# Effect of annealing on thermal and dynamic mechanical properties of poly(lactic acid)

Assia Zennaki, Latifa Zair, Khadidja Arabeche, Lina Benkraled, Ulrich Maschke, Abdelkader Berrayah

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Assia Zennaki, Latifa Zair, Khadidja Arabeche, Lina Benkraled, Ulrich Maschke, et al. Effect of annealing on thermal and dynamic mechanical properties of poly(lactic acid). Journal of Applied Polymer Science, 2022, Journal of Applied Polymer Science, 139 (44), pp.e53095. 10.1002/app.53095 hal-03871477

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# Effect of annealing on thermal and dynamic mechanical properties of poly(lactic acid)

Assia Zennaki<fnref n="1" />

Latifa Zair<fnref n="1" />

Khadidja Arabeche<fnref n="1" />

Lina Benkraled<fnref n="1" />

Ulrich Maschke<fnref n="2" /> ID

Abdelkader Berrayah<fnref n="1" />

## Correspondence

Ulrich Maschke, UMET – Unité Matériaux et Transformations, UMR 8207, University of Lille, CNRS, INRAE, Centrale Lille, Lille F-59000, France.
Email: ulrich.maschke@univ-lille.fr

## Funding information

University of Lille/France; Centre National de la Recherche Scientifique (CNRS); University of Tlemcen/Algeria; General Directorate of Scientific Research and Technological Development (DGRSDT); Algerian Ministry of Higher Education and Scientific Research (MESRS)

## Abstract

Effects of annealing temperature ($T_a$: 80–140°C) and time ($t_a$: 3-30 h) on the crystalline phase transition in poly(lactic acid) (PLA) were studied by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In the DSC curves, the sample annealed at $T_a$ = 80°C with time interval ($t_a$: 10–30 h) demonstrates a peculiarly small exothermal peak ($T_{exo}$) around 130°C, just prior to the melting point, corresponding to the disorder-to-order ($\alpha'$-to-$\alpha$) phase transition, while the sample annealed at temperature ($T_a$: 90–110°C) shows a double melting behavior considered as the $\alpha'$-$\alpha$ phase transition. Towards low temperatures, the glass modulus $E_g$ reported by DMA thermograms, shows an important increase ($\sim$30,000 MPa) at $T_a$ = 80°C for $t_a$ = 3 h, due to the extremely high self-nucleation density in low-crystallized PLA materials. After a sharp drop to 3600 MPa at $T_a$ = 110°C, a marked improvement of $E_g$ (15,900 MPa) is observed around $T_a$ = 120°C for all samples, regardless of time $t_a$. This interesting effect (improvement of $E_g$ in the range $T_a$ = 100–120°C) can be correlated with the grow of crystallinity in the same domain of $T_a$, and the $\alpha'$-$\alpha$ phase transition $T_a$ ($T_a$: 90–110°C) determined by the double $T_m$ melting DSC peak, which is confirmed by the increase of $T_g$ for $T_a$ = 90–110°C.

## KEYWORDS

biodegradable, crystallization, glass transition, mechanical properties, thermal properties

## 1 | INTRODUCTION

Poly(lactic acid) (PLA) is a biodegradable plastic, derived from renewable resources; it is a material of choice<fnref n="1" />⁻⁷ in applications of various domains such as biomedicine, food packaging, textile, electronics, and so forth. PLA is characterized by slow crystallization kinetics often

observed in conventional processing methods (extrusion, injection, etc.). This phenomenon is highly problematic, with significant consequences on thermal and mechanical properties of the final product. In many applications, increasing the crystallinity of PLA is desired because, in its amorphous form, the field of application of PLA is severely limited by its low glass transition temperature

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

© 2022 The Authors. *Journal of Applied Polymer Science* published by Wiley Periodicals LLC.

<footnote n="1">¹Laboratoire de Recherche sur les Macromolécules, Faculté des Sciences, Université Aboubakr Belkaïd, Tlemcen, Algeria</footnote>

<footnote n="2">²UMET – Unité Matériaux et Transformations, UMR 8207, University of Lille, CNRS, INRAE, Centrale Lille, Lille, France</footnote>
(T<sub>g</sub>). At temperatures higher than T<sub>g</sub>, only the crystalline phase of PLA can impart valuable mechanical properties. Thus, the crystalline form is necessary to increase the strength of the material.<sup>8–11</sup> Annealing is widely used for the recrystallization of PLA-based materials.<sup>12–15</sup> Improvement of thermo-mechanical properties is sometimes obtained by mixing PLAs of different molar masses or with other biodegradable polymers.<sup>16,17</sup>

Typically, PLA homopolymer has three crystal forms,<sup>8,9,18–20</sup> such as α-, β-, and γ-form, which depend on the crystallization conditions. Among all these crystal forms, the α-form is the most common and stable polymorph. It has been shown that PLA is polymorphic,<sup>21–26</sup> exhibiting two different crystalline phases termed α and α'. Crystallization of the relaxed melt at temperatures above 120°C leads to the formation of α-crystals with a 10<sub>3</sub> helical chain conformation where two chains are interacting in an orthorhombic unit cell.<sup>18,27,28</sup> At temperatures lower than 100°C only α'-crystals develop. In the α'-form, the molecule segments adopt the same helical structure as in α-crystals, however they exhibit conformational disorders.<sup>28</sup> Within the 100–120°C crystallization temperature range, a mixture of crystalline phases α' and α was present. The α'-crystals are metastable at their formation temperature but transform into the stable α-form upon heating at their stability limit of around 150°C.<sup>21–23,28–32</sup> Zhang and coauthors<sup>29,30</sup> confirmed that the small exothermic peak T<sub>exo</sub> in the DSC curve, detected just prior to the melting peak, is associated with the disorder-to-order (α' to α) phase transition. This exothermic signal, already reported by Ohtani et al.,<sup>33</sup> Chao et al.<sup>34</sup> and Zhao et al.,<sup>35</sup> has been confirmed by Zhang and coauthors<sup>29,30</sup> by DSC, WAXD, and IR techniques. In the DSC thermograms, two endothermic peaks appear, related to the melting of the α' and α phases. The peak at the lower temperature is related to the melting of the α'-form and its recrystallization into the α form, while the second peak corresponds to the melting of the α-form. The two processes, the melting of the α' form and the recrystallization into the α form, can be considered as the α'-α phase transition.

This study aims to understand the fundamental effect of annealing time t<sub>a</sub> and temperature T<sub>a</sub> on the thermal and mechanical behavior of PLA. The α'-α phase diagram as function of t<sub>a</sub> and T<sub>a</sub> was determined by melting peaks from DSC data. Crystallinity of annealed PLA as a function of T<sub>a</sub> for different t<sub>a</sub> will be determined and correlated with the α'-α phase diagram. Through the DMA data, the glass modulus E<sub>g</sub> and loss tangent tan δ of annealed PLA, as a function of T<sub>a</sub> for different t<sub>a</sub>, was determined and analyzed for particular conditions of T<sub>a</sub> and t<sub>a</sub>. Correlation between α'-α phase transition and crystallinity, mechanical properties (E<sub>g</sub>) and molar mass (M<sub>v</sub>) will be discussed.

## 2 | EXPERIMENTAL

### 2.1 | Materials

The poly(lactic acid), supplied by Nature Works (USA), was a semi-crystalline 4043D grade material comprising around 4% mol of D-isomer units. The PLA pellets were transparent and amorphous.

### 2.2 | Sample preparation

Poly(lactic acid) (PLA) granules were dried in a vacuum oven at 50°C for 24 h, then melted at 200°C and compressed into a sheet with a thickness of about 1 mm. The melted-compressed PLA sheet was immediately quenched into ice water to obtain an amorphous PLA sheet. Amorphous PLA is then annealed in a vacuum oven, at temperatures T<sub>a</sub>: between 80 and 140°C, and time t<sub>a</sub> between 3 and 30 h. In the present study, the PLA samples of un-annealed (amorphous) and annealed at different T<sub>a</sub> and t<sub>a</sub> are referred to as “un-aPLA” and “aPLA” (T<sub>a</sub>; t<sub>a</sub>); for example, aPLA (T<sub>a</sub> = 80°C; t<sub>a</sub> = 30 h) means annealed sheet at 80°C and 30 h.

### 2.3 | Molecular weight measurement

The molecular weight of PLA was determined from its intrinsic viscosity (η) measurement applying the Mark-Houwink equation, using an Ubbelohde-type capillary viscometer at 25°C. Chloroform was used as solvent. The viscosity average molecular weight M<sub>v</sub> was calculated from the intrinsic viscosity by using the following equation<sup>36</sup>:

$$[\eta] = 5.45 \times 10^{-4} M_v^{0.73} \quad (1)$$

### 2.4 | Thermal analysis

Differential scanning calorimetry (DSC) measurements were performed on a TA Q2000 DSC apparatus equipped with a Refrigerated Cooling System (RCS90) allowing cooling down to –90°C. Temperature and heat flow scales were calibrated with high purity indium standards. Hermetic aluminum pans (Tzero, TA Instruments) were used, which are able to stand an internal pressure of 300 kPa. Samples of about 10 mg were analyzed under nitrogen gas flow at a heating rate of 10°C/min from 25 to 200°C, kept at the latter temperature for 2 min; cooled back to 25°C, followed by an isothermal scan for 2 min. The crystallinity rate (X<sub>c</sub>) induced by the thermal treatment could be deduced from the heat flow signal obtained by DSC, using the following equation<sup>37</sup>:
$$X_c = \frac{\Delta H_m - \Delta H_c}{\Delta H_c^0} \qquad (2)$$

where $\Delta H_m$ and $\Delta H_c$ are the enthalpies of melting and crystallization of the PLA samples, respectively, and $\Delta H_c^0$ represents the enthalpy of fusion of the PLA crystal of infinite size, which was reported by Fischer et al.³⁸ to be 93.1 (J/g).

## 2.5 | Dynamic mechanical analysis

For the study of viscoelastic phenomena, dynamic stresses are used in the frequency domain rather than in the time domain. The method consists of applying a sinusoidal deformation $\varepsilon(t) = \varepsilon_0 \sin(\omega t)$ of low amplitude $\varepsilon_0$ to the material and measuring the resulting sinusoidal stress $\sigma(t)$ of amplitude $\sigma_0$. For a viscoelastic material, $\sigma(t)$ is out of phase by a loss angle $\delta$ ($0 \ge \delta \ge \pi/2$) with respect to the deformation:

$$ \begin{aligned} \sigma(t) &= \sigma_0 \sin(\omega t + \delta) \\ &= \left[ \frac{\sigma_0 \cos \delta}{\varepsilon_0} \right] \varepsilon_0 \sin(\omega t) + \left[ \frac{\sigma_0 \sin \delta}{\varepsilon_0} \right] \varepsilon_0 \cos(\omega t) \\ &= E' \varepsilon_0 \sin(\omega t) + E'' \varepsilon_0 \cos(\omega t) \\ &= E' \varepsilon_0 \sin(\omega t) + E'' \varepsilon_0 \sin\left(\omega t + \frac{\pi}{2}\right) \end{aligned} \qquad (3) $$

$E'(\omega)$, the modulus of conservation in phase with the deformation, characterizes the energy stored in the sample in elastic form. $E''(\omega)$, the phase shifted loss modulus of $\pi/2$ with the deformation, characterizes the energy dissipated in the sample in viscous form. Tan $\delta = E''/E'$ is defined as the loss tan.

In general, storage moduli $E'$ versus temperature, at a given frequency $\omega$, present a plateau exhibiting high values (around $10^9$ Pa) towards low temperatures, defined by $E_g$, the storage modulus in the glassy region. With increasing temperature, $E'$ drops sharply towards about $10^5$ Pa; this sudden drop, defined by the glass transition zone, is characterized by the $T_g$ which is defined by the peak of $\tan \delta = E''/E'$ versus temperature.

Dynamic mechanical analysis was performed using a DMA Q800 (TA, Inc.). Experiments were conducted in tensile mode at a frequency of 1 Hz. Curves displaying storage ($E'$) and loss ($E''$) moduli were recorded as a function of temperature between 25 and 180°C applying a heating rate of 3°C/min. The amplitude and the preload force were 5 µm and 0.010 N, respectively. The shape of the PLA films was rectangular, with dimensions $10 \times 6 \times 1$ mm³. The temperatures of various relaxation processes were determined from the $\tan \delta$ temperature curve at 1 Hz. The glass transition temperature ($T_g$) can

<figure><img src="image_2.png" /><figcaption>FIGURE 1 DSC thermograms of as-received poly(lactic acid) (PLA)-pellet and the un-aPLA sheet</figcaption></figure>

be defined as the maximum of the transition in the loss tangent curve.

# 3 | RESULTS AND DISCUSSION

## 3.1 | DSC analysis

Figure 1 illustrates a comparison of DSC thermograms of the as-received PLA-pellet dried in a vacuum oven at 50°C and the un-annealed sample (un-aPLA), taken as reference sample for all thermograms. un-aPLA exhibits amorphous behavior with a very low crystallinity rate ($X_c = 1.6\%$) and a glass transition at 60°C. An exothermic peak $T_{cc}$ associated with a cold crystallization process appears at 127.2°C, following by an endotherm event with a central peak at $T_m = 153.4°C$. The results from thermal analysis of un-aPLA sample and PLA-pellets are given in Table 1.

DSC curves of aPLA ($T_a$: 80–140°C; $t_a$: 3–30 h) samples were analyzed. Annealing PLA samples at 80°C and times $t_a$: 3–30 h were illustrated in Figure 2. The small exothermal peak $T_{exo}$, just prior to the melting point, associated with the disordered-order $\alpha$-$\alpha$ phase transition, clearly appears for $t_a = 10-30$ h. This exothermal event, previously reported by Zhang et al.,³¹ was found to be in agreement with reports from many authors.²⁰⁻²⁸ Thermal properties of aPLA ($T_a = 80°C$; $t_a = 3$ h) sample were given in Table 2.

DSC curves of PLA annealed at 90 and 100°C were shown in Figure 3. The small exothermal peak disappears, and an additional melting peak ($T_{m2}$) was observed in the form of a small shoulder (Figure 3a) that stretches
<table><thead><tr><th>Sample</th><th>T<sub>g</sub> (°C)</th><th>T<sub>cc</sub> (°C)</th><th>ΔH<sub>c</sub> (Jg<sup>-1</sup>)</th><th>T<sub>m</sub> (°C)</th><th>ΔH<sub>m</sub> (Jg<sup>-1</sup>)</th><th>X<sub>c</sub> (%)</th></tr></thead><tbody><tr><td>PLA-pellets</td><td>59.5</td><td>125.8</td><td>1.65</td><td>152.5</td><td>3.60</td><td>2.1</td></tr><tr><td>un-aPLA</td><td>60.0</td><td>127.2</td><td>3.41</td><td>153.4</td><td>4.86</td><td>1.6</td></tr></tbody></table>

Abbreviation: PLA, poly(lactic acid).

<figure><img src="image_3.png" /><figcaption>FIGURE 2 DSC curves of aPLA sheets at T<sub>a</sub> = 80°C for times t<sub>a</sub> = 3–30 h</figcaption></figure>

towards long annealing times in the case of aPLA (T<sub>a</sub> = 90°C; t<sub>a</sub>: 3–30 h). The two peaks, low-intensity T<sub>m2</sub> (α' phase) and high-intensity T<sub>m1</sub> (α phase), present a α'-α phase transition, with a α' phase in minority. In the case of annealing at 100°C (Figure 3b), the two peaks (T<sub>m2</sub> and T<sub>m1</sub>) appear for all annealing times (t<sub>a</sub>: 3–30 h), thus showing a α'-α transition of phases with the same order of magnitude. Results of thermal properties of aPLA (T<sub>a</sub> = 90,100°C; t<sub>a</sub>: 3–30 h) samples are given in Table S1.

Samples annealed at T<sub>a</sub> = 110°C for t<sub>a</sub> = 3 and 6 h show two peaks, T<sub>m2</sub> of high intensity and T<sub>m1</sub> as weak shoulder (Figure 4), indicating the presence of a α'-α phase transition. Annealing for t<sub>a</sub> = 10 h yields to a single high-intensity T<sub>m2</sub> peak indicating that the α' phase is completely transformed into the α phase. Results from thermal analysis of aPLA (T<sub>a</sub> = 110°C; t<sub>a</sub>: 3–30 h) are given in Table 3.

Figure 5 shows a single peak T<sub>m2</sub> of high intensity when annealing at T<sub>a</sub> = 120°C, thus indicating that the α' phase is wholly transformed into the α phase. Beyond T<sub>a</sub> = 120°C, the α phase is perfectly established, as shown in Figure S1a, b, corresponding to PLA samples annealed at 130 and 140°C. Results of the thermal properties of samples annealed at T<sub>a</sub> = 120, 130, and 140°C are given in Table S2.

## 3.2 | α'-α phase diagram

The α'-α phase diagram, built from temperatures of T<sub>exo</sub>, T<sub>m1</sub> and T<sub>m2</sub> peaks, is of great interest to better understand the evolution of the crystal structure as function of t<sub>a</sub> and T<sub>a</sub>. Figure 6a gives a follow-up of α' and α as function of T<sub>a</sub> for different t<sub>a</sub>. The lower and upper limits of the α'-α phase transition require special attention. The lower limit of the α'-α transition coincides with the appearance of T<sub>m1</sub> and T<sub>m2</sub>; but often it is below 5 to 10°C since the α'-form transforms into α at T<sub>exo</sub>. The upper limit is determined by linear extrapolation of T<sub>m2</sub>, which is considered to correlate with the melting phenomenon of the α-form. Giving the equilibrium melting temperature (T<sub>m</sub><sup>0</sup>) at T<sub>m</sub> = T<sub>a</sub>, the Hoffman-Weeks equation can be written as<sup>39</sup>:

$$T_m = T_m^0 \left[ 1 - \frac{1}{r} \right] + \frac{1}{r} T_a \quad (4)$$

where 1/r is the stability parameter depending on the crystal thickness, and r represents the ratio of the lamellar thickness at T<sub>m</sub> to the lamellar thickness of the critical nucleus at T<sub>a</sub>. Figure 6b shows the Hoffman-Weeks plots of aPLA, at 3 and 24 h for different T<sub>a</sub>, showing the upper limits towards T<sub>a</sub> = 110°C. The α'-α phase transition is therefore situated between 90 and 110°C with T<sub>m</sub><sup>0</sup> = 168.3 and 172.3°C for aPLA at t<sub>a</sub> = 3 and 24 h, respectively. Table 4 reports T<sub>m</sub><sup>0</sup> and 1/r for aPLA at different times (t<sub>a</sub>: 3–24 h).

Since α' crystals transform to α crystals upon heating, only annealing temperatures above T<sub>a</sub> ≥ 110°C were considered, where the α crystals are predominantly produced. It has been firmly established that crystallization at high supercooling of the melt, defined by ΔT = T<sub>m</sub><sup>0</sup> - T<sub>a</sub>, allows the development of increasingly smaller nuclei of supercritical size than crystallization at low supercooling. The aPLA (T<sub>a</sub> = 80°C; t<sub>a</sub> = 3 h) sample clearly shows high supercooling (ΔT ≈ 88°C) compared to the aPLA (T<sub>a</sub> = 120°C; t<sub>a</sub> = 24 h) sample which presents low supercooling (ΔT≈52°C). The aPLA (T<sub>a</sub> = 80°C; t<sub>a</sub> = 3 h) sample with high supercooling develops increasingly smaller nuclei of supercritical size.

The domain of the α'-α phase transition is often reported in the literature between 100 and 120°C,<sup>25,29–31</sup> which is highly desirable for molding and extrusion to
<table><caption>TABLE 2 Results from thermal analysis of annealed samples at $T_a = 80°C$ for $t_a = 3–30 h$</caption><thead><tr><th>Sample</th><th>t<sub>a</sub> (h)</th><th>T<sub>g</sub> (°C)</th><th>T<sub>cc</sub> (°C)</th><th>ΔH<sub>c</sub> (Jg<sup>-1</sup>)</th><th>T<sub>exo</sub> (°C)</th><th>T<sub>m1</sub> (°C)</th><th>T<sub>m2</sub> (°C)</th><th>ΔH<sub>m</sub> (Jg<sup>-1</sup>)</th><th>X<sub>c</sub> (%)</th></tr></thead><tbody><tr><td rowspan="5">aPLA (80°C-t<sub>a</sub>)</td><td>3</td><td>61.5</td><td>116.1</td><td>3.75</td><td>—</td><td>152.0</td><td>—</td><td>10.61</td><td>7.4</td></tr><tr><td>6</td><td>62.1</td><td>118.0</td><td>0.85</td><td>—</td><td>153.1</td><td>—</td><td>13.81</td><td>13.9</td></tr><tr><td>10</td><td>60.2</td><td>—</td><td>—</td><td>131.8</td><td>152.4</td><td>—</td><td>16.78</td><td>18.0</td></tr><tr><td>18</td><td>63.5</td><td>—</td><td>—</td><td>132.9</td><td>152.8</td><td>—</td><td>17.43</td><td>18.7</td></tr><tr><td>24</td><td>65.6</td><td>—</td><td>—</td><td>132.3</td><td>152.6</td><td>—</td><td>17.83</td><td>19.2</td></tr><tr><td></td><td>30</td><td>62.7</td><td>—</td><td>—</td><td>131.9</td><td>152.4</td><td>—</td><td>17.84</td><td>19.2</td></tr></tbody></table>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>FIGURE 3 DSC curves of aPLA samples for times $t_a$: 3–30 h, at (a) $T_a = 90°C$ and (b) $T_a = 100°C$</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>FIGURE 4 DSC curves of aPLA samples at $T_a = 110°C$ for $t_a$: 3–30 h</figcaption></figure>

get optimal crystallinity. However, this domain is sometimes shifted towards lower or higher temperatures, strongly depending on heat treatment, molecular weight, external induced effects, and so forth. Jalali et al.²⁵ observed the domain of the α'-α phase transition between 100 and 120°C for PLA samples, exhibiting the highest crystallization temperature, the highest nuclei

density, and the smallest spherulite size. Zhang et al.²⁹,³⁰ showed that for PLA of molecular weight $M_w = 150,000$ g/mol, the α'-α phase transition is located at $110°C < T_c < 130°C$ while for low molecular weights ($M_w = 50,000$ g/mol), it was found around $105–130°C$. Kawai et al.³¹ showed that from melt-crystallization at $T_c = 80°C$, the peak $T_{exo}$, characterizing the α' phase, appears clearly for different heating rates from 1 to $20°C/min$, and that the range of the α'-α phase transition can be estimated to be around $80–120°C$. Pan et al.²³ found that the α'-α transition depends entirely on $t_a$ (0–24 h) and $T_a$ (120–160°C), and $M_w$ affects the crystalline phase transition significantly, which can be explained by the mobility of polymer chains. Zhou et al.⁴⁰ studied deformational behavior and structural evolution of amorphous and aPLA samples stretched within $100–150°C$. The amorphous PLA stretched at $120°C$ exhibits excellent mechanical properties.

The aPLA ($T_a$; $t_a$) sample, considered in this study, presents a α'-α phase transition around $90–110°C$, as shown in Figure 6a that is slightly shifted towards lower temperatures, compared to the standard range $100–120°C$.
<table><caption>TABLE 3 Thermal properties of annealed samples at T_a = 110°C for t_a: 3-30 h</caption><thead><tr><th>Sample</th><th>t<sub>a</sub> (h)</th><th>T<sub>g</sub> (°C)</th><th>T<sub>cc</sub> (°C)</th><th>ΔH<sub>c</sub> (Jg<sup>-1</sup>)</th><th>T<sub>exo</sub> (°C)</th><th>T<sub>m1</sub> (°C)</th><th>T<sub>m2</sub> (°C)</th><th>ΔH<sub>m</sub> (Jg<sup>-1</sup>)</th><th>X<sub>c</sub> (%)</th></tr></thead><tbody><tr><td rowspan="6">aPLA (110°C-t)</td><td>3</td><td>62.1</td><td>—</td><td>—</td><td>—</td><td>152.8</td><td>150.2</td><td>17.96</td><td>19.3</td></tr><tr><td>6</td><td>62.1</td><td>—</td><td>—</td><td>—</td><td>151.6</td><td>150.1</td><td>19.17</td><td>20.6</td></tr><tr><td>10</td><td>59</td><td>—</td><td>—</td><td>—</td><td>—</td><td>152.9</td><td>20.84</td><td>22.4</td></tr><tr><td>18</td><td>59.3</td><td>—</td><td>—</td><td>—</td><td>—</td><td>151.5</td><td>21.17</td><td>22.7</td></tr><tr><td>24</td><td>62.4</td><td>—</td><td>—</td><td>—</td><td>—</td><td>152.6</td><td>23.42</td><td>25.2</td></tr><tr><td>30</td><td>59.4</td><td>—</td><td>—</td><td>—</td><td>—</td><td>152.4</td><td>23.67</td><td>25.4</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>FIGURE 5 DSC curves of aPLA samples at T_a = 120°C for annealed times (t_a = 3-30 h)</figcaption></figure>

### 3.3 | Effect of annealing temperature and time on crystallinity

Figure 7 shows the effect of T_a for different times t_a on the crystallinity rate X_c, evaluated using Equation (2). At T_a = 80°C, samples annealed during 3 and 6 h exhibit X_c = 7.4% and 14.8% with a cold crystallization at 116 and 117°C, respectively. This is in good agreement with the work of Mijovićet al.,⁴¹ Tsuji et al.⁴² and Park et al.⁴³ where grain like morphology was observed during cold crystallization at 80°C. Annealing times of 3 and 6 h are not sufficient to achieve complete crystallinity which occurs at times t_a = 10-30 h around X_c = 19%. For annealing temperatures in the range 80°C ≤ T_a < 90°C at t_a: 10-30 h, the cold crystallinity disappears, and X_c shows plateau values around 19% (Figure 7), which can be explained by the presence of the disordered α' phase (see thermograms of Figure 2). For annealing temperatures in the range 90°C ≤ T_a < 110°C, the crystallinity begins to increase at 90°C with more substantial growth from 100°C up to 110°C (X_c ≈ 25% for T_a = 110°C at t_a = 24 h). For T_a > 110°C, X_c grows to reach an optimal crystallinity (X_c ≈ 32%) at 120°C. The domain of α'-α

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>FIGURE 6 (a) Melting peaks T<sub>m1</sub> and T<sub>m2</sub> plotted as a function of T_a for t_a: 3-30 h; (b) Hoffman-Weeks plots for aPLA for 3 and 24 h at different temperatures</figcaption></figure>

phase transition (90°C ≤ T_a < 110°C), reported by the melting peaks T_m (Figure 6a), is shifted by approximately 10°C towards lower temperatures compared to that predicted by X_c versus T_a. In the latter case, above 110°C, X_c increases rapidly to an optimal value of 32% followed by a plateau above 120°C. On the other hand, the first case clearly shows a α'-α phase separation, with the presence of the two melting peaks limited to 110°C. The strong growth of X_c at T_a > 110°C is due to discontinuities and
<table><caption>**TABLE 4** Equilibrium melting temperature $T_m^0$ and stability parameter (1/r) for aPLA at different times</caption><thead><tr><th>Samples</th><th>t<sub>a</sub> (h)</th><th>T<sub>m</sub><sup>0</sup> (°C)</th><th>1/r</th><th>R<sup>2</sup></th></tr></thead><tbody><tr><td rowspan="5">Hoffman-Weeks plots for annealed PLA for 3–24 h</td><td>3</td><td>168.3</td><td>0.314</td><td>0.96</td></tr><tr><td>6</td><td>171.9</td><td>0.348</td><td>0.98</td></tr><tr><td>10</td><td>169.7</td><td>0.284</td><td>0.96</td></tr><tr><td>18</td><td>170.9</td><td>0.315</td><td>0.97</td></tr><tr><td>24</td><td>172.3</td><td>0.312</td><td>0.99</td></tr></tbody></table>

Abbreviation: PLA, poly(lactic acid).

<figure><img src="image_10.png" /><figcaption>**FIGURE 7** Crystallinity of aPLA as a function of temperature $T_a$ for different times $t_a$</figcaption></figure>

strong growth of the dimensions of the spherulites observed in the same temperature range, as already mentioned by several authors.⁴¹⁻⁴⁵ Mijovićet al.⁴¹ showed a series of optical micrographs taken at different times during melt- and cold-crystallization at 140 and 80°C, respectively. The resulting morphologies are quite different; larger and more perfect spherulites were formed during melt-crystallization at 140°C whereas a grain like morphology was observed during cold crystallization at 80°C. Tsuji et al.⁴² found that the radii of spherulites of melt-crystallized PLLA samples increased dramatically from 10 to 150 µm when the annealing temperature ($T_a$) was increased from 100 to 160°C. Park et al.⁴³ observed that density and size of spherulites increase with an increase of annealing time and temperature. For a semi-crystalline polymer, crystal growth rate is generally faster when $T_a$ is close to its crystallization temperature.⁴³⁻⁴⁵

## 3.4 | Dynamic mechanical behavior

Figures 8 and 9 illustrate storage modulus $E$ and tan $\delta$ (defined as $E'/E$) of PLA sheet as function of temperature, for un-aPLA and aPLA ($T_a$: 80–140°C; $t_a$:3–24 h), respectively. The shape of $E'$ thermograms (i.e., drop, rise, and

drop) of un-aPLA and aPLA ($T_a$: 80°C; $t_a$: 3,6 h), is associated with the recrystallization process as given by Figure 8a,b. Above $T_g$, the increase of the chain mobility favors the crystallization process, which proceeds due to a slow scan rate (3°C/min) coupled with sinusoidal solicitation.

Incompletely crystallized samples show around 100°C an increase in $E'$ ($T > T_g$), caused by the cold crystallization phenomenon. The recrystallization induces stiffening of the macromolecular chains, responsible for the increase of the storage modulus. Towards high temperatures, the storage modulus drops when the material starts to flow, corresponding to crystal melting of the material. The drop of $E'$ values of un-aPLA and aPLA ($T_a$: 80°C; $t_a$: 3–6 h) samples is much faster than that in the case of highly crystallized aPLA ($T_a$: 90–140°C; $t_a$: 3–6 h) and aPLA ($T_a$: 80–140°C; $t_a$: 10–24 h) samples, which do not exhibit any further recrystallization process ($T > T_g$) and behave as perfectly semi-crystalline material. Thus, annealing, and therefore crystallization, can improve the heat resistance of PLA mainly due to restriction of molecular motion by the formation of firm spherulites.

The behavior of tan $\delta$ around $T_g$ of un-aPLA and aPLA ($T_a$: 80–140°C; $t_a$: 3–24 h) are presented in Figure 9. The major relaxation process is associated with $T_g$, measured by the temperature of tan $\delta$ peak, where the sharpness and height are affected by the crystallinity. The un-aPLA sample behaves like an amorphous polymer showing a very sharp and intense tan $\delta$ peak because there is no restriction of the motion of the main chain. On the other hand, in semi-crystalline polymers, the dispersed crystalline regions hinder the chain mobility in the amorphous regions that is illustrated by a reduction of sharpness and height of the tan $\delta$ peak. A significant reduction of the height of the tan $\delta$ peak is observed for aPLA ($T_a$: 80°C; $t_a$: 3 h) (Figure 9a) with the same sharpness due to the low crystallinity ($X_c$: 7.4%) and the presence of cold crystallinity. Tan $\delta$ of aPLA ($T_a$: 90–140°C; $t_a$: 3 h) is represented by a sharp enlargement with a decrease of peaks due to the increase in crystallinity (16.5% to 32.1%), thus showing the semi-crystalline behavior of these samples. Tan $\delta$ of aPLA ($T_a$: 80–140°C; $t_a$) samples annealed at 6, 10, and 24 h, shows
<figure><img src="image_11.png" /><figcaption>FIG URE 8 Storage moduli of un-aPLA and aPLA ($T_a$: 80–140°C; $t_a$) as a function of temperature for different annealing times. The symbols represent: ☆ Un-aPLA, ○ aPLA80, △ aPLA90, ▽ aPLA100, ◊ aPLA110, ◁ aPLA120, ▷ aPLA130, □ aPLA140</figcaption></figure>

wide peaks with low amplitudes, thus characterizing a
perfectly semi-crystalline behavior as shown in
Figure 9b-d.

**3.5 | Effects of annealing temperature and time on PLA stiffness**

The salient fact, which emerges from thermograms of
Figures 8 and 9, is the behavior of glassy moduli E_g and
loss tan δ as function of annealing temperature T_a at dif-
ferent times t_a, observed for low temperatures and
around the glass transition. T_g's of different aPLA (T_a; t_a)
samples, illustrated in Table 5, show an increase for T_a
(T_a: 90–110°C) and t_a (t_a: 3–24 h), thus presenting greater
rigidity of the material, particularly for the sample
annealed at t_a = 24 h with T_g = 80.1°C. The increase of
T_g in the range T_a: 90–110°C is due to the growth of crys-
tallinity as shown in Figure 7. This can be correlated with
the α'-α phase transition domain (T_a: 90–110°C), given
by DSC data in Figures 6 and 7, thus confirming the
improvement of the stiffness of the material.²⁶,²⁹⁻³¹

The stiffness of aPLA (T_a; t_a) samples can be also
viewed through the typical glassy plateau E_g (Figure 8),
observed for all samples, depending on crystallinity and
molecular weight. Given the same molecular weight, E_g
should increase with crystallinity, but this trend was not
observed. Indeed, PLA degrades due to thermal cleavage
causing thus a substantial reduction of its molecular
weight. This is an inevitable effect that must be taken
into account when setting processing and annealing con-
ditions of the material. Through monitoring E_g versus T_a
at different t_a (Figure 10), two particular effects emerge
from aPLA (T_a = 80°C; t_a = 3 h) and aPLA (T_a = 120°C;
t_a): firstly, the very high value of E_g (29,900 MPa) of aPLA
(T_a = 80°C; t_a = 3 h), and secondly the improvement of
E_g around T_a = 120°C for all annealing times (t_a: 3–24 h).
This improvement can be represented by the reduced
glassy modulus E_gR equal to the ratio of the maximum
modulus (E_gmax120) at T_a = 120°C and the maximum
modulus (E_gmax) along the curve t_a (E_gR = E_gmax120/E_gmax).
The sample annealed at t_a = 24 h presents the highest
ratio (E_gR = 0.9), corresponding to a strong crystallinity
generated by the couple (T_a, t_a), which will be elucidated
<figure><img src="image_12.png" /><figcaption>FIGURE 9 Tan δ of un-aPLA and aPLA (T_a: 80–140°C; t_a) as a function of temperature for different annealing times. The symbols represent: ☆ un-aPLA, ○ aPLA80, △ aPLA90, ▽ aPLA100, ◊ aPLA110, ◁ aPLA120 ▷ aPLA130, □ aPLA140</figcaption></figure>

<table><caption>TABLE 5 DMA glass transition temperatures of aPLA (T_a: 80–140°C; t_a: 3–24 h). The T_g for un-aPLA corresponds to 75.2°C</caption><thead><tr><th>Annealing temperature (°C)</th><th>T<sub>g</sub>-3 h (°C)</th><th>T<sub>g</sub>-6 h (°C)</th><th>T<sub>g</sub>-10 h (°C)</th><th>T<sub>g</sub>-24 h (°C)</th></tr></thead><tbody><tr><td>80</td><td>74.8</td><td>73.6</td><td>71.9</td><td>77.6</td></tr><tr><td>90</td><td>75.6</td><td>77.5</td><td>76.9</td><td>79.3</td></tr><tr><td>100</td><td>77.3</td><td>79.5</td><td>76.1</td><td>75.0</td></tr><tr><td>110</td><td>77.8</td><td>78.1</td><td>76.8</td><td>80.1</td></tr><tr><td>120</td><td>73.5</td><td>73.9</td><td>75.7</td><td>76.0</td></tr><tr><td>130</td><td>73.6</td><td>74.3</td><td>73.6</td><td>77.3</td></tr><tr><td>140</td><td>68.6</td><td>71.9</td><td>71.1</td><td>71.2</td></tr></tbody></table>

Abbreviation: DMA, dynamic mechanical analysis.

in the next chapter. After a drop from T_a = 80°C, E_g begins to increase around T_a = 110°C for all annealing times (t_a: 3–24 h), to reach a maximum value at T_a = 120°C, corresponding to the maximum of crystallinity (Figure 7).

This improvement in rigidity around T_a = 120°C, independently of t_a, corresponds to the (α'-α) phase separation, obtained from DSC data (Figures 6 and 7). This can be explained by the fact that despite a high T_a (120°C), which is supposed to reduce the rigidity of the material thermally, an improvement of the modulus E_g is observed in the range T_a: 100–120°C. This effect, linked to the (α'-α) phase transition, is highly sought after in the molding and extrusion of PLA type materials.

## 3.6 | Effects of crystallinity and molecular mass on aPLA (T_a = 80°C; t_a) and aPLA (T_a; t_a = 24 h) samples

Crystallinity and molecular weight M_v directly influence the behavior of the glass modulus. If crystallinity tends to increase E_g, decrease of M_v, on the other hand, tends to make it fall. To better understand these relationships, E_g, X_c, and M_v were determined as function of t_a and T_a, to
<figure><img src="image_13.png" /><figcaption>FIGURE 10 Glass modulus $E_g$ of annealed PLA as a function of $T_a$ for $t_a$: 3–24 h. $E_{gR}$ of the $t_a$ curves is 0.5, 0.8, 0.8 and 0.9 relative to annealing times 3, 6, 10 and 24 h, respectively</figcaption></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>FIGURE 11 Glass modulus $E_g$, crystallinity $X_c$, and molecular weight $M_v$, as functions of (a) time $t_a$ for $T_a = 80^\circ C$; (b) temperature $T_a$ for $t_a = 24 h$</figcaption></figure>

analyze particular effects concerning aPLA ($T_a = 80^\circ C$; $t_a = 3$ h) and aPLA ($T_a = 120^\circ C$; $t_a = 24$ h) samples (Figure 10). Concerning the first sample, $E_g$ suddenly increases from the un-annealed state (2900 MPa) up to

29,900 MPa at $t_a = 3$ h despite the drop of $M_v$ from 40,200 to 15,700 g/mol (Figure 11a). Regarding crystallinity, the slight increase of $X_c$ (~8%) cannot be the cause of this spectacular increase of the modulus. Indeed, structure and molecular mobility of the amorphous phase are strongly affected by the crystalline morphology developed in semicrystalline polymers. The first indication comes from the theory of nucleation.46–55 More particularly, a self-nucleation effect47–51 could originate from smaller clusters or residual segmental orientation remaining in the melt, which play a role as precursors of the subsequent crystallization. From an industrial point of view, self-nucleation could be highly attractive to reduce time needed to process polymeric materials and to improve their final properties. The particularity of this process is that the critical size of the nucleus decreases with an increase in supercooling of the melt. Crystallization at high supercooling of the melt i.e. in the order of several tens of degrees, allows the development of increasingly smaller nuclei of supercritical size compared to crystallization at low supercooling. The aPLA ($T_a = 80^\circ C$; $t_a = 3$ h) sample responds to this crystallization scheme with a high supercooling of the melt exhibiting ($T_m^0 - T_a$) > 70°C. An extremely high nucleation density in low crystallization PLA materials around 80°C has been highlighted.34,50,51 The increase of the crystallization rate in the case of aPLA ($T_a = 80^\circ C$; $t_a$) samples, shown in Figure 11a, can thus be attributed to self-nucleation.

The multitude of spherulites of nanometric dimensions resulting from strong nucleation are considered as crosslinking nodes, thus significantly increasing the material's rigidity. This explains the strong growth of $E_g$ for aPLA ($T_a = 80^\circ C$; $t_a = 3$ h). A decrease of $E_g$ was observed for $t_a = 6$ h, beyond which it stabilizes around an average value of 8000 MPa. This proves that crystallinity strongly depends on $t_a$ and that a critical time exists, corresponding to a critical size of the nucleus, followed by another growth domain of larger crystals.45–51 The increase of $E_g$ also results from cold crystallinity observed on the thermograms of the samples annealed at 3 and 6 h, given by DSC (Figure 2) and DMA (Figure 8b). Above $t_a = 10$ h cold crystallinity disappeared, and the fall of moduli, caused by the heat treatment, was damped by the increase of crystallinity (19%). Beyond $t_a = 10$ h, the different properties, $E_g$, $M_v$, and $X_c$, tend towards plateau values around 6500 MPa, 37,500 g/mol and 19%, respectively. The sudden increase of molecular weight beyond $t_a = 6$ h can be attributed to the crystallinity effect where larger-sized crystallites greatly limit chain mobility and prevent polymer degradation. Another interesting effect is the improvement of the thermomechanical properties of aPLA ($T_a$; $t_a$) samples around $T_a = 110^\circ C$ (i.e., around $\alpha'-\alpha$ phase separation), in
particular for the aPLA ($T_a$: 100–120°C; $t_a$ = 24 h) sample. Figure 11b illustrates the evolution of $E_g$, $M_v$, and $X_c$, as function of $T_a$ at $t_a$ = 24 h. Between 80 and 90°C, the crystallinity (~19%) was sufficient to increase $E_g$ (from 6500 to 7800 MPa) despite the drop of $M_v$ (from 37,000 to 34,000 g/mol). Between 90 and 100°C, the strong decrease of $M_v$ (from 34,000 to 25,000 g/mol) causes a strong drop of $E_g$ (from 7800 to 4800 MPa), despite a slight increase of crystallinity. From 100 to 120°C, the strong increase of crystallinity (from 20% to 31.5%) directly led to an improvement of $E_g$ (from 4800 to 7800 MPa), and an increase of $M_v$ from 25,000 to 35,000 g/mol before stabilizing around 34,000 g/mol. This can be explained by the formation of large crystallites hindering chain mobility and, therefore, dramatically reducing the drop in molecular weight. The material exhibits optimal conditions around 120°C in terms of crystallinity ($X_c$ = 31.5%), molecular weight ($M_v$ = 35,000 g/mol) and rigidity ($E_g$ = 7800 MPa), thus very favorable conditions for the production of semi-crystalline materials.

## 3.7 | Polymorphism in aPLA ($T_a$ = 80°C; $t_a$) and aPLA ($T_a$; $t_a$ = 24 h) samples

In terms of polymorphism, two domains can emerge for aPLA ($T_a$ = 80°C; $t_a$: 3–24 h) (Figure 11a): the domain of self-nucleation below $t_a$ = 6 h with a very high rigidity particularly at $t_a$ = 3 h, and the domain of $\alpha$'-crystals corresponding to the appearance of the $T_{exo}$ peak beyond $t_a$ = 6 h as described by the DSC thermograms in Figure 2. The $\alpha$'-$\alpha$ transition region, showing an improvement of the glass modulus, is not accessible during 24 h, limited by the thermal stability of the material. For aPLA ($T_a$: 80–140°C; $t_a$ = 24 h) (Figure 11b), the annealing temperature scan ($T_a$: 100–120°C) shows a strong increase of the crucial parameters $E_g$, $X_c$, and $M_v$, thus predicting the same effects (improvement of rigidity) as the $\alpha$'-$\alpha$ phase separation in the range $T_a$: 90–110°C, determined by $T_m$'s peaks (DSC, Figure 6). Below 90°C, the increase of modulus is mainly due to the extremely high nucleation density in PLA materials, what may be associated with the $\alpha'$ phase. For $T_a$: 90–110°C, a drop of the modulus was observed due to the decrease of $M_v$ despite the strong crystallinity. Between 110 and 120°C the strong increase in crystallites induces a marked improvement of the modulus, limiting the movement of the chains and improving the mechanical properties of the polymer. Beyond 120°C, crystallinity and molecular weight stabilize, corresponding to the $\alpha$ phase.

# 4 | CONCLUSIONS

In the present report, the thermal behavior of various PLA samples annealed at various $T_a$ and $t_a$'s, were investigated by DSC and DMA measurements. It has been found that the small exothermic peak in the DSC curve just prior to the melting peak, associated with the ($\alpha$'-$\alpha$) phase transition, was detected at $T_a$ = 80°C and $t_a$ ≥ 10 h. This phase separation, reported by the double $T_m$ melting peak in the range $T_a$: 90–110°C, can be correlated with the increase of crystallinity for $T_a$: 100–120°C. DMA data show a sharp enlargement with a decrease of tan $\delta$ peaks of aPLA due to the increase in crystallinity, showing the semi-crystalline behavior of these samples. The salient fact of this work is the substantial growth (~30,000 MPa) of the glassy modulus at $T_a$ = 80°C for $t_a$ ≈ 3–6 h, due to strong nucleation. The second interesting effect is the improvement of $E_g$ for $T_a$: 100–120°C, regardless of time $t_a$, which can be correlated to the increase of crystallinity in the range of $T_a$: 100–120°C, and the $\alpha$'-$\alpha$ phase transition ($T_a$: 90–110°C).

## AUTHOR CONTRIBUTIONS

**Assia Zennaki:** Investigation (lead). **Latifa Zair:** Investigation (supporting); visualization (equal). **Khadidja Arabeche:** Conceptualization (equal); data curation (equal); validation (equal). **Lina Benkraled:** Investigation (equal); visualization (equal). **Ulrich Maschke:** Writing – review and editing (equal). **Abdelkader Berrayah:** Conceptualization (equal); methodology (equal); supervision (equal); writing – original draft (equal).

## ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of the Algerian Ministry of Higher Education and Scientific Research (MESRS), the General Directorate of Scientific Research and Technological Development (DGRSDT) of Algeria, the University of Tlemcen/Algeria, the CNRS, and the University of Lille/France.

## CONFLICT OF 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.

## DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available on request from the corresponding author.

## ORCID

Ulrich Maschke https://orcid.org/0000-0001-7970-1966
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How to cite this article: A. Zennaki, L. Zair, K. Arabeche, L. Benkraled, U. Maschke, A. Berrayah, *J. Appl. Polym. Sci.* **2022**, e53095. https://doi.org/10.1002/app.53095