# Effects of electron beam radiation and annealing on the crystallinity and tensile properties of a 3D-printed PLA-based material

A. Arias-Blanco, Gabriela G. Tataru, M. Marco, M.H. Miguélez, X. Coqueret

► To cite this version:

A. Arias-Blanco, Gabriela G. Tataru, M. Marco, M.H. Miguélez, X. Coqueret. Effects of electron beam radiation and annealing on the crystallinity and tensile properties of a 3D-printed PLA-based material. Polymer Degradation and Stability, 2025, 240, pp.111476. 10.1016/j.polymdegradstab.2025.111476. hal-05114712

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# Effects of electron beam radiation and annealing on the crystallinity and tensile properties of a 3D-printed PLA-based material

A. Arias-Blanco<sup>a,b</sup>, G. Tataru<sup>a</sup>, M. Marco<sup>b</sup>, M.H. Miguélez<sup>b</sup>, X. Coqueret<sup>a,*</sup>

## ARTICLE INFO

Keywords:

Poly(lactic acid) (PLA)

PLA blends

3D-printing

Crystallinity

Tensile properties

Radiolytic effects

Chemi-crystallization

Thermal healing

## ABSTRACT

This article reports on the influence of electron beam irradiation on the tensile properties and crystallinity of 3D-printed PLA-based materials. Monitoring the evolution of key tensile properties as a function of dose confirmed the detrimental effects of irradiation with a typical loss of about 50 % of tensile strength and ultimate displacement after a 75 kGy treatment. Particular attention was paid on the nature of crystalline domains and degree of crystallinity $\chi$ in samples analyzed by calorimetry and X-ray diffractometry, as obtained after 3D-printing, after irradiation and after annealing in optimized conditions. The $\chi$ value in irradiated samples was shown to initially increase from 19 to 24 % and return to 19 % at 100 kGy before a stronger decay at 300 kGy. The maximum crystallization capacity of the same samples was much higher ($\chi$>60 %) within this dose range. Crystallinity can therefore be adjusted after irradiation with possible beneficial effects to mitigate the undesired impact of irradiation. For samples irradiated at 75 kGy with a loss of 50–60 % on tensile strength and ultimate displacement, part of the mechanical damage due to irradiation was recovered by soft annealing at 60 °C, increasing the tensile strength by 20 % and ultimate displacement by 50 %. This remarkable result can be interpreted by considering the particular sensitivity to the evolution of tie chains. While chain scission under radiation is effective and mechanically impactful in domains closed to crystallites, chemi-crystallization is believed to occur with partial healing of the damages due to chain scission.

## 1. Introduction

Poly(lactic acid) and its abbreviation PLA are commonly used to design a large variety of biobased polymers that are now acknowledged as the most significant alternative to oil-based commodity plastics [1]. The unique features of PLA in terms of biocompatibility and bio-resorption have aroused considerable amount of research leading to a broad family of lactide homo- and co-polymers, PLA blends and composites [2]. The main initial driver of PLA development was indeed the perspective of biomedical applications. Due to its optimal content in renewable carbon, compostability under appropriate industrial conditions and economically sustainable production, PLA continues to gain market shares in major sectors such as packaging plastics and synthetic textiles [3]. Alongside these main uses of PLA as a commodity plastic, 3D-printing of PLA objects by fused filament fabrication (FFF) has recently emerged as an important niche application [4]. As an increasing number of products manufactured by conventional and additive processes are designed for biomedical purposes and bioengineering, the

impact of treatments by radiation sterilization needs being assessed in some detail [5,6]. The consolidation of 3D-printed objects by reducing their original anisotropy by a radiation treatment inducing post cure or cross-linking is another effect with promising perspectives [7]. Moreover, the localized modification by radiation processed of 3D-printed prosthetic materials can provide a useful means to modulate in a spatial manner the properties of implants, particularly their bio-resorption rate [8].

Ionizing radiation is indeed an effective method to modify polymers by inducing scission or cross-linking and by graft polymerization and a convenient mean to sterilize items produced for biomedical and biological applications [9]. Depending on the chemical structure and physical features of the irradiated polymer, the effects of radiation are different with positive or negative impacts, depending of the application. The molecular and physical changes observed in polymers are primarily a consequence of chain scission, cross-linking, these processes being accompanied by the formation of molecular gases [10].

Since decades, there have been a large number of studies addressing

<footnote><sup>a</sup> Institut de Chimie Moléculaire de Reims, Université de Reims Champagne Ardenne, CNRS UMR 7312, Reims 51097, France</footnote>

<footnote><sup>b</sup> Department of Mechanical Engineering, Universidad Carlos III de Madrid, Spain</footnote>

<footnote>* Corresponding author.
E-mail address: xavier.coqueret@univ-reims.fr (X. Coqueret).</footnote>
the radiolytic behavior of various types of PLAs [11–13] submitted to the different types of radiation sources commonly employed in industry and research laboratories, covering a broad range of dose and dose rates in various environmental conditions. These include many reports using γ radiation [14] and electron beam [15] and, in a lesser number of studies, X-ray radiation [16].

It is now well-established that the quantified responses in terms of molecular modifications and changes in physical properties induced by radiation treatments strongly depend on the particular PLA-based system under consideration [17]. A first series of factors exerting an influence of the response of the material to radiation processing relate to the particular chemical constitution of the PLA material, its molecular weight distribution, stereoregularity, presence of additives other polymers in the case of a thermoplastic blend or of fillers in the case of a composite materials. Other factors depend on the physical state of the material, particularly its degree of crystallinity as well as its morphological features, and then the radiation treatment by itself such as radiation dose and dose rate, time of exposure and thermal effect.

Qualitatively, chain scission is the prominent molecular effect of ionizing radiation on PLAs of any type in the absence of low- or high-molecular weight cross-linkers. This reduces its average molecular weight with a severe impact at high doses and leads to reduced mechanical strength, higher brittleness and lower thermal stability [18]. However cross-linking can take place within irradiated PLA as a minor effect in particular situations [19,20]. Of particular relevance is the monitoring of changes in crystallinity. PLAs of sufficiently high stereoregularity have the capacity to crystallize to a large extent, but in practice the rather high glass transition temperature of the amorphous component in the semi-crystalline material limits the development of ordered phases in samples cooled from the molten state without specific thermal control [21]. In such samples, the common feature of many semi-crystalline that have their degree of crystallinity enhanced by limited radiation treatments has been reported for PLAs, with some alteration of their mechanical behavior [22,23].

As the stereochemistry of LA repeat units is a key factor controlling the aptitude of the polymer to crystallize, the response to radiation treatments is strongly affected by this indirect effect of the structure [24]. Highly stereoregular PLA, as poly(L-lactic acid) or poly(D-lactic acid) with a molar content equal or higher than 95 mol-% are expected to be less responsive than irregular PLAs and racemic poly(DL-lactic acid) to ionizing radiation.

The spontaneous formation of PLA stereocomplex in which highly stereoregular PLA of opposed configuration, namely PLLA and PDLA, are tightly associated is a very specific situation resulting in enhanced thermal and mechanical stability. This often translates into improved resistance to the predominant radiation-induced degradation when compared with the individual stereoregular polymers [25]. A more common situation is the presence of cross-linking agents or other thermoplastics to enhance the properties of the unirradiated blended material and simultaneous to alleviate the detrimental effects of irradiation. Blending PLA with other polyesters like PBAT (polybutylene adipate terephthalate) or PCL (polycaprolactone) is primarily achieved to enhance its toughness and flexibility and can also alter its radiation response [26]. The extent of scission may be reduced in presence of another polymers exhibiting significant radiation cross-linking such as poly(caprolactone) [27]. Other changes related to phase morphology, potentially leading to phase separation or improved compatibility, may occur upon treatment of blends made of poorly miscible polymers.

The differences in chemical composition between the commercial material that was used and those described in literature reports, the specific conditions of the FFF-process and the treatment at high dose rate with a 10 MeV accelerator justify a precise evaluation of the changes of properties expectedly arising from the well-documented main chain scission mechanism occurring in blends free from cross-linker. The central objective of the present study was to revisit the effects of irradiation on the degree of crystallinity, crystallization behavior and

mechanical properties of a representative PLA-based material used in 3D-printing.

Preliminary chemical analysis revealed that the commercial filament is actually a polymer blend made of highly stereoregular PLLA (88 wt-%) and PBAT (12 wt-%). A study of the molecular and spectroscopic aspects of the radiolytic behavior of this particular material is ongoing and will give rise to a forthcoming article [28]. Being focused here on the mechanical and physical aspects of the subject, it is worth mentioning here recent papers by Malinowski, reporting on the EB irradiation of blends of PLA (3.5 % D unit) and PBAT in proportion 80:20 and 20:80 [29,30]. Due to the poor compatibility between the two components, the compositions with much higher content in PBAT are quite different from the present one, while the manufacturing processes used in the published research are also different. Moreover the semi-crystallinity of the PLA component in the material as a function of its thermal history and radiation treatment controls to a large extent its mechanical properties.

## 2. Materials and methods

### 2.1. Materials

The thermoplastic material employed in this work was a PLA-based filament commercialized by the company 3DGence (Poland) under the reference "PLA Commodity Material". The chemical composition of the filament was established by ¹H and ¹³C NMR spectroscopy at 500.28 and 125.8 MHz, respectively. The spectra recorded on a Bruker Advance Neo 500 MHz (Bruker Corporation, Germany) with samples dissolved in CDCl₃ (99.8 % D). Dissolution in chloroform of large samples was shown to yield optically clear and colorless solution. This strongly suggests the absence of inorganic particulate fillers in the blend.

### 2.2. 3D-printing of test samples

Printing of PLA objects designed for the study was performed with an FFF-type 3D printer 3DGence F340 with an M280 printing module with a 0.4 mm nozzle diameter. The employed parameters, recommended by the manufacturer and used in a previous study [31] were set as follows: extrusion temperature 210 °C, bed temperature 60 °C, build volume temperature 30 °C, layer thickness 0.25 mm. First layer and infill printing speed 25 and 50 mm s⁻¹, respectively. A concentric infill has been used in printed samples to avoid the influence of defects in mechanical properties.

Two different series of samples were printed for this work: dog-bone samples, according to ISO 527-1:2020 standard [32], for the mechanical characterization (Fig. 1-a) and a specifically designed dot-mesh sample (Fig. 1-b) that allows an easy separation of cutout disk-shaped tablets in view of calorimetric measurements. Samples were printed in the XY

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Sketch describing the geometry and dimensions (expressed in mm) of the 3D-printed samples used in this study: a) 4 mm-thick dog-bone specimen for mechanical tests, b) 2 mm-thick dot-mesh sample for calorimetric measurements.</figcaption></figure>
plane, being the Z layer-by-layer axis.

## 2.3. Electron beam irradiation

The samples were irradiated using a 10 MeV electron beam from a pulsed Linac accelerator (640 Hz) at an average dose rate of 15 kGy s⁻¹ (IONISOS, Chaumesnil, France). The vacuum-bagged samples were treated to absorbed dose of radiation of 25, 50, 75, 100, 150, 200 and 300 kGy by repeating unit passes of 25 kGy with an 5 min time interval so as to limit the temperature increase within the treated samples.

## 2.4. Mechanical characterization

Mechanical characterization has been carried out through tensile tests following the ISO 527-1:2020 [32] standard using a universal testing machine, Instron 3366 (Instron, United States), and a 10 kN load cell. The loading was applied in displacement control at a cross-head speed of 1 mm/min under quasi-static conditions. Measurement of strains in the samples has been carried out with an axial clip-on extensometer, Instron 2630-107. A minimum of 3 and a maximum of 6 samples were tested per radiation dose. Samples irradiated at 300 kGy presenting fixation problems during testing due to excessive brittleness were discarded from obtained data.

To take into account the influence of microvoids generated during the additive manufacturing process that result in a lower apparent density of the 3D-printed samples, a porosity correction factor of 0.93. This previously employed by the authors was introduced to calculate the intrinsic mechanical properties by a method described in [31].

The measured mechanical properties are Young's modulus, *E*, ultimate stress, $\sigma_{Max}$, and maximum displacement, Disp.$_{Max}$. As strain measurements have been carried out using a clip-on extensometer, strain measurement when reaching the ultimate strain, $\epsilon_{Max}$, might result in incorrect values or even in the break of the extensometer. Since the main purpose of this work is to analyze the evolution of the mechanical properties within different irradiation doses rather than on the actual values of magnitudes, Disp.$_{Max}$, provided by the testing machine can be used instead without loss of information of effects on the results. Besides, the displacements will be consistent as all the samples have the same shape with identical dimensions and have been tested under the same conditions. Nevertheless, it is worth noting that values provided by the extensometer have been indeed used in the calculus of Young's modulus.

## 2.5. Calorimetric measurements

Differential scanning calorimetry (DSC) analyses were performed using a TA Q100 (TA Instruments, UK) calibrated with indium. The analytical cell was flushed with nitrogen at a rate of 50 mL min⁻¹. The mass of tested samples placed in aluminum pans was adjusted to about 10 mg. The standard thermal cycle used to characterize the PLA-based samples. Unless indicated differently in section Results and Discussion, the samples were submitted to the following temperature profiles to record the heat flux and exploit the obtained thermogram: equilibration the samples at 20.0 °C for 5 min, heating from -80.0 to 210.0 °C at a rate of 10.0 °C min⁻¹, cooling down to 140.0 °C at a rate of 10 °C min⁻¹, isothermal stage at 140.0 °C for 120 min (or 240 min for some samples, when mentioned in the main text), cooling down to 20.0 °C at a rate of 10.0 °C min⁻¹, isothermal stage at 20.0 °C for 30 min (or longer, as mentioned in the main text), heating to 210.0 °C at a rate of 10.0 °C min⁻¹ (Supplementary material, #1).

## 2.6. X-ray diffractometry

X-Ray diffraction (XRD) experiments were carried out using a Bruker D8 Advance diffractometer (Bruker, Germany) working at 40 kV and 40 mA with Cu Kα radiation (λ$_{Cu}$ K$_{α}$ = 1.5418 Å), operating in Bragg-

Brentano geometry for 2θ ranging from 10° to 40°, with a step of 0.04° and a scan speed of 2 s by step. Diffractograms were recorded from samples in the form of fine powder produced by scrapping the 3D-printed tablet samples with an iron file. 8 mg of powder samples ground (< 20 µm) is placed on a sample holder and the surface of the powder is flattened to avoid any preferential orientation that would alter the intensity of the diffraction peaks.

For comparing the samples as a function of the absorbed dose of radiation and determining of the degree of crystallinity, the diffractograms were normalized with respect to the total area above the baseline. The relative area of particular diffraction peaks was calculated after subtraction of the contribution of halo from the amorphous component to estimate the crystallinity in each sample.

## 2.7. Annealing of irradiated PLA specimens

Annealing experiments were conducted on unirradiated and 75 kGy-irradiated dog-bone specimens as defined in Section 2.2. The samples were placed in a POL-EKO SLW 53 (Poland) drying oven set at temperatures between 50 and 90 °C and for various time durations (30 to 120 min) under ambient atmosphere.

# 3. Results

Damages induced by radiation into polymer materials are easily evidenced by tensile measurements that express at a macroscopic level the cumulative and complex changes induced by radiolysis into single macromolecules.

## 3.1. Tensile properties of 3D-printed samples

The samples printed with the selected PLA material were treated at doses up to 300 kGy. Tensile tests performed on series of specimens treated at the indicated dose allow to draw the force-displacement plots represented in Fig. 2. Limited plastic deformation is observed only with the non-irradiated samples, the relative elongation corresponding to approximately to 3.8 %, the ultimate displacement Disp.$_{Max}$ being calculated slightly above 4.0 %. The linear part of the plot allowed to calculate a Young's modulus *E* of 3.35 ± 0.26 GPa, while ultimate stress and maximum displacement were determined were $\sigma_{Max}$ = 58.28 ± 0.65 MPa and Disp.$_{Max}$ = 3.20 ± 0.05 mm, respectively.

As expected for such a polymer blend known to undergo scission predominantly, the general evolution in the plots of Fig. 2 indicates the concomitant decrease of maximum strength and displacement at break as a function of dose, with the typical profile of brittle materials. The dose-dependence of the three parameters is represented in Fig. 3a-c, each quantity being to normalized with respect to the value measured for unirradiated samples. Considering the precision of the measurements, the Young's modulus *E* (Fig. 3-c) does not seem significantly affected by the treatment under radiation up to a total dose of 300 kGy.

The decrease of both the maximum strength and displacement is not monotonic all over the explored dose range. Three different domains can be defined by the corresponding dose range, between 0 and 25 kGy, between 25 and 100 kGy, and between 100 and 300 kGy denoted (R1), 2 (R2), and 3 (R3), respectively. In domain R1, the strength $\sigma_{Max}$ is not significantly affected after application of a moderate dose of 25 kGy, however, Disp.$_{Max}$ exhibits a significant decrease of about 15 %. In domain R2, the damages are more severe as the dose increases and the strength and maximum displacement quite linearly down to 20 % of their initial values, limiting considerably the practical uses of the radiation processed PLA materials treated, depending on the performances required for the PLA base items. For doses exceeding 100 kGy, (Domain R3), the samples exhibit very low tensile properties below 20 % of those in the non-irradiated state. In this domain, the decrease in $\sigma_{Max}$ and Disp.$_{Max}$ with the dose tends to level off.
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Impact of the absorbed dose of radiation on the force-displacement plots recorded on representative 3D-printed PLA-based dog-bone specimens treated by EB radiation. Only one sample is shown as an example for each dose.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Evolution of normalized tensile properties of standard PLA-based dog-bone specimens as a function of the absorbed dose of radiation: ultimate stress, $\sigma_{max}$ (a), maximum displacement, Disp_max (b) and Young's Modulus, E (c).</figcaption></figure>

## 3.2. Calorimetric properties of PLA in 3D-printed samples

The changes in crystallinity in 3D-printed materials at the different stages of the process were first analyzed by differential scanning calorimetry (DSC) [33]. The objective was to determine the nature of the crystalline domains and to quantify the fraction of crystalline domains in the original filament, unirradiated and irradiated 3D-printed samples, as obtained. Additionally, it seemed important to monitor the evolution the maximum crystallization capacity of the PLLA fraction within the various samples by performing an extensive annealing of the samples under well-defined conditions.

DSC analysis of the original filament material and of a 3D-printed disk tested without additional thermal treatment provides information on the effects of the fused filament printing process (Fig. 4). In the temperature domain ranging from 20 to 200 °C, both types of samples present the same general profile showing the following sequence of transitions, as reported by many authors [34,35]. The glass transition is observed at 65 °C, appearing as a superimposed reversible change in PLLA heat capacity $C_p$ together with some irreversible enthalpic

phenomenon that have been evidenced separately by temperature-modulated DSC measurements [35]. The cold crystallization of amorphous PLLA is then observed between 80 and 115 °C, with an onset and a maximum for the exothermal peak shifted by 5–6 °C towards higher temperatures for the 3D sample, suggesting an easier crystallization process in the filament. The incipient cold crystallization at lower temperature is likely due to the orientation of polymer chains during the extrusion and stretching process that inevitably takes place during the production of such filaments. The thermograms then show a long domain without enthalpic transition (110–140 °C) for both samples, and then, at the same characteristic temperatures, the sequence consisting in the exothermic rearrangement for the α' allomorph into the more stable α form (155–170 °C) and to the final melting at 170–195 °C [36]. This high value for the melting is typical for a PLLA material of high stereoregularity with a content in L lactate units of at least 99.3 mol- % [34].

Additional experiments conducted with expended temperature range did not allow to observe the enthalpic transitions of pure PBAT expected at about –30 °C for $T_g$ and the melting at about 120 °C [37]. Taking into
<figure><img src="image_7.png" /><figcaption>Fig. 4. DSC thermograms recorded for the filament (solid line) and the corresponding 3D-printed sample (dashed) showing the main enthalpic transitions within the analyzed PLLA samples.</figcaption></figure>

account the low PBAT content in the blend (12 wt-%) and the presence of small, yet not negligible, amount of the other polymer in each phase separated-domain the thermal features of PBAT may be modified or weakened in the thermogram [38].

Several authors report a value of -57 J g⁻¹ for the enthalpy of crystallization of amorphous PLLA into the α' allomorph, whereas the melting enthalpy of completely crystalline PLLA ΔHₘ,PLLA is considered to be close to -93 J g⁻¹ [39–43]. By comparing the amplitudes of the transitions and measuring the values of the corresponding transition enthalpies in thermograms of Fig. 4, one can see that relatively to the cold crystallization exotherm area between 80 and 115 °C, the exotherm the α' to α conversion between 155 and 170 °C represents much less than the ratios 57/93 and 36/93, respectively, that would be expected if only the α' allomorph was produced from the amorphous PLLA [44]. It can be concluded that this is not the case and that both allotropic forms are present.

The degree of crystallinity of PLLA (χₔ₅₄) in the two types of original samples discussed was determined as follows. The measured enthalpies for the cold crystallization ΔHₘₚ and allotropic conversion ΔHₘ'→α were subtracted from the experimental value ΔHₘ,exp and then divided by the specific enthalpy of 100 % crystallized PLLA material, ΔHₘ,PLLA, weighed by a correction factor of 0.88, corresponding to the actual weight fraction wₚ,LLA of PLLA in the blend.

The overall formula is expressed by the following equation:

$$X_{DSC,DSC} = \frac{\Delta H_{m,exp} - \Delta H_{CC} - \Delta H_{\alpha' \to \alpha}}{w_{PLLA} \times \Delta H_{m,PLLA}} \quad (1)$$

Applying this relation, the calculated degree of crystallinity in the original filament was χₔ₅₄ = 0.154 whereas the corresponding value for the 3D-printed specimen was somewhat higher χₔ₅₄ = 0.193. The higher content in crystalline domains of the 3D-printed samples can be explained by their larger dimension of the fabricated objects (2 mm-thick, several square centimeters in surface, to be compared to 1.75 mm in diameter for the original filament) and also by the temperatures of the platform and printing chamber maintained at 60 and 30 °C,

respectively, during the fabrication process. The higher cooling rate of the filament would logically reduce its degree of crystallinity [43]. The effects of irradiation were then analyzed using the same analytical approach. The corresponding series of thermograms recorded for 3D-printed samples treated with doses up to 300 kGy are reproduced in Fig. 5.

The moderate to strong treatment under ionizing radiation affects gradually all the transitions previously detailed for the unirradiated materials. The irreversible enthalpic phenomenon associated with the glass transition at 55–65 °C disappears after application of the lower dose. The inflexion point corresponding to T₉ seems to be shifted slightly to lower temperatures, likely a consequence of the scission reactions leading to some plasticization effect by low molecular fragments and additional chain-ends. Similarly the first order transitions have their maximum shifted by 5–15 °C to lower temperatures with a non-monotonic evolution of the associated enthalpies. The precise values of the transition temperatures and associated enthalpies are gathered in Table 1. Due to the relatively weak enthalpy of the allotropic phase change and to its position no far below the melting temperature of the α form, the precise quantification of the transitions for the samples treated at 200 and 300 kGy is subject to some uncertainties. The values are certainly much lower than those calculated for the lower dose range, but the precise numbers may be questioned. Complementary information obtained by X-ray diffraction was expected to clarify this point (see Section 3.3).

Besides the trends in the general evolution of thermograms that was just presented, the most noticeable point is the limited evolution of the final crystallinity (0.21 ± 0.02) for doses up to 100 kGy. Within this range, the degree of crystallinity obviously increases initially with the dose. This quite general phenomenon in irradiated semi-crystalline polymers may have been limited in the particular case of PLLA since the glass transition is above the irradiation temperature, at least for samples receiving a low dose. At higher doses (D > 100 kGy), the damages caused to the radiation-sensitive PLLA seem to induce too much disorder in the different domains. However, the maximal
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Influence of EB irradiation (0–300 kGy) on the thermograms representing the enthalpic transitions measured with 3D-printed PLLA-based samples analyzed as obtained.</figcaption></figure>

<table><caption>**Table 1**

Thermal properties of 3D-printed PLLA-based materials treated at various doses of EB irradiation.</caption><thead><tr><th>Dose<sup>a)</sup> (kGy)</th><th>T<sub>g,PLLA</sub><sup>b)</sup><br>(°C)</th><th>T<sub>cc,PLLA</sub><sup>c)</sup><br>(°C)</th><th>ΔH<sub>cc,PLLA</sub><sup>d)</sup><br>(J/g)</th><th>T<sub>α'→α,PLLA</sub><sup>e)</sup><br>(°C)</th><th>ΔH<sub>α'→α,PLLA</sub><sup>f)</sup><br>(J/g)</th><th>T<sub>m,PLLA</sub><sup>g)</sup><br>(°C)</th><th>ΔH<sub>m</sub><sup>h)</sup><br>(J/g)</th><th>χ<sub>DSC</sub><sup>i)</sup><br>(%)</th></tr></thead><tbody><tr><td>0</td><td>57.0</td><td>92.3</td><td>23.9</td><td>160.6</td><td>3.36</td><td>178.6</td><td>43.1</td><td>19.3</td></tr><tr><td>25</td><td>55.7</td><td>89.2</td><td>25.7</td><td>158.2</td><td>2.93</td><td>176.2</td><td>44.7</td><td>19.6</td></tr><tr><td>50</td><td>54.8</td><td>89.3</td><td>25.4</td><td>159.3</td><td>1.84</td><td>175.9</td><td>46.6</td><td>23.7</td></tr><tr><td>75</td><td>54.3</td><td>87.8</td><td>26.1</td><td>157.7</td><td>1.20</td><td>174.2</td><td>44.6</td><td>21.1</td></tr><tr><td>100</td><td>52.5</td><td>87.7</td><td>26.1</td><td>158.2</td><td>0.98</td><td>172.8</td><td>42.9</td><td>19.3</td></tr><tr><td>150</td><td>53.7</td><td>87.8</td><td>26.3</td><td>155.9</td><td>0.49</td><td>170.1</td><td>38.5</td><td>14.3</td></tr><tr><td>200</td><td>51.9</td><td>86.9</td><td>25.6</td><td>154.3</td><td>0.12</td><td>168.2</td><td>35.3</td><td>12.4</td></tr><tr><td>300</td><td>50.5</td><td>86.3</td><td>27.7</td><td>150.6</td><td>-</td><td>163.2</td><td>31.0</td><td>4.0</td></tr></tbody></table>

a) Absorbed dose of 10 MeV EB radiation.

b) glass transition temperature.

c) temperature for the cold crystallization.

d) absolute value of the cold crystallization enthalpy.

e) temperature for the allotropic rearrangement $α'→α$.

f) absolute value for the allotropic rearrangement $α'→α$.

g) melting temperature of α phase.

h) melting enthalpy of α phase.

i) degree of crystallinity for the PLLA component in the samples (Plots in Supplementary material #2).

crystallization capacity of PLLA is expected to be much higher than the values observed for original samples. This first set of analyses was therefore completed by a series of enthalpic measurements performed after extensive annealing.

After preliminary tests performed on unirradiated samples, annealing treatments were conducted at a temperature of 140 °C and a duration of 2 h. This was shown to suppress the occurrence of cold crystallization observed in the untreated samples. Applying this procedure to irradiated samples, DSC analysis led to the thermograms of Fig. 6 indicating that annealing is complete for samples irradiated up to 150 kGy. The strong endothermic peak with a extremum located at a temperature of 175–178 °C is indeed matching the typical features of the α phase of PLLA.

The values measured for the melting enthalpy $ΔH_m$ correspond to a

degree of crystallinity between 0.71 and 0.77 which seem to increase with the dose up to 75 kGy in the initial stage, and then to decrease, as detailed in Table 2.

Obviously the traces recorded for samples treated at 200 and 300 kGy show the persistence of significant cold crystallization phenomenon, suggesting that the annealing conditions at 140 °C were not sufficient to complete the process. By extending the duration to 4 h at the same temperature, the cold crystallization exotherm is no more seen in the thermograms, thus establishing that treatments at high doses reduce the crystallization rate, while the maximum crystallinity to be measured with such samples is still important, $χ_{DSC}$ values of 0.625 and 0.632 being determined for the samples irradiated at a dose of 200 and 300 kGy, respectively. One cannot invoke the contribution of chain scissions to explain the kinetic impact of the dose, while the crystallization
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Thermograms of enthalpic transitions recorded with 3D-printed PLLA-based samples EB-irradiated at various doses (0–300 kGy) and subject to an annealing treatment for 2 h at 140 °C.</figcaption></figure>

**Table 2**

Thermal properties of 3D-printed PLLA-based materials treated at various doses of EB irradiation and then annealed for 2 or 4 h at 140 °C.

<table><thead><tr><th>Dose<sup>a)</sup><br>(kGy)</th><th>Annealing<br>duration<br>at 140 °C (h)</th><th>T<sub>cc,PLLA</sub><sup>b)</sup><br>(°C)</th><th>ΔH<sub>cc,PLLA</sub><sup>c)</sup><br>(J/g)</th><th>T<sub>m,PLLA</sub><sup>d)</sup><br>(°C)</th><th>ΔH<sub>m</sub><sup>e)</sup><br>(J/g)</th><th>χ<sub>DSC</sub><sup>f)</sup><br>(%)</th></tr></thead><tbody><tr><td>0</td><td>2</td><td>-</td><td>-</td><td>177.8</td><td>58.4</td><td>71.4</td></tr><tr><td>25</td><td>2</td><td>-</td><td>-</td><td>175.6</td><td>62.6</td><td>76.5</td></tr><tr><td>50</td><td>2</td><td>-</td><td>-</td><td>174.8</td><td>60.3</td><td>73.7</td></tr><tr><td>75</td><td>2</td><td>-</td><td>-</td><td>174.0</td><td>63.0</td><td>77.0</td></tr><tr><td>100</td><td>2</td><td>-</td><td>-</td><td>172.6</td><td>61.6</td><td>75.3</td></tr><tr><td>150</td><td>2</td><td>-</td><td>-</td><td>169.4</td><td>58.5</td><td>71.5</td></tr><tr><td>200</td><td>2</td><td>89.5</td><td>3.4</td><td>167.1</td><td>50.4</td><td>&lt;<br>60.0</td></tr><tr><td>300</td><td>2</td><td>88.2</td><td>25.1</td><td>157.9</td><td>34.7</td><td>&lt;<br>15.0</td></tr><tr><td>200</td><td>4</td><td>-</td><td>-</td><td>167.1</td><td>53.2</td><td>62.5</td></tr><tr><td>300</td><td>4</td><td>-</td><td>&lt; 0.5</td><td>157.9</td><td>52.4</td><td>63.2</td></tr></tbody></table>

a) Absorbed dose of EB radiation.

b) temperature for the cold crystallization.

c) absolute value of the cold crystallization enthalpy.

d) melting temperature of α phase.

e) melting enthalpy of α phase.

f) degree of crystallinity for the PLLA component in the samples.

capacity remains high since shorter chains should on the average favor the mobility necessary to arrange PLLA chains in ordered domains. It is more reasonable to consider the effect of cross-linking that may start to be significant, yet a minor process compared to scission, when the PLA-based materials are irradiated at temperatures starting to approach the glass transition. Indeed, a rise in temperature cannot be completely avoided during the cumulative application of 25 kGy doses. Another possibility is the decrease in nucleation sites if those are sensitive to the effects of radiolysis at higher doses.

### 3.3. Analysis by X-Ray diffraction

Powder X-ray diffractograms were recorded for 2θ values ranging between 10 and 40 from unirradiated and irradiated (25–300 kGy) 3D-printed samples to monitor the evolution of crystallinity in the PLA-based samples. The original diffractograms were normalized with respect to the total area below the curve to yield the series of plots appearing in Fig. 7.

Two noticeable peaks are observed above the large halo observed between 2θ values of 10° and 27° due to the scattering by amorphous domains of the PLLA materials. The first and central peak with the highest intensity is located at around 2θ = 16.2°, associated with the (110)/(200) reflection, and the second one with lower intensity is located at around 2θ = 18.5°, associated with the reflection (113)/(203). The observed patterns are typical of the features shown by semicrystalline PLLA [45–48] and supported by simulations and ab-initio predictions [49]. However, there are limits to the contribution of X-ray powder diffraction analyses technique the two forms of crystalline PLLA present here possess a similar helix conformation and just differ from the less perfect alignment and packing between chains in α' allomorph [37, 42]. Detailed crystallographic and calorimetric studies on these points however indicate measurable evolution of the position and width of specific lines in the course of crystallization experiments monitored at various temperatures. To analyze more precisely the characteristics of the main diffraction peaks, the scattering halo was subtracted from the normalized diffractograms resulting in the focused representation of Fig. 8.

The position of the higher peak is originally centered at 16.25° in the unirradiated sample and then shifts to 16.20° and further down to 16.10° as the dose is increased, this being accompanied by a slight enlargement of the width at mid-height. The relative intensity and area of this peak increases when the dose increases to 100 kGy and is then reduced with strong deformations of the diffractogram at lower 2θ values.

Comparing the position of the main peak observed here to those
<figure><img src="image_10.png" /><figcaption>Fig. 7. Wide angle X-ray diffractograms recorded from powdered 3D-printed PLLA-based samples unirradiated and EB-irradiated at various doses (25–300 kGy).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Diffraction lines separated from the halo observed in normalized wide angle X-ray diffractograms recorded from powdered 3D-printed PLLA-based samples unirradiated and EB-irradiated at various doses (25–300 kGy).</figcaption></figure>

reported for the thermal conversion for the α' form (peak centered at 2θ = 16.3 to 16.5°, monochromatic K<sub>α</sub> X-ray line,) to those of the α form (peak centered at 2θ = 16.6 to 16.8°) suggests the presence of both

phases [46] in the original samples. The changes in PLLA crystallinity allowed by the combined effects of radiolysis and temperature seem to lead first to the formation of additional α' phase up to a dose of 75–100
kGy [50]. At higher doses, the continuous increase of chain scissions and new defects reduces the aptitude of the modified PLLA chains to recrystallize in one of the well-defined allomorphs.

# 4. Discussion on structural changes due to irradiation

## 4.1. Mechanical properties

The general evolution of tensile properties as a function of dose 3D presented in Section 3.1 corresponds to the general features reported for PLA materials and other semi-crystalline thermoplastics. In the low dose range, radiolytic damages are essentially confined to the less-ordered, interlamellar amorphous regions. This preserves the overall integrity of the crystalline domains. Antagonistic effects may however take place, with recovery of some properties due to enhanced recrystallization while some tie chains having been cut by radiolysis, their contribution to plastic deformation is lost. At higher doses, in domain R2 of Fig. 3, further random scission of PLA chains leads to the steep decrease of tensile strength. The reduced sensitivity to doses above 100 kGy can result from the gradual depletion of highly reactive and mechanically impacting segments that have caused the large amount of damage at lower doses, while the remaining crystalline fraction is known to be less responsive to irradiation. One cannot exclude the occurrence of some cross-linking, due to in increasing number of free radicals trapped in the material, with a higher mobility due to the decrease of molecular weight and higher free volume. Thermal effects associated with the application of high doses and possible oxidative reactions that can counteract scission reactions may also contribute to the lower dependance to dose effects in the conditions of domain 3. This trend has been reported in the literature and merits a detailed comparison. A typical criterion used as an indicator to determine the practical lifetime of materials under various ageing conditions is the loss of 20 to 50 % of the original tensile strength [51,52]. In the present case, it seemed appropriate to consider a stronger loss in the steep slope expressing the decay of $\sigma_{Max}$ as a function of the dose. The series of data collected in Table 3 [39,53–58] allow to compare the dose D$_{50}$ % at which a 50 % reduction of $\sigma_{Max}$ was observed for somehow distinct PLA-based materials that were processed and irradiated under various conditions.

Quite interestingly, both the values reported for $\sigma_{maxD=0}$ unirradiated samples and critical value or dose range D$_{50}$ % are quite uniform for materials of varied origins with different degrees of stereoregularity, possibly blends or known to be blended with another polymer, the selected results show a rather convergent pair of values, with the strength at break in the range of 50 ± 8 MPa, while the critical dose was close 75 kGy in most of the cited reports, whatever the precise constitution of the PLA, the mode of fabrication, and the parameters of the irradiation process. Moreover, a common feature is the rather limited loss of properties for doses up to 25 kGy. This is particularly of interest since this dose corresponds to the dose applied to sterilize various items for use in the healthcare sector [6].

Another point is the specific contribution of PBAT as a component of the polymer blend used for this study. A detailed comparison of the plots already presented and discussed (Fig. 2) to those reported by Zhao et al. in ref [58] shows many common features both in terms of initial value of $\sigma_{Max}$ and the limited plastic deformation domain for the unirradiated samples, while after irradiation the samples from both studies exhibit a brittle fracture and similar variations of the dose-dependence of $\sigma_{max}$, as supported by the relative variations collected in Table 4. The presence of PBAT in the blend of this study, besides its primary role as a toughener to alleviate the intrinsic brittleness of PLA-based materials [59,60] is a polyester containing aromatic units that may induce chemically protective effect against the effects of radiation. Those two effects possibly explain the slightly higher values measured with the selected blend through various possible mechanisms since the fabrication method by compression molding is expected to provide specimens of better uniformity.

<caption>**Table 3**</caption>

Comparison of representative radiation treatments inducing a 50 % reduction of the strength at break $\sigma_{Max}$ of PLA samples as reported in literature.

<table><caption>**Table 4**</caption><thead><tr><th>Reference</th><th>Nature and type of PLA</th><th>Plastic processing method</th><th>Radiation type, conditions</th><th>Dose range</th><th>Tensile strength (MPa)</th><th>Dose inducing 50 % reduction of $\sigma_{max}$</th></tr></thead><tbody><tr><td>This work</td><td>Commercial filament of PLLA-PBAT<br>88:12 w/w<br>$T_{melting}$ 165–185 °C</td><td>3D printing (FFF)</td><td>10 MeV EB (vacuum sealed bags)</td><td>25–300 kGy</td><td>$\sigma_{maxD=0}$ = 58 MPa<br>D$_{50}$ % = 75 kGy</td><td></td></tr><tr><td>[53]</td><td>Commercial filament<br>$d$ = 1.24 g cm<sup>-3</sup><br>$T_{melting}$ 145–160 °C</td><td>3D printing (FFF)</td><td>$^{60}\text{Co}$ γ rays (no inerting)</td><td>25 kGy – 5.3 MGy</td><td>$\sigma_{maxD=0}$ = 48 MPa<br>D$_{50}$ % = 75–100 kGy</td><td></td></tr><tr><td>[54]</td><td>precise structure unknown<br>Commercial filament<br>$d$ = 1.24 g cm<sup>-3</sup><br>precise structure unknown</td><td>3D printing (FFF)</td><td>$^{60}\text{Co}$ γ rays (air)</td><td>10–175 kGy</td><td>$\sigma_{maxD=0}$ = 55 MPa<br>D$_{50}$ % = 70 kGy</td><td></td></tr><tr><td>[55]</td><td>Filaments extruded from commercial PLA pellets<br>$d$ = 1.26 g cm<sup>-3</sup><br>$T_{melting}$ 165–170 °C</td><td>3D printing (FFF)</td><td>10 MeV EB (zip lock bags)</td><td>25–200 kGy</td><td>$\sigma_{maxD=0}$ = 40 MPa<br>D$_{50}$ % = 75–100 kGy</td><td></td></tr><tr><td>[56]</td><td>Commercial filament reference and precise structure unknown</td><td>3D printing (FFF)</td><td>$^{60}\text{Co}$ γ rays<br>25–150 kGy</td><td></td><td>$\sigma_{maxD=0}$ = 44 MPa<br>D$_{50}$ % = 80–90 kGy</td><td></td></tr><tr><td>[39]</td><td>Pre-irradiated PLLA pellets for peroxidation<br>$d$ = 1.24 g cm<sup>-3</sup><br>$T_{melting}$, $D=0$ = 165–180 °C</td><td>Injection molding</td><td>$^{60}\text{Co}$ γ rays (no inerting)</td><td>80–175 kGy</td><td>$\sigma_{maxD=0}$ = 55 MPa<br>D$_{50}$ % = 75–80 kGy</td><td></td></tr><tr><td>[57]</td><td>unirradiated Commercial PLA pellets<br>$d$ = 1.31 g cm<sup>-3</sup><br>precise structure unknown<br>$T_{melting}$ 155–170 °C</td><td>Compression molding</td><td>175 keV EB*</td><td>20–120 kGy</td><td>$\sigma_{maxD=0}$ = 15 MPa<br>D$_{50}$ % = 60–80 kGy</td><td></td></tr><tr><td>[58]</td><td>Commercial pellets<br>$d$ = 1.24 g cm<sup>-3</sup><br>precise structure unknown<br>$T_{melting}$ 155–170 °C</td><td>Compression molding</td><td>10 MeV EB (zip lock bags)</td><td>25–100 kGy</td><td>$\sigma_{maxD=0}$ = 55 MPa<br>D$_{50}$ % = 75 kGy</td><td></td></tr></tbody></table>

Comparison of dose-dependence of normalized $\sigma_{Max}$ values obtained for PLLA-PBAT of this study with those reported in ref [58] for pure PLA material.

<table><thead><tr><th>Dose [kGy]</th><th>Normalized $\sigma_{Max}$ for molded PLA specimens from ref [58]</th><th>Normalized $\sigma_{Max}$ for 3D-printed PLLA specimens (this work)</th></tr></thead><tbody><tr><td>0</td><td>1.000</td><td>1.000</td></tr><tr><td>25</td><td>0.949</td><td>0.996</td></tr><tr><td>50</td><td>0.780</td><td>0.763</td></tr><tr><td>75</td><td>0.549</td><td>0.503</td></tr><tr><td>100</td><td>0.270</td><td>0.325</td></tr></tbody></table>
## 4.2. Crystal features and structural properties

DSC measurements have shown that the molecular effects of radi- olysis affect to a somehow limited extent the actual crystallinity in samples as obtained after irradiation, and their crystallization capacity after annealing. Conversely, the mechanical consequences start to decrease dramatically for doses exceeding 25 kGy, as reported for this study and in other recent reports [61,62]. To further quantify the structural changes induced by EB radiation, the area of the 2 main XRD diffraction lines above the halo (Fig. 8) was integrated, A<sub>peak1</sub> and A<sub>peak2</sub> respectively, to obtain an rough estimate the degree of crystallinity based on XRD, χ<sub>XRD</sub>, using Eq. (2), where A<sub>total</sub> is the total area of the diffractogram.

$$X_{XRD} = \frac{A_{peak1} + A_{peak2}}{A_{total}} \qquad (2)$$

The corresponding values are plotted in Fig. 9, together with the crystallinity values χ<sub>DSC</sub> determined by DSC for the irradiated samples before and after annealing. For non-annealed samples, the same general trend is observed for the evolution of the degrees of crystallinity of determined by the two methods. The extent of crystallinity determined by integrating numerically modified diffractograms after normalization and baseline corrections is understandably not as accurate as that based on DSC measurements. This may explain the less regular variations observed with the dose. In addition, the samples analyzed by XRD at room temperature may bear in their crystalline structure the sign of somewhat different thermal histories between RT and T<sub>g</sub>, whereas in DSC analysis, the first transition to be quantified for the determination of χ<sub>DSC</sub> starts in the calorimetric cell at 75–80 °C, after a slow heating process. This may erase some differences inherited from previous pro- cessing or storage conditions in a material showing the onset of the glass transition at 65 °C. In spite of these limitations, the general trend as well as the range of variations obtained from both methods provide a consistent picture of the changes in crystallinity due to the damages and thermal effects taking place during irradiation.

## 5. Healing of mechanical damages due to irradiation by soft annealing

The discussion of experimental results in the previous section high- lights two important features. The maximal crystallization capacity of annealed PLA-based samples is much higher than the value measured in the original state and not much affected by the dose, at least up to a value of 100 kGy (Fig. 9). Conversely, the mechanical consequences start to decrease dramatically for doses exceeding 25 kGy, as reported for this study and in other recent reports (see Table 3 and references therein). Chemi-crystallization consecutive to chain scissions is expected to be restricted by the reduced mobility of the shortened chain segments within the glassy domains at temperatures between RT and PLLA's T<sub>g</sub>. The same holds for broken tie-chains in the interphase between amor- phous zones and crystal lamellae. Investigations were continued to study the influence of annealing at moderate temperature on a series of irra- diated 3D-printed dog-bone specimens irradiated with a dose of 75 kGy, this value corresponding to a significant decrease of the ultimate stress σ<sub>Max</sub>. Preliminary tests were conducted to determine a sufficient dura- tion for an effective annealing on the considered samples. The treat- ments were performed for 120 min in aerated laboratory ovens at temperatures ranging from 50 to 90 °C with a 10 °C interval.

The evolution of the tensile profiles as a function of the thermal treatment together with the reference plots of the unirradiated and non- annealed irradiated samples can be compared in Fig. 10. Annealing treatments for 2 h at 50 °C, or between 70 and 90 °C have a detrimental effect on tensile properties, with a gradual decrease of both the ultimate strength and displacement of the tested specimens. A quite interesting behavior was observed with samples annealed for 2 h at 60 °C that a contrasting evolution. At this temperature, where the sample is placed at the center of the glass transition domain, there is a beneficial effect on tensile properties with a partial but noticeable recovery for the ultimate strength and displacement.

The changes can be quantitatively compared from the data collected in Table 5 that show that the radiation damaged sample showing 50 % of the initial strength is improved by some healing phenomenon with a recovery to 63 %. This treatment has even a larger effect on the ultimate

<figure><img src="image_12.png" /><figcaption>Fig. 9. Comparison of the degrees of crystallinity determined by DSC and by wide angle X-ray scattering of 3D-printed PLLA-based samples unirradiated and EB-irradiated at various doses (25–300 kGy).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Force – displacement plots for the different specimens unirradiated and irradiated at a 75 kGy dose showing the evolution of tensile properties as a function of the temperature of the 2 h-long annealing treatment.</figcaption></figure>

<table><caption>**Table 5**
Effects of the thermal treatment at 60 °C on the normalized tensile strength $\sigma_{max}$ of samples treated with a 75 kGy dose.</caption><thead><tr><th>Physical quantity</th><th>Before irradiation</th><th>Irradiated at 75 kGy before annealing</th><th>Irradiated at 75 kGy after annealing at 60 °C for 2 h</th></tr></thead><tbody><tr><td>Normalized σ<sub>Max</sub></td><td>1</td><td>0.50 ± 0.03</td><td>0.63 ± 0.06</td></tr><tr><td>Normalized Disp<sub>max</sub></td><td>1</td><td>0.40 ± 0.02</td><td>0.61 ± 0.08</td></tr><tr><td>Normalized E</td><td>1</td><td>0.95 ± 0.02</td><td>0.94 ± 0.03</td></tr><tr><td>Degree of crystallinity (weight fraction)</td><td>0.19</td><td>0.21</td><td>0.24</td></tr></tbody></table>

displacement which is increased from 40 to 61 % of the value measured for the unirradiated specimens. To support the interpretation based on a chemi-crystallization phenomenon, the samples were annealed in conditions just allowing a soft reorganization in the vicinity of the damages by the radiation treatment. This treatment modifies the DSC thermograms shown in Fig. 11. There is a significant increase of the degree of crystallinity, as shown in the bottom line of Table 5.

There is little information in the literature about the cumulative effects of annealing and radiolysis on PLA-based materials. Wach et al. observed an increase in the flexural strength of PLA by 17 % after a treatment at 85 °C for about 1 h. However, this very same treatment was ineffective with samples previously irradiated at 25 kGy, a decrease in flexural strength of 22 % being finally measured [23]. Annealing treatments being performed in the high temperature range or above of the glass transition, at the onset of the cold crystallization, one can anticipate that, as for the corresponding high temperature experiments of this study, a large recrystallization process takes place and leads to more brittle specimens.

One cannot exclude a possible contribution of recombination reactions between dangling macroradicals that would possibly repair some of the damages due to scission reactions. However, the significant, yet deliberately limited, enhancement of crystallinity suggests that the healing process is mainly due to chemi-crystallization. This

phenomenon induced by exposure to various types of degradative processes such as thermal oxidation, hydrolysis, photodegradation and radiolysis [63,64] continues to be investigated by few groups due to its importance of the properties of polymer materials with long service periods [65,66].

At macromolecular and mesoscopic levels, the structural and morphological changes taking place at the different stages of the sequential treatments can be represented by the sketch are shown in Fig. 12. The initial morphology is described by lamellae, tie chains and a large volume fraction of amorphous phase. The chemical damages induced by the random energy deposition are known to be more important in amorphous domains. This results in the reorganization of chains in the vicinity of crystalline domains and to the nucleation of additional crystalline domains. This rearrangement of PLLA chains is amplified by a soft annealing treatment that results in repairing some damages in critical sites that would impact mechanical properties. After annealing at high temperature, the strong enhancement of crystallinity renders the material event more brittle with much smaller critical elongations.

The keypoint for the healing process would therefore consist in inducing the chemi-crystallization in the areas peripheral to the crystalline lamellae to restore some links between crystalline and amorphous domains that were cut by scission reactions, while bulk recrystallization of glassy domains would remain limited by the thermal treatment at moderate temperature (60 °C).

## 6. Conclusions

The results presented in this report consolidate and expand the current knowledge about the response of PLA-based materials to high energy radiation. Initially motivated by the need for a detailed assessment of the effects of EB irradiation on 3D-printed PLA-based objects, the results obtained from tensile measurements, calorimetric analysis and X-ray diffraction allow to draw the following conclusions.
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Thermograms of enthalpic transitions recorded with 3D-printed PLLA-based samples EB-irradiated at 75 kGy before and after an annealing treatment for 2 h at 60 °C.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Sketch representing a possible evolution of the polymer chains in the different domains of PLA-based materials at different stages of their radiolytic treatment and thermal post-treatment: before irradiation (a), random energy deposition during irradiation (b), chemical damages more important in amorphous domains (c), morphological changes due to irradiation (d), limited recrystallization in mechanically impacting zones under soft conditions (e) and recrystallization after extensive annealing at high temperature (f).</figcaption></figure>

- 3D-printing by fuse-deposition has low impact on PLLA crystallinity in test samples, a consequence of the relatively fast cooling of the produced;

- the effects of irradiation for doses up to 300 kGy on tensile of PLA were quantified confirming the trends shown in several other reports, in spite of significant differences in terms of chemical

constitution, transformation processing and irradiation parameters. Doses around 75 kGy were found to reduce by 50 % the tensile strength of the PLA-based samples initially measured at about 58 MPa;

- the crystallization capacity of irradiated PLLA-based material is high and comparable to original sample the degree of crystallinity varying
in a non-monotonic way between 70, 80 and 60 % over the explored
dose range between 0 and 300 kGy.

Moreover, two important new findings have been identified:

- Irradiation kinetically reduces significantly the rate of crystallization in samples irradiated at doses higher than 150 kGy, very likely due to some cross-linking reactions which appear to strongly slower the crystallization process, while the reduction of the overall crystallization capacity is limited;

- Soft annealing allows for the partial recovery of tensile properties in irradiated PLLA-based samples, specimens irradiated at 75 kGy with a 50 % loss of their tensile strength recover to 63 % of the initial value after treatment at 60 °C for 2 h. This is explained by the positive effect of a moderate heating treatment at the onset on the glass transition. Radiation-induced chemi-crystallization that occurs in the glassy amorphous phase surrounding the lamellae is limited by the lack of chain mobility during the irradiation at RT, but starts to be effective after soft annealing. This finding can be advantageously used in future strategies for enhancing irradiated PLA's performance in critical applications.

On-going work is devoted to the analysis of molecular changes taking
place within PLLA samples and on the effects of the presence of PBAT as
a toughener and radiation protective component through various
mechanisms worth to be explored.

Another pending issue not receiving much attention in the literature is the influence of radiation and annealing on impact properties, which may be a critical point in 3D-printed objects, monolithic with complex shapes and scaffolds containing a large number of small volume domains concentrating both internal and external stresses.

CRediT authorship contribution statement

**A. Arias-Blanco:** Writing – original draft, Visualization, Investiga-
tion, Formal analysis, Data curation. **G. Tataru:** Writing – original draft,
Visualization, Investigation, Formal analysis, Data curation. **M. Marco:**
Writing – original draft, Funding acquisition, Formal analysis. **M.H. Miguélez:** Writing – original draft, Funding acquisition, Formal anal-
ysis. **X. Coqueret:** Writing – review & editing, Funding acquisition, Data
curation, Conceptualization.

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.

Acknowledgements

The work presented in this article was partially performed in the
frame of the IAEA's Coordinated Research Project "Strengthening the
Use of Biomass for Synthesis of Bioplastics and Other Compounds, Using
Radiation Technology" and of the MIPPI-4D research program granted
by Conseil Régional Grand Est and EU-program FEDER. Financial sup-
port by CNRS, Conseil Régional Grand Est, Ministry of Higher Education
and Research (MESR) to the PlAneT CPER project is gratefully
acknowledged. The authors express their thanks to the French Embassy
in Spain for having supported the research stay of AAB at URCA through
the program "Becas Doctorales y Postdoctorales de 2023" and to Spanish
Ministry of Science and Innovation (grant PID2020-112628RA-I00/AEI
/10.13039/501100011033. AAB is indebted to the iia association for
offering him the opportunity to attend the Leadership Workshop 2023.

**Supplementary materials**

Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.polymdegradstab.2025.111476.

Data availability

Data will be made available on request.

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