Physical explanations about the improvement of PolyHydroxyButyrate ductility: Hidden effect of plasticizer on physical ageing

Physical explanations about the improvement of PolyHydroxyButyrate ductility: Hidden effect of plasticizer on physical ageing

Accepted Manuscript Physical explanations about the improvement of PolyHydroxyButyrate ductility: Hidden effect of plasticizer on physical ageing Raph...

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Accepted Manuscript Physical explanations about the improvement of PolyHydroxyButyrate ductility: Hidden effect of plasticizer on physical ageing Raphael Crétois, Jean-Marc Chenal, Nida Sheibat-Othman, Alexandre Monnier, Clélia Martin, Olivier Astruz, Rafael Kurusu, Nicole Raymonde Demarquette PII:

S0032-3861(16)30806-0

DOI:

10.1016/j.polymer.2016.09.017

Reference:

JPOL 19023

To appear in:

Polymer

Received Date: 10 May 2016 Revised Date:

22 July 2016

Accepted Date: 6 September 2016

Please cite this article as: Crétois R, Chenal J-M, Sheibat-Othman N, Monnier A, Martin C, Astruz O, Kurusu R, Demarquette NR, Physical explanations about the improvement of PolyHydroxyButyrate ductility: Hidden effect of plasticizer on physical ageing, Polymer (2016), doi: 10.1016/j.polymer.2016.09.017. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Physical explanations about the improvement of PolyHydroxyButyrate ductility: Hidden effect of

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plasticizer on physical ageing Raphael Crétois, Jean-Marc Chenal*, Nida Sheibat-Othman, Alexandre Monnier, Clélia Martin, Olivier Astruz, Rafael Kurusu, Nicole Raymonde Demarquette

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Graphical abstract

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Physical explanations about the improvement of PolyHydroxyButyrate ductility: Hidden effect of

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plasticizer on physical ageing Raphael Crétois1,2, Jean-Marc Chenal*1,2, Nida Sheibat-Othman1,3, Alexandre Monnier1,3,

1 : Université de Lyon, CNRS, F-69621, Lyon, France

2 : MATEIS, INSA-Lyon, CNRS UMR 5510, F-69621, Lyon, France

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Clélia Martin1,2, Olivier Astruz1,2, Rafael Kurusu4, Nicole Raymonde Demarquette4

3 : LAGEP, CPE Lyon, CNRS UMR 5007, Villeurbanne, France

4 : Ecole de Technologie Supérieure, Mechanical Engineering Departement, Montréal, Canada 15

Abstract

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Firstly, specific aging and rejuvenating treatments are applied to PHB samples. This enables to clarify the roles of secondary crystallization and of physical ageing on PHB embrittlement over time. The reversibility of physical ageing and irreversibilty of secondary crystallization are demonstrated after annealing. Secondly, we focus on the effect of addition of plasticizer on the microstructure of PHB and its mechanical properties. The experimental results showed

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that the crystallinity of any plasticized sample is always higher than that of unplasticized, in spite of the greater ductility of the plasticized samples, indicating the importance of the

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amorphous phase on the mechanical properties of PHB. The experimental results demonstrate that a hidden major effect of plasticizer is in fact to prevent the physical ageing of the 25

amorphous phase.

Keywords: secondary crystallization, physical ageing, embrittlement, plasticizer, polyhydroxyalkanoate,

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*Corresponding author. Tel.: +33 47 343 6129; fax: +33 47 243 8528 E-mail address: [email protected]

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ACCEPTED MANUSCRIPT 1. Introduction PolyHydroxyButyrate (PHB) is a biopolyester, fully biodegradable and biocompatible 35

produced by various bacteria, which can be used for short time life packaging and medical devices [1,2]. This semi-crystalline polymer possess a low hydrophilic character, mechanical

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and barrier properties close to some petroleum plastics [3,4]. However, PHB is subjected to a progressive embrittlement, which leads to a drastic decrease of its elongation at break and consequently limits its potential applications [5,6].

This phenomenon is not clearly elucidated but it is assumed that the interlamellar

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amorphous chains are progressively “frozen” resulting in an increase of the amorphous rigid

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fraction [7,8]. Two explanations are proposed to account for this phenomenon. The first hypothesis considers that a secondary crystallisation takes place after the primary one [6,9]. This secondary crystallization slowly occurs during the storage at room temperature, the 45

appearance of small crystallites can bridge crystalline lamellae but also constrain the

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remaining amorphous PHB chains leading to the embrittlement of the material [10]. The second assumption is based on the physical ageing of the amorphous phase [11,12,13,14]. This phenomenon due to the non-equilibrium character of the glassy state was studied in

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thermoplastic semi-crystalline polymer by Struiks [15,16]. An explanation based on the coexistence of two types of amorphous phase was proposed: an undisturbed (far from

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crystalline lamellae) and a disturbed phase (close to crystalline lamellae) with a restrict mobility due to interaction with the crystal. Struiks [15,16] defines a boundary Tg for each phase ( TgL and TgU for undisturbed and disturbed phase respectively) and demonstrated that physical ageing happens below TgU. In the case of PP [16], this ageing is characterized by an 55

increase of storage modulus (18%) and a decrease of loss tangent (~30%) after 15h at 20 °C (TgL is found between -20°C and 0°C and TgU lies above 80°C). A similar interpretation of the crystal/amorphous interface led to the development of the three-phases model consisting of

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embrittlement of PHB would be the consequence of the amorphous phase densification due to physical ageing. However in this study, crystallinity according to storage time has not been

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measured, consequently the first hypothesis of a secondary crystallization cannot be rejected to explain Scandola’s results. Moreover some authors [6,9] consider that the glass transition of PHB is too low (around 0 °C) to enable the development of physical ageing at room temperature. Nevertheless, we have to keep in mind that ~ 0 °C corresponds to the glass

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transition of quenched PHB (100% amorphous). In the case of semi-crystalline PHB the glass

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transition is higher (it is commonly accepted that an increase of crystallinity rises the glass transition, PHB has usually a crystallinity around 60% and we could wonder if it is enough to increase the Tg above the ambient temperature to enable physical ageing). So far, as 70

demonstrated above, there is no consensus on the explanation about the physical origin of the

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PHB embrittlement. However, for many years, numerous teams proposed different ways to improve the ductility of PHB like annealing [6,12], plasticizing [22,23], blending with elastomers [24,25] or copolymerization [9,26,27]. Although annealing leads to improvement

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of the toughness by rejuvenating the material, this effect seems to be temporary [6]. De Koning et al. [6] studied the mechanical properties of an annealed PHB (10h at 110 °C), they

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measured, after 150 days of storage, a decrease of the elongation at break from 43% to 30%, an increase of storage modulus from 0.92 GPa to 0.94 GPa and a significant decrease of loss tangent from 0.10 to 0.065. Copolymerization of hydroxybutyrate (HB) with a comonomer like hydroxyvalerate (HV) or hydroxyhexanoate (HH) were performed by different authors 80

[9,26,22]. In each case, the crystallization of these copolymers induces a reject of the comonomer into the amorphous phases that could reduce the extent of secondary crystallization. This way can strongly improve the elongation at break, for example 12% of

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Young modulus lower than that of pure PHB. The blending with an elastomer (EPDM for example) improves the elongation at break and the impact resistance due to the presence of

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larger rubber domains in the material but like previously the Young’s modulus of the blend decreases significantly [24]. The use of plasticizers can also lead to similar effects [28], but the recent work of Kurusu et al. [22] have highlighted that the blends with plasticizers can also present poor mechanical properties.

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It is worth noting that in many studies about the improvement of PHB ductility [29,30],

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the authors don’t mention the lag time between processing and testing of the samples. This parameter is of prime importance, as demonstrated by de Koning et al. [6], because the embrittlement takes place gradually and the microstructure of sample reaches equilibrium 95

after around 150 days.

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In our work, we investigated the microstructural changes between neat and plasticized samples in equilibrium state in order to discriminate the multiple factors at the origin of embrittlement. From a practical viewpoint, different neat and plasticized PHB samples (called

microstructure to mechanical properties. All the experimental (DSC, SAXS, DMA) data enable us to propose a scenario describing the mechanism of PHB embrittlement, which helps

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“aged”, “rejuvenated” and “re-aged”) were prepared and characterized to correlate

us to explain effects of plasticizer.

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2. Experimental Section 2.1. Materials PHB powder (Mw =330 000 g.mol-1) was supplied by PHB Industrial (Brazil). This PHB is pure (no plasticizer or nucleating agent). The plasticizer added was the tri(ethylene glycol)

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bis(2-ethylhexanoate) (TEG) purchased from Celanese (U.S.A.) with a molecular weight of 402.6 g.mol-1.

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2.2. Samples preparation

Firstly, pure PHB and plasticized PHB (10 wt% and 30 wt% of TEG) were extruded into

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pellets using a twin screw extruder Haake Rheomix PTW16/Polylab (Germany). The temperature range applied between the die and the feed of the extruder varies from 160 °C and 170 °C with a screw speed fixed at 100 rpm. Finally, injected samples were moulded using a Demag Ergotech Pro 35-115 (Germany) at 170 °C and a screw speed fixed at 50 rpm.

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The injection pressure was established at 125 bar and the mould temperature at 50 °C. A first set of samples, named “aged” were obtained by aging neat and plasticized PHB 120

samples after the injection process at room temperature for 6 months. A second set of

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samples, named “rejuvenated”, were prepared by annealing the “aged” samples at 125 °C during 20 min (corresponding to optimum annealing conditions (unpresented data). The

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previous “rejuvenated” samples were kept 6 months at room temperature to obtain the last set of samples named “re-aged”. The “aged” and “re-aged” samples have reached their 125

microstructural equilibrium contrary to “rejuvenated” ones which were immediately tested after annealing.

2.3. Differential Scanning Calorimetry (DSC)

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beforehand calibrated with an indium standard. A heating step from -40 °C to 195 °C at 10 °C min-1 was applied to samples. From the melting enthalpy, measured between 120°C and 190°C, crystallinity (Xc) was calculated according to the following relationship:

∆H m ×100 ∆H m0 ×ωPHB

(1)

ω PHB is the weight ratio of PHB in the blend,

∆H m and ∆H m0 [31] are the measured and the

theoretical (crystallinity equal to 100%) melting enthalpy of PHB respectively. Theoretical

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Xc =

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0 thermodynamic data, like ∆H m , depend on temperature, in the case of PHB this value is

estimated to 146 J.g-1 at around 170°C [24]. If melting of crystallites takes place below or above 170°C, ∆H m should have to be corrected to take into account the gap between 170°C 0

and Tm [20,32,33]. Classically, this tricky correction is negligible when the gap between 170°C and Tm is small enough (~50°C). This explains why, in this study, we only measured

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the melting enthalpy between 120°C and 190°C. Consequently, crystallinity of aged PHB is underestimated (see 3.1).

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About the part of this study dedicated to physical ageing, samples were firstly heated to 195 °C and kept at this temperature 2 min to remove their thermal history. Then samples were quenched (to avoid crystallization) to -15 °C, aged at this temperature during 1h and finally

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heated to 50 °C at 10 °C.min-1 to highlight physical ageing.

2.4. Dynamic Mechanical Analysis (DMA) The DMA measurements were carried out on a homemade torsional pendulum previously 150

detailed [34]. This apparatus measures the values of the real and imaginary components of the shear modulus (storage modulus G’ and loss modulus G”). From these modulus, loss tangent, tan ϕ (= G”/G’), can be calculated. Parallelepipedic samples (Length*Width*Thickness:

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°C.min-1 and a strain frequency set to 1 Hz.

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2.5. Small Angle X-ray scattering (SAXS)

SAXS measurements were carried out on an apparatus equipped with a copper rotating anode (λ=1.54 Å) (Rigaku Corporation, Japan), a Gobel’s mirrors collimation system (Elexience, France) and a 2D detector (Princeton Instruments, USA). The patterns were

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acquired during 10 min, each pattern was integrated azimuthally and corrected from the

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background scattering. The corrected scattering intensity was normalized by the thickness and the absorption of each sample. Finally, Lorentz corrected scattering curves ( I ( q ) q 2 vs.q ) were plotted. The one-dimensional correlation function ( γ (r ) ), calculated from the Fourier 165

transformation of the Lorentz-corrected intensity, enabled to obtain the structural parameters

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Lp, lc and la [35]. The long period (Lp) corresponds to the sum of the average thickness of the crystalline lamella lc and the interlamellar amorphous layer la. The correlation function

sample:

∫ I (q)q γ (r ) = ∞

2

cos(qr )dq

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provides a description of the change of electron density as a function of distance within the

Q

with Q =





0

q2 I (q)dq

(2)

where r corresponds to the distance in the direction normal to the layer faces in the stack and Q is the scattering invariant.

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ACCEPTED MANUSCRIPT 3. Results and discussion

First of all, we investigated the two assumptions about the embrittlement of PHB to 180

determine the predominating phenomenon according to the type (aged or rejuvenated) of

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sample. Then we studied the effect of plasticizer addition on the microstructure and its consequences on the PHB’s embrittlement.

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3.1. What about the “Secondary Crystallization” phenomenon? 185

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To have an overview on the role of the secondary crystallization on the embrittlement, a study of the microstructural changes according storage time was performed thanks to DSC and SAXS. The crystallinity, the long period and the thicknesses of crystal and amorphous lamellae of different PHB samples are gathered Table 1.

LP

la

lc

(± 1 %)

(± 0.2 nm)

(± 0.2 nm)

(± 0.2 nm)

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6.3

2.4

3.9

Rejuvenated PHB

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8.1

3.3

4.8

Re-aged PHB

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8.0

3.3

4.7

Aged PHB/10%TEG

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7.4

3.0

4.4

Rejuvenated PHB/10%TEG

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9.0

3.5

5.5

Re-aged PHB/10%TEG

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3,5

5,5

Aged PHB/30%TEG

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(a)

(a)

(a)

Rejuvaneted PHB/30%TEG

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(a)

(a)

(a)

Re-aged PHB/30%TEG

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(a)

(a)

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Aged PHB

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Xc

Samples

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The X rays scattering due to domains of plasticizer hinders the measurement of the semi-

crystalline parameters of PHB. At high ratio of TEG (30%), phase segregation took place during sample processing.

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The measured crystallinity of aged PHB, around 60%, increases up to 66% after annealing.

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The DSC curve of the aged PHB displays a shoulder at 60°C (Figure 1) due to samples processing way. This shoulder corresponds to the beginning of the melting of the crystalline phase because the onset of crystallization took place in the mould of the molding injection

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machine pre-heated at 50 °C. After rejuvenation, this shoulder shifts above the annealing temperature (125 °C) because all the “small” crystallites populations (including the ones due to secondary crystallization) have melted and then recrystallized at this temperature. The

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small crystallites populations of aged PHB mainly explains the difference of crystallinity compared with rejuvenated PHB. Indeed, crystallinity of aged PHB is underestimated because the melting enthalpy is only calculated in the temperature range of 120 to 190 °C (“small”

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Otherwise, no significant change of crystallinity is observed after storage for six months between rejuvenated and re-aged samples. Thus, small crystallites populations erased with 210

annealing do not reappear over time. About the semi-crystalline microstructure, Table 1 displays the increase of the long period, due to annealing, associated to the thickening of crystalline and amorphous lamellae (lc and la). This phenomenon comes from an improvement of the crystalline perfection (lc increase) and the melting of small/defective crystallites (la increase) [10,36], some of these crystallites

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may come from to secondary crystallization. Concerning the effect of storage time on the

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samples.

Figure 1: DSC melting curves of aged, rejuvenated and re-aged PHB. 225

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DMA is sensitive to the glass transition and other secondary transitions of polymers and can therefore be used to detect the coexistence of dual amorphous phases (described in semi-

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crystalline models). The comparison between Figures 2 and 5 enables to conclude that tan φ peaks (Figure 2) mainly correspond to constrained amorphous phase. From a mechanical 230

viewpoint, annealing induces a decrease of storage modulus at room temperature and an increase of the magnitude (tan ϕ) of the main relaxation peak (~20 °C) between aged and rejuvenated PHB (Figure 2), but no shift to higher temperatures. These results are consistent with an increase of sample toughness and could be explained by the melting of small crystallites previously highlighted.

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Figure 2: Storage modulus G’ (filled symbol) and loss tangent tan ϕ (unfilled symbol) for the

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aged ( , ), rejuvenated ( , ) and re-aged PHB ( , ) versus temperature. * rejuvenated “re-aged PHB” sample ( ,

) is shown for complementary information

However, during ageing of rejuvenated PHB (to obtain re-aged sample) at room temperature,

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the values of storage modulus and tan ϕ changed again. The mechanical behaviour of re-aged sample evolves toward that of aged sample. This result indicates an increase of the stiffness with the storage time and confirms the temporary effect of annealing on mechanical properties. Otherwise, crystalline parameters (Table 1) are unchanged between rejuvenated

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and re-aged PHB, consequently as previously explained, a new secondary crystallization at

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room temperature can be excluded. We are concluding that the secondary crystallization phenomenon is not the key parameter to understand PHB embrittlement but only a parameter to take into account because it cannot justify the different thermo-mechanical behaviours 250

between rejuvenated and re-aged sample (without secondary crystallization how to understand the changes of G’ and tanφ). It’s worth noting that if re-aged sample is annealed again, the sample properties (DSC, SAXS and DMA) are the same than the ones of the previous rejuvenated sample (Figure 2), the reversibility of DMA properties enables to exclude the

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changes, physical ageing [11,12] could be postulated to induce a gradual embrittlement.

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3.2. What about the “Physical Aging” phenomenon?

To introduce the future discussion, a study of the effect of heating rate on the location of 260

PHB thermal transitions is performed. The DSC curves obtained at 2, 5, 10 and 20 °C.min-1

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are displayed on Figure 3. Beforehand the sample was heated 2 min at 195°C to remove its thermomecanical history and quenched to -40°C at a cooling rate of 100°C/min to avoid

Figure 3: Effects of heating rate on thermal transitions (Tg, Tc, Tm) of PHB.

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crystallization.

As expected, the higher the heating rate, the higher the glass transition temperature (Tg) ( Tg shifts from -5.6 to -1.7 °C when the heating rate increases from 2 to 20 °C.min-1). Exothermic peaks of crystallization are also observed between 30 °C and 80 °C. The results are in 270

accordance with the study of An et al. [37], the lower the heating rate, the lower the crystallization peak (Tc). At high temperature, endothermic peaks correspond to the melting of crystalline phase, it is interesting to notice that one or two melting peaks are observed

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have more or less time (according the heating rate) to melt and recrystallize to form more stable crystals with higher melting temperatures [38,39]. At the higher heating rate, two

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melting peaks are observed, because reorganization of the crystalline microstructure has not enough time to finish. [10].

Prior to evaluate if the physical ageing could be at the origin of the change of mechanical properties of rejuvenated PHB during the storage at room temperature, it is necessary to check

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the sensitivity of PHB to this phenomenon. DSC experiments with an amorphous PHB are

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performed (see 2.3): one sample is heated without aging; another one is aged 60 min at -15 °C before heating (Figure 4). The unaged and aged samples exhibit a glass transition around -4 °C, the aged one displays an additional endothermic peak (characteristic of physical ageing). 285

Biddlestone et al. [9] also observed the same phenomenon but with an astonishing shift of Tg

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from -6 °C to 3 °C after 60 min aging at -13 °C. It is worth noting that physical ageing also induces a shift of the crystallization peak toward lower temperature. This phenomenon was previously observed on aged PLA by Pan et al. [40] and should be due to conformational and

be characterised by a decrease of the crystallization peak temperature.

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microstructural rearrangement during ageing process. Thus, the PHB physical ageing can also

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Figure 4: DSC curves of unaged and aged (1h and 4h at -15 °C) PHB

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Now, we wonder if this phenomenon could take place at room temperature after the PHB crystallization. To address this question, the change of mobility of the remaining amorphous phase during crystallization was followed using DMA analysis (Figure 5). For this experiment, a PHB sample was firstly heated to 195 °C and kept at this temperature 2 min to remove its thermal history, then the sample was quenched (to avoid crystallization) in liquid

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nitrogen. This fully amorphous PHB is used to perform DMA analysis (the apparatus is located in a refrigerated room (~-5°C°), to avoid crystallization before the starting of the

Tα (Amorphous)

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experiment).

Figure 5: Changes of thermo-mechanical properties from amorphous to crystallized PHB: 305

Storage modulus G’ ( ), loss modulus G” ( ) and tan ϕ ( )

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For an easier understanding three areas are delimited on figure 5. From -150 °C up to 5 °C (area 1), PHB is amorphous, the changes of G’, G’’ and tan φ correspond respectively to β (~ -110 °C) and α (-3 °C) relaxations. The glass transition temperature of amorphous PHB measured by DSC (-4°C) at a heating rate of 2°C/min (Figure 3) corresponds to the α

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relaxation temperature. From 5 °C to 60 °C, (area 2) crystallization (characterized by an increase of G’) takes place, which explains the enlargement of tanφ peak originated by the

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mobility restriction of the remaining amorphous phase due to the constraint induced by the appearance and growth of the crystalline phase. This is also in good agreement with the very thin amorphous layer measured by SAXS analysis (Table 1). Thus, the glass transition of the

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semi crystalline PHB (impossible to measure by DSC without a specific protocol [20]) is shifted above room temperature and consequently physical ageing of the remaining amorphous phase can take place (in accordance with Struik’s theory). The location of the

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glass transition temperature of constrained amorphous phase is supported by temperaturemodulated calorimetry (TMDSC) analyses performed by Righetti et al. [20] on polyhydroxybutyrate. The authors also concluded that its value is higher than room

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temperature. Finally, from 60 °C to 110 °C (area 3) the shape of G’ and G’’ curves are typical

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of a semi-crystalline polymer (and in agreement with Figure 2) From DSC, SAXS and DMA data, we demonstrate that the physical ageing phenomenon induces the reversible change of 325

the mechanical properties between re-aged and rejuvenated PHB and has to be taken into account to explain the gradual embrittlement of PHB. As a conclusion, secondary crystallization and physical ageing are both responsible of the embrittlement of PHB after sample processing. Annealing is only effective to definitively erase the effect of secondary crystallization but, because annealing does not prevent subsequent physical ageing, it does not

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enable to keep the ductility of rejuvenated PHB over time.

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3.3 What about the effects of a plasticizer on PHB embrittlement?

The addition of a plasticizer is classically used to improve the ductility of materials. As demonstrated by Calvao et al. [41], the impact of TEG on the glass transition (or α relaxation)

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of PHB is very efficient compared to the others plasticizers usually used and furthermore it is considered more environmentally friendly. In our study two TEG ratio (10%wt and 30%wt)

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were added to PHB. The lower ratio enables to perform SAXS measurements and the higher ratio clearly illustrates the impact on thermo-mechanical properties. We mainly focus on the effect of plasticizer on the shifting of the main relaxation temperature between neat and

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plasticized PHB before and after rejuvenation. Beforehand the effect of plasticizer on crystalline microstructure was studied. Table 1 gathers the crystalline parameters of both plasticized samples. The addition of plasticizer induces an increase of crystallinity (+ 9% with

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30%wt of TEG) as already observed by Kurusu et al. [22]. Plasticizer improves the polymer chains mobility, which favours the crystallization of PHB. Like in the case of aged PHB and for the same reasons, crystallinity of both aged PHB/TEG increases after annealing and

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remains unchanged after 6 storage months at room temperature. Aged PHB/10%TEG exhibits

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a higher long period than that of aged PHB. This increase can be explained by those of the thickness of amorphous and crystalline lamellae (la and lc) respectively due to the insertion of 350

plasticizer in amorphous phase (swelling of amorphous phase) and to a higher molecular mobility, which facilitates the organization of crystallites. The rejuvenated PHB/10%TEG sample stored six months after annealing (re-aged PHB/10%TEG) keeps the same structural parameters (Table 1). DSC and SAXS measurements highlight the role of plasticizer on the crystalline microstructure, and also confirm that the presence of small crystallites (in part due

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to secondary crystallization) in aged PHB/Plasticizer is definitively erased after annealing in rejuvenated and re-aged samples. Figure 6 displays the thermo-mechanical properties of aged, rejuvenated and re-aged PHB/30%TEG. The peak at low temperature (~ -90 °C) corresponds to the main relaxation of

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plasticizer and corroborates its partial segregation during the blend processing. In the case of PHB/10%TEG no phase segregation took place (unpresented data), which means that TEG exhibits a solubility limit between 10%wt and 30%wt in PHB. As expected, the α relaxation

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temperature of aged PHB/30%TEG (-3 °C) is below that of aged PHB (20 °C), which seems consistent with the plasticizing of the amorphous phase already evidenced by its swelling

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measured by SAXS in the case of PHB/10%TEG sample. After annealing, storage modulus G’ decreases and an increase of tan ϕ, differently from than in the case of rejuvenated of aged PHB, goes along with a shift of the α relaxation temperature toward a higher temperature (from -3 °C to 5 °C) and an enlargement of this relaxation peak. Like in the case of PHB, the

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rejuvenation leads to improve the toughness by reducing the stiffness (decrease of G’). However, the changes of the relaxation peak are not so easy to explain. Several assumptions

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can be considered and will be discussed later.

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Figure 6: Thermo-mechanical properties of PHB/30%TEG: G’ (filled symbol) and tan ϕ (empty symbol) for aged sample ( , ), rejuvenated sample ( , ), re-aged sample ( , ).

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* rejuvenated “re-aged PHB” sample ( ,

) is shown for complementary information

Over time the mechanical properties of rejuvenated PHB/30%TEG comes back to the ones of aged PHB/30%TEG (= re-aged PHB/30%TEG). It’s worth noting that if re-aged sample is

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annealed again, the sample properties (DSC and DMA) are the same than the ones of the previous rejuvenated sample (Figure 6). Four assumptions about the shift and enlargement of 380

the main relaxation peak after rejuvenation could be considered: secondary crystallisation, phase segregation during annealing, change of mechanical coupling between amorphous and

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crystalline phase and physical ageing. The role of secondary crystallization can be excluded

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because it was previously demonstrated that the melting of small crystallites for pure PHB goes along with no change of α relaxation temperature (Figure 2). Phase segregation during 385

annealing would suggest that some of the plasticizer would have migrated during annealing and consequently PHB/30%TEG would have a mixed lamellar morphology with regions with and without plasticizer in the interlamellar amorphous layer, which could explain the shift of

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α relaxation temperature toward a higher temperature. However, phase segregation already took place during sample processing as demonstrated by the comparison between aged and rejuvenated PHB/30%TEG (Figure 6), ruling out this possibility. The reversibility of

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behaviours between aged and rejuvenated PHB/30%TEG excludes the change of mechanical

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coupling between amorphous and crystalline phase (it is worth remembering that secondary crystallization is definitively erased after the first annealing). Also, according the low temperature of the main relaxation (-3°C), physical ageing mainly responsible of 395

unplasticized PHB thermo-mechanical properties at room temperature seems impossible to invoke in presence of plasticizer. Finally, to understand the shift and the enlargement of the main relaxation, an heterogeneous microstructure composed of three phases could be considered: a pure phase of TEG, another pure phase of PHB and a last one made of a blend PHB/TEG. This type of microstructure is consistent with the values of solubility parameters

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calculated previously [42]. We consider that the regions made of pure PHB (Tα ~ 20 °C) are subject to physical ageing while the regions made of a blend PHB/TEG (Tα ~ -3 °C) remains unchanged. Thus the shift and the enlargement of the relaxation peak are only due to the rejuvenation of the pure PHB phase. In order to comfort this explanation, Figure 7 displays

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the loss tangent versus temperature for rejuvenated and re-aged samples (for the PHB and PHB/30%TEG). The subtraction of the rejuvenated and re-aged PHB/30%TEG curves enables to highlight the relaxation peak due to the rejuvenation of the pure PHB phase. A

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perfect correlation between this peak and that of re-aged PHB appears.

Figure 7: Loss tangent (tan ϕ) versus temperature: rejuvenated ( ) and re-aged ( )

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PHB/30%TEG, re-aged PHB ( ) and tan ϕ subtraction of rejuvenated and re-aged PHB/30%TEG samples (- -).

Physical ageing can also be evidence thanks to the DSC method presented in paragraph 3.2 415

(Figure 8). The glass transition temperature of amorphous PHB/30%TEG obtained by quenching at -50°C is -30°C. A shift of the crystallization peak towards the lower temperature is observed after aging at -15 °C (1h). This is the signature of physical ageing as already view

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To sum up, even if plasticizer (TEG) increases PHB crystallinity, PHB/TEG samples remain more ductile than unplasticized ones. This experimental data seems astonishing.

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However, we demonstrated that the effects of plasticizer are multiple. First, it obviously lowers the glass transition of the remaining amorphous polymer chains after crystallization. Second, thanks to the amplitude of the previous effect, it also avoids physical ageing of the amorphous phase over time at room temperature. However, phase segregation (between TEG

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and PHB) takes place during the sample processing (probably when samples are cooled down

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as demonstrated by the calculation of solubility parameters [42]) inducing the appearance of unplasticized domains subject to physically ageing. Consequently, the benefits of TEG plasticizer are not optimized and would have to be reinforced in particular by adjusting the processing temperature to minimize the elusion of TEG.

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Figure 8: DSC curves of unaged and aged (1h at -15 °C) PHB/30%TEG

4. Conclusion 435

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ACCEPTED MANUSCRIPT First we clarified the role of the secondary crystallization and that of the physical ageing (too often forgotten in many papers) on PHB embrittlement. The various analyses (DSC, DMA, SAXS) performed on aged (at room temperature), rejuvenated (annealing 20 min at 125 °C) and re-aged (at room temperature) samples enable to conclude that both phenomena affect PHB ductility and only secondary crystallization can be erase by annealing. Physical

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ageing at room temperature can happen because glass transition in constrained amorphous phases of PHB is above room temperature. Over time, each ageing step will induce the

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happening of physical ageing and each rejuvenating step will temporary enable its disappearance. From the previous analysis on the changing behaviours of neat PHB, we displayed an explanation about the unexpected shift of the main relaxation peak of plasticized

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PHB sample toward higher temperature after annealing. Again we highlight the main role of physical ageing, which expresses the inhomogeneity of PHB/Plasticizer blend evidenced by DMA experiments (only pure PHB domains are subject to ageing). This paper gives also

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others insights to improve the ductility of PHB (for example: by increasing the thickness of amorphous lamellae or by lowering the constraint on amorphous phase). In the future, much more attention will have to be taken to control the microstructure of PHB blends (phase

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segregation) but also that of PHB matrix (crystallinity, Lp….) to obtain synergetic effects against PHB embrittlement and to reach optimum mechanical properties. These approaches

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will be refined in next papers.

Acknowledgments

The authors thank the Institut Carnot [email protected] for the financial support.

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Highlights :

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Roles of secondary crystallization and of physical ageing on PHB embrittlement Irreversibilty of secondary crystallization are demonstrated after annealing Plasticizer prevents the physical ageing of the amorphous phase

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• • •