Tiphaine Messin

Nadège Follain

Alain Guinault

Guillaume Miquelard-Garnier

Cyrille Sollogoub

Nicolas Delpouve

Valérie Gaucher

Stéphane Marais
email: stephane.marais@univ-rouen.fr

Confinement effect in PC/MXD6 multilayer films: Impact of the microlayered structure on water and gas barrier properties

Keywords:

Films of PC and MXD6 were prepared via multilayer coextrusion. Films presenting stretched nodules/lamellas of MXD6 in the PC matrix, and films with quasi-continuous thin MXD6 layers alternating with PC layers were obtained.

The transport properties were specifically investigated from water and gas permeation kinetics, and the corresponding permeation parameters were determined. The confinement effect of MXD6 in the multilayer structure was highlighted by comparing the permeability predicted from the series model equation with the experimental data. Although the MXD6 layers were not homogeneous and continuous for all membranes, water and gas permeabilities of the PC/MXD6 multilayer film were largely reduced. Such improvement of barrier properties was related to the change of the MXD6 structure, considering that the confinement of the MXD6 layers induced by PC layers allowed to bring chain orientation and to reduce the chain segment mobility in the MXD6 amorphous constrained zones. In addition, in the case of a PC/MXD6 multilayer structure well defined with continuous layers and relatively homogeneous thicknesses, the degree of crystallinity was slightly increased.

Introduction

Polymer films find many applications in several fields such as packaging, medicine, cosmetic, coating, aeronautic or transport. Some key property requirements in these fields are good transparency, toughness, mechanical, thermal properties as well as barrier properties. To obtain such properties, blending or combining two (or more) polymers has been a classical top down approach, over many decades, to achieve polymer structures with properties larger than those of the individual components. Coextrusion process remains one of the most common industrial technique for elaborating polymer films consisting in a few (typically < 10) stacked layers of different polymers with usually opposite technical characteristics (protective coating, filmforming properties, barrier properties, food contact, etc.), possibly combined with a tie layer or a compatibilizer [START_REF] Dooley | Encyclopedia of Polymer Science and Technology[END_REF][START_REF] Lamnawar | Coextrusion of Multilayer Structures, Interfacial Phenomena[END_REF].

Recently, it has been reported that an innovative technique of film processing, namely multilayer coextrusion, could be used to obtain materials with enhanced macroscopic properties [START_REF] Ponting | Polymer nanostructures by forced assembly: process, structure, and properties[END_REF]. Multilayer coextrusion is derived from classical coextrusion and its industrial potential has already been demonstrated by Dow which patented it close to 40 years ago [START_REF] Schrenk | Method for Multilayer Coextrusion[END_REF][START_REF] Schrenk | Interfacial flow instability in multilayer coextrusion[END_REF]. With this process, starting from two (AB) or 3 layers (ABA) of polymers A and B, and by forcing the polymer flows through a series of Layer Multiplying Elements (LME), one can achieve films made of thousands of alternating A and B layers, in which all the layers have nanometric thicknesses (see Fig. 1) while the total thickness of the material remains those of a typical polymer film (0.1-1 mm). Baer's group in Case Western Reserve University widely used and developed this tool over the last 20 years and was able to produce materials with enhanced properties, in particular optical properties [START_REF] Kazmierczak | Polymeric one-dimensional photonic crystals by continuous coextrusion[END_REF], mechanical properties [START_REF] Ebeling | Delamination failure mechanisms in microlayers of polycarbonate and poly (styrene-co-acrylonitrile)[END_REF][START_REF] Kerns | Mechanical behavior of polymer microlayers[END_REF], gas barrier properties [START_REF] Lin | A new method for achieving nanoscale reinforcement of biaxially oriented polypropylene film[END_REF][START_REF] Wang | Confined crystallization of polyethylene oxide in nanolayer assemblies[END_REF][START_REF] Zhang | The effect of confined spherulite morphology of high-density polyethylene and polypropylene on their gas barrier properties in multilayered film systems[END_REF]. Those improvements were shown to arise from the multiplication of the interfaces and/or the confinement, induced by the process (named 'forced-assembly').

In particular, for semi-crystalline polymers, this innovative technique has been recognized as an effective way to induce one-dimensional confined crystallization of polymers [START_REF] Carr | Confined crystallization in polymer nanolayered films: a review[END_REF]: when the polymer layer thickness decreases, the crystalline morphology is gradually altered from a three-dimensional spherulitic morphology into one-dimensional crystalline lamellae. For example, in the case of poly(ethylene oxide) (PEO) [START_REF] Wang | Crystallization kinetics of poly(ethylene oxide) in confined nanolayers[END_REF] or poly(ε-caprolactone) (PCL) [START_REF] Ponting | Effect of substrate on the isothermal crystallization kinetics of confined poly (ε-caprolactone) nanolayers[END_REF], confined with an amorphous polymer (typically PS), an in-plane orientation of crystalline lamellae of PEO or PCL have been obtained. In those cases, a significant reduce of gas permeability has been measured (more than two orders of magnitude reduction). For other polymers, it happens that confinement can favor other crystal morphologies or orientation [START_REF] Carr | Confined crystallization in polymer nanolayered films: a review[END_REF][START_REF] Mackey | Confined crystallization of PVDF and a PVDF-TFE copolymer in nanolayered films[END_REF]. In such cases, permeability is not improved (or even may be worsened) but post-processing procedures, thermal annealing possibly combined with (biaxial) stretching, may allow to provide the desired crystal morphologies.

Geometric confinement may also have an effect on the amorphous phase, inducing chain orientation or modifying the amorphous phase dynamics. For example, molecular mobility changes have been observed in PC/PMMA multilayered films [START_REF] Arabeche | Study of the cooperativity at the glass transition temperature in PC/ PMMA multilayered films: influence of thickness reduction from macro-to nanoscale[END_REF], as the thickness of each component became thinner than 125 nm: PC exhibited a drastic decrease in cooperativity volume (measured with TM-DSC) whereas slight modifications were observed for PMMA.

Poly(m-xylene adipamide) (MXD6) is an aromatic polyamide with excellent barrier properties due to its benzene ring. It was first synthesized in the 50's, then produced industrially in the 80's by Mitsubishi using direct polycondensation [START_REF] Ammala | Multifunctional and Nanoreinforced Polymers for Food Packaging[END_REF], and is classically blended with PET for food packaging applications [START_REF] Hu | Improving gas barrier of PET by blending with aromatic polyamides[END_REF][START_REF] Ben Doudou | Relationship between draw ratio and straininduced crystallinity in uniaxially hot-drawn PET MXD6 films[END_REF]. MXD6, that crystallizes slowly [START_REF] Ben Doudou | Crystallization and melting behaviour of poly (m-xylene adipamide)[END_REF], is usually amorphous when extruded and needs post-processing treatment, such as thermal annealing or hot drawing, to induce crystallization and thus improve its gas transport properties. MXD6 also has ability to crystallize in contact with water, which will impact its barrier properties over time and/or exposure with water [START_REF] Ben Doudou | Barrier properties and microstructure modifications induced by liquid water for a semiaromatic polyamide[END_REF][START_REF] Hu | Effect of water sorption on oxygen-barrier properties of aromatic polyamides[END_REF][START_REF] Buquet | Permeation properties of poly(m-xylene adipamide) membranes[END_REF]. To date no attempt of using multilayer coextrusion to achieve thin layers of MXD6 and study the impact of this confinement on the barrier properties has been published in the literature. As it was exposed before, geometric confinement may have effect on the crystalline phase as well as on the amorphous one, and thus impact barrier properties.

In some cases it has been observed that this process is subject to instabilities which give rises to rupture of the layers [START_REF] Sollogoub | Instabilities in a layer-multiplying device[END_REF][START_REF] Zhu | Kinetics of thin polymer film rupture: model experiments for a better understanding of layer breakups in the multilayer coextrusion process[END_REF], especially with complex polymers like MXD6. However, if the structures created are still under the form of elongated lamellas, they may still be of interest for gas barrier applications [START_REF] Boufarguine | PLA/PHBV films with improved mechanical and gas barrier properties[END_REF].

In this context, we prepared in this study films with alternating polycarbonate (PC) and MXD6 layers, composed of numerous thin layers of MXD6. PC was chosen since it is amorphous and has a high glass transition temperature which should favor the confinement of the MXD6 and the understanding of the resulting phenomena. Quasicontinuous layers as well as elongated lamellae were obtained, depending on the number of LME used. A structural characterization was conducted on the different films to evaluate the influence of the resulting multilayer structure on crystallinity, crystal orientation and lamella size. A comparison was done with films obtained by the classical blending of the two polymers. Again, for the sake of comparison, monolayer films of MXD6 and of PC were also extruded to be used as references in order to highlight the properties of the multilayered structures and the corresponding changes. Moreover, for a better insight of the confinement effect induced by amorphous polymer layers, multilayer films containing only the amorphous polymer, noted PC/PC multilayer film, and only the semi-crystalline polymer, noted MXD6/MXD6 multilayer film, were prepared. Transport properties of gases and water were analyzed and compared to the PC monolayer and MXD6 monolayer films. The structuremorphology-property relationships will be discussed in the present study. In particular, a deep analysis of the transport properties, on the basis of thermodynamic and kinetic aspects, has been performed by testing four molecular probes (N 2 , O 2 , CO 2 , H 2 O), differing in size and interaction capacity. From permeation kinetics and the resulting permeation parameters, the structural effects within the multilayer films were clearly correlated to barrier properties.

Experimental

Materials

Amorphous polycarbonate, referenced as LEXAN 121R (Tg=145 °C) was supplied by SABIC (Saudi Arabia). Semi-crystalline aromatic poly(m-xylene adipamide), MXD6 grade 6007 (Tg=85 °C) was obtained from Mitsubishi Gas Chemical. They were chosen to have viscosity ratio as close as possible to 1. At a processing temperature of 260 °C and at shear rates equivalent of those imposed in the extruders (1-100 s -1 ), the viscosities of PC and of MXD6 polymers were measured equal to 822 and 1690 Pa s, respectively, which induces a viscosity ratio of 2. The PC and MXD6 pellets were dried at 120 °C overnight before processing. The residual moisture content was measured and found to be less than 0.02% for PC and 0.1% for MXD6.

Films preparation

The layer-multiplying coextrusion process consists of two singlescrew Scamex extruders (France) combined in a classical A/B/A coextrusion feed block, to obtain a 3-layers polymer flow, and equipped with a layer-multiplying elements-device placed at the end of the feed block (Fig. 1). The device aims to cut vertically the 3-layers flow into two parts, which are thereafter superposed and compressed in order to obtain a 5-layers flow. The number of layers was evenly calculated with the following Eq. ( 1) [START_REF] Li | Forced assembly by multilayer coextrusion to create oriented grapheme reinforced polymer nanocomposites[END_REF]:

Number of layers N = 2 +1 n ( +1) (1)
With n is the number of multiplying elements. Two series of PC/MXD6 films were prepared with two specific processing conditions with the aim of adjusting the composition ratio and the number of layers within the films.

For the first series at a 75/25 ratio, noted PC/MXD6-1 film, the multilayer film was coextruded (extruder 1: Scamex, 20 mm diameter, speed at 48 rpm with a temperature profile of 310/280/260/260 °C; extruder 2: Scamex, 20 mm diameter, speed at 60 rpm with a temperature profile of 310/270 °C) with a total polymer flow fixed at 2400g/h, to prevent PC bubbles initiated by atmosphere humidity, by using a 10 multiplying elements-device (to form theoretically 2049 layers within the film) at 265 °C. The melt flow was spread through the flat die at 260 °C and then onto the chill roll at 130 °C.

For the second series at a 80/20 ratio, noted PC/MXD6-2 film, the coextrusion process (extruder 1: Scamex, speed at 26 rpm with a 310/ 280/260/260 °C profile; extruder 2: speed at 31 rpm with a 310/ 260 °C profile) was performed using a 9 multiplying elements-device (giving film with 1025 layers) at 245 °C. Indeed, the decision to reduce the number of mixing elements was taken since layer break-up was observed when using 10 multiplying elements. The melt flow was spread through the flat die at 240 °C and then onto the chill roll at 130 °C.

The multilayer films with an average thickness of 200 µm were selected and were stored at room temperature in desiccators (under vacuum with P 2 O 5 ) for further characterizations.

Morphological characterization

Films were observed with an optical transmission microscope (Axio Imager.a2m, Zeiss) under polarized light to differentiate the two phases. The grey phase corresponds to PC polymer and the dark phase to MXD6 polymer. Samples were prepared by using a microtome (Leica RM2255 microtome) to obtain a 10-20 µm thickness sample for observation.

Wide Angle X-ray Scattering (WAXS) experiments were performed using a Genix microsource X-ray generator operated at 50 kV and 1 mA. The Cu-Kα radiation (λ=1.54 Å) was collimated with a FOX2D mirror and two pairs of Scatterless slits from Xenocs. The 2D-patterns were recorded on a CCD camera from Photonic Science. All the X-ray patterns were corrected from background scattering and normalized using the transmission factor, defined as the ratio between the transmitted and the incident intensities measured with WAXS data acquisition. Radial intensity profiles I(2θ) are obtained by azimuthal integration of the 2D-patterns using FIT2D software.

Thermal properties (TGA, DSC)

Thermal properties of the neat films and the multilayer films were analyzed by thermogravimetric analyses (TGA), by classical differential scanning calorimetry (DSC) and by modulated-temperature DSC (MT-DSC).

TGA analyses were performed with a Q500 TGA from TA Instrument, with a heating rate of 10 °C/min from 30 to 600 °C under nitrogen. The degradation temperature (T deg ) was determined for a loss of 5% by mass of the sample.

DSC experiments were performed on around 6-7 mg of sample with a Discovery series Differential Scanning Calorimeter from TA Instruments at a heating/cooling rate of 10 °C/min from -10 to 300 °C in aluminum pan, after temperature calibration using indium standard. The degree of crystallinity was calculated by:

Xc Hm Hc H = ∆ -∆ ∆ *100 m 0 (2)
Where Hm ∆ is the enthalpy of melting, Hc ∆ is the enthalpy of crystallization and H ∆ m 0 is the theoretical enthalpy of melting of the 100% crystalline polymer. For the MXD6, H ∆ m 0 is equal to 175 J/g [START_REF] Ben Doudou | Barrier properties and microstructure modifications induced by liquid water for a semiaromatic polyamide[END_REF].

The degree of crystallinity was determined from the first heating step because the value is supposed to be the crystallinity of the films when analyzing by permeation measurements. MT-DSC experiments were performed with a DSC Q2000 from TA Instruments, in "Heat-Only" mode, from 0 to 270 °C ( ± 0.318 °C for the oscillation amplitude, 60 s for the oscillation period and 2 °C min -1 as heating rate).

Mechanical properties

Uniaxial tensile tests were performed at room temperature on an Instron 5543 traction machine with a 500 N sensor. The samples were dumbbell-shaped with dimensions of 30 in length×4 mm in width under 0.20 mm of thickness. A crosshead speed was fixed at 5 mm/min in order to compare with values of MXD6 film already published in a separate paper [START_REF] Fereydoon | Rheological, crystal structure, barrier, and mechanical properties of PA6 and MXD6 nanocomposite films[END_REF]. The stress-strain curves were plotted and the tensile modulus, also noted Young's modulus was deduced from the low strain region. At least ten specimens per film were tested and the mean values were reported for ultimate mechanical properties, i.e. nominal elongation and nominal strength.

Transport properties 2.6.1. Gas permeation

Gas permeation measurements were performed at 25 °C with a labbuilt apparatus [START_REF] Joly | Residual solvent effect on the permeation properties of fluorinated polyimide films[END_REF]. The film was placed in the permeation cell composed of the upstream and the downstream parts, in which vacuum was applied for 15 h. Then, the upstream part compartment was fed with gas (N 2 , O 2 , or CO 2 at a pressure of 4 bars), while a pressure sensor monitored the increasing of pressure in the downstream part. This barometric method called "time-lag" method gives access to permeability coefficient P, expressed in Barrer (1 Barrer=10 -10 cm 3 (STP) cm cm -2 s -1 cmHg -1 ), by measuring the quan-tity of diffusing molecules which passed through the film as a function of time, according to:

P Jst L p = * ∆ (3)
With Jst is the stationary flux, L is the film's thickness and p ∆ is the difference of pressure between the upstream and the downstream parts of the permeation cell.

The stationary flux Jst is calculated from the slop α of the kinetic curve at long time:

Jst α V A R T = * * * (4)
With V is the volume of the downstream compartment, R is the ideal gas constant, A is the surface of film exposed to gas and T is the experimental temperature.

For the sake of comparison, a predicted permeability coefficient was calculated from two different approaches. One on the basis of the Maxwell model [START_REF] Nielsen | Predicting the Properties of Mixtures[END_REF][START_REF] Sadeghi | Modification of existing permeation models of mixed matrix membranes filled with porous particles for gas separation[END_REF][START_REF] Sadeghi | New permeation model for mixed matrix membrane with porous particles[END_REF] taking into account a two-phase mixtures with one continuous phase (PC) and a disperse phase (MXD6) as follows:

P P φ φ = * 1 + 2 (λ -1)/(λ + 2) 1 - (λ -1)/(λ + 2) Film PC MXD6 MXD6 (5)
where P Film is the permeability coefficient of the film of two phase mixture, PPC is the permeability of the continuous phase (PC), φ MXD6 is the volume fraction of the dispersed phase (MXD6) and λ is the relative permeability of the dispersed phase into the continuous phase (λ P P = / MXD6 PC ). And the other one, corresponding to the ideal case where the two phase mixture is composed of infinite lamellae and that can be defined as a multilayer structure for which each layer is a lamella, the wellknown series model [START_REF] Zhang | The effect of confined spherulite morphology of high-density polyethylene and polypropylene on their gas barrier properties in multilayered film systems[END_REF][START_REF] Gupta | Structure and gas barrier properties of poly(propylene-graft-maleic anhydride)/phosphate glass composites prepared by microlayer coextrusion[END_REF][START_REF] Prattipati | Effect of compatibilization on the oxygen-barrier properties of poly(ethylene terephthalate)/poly(m-xylylene adipamide) blends[END_REF][START_REF] Wang | Confined crystallization of PEO in nanolayered films impacting structure and oxygen permeability[END_REF] can be considered as follows:

P φ P φ P 1 = + Film MXD6 MXD6 PC PC ( 6
)
With φi is the volume fraction and Pi is the permeability coefficient, the index i corresponds to polymers, PC and MXD6.

Water permeation

Water permeation measurements were performed at 25 °C with a lab-made apparatus composed of a permeation cell in which the film separates the upside from the downside compartments. The two parts of the permeation cell were swept with dry nitrogen until a low constant dew point (≈-70 °C), monitored by a chilled mirror hygrometer (Elcowa®, France, General Eastern Instruments), is reached. Then, the upstream compartment was filled with liquid water (milli Q) and the downstream compartment was continuously swept by a nitrogen flux. The permeation flux of water passing through the film was measured by following the variation of the dew-point temperature as a function of time. At the stationary state, the permeation flux Jst is directly proportional to the permeability coefficient P (expressed in Barrer) according to:

P Jst L a = * ∆ ( 7
)
With Jst is the stationary flux and a ∆ is the difference of water activity between the upstream and the downstream compartments of the permeation cell ( a ∆ =1 in our case).

Results and discussion

It is generally accepted that permeation properties of polymer membranes are directly dependent on the structure of the polymer assembly resulting from the processing used and also on the ability of polymer chains to crystallize into semi-crystalline phase and/or to be oriented. Recently, Wang et al. [START_REF] Wang | Confined crystallization of polyethylene oxide in nanolayer assemblies[END_REF] have discovered a new crystalline morphology, resulting from constrained two-dimensional polymer crystallization and induced by forced assembly of a specific coextrusion process, which presents outstanding gas permeation properties superior, than those expected from the bulk polymers, due to the specific lamellar crystal orientation within the layered film. In the present work, it was really interesting to see how the confinement of MXD6 intended within the PC/MXD6 films could change the structure of the film and the resulting transport properties. Therefore, to evidence the properties of the confined MXD6 layers within the PC/MXD6 films, DSC and permeation measurements were performed on the PC monolayer and MXD6 monolayer films, also called the neat films, to serve as reference and likewise on PC/PC and MXD6/MXD6 multilayer films to evaluate the level of improvement in barrier properties.

Structure and morphology

The Polarized Optical microscopy (POM) observations were carried out to show the existence of a continuous alternating layer structure within the multilayered films and to highlight eventually a difference of crystallinity of the layers with the film thickness by means of polarized light. For the MXD6/MXD6 film, no apparent layered structure was observed in the cross-section. However, even if the structure of this film seems homogeneous in the entire thickness, it is possible that the confined layers of MXD6 (at least 1000 MXD6 confined layers with 10 multiplying elements) in such multilayer MXD6 structure, can lead to changes in permeation properties, compared to the neat MXD6 film. Concerning the PC/PC multilayer film, it was not possible to clearly observe the multilayer structure, because of its amorphous state. However, at a macroscopic scale, this multilayer film seems to be homogeneous.

From the POM image shown in Fig. 2, the PC/MXD6-1 film can be considered as a membrane made of a polymer blend with more or less stretched nodules of MXD6.

For the PC/MXD6-2 film, the layer continuity is evidenced in the middle of the film (except for the film edges). In addition, the layers seem to be thin but with some irregularities, in particular in thickness. The measured layer thickness of the film imaged varies from about 0.5-5 µm for MXD6 (in black) and PC (in grey) layers, respectively, while it was expected for theoretical thicknesses equal to 0.08 µm and 0.32 µm respectively for MXD6 and PC. It is interesting to see that the quality of the structure of the PC/MXD6 multilayer film is strongly dependent on the process conditions which are slightly changed. With a 9 multiplying elements-device, a continuous layer structure is obtained, while with the use of a 10 multiplying elements-device the layers are disrupted.

Wide Angle X-ray Scattering (WAXS) measurements were performed to detect changes in the crystalline structure of the different multilayer films, compared with monolayer films. Concerning the PC monolayer and PC/PC multilayer films, from the WAXS diffractograms (Fig. 3) and also as expected, an amorphous halo was observed, reflecting an amorphous structure. The same observation was made for the MXD6 in monolayer and multilayer forms due to a very low crystallinity rate. Indeed, it is worth noting that the limit of detection being around 5% to detect a crystalline structure from XRD data; it seems appropriate that no crystalline structure is observed. Concerning the two PC/MXD6 multilayers films, an amorphous halo was also obtained, similar to those observed for the neat monolayer films, meaning that no change in the MXD6 crystalline structure was generated by the use of the layer-multiplying coextrusion process, certainly due to the very low crystallinity rate of MXD6.

Thermal and mechanical properties

DSC measurements were performed to analyze the thermal behavior of the neat and the multilayer films and also to determine the degree of crystallinity. The DSC curves from the first heat are plotted in Fig. 4 and the characteristic temperatures (glass transition temperature Tg, melting temperature Tm, crystallization temperatures Tc) are gathered in Table 1. The first heat was considered because it is related to the thermal and structure properties of the material in the same state its barrier properties are characterized. It can be mentioned that the glass transition temperature of PC being relatively close to the cold crystallization temperature of MXD6 (110-130 °C), MT-DSC experiments were used to separate the two corresponding phenomena occurring in the PC/MXD6 multilayer films. Evenly, tensile tests were performed and characteristic parameters were obtained from the stress-strain curves.

It is worth noting that the thermal and mechanical behaviors (Table 1) of the PC monolayer film are consistent with the literature [START_REF] Feng | Effects of organoclay modifiers on the flammability, thermal and mechanical properties of polycarbonate nanocomposites filled with a phosphate and organoclays[END_REF][START_REF] Weibin | The effects of hydrothermal aging on properties and structure of bisphenol A polycarbonate[END_REF][START_REF] Zhou | The effect of crystallization on hydrolytic stability of polycarbonate[END_REF]. In addition, the behavior of the PC/PC multilayer film is quite similar to the behavior of the PC monolayer film, except for the degradation temperature, which is found lower. This result could be due to the preparation process, which can slightly reduce the average chain length, as generally observed when using conventional processing technique [START_REF] Canevarolo | Chain scission distribution function for polypropylene degradation during multiple extrusions[END_REF]. Besides, the similar thermal and mechanical values between the PC/PC films and the PC film suggest that the PC amorphous polymer chains were not constrained, indicating the absence of confinement effect of the PC layers within the obtained multilayer structure.

Then the evaluation of the confinement of MXD6 in the multilayer structure was performed from the MXD6 monolayer film and the MXD6/MXD6 multilayer film. The thermal and mechanical characteristics of the MXD6 monolayer film are in good agreement with those reported in the literature [START_REF] Hu | Improving gas barrier of PET by blending with aromatic polyamides[END_REF][START_REF] Ben Doudou | Relationship between draw ratio and straininduced crystallinity in uniaxially hot-drawn PET MXD6 films[END_REF][START_REF] Fereydoon | Rheological, crystal structure, barrier, and mechanical properties of PA6 and MXD6 nanocomposite films[END_REF], except for the glass transition temperature for which we found two glass transition temperatures, the first one at around 50 °C and a second one, located more classically at 85 °C. This unexpected low glass transition temperature was probably due to the extrusion processing conditions and certainly to the presence of the residual water within MXD6 pellets and that leads to the plasticization phenomenon. In fact, during the second heat in DSC experiments, only one Tg was measured at 85 °C, as observed in literature. As for PC and taking into account the error measurements, the MXD6/MXD6 multilayer film presents globally similar mechanical and thermal properties, compared with the MXD6 monolayer film. With our processing conditions, the monolayer MXD6 film as well as the multilayer MXD6-MXD6 and PC-MXD6 films present semi-crystalline MXD6 layers, with the degrees of crystallinity very low, varying from 3% to 8% (see Table 1). These results are in accordance with the XRD data indicating a rather amorphous state of the MXD6 monolayer and of the multilayers. The fact that a very low degree of crystallinity is revealed by DSC and not by XRD is due to the shortcomings by both techniques to calculate the degree of crystallinity. By XRD measurements, results have been obtained in one direction at the film surface for a limited thickness. The presence of crystallites at the film surface tends to deflect the X-ray penetration, which made difficult to determine the crystallinity of the entire film sample. In addition, the presence of crystallites, with small sizes, contributes to broadening the XRD peaks, leading to uncertainties. From DSC results, a slight increase of Tg and a decrease of the degree of crystallinity is obtained for the MXD6/MXD6 multilayer film, compared to the MXD6 monolayer film. This result could be explained by an effect of confinement of MXD6, which can lead to reduce the chain mobility, and evenly to reduce partially the crystallization phenomenon to be occurred. The inherent properties of MXD6, as obtained when processing the monolayer structure, were maintained in the multilayered structure. The formation of layers within the film did not weaken it and the thermo-mechanical properties are practically constant.

Concerning the PC/MXD6 multilayer films, same characteristic temperatures to the monolayer films were measured, except for the MXD6 crystallization temperature, which increases when in contact with the PC within the multilayer films. The increase of Tc reflects a strain-induced crystallization of the MXD6 by the PC layers. At this stage of discussion, it is worth mentioning that the degree of crystallinity, given in Table 1, originated from semi-crystalline MXD6, the values for both PC/MXD6 multilayer films were calculated by taking into account the weight fraction of MXD6 within the multilayer films. As shown in Table 1, a slight increase of the degree of crystallinity (8.2% vs 5.5%) was measured for the PC/MXD6-2 film, compared to the PC/MXD6-1 film and the MXD6 monolayer film. This increase could be not negligible as the weight fraction of MXD6 in the PC/ MXD6 multilayer film is rather low (20%). It can be explained by the presence of the continuous layers and by the confinement effect of the MXD6 by the PC, contrary to the MXD6/MXD6 multilayer film, for which the reduction of crystallinity is quite surprising. The thermal and mechanical behaviors are found similar for the two multilayer films, except for the Young's modulus, which is higher for the PC/MXD6-1 multilayer film, due to a higher weight fraction of MXD6 (25 wt% instead of 20 wt%). The MT-DSC curves of the PC/MXD6 multilayer films are reported in Fig. 4b. From the MT-DSC curves, a slight change in slope of the Inphase Cp can be observed at around 85 °C that was not observed with classical DSC experiments. An enlarged view is also reported to evidence the change in slope, which shows that the two Tg relative respectively to a fraction of dry amorphous phase and a fraction of water-plasticized amorphous phase coexist also in PC-MXD6 multilayer films as in MXD6 single layer film.

Barrier properties 3.3.1. Transport properties of the PC films

Barrier properties of the films were examined through gas and water permeation measurements. In addition to water as permeant, three gas molecules (N 2 , CO 2 and O 2 ), differing in diameter and critical temperature, were selected to highlight the change in barrier properties of the multilayer films, compared to the non-structured membranes, the reference films.

The permeability coefficients for the PC monolayer film and PC/PC multilayer films are gathered in Table 2. Similar values between the two films and those of literature [START_REF] Kim | Processing-property relationships of polycarbonate/graphene composites[END_REF][START_REF] Liu | Forced assembly of polymer nanolayers thinner than the interphase[END_REF][START_REF] Moon | Water vapor permeation resistance of polycarbonate at various temperatures[END_REF][START_REF] Moon | Outgassing of oxygen from polycarbonate[END_REF][START_REF] Shih-Hsiung | Gas sorption and transport properties of the membrane from polycarbonate/CoIII acetylacetonate blend[END_REF] are obtained, whatever the gas tested. Moreover, as stated by Van Krevelen [START_REF] Van Krevelen | Properties of Polymers[END_REF], the well-known tendency in the comparison of gas permeability ranking is observed: P CO2 > P O2 > P N2 . This ranking is the result of the double dependence of permeability with diffusivity, which depends mainly on the dynamic diameter of permeants, and with solubility, which depends on the critical temperature of permeants indicative of the ability of a molecule to be condensed in the material. In other words, the more the dynamic diameter is small, the more the diffusion is easier, and as a result the permeability increases. Otherwise, the higher critical temperature of CO 2 (31.15 °C), than those of N 2 (-146.94 °C) and O 2 (-118.56 °C) also explains the greater permeability to carbon dioxide.

Concerning water as permeant, the reduced water permeation fluxes are plotted as a function of the reduced time scale, to overcome the thickness effect, as shown in Fig. 5, and the permeability coefficients are deduced. Same permeability coefficients were obtained between the PC monolayer and the PC/PC multilayer films, but the diffusivity was found slightly lower for the PC/PC multilayer film (1.7×10 -8 cm 2 s -1 ), compared to the monolayer film (2.4×10 -8 cm 2 s -1 ), reflecting in the time-scale shift of the permeation flux. From results, it can be stated that no noticeable difference exists between the two films, despite the different techniques, which indicates that the confinement of amorphous chains of PC polymer has no effect on permeation properties towards water and gases.

Transport properties of the MXD6 films

The values of gas permeability coefficients obtained for the single layer MXD6 films are: P CO2 =P O2 =P N2 =0.03 Barrer (Table 3). These values are strongly higher than those usually given in literature (oxygen permeability 0.003-0.004 Barrer at 0% RH [18, [46] and 0.008 [START_REF] Wang | Novel preparation method for enhancing nanoparticule dispersion and barrier properties of poly(ethylene terephthalate) and poly(mxylylene adipamide)[END_REF] and than those we already measured to CO 2 from another MXD6 (0.001 Barrer) [START_REF] Buquet | Permeation properties of poly(m-xylene adipamide) membranes[END_REF]. However, according to literature, it must be considered that the gas permeability of MXD6 is strongly dependent on its structure (crystallinity, orientation) but also on its hydration state. Indeed, it has been reported that the value of the oxygen permeability coefficient can increase of one order of magnitude with the increase of the relative humidity (for example from 0.1 cc.mm/m 2 /day/ atm at 0% RH to 2 cc.mm/m 2 /day/atm at 100% RH, from data of Mitsubishi). From that, our higher values of permeabilities could be explained by the presence of residual water molecules able to plastify the material (as shown by DSC measurements) and so increase its permeability due to the increase of the free volume. The preliminary drying step of MXD6 is probably not sufficient to remove all water molecules and some of them would be trapped because of strong interactions with amide groups and their confinement in the rigid structure of MXD6. We must also keep in mind that MXD6 has also a rapid water sorption capacity, so that additional water molecules can be sorbed in the MXD6 before measurement. This phenomenon of water retention is even more likely that the thickness of our samples is high (260 µm) in comparison with those in literature (20-80 µm) [START_REF] Hu | Improving gas barrier of PET by blending with aromatic polyamides[END_REF][START_REF] Buquet | Permeation properties of poly(m-xylene adipamide) membranes[END_REF][START_REF] Thellen | Co-extrusion of multilayer poly(m-xylylene adipimide) nanocomposite films for high oxygen barrier packaging applications[END_REF]. Thus it would have been necessary to force the drying step by increasing the temperature (higher than 100 °C) but with the risk of changing the crystalline phase of the material. If the values of the gas permeability coefficients are higher than that expected, they are still low testifying the high barrier level of MXD6 owing to its high rigid structure and the formation of hydrogen bonding between adjacent chains via amide functions, leading strong cohesion between polymer chains. The semi-crystalline structure of MXD6, even at low level of crystallinity in our case, also contribute to the barrier effect, crystals acting as impermeable obstacles to gases. But this low crystallinity degree can also explain the lower gas barrier properties measured on the single layer MXD6 films.

Surprisingly, compared to the MXD6 monolayer film, a decrease of all gas permeability coefficients was observed for the MXD6/MXD6 multilayer film (Table 3) despite a same degree of crystallinity. It seems that the layered structure increase the barrier behavior of the MXD6 polymer.

In the case of water, atypical reduced flux curves were obtained from both the single layer MXD6 films and MXD6/MXD6 multilayer films (Fig. 6). These particular water permeation behaviors are due to the well-known phenomenon called water-induced crystallization. To evidence a crystallization phenomenon induced by water occurring during permeation course, as already observed in previous works [START_REF] Buquet | Permeation properties of poly(m-xylene adipamide) membranes[END_REF], two subsequent permeation measurements for the same film were performed to compare water permeation behavior. Before the second measurement within the film, a drying in an oven at 80 °C during 24 h was applied to the tested film, knowing that this temperature is largely lower than that for which the MXD6 crystallization can occur (130 °C). Fig. 6 presents the thickness-corrected water fluxes, also called reduced water fluxes, as a function of the reduced time. Two curves per film were reported; the first one corresponds to the first measurement (noted first passage of water) applied to the film and the second one corresponds to the second measurement (noted second passage of water) applied to the same film but having overcame the water-induced crystallization during the first measurement. As shown in Fig. 6 and in agreement with a previous work [START_REF] Buquet | Permeation properties of poly(m-xylene adipamide) membranes[END_REF], the two curves for a film did not exhibit similar profile and the second curve conforms to a typical permeation curve without dependence of time. The first curve highlights the presence of a maximum water flux (noted P max ), from which a decrease as a function of the reduced time is obtained until the stationary flux (noted P stat ), corresponding to the steady state of permeation, is reached. This decrease is explained by the occurrence of the crystallization induced by water, with a gradient of water concentration between both sides of the film due to a non-symmetrical crystallization. The degree of crystallinity measured at the end of the first passage of water was significantly increased for both MXD6 films (increase of 24%), the single layer MXD6 film (from 5.5% to 30%) and the MXD6/MXD6 multilayer film (from 2.9% to 27%). The reduced water fluxes obtained after the second passage of water show that the steady state is obtained at lower values, meaning that the transfer of water is reduced. Again, this finding is the result of the crystallization phenomenon by water. Indeed, an increase in the delay time of diffusion is measured for the curve corresponding to the second passage of water, due to the larger presence of obstacles to diffusion, which is a higher crystallinity, improving tortuosity effects. The permeability coefficients were determined at the steady state of permeation, i.e. at the longest time of measurement.

From data gathered in Table 3, it can be noted that the water permeability of the MXD6/MXD6 multilayer film decreases, compared to the monolayer film, again due to the multilayered structure, as observed for the three tested gases. It seems that the confinement of the MXD6 polymer chains in lower spaces within the multilayer structure have modify the MXD6 structure at point to increase tortuosity, as revealed by the decrease of the water diffusivity (9.8×10 -8 cm 2 s -1 for the monolayer film and 6.0×10 -10 cm 2 s -1 for the multilayer film) related to the time-scale shift of curves. As the MXD6 crystallinity is practically unchanged or slightly reduced between both MXD6 films (30% vs 27%), an orientation of crystals in the confined MXD6 layers is expected to be at the origin of the increase of the diffusion pathways owing to the increase of tortuosity. In addition, considering that polymer chains within the amorphous phase are constrained by confinement effect due to the forced assembly induced by the coextrusion process, the reduction of the chain mobility should lead to decrease the diffusivity of water molecules.

Because the degree of crystallinity of the MXD6 film was largely increased after the first water passage (Table 3) due to the waterinduced crystallization phenomenon, the permeability to water after the second measurement was reduced (Table 3). During the second passage of water, for both films (monolayer and multilayer films), it can be seen now the typical permeation curve, without water induced crystallization, meaning that the transport properties of both films are now stabilized.

Also from water permeation results, it can be noted that barrier effect due to the confinement of MXD6 crystals within the multilayered film is assumed to be equivalent to an increase of ~25% of crystallinity, if we compare the permeability coefficients obtained after the second passage of water through the MXD6 monolayer film and after the first passage of water through the MXD6/MXD6 multilayer film. It is interesting to see that due to the confinement of MXD6 in multilayer form, the gas and water barrier properties are strongly increased. However, despite this increase in moisture resistance, the waterinduced crystallization stills occurs in the MXD6/MXD6 multilayer film, leading to similar increase of crystallinity (Table 3).

Transport properties of the PC/MXD6 multilayer films

Concerning the gas permeation, the values of the permeability coefficients (Table 4) for the two PC/MXD6 films are slightly lower for the first series compared to the second one as it contains more MXD6 (25% vs 20%). However, the gas permeability ranking complies with the works of Van Krevelen [START_REF] Van Krevelen | Properties of Polymers[END_REF] since the P CO2 > P O2 > P N2 tendency is still obtained.

To demonstrate the possible effect of the confinement effect of the MDX6 polymer, resulting to the processing of the multilayer structure, on the gas barrier properties, permeability values were calculated using the Maxwell model (Eq. ( 5)) and the series model (Eq. ( 6)), taken into account the volume fraction φ and the experimental permeability coefficient P of each polymer, and then confronted with the experi- mental values. As it can be seen in Table 4, a significant difference is observed between the experimental and calculated permeability values for the two films. In comparison with the values calculated with Maxwell equation, the experimental values are widely inferiors. Also, the experimental values are found significantly lower to those calculated with the series model equation for N 2 and O 2 gases while for CO 2 gas, no noticeable difference is observed. In fact, for CO 2 gas, the ratio of the permeability coefficients of the PC over the MXD6 is so high (P P PC/ MXD6=230) that the effect due to confined MXD6 layers within the multilayer structure becomes negligible. These permeation results show clearly that the elaborated systems have a structure closer to a multilayer than to a classical binary system.

For water permeation, the PC/MXD6-2 film globally present a lower value of the permeability coefficient compared to that of the PC/ MXD6-1 film, although the mass fraction of MXD6 is lower. This result can be attributed to the fact that irregular, but continuous MXD6 layers were formed within the PC/MXD6-2 film. Moreover, the experimental value is close to the calculated one for the PC/MXD6-2 film, whereas the experimental value is found higher for the PC/MXD6-1 film. This finding can be explained by the slight gap in crystallinity of MXD6 (series 1:5.5% and series 2:8.2%) between the two films.

As above-observed with the MXD6 films, when applying two subsequent passages of water through the tested film, the permeability and diffusivity coefficients were slightly decreased for both PC/MXD6 multilayer films after the second water permeation measurement (Fig. 7). This result is again related to the water-induced crystallization of MXD6 occurring during the first passage of water, which slows down the water transfer during the second passage by an increase in crystallinity. This increase of crystallinity was confirmed by DSC as the degree of crystallinity was practically two times higher after the first passage of water.

Assuming that only the semi-crystalline structure of the MXD6 can be at the origin of the confinement effect induced by forced assembly during multilayer structure processing, it became interesting to evaluate the improvement of the barrier properties of MXD6 layers by recalculating the permeability of MXD6 within the multilayer film. For this, from the experimental permeability coefficient obtained for the PC film and those for the multilayer films, and also taken into account the volume fractions of PC and MXD6 within the multilayer structures, the permeability coefficient of MXD6 was re-calculated as follows:

P φ = - P φ P MXD6 MXD6 1 Film PC PC (8)
The new calculated permeability values were compared with the experimental values (Table 5) to highlight how the confinement in a layered structure can influence the barrier effect and that can be quantified through the Barrier Improvement Factor (BIF factor, expressed in %). This useful metric to normalize and compare permeability results can be determined as the difference between the permeability in the MXD6 monolayer film permeability with the permeability in MXD6 layers within the PC/MXD6 multilayer film divided by the permeability of the MXD6 monolayer film. For N 2 and O 2 gases, both PC/MXD6 films were characterized by a similar BIF around 60%, indicating that barrier performances were largely improved due to the tortuosity induced by the layered structure. For CO 2 gas, the improvement factor is close to 0, meaning that no effect on permeability was obtained. This result can be explained by the high solubility of CO 2 gas resulting to both its low diameter, the lowest value considering the two other gases tested, and its high critical temperature, the highest value considering the two other gas temperatures. In this case, the permeability of CO 2 was not improved by the spatial organization in the layered films. Regarding water as permeant, the BIF for the PC/MXD6-2 film was determined equal to 13%. Again, the BIF factor reflects an enhanced permeability of water, which proves the positive influence of the presence of continuous MXD6 layers on barrier properties compared to a film prepared from a classical polymer blending. In the case of the PC/MXD6-1 film, the BIF was found negative, meaning that the water transfer is not reduced but inversely is increased due certainly to the non-homogeneous structure characterized by non-continuous layers and by stretched nodules that could create diffusion pathways for these plasticizing molecules.

From all permeation data, it results that the confinement of the semi-crystalline MXD6 in multilayer form by using the amorphous glassy PC as confiner layer leads to highly increase gas barrier properties of MXD6. It was also shown that water barrier properties are increased when the multilayer structure is well defined with continuous layer and relatively homogeneous thickness (PC/MXD6-2 film). In that case, the degree of crystallinity was accordingly increased.

Conclusion

By applying the forced assembly of the layer-multiplying coextrusion process, immiscible polymers can be together combined in unique film stack with ultrathin layers. Ultrathin layer polymer membranes present reduced dimensions, which confine polymer crystallization to sizes assumed to be of the order of magnitude close to individual lamellar crystals. However, in the present work, the difficulty of processing a homogeneous multilayer film with continuous and regular ultrathin layers, composed of semi-crystalline MXD6 polymer, can be partly explained by a poor compatibility between the two polymers MXD6 and PC, or polymer-polymer affinity at the interface between adjoining layers, but also resulted to experimental processing conditions through the temperature applied during the coextrusion process. Therefore, it has been showed that it is possible to create multilayer structure of MXD6 in MXD6 that shows that the co-extrusion multiplying process permits to generate multilayer structure also with miscible polymers. Indeed, to develop a confined environment at a nano-scale for polymer crystallization, it is assumed that the semicrystalline polymer has to be sandwiched between two layers of a rigid amorphous polymer presenting a high melting temperature. However, in our case, the confined environment is provided by the amorphous PC polymer with a processing temperature around 230 °C, whereas the melting temperature of MXD6 is around 250 °C. This slight difference, in addition to the processing effect and the lack of compatibility, can mainly explain the non-homogenous organization of MXD6 layers within the PC/MXD6 multilayer films.

Independently to form ultrathin layers within a multilayer film, one of the most promising effect mediated by the spatial confinement occurring in two dimensions concerns the possibility to preferentially orientate polymer crystals parallel to the layer. This kind of orientation is a key factor in the field of barrier applications to specifically improve the resistance to diffusive species. But from DSC analyses, it can be deduced that MXD6 was mainly in its amorphous state in monolayer as well as in multilayer films. The geometrical confinement does not lead to a sufficient increase of the crystallization rate permitting to hope crystalline orientation. From permeation data, it clearly appears that the transport properties of the MXD6 are improved in multilayer form, either of pure MXD6 (MXD6/MXD6) or associated with PC (PC/ MXD6), compared to the MXD6 monolayer film. The permeability coefficient is reduced, as that the initial permeability was already low. Unlike gas molecules, the water-induced crystallization of MXD6 within MXD6/MXD6 film is highlighted due to achieving permeation profile with maximum threshold during water permeation measurements. This implies a significant increase in crystallinity and accordingly a reduction in water permeability. This phenomenon is not occurred in the second measurement because of the increased degree of crystallinity which has reached a maximum at the steady state of the first permeation. Regarding the PC/MXD6 films, this phenomenon is prevented due to the protective role of the outer PC layers, irrespective of the layer continuity within the multilayer films. Again, low permeability coefficients are measured, despite the large fraction of the amorphous PC polymer. The mobility of polymer chains is reduced in amorphous constrained zones. The barrier improvement of the MXD6 layers was evaluated by applying the well-known series model used to re-calculate the MXD6 permeability from the experimental data. Through the BIF factor, irrespective of permeants, the films' performances are pointed out. The correlation between the permeation properties and the morphology provides evidence that the 2-D confinement induced by the multilayer process, and as a result the improvement of barrier performances. The coextrusion process with the possibility to multiply ultrathin layers is an innovative system to create a direct polymer assembly presenting new morphology with hundreds until thousands of alternating layers of polymers and can exhibit properties multiplied many-fold by the layer number in the assembly.

Fig. 1 .

 1 Fig. 1. Schematic principle of the coextrusion process with a multiplying elements device.

Fig. 2 .

 2 Fig. 2. POM images of the multilayers films (PC in grey, MXD6 in dark): a. and b. (higher magnification) the PC/MXD6-1 (75/25 wt%) c. and d. (higher magnification) the PC/MXD6-2 (80/20 wt%).

Fig. 3 .

 3 Fig. 3. WAXS diffractograms of the PC monolayer film, the MXD6 monolayer film and the PC/MXD6 multilayer films.

Fig. 4 .

 4 Fig. 4. a) DSC curves of the PC monolayer film, the MXD6 monolayer film and the PC/MXD6 multilayer films, and b) MT-DSC curves of the two PC/MXD6 multilayer films.

Fig. 5 .

 5 Fig. 5. Reduced water permeation curves for the PC monolayer and PC/PC multilayer films.

Fig. 6 .

 6 Fig. 6. Reduced water permeation curves for the MXD6 monolayer and MXD6/MXD6 multilayer films.

  2.2.1. Elaboration of monolayer films: PC monolayer and MXD6 monolayer filmsThe two monolayer films were directly prepared from a classical single-screw extruder (20 mm diameter Scamex extruders with a barrel of length-to-diameter ratio of 20), without feed block and multiplying elements-device, with a temperature profile of 310/280/260/260 °C from the hopper to the die and a screw speed of 40 rpm. The chill-roll temperature was adjusted to 130 °C and 90 °C for PC and MXD6, respectively. The final thicknesses of the films were respectively 200 µm for PC and 260 µm for MXD6 films.

	2.2.2. Elaboration of multilayer films made of one polymer: PC/PC
	multilayer and MXD6/MXD6 multilayer films
	Multilayer films at a 75/25 weight ratio were coextruded by using a
	series of 10 multiplying elements, to theoretically obtain 2049 layers-
	film, with a total polymer flow fixed at 1200g/h to prevent thermal
	polymer degradation by adjusting the residence time. The PC/PC film
	was prepared from two single-screw extruders (extruder 1: Scamex,
	20 mm diameter, 25 rpm with a temperature profile of 330/300/260/
	260 °C; extruder 2: Scamex, speed at 22 rpm with a temperature profile
	of 330/300 °C), with the multiplying elements-device at 270 °C. The
	films were then spread through a flat die (width of 100 mm and
	thickness of 1 mm) at 260 °C and onto the chill roll at 130 °C. The final
	thickness was 200 µm. For the MXD6/MXD6 film, the films were
	coextruded (extruder 1:19 rpm with a temperature profile of 310/280/
	260/260 °C; extruder 2: speed at 31 rpm with a temperature profile of
	310/270 °C), and were then spread through the flat die (width of
	100 mm and thickness of 1 mm) at 260 °C and onto the chill roll at
	90 °C to allow relaxation of MXD6. The final thickness was measured
	equal to 200 µm.
	2.2.3. Elaboration of multilayer films made of the two polymers: PC/
	MXD6 multilayer film

Table 1

 1 Thermal and mechanical properties.

	Thermal properties (°C)

Table 2

 2 Gas and liquid water permeation parameters for the PC monolayer and PC/PC multilayer films.

		Permeability coefficients (Barrer a )
		N 2	O 2	CO 2	H 2 O
	PC monolayer	0.32 ± 0.02	1.50 ± 0.02	7.00 ± 0.02	1730 ± 40
	PC/PC multilayer	0.33 ± 0.01	1.52 ± 0.01	7.02 ± 0.01	1780 ± 30

a 1 Barrer=10 -10 cm

3

(STP) cm cm -2 s -1 cmHg -1 .

Table 3

 3 Gas and liquid water permeation parameters for the MXD6 monolayer and the MXD6/MXD6 multilayer films. P max is calculated from the maximum of the flux curve. ** P stat is calculated from the stationary state of the flux curve.

		Permeability (Barrer)
		N 2	O 2	CO 2	H 2 O
					1st passage		2nd passage
					P max	*	P stat **	P stat
	MXD6 Monolayer	0.031 ± 0.002	0.029 ± 0.001	0.029 ± 0.002	1110 ± 30 (Xc MXD6 =5.5%)	640 ± 50 (Xc MXD6 =30%)	489 ± 25
	MXD6/MXD6 Multilayer	0.020 ± 0.004	0.015 ± 0.001	0.015 ± 0.001	710 ± 20 (Xc MXD6 =2.9%)	485 ± 20 (Xc MXD6 =27%)	347 ± 90

*

Table 4

 4 Gas and liquid water permeation parameters of the two PC/MXD6 films.

		Permeability (Barrer)
		N 2	O 2	CO 2	H 2 O
					First passage	Second passage
	PC/MXD6-1 (75/25)	0.044 ± 0.004	0.055 ± 0.003	0.112 ± 0.003	1450 ± 200 (Xc=5.5%)	1350 ± 90 (Xc=10.2%)
	Calculated permeability (Maxwell model, Eq.(5))	0.228	1.022	4.721	1413	/
	Calculated permeability (series model, Eq.(6))	0.097	0.112	0.117	1220	/
	PC/MXD6-2 (80/20)	0.054 ± 0.001	0.059 ± 0.003	0.153 ± 0.007	1225 ± 250 (Xc=8.2%)	1210 ± 110 (Xc=14.6%)
	Calculated permeability (Maxwell model, Eq.(5))	0.245	1.107	5.128	1472	/
	Calculated permeability (series model, Eq.(6))	0.113	0.137	0.145	1295	/

Fig. 7. Reduced water permeation curves for the PC/MXD6 multilayer films.

Table 5

 5 Calculated permeability coefficients of MXD6 within the PC/MXD6 films and the corresponding BIF factor.

	Permeability (Barrer)
	N 2	O 2	CO 2	H 2 O

Acknowledgment

The authors thank the GRR Crunch (Grant No. EOTP N°427341) (supported by Upper Normandy region, France) for the financial support of the PhD fellowship of T. Messin.


Bastien Confais

Adrien Lebre

Benoît Parrein

Fog and Edge Computing infrastructure have been proposed to address the latency issue of the current Cloud Computing platforms. While a couple of works illustrated the advantages of these infrastructures in particular for the Internet of Things (IoT) applications, elementary Cloud services that can take advantage of the geo-distribution of resources have not been proposed yet. In this paper, we propose a first-class object store service for Fog/Edge facilities. Our proposal is built with Scale-out Network Attached Storage systems (NAS) and IPFS, a BitTorrent-based object store spread throughout the Fog/Edge infrastructure. Without impacting the IPFS advantages particularly in terms of data mobility, the use of a Scale-out NAS on each site reduces the inter-site exchanges that are costly but mandatory for the meta-data management in the original IPFS implementation. Several experiments conducted on Grid'5000 testbed are analyzed and confirmed, first, the benefit of using an object store service spread at the Edge and secondly, the importance of mitigating inter-site accesses. The paper concludes by giving few directions to improve the performance and fault tolerance criteria of our Fog/Edge Object Store Service.

An Object Store Service for a Fog/Edge Computing Infrastructure based on IPFS and Scale-out NAS

I. INTRODUCTION

The Internet of Things (IoT) brings new constraints: a huge number of limited devices with a need of low latency computing. While largely adopted, the Cloud computing paradigm is not well suited because it relies on only a small number of datacenters located far away from users. The Fog computing paradigm has been proposed by Cisco to cope with this latency requirement [START_REF] Bonomi | Fog Computing and Its Role in the Internet of Things[END_REF]. The idea is to deploy dedicated servers in micro/nano datacenters geographically spread at the Edge of the network, that is, close to the end-users.

Figure 1 illustrates a Fog/Edge infrastructure. Each Fog site is composed of a small number of servers providing computation as well as storage resources to users. Users/IoT devices that are located in the Edge and in the Extreme Edge are connected to servers of the closest site, enabling low latency access. Devices located in the Edge contact directly the servers located in the Fog whereas those located in the Extreme Edge have to cross a local network before reaching the Fog. The one-way network latency to reach the Fog (noted L F og ) is comprised between 10 and 100 ms (latency of a local wireless link [START_REF] Jorgensen | 3G Femto or 802.11g WiFi: Which Is the Best Indoor Data Solution Today?[END_REF]). The latency between the sites of Fog (noted L Core ) is comprised between 50 and 100 ms (mean latency of a Wide Area Network link [START_REF] Markopoulou | Loss and delay measurements of Internet backbones[END_REF]). Finally the latency to reach the Cloud (noted L Cloud ) is variable and unpredictable [START_REF] Zhang | The Cloud is Not Enough: Saving IoT from the Cloud[END_REF]. Although using Fog/Edge facilities to deliver more efficient storage repositories has been discussed as of 2012 [START_REF] Bonomi | Fog Computing and Its Role in the Internet of Things[END_REF], any solution has been proposed yet. We started to address this question in a preliminary study [START_REF] Confais | Performance Analysis of Object Store Systems in a Fog/Edge Computing Infrastructures[END_REF] with the ultimate goal of delivering a system similar to the Simple Storage Service (S3) of Amazon, a widely used service and a building block for hundred of Cloud services. The main contributions of this study were:

• A list of properties a Fog Object Store Service should meet; • An empirical analysis of three available storage systems, namely Rados [START_REF] Weil | RADOS: A Scalable, Reliable Storage Service for Petabyte-scale Storage Clusters[END_REF], Cassandra [START_REF] Lakshman | Cassandra: A Decentralized Structured Storage System[END_REF] and InterPlanetary File System (IPFS) [START_REF] Benet | IPFS -Content Addressed, Versioned, P2P File System[END_REF]. Among the three tested solutions IPFS filled most of the criteria expected for a Fog/Edge computing infrastructure. IPFS can be seen as an object store service built on top of the BitTorrent protocol [START_REF] Legout | Understanding BitTorrent: An Experimental Perspective[END_REF] and the Kademlia DHT [START_REF] Maymounkov | Kademlia: A Peer-to-Peer Information System Based on the XOR Metric[END_REF]. Both mechanisms are well-known protocols for their ability to scale to a large number of nodes. While the BitTorrent protocol is used to manipulate objects between the different peers of the system in an efficient manner, the Kademlia DHT is in charge of storing the objects location.

The main issue of using a DHT for the metadata management in IPFS is that each time a user wants to access an object, it generates a remote access if the object is not available on the requested node. This increases the access time in addition to generate an important amount of network traffic between the sites, especially when a large number of objects is manipulated. In this paper, we propose to tackle this issue by deploying on each site a local Scale-out Network Attached Storage system (NAS). This system acts as a storage backend for the IPFS nodes belonging to the site. This way allows nodes to access any object stored locally on the site without using the global DHT. Nodes interact with the DHT only when the object they looking for has been created on another site. To validate our proposal, we implemented a Proof-of-Concept leveraging IPFS on top of RozoFS [START_REF] Pertin | Distributed File System based on Erasure Coding for I/O Intensive Applications[END_REF], and conducted several experiments on the Grid'5000 testbed [START_REF] Balouek | Adding Virtualization Capabilities to the Grid'5000 Testbed[END_REF]. We emulated three sites and measured the access times as well as the amount of network traffic exchanged between the sites for both local and remote conditions to validate ubiquitous access. While the results are promising to deliver a well suited object store service for Fog/Edge infrastructures, we discuss current limitations of our changes and propose several directions to improve the performance while reducing as much as possible all the inter-site exchanges.

The remaining of the paper is organized as follows: Section II presents in details the limitation of using a DHT in IPFS for the metadata management. Section III gives an overview our solution that couples IPFS to a Scale-out NAS. Section IV deals with the experimental evaluations while Section V discusses two additional considerations related to our proposal. Related work are presented in Section VI. Finally, Section VII concludes this work and gives some perspectives.

II. IPFS: OVERVIEW AND LIMITATION

As briefly introduced, IPFS has been built on top of the Bit-Torrent protocol and the Kademlia DHT. The former enables the efficient relocation of objects between peers composing the infrastructure while the latter is used for the management of the metadata. The IPFS protocol is depicted in Figure 2. We underline that IPFS only enables the creation of immutable objects. Figure 2(a) depicts the creation of a new object: the client sends its object to any node on its nearest site. This node stores the object locally and put the location of the object in the DHT. Because the DHT does not provide locality, the node storing this metadata can be located in any node composing the Fog infrastructure. In our example, node3 belonging to Site 2 stores the location of the object that has been created on node1.

Figure 2(b) illustrates what happens when the client reads an object stored on its local site. Each time an IPFS node receives a request for a particular object, it first, checks whether this object is available on the node. In this case, the node sends the object directly to the client. Otherwise, the IPFS node should rely on the DHT protocol to locate the requested object. That is, it should (i) compute the hash based on the object id, (ii) contact the node in charge of the metadata, (iii) retrieve the object from the node(s) storing it (using the BitTorrent protocol), (iv) make a local copy while sending the data to the client, and (v) finally update the DHT in order to inform that there is a new replica of the object available on that node.

Figure 2(c) describes what does happen when an object is requested from another site (because the client can move from a site to another one or because the object can be accessed by a remote client). In any case, the client contacts a node on the site it belongs to. Because the node does not store the object, the protocol is similar to the previous one involving the extra communication with the DHT. We underline that, thanks to the implicit replication performed by the BitTorrent protocol, if the client wants to read the object a second time on the same node, the node will be able to satisfy the request without any interaction as described in Figure 2(b).

As explained in Section IV, IPFS provides better performance than an object store deployed in the Cloud computing. Nevertheless, it does not take advantage of the physical topology of the Fog infrastructure: When an object is created or available on one site due to BitTorrent copies, the other IPFS nodes that belong to this same site are not aware of the availability of the object and rely on the DHT to locate the object. This is illustrated by the second get operation on Figure 2(c from every node that owns a copy. Accessing the DHT to read an object stored locally on the site has two major drawbacks. First, it increases the access times as well as the amount of traffic exchanged between the sites and secondly, it avoids clients to access the objects stored locally in case of network partitioning because nodes of the DHT are unavailable. IPFS provides an efficient way to access objects remotely thanks to the BitTorrent protocol. But, it does not take the advantage of the Fog topology for local access. To tackle such a limitation, we propose to couple IPFS with a Scale-out NAS solution.

III. COUPLING IPFS AND SCALE-OUT NAS SYSTEMS

To improve IPFS in a Fog Computing context, we propose to deploy a Scale-out NAS system independently on each site. Each system acts as the default storage backend for all IPFS nodes that belong to the same site as depicted in Figure 3. This approach allows each IPFS node of one site to access data stored by others nodes of the same site without any intrusive changes in the IPFS code as explained in the two next sections.

A. Protocol Changes

The modifications in the protocol are illustrated in Figure 4. In Figure 4(a), we can observe there is no change in the protocol for the creation of a new object. The difference appears when a client wants to access an object. Figure 4(b) depicts these changes: Any IPFS node on a site can see all the objects manipulated by the other nodes of the same site thanks to the Scale-out NAS system. As a consequence, regardless the node on the local site the request is sent to, the requested node checks if the object exists in the Scale-out NAS and sends it to the client if it is available. Contrary to the original IPFS design, there is no need to access the DHT and to download the object from the other nodes located on the site.

Similarly, Figure 4(c) shows how the protocol behaves in case of remote access. When the client is connected to a remote site, the request is sent to any node. The node checks To sum up, our approach limits the amount of network traffic sent in case of local reads: when a client reads an object available locally on the site, there is no need to access the DHT, regardless the node the request is sent to. In the other situations, the scenario as well as the amount of traffic exchanged between the site remains the same as with the default approach. We discuss in the next section, the evaluation we performed to quantify this gain.

B. IPFS Code Modifications

We performed some modifications in the source code of IPFS 1 . First, we moved the folder used by IPFS to store the objects on the Scale-out NAS mountpoint without moving the local database used by each node. Secondly, we removed the cache used by the blockstore module of IPFS. The cache is designed to keep in memory the fact an object exists or not. This modification is mandatory in order to enforce a node to check in the Scale-out NAS system before using the DHT 1 https://github.com/bconfais/go-ipfs/tree/common backend v1a to locate the requested object. Finally, we modified the way IPFS requests objects to other nodes. By default, each time an IPFS node wants to read an object stored remotely, it looks for the peer responsible for the object by using the DHT and contacts the nodes storing it. However, it does not request only this object, but all the previous requested objects for which the download is not finished yet. This unnecessary increases the network traffic between sites. The modification consists to request only the object to the nodes specified in the DHT.

Finally, we disabled the replication of metadata in IPFS in order to measure the minimum amount of network traffic exchanged between the sites that is possible to have. Disabling the replication of metadata has been done for all the software architectures we considered. We also removed the limit of the number of threads created by IPFS in order to get the best access times as possible.

IV. EXPERIMENTAL EVALUATION

In this experimental evaluation, we compare the performance of (i) a Cloud based object-store service, (ii) the default IPFS solution and (iii) our approach coupling IPFS and Scaleout NAS systems. We underline the comparison with a Cloud based solution aims to illustrate the relevance of designing an object-store service which can benefit from the Fog/Edge specifics.

The first paragraph presents our experimental protocol, the second deals with local accesses and finally, the third discusses results we gathered for remote access scenarios.

A. Material and Method

Our evaluations have been performed using three different software architectures:

1) IPFS in its default configuration deployed into a regular Cloud; 2) IPFS in its default configuration deployed across a Fog/Edge infrastructure; 3) IPFS coupled with independent Scale-out NAS solutions in a Fog/Edge context. We selected RozoFS [START_REF] Pertin | Distributed File System based on Erasure Coding for I/O Intensive Applications[END_REF] as the Scale-out NAS solution. RozoFS is a POSIX compliant open-source solution that achieves good performance in throughput and I/O rate both for sequential and random access. Similarly to other Scaleout network file systems, it uses a metadata-server to locate the data. The distribution of the data across the I/O servers is realized by the Mojette erasure code that makes the system fault tolerant (2 Mojette projections out 3 are necessary to reconstruct the data). More details are given in the article of Pertin et al. [START_REF] Pertin | Distributed File System based on Erasure Coding for I/O Intensive Applications[END_REF]. We emphasize this aspect because it leads to an overhead in writing (50% overhead in size i.e., one redundant projection to combat one failure). This aspect is further discussed in Section V.

We consider that each storage node acts as both an IPFS node and a storage node of RozoFS (in fact, IPFS is a client of RozoFS through the rozofsmount daemon). To avoid any bias, we used tmpfs as the low level back-end for the three architectures and drop all caches after each write or get operation. Finally, we highlight that our proposal can be coupled with any kind of Scale-out network file systems such as GlusterFS [START_REF] Davies | Scale out with glusterfs[END_REF], Lustre [START_REF] Donovan | Lustre: Building a file system for 1000-node clusters[END_REF] or even Rados [START_REF] Weil | RADOS: A Scalable, Reliable Storage Service for Petabyte-scale Storage Clusters[END_REF]. We chose RozoFS because we have a good knowledge of the internal mechanisms of the system, which helped us for the analysis of the results. Moreover, RozoFS achieves good performance regarding I/O access.

The topology we evaluated corresponds to the one illustrated in Figure 3. The platform is composed of 3 sites, each containing 6 nodes: 4 storage nodes, a metadata server for RozoFS and a client. The Cloud-based IPFS is composed of 12 IPFS nodes (the same number of nodes as used in the Fog experiments). Topology does not vary so that we do not have any node churn. The one-way network latencies between the different nodes have been set in order to be representative to

• local wireless link [START_REF] Jorgensen | 3G Femto or 802.11g WiFi: Which Is the Best Indoor Data Solution Today?[END_REF], L f og = 10ms, latency between clients and the fog site they belong to; • wide area network link [START_REF] Markopoulou | Loss and delay measurements of Internet backbones[END_REF], L core = 50ms, latency between each fog site; • and the latency to reach a cloud [START_REF] Souza Couto | Network design requirements for disaster resilience in IaaS Clouds[END_REF]; L Cloud = 100ms. Inside each Fog site and inside the Cloud, the latency between the servers has been set to L site = 0.5ms. Latencies were emulated using the Linux Traffic Control Utility (tc). We did not modified the throughput of the network links, i.e., 10Gbps in our testbed for both intra and inter-sites links. According to Padhye et al. [START_REF] Padhye | Modeling TCP Throughput: A Simple Model and Its Empirical Validation[END_REF], the TCP throughput is depending on packet loss and link latency. Measuring the throughput using iperf shows 2.84 Gbps for clients to reach a Fog node, 9.41 Gbps between the Fog nodes of a same site, and 533 Mbps between the sites. The measured throughput to reach the Cloud platform is 250 Mbps.

We performed experiments on two scenarios:

• a first scenario where one client per site writes objects on the site it belongs to and reads them; • a second scenario where one client located on the first site writes objects and then, another client located on another site reads them. The first scenario is designed to evaluate the performance in terms of locality and isolation between the workloads whereas the second one enables us to quantify the cost of accessing objects from another location. For every write and read operation of each scenario, each client sends the request to an IPFS node selected randomly on the site it belongs to in order to balance the load among all the nodes of the site (for the cloud-based scenario, the selection is performed throughout the 12 nodes).

Experiments have been performed in the spirit of reproducible research2 . We measured access times using our own benchmark script and the amount of data exchanged between the sites using iptables. The access time corresponds to the time to complete the operation from the client point of view. We highlight that we previously performed experiments using the Yahoo Cloud System Benchmark [START_REF] Cooper | Benchmarking Cloud Serving Systems with YCSB[END_REF] with an IPFS module we developed [START_REF] Confais | Performance Analysis of Object Store Systems in a Fog/Edge Computing Infrastructures[END_REF]. However, YCSB adds several bias that prevent us to finely analyze the results. We voluntary choose to develop our own benchmark in order to improve the quality of our analysis.

Each scenario has been run 10 times for consistency in results.

B. Write/Read Operations from the Same Location

In this section, we discuss the results obtained for the local access scenario, i.e., when each client manipulates data simultaneously from their respective site. We also compare the access times we get in the case where objects are stored in a regular Cloud infrastructure.

1) Access Times: Table I shows the mean time to put or get an object in each software architecture. The number of objects as well as their size correspond to what is written and read on each site in parallel.

Table I(a) presents the access times for the Cloud architecture. Each client (3 in our experiment) writes objects on their closest site and reads them. The clients work simultaneously. Results show significant access times with respect to the amount of data that is manipulated. Creating a single object of 256 KB takes 1.72 seconds and it requires up to 3.07 seconds in average for one object of 10 MB. If we increase the number of access in parallel, the maximum throughput that can be obtained for write operations is around 36 MB/sec (100 × 10/27.58). Access times for read operations are better than the write ones for several reasons. First, some objects are retrieved without any indirection to the DHT (i.e., when the IPFS request is sent to the IPFS node that stores the requested object). Secondly, the TCP throughput is higher for downloading than uploading with a maximum around 90 MB/sec (100×10/11.24). This is due to the management of Linux TCP connections and the block granularity of IPFS for sending objects. In any case, these values are still important with respect to the amount of data that is manipulated. It confirms the impact of the latency on the TCP throughput [START_REF] Padhye | Modeling TCP Throughput: A Simple Model and Its Empirical Validation[END_REF].

Table I(b) presents the access times obtained when IPFS is deployed at the Fog level. Results clearly show the interest of deploying the object store service on Fog/Edge resources. First, access times are significantly better with a maximum throughput per client around 500 MB/sec (4 Gbps). That is, half of the optimal performance for a 10 Gbps network interface (5 Gbps). Secondly, the impact between the clients is only related to the DHT traffic whereas in the Cloud architecture, all accesses are distributed across all nodes.

Tables I(c) shows the times to write and read objects using IPFS on top of RozoFS. First, we observe in the two Fog architectures, the writing times are in the same order of magnitude. It takes 3.92 seconds per object to write 100 objects of 10 MB with the default approach and 3.97 seconds using the couple IPFS with RozoFS. In other words, using a Scale-out NAS system such as RozoFS does not add an overhead. This is a valuable result because we can erroneously believe that using a complex software brick such as a Scale-out network file system can add some penalties. For reading, we observe the couple IPFS/RozoFS has better access times than the default approach. This is particularly visible for accessing a small number of small objects. As an example, it takes 0.14 seconds to read a 10 MB object whereas it requires 0.25 seconds with the original IPFS (almost twice). As previously described in Figure 4(b), the use of a Scale-out NAS enables IPFS nodes to directly satisfy the request wherever the object has been created on the site. The gain becomes smaller for large objects as the cost of accessing the DHT becomes less important regarding the time to send the object to the client. Moreover, as the probability to contact the node storing the object is 1/4 in the original IPFS architecture, reading a large number of objects leads to a less significant gain (over 100 requests, 25 will be directly satisfied). With larger sites (i.e., composed of a more important number of servers), the probability to contact the right node directly will decrease significantly and the gain of using a Scale-out system will be more visible.

2) Network Traffic: Figures 5(a) and 5(b) show the amount of network traffic exchanged between sites while writing and reading for the IPFS and IPFS/RozoFS solutions. We do not present the traffic for the Cloud-based approach as the traffic is equal to the amount of manipulated data. Moreover, we observe the amount of traffic sent between the sites is depending only on the number of accessed objects and not on the object sizes. So, for a better readability, we arbitrary choose to present only the results for objects of 1 MB.

Figure 5(a) presents the value for write operations. As expected, the same amount of network traffic is sent using both approaches. In addition to the messages related to the DHT (heartbeats), the two approaches send the location of the written objects.. For reading, the traffic generated by the default approach is mainly the DHT requests: the more objects are manipulated, TABLE I: Mean time (seconds) to write or read one object under different conditions (the number on the left indicates the number of operations that are executed in parallel on each client). higher the traffic. In our approach using RozoFS, we only observe the traffic related to the management of the DHT. Thus according to the duration of the experiment and because the amount of data is really low, we can see some fluctuations monitored by the standard deviation.

To conclude, our approach using a Scale-out NAS is better than the two other approaches, both for access times and for the amount of traffic sent in reading. The Scale-out NAS removes the need to access the DHT when clients read objects already available somewhere on the site it is connected to.

C. Write from One Location, Read from Another One

In this section, we discuss the results obtained for the remote access scenario, i.e., when one client creates data on its site and another client from another site access the created objects twice. Only read results are presented because writing locally has been already discussed in the previous section. Also, we do not consider the Cloud-based architecture in this scenario. Similarly to the write results, the read values will follow the same trend as to the one previously presented (there is no change because all clients are at the same distance from the Cloud).

1) Access Times: Tables II(a) and II(b) show the access times for the first and the second read operations.

In the first read, the times are in the same order of magnitude. This is due because the two approaches use the DHT to retrieve the location of the object and then copy the object on the second site. This trend corresponds to what we describe in Figures 2(c) and 4(c): the exchanges between the sites are the same. The requested node looks for the object location in the DHT and downloads the object from the node it is stored on. The only difference is the remote node retrieves the object from the Scale-out NAS before sending it to the TABLE II: Mean time (seconds) to read one object twice with IPFS using the default approach (a) and using a shared backend in each site (b).

node that requests it. The object is finally sent back to the client. According to the presented access times, this indirection through RozoFS on the remote site does not add an important overhead.

For the second read, we observe our approach gets better access times than the default one. As an example, with RozoFS it takes 1.86 seconds per object to read 100 objects of 10 MB whereas it takes 6.08 seconds in the default approach. Although the object is copied locally in both approaches, the probability to contact the right node for the second read (i.e., the node that satisfied the first request) is 1/4. If the client does not contact the right node, the process will be the same as for the first read. The only difference is the DHT will inform the node that it can retrieve the object from both the node on the remote site and from the node that previously retrieved and copied the object in the first read. In the IPFS/RozoFS approach, because of the shared back-end, objects are seen as stored locally by the requested node, the DHT is not accessed and nothing is downloaded from the remote site. As a consequence, access times are much faster. This behavior has been explained in Figure 2 For the first read, the same amount of data is exchanged between the sites in the two approaches. As previously explained, the objects must be downloaded from the site they were created. We highlight the traffic related to the DHT management is not significant enough in comparison to the amount of exchanged data to be visible.

For the second read, a lot of traffic is sent between the sites for the default IPFS approach. As explained in addition to the DHT request, the object is retrieved from the neighborhood and once again from the remote node where the object was created. Our IPFS/RozoFS approach enables the object store service to satisfy the requests completely by the local NAS. This way reduces the amount of traffic between sites only to the mandatory one. Once an object has been retrieved it is available for any node of the site. The default approach uses a local replica too but it is not shared among all the nodes of the site. Thus, nodes need to access the DHT to be able to retrieve it.

Although the IPFS/RozoFS Proof-of-Concept is promising, we discuss in the next section two additional considerations.

V. ADDITIONAL ANALYSIS

In this section, we discuss the possible side effects of the storage spaces induced by the fault tolerance of the underlying Scale-out NAS. We also discuss a performance limitation that may occur in some cases and propose to solve it by clustering IPFS nodes within a site into one virtualized IPFS node.

A. Storage Space Control

It is noteworthy that an approach based on a Scale-out NAS may add an overhead in terms of storage space according to the underlying fault tolerance policy (full replication, erasurecode etc.). When IPFS is used alone, the object is written on the IPFS node receiving it, without any replication. In our IPFS/RozoFS implementation, RozoFS writes a 50% overhead due to the Mojette erasure code [START_REF] Pertin | Distributed File System based on Erasure Coding for I/O Intensive Applications[END_REF] in order to support one failure at the NAS level. However, our proposal becomes more space efficient than the original IPFS for subsequent reads: in the default approach the object is copied on each node receiving a request for it. As explained in Section II, Figure 2(b), the object is written both on the first node it was written on and on the node that read it. In our approach using a Scale-out NAS, an object is written once for all. This prevents multiple duplications of one object on the same site. The object is only copied on an other sites in case of remote access as described in Section III, Figure 4(c).

In other words, the default policy of IPFS consists of copying objects locally each time a node wants to access it can overpass largely the small side effect introduced by the fault tolerance capability of the underlying Scale-out NAS backend.

B. Performance Limitation and Future Work

From a performance viewpoint, it is important to underline that our current implementation can be improved. Indeed, when an IPFS node wants to access an object, it retrieves the object from all the nodes having it (see Figure 2(b) in Section II). This feature comes from the BitTorrent protocol is interesting as it enables IPFS to mitigate the negative impact of overloaded nodes: If one node is overloaded, any other node storing the object will be able to satisfy the request in an efficient way.

In our approach using a Scale-out NAS, the situation differs because the DHT knows at most one node per site: either the node where the client has written the object or the first node used to retrieve the object in case of a remote access. As described in Section III, Figure 4(c), the Site 3: IPFS node4 can download the object only by contacting Site 1: IPFS node1 because it is the original node that created the object (the DHT stores Site1:IPFS node1 as the node owning the object). Site 3: IPFS node4 does not know that Site1:IPFS node2 has also access to the object stored in the Scale-out NAS. This situation can lead to bottleneck by overloading Site1:IPFS node1.

Similarly, if Site1:IPFS node1 crashes, the object will be unreachable from any node belonging to remote sites because IPFS node1 is the only entry point for that object in the DHT. This is rather frustrating because the object is still available by contacting any other node belonging to Site1 (Site 1: IPFS node2, for instance).

A solution for this problem may be not to add the address of a node in the DHT but an identifier of the site the object is. In this situation, remote nodes will be able to download the objects from all the nodes located on the sites the object is. Another solution can be placing all the IPFS nodes of a site at the same place in the DHT, making all the nodes of a site responsible for the same range of keys so that there is no distinction between the nodes of a site (as an example, Site 1: IPFS node1 and Site 1: IPFS node2 will have the same identifier). Evaluating pros/cons of each possibility is let as future work.

VI. RELATED WORKS

There exists many approaches to manage the data location in an object store. But most of them either does not provide flexibility in data placement or they generate an important amount of network traffic by announcing the location of the objects to all nodes.

Using a hashing function or a placement function like CRUSH [START_REF] Weil | CRUSH: Controlled, Scalable, Decentralized Placement of Replicated Data[END_REF] is not easy because the topology has to be distributed in a consistent way between all the nodes. Also, the function does not provide a flexible way to specify the location of every objects. Indeed, with CRUSH, to place explicitly each object, users have to specify placement rule for each of them which is not scalable.

Gossip enables this flexibility by distributing the location of each object to all nodes. But the network traffic is depending on the number of stored objects. Also, nodes have to store the location of every object that may require a lot of storage space.

Some approaches like the approach used by Cassandra [START_REF] Lakshman | Cassandra: A Decentralized Structured Storage System[END_REF] and Dynamo [START_REF] Decandia | Dynamo: Amazon's highly available key-value store[END_REF] combine the gossip and hashing. As an example, in Cassandra, the ranges of keys the nodes are responsible for is gossiped between the nodes and then nodes determine the locations of objects by hashing the object key. This approach is a trade-off between gossiping the location of each object and not be able to move the objects because of a hashing function. The drawback of this approach is to manage the location of the objects by groups only. Placing each object independently leads to generate as much traffic as a traditional gossip.

VII. CONCLUSION

In this paper, we proposed to revise the IPFS proposal in order to cope with the Fog/Edge Computing context. Our proposal leverages a Scale-out NAS system deployed on each site in order to avoid the use of the global DHT when the requested object is available locally on the site. We implemented a Proofof-Concept using RozoFS, an efficient Scale-out network file system. Evaluation using Grid'5000 showed that leveraging a Scale-out NAS on each site reduces the access times as well as the amount of network traffic sent between the sites. We also showed our approach does not impact the IPFS mobility criteria. Accessing an object from a remote site is also efficient in addition to reducing inter-site network traffic. The Scaleout NAS also provides IPFS an access to local files in case of network partitioning because the DHT is only accessed when a read is performed from a remote site. We discussed two additional aspects related to the storage space and possible performance improvements.

As a future work, we plan to investigate the benefit of coupling IPFS with a Scale-out NAS system where all the IPFS nodes of a site are considered as a single node in the DHT, avoiding performance and reliability problems previously described. Moreover, we would investigate the interest of placement strategies across the sites. One can envision to apply replication strategies between nearby sites. These strategies can be defined by end-users according to their requirements and the underlying network topology.

Fog site 1 Fog site 3 Fog site 2 Fog site 4 Fig. 1 :

 13241 Fig. 1: Overview of a Cloud, Fog and Edge infrastructure.

Fig. 2 :

 2 Fig. 2: Network exchanges when a client writes or reads an object using IPFS (Cont).

Fig. 3 :

 3 Fig. 3: Topology used to deploy an object store on top of a Scale-Out NAS local to each site.

  n1: IPFS n2: DFS n1: DFS n2: DFS MDS: n1: IPFS n2: DFS n1: DFS n2: DFS MDS: IPFS n3: (b) -Read an object stored locally. IPFS n2: DFS1 n1: DFS1 n2: DFS1 MDS: IPFS n3: DFS2 MDS: DFS2 n1: IPFS n4: IPFS n5: client: (c) -Read an object stored remotely.

Fig. 4 :

 4 Fig. 4: Sequence diagrams of the network traffics when a client writes or reads an object using IPFS on top of DFS locally deployed on each site.

Fig. 5 :

 5 Fig. 5: Cumulative amount of network traffic exchanged between all the sites while clients write and read objects located on their sites.

Fig. 6 :

 6 Fig.6: Cumulative amount of network traffic exchanged between all the sites while clients read objects twice.

  (c) and 4(c). a) Network Traffic: Figures 6(a) and 6(b) show the cumulative amount of network traffic exchanged between the sites during the two reads.

  ): Although the object is already available on node4 of Site 3, node5 contacts the DHT and retrieves the object

	Site 1	Site 2	Site 3	Site 1	Site 2	Site 3
	IPFS node1: IPFS node2: IPFS node3: IPFS node4: IPFS node5: client:	IPFS node1: IPFS node2: IPFS node3: IPFS node4: IPFS node5: client:
		find	get object			find	get object
		location				location
		in DHT				in DHT
		get/read object			get/read object
		get/read object			get/read object
			send object			get/read object
		put	store object				send object
		location in DHT				put	store object
						location
						in DHT
			(c) -Read an object stored remotely.
					Site 1		Site 2
				client:	IPFS node1:	IPFS node2:	IPFS node3:
				put object
					store object
				done	put
						location in DHT
					(a) -Write an object.
					Site 1		Site 2
				client:	IPFS node1:	IPFS node2:	IPFS node3:
				get object
					read object
				send object
					get object	find
						locations in DHT
					get/read object
					send object
						store object
						put
						location in DHT
					(b) -Read an object stored locally.
				Fig. 2: Network exchanges when a client writes or reads an
					object using IPFS.

Our benchmark code as well as results are available at https://github.com/ bconfais/benchmark