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

M. Pliquet, M. Rapeaux, F. Delange, D. Bourgogne, J.L. Gardette, S.

Therias, P.O. Bussiere

► To cite this version:

M. Pliquet, M. Rapeaux, F. Delange, D. Bourgogne, J.L. Gardette, et al. Multiscale analysis of thermal degradation of polyamide 6,6 - Influence of temperature on oxygen diffusion-limited oxidation profiles. Polymer Degradation and Stability, 2021, 192, pp.109695. 10.1016/j.polymdegradstab.2021.109695. hal-03401853

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

M. Pliquet<sup>a,b</sup>, M. Rapeaux<sup>b</sup>, F. Delange<sup>b</sup>, D. Bourgogne<sup>a</sup>, J.L. Gardette<sup>a</sup>, S. Therias<sup>a</sup>, P.O. Bussiere<sup>a*</sup>

*Corresponding authors:

I.C.C.F, UMR 6296 - Institut de Chimie de Clermont-Ferrand, Université Clermont Auvergne - CNRS - SIGMA Clermont Campus des Cézeaux - 24, avenue Blaise Pascal - TSA 60026 - CS 60026 63178 Aubière Cedex, France. E-mail address: pierre-olivier.bussiere@sigma-clermont.fr (P.O. Bussiere).

<footnote><sup>a</sup> Université Clermont Auvergne, Clermont Auvergne INP, CNRS, ICCF, F-63000 Clermont-Ferrand, France</footnote>

<footnote><sup>b</sup> Schneider Electric, Site Electropôle, 31 rue Pierre Mendès France, F-38320 Eybens, France.</footnote>
**Abstract**

This paper is devoted to the impact of oxygen diffusion on thermal ageing of PA-6,6. Samples were subjected to ageing in air at temperatures ranging from 140 °C to 200 °C for different times. Depth profiling methods using AFM measurements were developed to characterise the heterogeneity of the thermal and mechanical properties of the oxidised polymer. A comparison of the results obtained with micro-FTIR profiles was performed to correlate the chemical and physical changes induced by oxidation (i.e., from molecular to macroscopic scales). As a result of diffusion-limited oxidation (DLO), it was shown that polymer oxidised much faster at the surface than in the bulk.

**KEYWORDS:** Polyamide 6,6, Thermo-oxidation, profile, AFM (atomic force microscopy), diffusion-limited oxidation (DLO)
# 1. Introduction

Aliphatic polyamides are engineering polymers. They can be used in various fields, such as textiles, automotive, and electrical insulation. Certain applications require high-temperature exposure, which can reduce the performance of the material during its service life. The thermal oxidation of polyamides in the bulk is a widely studied process since polyamides are commonly used in applications that require long-term/high-temperature exposure. [1][5] It is known that polyamide 6,6 oxidation is a chain radical mechanism that starts with the removal of the most labile hydrogen in the α-position to the amide nitrogen. Regarding the basic autoxidation scheme, hydroperoxides are the first oxidative products. [6][7] The decomposition of α-amino hydroperoxides then leads to the formation of imides, carboxylic acids, amines and aldehydes. [8][10] Changes in the chemical structure can cause a degradation of the macromolecular architecture [11][12] and microstructure [13][14] and a loss of the functional properties. [15][16] Beyond chemical changes, oxygen permeation can also spatially affect the degradation of polymer materials. [17][22] When the oxygen consumption rate is higher than the oxygen permeation, oxidation then becomes limited by oxygen diffusion (DLO). [23][24] As a result, an oxidation profile across the material thickness develops and is indicative of heterogeneous oxidation. Oxygen is primarily consumed during surface oxidation and thus does not penetrate deeper layers in the material.

Various experimental techniques are employed to investigate the oxidation heterogeneity in polymers, including infrared microspectroscopy [25][27], chemiluminescence [28][30] and oxygen absorption measurements. [18] These techniques allow for the characterisation of chemical changes induced by oxidation. Novel analytical methods are required to characterise degradation at the upper levels. Atomic force microscopy (AFM) is an advanced technique that determines material properties with nanometric precision. In this study, AFM is used to measure thermal and mechanical properties. Recent studies using AFM to characterise the
changes in material properties due to ageing have suggested that it can be an efficient tool.

[31][35]

In this paper, oxidation profiles of PA 6,6 thick samples were determined using different techniques that characterise the material changes from molecular to macroscopic scales. The aim was to determine pertinent degradation probes to validate the multiscale approach protocol.

## 2. Experimental methods

### 2.1. Materials

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

### 2.2. Oven ageing

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

### 2.3. Characterisation techniques

#### 2.3.1. FTIR microscopy
Infrared microspectroscopy was used to measure the oxidation profiles on the PA 6,6 plates.
The specimens were 20 µm-thick films from the cross section of oxidised PA 6,6 that were
cut using a LEICA RM2165 microtome with tungsten carbide blades. The micro-FTIR
analyses were performed on a Nicolet 6700 spectroscope coupled with a Nicolet continuum
FTIR microscope (Thermo Scientific). The dimensions of the infrared beam were a 30 µm
length and 100 µm height. The infrared beam scanned the cross-sectional film every 20 µm.
The spectra were collected for 128 scans at a resolution of 4 cm⁻¹.

**2.3.2. AFM nanoscale thermal analysis (VITA)**

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

The VITA system contained a probe (tip + cantilever) connected to an electric circuit. The
probe was heated by the Joule effect by applying an electric current on the system. A
calibration was mandatory to correlate the applied tension to the corresponding temperature.
The probe was calibrated with three standard polymers with known melting temperatures:
polycaprolactone (T<sub>m</sub> = 55 °C), polyethylene (T<sub>m</sub> = 116 °C) and polyethylene terephthalate (T<sub>m</sub> = 235 °C). The spring constant of the VITA probe was between 0.5 N/m and 3 N/m, and its cantilever was 200 µm long.

Measurements were performed on the profiles of non-aged and aged PA samples to determine
changes in the thermal properties with thermo-oxidative ageing. [36] After cross section
cutting, a mapping of the melting temperature variation was performed between depths of 0
and 350 µm. For each depth, nine measurements were performed. A distance of 15 µm was
fixed between two measurements points to prevent the thermal expansion effect. During these
experiments, the probe was heated from 30 °C to 350 °C at a heating rate of 5 °C/s. AFM
images of the zone under study were collected before and after testing to observe the shape
and the dimensions of the hole left behind the probe during melting.

2.3.3. AFM nanoscale mechanical measurements

The PeakForce QNM mode in AFM was used to determine mechanical properties, such as the
modulus, adhesion, roughness and deformation, with a nanometric resolution. The AFM
measurements were performed with a Bruker Multimode 8 model equipped with a Nanoscope
9.20 in tapping mode. The sample was placed on a piezoelectric, which assured XYZ
displacement. Topographic images were collected with a probe on silicon from Bruker. The
RTESPA 300-30 model was used that had a curvature radius of 30 nm and a spring constant
near 40 N/m. The topographic images were processed to determine the roughness (R<sub>q</sub>)
according to equation 1:

$$
R_q = \sqrt{\frac{\sum (z_t)^2}{N}} \qquad \text{Eq. 1}
$$

where z_t is the current height value and N is the number of points within the considered area.

While obtaining the topographic images, force curves were collected at every pixel of the
image. The analysis of the force curve data with the Nanoscope 9.20 software provided the
mechanical properties. For the study, the stiffness was tracked by the reduced Young's
modulus E* obtained in the DMT modulus images. The model used to calculate E* was
provided by Derjaguin et al. [37]

Measurements were performed on cross-sections of PA 6,6 samples in VITA mode to
determine the mechanical properties in the bulk of the material. Images with dimensions of 5
µm x 5 µm at a scan rate of 0,5 Hz with 512 scan lines were captured. The mapping was
conducted by collecting images between depths of 0 and 325 µm. At each depth, five areas
were measured, and 3 images were collected in each measurement area. For probe calibration,
the sample must be clean and hard with a known mechanical modulus. The probe was
calibrated with silicon (E = 130-185 GPa). A quantitative measurement of the reduced Young
modulus also requires a calibration of the cantilever spring constant and piezoelectric scanner
movement. As a polymer surface is rarely flat and the tip vertex geometry is not exactly
spherical, this can induce calculation errors not accounted for in the Derjaguin-Muller-
Toporov (DMT) model used to determine the mechanical module [38]. This is why the study
was limited to the measurement of the relative variation of the stiffness. [39][40]
# 3. Results and discussion

## 3.1. Chemical structure

Infrared microspectroscopy was used to investigate the distribution of the chemical changes in the thickness of PA 6,6 plates induced by thermal degradation and determine the influence of temperature on this distribution. Samples of 1500 µm thickness were aged in thermo-oxidative conditions for different times and temperatures. After ageing, the distribution of the oxidation products through the thickness of samples aged at 140 °C, 160 °C, 180 °C and 200 °C were monitored. The distribution of the oxidation products was recorded at 1734 cm⁻¹ which reflects the formation of imides. [41] The oxidation profiles which were obtained are displayed in Fig. 1 and represented in differential values according to the following equation:

$$ \Delta A = A_t - A_{ref} \qquad \text{Eq.2} $$

with A the imide absorbance. A$_{ref}$ corresponds to the absorbance measured in the middle of the unaged thick sample and A$_t$ the absorbance at a specific thickness.
<figure><img src="image_2.png" /><figcaption>Fig. 1 Oxidation depth profiles of samples aged in thermo-oxidation for different times at 140 °C (a), 160 °C (b), 180 °C (c) and 200 °C (d).</figcaption></figure>

The curves in Fig. 1 show an oxidation depth profile that reflects a diffusion-controlled oxidation process (DLO). [42] The aged samples present a typical skin-core oxidation profile with the formation of oxidation products at the surface of the sample while the core remains unchanged. The profiles are symmetric with the formation of imides at the surface, which increases progressively over time. The profile is marked, and the oxidation becomes more superficial as the ageing temperature increases. The oxidation depth can be defined as the distance from the surface at which no oxidation products are formed. [43] In Table. 1, the oxidation depth is indicated for when the absorbance at the surface exceeds 1 for each exposure temperature.
<table><caption>Table. 1 Total oxidation depth for each ageing temperature</caption><thead><tr><td>Ageing temperature (°C)</td><td>Total oxidation depth (μm)</td></tr></thead><tbody><tr><td>140</td><td>560</td></tr><tr><td>160</td><td>360</td></tr><tr><td>180</td><td>270</td></tr><tr><td>200</td><td>200</td></tr></tbody></table>

The data in Table. 1 clearly indicate that the thickness of the oxidised layer is temperature dependent. A decrease in oxidation depth is observed when the ageing temperature rises, which is in accordance with the results in the literature. [42][44][45] The increase in temperature and ageing time enhances the skin-core oxidation profile. The development of diffusion-limited oxidation profiles is the result of competition between the oxidation rate and oxygen diffusion. [46][47] Consequently, as the oxidation rate increases faster than the oxygen diffusion with temperature, one can suggest that the total amount of oxygen is consumed at the surface of polymer plates and then cannot diffuse deeply in the bulk when the ageing temperature increases.

## 3.2. Crystallinity

Infrared spectroscopy has also been used to investigate the microstructure properties of a polymer, such as the crystallinity and crystalline morphology. [48] In the literature, various IR bands have been assigned to the crystalline and amorphous phases in PAs. [49][50] The bands at 906, 936 and 1200 cm⁻¹ are attributed to the crystalline phase, while the bands at 924 and 1136 cm⁻¹ are assigned to the amorphous phase. [51][56] The bands at 936 and 924 cm⁻¹ that correspond to the C-C=O stretching in the respective crystalline and amorphous phases
overlap. A deconvolution is necessary to distinguish the absorbance of the two bands. For simplicity, the bands at 906 (am) and 1136 cm⁻¹ (cry), which are directly accessible in IR spectra, were chosen to determine the amount of crystalline material. The crystallinity ratio determined by FTIR spectrophotometry was calculated according to the following equation:

$$ \chi_c = \frac{A_{cry}}{A_{cry} + \alpha \times A_{am}} \qquad \text{Eq. 3} $$

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

$$ \alpha = \frac{\varepsilon_{cry}}{\varepsilon_{am}} \qquad \text{Eq. 4} $$

According to the literature, the coefficient α is equal to 0,57 for the selected band ratio. [57]

The crystallinity as a function of the thickness were determined before ageing and after ageing for 100 h at 180 °C. The resulting crystallinity depth profiles are displayed in Fig. 2.

<figure><img src="image_3.png" /><figcaption>Fig. 2 Crystallinity depth profiles determined by FTIR microspectroscopy for a sample before ageing (■) and for two samples aged at 180 °C for 100 h with (▲) and without oxygen (Δ).</figcaption></figure>

Interestingly, it should be noted that the relative crystallinity determined by infrared spectroscopy in the bulk (≈ 27 %) for the sample before ageing is close to the degree of
crystallinity measured by DSC (≈ 30 %) [58], which confirms the relevance of infrared to determine the crystallinity.

From the curves presented in Fig. 2, we can notice that the crystallinity amount is lower (≈ 19 %) in the first 200 µm than in the core (≈ 27 %) for the PA 6,6 sample before ageing. This may result from the processing of the samples. During injection moulding, crystallisation occurs during cooling of the melt. The skin parts that are directly in contact with the cold mould surfaces are quickly cooled. Many studies have investigated the influence of thermal history on the skin/core morphology. [59][64] Based on the literature, it is assumed that the core cooling rate ranges from approximately 400 to 600 K.s⁻¹ and is slower than that in the skin. [60][62][65] During slow cooling, the chains have sufficient time to arrange into crystals. In comparison, the rapid cooling in the skin inhibits the development of crystals and spherulites, which is reflected in a lower crystallinity ratio than that of the core. This result can directly explain why the mechanical properties of this kind of polymer present before aging a such strong difference from the surface to the bulk and why it is so difficult to directly compare directly hardness measurements (i.e. surface analysis) to tensile test.

After thermal ageing in air, a significant increase in the crystallinity can be seen over the entire thickness of the sample thermo-oxidised at 180 °C for 100 h. In addition, the crystallinity profile of the thermo-oxidised sample is heterogeneous; in other words, the crystallinity of the surface is significantly higher than that of the core. The crystallinity increase was previously observed in the bulk by DSC [66] and ascribed to chain scissions that release chain fragments that are able to integrate into the crystalline phase by chemicrystallisation or to reorganise in new crystals. [67] The crystallinity profile evolves from the surface up to 200 µm towards the core and then stays at approximately 40 %. The profile shape and depth are comparable to the infrared profile obtained by monitoring the
oxidation products. However, while no oxidation products are detected in the core of the material due to the DLO effect, an increase in the crystallinity from 27% to 40% occurs simultaneously. This significant increase in the crystallinity in few hours of aging can also explain why, in our previous paper [58], we have seen a drop in the tan δ peak maximum, which indicated a loss in molecular mobility in the first hours of aging when the chain-scissions mechanism was not the major phenomenon.

To explain this behaviour, we measured crystallinity after ageing for the same time (100 h) but in the absence of oxygen. The results obtained show a homogeneous crystallinity of approximately 37% over the entire thickness, which is higher than the crystallinity measured in the sample before ageing. The polymer is quenched during moulding as it is cooled rapidly, which results in the development of an unstable crystalline morphology. Submitting the sample to a temperature above its glass transition temperature (Tg = 65 °C) allows the macromolecular chains to rearrange and crystallise, which improves the order in the crystalline phase and increases the crystallinity. [68] This also occurs when the sample is subjected to oxidation, which explains the increase in the crystallinity in the bulk, despite no oxidation occurring as a result of oxygen depletion.

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

[69][70] These results permit also to better understand the consequences of the oxidation and to definitely separate the consequences of an oxidation process versus an annealing process.

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

**3.3. AFM nanoscale thermal analysis**

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

AFM thermal analysis (VITA®) is a technology that allows a local determination of thermal
properties, such as the melting point or glass transition temperature. [71][73] To collect the
mapping results, the melting temperature was chosen to evaluate changes in the
microstructure with thermo-oxidative ageing. First, thermal mapping was carried out on PA
6,6 before ageing. (Fig. 3)
<figure><img src="image_4.png" /><figcaption>Fig. 3 Melting temperature mapping of non-aged PA 6,6.</figcaption></figure>

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

Mapping was also performed on two samples aged at 140 °C for 2000 h and 180 °C for 197 h, and the obtained results are presented in Fig. 4. These ageing times were selected because they correspond to the maximum thickness of the oxidised layer for each of the temperatures selected.

<figure><img src="image_5.png" /><figcaption>Fig. 4. Melting temperature mapping of samples thermo-oxidised at (a) 140 °C for 2000h and (b) 180 °C for 197 h.</figcaption></figure>
Changes in the melting temperature are observed from the surface to the core (until 320 µm is reached). With thermal ageing, the melting temperature ranges from 125 °C to 275 °C. The hatched area corresponds to a region where no melting is observed. It is important to specify that the melting temperature measured by AFM corresponds to the onset of the melting peak measured by DSC. Indeed, the AFM probe deflects as soon as the material starts to melt. In Fig. 4a, we observe that the melting temperature of the sample aged at 140 °C gradually evolves across the whole profile. The melting temperature decreases as one approaches the surface. In the first 80 µm, the melting temperature is approximately 125 °C and 145 °C.

These findings are relevant to our previous results measured by DSC [58], where we observed a large distribution of the melting peak towards lower temperatures with thermal ageing. For the sample aged at 180 °C presented in Fig. 4b, the melting temperature gradient extends from the surface to a depth of 170 µm. From there, the melting temperature remains at 265 °C and does not change. The thermal property profile at 140 °C is deeper and more progressive than the profile determined at 180 °C, which is consistent with the results previously obtained by infrared microspectrometry. The melting temperature at the surface and near the surface could not be determined despite heating up to 350 °C (hatched area). The lack of melting in this particularly degraded area may be caused by crosslinking and/or carbonisation of the surface with thermal ageing. Authors that characterised ageing by SEC or rheology have shown that thermo-oxidation mainly causes chain scissions [12][13], but a crosslinking process cannot be excluded and has been observed for samples that underwent advanced ageing. [74]
3.4. AFM nanoscale mechanical properties

AFM in Peak Force QNM mode was employed to characterise the mechanical properties at
the surface and in the bulk of the polymeric materials. [75][78] This AFM mode gives access
to the DMT modulus, which can be likened to the reduced Young's modulus. [38] Mapping
was carried out on the same two samples as for those analysed with AFM thermal analysis but
in a different area because this analysis is destructive. The results of the DMT modulus
mapping are presented in Fig. 5.

<figure><img src="image_6.png" /><figcaption>Fig. 5 Stiffness mapping of samples thermo-oxidised at (a) 140 °C for 2000 h and (b) 180 °C for 197 h.</figcaption></figure>

A large distribution of the DMT modulus from the surface to the core can be seen in both
samples. The DMT modulus varies between 1,5 and 2,5 GPa in the core and can increase to
10 GPa at the surface. The effect of thermal oxidation on the mechanical properties can be
seen up to a depth of 200 µm in the sample aged at 140 °C, while it remains very superficial
in the sample aged at 180 °C. One can also notice that the modulus mapping does not show
like in the case of mapping of melting temperature a blank zone in the first micrometers
(surface). This zone corresponds to a degraded area with an important stiffness that can be
linked to a crosslinking of the surface with thermal ageing. Authors that characterised ageing
by SEC or rheology have shown that thermo-oxidation can also lead to a crosslinking process
and this has been observed for samples that underwent advanced ageing. [74] Thus, this
experiment allows us not only to specifically separate the crosslinking reactions from the
chain scission mechanism but also to identify areas where one of the reactions is predominant
over the other.
### 3.5. Multi-scales correlations

In a previous paper, we proposed a multiscale approach to characterise the PA 6,6 thermal degradation. [58] We showed that following the variations in the intensity of the band at 3080 cm⁻¹, assigned to the overtone of the amide II band, was a relevant chemical probe to correlate changes in the chemical structure with the resulting modifications of the microstructure and loss of functional properties. The variations in absorbance at 3080 cm⁻¹ fit well with the changes in the crystallinity, molar mass and microhardness. In the current study, the variation in the absorbance at 3080 cm⁻¹ as a function of the distance from the outer surface was also investigated. Fig. 6 shows the variations in absorbance at 3080 cm⁻¹ through the thickness of thick samples aged for various times at 140 °C.

<figure><img src="image_7.png" /><figcaption>Fig. 6 ΔA<sub>3080 cm<sup>-1</sup></sub> depth profiles of the samples aged with thermo-oxidation for different times at 140 °C.</figcaption></figure>
**Fig. 6** shows a similar skin-core profile as those reported above, with a **decrease in the** absorbance from the core to the surface. The drop in the amide absorbance reflects that oxidation occurs.

The absorbance at 3080 cm⁻¹ may be correlated to the modifications of other properties of the polymer to establish a multiscale characterisation of the profiles after ageing. Fig. 7 displays the thermo-oxidation profiles determined by AFM thermal mode and micro-FTIR in the first 400 µm of a thick sample (1.5 mm) aged at 180 °C for 197 h.

<figure><img src="image_8.png" /><figcaption>Fig. 7 Thermo-oxidation profile of a PA 6,6 sample measured by micro-FTIR (■) and AFM – VITA (●) after 197 h of ageing at 180 °C.</figcaption></figure>

As shown above in Fig. 7, the loss of the amide bond is only observable in the first 200 microns and is more apparent as the surface is reached. Correspondingly, in the first 200 microns, the melting temperature measured using AFM nanoscale thermal analysis is decreasing. According to the oxidation mechanism, chain scissions occur at the C-N bond of the amide, resulting in the formation of aldehydes, carboxylic acids and primary amides as oxidation products. [79][80] The resulting crystallites composed of chain fragments are small and thus tend to melt at a low temperatures, which can be seen in the nanothermal profile measured by AFM.
The absorbance at 3080 cm⁻¹ is also plotted with the DMT modulus in Fig. 8 to compare changes in the chemical structure with those of the mechanical properties during thermal ageing.

<figure><img src="image_9.png" /><figcaption>Fig. 8 Profile measured by micro-FTIR (■) and AFM peak force (▲) of a sample thermo- aged at 180 °C for 197 h</figcaption></figure>

Fig. 8 shows that the DMT modulus increases from the core to the surface, contrary to the absorption band that decreases at the same depth. The change in slopes occurs at a depth of approximately 125 µm. According to the thermo-oxidation mechanism, chain scissions occur at the C-N bond of the amide group and thus lead to a decrease in this absorbance band. Previously, we found that chain scissions caused an increase in the crystallinity. As crystalline regions have higher mechanical properties than amorphous regions, this increase results in a rise in the stiffness, represented in this study by the DMT modulus.

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

Fig. 9 compares the infrared and nanomechanical AFM profiles of a sample oxidised for 197 h at 180 °C.
<figure><img src="image_10.png" /><figcaption>Fig. 9 Crystallinity gradient (▶) and AFM peak force (▲) as a function of the thickness of a sample thermo- aged at 180 °C for 197 h</figcaption></figure>

Fig. 9 compares the profiles of the crystallinity and DMT from the surface to the core (400 µm depth). This chart provides evidence of a correlation between the two profiles. Chain scissions that were previously correlated to a drop in the melting temperature cause an increase in the crystallinity. These parallel evolutions confirm a quantitative and direct correlation between the modification of the chemical structure and that of the mechanical properties.
## 4. Conclusion

Oxidation depth profiles were obtained for PA 6,6 samples thermally aged from 140 °C to 200 °C at different times by measuring the distribution of imides. We show that PA 6,6 degradation is controlled by oxygen diffusion, resulting in a higher oxidation degree at the surface than in the bulk. The heterogeneity of the degradation is temperature dependent, which means that oxidation becomes more superficial when the temperature increases. FTIR results also show the existence of a crystallinity profile where the surface has a higher crystallinity than the core. Additionally, we have shown that changes in the thermal and mechanical properties as a function of sample thickness can be measured by AFM. The comparison of the chemical structure changes with the thermal and mechanical properties shows a good correlation. Finally, the results presented in this study clearly show that AFM is a convenient and complementary method for determining the oxidation depth profiles of polyamide 6,6.

## Acknowledgements

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

<figure><img src="image_11.png" /><figcaption>Figure 11: Pictures of samples: (left) before aging and (right) thermo-oxidised at 140 °C for 2000h.</figcaption></figure>

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