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Layered poly(ethylene-co-vinyl acetate)/poly(ethylene-co-vinyl alcohol) membranes with enhanced water separation selectivity and performance Jorge Arturo Soto Puente, Kateryna Yu. Fatyeyeva, Corinne Chappey, Stéphane Marais, and Eric Dargent ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14909 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017

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ACS Applied Materials & Interfaces

Layered poly(ethylene-co-vinyl acetate)/poly(ethylene-co-vinyl alcohol) membranes with enhanced water separation selectivity and performance J. A. Soto Puente 1,2, K. Fatyeyeva 1 *, C. Chappey1, S. Marais 1, E. Dargent 2 1 Normandie Univ, UNIROUEN, INSA Rouen, CNRS, PBS, 76000 Rouen, France 2 Normandie Univ, UNIROUEN, LECAP, 76000 Rouen, France *Corresponding author: [email protected] ABSTRACT A three-layered membrane based on poly(ethylene-co-vinyl acetate) (EVA) and hydrolyzed EVA – poly(ethylene-co-vinyl alcohol) (EVOH), was elaborated by the surface hydrolysis of a dense EVA membrane. Due to the chemical modifications, the three-layered EVOH/EVA/EVOH membrane was characterized by the particular microstructure (amorphous EVA and semi-crystalline EVOH) and the tunable hydrophilic/hydrophobic balance. Also, these modifications led to the membrane with the selective barrier properties compared with the pure EVA and completely hydrolyzed EVOH membranes. The water barrier behavior was related to the strong hydrogen bond interactions of water and vinyl alcohol groups whereas the weak chemical interactions were revealed for gases (N2 and O2). Besides, the influence of the polymer rubbery or glassy state on the permeation kinetics was established. In the case of the threelayered membrane the considerably high selectivity values were obtained for H2O/O2 (~ 11900) and H2O/N2 (~ 48000) at 25 °C. In addition to these highly selective properties, the three-layered structure does not present delamination features due to its elaboration procedure. Thus, these new layered membranes are very promising as selective materials for the water and gas separation and can be potentially used in food packaging or for the gas dehydration. KEYWORDS EVA, EVOH, water and gas permeation, selectivity, tortuosity effect INTRODUCTION The research of polymers used as barrier materials for food packaging and separation processes 1

has been extensively developed. In the case of food packaging, several strategies are used to improve the barrier properties of polymers, such as co-extrusion or metallization to obtain layered structures, addition of nanoparticles to modify the diffusion pathway, and surface treatment (for example plasma treatment) to

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change the membrane hydrophilicity or hydrophobicity. However, the layered structures can present a risk of delamination, the nanoparticles in the composite membrane can release and the efficiency of surface treatment can decrease with time. On the other hand, the performance of materials used for separation processes is generally based on the membrane pore size and is explained by concentration/pressure gradient. However, the limited mechanical and chemical stability restricts their applications. Usually the “dissolution-diffusion” mechanism is used to describe the transport phenomena through the dense polymer membranes.2 The transport of small molecules, i.e. a random molecular motion of these molecules, so-called diffusivity, is mainly controlled by the polymer free volume. Besides, it is known that the chemical modification of the membrane can generate different interactions with diffusing molecules (solubility) and allow controlling the selectivity. Poly(ethylene-co-vinyl acetate) (EVA) is a commercially available polymer with several applications, 3

ranging from hot melt adhesive to cable sheathing. The EVA microstructure changes as a function of the vinyl acetate (VAc) monomer content.4

- 6

It is reported that EVA with the VAc content up to 10 wt. % is

more transparent and flexible than low density polyethylene (LDPE) and when the VAc content is higher than 30 wt. %, EVA shows rubber-like properties.7 Moreover, the acetate group hinders the crystallization of the ethylene part at the VAc content higher than 40 wt. % and EVA becomes completely amorphous.

8

EVA can be chemically modified by the hydrolysis reaction where the vinyl acetate group on the polymer backbone is replaced by the vinyl alcohol (VOH) group and thus enhances the polymer hydrophilicity.

9, 10

The partial hydrolysis of EVA allows obtaining a material with tunable properties, for which the vinyl alcohol groups ensure the hydrophilic behavior and the ethylene and VAc groups – the hydrophobic character. The completely hydrolyzed EVA, i.e. EVOH, is a polymer with a semi-crystalline structure.10, 11 The transport properties of EVA copolymers towards water and gases are widely studied; it has been concluded that an increase of the VAc content improves the H2O/O2, CO2/O2 and CO2/N2 selectivity for food packaging and the gas separation technology.

12, 13

On the other hand, among the polymers usually

used in packaging EVOH has one of the lowest gas permeability, particularly towards oxygen (0.01 Barrer).

11, 14

However, as a function of the vinyl alcohol content, EVOH can be a hygroscopic material and

at elevated humidity the water molecules plasticize the polymer,15 thus significantly decreasing the gas 16 - 18

barrier performance.

Only polyvinyl alcohol (PVA) has a lower permeability (0.002 Barrer) than EVOH,

but PVA is soluble in water and the melting temperature of PVA is higher than the degradation one, which

2


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limits its application in food packaging.17 Moreover, the barrier properties and, consequently, the selectivity parameter are reported to vary as a function of temperature. The efficiency of separation processes, such as post-combustion capture and metal separation, is based on a high selectivity parameter or a separation factor.

19, 20

The selectivity values of materials used

for food packaging are in the wide range from 10 to 4000 and from 10 to 10000 for H2O/O2 and H2O/N2, respectively. For example, the H2O/O2 selectivity of polyethylene (PE) and polypropylene (PP) commonly used in food packaging are 1821 and 4022, respectively. The gas O2/N2 selectivity of polyimide (PI) is ~ 23

170

® 24

and the selectivity between hydrocarbons C3H6/C3H8 is ~ 20 for commercial PI (Matrimid ).

Typically, the high selective materials for the gas separation present a separation factor higher than 8 for 25

26

O2/N2 and higher than 50 for CO2/N2 and CO2/O2.

In our previous work, a new strategy was used to obtain a three-layered structure (EVOH/EVA/EVOH) by the surface hydrolysis reaction of the EVA membranes.

10

A gradient of

amorphous/semi-crystalline structure and a tunable hydrophilicity/hydrophobicity behavior can be obtained by controlling the hydrolysis time. Besides, the delamination problem existing in other layered membranes is not observed in this three-layered structure due to the chemical bonds at the interfaces of the EVA and EVOH layers. The present study aims to correlate the chemical structure and, consequently, the microstructure (amorphous EVA and semi-crystalline EVOH) of the hydrolyzed EVA membranes with their barrier properties (permeability, diffusion and selectivity) towards water and gases. These properties were also evaluated as a function of the rubbery (above the glass transition temperature Tg) and glassy (below Tg) state by temperature T increasing. The selectivity performance of water and gases will be discussed on the basis of both the molecular mobility of the amorphous phases of the EVA and EVOH layers and the specific interactions between the water molecules and the polar EVOH groups (hydrophilic/hydrophobic balance). EXPERIMENTAL Materials -1

®

The EVA pellets with 70 wt. % (43 mol %) of VAc and Mw = 268 000 g mol (LEVAPREN 700 ) were kindly provided by Lanxess Co. Dichloromethane (99 % purity) and methanol (99.5 % purity) were purchased by Alfa Aesar and VWR, respectively. Glycerol (99.5 % purity, Alfa Aesar), diiodomethane (99 %

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purity, Acros), sodium hydroxide (Acros) and 37 wt. % hydrochloric acid solution (Merck) were used as received. Deionized water was obtained from a Millipore Milli-Q purification system. N2 (99.99 % purity, Air Products®), O2 (99.99 % purity, Air Liquide®) and CO2 (99.5 % purity, Air Products®) were used as received. Membrane elaboration and hydrolysis procedure The detailed explanation of the membrane preparation and the hydrolysis procedure is given in10. Briefly, a solution of EVA (10 wt. %) in dichloromethane was stirred at 16 ± 2 °C for 24h. The solution was cast on a glass plate with an applicator and after the solvent evaporation for 16h at 20 °C and atmospheric pressure the membrane was dried in an oven for 7h at 80 °C. The average thickness of the EVA membrane was ~130 µm. The free-standing and dense EVA membranes were submerged in a 0.8 M NaOH solution in methanol/water mixture (75 vol. % / 25 vol. %) at 16 ± 2 °C for the hydrolysis reaction during a definite time period, i.e. hydrolysis time (th). When th increasing, the thickness of the hydrolyzed layers (on the top and bottom surfaces of the membrane) increases. Then, the membranes were rinsed with deionized water and submerged in a 3.0 M HCl solution to stop the hydrolysis. Finally, the membranes were washed with deionized water until pH=7 was reached. Before the measurements, the membranes were stored in a desiccator under vacuum over phosphorus pentoxide P2O5 (Acros) at 16 ± 2 °C. Characterization techniques The chemical characterization of the membrane surface was carried out by Fourier transform infrared spectroscopy (FTIR) with attenuated total reflection (ATR) using a Nicolet Avatar 360 (Thermo -1

Fisher) spectrometer. Spectra were obtained in the range from 600 to 4000 cm using 128 scans and a resolution of 4 cm-1. Spectra were smoothed by using Omnic® software (version 5.2a). The hydrophilicity of the membrane surface was characterized by the contact angle measurements using a Multiskop goniometer (Optrel) at 20 ± 1 °C and three different liquids (deionized water, glycerol and diiodomethane). For each measurement, a 3 µL drop of liquid was formed at the tip of the syringe. After the liquid dripped onto the membrane surface, the contact angle was measured within 5 s by a sessile drop method. For each sample five drops were placed at different locations uniformly on the membrane surface and the average value of measurements was calculated.

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The thermal characterization of the membranes was carried out by a differential scanning calorimetry (DSC) using a Q100 apparatus (TA Instruments). The temperature calibration was performed with benzophenone and indium standards and the energy calibration was performed with indium standard. The measurements were carried out under nitrogen atmosphere with a sample size of about 10 mg in an aluminum hermetic pan. The measurements were performed in a temperature range from -50 to 180 °C -1

st

with a heating rate of 10 °C min and a heat/cool/heat cycle (only the 1 heating ramp will be used to correlate the membrane microstructure with the transport properties). The membrane crystalline structure was analyzed by wide angle X-ray diffraction (WAXD) measurements using a D8 Advance AXS machine (Bruker) equipped with a CoKα radiation source (λ = 0.179 nm). X-ray spectra were obtained for the angles between 5° and 40° with the scanning speed of 0.5 s/step and the angle increment equal to 0.04°/step. The patterns were decomposed into crystalline peaks ®

and amorphous halo by using Grafity software (version 0.4.5). The degree of crystallinity Xc was calculated from DSC curves (Equation 1) and from X-ray diffraction patterns (Equation 2).

 ∆H m  X C DSC (%) =  0  ⋅ 100 ∆ H m 

(1)

A  ⋅ 100 X C WAXD (%) =  C   ( AC + AA )

(2)

where ∆Hm is the experimental melting enthalpy, ∆H

0 m=

-1

157.8 J g is the PVA equilibrium melting enthalpy

(theoretical value of the polymer supposed to be 100 % crystalline)27, 28, AC is the area under the crystalline peak and AA is the area of the amorphous scattering contribution. The permeation measurements were performed by using the home-made setup described previously.

29

In brief, the dried polymer membrane is placed in the permeation cell. The downstream and

upstream compartments are under the dry nitrogen flow (560 mL min-1) until a dew point value below -70 °C (2.5 H2O ppm at pressure = 1.013 bar) is reached. Then, a stream of deionized water (Milli-Q) is added in the upstream compartment and water concentration is monitored in the downstream compartment using a mirror hygrometer (General Eastern). When the first water molecules crossed the entire membrane thickness and were conducted by dry nitrogen in the downstream compartment to a mirror hygrometer, the

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dew point value increased as a function of time (transient state) and after certain time, the dew point reached a constant value corresponding to the stationary state (Figure 1a). The measurements were performed in a temperature-controlled oven (± 1 °C) for at least three different samples for each temperature condition (from 15 up to 75 °C). The average standard deviation of the permeability coefficient (P) was estimated at 5 %. P was calculated according to:

P=

J st L

(3)

∆p

where Jst corresponds to the stationary flux, L is the thickness of the membrane and ∆p is the difference of vapor pressures between the upstream and downstream compartments. The plasticization effect caused by the permeant inside the polymer increases the polymer free volume. Thus, the diffusion coefficient D dependency on the permeant concentration is defined as follows: 30, 31

D = D0 e γ C

(4)

where D0 is the value of the diffusion coefficient when the water concentration in the membrane C approaches zero (C→0) and γ is the plasticization coefficient. The water concentration in the stationary state (C = Ceq) (Figure 1a) allows obtaining the maximal diffusion coefficient DM (= D0 eγCeq) with γCeq as a plasticization factor. In addition, the mean integral diffusion coefficient summarizes the plasticization effect and the diffusion coefficient variation when the polymer/diffusing molecules interactions are strong by the following equation:

D =

1 Ceq

Ceq

DM − D0

∫ D (C )dC = γ C 0

(5)

eq

31, 32

Gas permeation properties were evaluated by the “time-lag” method

at different temperatures

(from 25 to 75 ± 1 °C). Briefly, the thermostated permeation cell was evacuated by applying vacuum on the downstream and upstream compartments. Then, the upstream side was provided in gas under the manometric pressure at 3 bar (absolute differential pressure ∆p = 4 bar). The pressure (p) in the ®

downstream side was measured with a sensitive pressure gauge Druck (0 - 10 mbar) connected to a data acquisition system. At the beginning, the downstream pressure is negligible compared to the upstream pressure (pupstream >> pdownstream) and during the measurement, the downstream pressure increases as

6


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Figure 1. (a) Dew point in the downstream compartment vs. time and (b) pdownstream vs. time during the water and the gas permeation measurements, respectively as function of time t (transient state) until the steady state is reached (i.e. linear variation of pdownstream with time t dpdownstream/dt) (Figure 1b). Reported results are the average values of at least four measurements for each gas and each temperature. The permeability coefficient P was calculated according to the variable pressure method with the following equation:33

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P=

L V T0  dp downstream    S pupstream p0 T  dt 

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(6)

where V is the volume of the downstream compartment, S is the active surface of the membrane and T is the experimental temperature. Finally, T0 and p0 correspond to standard conditions of temperature and pressure, 273.15 K and 76 cm Hg, respectively. The permeability coefficients (Equations 3 and 6) are generally expressed in Barrer = 10-10 cm3 (STP) cm s-1 cm-2 cmHg-1. The gas diffusion coefficient D was calculated from the time-lag tL value (extrapolation of dpdownstream/dt straight line to pdownstream equal to zero as shown in Figure 1b) by the following equation:34

D=

L2 6 tL

(7)

The water vapour sorption kinetic measurements were performed using an automatic gravimetric dynamic vapour sorption system DVS1 Advantage (Surface Measurement Systems) equipped with a Cahn microbalance. The measurements were carried out at the constant temperature (25.0 ± 0.2 °C) and for different water activity (aw) values ranging from 0 to 0.95.Approximately 10 mg of membrane was loaded onto the stainless steel mesh pan that was placed in a closed chamber. First, the membrane was completely dehydrated (aw = 0) until a constant dry weight (m0) was reached. Then, the membrane was submitted to a hydration cycle. For this purpose, humidified nitrogen of known water activity was passed through the chamber constantly. The change of the membrane weight was recorded for each water activity tested and the weight at each equilibrium state meq was used to construct the sorption isotherm. The water concentration Ceq (in mmol/g) was estimated as follows:

C eq =

1000 (meq − m0 )

(8)

MWw m0

where MWw is the molecular weight of water. In literature, several models are reported to fit such isotherms taking into account the mechanisms of sorption and possible interactions between the sorbed water molecules and the polymer chains.

35, 36

In our

case, the experimental sorption isotherms were fitted to Park’s sorption isotherm model (Equation 9) that supposes the combination of three mechanisms: the specific sorption on special sites (Langmuir’s mode) at

8


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low water activity aw (aw < 0.2); the non-specific sorption (Henry’s law) for water activity range up to 0.5 and the water molecule clustering at high water activity (aw > 0.5).35

C eq =

AL bL a w + k H a w + n k a k H a Hn 2O 1 + bL a w

(9)

where AL is the average concentration of Langmuir’s sites, bL is an affinity constant, kH is the Henry’s law coefficient, n is the average number of molecules per cluster and ka is a clustering equilibrium constant. RESULTS AND DISCUSSION Membrane characterization In our previous study, the effect of the hydrolysis time th on the microstructure of the EVA membranes and its impact on the water sorption and permeability behavior were reported.

10

It was found

that the tunable layered structure may be obtained by controlling the hydrolysis time (Figure 2). For example, the formation of a three-layered membrane (EVOH/EVA/EVOH) was revealed for the th value between 2h and 12h by means of FTIR (Figure 3a) and fluorescent microscopy (see Supporting Information Figure S1) measurements. FTIR spectroscopy was used to study the chemical modification of the EVA membrane during the hydrolysis reaction as the VAc groups are replaced by the VOH groups (Figure 2). The notable changes of FTIR spectrum can be clearly observed during EVA hydrolysis (Figure 3a). The CH2 stretching peak at 2930 cm-1, CH2 scissoring peak at 1420 cm-1 and CH2 deformation at 1330 -1

cm were observed for all samples. For the EVA membrane, the characteristic peaks related to the VAc group are clearly visible at 1735 cm-1 (C=O stretching) and at 1090 cm-1 and 1250 cm-1 (C-O stretching).37 For the three-layered (EVOH/EVA/EVOH) and completely hydrolyzed (EVOH) membranes, the characteristic peak of -OH stretching of the intermolecular hydrogen bonding at 3300-3400 cm-1 revealed -1

the presence of the VOH groups on the membrane surface. Besides, the intensity of C=O (at 1735 cm ) and C-O (at 1250 cm-1) stretching peaks decreased for the three-layered membrane and these peaks completely disappeared for the completely hydrolyzed membrane confirming the presence of pure EVOH on the membrane surface.

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Figure 2. Schematic representation of the surface hydrolysis reaction of the EVA membranes Such a three-layered membrane has a gradient chemical structure due to the surface hydrolysis without the risk of delamination problem on the EVA/EVOH interfaces. The three-layered structure obtained after th = 8h with a thickness of each hydrolyzed layer (Lh) (EVOH) equal to 28 ± 4 µm and the EVOH/EVA/EVOH structure corresponding to one fourth (EVOH), one half (EVA) and one fourth (EVOH) of the membrane total thickness, has shown a high water capacity sorption (~ 120 %) and the improved water management control.10 A completely hydrolyzed (EVOH) membrane structure confirmed by fluorescent microscopy (see Supporting Information Figure S1) and 1H NMR analysis (not shown here) was obtained for th ≥ 72h (Figure 2) without any membrane thickness variation. In addition, the microstructure of the membrane was also modified during the hydrolysis as confirmed by DSC (Figure 3b) and wide X-ray diffraction (Figure 3c) measurements. As observed in Figure 3b, The EVA film with 70 wt. % of VAc groups demonstrates a characteristic thermogram of a fully amorphous material with an endothermic heat capacity step due to the glass transition at Tg = -15 °C. This result is in good agreement with the X-ray pattern presenting the amorphous halo for EVA (Figure 3c). For the three-layered membrane, a first glass transition referring to EVA was observed at -15 °C and a broader glass transition at higher temperature (~ 35 – 55 °C) related to the EVOH presence in the external layers of the membrane was detected at the heating rate of 10 °C min-1 and confirmed by the fast -1

calorimetry measurements at the heating rate of 50 °C min (inset to Figure 3b). However, EVOH in the hydrolyzed layers does not have a high degree of crystallinity and, thus, the endothermic peak related to the crystal melting at the temperature range from 80 to 130 °C is broad and weak. The average value of Xc calculated from DSC measurements for this membrane is lower than 5 %. This result is confirmed by the wide angle X-ray diffraction pattern (Figure 3c) exhibiting a crystalline peak of slight intensity related to the plane (200) of the orthorhombic EVOH crystals at 2θ = 24.0 ± 0.2°.38 For the completely hydrolyzed (EVOH) membrane, the presence of a semi-crystalline microstructure is clearly seen by the endothermic

10


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Figure 3. (a) FTIR spectra; (b) DSC curves (heating rate: 10 °C min-1), and (c) WAXD patterns for the EVA, EVOH and three-layered (EVOH/EVA/EVOH, th = 8h) membranes. Inset to Figure 3b: DSC curve for the three-layered membrane (EVOH/EVA/EVOH, th = 8h) (heating rate: -1

50 °C min ) peak related to the melting of the EVOH crystals (melting temperature Tm is about 130 °C) (Figure 3b) and by the intense crystalline peak (Figure 3c). In this case the glass transition of the amorphous phase in EVOH is detected at ~ 35°C and the degree of crystallinity Xc determined from DSC and WAXD measurements is estimated at ~ 33 %. The influence of the hydrolysis reaction on the hydrophilic/hydrophobic balance of the elaborated membranes was studied by the contact angle measurements. Based on the contact angle data, the polar γp and dispersive γd components of the polymer surface free energy are calculated according the Owens39

Wendt approach (Table 1).

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d

p

Table 1. Water contact angle (θw) and surface free energy dispersion γ and polar γ components for the EVA, EVOH and three-layered (EVOH/EVA/EVOH, th = 8h) membranes Membrane

θw (°)

γd (mJ·m-2)

γp (mJ·m-2)

EVA

97 ± 3

31.1 ± 0.1

0.3 ± 0.2

EVOH/EVA/EVOH

78 ± 2

33.5 ± 0.2

4.4 ± 0.3

EVOH

93 ± 1

31.4 ± 0.3

0.5 ± 0.2

The EVA water contact angle value (θw ~ 97°, Table 1) can be explained by the low affinity of the water molecules and the VAc groups. This explanation is confirmed by the extremely low polar component p

p

-2

value γ (γ = 0.3 ± 0.2 mJ·m ). For the three-layered membrane, a significantly lower θw value (θw ~ 78°, Table 1) is obtained. This result is related to the presence of VOH groups on the hydrolyzed surface that have strong interaction with the water molecules through the hydrogen bonds. That’s why the increase of p

p

-2

the polar contribution to the free energy surface γ is observed (γ = 4.4 ± 0.3 mJ·m ). Really, the low crystallinity degree of the three-layered membrane (Xc ~ 5 %, Figure 3b and 3c) testifies to the presence of the greater number of the available VOH groups capable to interact with the water molecules compared to the completely hydrolyzed membrane. Indeed, the higher degree of crystallinity (Xc ~ 33 %, Figure 3b and 3c) obtained for the EVOH membrane explains the reduced access to the VOH groups and, so, the higher θw and lower γp values (Table 1). Such chemical and microstructural modifications, namely hydrophilicity of the VOH groups and hydrophobicity of the VAc and ethylene ones and Xc values, may have influence on the membrane barrier properties. Moreover, the change of the amorphous phase dynamics revealed by the glass transition increase can modify the permeability parameters at a given temperature due to the modification of the macromolecular mobility and free volume. The driving force in the transport phenomenon is a concentration gradient between two phases, the first one containing the diffusing molecules and the second one without or with a significantly lower concentration of these molecules.

40

During the permeation, the concentration difference (in the case of

water permeation) or the pressure difference (in the case of gas permeation) of penetrant molecules between two phases separated by a membrane decreases and this difference becomes negligible in the stationary state (Figure 1). The transport process involves three steps – sorption, diffusion and desorption.

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At the beginning, the molecules are absorbed at the membrane surface, then the molecules start to diffuse through the membrane and, finally, the diffused molecules are desorbed in the phase with the lowest penetrant concentration.30, 41 The material transport properties depend on the possible interactions of the polymer and the diffusing molecules. In addition, the transport behavior for a given penetrant depends on the free volume inside the polymer and on the segmental mobility of the polymer chains. As reported for different polymers, such as natural rubber42, poly(ethylene terephthalate) (PET) and several types of poly(ethylene) (PE)43, the permeability P decreases when the crystallinity degree increases. Indeed, the penetrant molecules diffuse only through the amorphous phase and the crystalline phase hinders the transport phenomenon by 44

increasing the diffusion pathway, also known as the tortuosity effect.

For example, the water vapor

permeability of semi-crystalline PET (Xc ~ 40 %) is four times lower than that of amorphous PET.43 Moreover, it is widely admitted that the temperature variation of the permeability and diffusion coefficients follows an Arrhenius law:2, 31

P = P0 e

 − EP   RT  

(10)

 − ED   RT 

D = D0 e

(11)

where EP and ED are the activation energy of permeation and diffusion, respectively, P0 and D0 are the pre-1

-1

exponential factors and R is the gas constant (8.314 J·K ·mol ). Water permeation The water molecules can be sorbed by the material in two ways, namely they can be randomly dispersed into the membrane or they can interact with the specific sites (if present) of the macromolecular backbone. From the permeation kinetics, a constant value of a dew point in the downstream compartment is reached in the stationary state and the stationary flux Jst through the polymer membrane under a vapor pressure gradient is obtained (Equation 3). This flux allows calculating the water permeability coefficient P for the different membranes (Table 2). By comparing values of P at any given temperature, a significant decrease can be observed with the EVOH content increasing. For example, the EVA membrane shows the P value equal to 16100 Barrer,

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whereas the EVOH membrane has a much smaller permeability coefficient (130 Barrer) at 25 °C. Similar behavior is revealed for the mean integral diffusion coefficient (Table 2). For example, is equal to 8.010-8 cm2·s-1 for the EVA membrane and 1.010-9 cm2·s-1 for the EVOH membrane at 25 °C. This temperature evolution of P and can be explained by the different microstructure and mobility of the polymer chains, as EVA is an amorphous material with Tg = -15 °C (i.e. in the rubbery state for the whole experimental temperature range) whereas EVOH is a semi-crystalline polymer with Tg ~ 35 °C (i.e. in the glassy state at temperatures below 35 °C and in the rubbery state at temperatures above 35 °C).10 For the three-layered membrane, the P value is almost 6 times lower compared to the EVA membrane even if the global Xc remains lower than 5 %. However, due to the low Xc value, the P coefficient is more than 20 times higher in comparison with the completely hydrolyzed membrane (EVOH), for which Xc ~ 33 %. Such behavior can be also correlated to the semi-crystalline microstructure of EVOH that increases the tortuosity of the diffusion pathway and to the presence of a glassy amorphous phase of EVOH (Tg ~ 35 °C), i.e. the low macromolecular mobility. Table 2. Temperature dependence of the water permeability P and the mean integral diffusion coefficient for the EVA, EVOH and three-layered (EVOH/EVA/EVOH, th = 8h) membranes Experimental temperature T (°C)

108 (cm2·s-1)

P (Barrer) State*

EVA

EVOH/EVA/EVOH

EVOH

EVA

EVOH/EVA/EVOH

EVOH

15

EVA-r EVOHg

10900 ± 100

610 ± 10

30 ± 5

5.5 ± 0.3

0.30 ± 0.05

0.05 ± 0.01

25

EVA-r EVOHg

16100 ± 500

2900 ± 400

130 ± 20

8.0 ± 0.3

1.00 ± 0.05

0.10 ± 0.01

35

EVA-r EVOHg

29000 ± 600

3800 ± 300

300 ± 30

14.0 ± 0.5

1.40 ± 0.05

0.30 ± 0.03

45

EVA-r EVOHr

42300 ± 500

5200 ± 300

700 ± 50

46.0 ± 0.5

2.50 ± 0.10

0.80 ± 0.05

60

EVA-r EVOHr

72500 ± 600

9000 ± 300

1100 ± 90

74.0 ± 0.4

8.00 ± 0.10

1.80 ± 0.05

75

EVA-r EVOHr

102700 ± 600

10800 ± 400

1700 ± 80

260.0 ± 0.5

18.00 ± 0.20

8.10 ± 0.10

* Rubbery (r) or glassy (g) state of the amorphous phase of EVA and EVOH in the membranes at the experimental temperature T

14


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Moreover, the permeability coefficient increases with the temperature increasing (Table 2). Such behavior may be explained by a higher mobility of the macromolecular chains due to the thermal energy increasing. The evolution of the permeation parameters as a function of temperature will be described in details together with the gas permeation results and taking into account the activation energy values. Gas permeation The driving force for the gas transport through the membrane is the pressure gradient between the 45

upstream and downstream compartments.

The gas barrier behavior at a given temperature is dominated

by the size (dk) (i.e. the kinetic diameter in Å, for N2 = 3.64, O2 = 3.46 and CO2 = 3.30)46, the Van der Waals 3

-1

molar volume (VVdW) (the solubility parameter in cm ·mol , for N2 = 39.1, O2 = 31.8 and CO2 = 42.7)

47

and

the critical temperature (TC) (the condensability parameter in °C, for N2 = -147, O2 = -118 and CO2 = 31)48 of the penetrant molecules. The diffusing molecules are known to have poor interactions with polymers, thus allowing us to study the effect of the membrane structure on the permeability P and diffusion D coefficients as a function of temperature. For the pure EVA membrane, the pressure in the downstream compartment increases too fast due to the mechanical deformation caused by the vacuum. And so, the EVA membrane was sandwiched between two highly permeable silicon sheets (SS). In that case, the permeability coefficient of EVA (PEVA) was calculated from the series resistance model, also called the ideal laminate theory, taking into account a multilayered system with three layers:49, 50

LT L EVA L = + 2 SS PT PEVA PSS

(12)

where LT and PT are the total thickness and the permeability coefficient of the sandwiched system, respectively; LEVA is the thickness of the EVA membrane, and LSS and PSS are the thickness and the permeability coefficient of one silicon sheet, respectively. However, the diffusion coefficient D cannot be determined for this system, therefore only the variation of P with temperature will be discussed. According to the literature, the permeability coefficient P values for EVA with 70 wt. % of VAc are 1, 4 and 40 Barrer for N2, O2 and CO2, respectively.

12

Such results were obtained at 22 °C with a constant

pressure (7 bar)/variable volume (from 40 to 2500 cm3) setup whereas our measurements were performed

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Page 16 of 34

with a constant volume (70.3 cm3)/variable pressure (from 1 to 3 bar) setup. However, it can be observed that the results obtained by using the series resistance model at 25 °C are in good agreement with the reported permeability coefficients (See supporting information, Figure S2). For the EVA membrane at any temperature, the decreasing value order of P and D coefficients (i.e. CO2 > O2 > N2) is in good agreement with the kinetic diameter order of the diffusing molecules without specific interactions with the polymer. The same tendency can be observed for the three-layered and the completely hydrolyzed EVA (EVOH 43 mole % of VOH) membranes. Besides, it can be noticed that the EVA membrane has the highest permeability coefficient for a given temperature while the EVOH membrane has the lowest P value. For the three-layered structure the P value is significantly lower compared to EVA and only three to five times higher compared to EVOH for any diffusing molecule (Figure 4). This behavior can be explained by the membrane microstructure changing during the hydrolysis reaction – from amorphous EVA to semi-crystalline EVOH. In this case, the higher degree of crystallinity increases the tortuosity and, consequently, decreases the P coefficient value. In the case of the oxygen permeability the P value for the EVOH membrane is similar to the values reported for extruded EVOH with 71 mole % of VOH (P (O2) = 0.01 – 0.4 Barrer).17 Besides, the determined oxygen permeability value agrees well with the values for sandwiched structures with coextruded EVOH as a middle layer, for example PET/EVOH/PP and PET/PP/Nylon 6/EVOH/PP/PET (P ~ 0.005 Barrer).51 The permeability coefficient P increases with temperature increasing for all membranes for any diffusing molecule (see Supporting Information, Figure S2). Such behavior can be explained by the increase of the segmental mobility or free volume at higher temperatures. Activation energy To calculate the activation energy of the water and gas permeation processes, Equations 10 and 11 were linearized as follows:

ln P = ln P0 −

1   T 

(13)

ED  1    R T 

(14)

EP R

ln D = ln D0 −

16


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Figure 4. Gas permeability coefficients for the EVA, EVOH and three-layered (EVOH/EVA/EVOH, th = 8h) membranes at 25 °C Some deviations from these relations have been already reported, especially in the glass transition temperature region.52 A break between two straight lines in the Arrhenius plot is more often observed with the higher activation energy values below Tg than above it. Indeed, in the glassy state (i.e. below Tg) the mobility of the polymer chains and, consequently, the free volume available for the diffusion is lower than in the rubbery state (i.e. above Tg).

53

Figure 5 presents the obtained temperature behavior for the EVA,

EVOH/EVA/EVOH and EVOH membranes for four diffusing molecules (H2O, CO2, O2 and N2). The presence of only one straight line in the water permeation results for the EVA membrane (Figure 5a) may be explained by the fact that the permeability measurements were performed at temperatures much higher than the Tg value of EVA (~ -15 °C), i.e. no significant change of the macromolecular mobility occurs. At the same time, a slope break at the temperature between 35 and 40 °C was observed when EVOH was present in the membrane, i.e. for the three-layered membrane (Figure 5b) and for the completely hydrolyzed membrane (Figure 5c), due to the glassy/rubbery state transition of EVOH (Tg ~ 35 °C). Therefore, two values of the permeation activation energy EP were obtained and given in Table 3. Besides, when plotting ln as a function of 1/T for the water permeability measurements (see Supporting Information, Figure S3), any deviation of the linear behavior was revealed and a single value of the diffusion activation energy ED was calculated (Table 3). The slope break in Figures 5b and 5c could be

17



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Page 18 of 34

related to the variation of the solubility parameter S with the temperature increasing according to the general equation:

P = D⋅S

(15)

Finally, the value of decreased with the hydrolyzed layer thickness (i.e. with the EVOH content) increasing at any studied temperature. This result is explained by the increased tortuosity of the diffusion pathway due to the presence of the EVOH crystals. As for the water permeation results, the permeability P and diffusion D coefficients towards gases (N2, O2 and CO2) are plotted as a function of reciprocal temperature according to Equations 13 and 14 and presented in Figure 5 and Figure S3 (see Supporting Information), respectively. The permeability coefficient P variation with temperature reveals a nonlinear behavior in the Tg region for the EVOH membrane whatever the gas diffusing molecule. It can be noticed in Figures 5b and 5c that the P values at the temperature close to 45 °C are different from the expected values (dash-line) in the case of a single activation energy. Besides, the tendency does not allow estimating two activation energy values, i.e. below and above Tg, as no clearly defined slope break is observed. This behavior can be related to a “transport delay” due to the specific interactions and increasing segmental mobility in the wide range of temperature for the EVOH glass transition (Figure 3b). For the three-layered membrane such delay is detected at above 35 °C (dashed zone in Figure 5b) as this membrane contains less EVOH, whereas for the completely hydrolyzed membrane the delay is observed above 45 °C (dashed zone in Figure 5c). The diffusion coefficient D variation reveals a nonlinear behavior towards gas for the three-layered and EVOH membranes (see Supporting Information, Figure S3). Besides, it can be noticed that the deviation from Arrhenius behavior is less marked when the diffusing molecule can be condensable, for example CO2 and H2O, as opposed to the gases with a lower condensability, such as N2 and O2, i.e. with a very low TC. According to the literature, the inflection depends not only on the glassy/rubbery state transition, but also on the polymer free volume and the size of the diffusing molecule.54, 55 For example, no slope break of the permeability variation as a function of temperature was reported for the polymers, such as poly(vinyl acetate) (PVAc),54 poly(vinyl chloride) (PVC),55 and poly(ethyl methacrylate),56 for He, H2, O2 and N2 at the glass transition. However, the slope deviation was observed when CO2 and Ar were used as diffusing

18


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molecules. The kinetic diameter for O2 (3.46 Å) is close to that of CO2 (3.30 Å) and Ar (3.40 Å).57 Therefore, the simple comparison of the size of the diffusing molecule is not sufficient to explain such a complex phenomenon close to the glass transition.

Figure 5. Temperature variation of the permeability P coefficient towards water and gases (N2, O2 and CO2) for: a) EVA; b) three-layered (EVOH/EVA/EVOH, th = 8h); c) EVOH membranes. Dashed zones represent the EVOH glass transition temperature region. The calculated activation energy values (EP and ED) for the different diffusion molecules are gathered in Table 3. It can be noticed that the permeation activation energy EP decreases with the decreasing of the kinetic diameter of the diffusing molecule as follows EP(N2) > EP(O2) > EP(CO2) for the EVA and threelayered membranes. Besides, for the completely hydrolyzed EVA membrane (i.e. for pure EVOH) this tendency has a different character with the minimum value for the O2 molecules. The main difference in EP is detected when CO2 is used as the diffusing molecule. This can be explained by a higher energy

19



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Page 20 of 34

necessary for this condensable molecule to be sorbed and to diffuse into the semi-crystalline EVOH microstructure that strongly delays the molecule transport and, thus, reduces the sorption capacity. Table 3. Activation energy of the permeability and diffusion processes towards different diffusing molecules for the EVA, EVOH and three-layered (EVOH/EVA/EVOH, th = 8h) membranes -1

-1

EP (kJ·mol )

ED (kJ·mol )

Membrane / Penetrant

N2

O2

CO2

H2O

N2

O2

CO2

H2O

EVA

43 ± 1

37 ± 3

22 ± 5

75 ± 3

-

-

-

54 ± 4

EVOH/EVA/EVOH

53 ± 2

46 ± 2

42 ± 2

70 ± 5 / 112 ± 7

56 ± 5

41± 6

59 ± 3

55 ± 2

EVOH

47 ± 4

44 ± 7

55 ± 7

68 ± 5 / 128 ± 3

72 ± 3

60 ± 6

63 ± 7

70 ± 2

In the case of water as a diffusing molecule, the EP value above the glass transition is similar for EVA and EVOH (Table 3), as the polymer chain mobility and free volume are high enough to minimize the effect of the microstructure or the polymer-diffusing molecule interactions. On the other hand, the EP value below the glass transition for the EVOH membrane (~ 128 kJ·mol-1) is slightly higher compared to the -1

three-layered membrane (~ 112 kJ·mol ). Such a result highlights a higher energy barrier for the permeability process in the case of semi-crystalline EVOH due to the lower chain mobility compared to EVA present in the non-hydrolyzed and three-layered membranes. For the diffusion activation energy ED, no clear tendency can be established on the basis of the hydrolysis degree or the size of diffusing molecules. As explained previously, it was possible to determine the gas diffusion coefficient D values for the EVA membrane. The ED values for the three-layered -1

-1

membrane are in the range from 41 kJ·mol to 59 kJ·mol depending on the diffusing molecule and this value is between 60 kJ·mol-1 and 72 kJ·mol-1 for the EVOH membrane (Table 3). This result reveals a higher energy barrier for the diffusion phenomenon due to the different microstructure and chain mobility of EVOH compared to EVA. It is well known that the water sorption isotherms give additional information about the membrane structure. Therefore, in order to have a deeper insight into the water sorption mechanism related to the diffusing molecule/polymer interactions, the water vapour sorption isotherms were measured (Figure 6). As one can see, a nonlinear behavior of Ceq is observed for low aw values (below 0.2) for all studied membranes (inset to Figure 6). With further aw increase from 0.2 to 0.6, linear Ceq increase can be

20


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observed. Finally, with increasing water activity aw (aw > 0.6) the equilibrium water content Ceq sharply rises, especially for the three-layered membrane (Figure 6). As the aw increase is accompanied by a nonlinear increase of the water content, the experimental water sorption isotherms are fitted according to Park’s model (Equation 9).

35

The model fitted parameters

are gathered in Table 4.

Figure 6. Water vapour sorption isotherms for the EVA, EVOH and three-layered (EVOH/EVA/EVOH, th = 8h) membranes at 25 °C. Inset to Figure 6: Zoom for water sorption isotherms at low water activity (aw < 0.3) Table 4. Parameters of Park’s model for the EVA, EVOH and three-layered (EVOH/EVA/EVOH, th = 8h) membranes

Membrane

AL (mmol·g-1)

kH (mmol·g-1)

n

ka (mmol·g-1)1-n

EVA

0.04

0.5

4

1.9

EVOH/EVA/EVOH

0.06

0.8

6

2.0

EVOH

0.34

0.8

4

0.1

According to Park’s model (Equation 9) the parameter bL allows to evaluate the affinity of the diffusing molecules and the specific Langmuir’s sites. However, it should be noted that it was impossible to

21



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Page 22 of 34

give a significant value of this parameter because of the lack of the experimental points for the water activity aw < 0.1. The calculated value of AL related to the concentration of the specific sorption sites was accurately determined from the stationary state of the Langmuir’s mode. The AL value for the completely -1

hydrolyzed membrane (EVOH) (AL ~ 0.34 mmol·g , Table 4) is significantly higher than the values for other membranes (AL ~ 0.05 mmol·g-1, Table 4). This fact testifies to the presence of numerous specific sorption sites in the EVOH membrane. The linear variation of Ceq with the aw increase is related to the random sorption of the water molecules in the polymer (Henry’s law). The calculated values of the parameter kH confirm the stronger interactions of the water molecules and the VOH groups in the three-layered and EVOH membranes (kH ~ -1

0.8 mmol·g , Table 4) compared to the interactions of the water molecules and the VAc groups in the EVA membrane (kH ~ 0.4 mmol·g-1, Table 4). Finally, the exponential increase of Ceq values for aw > 0.6 is due to the aggregation (clustering) phenomenon (Figure 6). It can be noted that the average number of the water molecules per cluster varies from 4 to 6 for the studied membranes. Despite the fact that the n value may be questionable as it is deduced from a phenomenological model35, it is reasonable to suppose that the size of the water clusters is quite similar whatever the membrane chemical composition and microstructure. A significantly low value of the aggregation constant ka is obtained for the completely hydrolyzed (EVOH) membrane (ka ~ 0.1 (mmol·g-1)1-n, Table 4). This fact can be explained by the strong effect of the EVOH crystals presented in this membrane (Xc ~ 33 %, Figure 3b and 3c) which hinder the water cluster formation. On the other hand, the higher ka value for the three-layered membrane (ka ~ 2.0 (mmol·g-1)1-n, Table 4) is due to the low Xc value (~ 5 %). Selectivity The choice of materials for the barrier application (in which the membrane is exposed to a mixture of gas or vapors), such as protective coatings, food packaging and a gas dehydration process, is generally based on the ability to separate the mixture or to be selective for the given diffusing molecule. The ideal selectivity coefficient (αA/B) can be estimated from the experimental permeability coefficients (P) towards two different diffusing molecules (A and B) as follows:

22


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α A = PA P B

(16)

B

The selectivity values are determined for the different pairs of diffusing molecules and given in Table 5. For the three studied membranes and the range of temperature from 25 to 75 °C, the selectivity values for the gas pairs are of the same order of magnitude, namely between 2 and 8 for CO2/O2, from 7 to 20 for CO2/N2 and from 1 to 5 for O2/N2. As the gas/gas selectivity values remain in the same order at all studied temperatures, only the average selectivity for each membrane and gas pair is reported in Table 5. These results are in accordance with the results obtained for poly[bis(2-(2methoxy)ethoxy)phosphazene] (αCO2/N2 ~ 6026) and PI (αO2/N2 ~ 2058) used for the gas mixture separation. Also, for the membrane based on the carbonized polymer precursor the similar selectivity value for hydrocarbons was found (αC3H6/C3H8 ~ 2559). According to the literature, an upper bound of selectivity for the different gas pairs (Robeson’s plot) has been determined on the basis of the gas kinetic diameter and considering that no specific interactions exist between the diffusing molecules and the polymer membrane.26 However, this trade-off relationship in not adapted for water as a diffusing molecule and, so, no upper bound has been reported for water/gas pairs. In our case, the water/gas selectivity values are considerably higher than the gas/gas selectivity (Table 5). The selectivity values for H2O/O2 and H2O/N2 separation for the all studied membranes and temperatures are much higher than those for H2O/CO2 separation. This can be related to the plasticization effect of water vapors and CO2 molecules, while N2 and O2 can be easily separated from the water molecules due to the low interactions with the polymer membrane and the poor plasticization capacity. This selectivity behavior is a key parameter for the different application fields, such as gas dehydration or food packaging. The performance of H2O/O2 and H2O/N2 selectivity versus oxygen and nitrogen permeability at 25 °C for the studied membranes is compared with that of other polymers, such as poly(propylene) (PP)22, PVC55, 60

61

62

PET , poly(carbonate) (PC) , poly(styrene) (PS) , low and high density PE (LDPE and HDPE, respectively)63, 64, poly(amide) 6 and 12 (PA6 and PA12, respectively)65, the EVA copolymers with different 66

VAc content , and some biodegradables polymers (poly(hydroxybutyrate) (PHB), poly(hydrobutyrate-cohydroxyvalerate) (PHBV)67, poly(lactic acid) (PLA)68, poly(butylene succinate) (PBS) and poly(butylene 69

succinate adipate) (PBSA) ) (Figure 7).

23



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Table 5. Selectivity coefficient

α

Page 24 of 34

of different pairs for the EVA, EVOH and three-layered

(EVOH/EVA/EVOH, th = 8h) membranes at different temperatures Membrane

EVA

EVOH/EVA/EVOH

EVOH

T (°C)

H2O/O2

H2O/N2

H2O/CO2

25

3400 ± 150

8500 ± 600

350 ± 15

35

3450 ± 130

8300 ± 600

630 ± 20

3800 ± 70

9600 ± 500

730 ± 10

60

2870 ± 60

5800 ± 220

650 ± 15

75

2570 ± 50

4400 ± 90

710 ± 10

25

11900 ± 2000

47500 ± 9000

2500 ± 400

35

9900 ± 1000

42000 ± 7000

2000 ± 190

3200 ± 280

10800 ± 800

910 ± 60

60

5700 ± 400

18000 ± 1800

1400 ± 60

75

3500 ± 170

8500 ± 1700

800 ± 40

25

1600 ± 290

6500 ± 2000

600 ± 120

35

2200 ± 330

7000 ± 1400

1300 ± 180

2800 ± 600

6900 ± 2000

580 ± 70

60

3100 ± 300

10600 ± 1600

900 ± 100

75

900 ± 50

3000 ± 1200

300 ± 20

45

45

45

CO2/O2

6±2

4±1

3±1

CO2/N2

13 ± 6

15 ± 5

11 ± 4

O2/N2

2±1

3±1

4±1

A significantly higher selectivity value than that for the common polymer materials can be clearly observed for the three-layered membrane (Figure 7). When compared with the non-hydrolyzed pure EVA membrane and the completely hydrolyzed EVOH membrane, the EVOH/EVA/EVOH membrane reveals a maximum selectivity value (αH2O/O2 = 11900 and

αH2O/N2 = 47500) at any studied temperature (Table 5 and

Figure 7). In general, the polymer glassy state (i.e. below Tg) induces a selectivity process governed by diffusivity, i.e. the size of the penetrant molecules has a dominating influence on the separation performance. On the other hand, when a polymer membrane is used in its rubbery state (i.e. above Tg), the selectivity depends on the solubility parameter as the free volume and mobility is high enough for any size of a diffusing molecule.2, 70

24


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Figure 7. Selectivity performance at 25 °C of: a) H2O/O2 and b) H2O/N2

Figure 8. Schematic representation of the molecules’ (water and gases) diffusion through the EVA, EVOH and three-layered (EVOH/EVA/EVOH, th = 8h) membranes The poor selectivity of the EVA membrane can be explained by the EVA rubbery state in the experimental temperature range (i.e. high molecular mobility), by the absence of the tortuosity effect (as EVA is amorphous polymer, Figure 3b and 3c) and by the weak interactions of the diffusing molecules with the EVA backbone (Figure 8). On the other hand, the low selectivity of the EVOH membrane can be caused by the low availability of the VOH groups and by the significant tortuosity of the diffusion pathway whatever is the polymer state (i.e. glassy or rubbery) due to the high Xc value (~ 33 %, Figure 3b and 3c). For the three-layered membrane at 25 °C, the amorphous EVA layer is in the rubbery state, whereas the EVOH layers with low degree of crystallinity are in the glassy state (Figure 8). The strong interactions of the water molecules and the VOH groups were revealed by the water contact angle analysis (θw ~ 77° with p

-2

the polar contribution γ equal to 4.4 mJ·m , Table 1). Also, the increased number of the hydrogen bond -1

-

interactions (the high random sorption kH ~ 0.8 mmol·g and elevated cluster formation ka ~ 2.0 (mmol·g 1 1-n

)

values, Table 4) defined by the water vapour sorption measurements allowed us to explain the

25



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Page 26 of 34

selective behavior of the three-layered membrane. Thus, the low tortuosity effect induced by the presence of the EVOH crystals, the segmental mobility (rather high for the EVA layer and reduced for the EVOH layers) and the availability of the VOH groups for the interaction with the water molecules lead to the alterations of the kinetic (diffusivity) and the thermodynamic (solubility) parameters. Therefore, the threelayered membrane can be potentially used for drying wet gas mixtures, for limiting the oxygen diffusion through the membrane and for controlling the moisture level without any delamination problems.

CONCLUSION The tunable microstructure of the membranes elaborated by the EVA surface hydrolysis, i.e. from a non-hydrolyzed and amorphous EVA to a completely hydrolyzed and semi-crystalline EVOH, was correlated with their water and gas (N2, O2 and CO2) barrier parameters in a wide temperature range (from 15 °C to 75 °C). A significant increase of the water permeation coefficient P and a reduction of the mean diffusion coefficient were observed with the hydrolysis degree increasing. This tendency was confirmed for the all studied temperatures. The gas (N2, O2 and CO2) permeation measurements revealed the enhanced barrier properties with the increase of the hydrolysis degree. The increasing degree of crystallinity and the higher glass transition temperature, i.e. the lower molecular mobility, of EVOH compared to EVA can explain such an improvement of the barrier properties. Furthermore, the permeability values follow the order PCO2 > PO2 > PN2 for all the studied membranes and temperatures. This permeability order is explained by the kinetic diameter order for diffusing molecules. The Arrhenius behavior of the permeability coefficient variation was clearly observed for the pure EVA membrane for all studied diffusing molecules. Such behavior may be explained by the rubbery state of EVA in the studied temperature range and by the absence of the specific interactions between the small diffusing molecules and EVA. The situation is more complex for the EVOH and three-layered membranes as nonlinear behavior is observed. The deviation from linearity is revealed at about the EVOH glass transition temperature and leads to two water permeation activation energy EP values. The EP value below -1

-1

Tg (~110 - 130 kJ·mol ) is found to be higher than that above Tg (70 kJ·mol ) as the increasing chain segment mobility reduces the energy barrier for the transport process. In the case of N2, O2 and CO2 diffusing molecules (especially for the permanent gases (N2 and O2)), a complex behavior was revealed for the EVOH and three-layered membranes. Such behavior is related to the influence of the rubbery/glassy

26


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state interfacial phenomena (different relaxation time, i.e. segmental mobility) in the EVOH/EVA/EVOH membrane and to the higher crystallinity degree of EVOH that affects the transport behavior. The high water/N2 and O2 selectivity was obtained for the three-layered EVOH/EVA/EVOH membrane at the all studied temperatures. The selectivity values at 25 °C (αH2O/O2 = 11900 and

αH2O/N2 =

47500) are significantly higher than those of the common polymers. The remarkable transport properties of this membrane testify of a high selective material for water/gas pairs. Such performance is related to selectivity of both diffusivity and solubility which results from the association of the rubbery amorphous EVA layer (high segmental mobility and free volume) and the glassy EVOH layers (low segmental mobility, low degree of crystallinity and enhanced hydrophilicity). Besides, the access of the water molecules into the three-layered structure is improved due to the hydrophilic nature and low crystallinity degree of the EVOH layer. Such a promising membrane structure can be potentially used in different application fields due to the existence of the chemical bonds at the interfaces between the layers (i.e. no delamination) and to the tunable properties by controlling the thickness of the hydrolyzed layers. SUPPROTING INFORMATION Cross-section fluorescent images, temperature dependence of the gas permeability coefficient towards towards N2, O2 and CO2, and temperature variation of the diffusion D coefficient towards water and gases of the EVA, three-layered (EVOH/EVA/EVOH, th = 8h) and EVOH membranes. ACKNOWLEDGMENTS The authors are grateful to GRR EEM (Upper Normandy Region, France) for the financial support (My Peace project) and to Lanxess Co. for free giving the EVA pellets. REFERENCES [1]

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