing time. As expected, for the same time of thermo-oxidation, the higher the ageing temperature was, the higher the oxidation product quantity. Fig 4 .c indicated a marked decrease in the absorbance at 3080 cm -1 at an early stage after exposure (loss of more than 50% of the absorbance after only 125h). The rate at which the absorbance of amides decreases depends on the temperature. Since one expects a correlation between the disappearance of amides and the formation of oxidation products, the variations in A 1734cm -1 vs A 3080cm -1 and A 1712cm -1 vs A 3080cm -1 were plotted, as shown in Fig. 5. This graph indicates that the correlation between the oxidation bands and the amide band depended on the temperature.

0
At the same extent of amide band intensity decrease resulting from oxidation, the same quantity of imides/acids should be formed, independent of the temperature. Complementary results obtained by TGA and thickness measurements (see supplementary information) showed that the mass loss was approximately 4% at 140 °C and below 1% at 200 °C for the same decrease in the absorbance at 3080 cm -1 . The cross-sectional microscopy images of samples oxidized at various temperatures (see supplementary information) clearly indicate that a non-negligible part of the sample has been lost in the case of the oxidation at 140 °C (500 hours at 140°C) whereas no significant loss was observed in the case of oxidation at 200 °C (25 hours at 200°C), for the same decrease in the absorbance at 3080 cm -1 . As shown in Fig. 5, at the same extent of amide band intensity reduction (3080 cm -1 ), the amount of oxidation products formed was lower at the lowest temperature. On the basis of the results found in the literature, an explanation can could be proposed, which involves the progressive loss of volatile products containing imide and acid functions, the loss being more important at 140°C since the oxidation time is much more important. The formation of volatile products has been reported by Gröning et al. [START_REF] Gröning | Headspace solid-phase microextractiona tool for new insights into the long-term thermo-oxidation mechanism of polyamide 6.6[END_REF].

3.1.b) SEC measurements

Molar mass variation is expected to result from thermal oxidation as degradation leads to crosslinking or chain scission. [START_REF] Chionna | Thermal oxidation products of nylon 6 determined by MALDI-TOF mass spectrometry[END_REF][START_REF] Okamba-Diogo | Investigation of polyamide 11 embrittlement during oxidative degradation[END_REF] It is then important to evaluate how the average A decrease in M n was observed with exposure time suggesting that a chain scission mechanism was predominant. As reported above in the case of the C-N-H bonds (Fig. 4.c), the rate was maximum at the beginning of exposure and decreased gradually with ageing time. In fact, 37% of the total decrease in molar mass was reached at 25 h of exposure at 140 °C, this decrease was more drastic with increasing temperature.

Furthermore, SEC measurements can provide access to the quantification of chain scission and crosslinking processes. The number of chain scission (s) and crosslinking (x) events per mass unit can be assessed according to the Saito relationship assuming random chain scission and crosslinking processes: [START_REF] Saito | On the effect of high energy radiation to polymers I. Cross-linking and degradation[END_REF] Eq.3

Eq.4

Fig. 6 displays the numbers of chain scissions (s) and crosslinking (x) events as a function of exposure time during oxidative ageing at 140 °C. This figure compares thin films (300 microns) with thick plates (1500 microns). In the case of thick plates, oxidation is limited to the outer layers whereas in the case of the thin films, no oxidation profile is observed. One of the consequences is an under evaluation of the modifications of molecular mass in the case of thick plates, since SEC measurements take into account the whole thickness of the samples, which means a large part of unoxidized polymer.

Fig.6 Chain scission (s) and crosslinking (x) numbers as a function of ageing time under air exposure at 140 °C (thin films vs thick plates)

The number of chain scission events increased until 1000 h of ageing and then reached a plateau, while there was a slight increase in the number of crosslinking events, which also achieved a plateau at 1000 h of ageing. Saito's equations confirmed that chain scissions events dominated over crosslinking; however, crosslinking still occurred to some extent. For samples aged under a long period of time (>500 h), the oxidation process was heavily advanced and led to the presence of insoluble parts in the material. These insoluble parts suggest that crosslinking is occurring at a higher amount than calculated.

3.2)

Modification of the macromolecular architecture

The morphological modifications, which occurred as a result of the thermo-oxidation process in the PA 6,6 samples, were analysed by differential scanning calorimetry (DSC). DSC ) / measurements were performed with PA 6,6 plates. Fig. 7 shows the DSC thermograms of the samples aged for 30 min at each temperature. In each thermogram, one can observe an exothermic peak prior to the endothermic melting peak at 265 °C. This peak, previously observed by Khanna in PA 6, PA 6,6 and PA 12, was attributed to a new sub-melting temperature transition. [START_REF] Khanna | Overview of transition phenomenon in nylon 6[END_REF] In the literature, the development of this exotherm is attributed to the rapid cooling of polymers. [START_REF] Khanna | Overview of transition phenomenon in nylon 6[END_REF][START_REF] Khanna | Measurement of crystalline index in nylons by DSC: complexities and recommendations[END_REF][START_REF] Li | Investigation of the sub-melting-temperature exotherm in melt-quenched polyamide-6/clay nanocomposites[END_REF] During processing, polyamide melted at 320 °C and then moulded at 70 °C, which caused quenching of the material. When the samples were aged at a temperature above the glass transition temperature (T g = 65 °C), there was a release of the internal stresses induced by quenching.

Moreover, a smaller endothermic peak was recorded approximately 10-30 °C above the ageing temperature (indicated by the arrows in Fig. 7.). Some authors have attributed this peak to the melting of microcrystals formed in the amorphous region. [START_REF] Khanna | Evaluation of thermal history of polymeric films and fibers using DSC/TMA/DMA techniques[END_REF][START_REF] Oswald | Development of a middle endotherm in DSC thermograms of thermally trated drawn PET yarnd and its structural and mechanistic interpretation[END_REF] The formation of these microcrystallites can be attributed to a post-crystallisation effect, as the samples are aged at a temperature above T g . Structural changes during post-crystallisation can occur as chain rearrangements caused by hydrogen bond breaking, relaxation of tie-molecules and increased chain motion. [START_REF] Eriksson | Effect of thermaloxidative aging on mechanical, chemical and thermal properties of recycled polyamide 66[END_REF] According to Shu et al., mobile chains in the amorphous region can combine with the crystallites as the chain motion increases, causing an increase in the crystallinity. [START_REF] Shu | Study on the long-term thermal-oxidative aging behaviour of polyamide 6[END_REF] To discriminate between the post-crystallisation effect and thermo-oxidation, some samples were aged in the absence of oxygen (i.e. experimental part). The DSC first heating curves of the thermo-oxidized samples (straight lines) and the aged samples without oxygen (dotted lines) at different ageing times at 160 °C were then compared and are shown in Fig. 8. The crystallinity evolution calculated from the thermograms is presented in Fig. 9. In the early stage of exposure, corresponding to the first 48 h at 160 °C, the significant increase in the crystallinity can be attributed to the post-crystallisation process, as the DSC curves and the crystallinity values are similar between the samples aged in the presence or absence of oxygen. We can assume that until 48 h of exposure at 160 °C, the postcrystallisation effect is predominant over oxidation (although the oxidation process cannot be excluded). Then, as shown in Fig. 9, the crystallinity ratio of samples aged without oxygen reached a plateau at 35% while the crystallinity ratio of thermo-oxidized samples still increased. This finding led us to assume that the end of the post-crystallisation process was reached after 48 h of exposure at 160 °C.

After 48 h, the melting endotherms of the thermo-oxidized samples contained a large melting peak at the lower temperatures, indicating a broad distribution in the crystal thickness, while the sub-melting temperature peak disappeared. With increasing ageing time, one can observe a new peak at 170 °C close to the main melting peak at 265 °C, suggesting a modification of the crystalline morphology. During thermo-oxidative ageing, chain scissions occur exclusively in the amorphous phase since crystalline fractions remain impermeable to oxygen. [START_REF] Shu | Study on the long-term thermal-oxidative aging behaviour of polyamide 6[END_REF][START_REF] Hawkins | The morphology of semi-crystalline polymers. Part I. The effect of temperature on the oxidation of polyolefins[END_REF] The literature indicates that chain scission can result in an increase in crystallinity, as chain fragments released by chain scissions in the amorphous phase can integrate into the crystalline phase if their mobility is high enough, which is known as chemi-crystallization The discolouration of the PA 6,6 plates of 1500 µm thickness with ageing time was examined by UV-VIS spectroscopy. Spectra were recorded for samples aged at 140 °C at various times of exposure. A large shoulder between 280 nm and 480 nm appeared in conditions of thermaloxidation, which increased in intensity during exposure. The samples became progressively yellow. Monitoring the absorbance at 450 nm allowed for a good perception of the colorimetric evolution of the material. [START_REF] Real | Ageing and characterisation of PVC-based compounds utilised for exterior applications in the building construction field: 1: Thermal ageing[END_REF] The kinetic curve of the changes in the absorbance at 450 nm of oxidized PA 6,6 at 140 °C is shown in Fig. 10. Fig. 11 shows a marked increase in the sample hardness during the first 100 h of exposure (see insert). This large variation was observed for all ageing temperature. At temperature below 200 °C, the Vickers microhardness seemed to reach a plateau for long-term exposure. In the case of thermo-oxidative ageing at 140 °C, the hardness increase levelled off after 750 h of ageing.

These findings can be explained considering the increase in crystallinity resulting from oxidation. Lorenzo et al. previously found a linear relationship between micro-hardness and crystallinity for PP and PE [START_REF] Lorenzo | Vickers microhardness related to mechanical properties of polypropylene[END_REF]. The mechanical properties of semicrystalline polymers depend on the degree of crystallinity as crystals are stiffer and tougher than amorphous regions. It would therefore be expected that crystallinity and hardness evolve similarly under thermal ageing. [START_REF] Bayer | Crystal hardness and average distance between stable entanglements in melt crystallized polyethylene[END_REF] In addition, PA 66 forms different crystal polymorphs whose development depends on the moulding conditions. The more stable α-phase is developed upon slow cooling and the chains are organized in a triclinic unit cell. A pseudohexagonal γ-mesophase is formed upon quenching the melt and is only metastable at lower temperatures. [START_REF] Russel | Structure and properties of injection-moulded nylon-6: Part 1 Structure and morphology of nylon-6[END_REF] Upon heating, this γmesophase converts irreversibly into the α-phase. [START_REF] Androsch | Crystallization of amorphous polyamides from the glassy state[END_REF] It has been previously shown that micro-indentation hardness values can be used to distinguish changes in the crystalline lattice of a polymer. [START_REF] Calleja | Phase changes in isotactic polypropylene measured by microhardness[END_REF][START_REF] Azzurri | Polymorphism of isotactic polybutene-1 as revealed by microindentation hardness. Part II: correlations to microstructure[END_REF] The initial steep increase in the microhardness with ageing time can also be associated with the transformation of the γ-phase into the α-phase, the latter having a larger hardness value. These variations in the hardness values between the two phases can be due to a difference in the densities of crystal nucleation. [START_REF] Azzurri | Polymorphism of isotactic poly(1butene) as revealed by microindentation hardness. 1. Kinetics of the transformation[END_REF]

4) Multiscale correlations

The aim was to establish correlations between the chemistry of thermo-oxidation and the deterioration of the functional properties. It is particularly important to determine relevant probes to quantify the oxidation of PA 6,6 and reflect changes in the macromolecular properties. The carbonyl index is commonly used to monitor chemical changes induced by ageing and is often considered to be well-connected with the mechanical properties. [START_REF] Forsstrom | Thermo oxidative stability of polyamide 6 films. I. Mechanical and chemical characterization[END_REF] However, its reliability is questioned in certain cases, for instance, in the case of polypropylene photooxidation, where degradation induces the formation of volatile oxidation products, which are not detected by infrared analysis of the solid polymeric matrix. [START_REF] Rouillon | Is carbonyl index a good probe to monitor polypropylene degradation ?[END_REF] In semicrystalline polymers, the complex morphology and chain scissions largely impact the mechanical properties such as elongation at break, affecting the correlations between both properties. [START_REF] Celina | Review of polymer oxidation and its relationship with materials performance and lifetime prediction[END_REF] As discussed in the previous section, the degradation of PA 6,6 during thermo-oxidation arises from a chain radical mechanism, which leads to the disappearance of an amide function at 3080 cm -1 in addition to the following:

-Formation of oxidation products, mainly imides and carboxylic acids -Chains scission responsible for the formation of carbonyl compounds -Crystallinity and µ-hardness increase On the basis of our experimental results, some comparisons can be made to gain a better understanding of the relationship between the modifications of various properties of polyamide 6,6 oxidized at a relatively high temperature (140 °C -200 °C). The aim was to determine whether the loss of functional properties was correlated with the chemical changes, which were monitored by measuring the absorbance at 3080 cm -1 ("overtone" amide II band).

In Fig. 12, the absorbance of amides is plotted as a function of ageing time (left scale).

Simultaneously, the changes in the molar mass are plotted (right scale) for the same conditions of ageing. To evaluate the interconnection between macromolecular architecture and microstructure, we plotted the variations in the crystallinity (left scale) and molar mass (right scale) with ageing time in Fig. 13-left. We also plotted the variation of crystallinity with molar mass (Fig. 13right). To evaluate the modifications of the macromolecular architecture, the thermo-oxidative degradation was characterized by DSC and SEC, shown above, and by DMA analysis to determine tan δ. The complete results of the DMA analysis are presented in the supporting information. We observed a decrease in the tan δ peak maximum, which indicated a loss in molecular mobility. [START_REF] Menczel | Thermal Analysis of Polymers: Fundamentals and Applications[END_REF] The variations in the crystallinity and maximum of tan δ peak with thermal ageing at 140 °C are shown in Fig. 14

Fig.14 Crystallinity and maximum of tan δ peak as a function of thermal ageing time at 140 °C (left) and variation of crystallinity in function of maximum of tan δ peak (right)

As expected, the decrease in the molecular mobility represented by the maximum of tan δ was strongly related to the increase in crystallinity. As the crystallinity modification was strongly interconnected with both molar mass and molecular mobility, one can conclude that the microstructure and the macromolecular architecture of PA 6,6 were closely correlated and thus impacted in the same manner by thermal degradation. The microhardness is directly related to the morphological and structural changes in a semicrystalline polymer. [START_REF] Bartolomeo | Dynamic mechanical analysis and Vickers micro hardness correlation for polymer coating UV ageing characterization[END_REF][START_REF] Katare | Microhardness testing to detect radiation-induced crosslinking in polystyrene: poly(methylmethacrylate) polyblends[END_REF][START_REF] Ania | Physical ageing and glass transition in amorphous polymers as revealed by microhardness[END_REF][START_REF] Santa-Cruz | Relating microhardness of poly(ethylene terephthalate) to microstructure[END_REF] Eq.5

where H is the theoretical hardness, ν is the volume fraction of crystallized spherulites and H c and H a are the hardness of the crystalline and amorphous phases. The hardness of the crystalline and amorphous phases of PA 6,6 are equal to 463 MPa and 65 MPa respectively.

[69]

In Fig. 15, the crystallinity (X c ) and Vickers micro-hardness (H v ) are plotted as a function of ageing time at 200 °C and compared to the theoretical hardness H calculated from Eq.6. The inset shows the linear increase in the crystallinity with Vickers µ-hardness. The data in Fig. 15 indicate that the crystallinity increase was closely correlated to that of the microhardness. It is also important to note that the calculated hardness values are in complete agreement with the experimental hardness data. This result shows a pseudo-quantitative correlation between these two measurement scales and between the physical properties and the macromolecular architecture. On this basis, we can establish a relationship between the crystallinity and the corresponding local mechanical properties measured through the microhardness. The relation was determined after exposure of the PA 6,6 samples at 200 °C and can be written as follows:

Eq.6

Direct correlations were drawn between changes in the chemical structure, modification of the microstructure and thermal/mechanical properties and this whatever the measurement scales, ranged from the molecular to the macroscopic scale. The same correlations (chemical structure-macromolecular properties) were found for all the studied temperatures and the results are presented in the supporting information.

5) Conclusion

The decrease in absorbance of the overtone of the amide II band at 3080 cm -1 was shown to be the best probe for oxidative degradation. At the molecular level, chain scissions predominated largely over crosslinking, regardless of the exposure conditions. At the macromolecular scale, the post-crystallisation process predominated at first, and then oxidation dominated. Both processes led to an increase in crystallinity. At scales corresponding to the functional properties, the oxidation caused yellowing, as shown by the increase in the absorbance at 450nm, and an increase in the Vickers microhardness. The modification of the overtone of the amide II band was correlated with the change in the molecular weight, which was directly connected to the increase in the crystallinity. This last parameter was also reflected by the microhardness. All of these PA 6,6 properties were proven to be reliable criteria for the multiscale characterization of the thermo-oxidative degradation of polyamides. The multiscale approach shows good correlations between the various scales under study and the exposure conditions (see supporting information).

In a next communication, the influence of oxygen diffusion on the thermo-oxidation of polyamides will be investigated in a multiscale approach.

Fig. 7

 7 Fig.7 DSC thermograms of PA 6,6 aged for 30 min at each temperature (first heating step)

Fig. 8 2 Fig. 9

 829 Fig.8 DSC thermograms of PA 6,6 aged at 160 °C in an oven with or without O 2

  [START_REF] Craig | Crystallization and chemi-crystallization of recycled photodegraded polypropylene[END_REF][START_REF] Rabello | Crystallization and melting behaviour of photodegraded polypropylene -I[END_REF][START_REF] Zhao | Effect of irradiation on crystallinity and mechanical properties of ultrahigh molecular weight polyethylene[END_REF]. However, since DSC heating curves show a wide crystal distribution at lower temperatures, it is noteworthy to consider the formation of new crystals arising from the reorganization of chain fragments. These crystals are smaller than those formed by lamellar thickening, and thus melt at lower temperatures, as already demonstrated by Thanki et al. in their study of PA 6,6 photooxidation.

Fig. 10 1 ,

 101 Fig.10 Changes in absorbance at 450 nm of the samples vs. oxidation time at 140 °C

  According to the literature, the products responsible for yellowing of PA and their formation mechanisms are not fully understood and still subject of extensive researches.[START_REF] Li | Study on discoloration mechanism of polyamide 6 during thermooxidative degradation[END_REF][START_REF] Karstens | Thermo-oxidative degradation of polyamide 6 and 6,6[END_REF][START_REF] Marek | Photodegradation and yellowing of polyamides[END_REF][START_REF] Fromageot | Thermooxidation yellowing of aliphatic polyamides[END_REF] 3.3.b) MicrohardnessMicrohardness is a convenient way to determine the mechanical properties on the surface of a polymer. The method consists of applying a sharp indenter with a given load onto a material surface and measuring the local deformation produced. Microhardness tests were performed on the surface of blank samples and samples aged at temperatures between 140 °C and 200 °C. The shapes of the diamond marks left by the indenter on the PA 6,6 surfaces are shown in the supporting information. The modification of micro-hardness as a function of ageing time is shown in Fig.11.

Fig. 11

 11 Fig.11 Vickers µ-hardness variations vs ageing time (from 140 °C to 200 °C in the presence of oxygen)

Fig. 12 Δ

 12 Fig.12 Δ A 3080 cm-1 and molar mass as a function of thermal ageing time at 140 °C

Fig. 13

 13 Fig.[START_REF] Gröning | Headspace solid-phase microextractiona tool for new insights into the long-term thermo-oxidation mechanism of polyamide 6.6[END_REF] Variations in molar mass and crystallinity during oven ageing at 140 °C (left) and variation of crystallinity with molar mass (right)

  -left and the variation of crystallinity in function of maximum of tan δ peak is shown in Fig 14-right.

Fig 14 -

 14 right indicates that the crystallinity varies linearly with the maximum of tan δ peak.

Fig. 15

 15 Fig.15 Kinetics of crystallinity and µ-hardness Vickers under thermal ageing at 200 °C

Table . 1

 . . Band assignments and corresponding wavenumbers of the oxidative species in polyamides

	Species	Wavenumber (cm -1 )
	"Free" carboxylic acids	1756
	Imides	1734
	"Bonded" carboxylic acids	1712

  weight is impacted by the chemical changes of the matrix. The molecular weights were determined by performing SEC measurements after the ageing experiments at 140 °C and 180 °C. This technique is certainly the most appropriate, as rheometry and viscosity measurements require the polymer to be in the molten state, which is thus further exposed to heat.[START_REF] Sagar | Autoxidation of n-alkyl amides. Part III. Mechanism of thermal oxidation[END_REF][START_REF] Chen | Analysis of polyamides by size exclusion chromatography and laser light scattering[END_REF] Thin films were analysed to avoid the effects of oxygen diffusion. The obtained M n values are presented in Table.2.

	/ 140 °C
	/ 200 °C
			-1
			cm
			A 1712
	Table.2. Modification of the M n of the samples aged under air at 140 °C
	Ageing time (h)	M n, 140 °C (kg.mol -1 )	M n, 180 °C (kg.mol -1 )
	0	30	30
	4		21
	10		15
	25	19	14
	100	12
	200	8

molecular

Acknowledgments

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


Pierre-Olivier Bussière
email: pierre-olivier.bussiere@sigma-clermont.fr

Jean-Luc Gardette
email: luc.gardette@uca.fr

Géraldine Rapp
email: geraldine.rapp@uca.fr

Claire Masson

Sandrine Therias
email: sandrine.therias@uca.fr

New insights into the mechanism of photodegradation of chitosan

Keywords: chitosan, photodegradation, oxidation, mechanism

Chitosan films were subjected to accelerated artificial weathering at  > 300 nm and 60°C in the presence of O 2 . The resulting variations in the chemical structure were characterized by IR spectroscopy and UV-visible spectroscopy, and a photooxidation mechanism was proposed based on the identified oxidation photoproducts. The formation of gluconolactone derivatives leading to chain scissions was shown. In addition, low molecular weight photoproducts, which accounted for chitosan deacetylation, were detected. Furthermore, crosslinking reactions occurred, as revealed by gel fraction characterization. Variations in the mechanical and surface properties were characterized by AFM, and the reduction in macroscopic properties was correlated with the structural changes observed at the molecular scale by a multiscale approach.

Introduction

Chitosan is a natural polysaccharide composed of N-acetyl glucosamine and glucosamine units, two derivatives of glucose, that are linked together by glycosidic bonds [START_REF] Synytsya | Modification of Chitosan-Methylcellulose composite films with meso-Tetrakis(4-sulfonatophenyl)porphyrin[END_REF] (Scheme 1). The presence of hydroxyl and amino groups promotes H bonds between macromolecular chains. Thus, chitosan can crystallize into a hydrated form and an anhydrous form [START_REF] Ogawa | Three D structures of chitosan[END_REF]. Due to its solubility in water, biodegradability, biocompatibility and non-toxicity, many studies have focused on the use of chitosan for medical purposes such as soft tissue engineering [START_REF] Tchemtchoua | Development of a chitosan nanofibrillar scaffold for skin repair and regeneration[END_REF], bone tissue engineering [START_REF] Nazeer | Intercalated chitosan/hydroxyapatite nanocomposites : Promising materials for bone tissue engineering applications[END_REF] and drug release [START_REF] Lien | In Vitro Assessment of Alkylglyceryl-Functionalized Chitosan nanoparticles as permeating vectors for the bloodbrain barrier[END_REF]. Due to the same "green" characteristics, chitosan was studied to be employed as a scaffold for water treatment [START_REF] Moczek | Novel water-soluble photosensitizers from chitosan[END_REF] and as packaging material in the food industry [START_REF] Aider | Chitosan application for active bio-based films production and potential in the food industry: review[END_REF].

Scheme 1: Chemical structure of chitosan

The dependence of the thermal degradation of chitosan on the degree of deacetylation was studied [START_REF] Gamiz-Gonzalez | Kinetic study of thermal degradation of chitosan as a function of deacetylation degree[END_REF], [START_REF] Villar-Chavero | Thermal and kinetics of the degradation of chitosan with different deacetylation degrees under oxidizing atmosphere[END_REF]. Changes in the degree of deacetylation of chitosan were reported during thermal treatment above 200°C (Zawadzki &Kaczmarek, 2010). The effect of the degree of deacetylation on the radiation-induced degradation of chitosan [START_REF] Taskn | The effect of degree of deacetylation on the radiation induced degradation of chitosan[END_REF] was also investigated and revealed that deacetylation is an important factor for controlling the degradation rate of chitosan. Degradation of chitosan under different stresses (microwave [START_REF] Wasikiewicz | Green » molecular weight degradation of chitosan using microwave irradiation[END_REF] and cavitation [START_REF] Huang | Degradation of chitosan by hydrodynamic cavitation[END_REF] has also been reported. Regarding photodegradation, a few papers report the use of UV irradiation as a sterilizing agent at a wavelength of 254 nm: chitosan [START_REF] Sionkowska | Influence of UV-irradiation on molecular weight of chitosan[END_REF] [START_REF] Andrady | Spectral Sensitivity of chitosan photodegradation[END_REF]) [START_REF] Mucha | Complex study on chitosan degradability[END_REF]; blends of chitosan/poly(vinyl pyrrolidone) [START_REF] Sionkowska | The influence of UV irradiation on the mechanical properties of chitosan/poly(vinyl pyrrolidone) blends[END_REF] and of chitosan/poly(ethylene oxide) [START_REF] Kowalonek | Surface and thermal properties of UV-irradiated chitosan/poly(ethylene oxide) blends[END_REF]; and chitosan modified with keratin [START_REF] Sionkowska | The influence of UV irradiation on the properties of chitosan films containing keratin[END_REF], with silk fibroin [START_REF] Sionkowska | The influence of UV-irradiation on thermal and mechanical properties of chitosan and silk fibroin mixtures[END_REF] or with tannic acid [START_REF] Sionkowska | The influence of UVirradiation on chitosan modified by the tannic acid addition[END_REF]. The effect of UV degradation was mainly characterized on the thermal or mechanical properties but also on the surface of chitosan films [START_REF] Sionkowska | The influence of UV irradiation on the surface of chitosan films[END_REF]. UV irradiation at = 254 nm was shown to provoke a decrease in molecular weight as a result of chain scissions. Changes in IR spectra were observed, such as the formation of bands attributed to carbonyl groups at 1730 cm -1 and an increase of bands attributed to amide at 1655 cm -1 (C=O) and 1560 cm -1 (N-H) accompanied with a decrease of the IR band at 1590 cm -1 corresponding to the amine group (Sionkowska et al;2013) [START_REF] Mucha | Complex study on chitosan degradability[END_REF].

One study reports the TiO 2 photocatalytic-oxidation of solid-state chitosan under fluorescent irradiation [START_REF] Nawi | Photocatalytic-oxidation of solid state chitosan by immobilized bilayer assembly of TiO2-chitosan under a compact household flurescent lamp irradiation[END_REF]; more recently, the effect of UV irradiation on the physicochemical characteristics of chitosan modified with nanoparticles of silver vanadate [START_REF] Abdelghany | The effects of prolonged UV irradiation on the physicochemical characteristics of chitosan lamellar films modified with nanoparticulate silver vanadate nanorods[END_REF] was reported. However, weathering of chitosan films under UV-light irradiation at long wavelengths (>300 nm) under accelerated conditions of natural outdoor weathering has received little attention [START_REF] Sionkowska | Weathering of chitosan films in the presence of low-and high-molecular weight additives[END_REF], and no mechanism of photooxidation has been proposed.

The aim of this work is to propose a photodegradation mechanism of chitosan in photooxidative conditions at  > 300 nm under accelerated artificial conditions (at 60°C in the presence of O 2 ), which can explain the variations in material properties. The main objective of the paper is to provide evidence of the consequences of photochemical reactions on the macroscopic properties of chitosan and to qualitatively and quantitatively confirm the direct relationship between variations in the chemical structure and variations in the mechanical properties due to a multiscale approach of the degradation.

Experimental

Materials

Chitosan was purchased from Polysciences Inc. as a purified powder with an average molecular weight of 15 000 g.mol -1 and a minimum degree of deacetylation of 85%. -Thus, the R group in Scheme 1 is defined as -H for 85 % of monomer and as acetamide group for 15 % of monomer. The deacetylation degree of the chitosan film (after Soxhlet treatment) was monitored using IR spectra by following the formula given by Kasaai [START_REF] Kasaai | A review of several reported procedures to determine the degree of Nacetylation for chitin and chitosan using infrared spectroscopy[END_REF] for the ratio of absorbance at 1655 cm -1 and 3450 cm -1 , at 1320 cm -1 and 1420 cm -1 , and at 1560 cm -1 and 2875 cm -1 . The degree of deacetylation was then estimated to be 82  4 %, and the NH 2 group predominated the acetamide group in this chitosan sample.

To prepare the films, chitosan powder was dissolved in acidified water (acetic acid 1% v/v, pH = 5) at a concentration of 10-20 mg.mL -1 and stirred for 3 h at 60°C. Solutions were then deposited in Petri dishes, and self-standing films were obtained after drying in ambient air for a few hours. Films of thickness in the range 25 -50 µm or 150 µm were obtained for the different subsequent analyses. Purification of the films was performed by Soxhlet extraction in MeOH for 24 h. The films were then dried at 60°C under vacuum for 14 hours.

Irradiation

UV-visible light irradiation ( > 300 nm) of thin films was performed in a SEPAP 12/24 unit, which was designed for studying polymer photodegradation during artificial ageing with medium-accelerated conditions [START_REF] Philippart | Influence of light intensity on the photooxidation of polypropylene[END_REF]. The chamber consisted of a square reactor equipped with four medium-pressure mercury lamps situated vertically at each corner of the chamber. Wavelengths below 300 nm were filtered by the borosilicate envelope of the lamps, acting as a UV filter below 300 nm (see Fig S1). In the centre of the chamber, the samples were fixed on a 13 cm-diameter rotating carousel that can hold 24 samples. The temperature in the chamber of SEPAP 12/24 is controlled at the surface of a polymer film through a Pt 1000 probe and continually regulated by air circulation. The temperature at the surface of the samples was set to 60°C.

Characterization Techniques

Spectroscopic analysis

Changes in the UV-visible spectra were monitored with a Shimadzu 2600 plus spectrophotometer equipped with an integrating sphere. IR spectra were recorded in transmission mode with a Nicolet 6700 Fourier transform infrared (FTIR) spectrophotometer operated with OMNIC software. The spectra were obtained with 32 scan summations at 4 cm - 1 resolution. IR analysis of chitosan powder was performed by mixing 2 mg of this powder with 200 mg of KBr.

Chemical derivatization reaction

Most of the oxidation products were identified by performing chemical derivatization treatments that selectively convert oxidation products into chemical groups with different IR characteristics [START_REF] Wilhelm | Infrared identification of carboxylic acids formed in polymer photooxidation[END_REF]. Ammonia (NH 3 ) reacts with carboxylic acids to generate carboxylate ions and with esters to generate amides. NH 3 treatment was performed at room temperature in simple flow reactors that could be sealed off to allow the reaction to proceed. 2,4-Dinitrophenylhydrazine (2,4 DNPH) was used to identify aldehydes and ketones.

The formation of 2,4-dinitrophenylhydrazone can be detected by two IR absorption bands at 1595 cm -1 and 1620 cm -1 and a very intense UV-Vis absorption band at 365 nm.

Solid Phase Micro Extraction (SPME)

Films were irradiated in sealed vials to collect the volatile photodegradation products. SPME analysis was performed using Carboxen-PDMS fibres (75 mm) from Supelco (Bellefonte, PA, USA) with an extraction time of 5 min at 60°C, and gas chromatographymass spectrometry (GC-MS) was performed with a PEG Supelcowax® 10 column (30 m x 0.25 mm x 0.25 mm) from Supelco.

Gel fraction

Crosslinking of polymers was evaluated by gravimetric measurements of the gel fraction (Gf). Irradiated films (approximately 5 mg) were immersed in acidified water (acetic acid 1% v/v) (5 mL) for 48 h. The solutions were then filtered using a Büchner funnel. The insoluble fractions were dried at 60°C and then weighed. The gel fraction is defined by equation 1:

(1)

Atomic force microscopy

AFM measurements were performed in tapping mode using an atomic force microscope (AFM) Bruker Multimode driven by a Nanoscope V control unit. The surface topographical images were analysed by the Nanoscope 7.20 software. Images were obtained at three different locations, and the average roughness was determined from these images at four different locations for each image. The tip was an RTESPA-300 cantilever (Bruker) with a spring constant of approximately 40 N/m and a radius of curvature of approximately 8 nm.

AFM nano-indentation

The force-distance curves were measured at a constant deflection (or constant load). Nanoindentations were performed using a diamond tip (64.05 kHz) with a spring constant of 234.5 N/m and a curvature radius of 25 nm. Three tests of nano-indentation were performed on different areas of the polymer film. Each test corresponded to nine indentations; thus, 3×9 force curves were used with a load that varied from 1 to 5 μN. The force-displacement curves obtained via AFM nano-indentation measurements were analysed using the model proposed by [START_REF] Oliver | An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments[END_REF] (procedure (O&P)).

AFM nanoscale thermal analysis

Variations in the thermal properties of the films were monitored at the surface using an AFM nanoscale thermal analysis module (Vita). The probe was heated from 20°C to 300°C at a rate of 5 °C/s, and each measurement was performed 3 times to ensure good reproducibility.

The Vita probes can also be used to obtain AFM images of the surface [START_REF] Collin | Physicochemical and mechanical impacts of photo-ageing on bisphenol a polycarbonate[END_REF].

Results and discussion

Characterization of chitosan films before irradiation

The main spectral features of chitosan and assignment of the IR bands according to the literature [START_REF] Mucha | Complex study on chitosan degradability[END_REF] (Valentin, Bonelli, Garrone, Di Renz &, Quignard, 2007) are the OH and NH stretching bands at 3435 cm -1 and between 3300-3150 cm -1 , respectively, the C=O stretching band at 1655 cm -1 , the NH 2 scissoring and NH deformation bands at 1590 cm -1 and 1560 cm -1 and the C-O-C stretching band attributed to the osidic bond or pyranose ring at 1150 cm -1 . Figure 1 The IR spectra shown in Figure 1 indicate that Soxhlet extraction of the chitosan film in MeOH provokes the disappearance of three IR bands at 1710 cm -1 , 1550 cm -1 and 1410 cm - 1 . This finding can be attributed to the removal of acetic acid and acetate ions. These two products are retained from the process that was used to obtain the films and involved preliminary dissolution in acetic acid-containing solution, which could therefore be trapped within the films at low concentrations even after drying. Acetate ammonium groups are likely generated as a result from the reaction of acetic acid with the amine functions of chitosan [START_REF] Rinaudo | Influence of acetic acid concentration on the solubilization of chitosan[END_REF]. The IR spectrum of chitosan film after Soxhlet extraction is similar to the spectrum of chitosan powder that was pressed into KBr pellets. The experiments reported in this paper were performed for chitosan films that were purified by Soxhlet extraction in MeOH for 24 h.

Photooxidation of chitosan

Analysis of chemical variations by IR spectroscopy

Irradiation in accelerated artificial conditions at  > 300 nm in the presence of O 2 (photooxidation) resulted in noticeable modifications to the IR spectra (Figure 2a). Irradiation provoked a marked decrease in the absorption in the domain 3500 -3300 cm -1 , which indicates a loss of constitutive water or water generated from the dissolution process and/or breaking of hydrogen bonds present in the chitosan film [START_REF] Ogawa | Three D structures of chitosan[END_REF]. A decrease in the IR absorption band of C-O-C bonds at 1150 cm -1 was also observed, which could reflect chain scission reactions and/or ring opening of glucosamine and N-acetyl glucosamine. To visualize the changes in the IR spectra in the carbonyl domain, the spectra were subtracted from those of the unexposed film in the domain 2000-1500 cm -1 and are presented in Figure 2b. A relatively broad band was observed, with a maximum at 1675 cm -1 and shoulders at 1730 cm -and 1780 cm -1 . This result is similar to the 254 nm irradiation-induced spectral The decrease in absorbance at 1150 cm -1 (Figure 2) reflects the disappearance of the pyranose ring/acetal bond and can be correlated with the accumulation of oxidation products at 1730 cm -1 . As proposed by several authors [START_REF] Mucha | Complex study on chitosan degradability[END_REF]) [START_REF] Sionkowska | Weathering of chitosan films in the presence of low-and high-molecular weight additives[END_REF], the accumulation of oxidation products at 1730 cm -1 could be related to the formation of an ester function and simultaneous disappearance of the 1,4 osidic bond. Taking into account the correlation between the increase in absorbance at 1730 cm -1 and the decrease at 1150 cm -1 , which reflects the disappearance of the pyranose ring/acetal bond (hydrogen abstraction on carbon atom (b) in Scheme 2), this ester bond formation would be consistent with the generation of gluconolactone.

The IR band at 1675 cm -1 can be attributed to the vibration νC=O of a carbonyl function of amide I. Considering the chemical structure of chitosan, the formation of a primary amide can also be explained by a mechanism involving oxidation in the  position of the amine function (hydrogen abstraction on carbon atom (c)). This mechanism is represented in Scheme 2 and discussed at a later point in the manuscript.

Derivatization treatment

Chemical derivatization treatments with NH 3 were performed to identify the degradation products observed by IR analysis at 1730 cm -1 . NH 3 reacts with carboxylic acids to generate carboxylate ions and reacts with esters, lactones and anhydrides to generate amides. First, it was verified that no reaction with NH 3 occurred with pristine chitosan. NH 3 treatment of photooxidized films provoked a decrease in the broad absorption at 1730 cm -1 and the formation of two weak bands at 1640 cm -1 and 1595 cm -1 (see Fig. S2). These bands indicate the formation of amides (1640 cm -1 ) and ammonium carboxylates (1595 cm -1 ).

Amides were formed from the reaction of NH 3 with esters or anhydride, and ammonium carboxylates resulted from the reaction of NH 3 with carboxylic acid or anhydrides. NH 3 treatment allows the identification of esters and carboxylic acids as oxidation photoproducts.

Ester formation was suggested by several authors [START_REF] Sionkowska | Influence of UV-irradiation on molecular weight of chitosan[END_REF], [START_REF] Andrady | Spectral Sensitivity of chitosan photodegradation[END_REF] [START_REF] Mucha | Complex study on chitosan degradability[END_REF]) [START_REF] Wasikiewicz | Degradation of chitosan and sodium alginate by gamma radiation, sonochemical and ultraviolet methods[END_REF], but to the best of our knowledge, direct evidence of gluconolactone formation has not yet been reported. For comparison and to confirm the presence of gluconolactone in the photooxidized chitosan film, molecular gluconolactone was blended with pristine chitosan, and the blend was analysed by IR spectroscopy. The IR spectrum is shown in Figure 3a

h before DNPH h after DNPH

To complete the identification of the oxidation photoproducts, chemical treatment using 2,4-DNPH was performed on photooxidized chitosan films. The UV-visible spectra of photooxidized chitosan films after 2,4-DNPH treatment (Figure 3b) show a broad band with a maximum at 365 nm corresponding to the phenylhydrazone derivative. This result indicates that aldehydes and/or ketones were formed by the photooxidation of chitosan.

Analysis of volatile products

Photooxidation of polymers is known to be responsible for the formation of low molecular weight products that are likely to migrate from the solid polymer samples and are not detected by the analysis of the films by the spectroscopic methods reported above (Gaume, Wong-Wah-Chung, Rivaton, Therias, Gardette, 2011). To identify the volatile degradation products, SPME-GC/MS was performed. Formamide and acetamide were detected as the main degradation products (see Figure S3). It is also important to note the presence of acetic acid, which was used to solubilize chitosan. As formamide and acetamide molecules contain nitrogen atoms in their structure and therefore are not formed by photooxidation of the cellulose backbone, these products can be attributed to the degradation of the pending groups of chitosan. As a consequence, the deacetylation degree of the chitosan film is changed.

The formation of volatile products (such as acetamide) and the accumulation of ketone/aldehyde groups can be connected to the cleavage of pending groups on carbon atoms (a), following the mechanism given in Scheme 2.

Characterization of chain scission/crosslinking

Gel fraction

To provide evidence of the variations in polymer architecture, the insoluble/gel fractions of photooxidized chitosan films were measured for irradiation times as long as 400 hours. The gel fraction of photooxidized chitosan films increased progressively from 0 (no insoluble fraction before irradiation) to 96 % after 400 hours of irradiation, which indicates that the polymer becomes almost insoluble. This high gel fraction value can be attributed to the formation of a three-dimensional network from crosslinking during the irradiation of chitosan.

Degradation mechanism

A photooxidation mechanism that explains the formation of the identified photoproducts was proposed (Scheme 2). This mechanism is based on a radical chain reaction with initial hydrogen abstraction on the carbon atoms noted (a), (b) and (c) by radicals from chromophoric impurities, or defects that in commercial chitosan could include proteins and metals. The photodissociation of acetamide groups attached to glucosamine unit, (broad band from 250 till 350 nm) could also be a potential source of radicals able to initiate the chain reaction. However, despite the many articles published in the literature, no real proof was brought that this reaction occurs on exposure to wavelengths above 300 nm. [START_REF] Rideal | Photochemical reactions in monolayers I-Photochemical properties of the ketoimino linkage[END_REF]. However, no evidence was obtained that this reaction would occur (Sharkey & Mochel,1959). [START_REF] Moore | The photochemical degradation of polyamides and related model Nalkylamides[END_REF]. Radical Oxygenated Species such as HOO•, HO•, R-OO• can be formed and initiate the oxidation by hydrogen abstraction on the polymer chains. The initiation processes in polymer degradation have been reviewed many times in the literature (see for example [START_REF] Scott | Initiation processes in polymer degradation[END_REF]. This is followed by the addition of dioxygen, the formation of hydroperoxides and the decomposition of hydroperoxides to generate macroalkoxy radicals (Scheme 2). Hydrogen abstraction from the carbon atom (a) generates the macroalkoxy radical A, which may decompose to generate gluconolactone (its IR band at 1730 cm -1 was identified by NH 3 treatment). This reaction generates lactones such as gluconolactone and is accompanied by chain scissions. Hydrogen abstraction from carbon (c) leads to the formation of radical C, which may decompose to form amide groups via ring-opening (IR band at 1675 cm -1 ).

An important issue is the role played by the temperature in the photooxidation performed at 60°C. Thermal ageing at 60°C in a ventilated oven in absence of light (dark control) during the same time (up to 420 h) was then performed. Comparing the IR spectra after exposure for 420 h to thermooxidation at 60°C or 400 h to photooxidation at  > 300 nm-60°C (see Fig. S4) clearly indicates that the IR spectra was only slightly modified after thermooxidation for 420 h, whereas exposure to light at  > 300 nm-60°C resulted in dramatic modification of the infrared spectra with the formation of the photproducts which formation has been described above (part 3.2.1). This result clearly indicates that in the current ageing conditions, the oxidation mainly results from irradiation.

Scheme 2: Simplified photooxidation mechanism of chitosan

Consequences of photooxidation on the properties of chitosan

UV-visible analysis

Analysis of a chitosan film by UV-visible spectroscopy was carried out during photooxidation (Figure 4). The absorption front of the UV-vis spectra progressively shifted towards longer wavelengths during photooxidation, leading to yellowing of the chitosan film, and a shoulder with an absorption maximum could be observed at 270 nm. This feature was attributed to the formation of carbonyl groups and the cleavage of glycosidic bonds by several authors [START_REF] Synytsya | Modification of Chitosan-Methylcellulose composite films with meso-Tetrakis(4-sulfonatophenyl)porphyrin[END_REF]) [START_REF] Wang | Study on the synergetic degradation of chitosan with ultraviolet light and hydrogen peroxide[END_REF].

Mechanical properties

Variations in the mechanical properties at the surface of chitosan films during photooxidation were assessed by elasticity measurements at the nanoscale (Table 1). To confirm the behaviour at the nanoscale, the polymer surface was analysed by AFM nanoindentation measurements. The loading/unloading indentation curves were measured for two irradiation times (see Figure S5).

The distance or displacement to reach the maximum force decreased after 200 h of irradiation. This is characteristic of an increase in the surface nano-hardness and Young's modulus. The increase in the nano-hardness of chitosan observed under irradiation can be correlated to crosslinking reactions. One can also notice a small negative deflection after 200 h of irradiation that is consistent with the increase in the adhesion work. The increase in the adhesion work with the exposure time has already been connected [START_REF] Bussiere | Multiscale Investigation of the Poly(N-vinylcarbazole) Photoageing Mechanism[END_REF] to the accumulation of polar species at the surface of the sample and thus to the formation of oxidation products. In addition, the nano-hardness of the polymer sample can be calculated from these loading-unloading curves.

AFM nanoscale thermal analysis

In addition, variation in the glass transition temperature resulting from UV light exposure were monitored by AFM thermal analysis (Vita). The deflection-temperature curves (Figure S6) after 100 h and 300 h of irradiation were compared with the deflectiontemperature curve before exposure. Nano-T g values are reported in Table 1. After irradiation, the deflection shifts to higher temperatures. As T g is related to the deflection, this result indicates that T g increased with irradiation time. The Flory theory (Fox & Flory,1950) relates T g to the molecular weight, reflecting crosslinking reactions. The increase in nano-hardness and nano-T g are consistent with crosslinking reactions leading to higher molecular weights, with a marked increase in the gel fraction (chitosan that became almost insoluble after 400 h of irradiation at  > 300 nm in the presence of oxygen).

Surface properties

Roughness values (RMS) of a chitosan film surface were calculated before and after irradiation. Modifications of the roughness (RMS) displayed in Figure 5 show that RMS increased during irradiation. As previously reported, these variations arise from the formation of oxidation products of the polymer sample and of volatile products [START_REF] Berthumeyrie | Photooxidation of cellulose nitrate: new insights into degradation mechanisms[END_REF].

Multiscale correlation

As shown above, the degradation of chitosan provoked by exposure to UV light in the presence of air led to variations in the following:

• chemical structure (oxidation, chain scission/crosslinking)

• macromolecular architecture (increase in network density, i.e., T g )

• mechanical properties (increase in hardness)

• surface properties (increase in roughness and yellowing). Therefore, the degradation caused by exposure to UV light can be characterized by the following parameters: amide and gluconolactone formation, C-O-C conversion rate, tanδ, nano-indentation and increased hardness. To check the inter-connection between the evolutions of these parameters during irradiation, a quantitative description was attempted.

Figure 6 shows the variations in the concentrations of amide groups (IR band at 1675 cm -1 ), gluconolactone (IR band at 1730 cm -1 ), and ethers (IR band at 1150 cm -1 ) during irradiation (the film thickness was 25 microns, and the absorption coefficients were as follows: CONH: 340-380 L.mol -1 .cm -1 , gluconolactone: 650-700 L.mol -1 .cm -1 , COC: 40-50 L.mol -1 .cm -1 ) (from the literature) [START_REF] Okamba-Diogo | Molecular and macromolecular structure changes in polyamide 11 during thermal oxidation[END_REF]. Figure 6 shows that the concentration of amide groups (1675 cm -1 ) produced during photooxidation was higher than the concentration of gluconolactone (1730 cm -1 ). Indeed, the concentration of gluconolactone generated after 600 hours of ageing corresponded to the concentration of amide group after 100 hours of ageing. To connect all the degradation criteria from the molecular scale to the macroscopic scale, we aimed to find kinetic information from the different parameters responsible for the degradation versus exposure time. We used a phenomenological model [START_REF] Rose | Thermal oxidative degradation of epoxy resins: evaluation of their heat resistance using invariant kinetic parameters[END_REF]) [START_REF] Musto | Photo-oxidation of high performance epoxy networks: Correlation between the molecular mechanisms of degradation and the viscoelastic and mechanical response[END_REF] to describe the kinetic processes of degradation as follows:

where X(t) is the value of a selected parameter obtained for various times t, X (t=0) is the parameter value at t = 0 and k is the Arrhenius-type kinetic constant. The equation was applied to the experimental parameters X, such the absorbance A at 1730 cm -1 , nanohardness, and T g . The results in Table 2 indicate that only the appearance of amide groups and the increase in yellowing follow zero-order kinetics. The accumulation of gluconolactone and the various parameters that were selected in an attempt to establish a correlation between the variation in chemical structure and mechanical properties follow a first-order kinetic, and the k values for the main parameters are similar (the average k value was 2.10 -3 .h -1 ). The half-life time was calculated from the kinetics, and the average t 1/2 value was approximately 200 hours.

The good agreement between the degradation-rate k-values of the different criteria confirms the cross-correlation established. These results underline the difference in reactivity of all the detected degradation species and confirm diverse routes of degradation.

Conclusion

Photooxidation of chitosan resulted in yellowing, crack formations, and an increase in roughness. The increases in network density and nano-hardness are relevant in terms of a reduction in the material's functional properties. A mechanism accounting for the main degradation routes for chitosan photooxidation at the molecular scale was proposed. We demonstrated the consequences of the chemical modifications on the material properties, and we showed correlations between these chemical modifications and the evolution of the mechanical properties. Our results allowed a « quantitative » correlation between the relevant degradation criteria.

Declaration of Interest

  compares the carbonyl domain in the IR spectrum of the commercial chitosan powder pressed into a KBr pellet to the same domain in the IR spectra of the pristine films before and after Soxhlet extraction in MeOH for 24 h.

Figure 1 :

 1 Figure 1: IR spectra of chitosan films (e = 25 µm) before and after 24 h Soxhlet extraction in MeOH and the chitosan powder in KBr pellets.

  Figure 2: a) IR spectra of a chitosan film (e = 25 µm) during photooxidation b) Subtracted IR spectra in the carbonyl domain

  Figure 3a: IR spectrum of a film (e=25 µm) of chitosan blended with gluconolactone.

Figure 4 :

 4 Figure 4: UV-Visible spectra of chitosan films during photooxidation (e = 25 µm)

Figure 5 :

 5 Figure 5: a) Change in roughness () versus irradiation time (line is obtained by an exponential fit). b) Image of the chitosan film surface before and after 300 hours of irradiation.

Figure 6 :

 6 Figure 6: Concentration of the oxidation products (detected at 1675 cm -1 and at 1730 cm -1 in IR spectra) and disappearance of the C-O-C bond (detected at 1150 cm -1 in IR spectra) versus irradiation time.

Figure 1 :

 1 Figure 1: IR spectra of chitosan films (e = 25 µm) before and after 24 h Soxhlet extraction in MeOH (domain 1800-1000 cm -1 ) and the chitosan powder in KBr pellets.

Figure 2 :

 2 Figure 2: a) IR spectra of chitosan film (e = 25 µm) during photooxidation, b) Subtracted IR spectra in carbonyl domain.

Figure 3 :

 3 Figure 3: a) IR spectrum of a film (e = 25 µm) of chitosan blended with gluconolactone. b) UV-visible spectra of a 600h photooxidized chitosan film before and after treatment with 2,4-DNPH.

Figure 4 :

 4 Figure 4: UV-Visible spectra of chitosan film during photooxidation (e = 25 µm).

Figure 5 :

 5 Figure 5: a) Change in roughness () versus irradiation time (line is obtained by an exponential fit). b) Image of the chitosan film surface before and after 300 hours of irradiation.

Figure 6 :

 6 Figure 6: Concentration of the oxidation products (detected at 1675 cm -1 and at 1730 cm -1 in IR spectra) and loss of the C-O-C bond (detected at 1150 cm -1 in IR spectra) versus irradiation time.

Table 1 :

 1 Evolution of nano-hardness (nano-Hv) and nano-T g as a function of

	irradiation time.
	Irradiation time	Nano-Hv (at 160 nm)	Nano-T g (K)
	0 h	90  8 MPa	430  5
	100 h	150  15 MPa	465  5
	200 h	230  25 MPa	495  10
	300 h	280  25 MPa	510  5
	400 h	330  45 MPa	530  15

Table 2 :

 2 Degradation rate (k) for the main degradation criteria of photooxidation

		k	standard error	C.coeff
	Parameters	Kinetic model
		(h -1 .10 -3 )	(h -1 .10 -3 )	χ 2
	α C=O (1675 cm -1 )	0.475	0.05	0.999
	Yellowing (A 400 nm)	0.475	0.07	0.989
	α C=O (1730 cm -1 )	2.8	0.1	0.996
	α COC	1.6	0.1	0.995
	nano-Hardness/	1.7	0.2	0.992
	Hardness
	nano-Tg	2.5	0.1	0.98
	Roughness	2.8	0.15	0.996

Table 1 :

 1 Evolution of nano-hardness (nano-Hv) and nano-T g as a function of irradiation time

Table 2 :

 2 Degradation rate (k) for the main degradation criteria of photooxidation Scheme 1: Chemical structure of chitosan Scheme 2: Simplified photooxidation mechanism of chitosan


Eric Savier

Chetana Lim

Olivier Scatton

come

Liver transplant from controlled donation after circulatory death donors with normothermic regional perfusion versus donation after drain death donors

Liver transplantation from controlled donation after circulatory death donors with normothermic regional parfusion vs. donation after drain death donors

Eric SAVIER 1,2 MD; Chetana LIM 1 We read with great interest the recent article of Ruiz et al. [START_REF] Ruiz | Similar Results in Liver Transplantation From Controlled Donation After Circulatory Death Donors With Normothermic Regional Perfusion and Donation After Brain Death Donors: A Case-Matched Single-Center Study[END_REF] and would like to congratulate the authors. Also, we would like to make some important comments. Ruiz et al. reported the outcomes of liver transplantation (LT) from controlled donation after circulatory death (cDCD) donors with normothermic regional perfusion (NRP) (from 2015 to 2019, 100 patient) versus donation after brain death (DBD) donors (from 2015 to 2019, 200 patients). Using the same design (case match study with a 1:2 ratio) and endpoints (graft survival and death noncensored graft survival, patient survival, biliary complications, early allograft dysfunction and acute kidney injury) we published same results in Transplantation last year [START_REF] Savier | Favorable Outcomes of Liver Transplantation from Controlled Circulatory Death Donors Using Normothermic Regional Perfusion Compared to Brain Death Donors[END_REF]. Unfortunately, our study was not cited in the Spanish one. Although the NRP techniques (premortem cannulation in this study vs. postmortem cannulation in ours) and the minimum follow-up period (6 months in the Spanish vs. 24 months in the French Study) were different, this study confirms the French results: altogether, these two studies demonstrate the benefits of NRP when applied in the context of cDCD donations.

We observed an interesting point in both studies about the transplant survival (event = retransplantation or death of the patient) which was superior of the NRP-cDCD group compared to the DBD group during the first years of the follow up (significant in the Spanish study only). Both studies were retrospective with inherent source of bias. Therefore, to progress in the analysis, we tested another endpoint: the arterial or biliary complication-free survival (ABCFS) (3) and found no difference (Figure 1). Instead of increasing the number of LT, ABCFS improves the power of the statistical test by taking account of retransplantation or death of the patient or ABC (arterial or biliary complication of Dindo/Calvien ≥III). ABC event, which included ischemic cholangiopathy, is a particular interest in LT from cDCD. It could be interesting to test ABCFS in the Spanish study or in both cohorts. The next step will be to test, with the same methodology, the association of a hypothermic perfusion (HPM) to the NRP (NRP-cDCD-HPM).

Figure 1

Following LT with NRP-cDCD (n = 50) versus DBD donor (n = 100), 1-year ABCFS (event = ABC or retransplantation or death) was 84% and 82% respectively and 1-year transplant survival rate (event = retransplantation or death) was 96% and 88% respectively.

  MD,PhD; Olivier SCATTON 1,2 MD,PhD 1: Service de chirurgie digestive et hépato-bilio-pancréatique, transplantation hépatique, CHU Pitié-Salpêtriere, Assistance Publique-Hôpitaux de Paris (AP-HP), Sorbonne Université, Paris, France 2: Centre de Recherche Saint-Antoine (CRSA), UMRS-938, Institute of Cardiometabolism and Nutrition (ICAN), Sorbonne Université, INSERM, Paris, France