Poly[(butylene succinate)-co-(butylene adipate)]-Montmorillonite

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Poly[(Butylene Succinate)-Co-(Butylene Adipate)]Montmorillonite Nanocomposites Prepared by WaterAssisted Extrusion: Role of the Dispersion Level and of the Structure-Microstructure on the Enhanced Barrier Properties Sébastien Charlon, Nadege Follain, Eric Dargent, Jérémie Soulestin, Michel Sclavons, and Stéphane Marais J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00339 • Publication Date (Web): 27 May 2016 Downloaded from http://pubs.acs.org on May 30, 2016

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Poly[(Butylene Succinate)-co-(Butylene Adipate)]-Montmorillonite Nanocomposites Prepared by Water-Assisted Extrusion: Role of the Dispersion Level and of the Structure-Microstructure on the Enhanced Barrier Properties

Sébastien CHARLON1,2,3, Nadège FOLLAIN1,2,3, Eric DARGENT1, 4, Jérémie SOULESTIN5, Michel SCLAVONS6, Stéphane MARAIS1, 2, 3, *

1

Normandie Univ. France

2

Université de Rouen, Laboratoire Polymères, Biopolymères et Surfaces, Bd. Maurice de

Broglie, 76821 Mont Saint Aignan Cedex, France 3

UMR 6270 CNRS & FR 3038 INC3M, Bd. Maurice de Broglie, 76821 Mont Saint Aignan

Cedex, France 4

AMME-LECAP EA 4528 International laboratory, Université de Rouen, Av. de l’Université

BP 12, 76801 Saint Etienne du Rouvray Cedex, France 5

Mines Douai, Department of Polymers and Composites Technology and Mechanical

Engineering, 941 rue Charles Bourseul, CS 10838, F-59508 Douai, France 6

Bio and Salt Matter, Institute of Condensed Matter and Nanosciences, Université catholique

de Louvain, Croix du Sud 1, B-1348 Louvain-La-Neuve, Belgium

*Corresponding author: [email protected] Mail : [email protected] Phone : 33 (2) 35.14.67.02

1 ACS Paragon Plus Environment


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ABSTRACT Composite films composed of a biodegradable Poly[(Butylene Succinate)-co-(butylene Adipate)] (PBSA) matrix with several contents (2.5; 5; 7.5; 10 wt%) of native (MMT) or organo-modified montmorillonite (OMMT) were successfully extruded using an innovative water-assisted extrusion process. As expected, better dispersion and exfoliation levels of nanofillers were obtained in PBSA+OMMT than in PBSA+MMT composite films, whatever the nanofiller content. Otherwise, the water-assisted extrusion process has also clearly enhanced the dispersion and exfoliation levels of native montmorillonite. This process was particularly efficient to delaminate and to disperse high amounts (≥ 5 wt%) of MMT in the PBSA matrix, which has highly increased tortuosity effects in the resulting composite films. The films prepared via the water-assisted extrusion have consequently exhibited better barrier properties to water and to gases than the films classically extruded, and as the nanofiller content increases.

1. Introduction Environmental concerns are major issues for the last decades. To limit the accumulation of plastic wastes into land and marine areas1, the use of biodegradable polymers is recommended2,3,4. A promising polymer enables to substitute common non-biodegradable plastics, such as polyolefins, with similar properties could be the Poly[(Butylene Succinate)co-(butylene Adipate)] (PBSA) because of good properties like processability5, chemical resistance, mechanical properties6, thermal stability similar to polyethylene and polypropylene ones7 and biodegradability8. However, even though the PBSA potentials for food packaging were recently demonstrated9, its barrier properties to gases and to water were found insufficient10, that limits its use for some practical applications. One way to improve barrier properties of a polymer is the incorporation of inorganic lamellar nanofillers in the polymer


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matrix11,12,13. This approach is more and more considered as it can improve the thermal and the mechanical properties of the matrix owing to modification in bulk and can also be industrialized. Since recent years, there is a growing interest in polymer nancomposites (PNCs) for improving the inherent properties of biodegradable polymers such as PBSA, PBS (poly(butylene succinate)), PCL (poly(ε-caprolactone)), PBAT (poly(butylene adipate-coterephtalate)) that derived from peroleum sources, but also PLA manufactured from renewable sources. Such materials called environmentally friendly PNCs (EFPNCs) are more and more considered to be used in packaging, automotive, construction and agriculture applications14. Nanofillers as native montmorillonite (MMT) are often used in works reported in the literature because of high aspect ratio, natural abundance and low cost. As the uniform dispersion of clays in polymer matrices is of prime importance for getting enhanced mechanical and physical properties, the clays need to be modified for improving compatibility with the polymer matrices15. Some organo-modified montmorillonite (OMMT) are commercially available. Okamoto et al.16 have prepared poly(butylene succinate)/layered silicate nanocomposites by simple melt extrusion of PBS and different types of OMMT and have shown improvements of their properties when compared with pure PBS. Although OMMT nanoplatelets are generally more dispersed and exfoliated in the polymer matrix than the MMT ones17, the poor thermal stability of OMMT18 often induces polymer chains degradation during the nanocomposite preparation19. Therefore, several research works were focused on the enhancement of the dispersion and exfoliation levels of native montmorillonite in polymer matrices. In the critical review of Ojijo et al.20, the most important methods used in the processing of bionanocomposites are described, in particular the melt processing techniques such as extrusion and injection molding, those are current industrial processes. Authors have shown the correlation between the processing procedures and the structures of the resulting nanocomposites. In 1999, Korbee et al.21 have described for the first time the



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well exfoliation of MMT in a polyamide matrix using a water-assisted extrusion process. This process consists in mixing inorganic platelets with water which acts as a swelling agent to facilitate the water penetration between the platelets for increasing the interfolliar distance. This mixture called “slurry” is thereafter injected in the extruder barrel containing the melted polyamide, decreasing the polymer viscosity. By this way, the risks of gel formation and of the polymer chain degradation are consequently limited with the increase of polymer chain mobility so that macromolecules can more easily penetrate between the platelets which are more or less aggregated. Water is then removed by vacuum degassing during extrusion process. Several approaches based on the water-assisted extrusion process were then developed22. Hasegawa et al.23 have proposed in first the use of a specific screw profile in order to inject the slurry under high pressure in the high compression zone of the extruder. They have also obtained well-dispersed and exfoliated MMT in a PA-6 matrix. According to the authors, this technique is very efficient to disperse and to exfoliate fillers in many polymer matrices (PA1224, PA623, LDPE26, PP25 …). However, several drawbacks make this technique less suitable in particular because of the discontinuity of the process, the need of using a high amount of liquid water to reduce the viscosity of the slurry, the risk of sticking between the slurry and the screw, the low filler content usable and the low throughput26. Therefore, based on a similar principle, another extrusion process was developed and consists in injecting liquid water under high pressure into the extruder barrel in which the fillers are already incorporated in the melted polymer matrix. Such process exhibits the benefits to be a continuous process which consumes a low quantity of water and to be compatible with a higher amount of filler usable26. As a result, the dispersion and exfoliation levels of MMT in polymer matrices like PP17, PA627 or SAN28 have been strongly enhanced. Due to the efficiency of the water-assisted extrusion process on MMT-based composites, some authors have applied it to prepare OMMT-based composites. For example, Dini et al.29


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have incorporated OMMT in a PET matrix. The degradation of PET chains caused by the surfactant presence was counterbalanced by a subsequent solid state polymerization which leads to intercalated structures composed of 1 to 3 nanoplatelets28. In the present work, the PBSA-based composites were successfully proceeded using an original water-assisted extrusion process which allows to enhance the dispersion and exfoliation levels of native and organo-modified montmorillonite. The work is aiming at evaluating the efficiency of the water-assisted extrusion process on the barrier properties of the composite films loaded with several contents of MMT or OMMT (those are 2.5, 5, 7.5, 10 wt%). In order to understand the action mechanism(s) of the injection of water on barrier properties of the resulting films, the polymer microstructure and the dispersion and exfoliation levels of the fillers in the PBSA matrix were analysed and correlated to transport properties of the nanocomposites. Water (interacting molecule) and permanents gases such as nitrogen, dioxygen and carbon dioxide penetrants were used as very sensitive structure probes, differing in size, shape and chemical nature, to investigate physico-chemical properties of the PBSA and to evaluate the PBSA/clays interfaces (in terms of free volume, rigid and mobile amorphous structures…).

2. Experimental 2.1 Materials PBSA pellets were supplied by Natureplast (France) under the trade name “PBE001”. The chemical structure of the PBSA is given in Figure 1. 1H NMR measurements have confirmed the presence of ~ 4 butylene succinate (BS) group per 1 butylene adipate (BA) group. The Melt Flow Index (MFI) is 5g/10min at 190°C.



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Figure 1 : Chemical structure of poly[(Butylene Succinate)-co-(butylene Adipate)] Native montmorillonite [Cloisite Na+ noted CNa], with a cation-exchange capacity (CEC) of 92.6 meq/100g, and organo-modified montmorillonite [Cloisite 30B noted C30B], with a

CEC of 90.0 meq/100g were provided by BYK Additives, Germany. Cloisite C30B containing ~30 wt% of surfactants ((methylbis-2-hydroxyethyl) tallow quaternary ammonium) was chosen because of good dispersion in the PBSA matrix, as mentioned in the literature30.

2.2 Elaboration of the composite films The composite films were prepared using a protocol modified from that applied to well disperse native and organo-modified clays in a polyamide 6 matrix, as recently described31. Initially, PBSA pellets were dried at 70 °C for 15 hours while CNa and C30B clays were used as-received without drying step in order to optimize the dispersion and exfoliation levels of fillers in matrix, as already shown by Tenn et al.32. PBSA pellets and 15 wt% of inorganic platelets (of CNa or C30B) were melt-mixed in a corotating Clextral BC45 twin-screw extruder, using a screw speed equal to 110 rpm, a polymer throughput adjusted to 10 kg.h-1 and a temperature profile from 120 to 160 °C between the feed to the die. At the end of the extruder die, rushes were cooled down in liquid water and then pelletized. The resulting pellets were rapidly dried at 70 °C for 24 hours to prevent the degradation of polymer chains of the biodegradable matrix. In order to obtain loaded-composite films with 2.5, 5, 7.5 and 10 wt% of fillers, the composite pellets were diluted with unfilled PBSA pellets in a corotating twin-screw extruder Krupp WP ZSK25, using a screw speed equal to 400 rpm, a polymer throughput adjusted to 8 kg.h-1 and a temperature profile from 120 to 180 °C between the feed and the die. During the extrusion process, liquid water was injected under high pressure (20-30 bar) into the high compression zone of the extruder (Figure 2). The 1.8 kg.h-1 throughput of water was experimentally 6 ACS Paragon Plus Environment

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adjusted in order to obtain homogenous rushes at the end of the extruder die, which was immediately cooled in liquid water and pelletized. Again, the resulting pellets were dried at 70 °C for 15 hours.

Figure 2 : Schematic representation of the water-assisted extrusion process33 Finally, the composites pellets were transformed into films with a single-screw Haake Thermo extruder, using a screw speed adjusted at 40 rpm and a profile temperature from 100 to 135 °C between the feed and the die. At the end of the die, the matter was calendared to obtain the composite films having an average thickness equal to 250 µm. Samples extruded using or not water-injection

(W) were

referenced as

PBSA+filler(filler content wt%)-W

and

PBSA+filler(filler content wt%), respectively.

2.3 Structure-microstructure analysis 2.3.1 Gel Permeation Chromatography (GPC) The molecular weight of PBSA chains of the composite films was determined on a Varian PL 50 plus using a PLgel 5 µm Mixed-C column. The flow rate of the solvent (dichloromethane) was adjusted to 1 mL.min-1 and polystyrene standards were used for calibration. Fillers were previously removed from the composite films by means of two filtrations of dissolved films in chloroform using filters of 0.45 µm pore size. Each measurement was duplicated.



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2.3.2 Differential Scanning Calorimetry (DSC) Thermal analyses of samples were performed using a TA Instrument DSC-Q100 under nitrogen atmosphere. 5-7 mg of films were placed on aluminum pan and heated at 10 °C.min-1 from -60 to 150 °C. The glass transition temperature (Tg), the heat capacity step at Tg normalized to the polymer fraction (∆Cp), the melting temperature (Tm) and the degree of crystallinity (Xc) of films were determined from the first heat, which corresponds to the microstructure of the film tested in permeation measurements. The degree of crystallinity was calculated as follows: ∆Hm − ∆H

Xc (%) = 100 ⋅

cc 0 ∆H ⋅ (1 − W ) m f

Equation 1

where ∆Hm is the melting enthalpy, ∆ H cc is the cold crystallisation enthalpy, ∆ is the

theoretical melting enthalpy of the completely crystalline PBSA, estimated to 113.4 J.g-1

34

and the nanofiller content determined by thermogravimetric measurements. As mentioned in the literature, the amorphous phase of the PBSA can be divided into two interconnected fractions, those are the mobile amorphous fraction (MAF) and the rigid amorphous fraction (RAF)33. The MAF was calculated as follows: % =

∆

∆

. 100

Equation 2

The RAF was determined by: % = 100 − +

Equation 3

where ∆C is the heat capacity step of the amorphous PBSA determined by the mean of flashDSC in a previous work33.

2.3.3 High Pressure Differential Scanning Calorimetry (HPDSC) A high pressure differential scanning calorimeter (Mettler Toledo HPDSC 827e, maximum pressure: 100 bar) was used in order to study the phase separation or miscibility of PBSA and 8 ACS Paragon Plus Environment

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water at high pressure and high temperature. The measuring chamber is connected to a pressure controlling valve (Brooks P.C. 5866 series) regulated by a valve controller (ReadOut & Control Electronics 0152). Thanks to this device, pressure and temperature can be independently set in the DSC oven and the heating/cooling curves measured at constant pressure, allowing the simulation of the processing conditions. In order to reproduce the conditions within the extruder, PBSA powder and water were blended at a weight ratio of 80/20, the full sample weighing around 10 mg. PBSA pellets were freeze-ground in a grinding device (Pulverisette14, Fritsch) at 14,000 rpm.

2.3.4 ThermoGravimetric Analysis (TGA) TGA analyses were performed on a TA Instruments Q500 TGA. Films were heated at 10 °C.min-1 from 30 °C to 800 °C under an oxidative atmosphere (N2 80%, O2 20%) in order to determine the inorganic part of filler Wf incorporated into the composite films after the complete degradation of the PBSA matrix.

2.3.5 Transmission Electron Microscopy (TEM) The dispersion and exfoliation levels of clays were imaged by using a transmission electron microscope LEO 922 (Zeiss) with a 200 kV acceleration voltage. The films were cut at -30 °C with a Reichert cryo-microtome at -30 °C. Ultrathin sections of about 95 nm were obtained using a cryo-diamond knife (Diatome) with a cut angle of 35°. Then, the resulting sample was transferred to copper grids of 400 mesh.

2.3.6 Wide Angle X-ray Scattering (WAXS) Basal distances relative to clays present in the PBSA matrix were evaluated on a AXS Brucker AXS Advance diffractometer using a cobalt radiation source (λ = 1.789 Å), a voltage



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of 35 kV and a current of 40 mA. Analyses were performed with diffraction angles 2θ ranging from 2° to 14° with an increment of 0.04 °/step. 2.3.7 Melt rheology Exfoliation levels of nanoclays into the composite films were studied on a Thermo Fisher Scientific Haake Mars rheometer III equipped with a 35 mm cone-plate diameter, using an oscillatory mode at 120 °C. Measurements were carried out under N2 atmosphere to prevent the degradation of polymer chains. Before analyses, the films were dried at 70 °C for 15 hours at least to remove eventual plasticization effects induced by water molecules. The linear domain of the tested films was determined using a 100 rad.s-1 angular frequency.

2.3.8 Tensile tests The unfilled film and the composite films were dumbbell-shaped with dimensions of 30 in length × 4 mm in width under 0.25 mm of thickness. Uniaxial tensile tests were conducted at room temperature (~ 23 °C) and at room relative humidity (~ 50 % RH) on an Instron 5543 (USA) tensile testing apparatus equipped with a 500 N captor at a crosshead speed of 50 mm.min-1, as indicated by Ray et al.35. From at least ten specimens per each film, the average value of the Young modulus (E) was deduced.

2.3.9 Gas permeation (N2, O2, CO2) Transport properties of films towards gas molecules were studied at 25 °C applying the variable pressure method called “time-lag method” on a lab-made apparatus described by Marais et al.36. The film (active surface of 11.3 cm2) was introduced in the cell measurement composed of two compartments called upstream and downstream compartments. First, a high vacuum (< 10-3 bar) was performed in the two compartments for 15 hours in order to remove impurities. Then, 4 bar pressure of gas was applied in the upstream compartment while the


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downstream compartment was maintained under vacuum. The driving force initiated by the pressure gradient applied in the cell measurement led to the film to be crossed by the gas. Therefore, the pressure in the downstream compartment measured with a pressure sensor (Druck, Effa AW-10-TA, Germany) was increased. The increase in pressure allows to determine the gas flux J which passed through the film as function of time. At the steady-state of permeation, a stationary flux Jst was reached and gives access to the permeability coefficient P as follows: P=

J st ⋅ L ∆p

Equation 4

where is the stationary flux of gas molecules, ! is the thickness of the film and ∆" is the pressure difference between the upstream and the downstream compartments. The intercept of the asymptote flux curve with the time axis corresponds to the time “timelag” #$ . By assuming no plasticizing effect induced by gases, the diffusion coefficient % is calculated by: D=

L2

Equation 5

6 .t l

As usually admitted for permeation process of small gas molecules in rubbery polymers, the solubility coefficient S to gas molecules can be deduced from: S =

P D

Equation 6

2.3.10 Water permeation Water transport properties of the PBSA-based films were studied at 25 °C applying the pervaporation method on a lab-built apparatus, as described by Alexandre et al.37. The film with an active surface of 19.74 cm2 was introduced in the cell measurement, separating the upstream and the downstream compartments. First, the two compartments were dried with a flow rate of 560 mL.min-1 of dry nitrogen until obtaining a very low water concentration (< 11 ACS Paragon Plus Environment


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2.5 ppmV) measured by a mirror hygrometric detector (General Eastern Instruments, USA) placed at the end of the downstream compartment. Then, the nitrogen flux in the upstream compartment was substituted by liquid water (Milli-Q®) so that water flux J penetrating across the film was measured in the downstream compartment as function of time. At the steady-state, the J flux reached a constant value noted Jst (corresponding to the stationary flux) from which the permeability coefficient is determined: &=

'() .*

Equation 7

∆+

where ∆,, the water activity difference between the upstream and the downstream compartments, can be considered as equal to 1 as the water activity in the downstream compartment is close to 0. It is well-known that water often induced plasticizing effects in polymers38,39. Therefore, the diffusion coefficient dependence with the water concentration was analysed according to the method developed by Follain et al.40, briefly described below. The calculated dimensionless curve of flux with time - = . / is plotted by solving the Fick’s diffusion equations, where -=

'

'()

and / =

0.

*1

. By assuming % constant, the diffusion coefficient was determined at two

characteristic points of the - = . / curve; the inflexion point I (/2 = 0.0091; - = 0.24) and the time-lag point L (/* = 1/6; - = 0.62). The two points corresponding to two different times of the extended permeation flux, gives access to two diffusion coefficients named %2 and %* , as given below: %2 =

89 .*1

Equation 8

%* =

8: .*1

Equation 9

9

:

A %* value superior to %2 is usually explained by plasticizing effects induced by water, which increase the diffusivity of water molecules through the film by increasing the free volumes. This behaviour can be simulated by considering the exponential law of D:


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% = % . ; =

@ 1 A % B . CB ∆ D

Equation 11

where B@ (upstream side) and BE (downstream side) are the water concentrations at the boundary conditions of the permeation process. In our case, < % >=

0F G0 &~+G > &~ > &~G . It is worth noting that the water-assisted extrusion is particularly efficient for the PBSA+CNa-W films, especially at high filler contents (≥ 5 wt%). All the results can be directly correlated with the dispersion and exfoliation levels of fillers in the PBSA matrix. Indeed, the more the filler dispersed and exfoliated, the more the



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composites barrier to gases. These results are explained by the tortuosity effects induced by lamellar fillers. Clays are usually considered as impermeable entities to gases acting as obstacles to the diffusion of gas molecules, which increases the diffusion pathway and accordingly improves the barrier properties. Many models have been developed based on the tortuosity concept, particularly the well-known Nielsen-Bharadwaj model described below: &J$+

J

=

~()H ~)H

=

@G

Equation 14

. D @ . 1

where is the filler orientation factor (} = . l. ′ − o) with ′ is the angle formed

by the lamellar fillers with the plan of the diffusing molecules flux, \G is the filler aspect ratio, and ¡ is the filler volume fraction calculated as follows : ¡ = ¢1 +

£ @G £¤ .

G@

¥

Equation 15

where ¦ and ¦£ are the bulk density of fillers (¦+ = 2.86 g. cmG and ¦ = 1.98 g. cmG) and of the matrix (¦~ = 1.26 g. cmG ) respectively, and ª is the filler mass fraction. Considering the geometrical parameters and \G as being adjustable parameters of the model, two approaches can be used to fit the experimental results of the relative permeability. The first approach consists in fixing the aspect ratio \]^L (determined from TEM images) and to determine the mean orientation factor using the residual sum of square (RSS) method. The second approach consists in fixing the value of the orientation factor between 0 (lamellar filler randomly oriented) to 1 (lamellar fillers oriented in the parallel direction to the plane surface of the film). The mean aspect ratio \G was then deduced from the best fitting of experimental data. As the aspect ratio \ ]^L obtained from TEM images is usually underestimated37,45,46, it was more judicious to apply the second approach. The orientation factor was fixed to 1 for all composite films, except for the PBSA+CNa films. In this case, the lowest values of the RSS (Figure 10) were obtained for 0 ≤ ≤ 0.4, in good accordance with TEM images. Indeed, large aggregates were observed without particularly 28 ACS Paragon Plus Environment

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orientation. For each film, the average aspect ratio determined from the Nielsen-Bharadwaj model is found higher than the aspect ratio obtained from TEM images, which tends to confirm the underestimation of the filler dimensions from TEM images. Also, it must be taken into account the fact that the aspect ratio depends on the filler content so that the calculated aspect ratio represents an average estimation of the aspect ratio of fillers in the composite films. The confrontation of \]^L and \G values only allows to validate the tendency of the variation of the average aspect ratios of the composite films as function of montmorillonite (CNa or C30B) and of the process used. The average aspect ratio values are ordered as follows: \~+ < \~+G < \~ < \~G , which confirms that the water injection during extrusion enables to improve the dispersion and exfoliation levels of native and organo-modified montmorillonites in the PBSA matrix and that C30B platelets are more exfoliated than CNa ones, whatever the process used. Nevertheless, NielsenBaradwaj model is based on the tortuosity concept considering only the influence of the average aspect ratio and the orientation of fillers. The transport properties of nanocomposite films are however more complex and can also notably be controlled by other physicochemical parameters like the filler/polymer chains interfaces, the local rigidity in the vicinity of fillers, the variation of free volumes and/or the microstructure. To have a better insight in mechanisms occurring in the composite films, it is therefore necessary to determine the kinetic and the thermodynamic parameters, which both influence the permeability. The solubility coefficients are gathered in Figure 11.



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Figure 11 : Solubilities to gas molecules in the PBSA-based composite films

By considering the measurement uncertainties, the solubility to gas molecules in the PBSAbased films does not seem to be highly impacted neither by the incorporation of CNa and of C30B, nor by the preparation process used. Therefore, it can be inferred that the variation of the gas permeabilities for the PBSA-based films is not mainly governed by the thermodynamic parameter. The diffusion coefficients were consequently determined and are plotted in Figure 12.


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Figure 12 : Gas diffusivities in PBSA-based composite films

The decrease of the diffusion coefficients as function of the nanofiller content is consistent with the permeability results. Besides, for each gas tested, the values of the diffusion coefficients can be ranged as follows: %~+ > %~+G > %~ > %~G . These results show clearly that the barrier properties are mainly dependent on the kinetic parameter which is related to the tortuosity effects. All the permeametric results are in good correlation with the dispersion and exfoliation levels of fillers in the PBSA matrix. Indeed, the higher the dispersion and exfoliation levels of fillers, the lower the diffusivity and consequently the permeability. To complete the investigation on transport properties, the transport properties to gases are usually associated to 31 ACS Paragon Plus Environment


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water transport properties because water permeation kinetics could depend not only on the structure-microstructure morphology of films but also on the chemical nature (polar groups) of the fillers and the matrix, which can induce new interactions.

3.3.2 Water permeation Water permeabilities of the PBSA film and the PBSA-based composites were measured and are plotted in Figure 13 as function of the -filler content.

Figure 13 : Permeabilities to water of the PBSA-based composite films

The profiles of the curves representative of the permeability to water and to gases are quite similars. Indeed, the water permeabilities of the PBSA+C30B films are lower than those of the PBSA+CNa films and the use of the water-assisted extrusion enables to reduce the water permeability of the PBSA film, whatever the montmorillonite. Moreover, as for gas permeation, the efficiency of the water-assisted process seems to be higher for the CNaloaded composite films than for the C30B-loaded composite films, especially at high filler 32 ACS Paragon Plus Environment

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content (≥5 wt%). One difference is shown in Figure 13: a slight increase of the permeability is measured for the PBSA+CNa films at the highest filler content. In order to understand these results, the water diffusion coefficients were determined at different times of the extent of water permeation kinetics and are gathered in Figure 14.

Figure 14 : Water diffusion coefficients ¬w , ¬w. , ¬w.®, ¬z corresponding to various water concentrations and average water diffusion coefficient < % > in PBSA-based composite films

The diffusion coefficients named % , %.E¯ , %.°E , and %L correspond to the diffusion coefficients calculated according to the concentration of water sorbed in the film during permeation; from 0 (to calculate % ) to the maximum water concentration at equilibrium state BJK (to calculate %L . < % >, corresponding to the mean integral diffusion coefficient, can be considered as the average water diffusion coefficient. The variation of the water diffusion coefficients as function of the filler content are rather similar and is compatible with the tendency exhibited from gas diffusion coefficients (Figure 12). Again, it is worth noting that diffusivity is directly correlated with the dispersion and exfoliation levels of fillers in the PBSA matrix. Surprisingly, the water diffusivity in the PBSA+CNa films continuously decreases, even at high filler contents, while the permeability 33 ACS Paragon Plus Environment


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increases. To explain this unobvious result, others phenomena took place which have counterbalanced the tortuosity effects induced by clays. From Figure 14, it is clear that the water diffusivity increases with the water concentration (diffusivity ranking is %L > %.°E > %.E¯ > % ). This result is usually attributed to the plasticization of polymer chains induced by water sorbed during permeation process. In addition, the affinity of water molecules according to the clay content can also be highlighted through the change in water concentration in the film at the stationary state BJK (as plotted as a function of filler content in Figure 15).

Figure 15 : Water concentration Q±² in the PBSA-based composite films at the stationary state

As expected, the strong affinity between montmorillonites and sorbed water leads to higher BJK values in the composite films than in the unfilled film so that the higher the filler content, the higher the BJK . By increasing the filler content, a higher water concentration within the PBSA matrix is measured, which accordingly can induce a higher plasticizing effects. To evaluate this phenomenon, the plasticization factor =. BJK was thus determined and shown in Figure 16. 34 ACS Paragon Plus Environment

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Figure 16 : Plasticization factor ³. Q±² of water in PBSA-based composite films

Surprisingly, even if the water concentration increases as filler content increases, the plasticization factor decreases in most cases. From this finding, it seems that the highest =BJK values are obtained for the CNa-loaded films and in the case of the water-assisted extrusion process is applied to prepare composite films. In other words, the plasticizing effect of water is more pronounced for better dispersed filler with more hydrophilic nature in the PBSA matrix. It is well-known that the introduction of montmorillonite in a polymer matrix often leads to an increase of the stiffness of polymer chains47, limiting also the macromolecular mobility and thus increase the permanent free volumes. An improvement of the global rigidity of the PBSA chains (through the improvement of the Young modulus) and/or of the local rigidity (in the RAF) could also limits the solubility of water molecules as previously mentioned in a separate paper48. From tensile tests, the variation in Young modulus E was determined and is plotted in Figure 17. The variations of E as function of filler content are clearly consistent with rheological results (Figure 9), which indicates that the stiffness of polymer chains in the composite films is highly increased, especially when the fillers are well



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dispersed and exfoliated. Also, the highest E values are found for the C30B-loaded composite films prepared with the help of the water-assisted extrusion process.

Figure 17 : Young modulus E of the PBSA-based composite films

It is interesting to see that a very good correlation between the water permeability P and the Young modulus E can be stated, the variations of P and E being opposite as function of the filler content. Indeed, as previously mentioned the better dispersion of clays obtained with the water-assisted extrusion (-W) is at the origin of the better water barrier properties and that explains why the C30B-W and CNa-W samples have lower permeability compared to their respective homologous C30B and CNa. Also, the C30B filler being better dispersed and exfoliated in the PBSA than CNa load, the Young modulus of the nanocomposites increases much more in the presence of C30B, reflecting larger nanoclays-chain polymer interfaces. The use of water injection during extrusion significantly increases the stiffness of PBSA films loaded with CNa while it affects very little those loaded with C30B. This is consistent with observations made with TEM images, which showed the disintegration of CNa agglomerates by the use of water injection while it is difficult to distinguish the differences between C30B 36 ACS Paragon Plus Environment

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based-nanocomposite films prepared with and without water-assisted injection. Despite the improvement made by the injection of water on the dispersion of CNa fillers, C30B loaded films remain much more rigid.

4. Conclusion Composite films based on a Poly[(Butylene Succinate)-co-(butylene Adipate)] (PBSA) matrix and loaded with native or organo-modified montmorillonite (CNa and C30B) were prepared by the mean of an innovative water-assisted extrusion process. If the incorporation of C30B has induced a slight degradation of the PBSA chains during the preparation process, the incorporation of CNa or the use of the water-assisted extrusion has no degradation effect. The microstructure of the polymer chains was not strongly affected neither by the incorporation of fillers (CNa or C30B) nor by the injection of water. An increase of the rigid amorphous fraction RAF was clearly observed in respect with the filler content, particularly when well dispersion. C30B nanoplatelets are more dispersed and exfoliated in the PBSA matrix than the CNa ones, whatever the preparation process. The water-assisted extrusion process enables to improve the dispersion and exfoliation levels of both CNa and C30B platelets in the PBSA matrix. In the present study, the structure-microstructure-transport properties relationship was remarkably established and well correlated to the dispersion and exfoliation levels of nanoclays depending on the montmorillonites and on the extrusion process. Barrier properties of the composite films are directly dependent on the state of dispersion and exfoliation. The higher the platelets dispersed and exfoliated, the higher the barrier properties to gases and to water, so that the best performance was obtained with the PBSA matrix loaded with 10 wt% of C30B and extruded with water injection . Even if the water-assisted extrusion process is not sufficiently efficient as to compete with the organo-modification of the montmorillonite, it enables to considerably increase the barrier properties to composites loaded with native



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montmorillonite, especially at high filler content. Therefore, the use of the water-assisted extrusion process can be assumed as a promising way to prepare nanocomposite films due to its efficiency and to its rapid industrialization. For future works, water sorption measurements will be performed to determine the water solubility in the composite films and its variation according to the composite composition, which should confirm the hypothesis suggested for explaining both the increase in permeability for the PBSA+CNa films at high filler contents and the decrease in plasticizing effect when the filler content increases.

Acknowledgements The authors are thankful to the French Ministry for Research and Technology for financial support and M. Florian Bailly for mechanical tests during training course.

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Poly[(butylene succinate)-co-(butylene adipate)]-Montmorillonite

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