Structure and Barrier Properties of Biodegradable ... - ACS Publications

Article pubs.acs.org/JPCC

Structure and Barrier Properties of Biodegradable Polyhydroxyalkanoate Films Nadège Follain,*,†,‡,§ Corinne Chappey,†,‡,§ Eric Dargent,†,∥ Frédéric Chivrac,⊥ Raphael̈ Crétois,†,‡,§ and Stéphane Marais†,‡,§ †

Normandie Université, France Laboratoire Polymères, Biopolymères et Surfaces, Bd. Maurice de Broglie, Université de Rouen, F-76821 Mont Saint Aignan Cedex, France § CNRS, UMR 6270 & FR 3038, F-76821 Mont Saint Aignan Cedex, France ∥ AMME-LECAP EA 4528 International Laboratory, Université et INSA de Rouen, Avenue de l′Université BP 12, 76800 Saint Etienne du Rouvray, France ⊥ CREAGIF Biopolymères, 14760 Bretteville-sur-Odon, France ‡

ABSTRACT: The water vapor sorption and permeation to water and gas molecules of films prepared from industrially available polyhydroxyalkanoates (PHAs) were analyzed in detail. Two film-forming processes were compared through their transport properties. The presence of additives, which can alter the barrier properties of films, was highlighted in some commercial-grade PHAs by investigating the physicochemical properties of these films. The water vapor sorption results were simulated using the appropriate Park model, highlighting the moisture resistance, the entrance of water molecules into films, and the impact of the process used and were then discussed through the analysis of diffusion coefficients. A significant reduction in water permeability was evidenced with the increase of degree of crystallinity of the biopolyesters. Water diffusivity was found to be dependent on water concentration and this was due to the plasticization effect induced by water molecules. In terms of gas permeation, a singular behavior peculiar to these PHA films was revealed showing a potential interest for packaging or biomedical applications.

1. INTRODUCTION With the growing awareness of environmental issues, the replacement of nondegradable petroleum-based plastics with materials produced from renewable resources intended for a range of short-life applications, especially packaging and disposable applications, and for biomedical uses, has been attracting much attention in recent years. Among the commercially available biodegradable polymers, polyhydroxyalkanoates (PHAs), the family of natural semicrystalline polyesters produced by bacterial fermentation with the same three-carbon backbone structure but differing in the type of alkyl group,1 appear to be a promising class of biopolymers, although a wide range of applications has not yet been found. They are attractive as biomaterials because they are biodegradable and biocompatible,2 can be recycled into organic waste, and through their semicrystalline structure,3 present better mechanical and barrier properties. In the PHA family, the properties of poly(3-hydroxybutyrate) (P3HB), the most common and first discovered PHA,1 are often compared to those of conventional synthetic polymers in terms of melting and glass transition temperatures and degree of crystallinity.4 The random insertion into the P3HB polymer of 3-hydroxyalkanoate (3HA) monomers having bulky side chains, such as (R)-3hydroxyvalerate (3HV),5,6 (R)-3-hydroxyhexanoate (3HHx)7,8 and longer 3HA,9,10 overcame some undesirable characteristics that had prevented its development: both the crystallinity and the © 2014 American Chemical Society

melting temperature of 3HB-based copolymers were lowered while preserving the low hydrophilic character. The widely used copolyesters containing 3HV units (PHBV) displayed toughness, thermal stability, elongation at break, and an enlargement of the processing-temperature window as a function of 3HV content.11 Moreover, a very good balance of barrier properties toward water and gas molecules is reported for PHBV copolymers.12 In order to be used in food packaging applications, an essential requirement concerns the barrier properties exhibited by polymer films; the transfer of small molecules has to be hindered by increasing the diffusion pathways taken by the diffusing molecules (water and gas molecules). The permeability is thus reduced. To our current knowledge, only a few studies dedicated to transport mechanisms of water and gas molecules have been specifically conducted on PHA films12−15 and most of them concerned either unformulated PHAs or films prepared by the solvent-casting method.16,17 In the literature, many investigations have been focused on structural characterization after incorporation of additives, such as nucleating agents,18,19 plasticizers,20 nanofillers,21,22 and blending with other biobased polymers,23,24 for instance. Such routes are also known to Received: August 14, 2013 Revised: February 14, 2014 Published: February 21, 2014 6165

dx.doi.org/10.1021/jp408150k | J. Phys. Chem. C 2014, 118, 6165−6177

The Journal of Physical Chemistry C

Article

4 h in order to get a 5 w/v % solubilization. The resulting solution was cast on a 85 mm-diameter Petri dish which was covered with a lid of larger size and placed under the fume cupboard at ambient temperature for at least 3 days to control the solvent evaporation. FTIR analysis of films was carried out to ensure the complete removal of solvent. 2.2.2. Conventional Plastic-Forming Process. Films with thicknesses of 200−250 μm were prepared by hot compression molding (SCAMEX press, France) at 190 °C under 200 bar. Polymer pellets were placed between stainless steel plates for 15 min under atmospheric pressure until the melt state of polymers was reached. Then, a pressure was applied by increments to avoid the formation of air bubbles within the films. Finally, the films were cooled from the melt state to ambient temperature under atmospheric pressure. 2.3. Characterization. 2.3.1. GPC Measurements. Biopolyester films were analyzed by GPC measurements at 303 K using dichloromethane as eluent (flow rate of 1 mL·min−1) to determine mass distributions. The GPC was composed of a Varian PL-GPC 50 Plus associated to a refractometer as detection system and a PL gel column (porosity of 5 μm). The elution profiles were analyzed allowing the average molecular weights and the polydispersity index (PDI) to be calculated using a calibration curve obtained from the polystyrene standards range. 2.3.2. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was carried out using a Netzch TG 209 apparatus. Data were collected on ∼15 mg of samples placed in an open aluminum crucible and heated from 20 to 500 °C with a 10 °C min−1 rate under nitrogen atmosphere (gas flow = 20 mL·min−1). Two characteristic temperatures, Tonset and Tdp, were collected, corresponding to the onset degradation temperature and to the temperature of the extremum of the first derivate weight versus temperature peak, respectively. 2.3.3. Differential Scanning Calorimetry. Differential scanning calorimetry (DSC) experiments were performed with a DSC 2920 (TA Instruments) equipped with an intracooler system. Around 8−12 mg of samples were placed in closed DSC devices under nitrogen (gas flow = 20 mL· min−1) to prevent oxidative degradation. Before each analysis, the calorimeter was calibrated from the melting temperature and enthalpy of high-purity indium and zinc standards. The DSC curves were normalized by taking into account 1 g of matter. The temperature ranged from 0 to 190 °C with heating and cooling rates at 10 °C·min−1. The melting point (Tm) was taken as the extremum of the melting endotherm peak. The crystallinity (χ) was therefore determined by using:

strongly modify the barrier properties of the resulting formulated PHA films, whatever the polymer type.25 The present work is aimed at investigating the transport properties of PHA films prepared from commercially available biopolyesters (Biomer P209 and P226 and TiaNan Enmat Y1000P) by means of two film-forming processes: the solventcasting method and the compression molding process. In addition to the usual characterization of film structure (thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and X-ray diffraction (XRD)), water vapor sorption and permeation to water and gas molecules were investigated. The diffusing probes (water, nitrogen, carbon dioxide, and dioxygen) were selected for the following reasons: gas molecules, which have different molecular diameters, are known not to interact (or very weakly) with polymer structures, while water interacts with polymer and its functional groups. The moisture resistance of PHA films was highlighted from sorption measurements and correlated with the crystallinity and formulation of PHAs. The sorption data and their modeling using an appropriate mathematical model were discussed by analyzing the diffusion coefficients. The water permeation results were examined in detail and the dependence of diffusivity on water concentration was determined. The gas permeation results underlined the impact of commercial PHA formulation and of the film-forming process on gas permeability. The ranking of permeation parameters was determined considering the gas molecules and crystallinity characterizing PHA films.

2. EXPERIMENTAL SECTION 2.1. Materials. Two poly(3-hydroxybutyrate) (PHB) polymers purchased from Biomer (DE), under the trade names P209 and P226, were used as received (Figure 1). Poly(3-hydroxybutyrate-

Figure 1. Chemical structures of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with x = 0.97 and y = 0.03.

co-3-hydroxyvalerate) with 3 mol % of hydroxyvalerate repeating units (as attested by 1H NMR measurements) was supplied by TiaNan Biologic Material (CN) under the trade name Y1000P and used as received (Figure 1). The average molecular weights of the PHA pellets were determined by GPC measurements at 303 K. Chloroform as solvent for the casting method was obtained from Acros Organics. Deionized water (Milli-Q water system, resistivity 18 MΩ cm−1) was used throughout. All chemicals used as received were of analytical grade. 2.2. Preparation of the Biopolyester Films. Two types of processing were considered to prepare the biopolyester films. The films prepared by the solvent-casting method are noted cast films, while those prepared by the compression molding process are noted compressed films. The films, with an average thickness of 200−250 μm were stored at 25 °C under vacuum with P2O5 powder before experiments. 2.2.1. Chloroform-Solvent Casting Method. About 3 g of polymer pellets were dissolved in chloroform at 50−55 °C for

χ=

ΔHm ΔHm0

× 100 (1)

where ΔHm is the melting enthalpy calculated from the endothermic peak recorded during the heating and ΔH0m = 146 J·g−1 is PHB equilibrium melting enthalpy (i.e., the theoretical melting enthalpy of the polymer assumed to be 100% crystalline).26,27 2.3.4. X-ray Diffraction. Wide-angle X-ray diffraction (XRD) measurements were performed on film samples using a Bruker D8 advance diffractometer in the Bragg−Brentano geometry operating with Co Kα radiation (λ = 1.78897 Å) generated at a voltage of 35 kV and current of 40 mA under ambient temperature. The 2-θ diffraction diagrams ranged from 10 to 6166



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After a preliminary high vacuum desorption of the system, the upstream side was thereafter provided with carrier gas at pressure p1, while the increase of pressure (p2) was measured in the downstream compartment as a function of time using a pressure sensor (Pfeiffer Vacuum). From the steady state, the permeability P (expressed in Barrer units) was calculated, as previously described.30

60° with 2-θ steps of 0.05°. The samples were studied under similar experimental conditions. 2.3.5. Water Transport Measurements. A complementary approach was used to investigate the water barrier performances of PHA films by using water sorption and water permeation experiments performed at 25 °C. The results are consistent, indicating that the values are representative of the behavior of these films. 2.3.5.1. Water Sorption. Water sorption experiments described the capacity of polymer films to sorb water molecules depending on the water activity domain. The liquid water sorption measurements were performed by immersing in deionized water a dried sample (Md) which was periodically weighed after being blotted dry with a lint-free paper for removing excess solvent. When no change in mass was measured, the equilibrium state of water concentration (Mw) was reached allowing the water mass gain to be calculated: water mass gain =

M w − Md × 100 Md

3. RESULTS AND DISCUSSION 3.1. Impact of Film-Forming Processes. XRD analysis was carried out to characterize the structure of the polymer film. The profiles for films obtained after the compression molding process are given in Figure 2. The same XRD profiles

(2)

The water vapor sorption measurements were carried out by means of a Dynamic Vapor Sorption system (Surface Measurement Systems Ltd., U.K.) equipped with an electronic Cahn D200 microbalance (resolution of 0.1 μg) enclosed in a temperature-regulated device, as previously detailed.28 Briefly, after sample drying until no further change in dry weight was measured, the measurement was performed using water activities varying stepwise from 0 to 0.95. From sorption kinetics, the equilibrium water mass gain was deduced for each water activity, allowing the sorption isotherm curve to be plotted. Taking into account the usual boundary conditions for gas and vapor sorption measurements,28 kinetic data were analyzed in terms of water diffusivity by applying a mathematical approach based on Fick’s diffusion laws describing transport mechanisms. The two diffusion coefficients were determined and are referred to as D1 and D2 coefficients as a function of the sorption period. For the first-half sorption, the D1 coefficient was determined when the water mass gain was lower than 50%, while for the second-half sorption, the D2 coefficient was calculated for mass gain higher than 50%. 2.3.5.2. Water Permeation Process. The water permeation measurements were performed by means of a home-built device based on the pervaporation method, as previously reported.29 Briefly, the dried film was mounted in the measurement cell consisting of two compartments (downstream and upstream). A stream of pure water (Milli-Q) was pumped through the upstream compartment and the transfer of water molecules through the polymer films from the upstream compartment to the downstream compartment was monitored by the humidity sensor (mirror hygrometer from General Eastern Instruments, USA). The water diffusivity (from the transient regime) and the water permeability (from the stationary flux) of the film were thereafter determined by plotting the water flux curve. 2.3.6. Gas Transport Measurement. The diffusion of gas molecules through the film sample was investigated using a variable pressure method, referred to as the “time-lag” method.30 Several diffusing probes, such as carbon dioxide, nitrogen and dioxygen (99.999% of purity, Air Liquide), were used.28 It can be noted that the results for gas properties were validated by a very good reproducibility of the measurements.

Figure 2. XRD profiles for PHA films obtained after the compression molding process.

were detected for cast films and for raw pellets (not shown here) without significant changes in peak intensity and position, indicating a similar crystalline phase. The characteristic diffraction patterns arising from the PHB orthorhombic phase were clearly observed for all film samples in the typical region of 2θ = 10−35°. No significant difference was detected, except for two additional sharp diffraction peaks located at 2θ = 11° and 33° for the two PHB polymers (P209 and P226). It has been reported in the literature a similar diffractogram to the PHB polymer for PHBV copolymers, with up to 40 mol % HV units, because the HV units are excluded from the PHB crystalline phase.5,31 For the films made from Biomer formulations, the additional peaks can be attributed to additives, as suggested by GPC results. This point will be discussed later in connection with thermogravimetric and DSC measurements. In fact, some authors have reported the addition of low molecular additives, such as biodegradable citrate, to the as-received Biomer pellets, although without complementary information.32,33 Thermal characterization was investigated by means of thermogravimetric analysis and DSC measurements. The mass loss and the corresponding derivative thermogravimetric (DTG) curves for PHA pellets are plotted in Figure 3. It is interesting to note that the degradation curves for processed PHA films present similar profiles and their characteristic temperatures are maintained. For the sake of clarity, the degradation profiles for cast and compressed films were not plotted here. The characteristic temperatures, Tonset and Tdp, for the as-received pellets, cast films, and compressed films are gathered in Table 1. The degradation of PHBV copolymer is less complex than for PHB samples. As shown in Figure 3, the degradation of 6167



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Figure 3. Thermogravimetric profiles of the as-received PHA pellets under nitrogen atmosphere.

Table 1. Characteristic Temperatures and Percentage of Residual Mass Obtained from TGA Experiments Tonset (°C) Y1000P P209 P226 Y1000P P209 P226 Y1000P P209 P226 a

Tdp (°C)

As-Received PHA Pellets 262 280 281 Compression Molding 264 280 282 Solvent-Casting Method 265 281 282

RM %a

274 291 291

0 2.4 2.1

276 293 292

0.3 2.8 3

279 293 292

0.3 2.4 2.5

Residual mass calculated at 500 °C with an accuracy of ±0.5 wt %.

PHBV occurred between 220 and 280 °C in one step, while several steps occurred for the two PHB samples starting at temperature close to 150 °C. For the main degradation, similar profile was exhibited by both Biomer pellets, with a Tdp temperature equals to 291 °C. It can be observed that the thermal degradation profile of PHB polymers was shifted to higher temperature compared with that of the PHBV copolymer (Tdp = 274 °C), as reported in the literature. The addition of an HV unit on the three-carbon backbone structure of the PHB polymer enlarge its processing window. Additional thermal degradation occurred at Tdp values of 396 and 398 °C, indicating additional organic additives in the as-received P209 and P226 pellets, respectively. The obtained results agree well with results previously reported in the literature.34 It can be noted that the difference between PHB and PHBV polymers is significant. At 320 °C, Y1000P was completely degraded, while the residual percentages were of about 12% and 8% for P209 and P226 samples, respectively. Moreover, the percentages of residual mass (RM %) reported in Table 1, that is the nonvolatile fraction at 500 °C (corresponding to inorganic elements), was close to 2.5% for the Biomer samples and was 0% for Y1000P. these results reveal the presence of nonvolatile additives (inorganic additives) for the Biomer samples (P209 and P226), while a monocomponent degradation was found for Y1000P. Similar results have recently been mentioned for commercial PHA pellets provided by different manufacturers, including the Biomer supplier.14 Regarding the DSC measurements performed during heating up to 190 °C for PHA pellets, the thermal phenomena also

Figure 4. DSC curves of the as-received PHA pellets: (a) heating rate of 10 °C·min−1 and (b) cooling rate of 10 °C·min−1.

look different for Y1000P and for P226 and P209 polymers (Figure 4a). For the Y1000P copolymer, only one melting peak is observable at Tm = 170 °C while for the P226 and P209 samples, the main melting peaks are observable at Tm = 165 °C and additional endothermic peaks are found close to 50 °C. After these analyses, DSC curves were obtained during cooling, and exothermic peaks characteristic of the crystallization from the melt are displayed in Figure 4b. The crystallization occurs at a lower temperature for Y1000P pellets, about 61 °C, compared with values close to 110 °C for Biomer pellets. It is well-known that the HV units present in PHBV decrease the regularity of the polymer chains and lower the crystallization rate. Nevertheless, it has recently been shown that the difference in crystallization temperature between pure PHB and PHBV is small (close to 5 °C).18,19 In previous papers, we have shown that addition of boron nitride nucleating agent in PHB18 and in PHBV19 allows crystallization to occur at a higher temperature. So, the difference in crystallization temperature between both the PHB and the PHBV polymers has to be attributed to the presence of additives (acting as nucleating agents). In addition, it is worth pointing out that the additional melting peak appeared at 50 °C for P209 and P226 films could be related to the presence of these additives. This phenomenon can be 6168



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From our results, we can point out that the melting phenomena in PHBV and PHB films are film-preparationdependent. The dependence of the melting temperature on the thickness of crystalline lamellas is well-established.38,39 So, the thermo-mechanical history followed by the PHBV copolymer during the compression molding process leads to the formation of crystalline lamellas with greater thickness than for the other kinds of film. From the data, it can be stated that the thermal features displayed by cast films and compressed films have no effect on PHB samples, while a modification of the crystalline thickness occurs for the PHBV copolymer. The Y1000P pellets exhibit a degree of crystallinity of about 58% (Table 2). Similar values have previously been reported in studies on PHBV-based nanocomposites.22 Concerning the Biomer pellets, the degree of crystallinity determined at about 41% and 47% for P209 and P226 samples, respectively, can be considered low compared with values reported in studies on pure PHB polymer, i.e., in the range of 60−90% when storing at a temperature above the glass transition. These lower values can certainly be correlated to the additive-induced effect. In fact, the P209 films display a higher percentage of additives and a lower degree of crystallinity compared with the P226 films. Moreover, the effect of additive content is also evidenced: the lowest crystallinity is associated to the lowest melting temperature Tm2. The smallest crystal lamellar size is obtained for P209 pellets. When using typical DSC protocols, the glass transition was not clearly defined because of a poor peak resolution due to the high crystallinity characterizing PHA polymers. A specific DSC protocol has recently been developed.18,19 The Tg values are in the range of −10 to 0 °C for all the samples. These values agree well with the Tg values for pure PHB and PHBV polymers reported in previous studies.16,22,40,41 The authors have calculated Tg values at about 1.5 to 4 °C for pure PHB polymer and at about −2 to +3 °C for pure PHBV with 4, 6, 12, and 18 mol % of HV units. According to Holmes,42 both the HB and HV homopolymers exhibit similar Tg values which reveal that the glass transition is not affected by the content in HV units in PHBV copolymers. However, this transition is shifted to higher temperatures with the increase of the degree of crystallinity of the samples.42 The changes in the weight-average molecular weight (Mw) and in the polydispersity index (PDI) determined by GPC measurements are briefly discussed. As commonly mentioned in the literature,14,41,43 the Mw values of the PHB Biomer pellets are found to be higher than those of the PHBV pellets. After applying the film-forming processes, the Mw values decreased, with a greater reduction for the Biomer films (of about 30%). This Mw reduction is in agreement with the phenomenon of degradation occurring during the film preparation14,22,44 and driven by a random chain scission mechanism (by hydrolysis reaction and/or free radical mechanism) from the ester functions in the polyester chain backbone, inducing the formation of olefinic and carboxylic acid groups.44 Since no variation of the PDI index is observed (PDI values of about 2.4, 3.8, and 3.7 for Y1000P films, P209 films, and P226 films, respectively), one can assume that the chain scission mechanism is limited. Indeed, as evidenced by some authors,33,44,45 the Biomer film samples are more sensitive to the degradation process. This finding related to the presence of thermal degradant molecules, such as plasticizers,33 was evidenced by an additional GPC peak located at 520 g·mol−1.

Table 2. Thermal Properties of the Processed PHA Film Samples Determined from DSC Measurements Tm1 (°C)/ ΔHm1 (J·g−1)

Tm2 (°C)

Tm3 (°C)

ΔHm2+3 (J·g−1) χ (%)

Compression Molding Y1000P P209 P226 Y1000P P209 P226

51/6 155 54/4.5 162 Solvent-Casting Method 153 45/3.5 144 50/2.5 150

169 165 170

83.5 60 65.5

57 42 46

172 164.5 169

86 58 68

59 41 48

correlated with the extra-thermal degradation observed by TGA analysis, and the additional XRD diffraction peaks. The temperature and enthalpy (Tm1/ΔHm1) of this melting peak are reported in Table 2: the melting enthalpy is higher for the P209 film than for the P226 film. So, TGA and DSC measurements showed that the P209 films contain more additives than the P226 films, while XRD and DSC analyses reported that the additives present a distinct crystalline phase. The DSC curves obtained from the heating ramp for compressed and cast films are plotted, as represented for P209 in Figure 5. The DSC curves were found to be similar for

Figure 5. DSC curves for cast P209 film and compressed P209 film (heating rate of 10 °C·min−1).

compressed and cast P226 films. The peak characteristic of the additives is presented, but the melting phenomenon appears different in comparison with that of the pellets (Figure 4a). An additional melting peak close to 150 °C is presented for all the samples. The thermogram for the cast Y1000P film also presented two melting peaks instead of only one melting peak for the compressed Y1000P film (as in Figure 4a). The characteristic temperatures and the enthalpies associated to the peaks are reported in Table 2. According to the literature, the lower melting temperature of the PHB crystals (referred to as Tm2 in Table 2) is attributable to crystals formed during the process (cooling ramp for compressed films, solvent evaporation for cast films), whereas the higher melting temperature is attributable to more perfect crystals formed during the process or by recrystallization (or crystal rearrangement) during the DSC heating.35−37 6169



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shape50 (regularly referred to as BET II51), some differences between the Y1000P films and Biomer films can be observed. A linear increase of water mass gain is obtained for the Y1000P films instead of an upswing of water mass gain concave to the activity axis, as exhibited by the Biomer films. This trend can be explained in two ways. The first involves an “antiplasticization” effect of water on sorption usually taking place at low sorbed water concentrations. A strong cohesion arising from waterpolymer hydrogen-bonding interactions was established, which reduced the free volume. The second explanation is based on the effect of plasticizer additives on water sorption. Due to an increased free volume, the additives favor the entrance of water and locally weaken the polymer−polymer hydrogen-bonding interactions. It is clearly evidenced that the films obtained by the solventcasting method exhibit a greater water sorption than the films obtained by the compression molding process (Figure 6), for the whole water activity range. The casting method obviously favors the entrance of water molecules into the films. During solvent evaporation in the casting method, the polymer chains have retained sufficient motion levels (such as translation and rotation motions) to create additional free volume, making the films less dense and more permeable. Generally, higher viscosities with higher shear rates at the processing temperature are observed during the hot plastic-process, which constrains the polymer chain motions. So, the polymer structure is thus more easily opened and plasticized by water molecules, which behave as mobility enhancers during water sorption. This causes the greater water sorption at high water activities (aw > 0.75). As observed in Figure 6, a greater effect was measured for the Biomer films by virtue of the water-affinity of plasticizers. 3.2.2. Modeling Using Appropriate Mathematical Models. To examine the water vapor sorption process in polymers, the well-known Park model52 (eq 3) is applied to the experimental data (Figure 6) because of the sigmoidal shape of isotherm curves.

3.2. Hydration Properties of Films by Sorption Measurements. Because materials with polar functions can interact with water, water vapor sorption measurements were performed. The dimensions and the thickness of the films remained unchanged after sorption measurements. 3.2.1. Water Vapor Sorption Isotherms. The water sorption isotherm curves of the films (Figure 6) deduced from kinetic

Figure 6. Water vapor sorption isotherms for cast films (− −) and compressed films (): P209 (◆,◊), P226 (■,□), and Y1000P (▲,Δ). The isotherm curves were fitted from the Park model. The open symbols are used for cast films and the filled symbols for compressed films.

curves reflect a poor affinity to water since the maximum mass gains were below 2% in spite of the presence of water-sensitive functions (ester functions). Moreover, these results agree well with recently published values for widely used biodegradable polyesters, such as PLA46 or PCL films,47 and are close in magnitude to those of PET48 and PVC.49 So, bacterial polyesters can be assimilated to nonpolar polymers. A gradation of sorption isotherm profiles exists from TiaNan films to Biomer films for the whole range of water activity (Figure 6), and it can be clearly correlated to the degree of crystallization. It is usually reported that the more crystalline a material, the lower the water absorption of that material.40 Moreover, according to Miguel,12 the water sorption ability of a PHBV film was independent of the HV unit content of the copolymer. The Y1000P films characterized by the highest degree of crystallinity (about 59%) are the most moistureresistant films (mass gain lower than 0.9%). The ester functions are involved in hydrogen-bonding interactions with adjacent functions, instead of interacting with water molecules. In contrast, the Biomer films are characterized by higher mass gains under similar conditioning of water activities, and also by lower crystallinities. Moreover, the isotherm profiles for the P209 films are shifted to higher values compared with the P226 films (Figure 6). This observation is consistent with the degree of crystallinity (Table 2), and also with the additive contents. Based on this result, the additives in Biomer formulations probably alter the water affinity of the resulting films by establishing hydrogen-bonding interactions with water or disrupting the water-polyester interactions to allow the ester functions to be accessible to water. This is clearly evidenced when the films are submitted to high water vapor concentrations. To clarify the water sorption behavior of films at a low water activity range (aw < 0.20), an expanded view is reported in Figure 6. Although the isotherm curves display a sigmoid

C=

AL bL a w + KHa w + K agra wn 1 + bL a w

(3)

where AL and bL are the Langmuir-type adsorption terms, KH is the Henry-type solubility coefficient, n and Kagr (Kagr = KnHKan with Ka the aggregation constant) are the average aggregate size and the aggregation equilibrium constant for the clustering reaction. The fitted values (AL, bL, KH, Kagr, and n) and the accuracy of the fit (based on the average deviation modulus E53 and the regression coefficient (R2 = 0.999) are gathered in Table 3. A nonlinear regression was carried out using Table Curve 2D software. It should be noted that the lack of experimental points Table 3. Parameters of Park’s Model for Polyester Films AL (mmol·g−1) KH (mmol·g−1) Y1000P P209 P226 Y1000P P209 P226 6170

Compression Molding 0.28 0.12 0.76 0.21 0.77 Solvent-Casting Method 0.30 0.24 0.89 0.54 0.87

n

Kagr

E (%)

4 7 8

0.33 0.96 0.79

8.5 3.7 4.3

11 15 10

0.77 2.04 1.05

3.1 7.2 3.9



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the two diffusion coefficients (noted D1 and D2) for the whole water activity range.28 These two coefficients provide useful information in terms of the rate of water vapor diffusion in films. In fact, it is usually claimed that the coefficient D1 is characteristic of the diffusion of water through the surface of the film (determination of D1 during the first-half sorption period), whereas the coefficient D2 is representative of the diffusion of water in the core of the film (D2 determined during the second-half sorption period). The evolution of the two coefficients as a function of water activity, in the semilogarithmic scale (log D), is reported in Figure 7. Although a similar profile

for water activity aw < 0.1 prevented the parameter bL being correctly determined. Park’s mathematical model (eq 3) is consistent with a multistep sorption mode, usually observed for polymers,46,54,55 and can be divided into three terms. The Langmuir-type sorption relates to the adsorption of the first water molecules at the surface of specific sorption sites within films, such as polar sites, or microvoids in the case of glassy polymers. The Henrytype law sorption describes the dissolution of sorbed water molecules in the amorphous phase of the polymer for an intermediate water activity range. The third term corresponds to the water aggregation reaction displayed by an exponential change in water mass gain, reflecting the sorption of water molecules layer by layer within the films. The analysis of Park’s parameters points out some differences existing between biopolyester films (Table 3). At a low water activity range (aw < 0.2), the Langmuir-type sorption terms are low. This is in agreement with the very low water affinity of bacterial polyesters, as highlighted from sorption isotherms. In addition, the AL terms are not measurable for the Y1000P films as the isotherm has no convex part, indicating that the accessibility of water molecules to polar functions of polymer is hindered. The crystalline phase could be at the origin of this limited access. However, at these low sorbed water concentrations, an “antiplasticization” effect of water is regularly observed, which is the result of a strong cohesion through water-polyester hydrogen-bonding interactions.46 This strong cohesion contributes to reducing the free volume, and hence the water mobility. The effect is less with the Biomer films (convex shape of the isotherm curve), indicating that the additives in Biomer formulations present a certain water affinity. Indeed, using the solvent-casting method to prepare Biomer films induced a reincrease of AL values, showing the preferential affinity to water. At an intermediate water activity range, the KH parameter is systematically found higher for the Biomer films. This result can be directly related to the crystallinity of the films. When using the solvent-casting method, the KH parameter is again increased. Additional microcavities, or changes in their size, are initiated during the film preparation. At a high water activity range (aw > 0.7), the access of water molecules was clearly changed since the Kagr and n parameters were strongly affected by the film components and also by the film process. For the Biomer films, the n and Kagr terms were larger that those calculated for the Y1000P films, whatever the film-forming process used. This result suggests a higher possibility to aggregate water molecules due to the solubilization of additional water molecules during the Henry-type sorption. Consequently, the water aggregate size is impacted. The additive content being higher in the P209 films, the aggregation parameter is higher, compared with values obtained for the P226 films. The presence of additives, taken as watersensitive moieties, impacted the ability to form water clusters in the films. When the solvent-casting method was used, the Kagr and n parameters were increased, showing a higher ability to aggregate water molecules. The free volume in the films was altered due to conformational changes of polymer chains. In the light of the sorption results, the Biomer films seemed to be less efficient in terms of moisture resistance, compared with the Y1000P films. 3.2.3. Water Vapor Diffusivity. To evaluate the dynamic aspect of water vapor sorption (sorbed water/film interactions), the sorption kinetics were analyzed through the calculation of

Figure 7. Evolution of D1 (a) and D2 (b) coefficients as a function of water activity for cast films (− −) and compressed films (): P209 (◆,◊), P226 (■,□), and Y1000P (▲,Δ). The lines serve as a guide. The open symbols are used for cast films and the filled symbols for compressed films.

was obtained with coefficients in the same range, some variations can be detected considering both the film-forming processes and the selected polymers. A first observation can be made about the values of coefficients D1 and D2. The coefficients D1 are found to be lower than the coefficients D2, revealing that the diffusion is not constant. In this case, the entrance of water in films contributed to the plasticization phenomenon of the films by water. The diffusion thus depends on the sorbed water concentration. Moreover, for low water activities, the diffusion coefficients increased until a plateau was established and then decreased for water activities higher than 0.7. The water clusters reduce water mobility and hence water diffusivity.56 For the Y1000P films, specifically, a decrease of the D values initiated at very low 6171



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sorbed water concentrations was observed, irrespective of the film-forming process. This highlights an antiplasticization effect of water in the films (reduction of water mobility). A similar finding has already been published for plasticized polymers57 and, to a greater extent, for materials containing carbohydrate components with covered polar functions.58 For aw > 0.15, water molecules revert to their usual role as plasticizers. Concerning the impact of the film-forming process, the values of coefficients D for the cast films were systematically higher than those for the compressed films, indicating that water mobility is increased when using the casting method. In addition, an outstanding behavior is observed for the Biomer films: a wide difference of coefficients D from the cast film to the compressed film is found for the P209 films, whereas a very weak gap is observed for the P226 films. This result might be related to the formulation. It seems that the higher the additive content, the more the diffusion of sorbed species is favored, and hence the greater the water accumulation in the film. 3.3. Barrier Properties of Films Shown by Permeation Measurements. In order to determine the capacity of small molecules to diffuse through the films, the barrier properties were investigated by means of measurements of permeation to water and gas molecules. The improvement of barrier properties is usually correlated to the tortuosity effects induced by the impermeable domains (crystalline phase) within the material. The diffusion coefficients were determined by taking into account widely adopted assumptions.59,60 It is generally assumed that the diffusion is found to be constant for gas permeation, while a variation is observed for liquid or vapor permeation. 3.3.1. Water Permeation Parameters. From water permeation kinetics, and assuming the plasticization effect of water in the films, a mathematical treatment was applied on the data to determine the water permeation parameters.61 The water permeability was deduced from the steady state of the curve as the product of the limit flow (Jst) and the film thickness (L) divided by the activity difference between the two faces of the film. To compare the water permeability of the different films, the influence of the thickness effect was overcome by using the JL versus t/L2 curve, as reported in Figure 8. The permeation data are given in Table 4. In terms of the barrier effect, a reduction in permeability was obtained by increasing the diffusion pathways, due to the crystallinity of films (Table 2). This result is in agreement with the sorption results since the least permeable films are characterized by the highest moisture resistance. Again, the permeability was higher for the cast films, compared with the corresponding compressed films.62 This higher permeability is promoted by the additional free volume, or microcavities, formed during casting, as mentioned above. Moreover, the statement emphasizing that the barrier effect of a polymer film is mainly caused by the tortuosity effects should be somewhat qualified because it does not take into account the impact of the film components. Although the crystallinity values of the PHB Biomer films are lower than those of the Y1000P films, the influence of additives that favor the penetration of water molecules cannot be neglected. Specific interactions through water-additive hydrogen-bondings can explain the permeability values, in agreement with sorption data. Under similar conditioning and experimental conditions, the values reported for the PHA films are of the same magnitude as those recently published for polyesters such as PLA (1957 Barrer46), and PCL films (5959 Barrer47). In the literature, comparable values for commercial PHA polymers

Figure 8. (a) Reduced flux (JL) as a function of reduced time (t/L2), and (b) normalized water permeation flux for polyester films: P209 (◆,◊), P226 (■,□), and Y1000P (▲,Δ). The open symbols are used for cast films and the filled symbols for compressed films.

are reported using water vapor permeability tests based on gravimetric methods: at 23 °C and 50% RH, 486 and 2880 Barrer (2.7 × 10−12 and 1.6 × 10−11 g·m−1·s−1·Pa−1);14 at 24 °C and 40% RH, 2095 Barrer for PHBV with 12 mol % of HV unit (0.127 × 10−13 kg·m1·s−1·m−2·Pa−1).21 However, as expected, these values are higher than those obtained for nonpolar polymer films such as LDPE films with a value of about 72 Barrer.63 In terms of diffusivity, the time-scale shift of the permeation flux curve for the compressed films shows an increase in the delay time of the water diffusion. A greater shift of the curve was obtained for the Y1000P film. This result is observed by plotting the dimensionless curves of water flux J/Jst versus reduced time t/L2 (Figure 8b). This indicates that the film/ water interactions are reduced for the compressed Y1000P film, whereas the penetration of the water molecules is favored for the other films. This point will be discussed later through diffusivity parameters. 3.3.2. Water Diffusivity. The diffusion coefficient is determined from two specific times during the permeation process, as previously described. Briefly, the coefficient DI is calculated at time tI corresponding to J/Jst = 0.24, while the coefficient DL is obtained at the time-lag tL corresponding to J/Jst = 0.62.61,64 The values of DL are higher than those of DI (Table 4), whatever the film tested. From this experimental result, it can be stated that the diffusion coefficient is not constant, highlighting that a relationship with water concentration occurs due to the plasticization effect of water.65 In this case, it is usually postulated that the diffusivity follows an exponential law with the local water concentration in the film during the water permeation (increased free volume).61 6172



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Table 4. Water Permeation Parameters for the Polyester Filmsa P (Barrerb)

a

D0 × 108 (cm2·s−1)

Y1000P P209 P226

562 4663 2330

0.19 0.11 0.18

Y1000P P209 P226

1954 6142 2305

0.77 1.29 0.58

DI × 108 (cm2·s−1) Compression Molding 0.26 1.13 0.76 Solvent-Casting Method 0.92 2.58 0.94

DL × 108 (cm2·s−1)

⟨D⟩ × 108 (cm2·s−1)

γCeq

0.28 1.81 1.10

0.46 3.13 2.17

1.70 4.92 3.85

0.99 3.19 1.10

1.31 5.92 1.88

0.97 2.53 2.01

Accuracy ±0.4%. b1 Barrer =10−10 cm3(STP)·cm·cm−2·s−1·cmHg−1.

The relationship P = DS, assuming D to be constant, is therefore not valid when using water molecules as the probe. Based on the free volume theory, the following exponential law of dependence of the D coefficient with water concentration C was used. D = D0 exp(γC)

nitrogen (N2), dioxygen (O2), and carbon dioxide (CO2), differing in their van der Waals molar volume,66 were selected because they are known not to interact (or only very weakly) with organic polymers. In the case of rubbery polymer films, such as PHA films, the permeation behavior conforms to a linear Henry’s sorption law, which involves a constant diffusion coefficient. The diffusion coefficient is usually determined from the time-lag tL given by the intercept of the steady-state asymptote on the time axis via the time-lag method.30 The permeation data (Table 5), resulting from a multiplicity of permeation measurements, are calculated from permeation kinetic curves and thus reflect well the film behavior. Similar rankings for permeation parameters (permeability P, diffusivity D, and solubility S) for both the cast and compressed films are obtained (Table 5 and Figure 9).62 Indeed, it is clearly evidenced that the permeability of the PHA films was lower than that exhibited by nonpolar polymer films, such as LDPE (Figure 9), and by well-studied polyester films, such as PLA films (Figure 9). The films are characterized by permeabilities close to those recently evaluated in the literature by monitoring an oxygen transmission rate (OTR): at 23 °C and 50% RH, 0.039 Barrer and 0.27 Barrer for the Y1000P and P226 films after injection molding (0.26 and 1.93 cm3·μm·m−2·day−1·atm−1), respectively14 and at 23 °C and 0% RH, 0.054 to 0.084 Barrer with increasing HV unit content.15 It is usually widely accepted for the gas permeation process that P is the product of D and S, based on a solution diffusion mechanism. The diffusivity D, considered as a kinetic factor in the permeation process, is related to the mobility of gas molecules, while the solubility S, a thermodynamic factor, is relative to the interactions established between the polymer and gas molecules.67 From this statement, it can be indicated that the gas permeability P is governed by the dynamic diameter of the diffusing molecules, but also by their critical temperatures. With the increase of the boiling or critical temperatures, the condensability of gas molecules and therefore their solubility is increased. In this case, the gas solubility S for a film will be ranked considering the temperature values.68 As shown in Table 5, the gas solubility follows the SCO2 > SO2 > SN2 ranking (except for the cast P209 film). Indeed, Van Krevelen68 has previously reported a similar ranking, PCO2 > PO2 > PN2, for the gas permeability P of polymer films, as measured in this study, considering the nature and the size of gas diffusing molecules. CO2 gas is generally the molecule inducing the highest permeability for a polymer film because of its low kinetic diameter and higher critical temperature. For O2 and N2 gases, the permeability is driven by the diffusivity (dynamic diameter), whereas the permeability to CO2 and O2 gases depends on the solubility (critical temperature). For the gas diffusivity D, the

(4)

where D0 is the limit diffusion coefficient at null concentration and γC is the plasticization factor. At the highest water concentration called Ceq, this factor becomes γCeq. All the permeation curves were well-fitted by using this exponential law, which is a mathematical procedure described in more detail in separate papers.28,61 The specific parameters (D0, the mean integral diffusion coefficient ⟨D⟩, and γCeq) included in the exponential law of D are gathered in Table 4. The positive values obtained for γCeq highlight the concentration-dependence of coefficient D.28 The DL and DI values increased with the decrease of the crystallinity of the PHA films (Table 4), irrespective of the filmforming process. As diffusion occurs through the amorphous phase of the polymer, the water diffusion was greater in the PHB Biomer films. Indeed, the values were higher for the cast films than for the compressed films due to the increased free volume. To explain the difference in diffusion for the Biomer films, in addition to the change in crystallinity and to the water plasticization effect, the main factor concerns the ability of water-sensitive additives to absorb water. These results are in agreement with the diffusion behavior in sorption measurements. Moreover, the water aggregates formed within the films were the cause of the diffusivity of water along a new pathway. The D0 coefficient altered in the same manner. Concerning the change in the plasticization coefficient γCeq (Table 4), two antagonist effects are observed: a reduction of γCeq values for the cast films compared with values obtained for the compressed films, and an increase in γCeq values for the Biomer films in comparison with the values of the Y1000P films. This last effect can be clearly attributed to the water plasticization effect. The reduction in the γCeq factor is in turn attributed to the reduction in chain mobility that also limits the water clusters. For the cast Biomer films, the pre-existing free volume initiated during the casting method facilitated the condensation of water within the film, as suggested by the n values in water vapor sorption, thus making possible specific water/film components interactions. As a consequence, in the pre-existing free volume of Biomer films the additional free volume is weaker and that leads to lower γCeq values when compared to those calculated for the compressed Biomer films. 3.3.3. Gas Permeation Parameters. The transfer of several inert gas molecules across the biopolyester films was investigated through gas permeation measurements to evaluate the structural effects of the films. Three diffusing gas molecules, 6173


Figure 9. Gas permeability (expressed in Barrer) for compressed and cast films, compared with LDPE63 and PLA (L:D ratio of 96:4) films.63

2.01 ± 0.23 1.98 ± 0.88 1.07 ± 0.17 1.74 ± 0.09 9.86 ± 1.76 4.62 ± 0.21 0.35 ± 0.05 1.95 ± 0.44 0.49 ± 0.10

0.31 ± 0.02 1.98 ± 0.30 1.45 ± 0.17 2.22 ± 0.50 6.98 ± 0.70 5.26 ± 0.40

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12.00 ± 0.80 15.10 ± 2.47 14.6 ± 2.12

ranking is DO2 > DN2 > DCO2 (Table 5), in agreement with the increase of the van der Waals molar volume of gas molecules66,69 or with the Lennard-Jones collision diameters,70 except for the P209 films. As reported by Matteucci,71 when considering many molecules of varying size and shape, the dependence of diffusivity on van der Waals volume is frequently reported in the literature. In the case of the P209 films, the experimental data suggest that it is more appropriate to correlate diffusivity with the kinetic diameters of gas molecules.72,73 The diffusivity ranking is thus DCO2 > DO2 > DN2, as reported in Table 5. A distinct mechanism in the P209 films involving the mobility of gas molecules within the film could explain this separate result, attributable to the presence of additives (at a higher content than in the P226 films). In this case, the size of the diffusing molecules is responsible for the diffusivity ranking. Indeed, as Breck72 indicates, in some cases kinetic diameter can give a faithful scaling of diffusivity according to the size of the molecules when using light gases (i.e., H2, He, N2, O2, CH4, and CO2). To examine in more detail the behavior of polymer films, the results for S, D, and P are discussed taking into account the film-forming processes and their crystallinities. In fact, the adsorption of gas molecules being dependent on the crystalline phase fraction available within films,74 P, D and S coefficients are therefore decreased. This result is observed for both the cast and compressed films. The P, D and S coefficients are reduced when the degree of crystallinity of the polymers increases: PP209 film > PP226 film > PY1000P film (Figure 9). Similar rankings are obtained for diffusivity and solubility (Table 5). Considering the impact of the film-forming process, it can be seen that all of the values for the cast films are higher than, or equal to, those for the compressed films. Due to the crystallinity of the films, the permeation data is ranked as follows, whatever the process used: PP209 film > PP226 film > PY1000P film. In addition, the permeability is also calculated under values characterizing LDPE and PLA films. The cast films were more permeable than the compressed films, with solubility and diffusivity also increased. Polymer chain motions being enhanced during the solvent evaporation, the formation of microcavities, free volume and spaces between polymer chains available for gas molecules were favored. Concerning the gas solubility, the coefficients for the cast and the compressed films decreased with the increase of crystallinity, as was the case for permeability. It is well-reported that the solubility of a polymer film is calculated by considering the volume fraction of the crystalline phase as the product S0(1 − ϕc) (S0 = solubility of amorphous domain and ϕc = volume fraction of crystalline phase for a polymer).67 With the

1.20 ± 0.15 12.30 ± 1.54 3.04 ± 0.24

1 Barrer = 10−10 cm3(STP) cm·cm−2·cmHg−1·s−1. a

1.41 ± 0.46 3.84 ± 0.83 3.28 ± 1.83 0.25 ± 0.03 0.66 ± 0.01 0.16 ± 0.04 Y1000P P209 P226

1.77 ± 0.09 1.69 ± 0.50 0.56 ± 0.19

1.64 ± 0.36 5.75 ± 2.58 3.82 ± 1.17 0.02 ± 0.01 0.64 ± 0.14 0.13 ± 0.02 Y1000P P209 P226

0.13 ± 0.01 1.12 ± 0.15 0.37 ± 0.18

0.15 ± 0.06 9.32 ± 0.15 2.30 ± 0.08

Compression Molding 1.39 ± 0.49 8.70 ± 1.07 2.14 ± 0.60 Solvent-Casting Method 1.00 ± 0.20 8.20 ± 0.31 2.04 ± 0.27

0.63 ± 0.17 10.80 ± 1.51 11.10 ± 2.76

0.10 ± 0.03 1.39 ± 0.04 0.50 ± 0.05

S × 103 (cm3(STP)·cm−3·cmHg−1) D × 108 (cm2·s−1) P (Barrer) S × 10 (cm3(STP)·cm−3·cmHg−1) D × 108 (cm2·s−1) P (Barrer)a

3

N2 gas

Table 5. Gas Permeation Parameters for the Polyester Films Tested

CO2 gas

3

S × 10 (cm3(STP)·cm−3·cmHg−1)

P (Barrer)

O2 gas

D × 108 (cm2·s−1)


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The Journal of Physical Chemistry C increase of crystallinity, the mobility of the amorphous phase is restrained, which thus limits the solubilization of gas molecules. In view of these results, it clearly appears that the biopolyester films, PHB polymer and its PHBV copolymer, exhibit interesting gas barrier properties and are considered to be biodegradable polymers with potential use in food packaging in terms of barrier performances.

ACKNOWLEDGMENTS

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REFERENCES

Creagif Biopolymères has provided valuable financial support and various native PHA pellets. The authors gratefully acknowledge F. Cuvilly for his help with X-ray diffraction analysis (UMR CNRS 6634, University of Rouen, France) and Dr. N. Kebir with GPC experiments (UMR 6270 & FR 3038 CNRS − INSA Rouen, France). The authors are thankful to Nicolas Jouen (University of Rouen) for the helpful work done during his training course.

4. CONCLUSION The structure of polyester films made from commercial PHA polymers and their barrier properties against small molecules were investigated through the usual physical characterizations: measurements of water vapor sorption and permeation to water and gas molecules. The presence of additives in the Biomer formulations was evidenced from TGA and DSC analyses, while no additive was detected in the Y1000P pellets. The crystallization process was found to occur at a lower temperature for the Y1000P polymer than for the Biomer formulations. Indeed, the melting phenomenon for films was detected at similar temperatures to those for the PHA pellets, but with some variations linked to more or less ordered crystalline phases. Moreover, it can be suggested that the impact of the film-forming process on the thermal features displayed by the PHA films is negligible. In terms of water barrier properties, the Biomer films seem to be less efficient concerning moisture resistance, compared with the PHB3 V films. The water mass gain increased. This result was obtained to a greater extent with the PHA films prepared by the solvent-casting method. The polymer structure was thus more easily opened and plasticized by water molecules, which act as mobility enhancers during the water sorption. However, the water mass gain was found to be under 2% for the whole range of water activities, indicating that the PHA films can be considered as nonpolar polymers. Concerning the water permeation properties, the film with the highest crystallinity has exhibited the lowest water permeability due to the tortuosity effects induced by the crystalline phase. Again, in comparison with the compressed films, higher water permeability was found for the cast films, facilitated by the additional free volume created during processing. In all cases, the dependence of water diffusivity on water concentration was evidenced. In terms of gas barrier properties, a detailed analysis showed that the permeation parameter ranking was governed by the nature and size of the diffusing probes. When the crystallinity degree of the polymers increased the P, D, and S coefficients decreased as follows: PP209 film > PP226 film > PY1000P film for P coefficient. Additional free volume and spaces between polymer chains in the cast films favored the transfer of gas molecules. However, although the cast films were more permeable than the compressed films with a similar parameter ranking, their permeability was lower than that of nonpolar polymer films. Because of their good moisture resistance and high gas barrier properties, PHB and its PHBV copolymer appear to be very promising biopolymers for future ecological packaging.

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*Tel: + 33 235146698. Fax: + 33 235146704. E-mail: nadege. [email protected]. Notes

The authors declare no competing financial interest. 6175



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