Clément Freymond

Xavier Mackré-Delannoy

Alain Guinault

Carole Charbuillet

Bruno Fayolle

Thermal oxidation of acrylonitrile-butadiene-styrene: Origin of the ductile/ brittle transition

Keywords:

The accelerated thermal ageing of ABS films has been investigated over a wide range of exposure temperatures in air, by FTIR, GPC, AFM and tensile testing in order to determine the origin of the ductile/brittle transition during exposure. Besides formation of oxidation products and decrease of unsaturated bonds in butadiene phase, molar mass changes during oxidation have highlighted a chain scission process. If chain scission is associated to SAN phase, crosslinking occurs in butadiene phase, which is confirmed by measuring by AFM the relative modulus increase of the nanometric butadiene domains. At last, strain at break changes during exposures has been investigated taking into account the modifications of each phase on ABS embrittlement. It appears that ultimate elongation as a function of chain scission number follows a power law regardless of the other parameters showing that chain scission occurring in the SAN phase is the main process responsible for ABS embrittlement.

Introduction

Acrylonitrile-butadiene-styrene (ABS) is a so called "terpolymer" found in many fields such as automotive, additive manufacturing, electrical and electronic equipment (EEE) [START_REF] Maris | Characterizing plastics originating from WEEE: a case study in France[END_REF][START_REF] Alassali | Towards a high quality recycling of plastics from waste electrical and electronic equipment through separation of contaminated fractions[END_REF] or the well-known Legos bricks because of its good impact and chemical resistance combined to a good melt processing. ABS in EEE is challenging to recycle especially because of the presence of additives which are highly toxic for the environment and human health such as brominated flame retardant derivative (around 5-25 %wt) [START_REF] Freegard | Develop a process to separate brominated flame retardants from WEEE polymers-final report[END_REF][START_REF] Hahladakis | An overview of chemical additives present in plastics: Migration, release, fate and environmental impact during their use, disposal and recycling[END_REF] and heavy metals (around 0.5-3 % wt) [START_REF] Stenvall | An analysis of the composition and metal contamination of plastics from waste electrical and electronic equipment (WEEE)[END_REF][START_REF] Oguchi | Toxic metals in WEEE: Characterization and substance flow analysis in waste treatment processes[END_REF]. If ABS recycling is used to manage EEE wastes (WEEE), it can induce a loss of mechanical properties after several cycles of reprocessing [START_REF] Mantaux | Régénération d'ABS et ABS-PC issus de DEEE -Résilience et mise en oeuvre[END_REF][START_REF] Fiorio | A statistical analysis on the effect of antioxidants on the thermal-oxidative stability of commercial mass-and emulsionpolymerized ABS[END_REF][START_REF] Boldizar | Degradation of ABS during repeated processing and accelerated ageing[END_REF][START_REF] Salari | Study on the recycling of ABS resins: simulation of reprocessing and thermo-oxidation[END_REF]. The causes of this embrittlement remains unclear since many mechanisms such as thermo mechanical degradation and/or oxidation can occur. Therefore, we propose to investigate possible relationships between oxidation and mechanical properties. If ABS oxidation has been studied for many decades mainly from a chemical point of view [START_REF] Gesner | Environmental surface effects on ABS resins[END_REF][START_REF] Davis | Rapid assessment of weathering stability from exposure of polymer films. I. Real and simulated weathering of commercial ABS terpolymers[END_REF][START_REF] Adeniyi | Clarification and discussion of chemical transformations involved in thermal and photo-oxidative degradation of ABS[END_REF][START_REF] Signoret | MIR spectral characterization of plastic to enable discrimination in an industrial recycling context: I. Specific case of styrenic polymers[END_REF][START_REF] Signoret | MIR spectral characterization of plastic to enable discrimination in an industrial recycling context: II. Specific case of polyolefins[END_REF], embrittlement mechanisms including macromolecular modifications have not been studied extensively.

Previous works on ageing of ABS induced by photo and thermal oxidation have shown to alter predominantly the butadiene (PB) phase over styrene-acrylonitrile (SAN) phase where substitution occurs on allylic hydrogen [START_REF] Shimada | The mechanism of oxidative degradation of ABS resin. Part I. The mechanism of thermooxidative degradation[END_REF][START_REF] Ghaemy | Photo-and thermal oxidation of ABS: correlation of loss of impact strength with degradation of the rubber component[END_REF][START_REF] Salman | Effect of thermal aging on the optical properties of ABS plastics[END_REF][START_REF] Suzuki | The thermal degradation of acrylonitrile-butadienestyrene terpolymei as studied by TGA/FTIR[END_REF]. During processing, it has been mentioned that oxidation of PB occurs between 200 and 250 • C under oxygen atmosphere while SAN oxidation occurs between 200 and 470 • C [START_REF] Shapi | TG and DSC studies of some thermal properties and stability aspects of poly (acrylonitrile butadiene styrene), polystyrene and poly (acrylonitrile styrene) plastics[END_REF]. Let us remember that ABS presents a specific morphology where butadiene nodules grafted with SAN are incorporated into the SAN phase [START_REF] Kulich | Acrylonitrile-Butadiene-Styrene Polymers[END_REF]. Chang et al. have shown by DSC under oxygen the SAN phase is more stable than the PB phase [START_REF] Chang | Methods for study of thermal aging and stabilization of grafted acrylonitrile-butadiene-styrene (ABS) terpolymers[END_REF]. Crosslinking phenomenon could be expected in ABS due to the presence of PB phase [START_REF] Coquillat | Thermal oxidation of polybutadiene. Part 3: Molar mass changes of additive-free non-crosslinked polybutadiene[END_REF]. Regarding the SAN phase, it has been reported that SAN undergoes chemical degradation above glass transition (T g ) and physical ageing under T g that can drastically affect elongation at break or impact resistance [START_REF] Wyzgoski | Effects of oven aging on ABS, poly(acrylonitrile-butadienestyrene)[END_REF][START_REF] Tiganis | Thermal degradation of acrylonitrile-butadiene-styrene (ABS) blends[END_REF]. It has been proposed that SAN phase degradation would be triggered by the PB one [START_REF] Piton | Photo-oxidation of ABS at long wavelengths (λ>300 nm)[END_REF][START_REF] Jouan | Photo-oxidation of ABS: Part 2-Origin of the photodiscoloration on irradiation at long wavelengths[END_REF]. Nevertheless, another study has reported molar mass decrease after ageing 3h at 190 • C [START_REF] Wolkowicz | Effect of thermal aging on impact strength acrylonitrile-butadiene-styrene (ABS) terpolymer[END_REF], suggesting a chain scission phenomenon in rigid fraction, which is the mechanism reported for SAN [START_REF] Grassie | Thermal degradation of copolymers of styrene and acrylonitrile. III. Chain-scission reaction[END_REF] and PS [START_REF] Shapi | TG and DSC studies of some thermal properties and stability aspects of poly (acrylonitrile butadiene styrene), polystyrene and poly (acrylonitrile styrene) plastics[END_REF][START_REF] Cameron | Thermal degradation of polystyrene-I. Chain scission at low temperatures[END_REF][START_REF] De Souza | ABS/HIPS blends obtained from WEEE: Influence of processing conditions and composition[END_REF]. Regarding the PB phase in HIPS, a butadiene signal modification is observed by FTIR since PB phase is more sensitive to oxidation than PS phase whatever its ratio [START_REF] Israeli | Photo-and thermoinitiated oxidation of high-impact polystyrene. I. Characterization by FT-IR spectroscopy[END_REF]. Prasad and Villaplana have noticed an intensity diminution infrared signal of butadiene function after 50h of UV exposure [START_REF] Prasad | Recent developments in the degradation and stabilization of high-impact polystyrene[END_REF] and 16 days of thermal ageing at 90 • C [START_REF] Vilaplana | Degradation of recycled high-impact polystyrene. Simulation by reprocessing and thermo-oxidation[END_REF] but there is few chemical structure changes indicating proof of crosslinking. An oxidation of the PB phase of ABS after ageing at 80 • C

Atomic force microscopy (AFM)

ABS films edge surface were probed by AFM Nanoscope V from Veeco Instruments (Santa Barbara, CA, USA) using the Peak Force QNM mode. The surface was prepared with Ultramicrotome from Leica using diamond knife from Diatome. AFM images were recorded with silicon nitride cantilevers (0.4 N/m) with silicon tips (designated ScanAsyst Air, Bruker) oscillating at 2 kHz and Peak Force amplitude of 150 nm. At least 3 images per samples of different zones were recorded at 5×5 µm 2 size with 256×256 lines per image. The topographical and mechanical analysis of AFM images including particles size and modulus variation was carried out by using Image J and Nanoscope Analysis software respectively.

Tensile test

Uniaxial traction were performed on ABS specimens of 15 mm useful length and 4 mm width using an Instron 4301 system 100 N force cell at 10 mm/min speed. Tests were carried out at 23 • C and 50% relative humidity on series of 7-10 samples. Variation of ultimate elongation was followed with ageing time.

Results

Chemical modifications during ageing

FTIR spectra of ABS films aged at 120 • C are shown on Fig. 1. ABS is mostly characterized by the nitrile C ---N signal at 2237 cm -1 and the two peaks of aromatic C-H bending located at 705 and 765 cm -1 . Stretched aromatics are visible above 3000 cm -1 at 3005, 3028, 3062, 3084 and 2105 cm -1 then aliphatics just below at 2851, 2923 cm -1 [START_REF] Suzuki | The thermal degradation of acrylonitrile-butadienestyrene terpolymei as studied by TGA/FTIR[END_REF][START_REF] Figueira | Coupled matrix kinetic Monte Carlo simulations applied for advanced understanding of polymer grafting kinetics[END_REF]. It appears that the signals carbonyl C=O (1650-1850 cm -1 ) and hydroxyl OH (3200-3600 cm -1 ) increase with time and a decrease in both trans-1,4 (967 cm -1 ) and vinyl -1.2 (911 cm -1 ) unsaturated bonds C=C is noticeable while the nitrile, aromatic and aliphatic groups are less affected. The same trend is also visible for higher and lower aging temperatures at shorter and longer time respectively. These measurements are not reported here. As a result, all modifications in the FTIR spectra are consistent with an oxidation mechanism.

As expected, the unsaturated bond linked to the PB fraction preferentially undergoes the oxidation. Even below the SAN glass transition temperature, without drastically affecting the nitrile, aromatic and aliphatic groups which correspond to the SAN phase. This is in line with Fig. 1. Infrared spectrograms of virgin ABS at 120 • C during ageing. [START_REF] Salman | Effect of thermal aging on the optical properties of ABS plastics[END_REF] was also observed. Let us mention that dispersion of butadiene in ABS and HIPS may differ one from each other, and thus the comparison between the two materials is difficult. Considering the previous state of the art, it appears that no explanation about the origin of the embrittlement and its mechanism have been reported for ABS.

The aim of this study is to investigate the thermal oxidation of a commercial ABS in order to determine the main mechanism which could explain its embrittlement. ABS films were extruded with a 100 µm thickness in order to avoid diffusion limited oxidation (DLO). Indeed, Tiganis et al. have been observed a critical depth of degradation close to 80 µm [START_REF] Chang | Methods for study of thermal aging and stabilization of grafted acrylonitrile-butadiene-styrene (ABS) terpolymers[END_REF]. Like the Tiganis study, we chose air exposures between 90, 100, 110, 120, 130 and 140 • C. The choice of this temperature range was chosen to study ABS ageing both under and above SAN T g (T g ~105 • C). In order to investigate possible structure-properties relationships, changes of chemical, morphological and mechanical properties were followed by GPC, DSC, AFM measurements and uniaxial tensile tests respectively.

Materials and method

. Materials and processing

A commercial acrylonitrile-butadiene-styrene (ABS) HI121H, (T g = 105 • C; ρ = 1,04 g/cm 3; MFI = 23 g/10 min (220 • C-10 kg)) provided by

LG Chem has been chosen for the study because it is an injection grade widely used in EEE domain. According to [START_REF] Li | Synergistic effect between a hyperbranched charring agent and ammonium polyphosphate on the intumescent flame retardance of acrylonitrile-butadiene-styrene polymer[END_REF], the ABS resin (HI-121H) was obtained from LG Industries, Korea and is composed of acrylonitrile (25%), butadiene (30%) and styrene (45%).

ABS pellets were first dried 16 h at 70

Infrared spectrometry

Infrared spectrometry experiment was carried out with a Bruker IFS 28 at 4 cm -1 resolution. Fourier Transform InfraRed spectroscopy (FTIR) in transmission mode was used with a total of 16 scans. Absorbance of carbonyl and diene species are followed through peak height at 1730, 967 and 911 cm -1 respectively.

Gel permeation chromatography (GPC)

GPC measurements were realized using a Waters 717plus chromatograph calibrated with styrenic samples equipped two columns from Waters as stationary phase, and refractive index (RI) then ultra violet (UV) detectors in tetrahydrofuran (THF) as mobile phase. Elution method consisted in isocratic step at 1 ml/min during 30 min at 35 • C. Calibration was performed using SM-105 PS standards from Shodex from which values were implemented in Breeze software, which enable to calculate molecular masses relative to PS standards.

Differential scanning calorimetry (DSC)

Thermal properties were analysed under nitrogen using A DSC Q10 from TA Instruments (Waters, New Castle, DE, USA). Temperature and base line calibration was performed using sapphire and indium. Since the chain scission process is clearly captured by the molar mass measurements for ageing temperatures higher than 100 • C, we propose to assess chain scission number and to correlate this number with an oxidation product. Chain scission number can be calculate by using the following equation

s = 1 M n - 1 M n0 (1)
In Fig. 5, carbonyl absorbance is plotted as a function of chain scission number. The Fig. 5 shows the link between C=O absorbance and scission at all temperatures above the glass transition temperature. The slope close to 1 (0.99 added in the figure legend) clearly indicates a strong correlation whatever the temperature of exposure. As a result, we propose here that chain scission occurs mainly in the SAN phase and can be associated to carbonyl formation.

As previously mentioned [START_REF] Coquillat | Thermal oxidation of polybutadiene. Part 3: Molar mass changes of additive-free non-crosslinked polybutadiene[END_REF], the branching mechanism can be associated to the oxidation of the PB phase, i.e. the reaction of radicals on C=C bonds as evoked formerly with FTIR results. Indeed, previous works have proposed that thermal oxidation of non-stabilized polybutadiene involves consumption of C=C and crosslinking which occurs mostly by intramolecular reactions [START_REF] Salman | Effect of thermal aging on the optical properties of ABS plastics[END_REF]. However, the C=C the same conclusion made in previous studies on thermal oxidation of ABS at different aging temperatures. Indeed, the growth of C=O and the disappearance of C=C were observed after 168 h at 120 • C [START_REF] Tiganis | Thermal degradation of acrylonitrile-butadiene-styrene (ABS) blends[END_REF] suggesting a predominant oxidation of the butadiene fraction.

In order to follow oxidation, only absorbance of carbonyl and double bond trans-1,4 absorbance are reported in Fig. 2 through peak height at 1730 and 967 cm -1 respectively. It appears clearly that the raise of carbonyl and the drop of trans-1,4 unsaturated groups occurs for all ageing temperatures, their rate being temperature driven. The same trend was observed for the double bond viny-1,2 (not reported). At last, no significant induction period is detected on carbonyl formation or double bond consumption.

Impact of oxidation on molar mass

First, we would like to indicate that if ABS has been solubilized in THF before GPC measurements, and insoluble PB nodules were removed by filtration. Only a small, originally soluble part of the PB fraction is measured here. Fig. 3 shows the raw data of GPC measurements carried out on ABS samples aged at 120 • C. At first glance, we observe a shift of the peak from the shorter to longer retention times corresponding to shorter chains. At the same time, broadening at the shorter retention times occurs indicating a possible branching mechanism between styrene and butadiene chains during exposure. The branching associated to the left region of the retention is probably due to the fact that grafting kinetic is diffusional limited on termination reactions since it involves macroradicals. However, the M n drop indicates clearly a dominant chain scission [START_REF] Figueira | Coupled matrix kinetic Monte Carlo simulations applied for advanced understanding of polymer grafting kinetics[END_REF].

The shift of the peak can be translated into number average molar masses M n . As reported in Fig. 4, the fact that M n is reduced from 63 to 35 kg/mol suggests clearly a chain scission mechanism for the exposures having a temperature ranged between 110 and 140 • C. These numerical values are relative as they are obtained with PS calibration standards. For reasonable oxidized state (i.e. embrittlement where Mn < 35 kg/ mol), the polydispersity index increases from 2 to 4. This increase can be attributed to a crosslinking mechanism as discussed below.

For the temperature of exposure 90 • C and 100 • C, the scattering values of M n do not allow to observe a clear trend as witnessed with FTIR data shown in the previous part. It is of interest to note that the points corresponding to absorbance below 0.4 on the FTIR data (Fig. 2 a)) do not follow the general Mn trends in Fig. 4 regardless of exposure temperature. This implies that such datapoint correspond to levels of degradation too low to show a significant change in term of molar mass. The guidelines are provided only for films having FTIR absorbance over 0.4, which corresponds to M n below ~ 50kg/mol. consumption observed by FTIR cannot be correlated with the GPC measurements since the majority of the PB phase is excluded from the GPC measurements.

If it is known that the crosslinking phenomenon is associated to an increase of Ɖ as pointed out by Choupin et al. with anaerobic thermal degradation of PEKK [START_REF] Choupin | Macromolecular modifications of poly(etherketoneketone) (PEKK) copolymer at the melting state[END_REF]. We believe that, in our case, M n drop indicates clearly a predominant chain scission in the SAN phase. However, we cannot exclude a possible crosslinking process as it has been mentioned in the case of ABS recycled from WEEE [START_REF] Wang | Recycling of acrylonitrile-butadiene-styrene (ABS) copolymers from waste electrical and electronic equipment (WEEE), through using an epoxy-based chain extender[END_REF].

Impact of ageing on thermal properties

The objective of the part 3.3 is to check out if (i) physical ageing can be observed for ageing temperatures below SAN T g (ii) if T g is affected by oxidation for ageing temperatures above SAN T g , knowing all temperatures of ageing will lead to embrittlement (see Part 3.5). Indeed, it may be useful to distinguish between physical and chemical ageing. In the first case, there is no alteration of the chemical structure of the macromolecules, only their spatial configuration is affected. Here the main mechanism is called structural relaxation ageing and can be detected by DSC. This physical ageing is associated to the appearance of an endothermic peak in the immediate vicinity of T g . In the second case, the chemical ageing, there is a modification of the chemical structure of the macromolecules leading chain scission or crosslinking.

The most exposed samples at 90 and 100 • C, as well as the samples showing a significant degree of oxidation (M n < 45 kg/mol in Fig. 4) after aging at 110, 120, 130 and 140 • C were analyzed by DSC (Fig. 6). On the one hand, the unaged sample and the ABS aged at 90 • C for 256 days show an endothermic peak at 105 and 114 • C, respectively, with this peak disappearing after a second heating scan. This result clearly indicates physical aging of the SAN phase [START_REF] Wyzgoski | Effects of oven aging on ABS, poly(acrylonitrile-butadienestyrene)[END_REF].

However, no significant change in SAN T g is noted for the oxidized samples performed between 100 and 140 • C. If a drop in T g with molar mass is expected because M n drop, this technique does not clearly detect it in our case. To confirm that the changes in T g of the SAN phase with oxidation do not allow us to put in evidence chain scission in our case. Let us recall the Fox-Flory approach linking T g and M n : Between an initial state (M n0 = 65 kg/mol) and a significant oxidized state as defined previously (M n = 40 kg/mol according to Fig. 4), the variation in T g between both states is equal to K/M n -K/M n0 . Even if K is taken as high as for PEEK [START_REF] Prasad | Recent developments in the degradation and stabilization of high-impact polystyrene[END_REF], the change in T g is less than 2 • C which is not consistent with the accuracy of the T g measurement obtained by DSC.

Modulus changes in the PB domains

The morphology of SAN and PB phases and their relative modulus changes with thermal oxidation has been probed by AFM peak force mode (Fig. 7). The contrast of the modulus channel enable to identify SAN (bright) and PB phase (dark) especially on Fig. 7a and 7b. On one hand, it is worth noting that the average diameter of PB particles remains equal to 200 ± 100 nm after a thermal oxidation of 256 and 11 days at 90 and 130 • C respectively. On the other hand, if one looks closer to the morphology of the Fig. 7a andb, the PB nodules appear to have an elliptic shape due to a shear deformation induced by the diamond knife cut, whereas the PB morphology of the aged ABS at 130 • C (Fig. 7c) highlights nearly perfect circular shape. This observation suggests a possible increase of the PB modulus, i.e. a higher rigidity, due to thermal oxidation. In order to go further, the variation of normalized modulus between PB (noted 1) and SAN (noted 2) phases has been measured for unaged and aged ABS (Fig. 7d).

First, comparing Fig. 7a and 7b, we observe a clear contrast in terms of relative modulus between PB nodules and continuous SAN phase for initial ABS (green curve in Fig. 7d) and the sample aged at 90 • C during 256 days (blue curve in Fig. 7d). Hence, neither the physical ageing of the SAN phase induced at 90 • C as evidenced by DSC, nor the low amount of chain scission/cross-linking seems to affect the module contrast.

Secondly, in ABS aged at 130 • C during 11 days (red curve in Fig. 7d), modulus contrast disappears completely. Assuming that SAN phase (in glassy state) modulus is not affected by the chain scission, the loss of contrast must be caused by PB phase (in rubbery state) modulus increase. Indeed, it is known that the rubbery modulus is proportional to the number of branching x according the classical rubber theory [START_REF] Le Gac | Predictive ageing of elastomers: Oxidation driven modulus changes for polychloroprene[END_REF].

Impact on ultimate elongation

From tensile test measurements, the main modifications are related to ultimate elongation ε b as expected. No significant changes in terms of modulus or stress at yield are reported here. ε b has been represented as a function of time for all ageing temperatures on Fig. 8. One can see that an embrittlement is observed for the entire span of ageing temperatures, from 74 to 0.5% for the lowest value. Two mechanisms can explain the embrittlement process: physical ageing and oxidation.

Embrittlement observed at 90 • C can be associated to physical ageing since no chemical modifications are observed whereas DSC evidenced a classical endotherm attributed to this ageing. The same conclusion was proposed by Wyzgoski et al. [START_REF] Wyzgoski | Effects of oven aging on ABS, poly(acrylonitrile-butadienestyrene)[END_REF]. A loss of strain at break by a factor 6 is noticed after ageing of 50 h apparently due to physical ageing of the glassy phase. Since PB phase is in rubbery state at 90 • C, the physical ageing is located in the SAN phase which is under its T g .

Finally, for the conditions where chemical modifications (FTIR and GPC) occur, embrittlement can be associated to oxidation in both phases. Tiganis et al. have also observed a reduction of the elongation at break after 28 days of exposure at 90 and 120 • C [START_REF] Tiganis | Thermal degradation of acrylonitrile-butadiene-styrene (ABS) blends[END_REF]. The authors proposed that the thermal oxidation of PB phase is mainly responsible of the mechanical properties loss. However, in our case, oxidation has been witnessed in both phase: crosslinking in PB phase and chain scission in SAN phase. As a result, embrittlement process or ductile/brittle transition can involve many mechanisms. The following discussion part will focus on separating those mechanisms by quantifying them in order identify the main mechanism responsible for ductile/brittle transition during thermal ageing.

Discussion

After the analysis of the experimental results obtained with our multi scale analysis, the discussion will be devoted to the formulation of different hypothesis for the origin of ABS embrittlement. More precisely, the main objective is to check if our results are consistent with some theoretical relationships linking macromolecular properties with embrittlement process available in the literature. As a result, we will propose the predominant modification in ABS embrittlement.

In literature, previous works have pointed out the existence of a critical molar mass related to the ductile/brittle transition during thermal oxidation which is working for linear polymers in a rubbery state which undergo random chain scission such as PP [START_REF] Fayolle | A critical molar mass separating the ductile and brittle regimes as revealed by thermal oxidation in polypropylene[END_REF], PS [START_REF] Fayolle | Review: degradation-induced embrittlement in semi-crystalline polymers having their amorphous phase in rubbery state[END_REF], PA [START_REF] Okamba-Diogo | Investigation of polyamide 11 embrittlement during oxidative degradation[END_REF] or PE [START_REF] Wang | Recycling of acrylonitrile-butadiene-styrene (ABS) copolymers from waste electrical and electronic equipment (WEEE), through using an epoxy-based chain extender[END_REF]. This critical molar mass is directly linked to entanglements and can become irrelevant when crosslinking occurs in the case of non-linear polymer. For instance, during thermal oxidation, PE shows the same behavior as in Fig. 4 with scissions outnumbering crosslinks [START_REF] Fayolle | Mechanism of degradation induced embrittlement in polyethylene[END_REF]. However, in the case of polymers having a glassy state such as PEKK, Roland et al. have pointed out that embrittlement occurring during thermal oxidation is due to a predominant crosslinking over chain scission [START_REF] Roland | Evolution of mechanical properties of aged poly(ether ketone ketone) explained by a microstructural approach[END_REF]. In our case, ABS has a biphasic morphology composed of SAN and PB phases which is exhibited during thermal oxidation chain scission and crosslinking mechanism respectively. These two phases, and the associated aging process, can thus have an impact on the ultimate mechanical properties, thus ABS will be treated as heterogeneous material.

Impact of the PB phase oxidation on embrittlement

We have seen that the oxidation of the PB phase leads to a crosslinking process. If some evidences of this crosslinking process can be detected by GPC with the appearance of high molar masses, the best way to follow this process is to monitor modulus changes (by AFM here) according to the classical rubber theory [START_REF] Bouaziz | Elastic properties of polychloroprene rubbers in Tension and compression during ageing[END_REF]:

ΔE = 3xRT (2)
With ΔE and x the modulus and crosslinks number during oxidation respectively.

According to the rubber elasticity approach, another relationship between the branching number x related to PB phase and the ultimate elongation is possible considering first that ε r is related to chain dimension [46] with initial Gaussian conformation as followed :

ε r ∝ R max R 0 ∝ 1 ̅̅̅ ̅ E √ (3)
Where R 0 is the chain end-to-end distance, R max the maximal value of chain extension.

Considering (1) and ( 2), one can expect that crosslinking modify significantly the mechanical behavior of the butadiene phase. The main question is now if these modifications are responsible of ABS embrittlement. In our case, the fact that the PB particles become rigid and brittle cannot explain this ABS embrittlement if the SAN phase remains ductile. This point is discussed below.

Effect of SAN phase on the embrittlement

The thermal oxidation of SAN copolymer is generally known to lead to a chain scission process [START_REF] Grassie | Thermal degradation of copolymers of styrene and acrylonitrile. III. Chain-scission reaction[END_REF]. We propose here to focus on the change of ultimate elongation (ε b ) as a function of s. It appears that ε b (Fig. 9) and the scissions s are correlated. A linear fit in a log-log scale has been applied solely on data associated to ageing above T g in order to exclude the physical ageing effect on ε b as shown by Fig. 9. The calculated R 2 for the power law fit has an acceptable value of 0.85.

In order to go further, the critical conditions of SAN embrittlement in terms of molar mass has been investigated considering that it is a linear amorphous copolymer. A value of molar mass between entanglements M e equal to 11.5 kg/mol has been proposed by Wu et al. for SAN [START_REF] Wu | Chain structure and entanglement[END_REF]. From this value, the relationship M' c ≈ 5M e determined for linear amorphous polymers [START_REF] Fayolle | Review: degradation-induced embrittlement in semi-crystalline polymers having their amorphous phase in rubbery state[END_REF] enables to determine a value of M' c close to 57 kg/mol. In the case of amorphous linear polymers, M' c corresponds to the number average molar mass M n parameter [START_REF] Okamba-Diogo | Investigation of polyamide 11 embrittlement during oxidative degradation[END_REF]. As a result, one can remark that M n of ABS decreases from 63 to 42 kg/mol (Fig. 4) going thus below M' c of the SAN phase, this suggests that chain scission occurring in this phase is the predominant process responsible for ABS embrittlement.

Conclusion

The effect of thermal ageing of ABS has been investigated on wide range of temperatures going from 90 to 140 • C in order to determine the origin of the ductile/brittle transition. As expected, FTIR have indicated the apparition of carbonyl groups while unsaturated groups tend to disappear after ageing, which proves that butadiene phase undergoes oxidation. The analysis of the molar mass and dispersity by GPC points out mainly chain scission on the SAN phase. More importantly, the morphological characterization performed by AFM shows an increase in the modulus of the PB phase detected by a loss in modulus contrast. In other words the use of peak force mode allows a new way to characterize PB phase during oxidation. As a result, if crosslinking occurs in the butadiene phase, chain scission is associated to SAN phase oxidation.

Knowing ABS is heterogeneous at microscopic scale, macroscopic embrittlement is driven by modifications occurring in both phases: chain scission in SAN phase, crosslinking in PB phase. However, we have proposed a separate analysis for each phase in order to put in evidence the main modification at play. By correlating the macromolecular changes with the fracture properties during oxidation, it appears that the ultimate elongation as a function of chain scission clearly follows a power law. Moreover, this embrittlement occurs below a critical molar mass related to the entanglement molar mass of the SAN. These results suggest that the embrittlement of ABS is primarily controlled by chain scission in the SAN phase rather than by cross-linking in the butadiene phase

  • C then films of 100 µm thickness were extruded with a Scamex single 20-20D screw extruder rotating at 20 tr/min and a 2×100 mm flat die. Barrel temperature profile is settled to 200 220 220 220 220 210 210 • C, and 210 • C for the die. Chill rolls speed is 175 cm/min and they are heated to 75 • C. To accelerate ageing, films were put in ventilated oven under air at 90, 100, 110, 120, 130 • C and 140 • C.

  6-7 mg of different ABS samples were heated from 30 to 200 • C at 10 • C/min and maintained during 3 min, cooled from 200 to 30 • C and finally heated a second time to 200 • C.

Fig. 2 .

 2 Fig. 2. Evolution of the absorbance of carbonyl a) and double bond trans-1,4 b) bands during thermal oxidation, the dotted lines as a guide for eyes.

Fig. 3 .

 3 Fig. 3. GPC experimental curves obtained for an ABS after thermal oxidation at 120 • C for different exposure times. The dotted line indicates the peak corresponding to the initial ABS sample.

Fig. 4 .

 4 Fig. 4. M n changes as a function of time for different temperature. The grey zone indicates the initial value of molar mass with its error.

Fig. 5 .

 5 Fig. 5. Carbonyl absorbance as a function of chain scission number at all temperatures above the glass transition temperature. A linear fit applied for the data in log scale leads to a slope of 0.99.

Fig. 6 .

 6 Fig. 6. Thermogram representing the first (solid line) and the second (dotted line) heating of unaged ABS film and aged ABS by thermal oxidation at 90, 100, 110, 120, 130 and 140 • C during 256, 156, 111, 72, 11 and 4d respectively. The vertical dotted line indicates the glass transition temperature of the initial sample.

Fig. 7 .

 7 Fig. 7. AFM DMT Modulus channel images recorded for unaged ABS film a), after 256 d of thermal oxidation at 90 • C b) and 11 j at 130 • C c). Butadiene and SAN phases are indicated with yellow arrows. Topographies of modulus variation were measured on each images and represented by the curves d).

Fig. 8 .

 8 Fig. 8. Evolution of ultimate elongation (ε b ) with time for different ageing temperatures. The grey area indicates initial value of ultimate elongation with its errors.

  CRediT authorship contribution statementClément Freymond: Investigation, Data curation, Writingoriginal draft. Xavier Mackré-Delannoy: Investigation, Writingreview &

Fig. 9 .

 9 Fig. 9. Relationship between ultimate elongation and chain scission number regardless the temperature. The solid line corresponds to the linear fit applied for the data range above the glass transition temperature.

Data Availability

Data will be made available on request.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.polymdegradstab.2022.110186.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.