Improvement of Water Barrier Properties of Poly(ethylene-co-vinyl

Article pubs.acs.org/JPCC

Improvement of Water Barrier Properties of Poly(ethylene-co-vinyl alcohol) Films by Hydrophobic Plasma Surface Treatments Nadine Tenn,† Nadège Follain,† Kateryna Fatyeyeva,*,† Jean-Marc Valleton,† Fabienne Poncin-Epaillard,‡ Nicolas Delpouve,§ and Stéphane Marais† †

Laboratoire Polymères, Biopolymères et Surfaces, UMR 6270 & FR 3038, CNRS, Université de Rouen, Bd. Maurice de Broglie, 76831 Mont Saint Aignan Cedex, France ‡ LUNAM Université, UMR 6283 CNRS, Institut des Molécules et Matériaux du Mans, département Polymères, Colloïdes et Interfaces, Av. Olivier Messiaen, 72085 Le Mans Cedex, France § Laboratoire d’Etude et de Caractérisation des Amorphes et des Polymères, EA 4528, Université de Rouen, Av. de l’Université, 76801 Saint Etienne du Rouvray, France

ABSTRACT: Poly(ethylene-co-vinyl alcohol) (EVOH) films with two different ethylene contents (29 and 44 mol %) have been treated by hydrophobic plasma (CF4, tetramethylsilane (TMS), CF4/H2, and CF4/C2H2). Conditions of the cold plasma treatment were optimized by the water contact angle measurements as a function of the different plasma parameters (plasma power, gas flow, and treatment time). Chemical changes of the film surface were characterized by X-ray photoelectron spectroscopy. The obtained results revealed the presence of fluorine containing functional groups such as CF, CF2, and CF3 in the case of CF4, CF4/H2, and CF4/C2H2 plasma treatment and the presence of SiOxCy compounds after TMS treatment. The morphology of the plasma treated EVOH films was examined by atomic force microscopy, which indicated an increase of the film roughness after treatment. Negligible changes of thermal properties of the modified EVOH films were observed by means of the temperature modulated differential scanning calorimetry. The barrier properties of films were characterized by water permeability measurements. It was found that the hydrophobicity was significantly improved after plasma treatment and for some treated films the water permeability was decreased up to 28%.

1. INTRODUCTION Barrier properties of polymer materials are currently in great demand, particularly in the packaging industry, where polymers are replacing many traditional materials.1 However, in comparison with traditional materials (metals, glass, and paper), plastic packaging is more permeable to gases, water vapor, and aroma compounds. The ingress of water leads to a permanent change in the nature of food products. Thus, the protection of the packaged goods against oxygen and water vapor is an essential prerequisite for achieving a long shelf life.1,2 Ethylene-co-vinyl alcohol (EVOH) copolymers are a family of random semicrystalline materials with excellent barrier properties to gases and hydrocarbons and with outstanding chemical resistance.3 The permeability of semicrystalline EVOH films depends on the copolymerization ratio of ethylene and vinyl alcohol groups. The increase of the ethylene group content leads to the decrease of EVOH gas permeability.4 The presence of EVOH in the packaging material is a key to food © 2012 American Chemical Society

quality and safety, as it reduces the access of oxygen and the loss of aroma components during extended package shelf life. However, despite their low gas permeation, EVOH copolymers show poor moisture resistance.5 This fact can lead to a high water uptake and, as a result, to the deterioration of the gas barrier performance at high relative humidity above a certain threshold (usually more than 50%). The water molecules can act as a plasticizer, weakening the strong interchain forces.5 This plasticization increases the void spaces and thus decreases the barrier properties of the material. As a result, EVOH has limited use for packaging application where the material could be exposed to high humidity level. One of the ways to decrease the water permeability of EVOH films is to produce a multilayered structure.6 In this case, a layer of EVOH film is positioned as a middle layer and polyolefin Received: March 7, 2012 Revised: April 28, 2012 Published: April 30, 2012 12599

dx.doi.org/10.1021/jp302223j | J. Phys. Chem. C 2012, 116, 12599−12612

The Journal of Physical Chemistry C

Article

adjustable. The treatment gas was introduced into the plasma chamber and then the glow discharge was initiated. 2.2.2. Water Contact Angle Measurements. The water contact angles were measured using a goniometer Rame-Hart N 100-00 (visual reading) at room temperature (23 ± 2 °C). For each measurement, a 5 μL drop of water (Milli-Q Water System, resistivity 18 MΩ cm−1) was formed at the tip of the syringe. After the water dripped onto the surface of the plasma treated film, the contact angle was measured within 5 s by a sessile drop method. The water drops were placed onto six different sites on the film surface to provide a statistical average for each sample and the average value of measurements was calculated. 2.2.3. Atomic Force Microscopy (AFM). A Nanoscope IIIA (Veeco − Digital instruments, California, USA) was used to analyze the topography of the polymer films. Images were obtained in the contact mode at a constant force (feedback loop on) in air at room temperature (23 ± 2 °C). The piezoscanner used was J-type (100 μm scan range); the microcantilevers used had a spring constant of 0.06 N·m−1. The tip was a standard tip made of silicon nitride. 2.2.4. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed with the VG 220i-XL ESCALAB photoelectron spectrometer. A nonmonochromatized Mg Kα (1253.6 eV) source was used for X-rays. Acquisition of high-resolution spectra was done at a pass energy of 20 eV. A flood gun was activated for charge compensation. All core-level spectra were referenced to the C1s neutral carbon peak (C−C and C−H bonds at a binding energy of 285.0 eV). The binding energies were reliable to ±0.1 eV. Fitting of the XPS spectra was performed using the Avantage software provided by ThermoFisher Scientific. Depth profile measurements were performed with an argon gun and with an ion current leading to a sputter rate of approximately 0.2 nm·s−1. 2.2.5. Temperature Modulated Differential Scanning Calorimetry (TMDSC). TMDSC experiments were performed with the TA Instruments apparatus (DSC 2920) equipped with a low-temperature cell (minimal temperature = −50 °C). Nitrogen was used as a drying gas (70 mL·min−1). The sample (about 2 mg) was encapsulated in a standard DSC aluminum alloy pan and the pan was disposed in order to have the best possible thermal contact. Before the experiments, the samples were stored in vacuum desiccators over P2O5 for at least 2 weeks to avoid moisture sorption effects. The temperature and energy calibrations were carried out using the standard values of melting temperature (Tm) and fusion enthalpy (ΔHm) for indium (Tm = 156.6 °C and ΔHm = 28.66 J·g−1).14 The specific heat capacities for each sample were measured using sapphire as a reference. TMDSC experiments were performed with an oscillation amplitude of 0.318 °C, an oscillation period of 60 s, and a heating rate of 2 °C·min−1. The analysis was carried out in the heat-only mode which is widely used for the study of crystallization and fusion processes in semicrystalline polymers.15 The deconvolution procedure proposed by Reading et al.16,17 was used, so it was possible to separate the average heat flow signal into its real (reversing heat flow) and imaginary (non-reversing heat flow) parts. 2.2.6. Water Permeation Measurements. The water permeation measurements were performed with a home-built apparatus at 25 ± 1 °C.18 The permeameter consists of a measurement cell, a dry nitrogen supply, and a chilled mirror hygrometer (General Eastern Instruments, Massachussetts,

resin, with high water barrier properties, occupies outer layers in a typical three-layer film. The other way to improve the barrier properties is the synthesis of nanocomposite based on EVOH film and different types of inorganic clays.7 In this case, an improvement is obtained by increasing the tortuosity of the diffusion path, so the result is strongly dependent on the morphology of the nanocomposite film. Alternatively, the use of plasma treatment appears to be suitable for industrial application, as it is a dry (without solvents) and sterile process and achieving the necessary changes in the surface properties takes a short time period.8−10 Besides, the use of cold plasma treatment allows the sterilization and surface modification without affecting the bulk properties.8 Moreover, the composition of a plasma deposited layer is easily adjustable by controlling the plasma parameters and the gas composition. Therefore, in order to improve the water barrier properties of EVOH film, the hydrophobic cold plasma treatment with the use of tetrafluoromethane2,11,12 or tetramethylsilane (TMS)13 can be applied. The influence of the different plasma parameters (gas composition, power, and gas flow as well as the treatment time) was studied for two types of EVOH films with different ethylene group contents (29 and 44 mol %). The optimum plasma parameters were established according to the water contact angle measurements. The effects of the plasma treatment on the surface morphology and composition as well as the thermal properties of EVOH films were evaluated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and temperature modulated differential scanning calorimetry (TMDSC) measurements.

2. EXPERIMENTAL SECTION 2.1. Materials. Two different commercial poly(ethylene-covinyl alcohol) copolymer grades (Soarnol) supplied by the Nippon Gohsei (UK) were used: EVOHDT29 and EVOH AT44 with 29 and 44 mol % of ethylene groups in the copolymer composition, respectively. The films 60 μm thick were prepared using a screw extruder (Scamex) operating at a constant rotating speed of 20 rpm and a temperature profile of 255, 256, 280, and 280 °C. Before measurements, the films were stored in a desiccator under vacuum over P2O5 at room temperature (23 ± 2 °C). CF4 (99.99% purity, Messer), C2H2 (99.95% purity, Air Liquide), Ar (99.99% purity, Air Liquide), H2 (99.99% purity, Air Liquide), and N2 (99.99% purity, Air Liquide) gases and TMS (>99.0% purity, Sigma-Aldrich) were used as received. 2.2. Experimental Techniques. 2.2.1. Cold Plasma Treatment. The plasma treatment of the polymer surface was carried out in a capacitively coupled radio frequency (RF) plasma reactor. The reactor is an aluminum rectangular chamber with a volume of around 13 L. Two horizontal electrodes which are driven by a 13.59 MHz generator (SAIREM) with a variable power (from 0 to 600 W) are placed inside the reactor. The incident power and the reflected power are measured with power meters (METRIX). The impedance of the generator and that of the reactor are adjusted until the reflected power is minimal. A primary pump (CITALCATEL Pascal N82010 SD) and a turbomolecular pump (ALCATEL ATP 80/100) allow obtaining a base pressure in the reactor of 10−7 mbar. The pressure is measured using a Pirani gauge (ACC 1009). Mass flowmeters (Aera FC 7700CDC) control the amount of gas injected. In this type of reactor, the pressure depends on the gas flow rate and is not 12600



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USA). The previously dried film with a thickness of L was mounted in the cell, and dry nitrogen was flushed into both compartments (downstream and upstream) for hours until a dew point lower than −65 °C was obtained. Then, a stream of pure water (Milli-Q water system, resistivity 18 MΩ·cm−1) was pumped through the upstream compartment and the transfer of water through the film, i.e., from the upstream compartment to the downstream one, was monitored by the humidity sensor. For permeation measurement, the initial and boundary conditions are defined as C(x , t ) = 0,

0 < x < L at t = 0

C(0, t ) = Ceq (constant)

and

It was found that the CF4 flow also influenced the water contact angle value for EVOH films (Figure 2). The water

(1)

C(L , t ) = 0 at t > 0 (2)

The presented results were averaged out of two measurements performed for each sample.

3. RESULTS AND DISCUSSION 3.1. Optimization of the Plasma Treatment Conditions. The cold plasma can be used for the modification of the material surface, for the grafting as well as for the polymerization. The surface functionalization implies the chemical reactions between the material and the plasma species. A change of the material surface properties is observed as a result of the surface modification.19 Depending on the plasma conditions, either a hydrophobic or hydrophilic surface can be obtained. In our case, in order to enhance the barrier properties of EVOH films in the presence of water, the hydrophobic plasma treatment was used. Figure 1 shows the variation of water contact angle for EVOH films treated with CF4 plasma as a function of the RF

Figure 2. Water contact angle as a function of the gas flow for CF4 plasma treatment. EVOHAT44 film (full circles): RF power = 20 W, treatment time = 10 min. EVOHDT29 film (empty circles): RF power = 15 W, treatment time = 10 min.

contact angle of treated EVOH film increased rapidly to a maximum of 100 ± 3° and 115 ± 3° for EVOHDT29 and EVOHAT44, respectively, for 25 sccm of CF4 and then stayed practically constant. The variation of the water contact angle of the treated EVOH film surface as a function of the treatment time is presented in Figure 3. It can be seen from the obtained

Figure 3. Water contact angle as a function of the treatment time for CF4 plasma treatment. EVOHAT44 film (full circles): RF power = 20 W, gas flow = 25 sccm. EVOHDT29 film (empty circles): RF power = 15 W, gas flow = 25 sccm.

Figure 1. Water contact angle as a function of the RF power for CF4 plasma treatment. EVOHAT44 film (full circles): gas flow = 20 sccm, treatment time = 10 min. EVOHDT29 film (empty circles): gas flow = 25 sccm, treatment time = 10 min.

results that the behavior of two types of EVOH is different during the short treatment time. In the case of EVOHAT44 film, one can observe that the water contact angle increases with the treatment time and reaches the maximum value (112 ± 3°) at a treatment duration of 10 min, whereas the water contact angle for EVOHDT29 film initially decreased with the treatment time increase (Figure 3). The maximum value of the water contact angle measured for EVOH films is obtained after 10 min of treatment. After 10 min of plasma discharge, the

power. Water contact angle for untreated EVOH film was found to be 73 ± 3° and 85 ± 1° for EVOHDT29 and EVOHAT44, respectively. One can see that the water contact angle initially increased with the increase of the RF power up to ∼15 W for both types of EVOH (Figure 1). When the RF power was higher than 20 W, the water contact angle for EVOH films decreased drastically up to ∼92° and remained almost constant with the further increase of the RF power. 12601



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Table 1. The Optimized Plasma Conditions for EVOH Films EVOHAT44 gas CF4 CF4/H2 CF4/ C2H2 TMS

EVOHDT29

RF power (W)

gas flow (sccm)

treatment time (min)

content of H2 or C2H2 (%)

pressure (mbar)

RF power (W)

gas flow (sccm)

treatment time (min)

content of H2 or C2H2 (%)

pressure (mbar)

20 20 20

25 21.3/3.7 24.2/0.8

10 10 10

15 3

0.0040 0.0036 0.0034

15 15 15

25 23.8/1.2 24.2/0.8

10 10 10

5 3

0.0029 0.0033 0.0030

60

10

0.67

60

10

2

(3)

e− + CF4 → e− + CF2• + 2F•

(4)

e− + CF4 → e− + CF• + 3F•

(5)

0.67

CF4 plasma, F• and CF3• are the major species which do not contribute to the polymerization. D’Agostino et al. studied the effect of the hydrogen addition, a scavenger of fluorine atoms, to CnF(2n+2) discharges.8,25,26 The fluorine atom concentration was found to decrease and the CF• and CF2• radical concentrations were found to increase with increasing hydrogen percentage. The hydrogen in the plasma phase reacts with the atomic fluorine that leads to the creation of very stable HF molecules which are inactive in the plasma atmosphere.25−27 On the other hand, it is known that the presence of C2H2 in plasma improves the barrier properties by the formation of a polymerized layer on the treated surface. This hydrophobic layer has a compact tridimensional network and may serve as a barrier for permeant diffusion.28,29 Therefore, mixtures of CF4 with H2 and C2H2 gases were used in order to enhance the hydrophobicity of EVOH films. As one can see, the low content of H2 (Figure 4a) or C2H2 (Figure 4b) gases in CF4 plasma leads to the initial increase of the water contact angle value. However, further increase of H2 or C2H2 content provokes a drastic decrease of the hydrophobicity that may be caused by the decreasing fluorine reactive species quantity. The similar results were found in the case of other fluorocarbon films obtained for CnF(2n+2)/H2 mixtures.25−27 The results of optical emission spectroscopy showed that the fluorine atom concentration passed through a minimum between 10 and 18% of hydrogen owing to the formation of the unreactive stable HF compound and then increased again.25,26 The optimized plasma conditions for CF4/ H2 and CF4/C2H2 gas mixtures are presented in Table 1. TMS plasma was used to study the influence of another type of hydrophobic plasma treatment. Organosilicon compounds are of particular interest for many deposition applications, as the nature of plasma deposited polymers can be shifted from organic to inorganic by controlling the plasma parameters.30 This feature seems suitable for barrier properties, since inorganic silicates, such as glass, are not permeable to any solvent. Preliminarily, it was found that the surface activation by Ar gas would ameliorate the deposition process. Taking this into account, the plasma treatment was performed in two steps. The first step was EVOH surface activation by Ar plasma, and the second step was the treatment of EVOH film by TMS/Ar mixture. Ar plasma was added to promote gas ionization, radical formation, and fragmentation of monomer molecules.31 Besides, in the plasma polymerization of TMS, the monomer may be considered to be multifunctional, because the dissociation of Si−C and C−H bonds takes place by the electron impact.30 It was found that 2 min of Ar plasma activation was sufficient for the next effective deposition. Thus, this activation time was used for further measurements. The influence of the RF power on the water contact angle values in the case of TMS plasma is shown in Figure 5. For both types of EVOH, the initial decrease of the water contact angle was

decrease of water contact angle is observed for both types of EVOH. The optimum conditions for CF4 plasma treatment that correspond to the maximum water contact angle value are presented in Table 1. One can see that the found parameters are the same for both types of EVOH film except the RF power value (15 W for EVOHDT29 film and 20 W for EVOHAT44 film). Thus, one may conclude that plasma parameters are not very influenced by the content of the ethylene groups in the studied polymer films. The fluorocarbon polymers are characterized by the F1s/C1s ratio (atomic fluorine number (F) to atomic carbon number (C)). If this ratio is close to 4, the non-polymer formation is observed.20 The reason why fluorocarbons with higher F1s/C1s ratio are not polymerized is that the F atoms can be etching species for hydrocarbon polymers.21,22 In CF4 plasma, the fluorinated species responsible for the plasma reactivity are the radicals (like CxFy•, CFy•, F•), the ions resulting directly from the ionization (like CF4, CF3+, CF2+, F+, F−), the ions resulting from the ionization and derived from the recombination (like CF2−CF2+), the neutrals, the electrons, and the photons. The dissociation mechanism of electronic impact of CF4 is represented below:23 e− + CF4 → e− + CF3• + F•

2

It is known that the fluorinated plasma leads to decreasing wettability, i.e., to increasing the hydrophobic character of the treated surface.2,22 The CF4 modification can be described as the combination of two mechanisms: degradation and fluorination.23 Besides, these two reactions seem to be parallel and competitive. Therefore, the observed decrease of water contact angle for EVOH film surfaces indicates that the degradation process is a predominated reaction at the surface (Figure 3). The competition between CF4 plasma fluorination and ablation reaches equilibrium faster at the lower RF power (15−20 W, Figure 1). This can be explained by the relatively low density of reactive species in plasma atmosphere. Thus, the fluorination reaction dominates the plasma modification. Similar results were obtained for CF4 RF plasma surface modification of silicone rubber.24 It was shown that the ablation process increases more quickly than the fluorination with the increase of treatment time. It was found that the degradation process in the case of CF4 plasma treatment was caused by the presence of the atomic fluorine in the plasma phase.8,23 Thus, to overcome this degradation reaction during the plasma treatment, the use of a CF4 mixture with hydrogen or acetylene can be proposed. In 12602



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It is found that TMS/Ar mixture plasma is used to produce silicon nitride or silicon carbide films.32 After the ionization of TMS, different species are present in the plasma phase: e− + SiMe4 → SiMe2+ + 2e− + 2Me

(6)

SiMe2+ → SiMe+ + Me

(7)

The dissociation of TMS is a very efficient process. At 40 W, the main dissociative process is Si−C bond breaking.32 The optimum conditions found for TMS plasma treatment are presented in Table 1. The effect of fluorinated and TMS plasma treatments on the EVOH surface was followed by AFM, XPS, TMDSC, and water permeability measurements. EVOH films treated under the optimum conditions (Table 1) were chosen for further investigation. 3.2. AFM Analysis. Contact-mode AFM analysis was performed for untreated and plasma treated EVOH films. The topographic images of EVOHDT29 film surfaces are shown in Figure 6. In the case of untreated and treated EVOHAT44 film surfaces, similar images were observed (Figure 7). Table 2 summarizes roughness values for untreated and treated EVOH films. One can easily see that plasma treatment affects the topography of the polymer film (Figures 6 and 7). The presence of nodular structures depressed in the center is observed for all treated films. Besides, the roughness of the treated surfaces was found to increase for both types of EVOH film (Table 2). Such an increase may be explained either by the deposition process during CF4 and TMS plasma or by the degradation process. Also, the change of the grain size of the nodular structure was observed as a function of the treatment gas used (Table 2). Lue et al. found that the films deposited from CxHy plasma during a short time (less than 90 s) were smooth and their shape was less pronounced.33 However, the film with longer exposure time had a rough surface covered with micrometer size particles. Similar results were obtained by Gao et al. in the case of CF4 plasma modification of silicone rubber.24 As it was stated before, CF4 plasma modification consists of two competitive and parallel processesdegradation and fluorination. Thus, the increased roughness can be attributed to fluorination over the ablation process (Table 2). For example, in the case of CF4 plasma, the increase of the average grain dimension from 0.85 ± 0.25 μm to 1.85 ± 0.65 μm and from 1.45 ± 0.55 μm to 2.25 ± 1.15 μm was observed for EVOHAT44 and EVOHDT29 films, respectively. These topological changes can result from chemical reactions occurring between CF4 or TMS plasma species and the EVOH surface and/or from the thermal effects, as this kind of polymers is rather thermosensible with a glass transition temperature close to 50−60 °C. 3.3. XPS Analysis. In order to verify the chemical changes that occurred on the polymer film, X-ray photoelectron spectra for both untreated and treated EVOH films were acquired. Surface quantification is given in Table 3. As one can see from the obtained results, the higher carbon concentration found in the case of EVOHAT44 film compared to EVOHDT29 film is in good agreement with the chemical formula; i.e., EVOHAT44 has more ethylene groups. The presence of Si on the film surface may be explained by the method used for film preparation. Fitting of the C1s spectra gives the information about the bonding environment of carbon atoms in the film (Figure 8).

Figure 4. Water contact angle as a function of H2 (a) and C2H2 (b) content in CF4 plasma. EVOHAT44 film (full circles): RF power = 20 W, treatment time = 10 min. EVOHDT29 film (empty circles): RF power = 15 W, treatment time = 10 min.

Figure 5. Water contact angle as a function of RF power for TMS plasma treatment. EVOHAT44 film (full circles): gas flow = 10 sccm, treatment time = 2 min. EVOHDT29 film (empty circles): gas flow = 10 sccm, treatment time = 2 min.

observed followed by the increase of the surface hydrophobicity starting from ∼25 W. The water contact angle stayed practically the same beginning from 60 W (Figure 5). 12603



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Figure 6. AFM images of EVOHDT29 film (a) before and after plasma treatments: (b) CF4 plasma; (c) CF4/H2 plasma; (d) CF4/C2H2 plasma; (e) TMS plasma.

commercial material derived from the parent vinyl acetate polymer. The surface composition was found to be influenced by the plasma gas used (Table 3). The change of chemical composition after the fluorinated plasma treatment was shown through an increase of fluorine and the decrease of oxygen and carbon concentrations. TMS plasma led to a significant amount of Si on the film surface.

The spectra for untreated EVOH were decomposed into six components (Figure 8a), assigned in order of increasing binding energy to CC (sp3) (284.6 eV), CCO (285.4 eV), CO (286.4 eV), CO (287.4 eV), COOR (288.5 eV), and COO− (289.4 eV). The presence of CO, COOR, and COO− peaks in small quantity (less than 2.5 at. %) may come from the residual vinyl acetate functions, since EVOH is a 12604



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Figure 7. AFM images of EVOHAT44 film (a) before and after plasma treatments: (b) CF4 plasma; (c) CF4/H2 plasma; (d) CF4/C2H2 plasma; (e) TMS plasma.

As one can see, the C1s spectrum of EVOHDT29 film after CF4 plasma treatment became more complicated (Figure 8b). In addition to C−Cn (sp3), C−CO, and C−O peaks from the EVOH structure, numerous peaks appear at higher binding energy, which correspond to C−Fn bonds.34−36 The film formed from CF4 gas shows a high concentration of fluorine on the film surface (44 at. %, Table 3). The detected amount of CF, CF2, and CF3 groups showed the predominant CF and CF2

species, supposing that, in the course of CF4 plasma treatment, the dissociation mechanism takes place according to eqs 4 and 5. Due to the presence of hydrogen and acetylene in CF4 plasma, biradicals are formed as the main source of free radicals.34,37 The presence of long-lived radicals on the film surface allows reacting the film with atmospheric oxygen, when the film is removed from the reactor chamber. The increase of 12605



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component at lower binding energy (685.7 eV) may be attributed to CF groups.36 In the case of gas mixtures (CF4/ C2H2 and CF4/H2), the presence of three components on the F1s spectrum was also observed (Figure 9b and c). However, for EVOHDT29 film treated by CF4/H2 plasma, the peak attributed to the presence of CF3 groups on the film surface (at 688.7 eV) was found to be twice higher than that of CF groups (at 686.1 eV) (Figure 9b). This confirms that the presence of hydrogen in CF4 plasma gas leads to the creation of very stable HF molecules.26,27 Besides, it is known that CF3 and CF2 groups make the main contribution to the hydrophobic nature of the material.11 To better understand the fluorined plasma treatment and to know about the fluorine plasma effect in depth, XPS sputtering has been carried out. A decrease of the C1s component at higher binding energies (more than 289 eV), assigned to various C−F bonds, is observed in the depth of plasma treated EVOH films, which means that fluorocarbon species disappear straight in the subsurface. The performed calculation gives an estimation of less than 9 nm for the average thickness of the fluorinated layer. The occurrence of this surface fluorination was also evidenced by the changes in water contact angle values (Figures 1−4). The high resolution C1s spectrum in the case of TMS plasma treatment can be fitted into six components (Figure 8e), assigned to CC (sp3) (284.3 eV), CSiO3 (285.0 eV), C O (286.2 eV), CO (287.2 eV), COOR (287.9 eV), and COO− (288.8 eV). The deposition of the SiOxCy layer on the EVOH film surface results in a lesser atomic composition of carbon than for untreated EVOH films (Table 3). For example, 72.2 at. % of carbon was found for the untreated EVOHDT29 film and only 52.5 at. % after TMS plasma treatment. The C3 SiO structure was found to be intermediate between the polydimethylsiloxane-like and silica SiO2-like chemical bonds,38 since the silicone in C3SiO has only one bond with oxygen and three with carbon. The fitted high resolution Si2p spectrum is shown in Figure 9d. The binding energies of silicon chemical bonds can be located at 101.3 eV for C3SiO, 102.3 eV for C2SiO2, and 103.3 eV for SiO4. It is established that the SiO4 chemical bond has the highest atomic concentration. The higher quantity of silicon found for EVOHAT44 film (15.9 at. %) compared to EVOHDT29 film (8.2 at. %) (Table 3) may be explained by the different contents of ethylene groups (44 and 29 mol %, respectively). 3.4. TMDSC Measurements. Cold plasma treatment usually modifies the extreme surface of polymer films, but in some cases, it can slightly change the crystallinity of a polymer material by increasing the temperature.2 Thus, in order to examine the influence of the plasma treatment on the EVOH film structure, TMDSC experiments were performed. Preliminarily, it was found that the glass transition temperature (Tg) was difficult to be determined from the average heat flow signal in the case of all studied samples. Janssens et al.39 proposed to determine the glass transition temperature value from the reversing heat flow signal, while the crystallization enthalpy (ΔHc), the fusion enthalpy (ΔHm), and the fusion temperature (Tm) are determined from the endothermic and exothermic peaks on the average heat flow signal. Usually, the crystallinity degree is calculated from the equation

Table 2. Root-Mean-Squared Roughness (rms) Estimated from AFM Measurements for the Plasma Treated EVOH Films sample

rms (nm)

grain size (nm)

EVOHAT44 treated by CF4 plasma treated by CF4/H2 plasma treated by CF4/C2H2 plasma treated by TMS plasma

15 27 22 16 19

60−110 1200−2500 450−600 750−1250 800−2100

EVOHDT29 treated by CF4 plasma treated by CF4/H2 plasma treated by CF4/C2H2 plasma treated by TMS plasma

26 29 30 30 38

900−2000 1200−3500 850−3300 1000−3000 1350−4000

Table 3. Elemental Surface Composition of EVOH Films Determined through XPS Analysis before and after Plasma Treatments atomic percentage EVOHAT44 treated by CF4 plasma treated by CF4/ H2 plasma treated by CF4/ C2H2 plasma Treated by TMS plasma EVOHDT29 treated by CF4 plasma treated by CF4/ H2 plasma treated by CF4/ C2H2 plasma treated by TMS plasma

F

Si

atomic ratio

C

O

76.8 43.1

22.5 11.6

Other

F/C

44.5

0.8

1.03

53.4

20.1

21.9

4.2

0.41

69.4

14.4

16.2

61.7

22.4

72.2 41.4

27.4 13.9

43.9

0.8

1.06

53.8

21.5

21.5

3.2

0.39

46.3

21.8

28.8

1.7

1.4

0.62

52.5

35.6

8.2

3.7

0.7

0.4

Si/C 0.01

0.01

0.23 15.9

0.26

0.4

0.01

0.04 0.16

oxygen concentration for CF4/H2 and CF4/C2H2 plasma treatments compared to CF4 plasma treatment suggests that these films have a greater number of long-lived radicals on the surface (Table 3) and, as a consequence, a larger amount of oxygen-containing moieties was formed on the film surface (Figure 8c and d). The higher hydrogen level increases scavenging of reactive fluorine species and inhibits the CFx→CFx+1 reaction that produces more highly fluorinated species for incorporation into the film. Furthermore, the higher carbon level decreases the F1s/C1s ratio in the feed gas (Table 3), leading to the formation of films with highly fluorinated species. F1s core-level spectra of the plasma treated polymer surface (Figure 9a−c) confirm that the fluorination is mainly connected to the carbon structure of EVOH by detection of F−C bonds. Besides, the general shape of the F1s envelope is more symmetrical than that of C1s and can be fitted into three components. In the case of CF4 plasma treatment, the spectrum is centered at 687.2 eV (Figure 9a). One can see that the component at 688.5 eV is due to the highly fluorinated carbon atoms up to CF3 groups,35 the component at 687.2 eV to CF2 groups in more or less fluorinated environments, and the

Xc = 12606

ΔHm − ΣΔHc ΔHm0

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Article

Figure 8. Fitted high resolution C1s XPS spectra of EVOHDT29 film (a) before and after plasma treatments: (b) CF4 plasma; (c) CF4/H2 plasma; (d) CF4/C2H2 plasma; (e) TMS plasma.

where ΔHm is the melting enthalpy, ΔHc is the crystallization enthalpy, and ΔH0m is the theoretical melting enthalpy with 100% of crystallinity. However, as EVOH is a copolymer, it is difficult to determine the crystallinity degree of EVOH, since copolymer with 100% crystallinity does not exist. Some authors, for example, Faisant et al.40 and Hwang et al.,41 have determined the theoretical enthalpy (ΔH0m) of EVOH only by the fusion heat of a perfect monoclinic crystal of the poly(vinyl alcohol) (PVA) from the relation ΔHm0(EVOH) = αΔHm0(PVA)

where α is the mole fraction of PVA in the copolymer. However, according to Lagarón et al.,3 this methodology is not justified because the ΔH0m of polyethylene crystallized structure should also be taken into account. Besides, Cerreda et al.42 have shown that the composition and the thermal history of EVOH film also influence the crystalline structure. Therefore, it would be better to estimate the degree of crystallinity from the X-ray diffraction measurements by calculating the ratio of the crystalline peak area to the total area.43 In our case, the influence of plasma treatment on crystallinity rate can be discussed on the basis of the quantity of ΔHm − ΔHc calculated from TMDSC results (Table 4).

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Figure 9. Fitted high resolution F1s or Si2p XPS spectra of EVOHDT29 film after plasma treatments: (a) CF4 plasma; (b) CF4/H2 plasma; (c) CF4/ C2H2 plasma; (d) TMS plasma.

decrease of Tg for CF4 plasma treated samples (Table 4). This fact may be explained by the presence of highly reactive atomic fluorine in the plasma atmosphere that breaks the polymer chains and makes the relaxation between the polymer chains easier. Gao showed the etching action of the radical F and ionized species on polyamide 6 films during He/CF4 plasma modification.44 The etching effect involves physical removal of molecules or fragments, breaking up of bonds, chain scission, and degradation processes. A similar degradation effect of CF4 plasma treatment also was observed on poly(ethylene terephtalate)45 and on polyimide46 films. In the latter case, the scission of imide bonds led to the formation of radicals, which initiated the degradation.46 In the case of EVOHDT29 film, plasma treatments (especially CF4/C2H2 and TMS) slightly reduce the Tg value (Table 4). Probably it is due to the fact that the structure of EVOHDT29 copolymer contains more hydroxyl groups that can be modified during the plasma treatment. For example, they may be transformed to Si−O groups after TMS treatment. Therefore, the quantity of the hydrogen bonds between the hydroxyl groups decreases, leading to a decrease of Tg value. This fact may also explain the slight decrease of crystallinity for CF4/C2H2 and TMS treated EVOHDT29 films (ΔHm − ΔHc value, Table 4). Indeed, a part of the crystalline phase of EVOH can be explained by the presence of hydrogen bonds.3,42,47 It is revealed that the plasma treatment practically does not influence the melting temperature (Tm) of EVOH films (Table

Table 4. Thermal Properties for EVOH Films before and after Plasma Treatments Tg (°C)

ΔHC (J·g−1)

Tm (°C)

ΔHm (J·g−1)

ΔHm − ΔHC (J·g−1)

EVOHAT44 treated by CF4 plasma treated by CF4/H2 plasma treated by CF4/ C2H2 plasma treated by TMS plasma

48.0 44.5

13 ± 3 5±3

164.5 165.0

66 ± 3 70 ± 3

53.0 ± 6 64.8 ± 6

45.5

13 ± 3

164.0

72 ± 3

59.2 ± 6

47.0

13 ± 3

164.0

65 ± 3

51.6 ± 6

49.0

4±3

165.0

65 ± 3

61.2 ± 6

EVOHDT29 treated by CF4 plasma treated by CF4/H2 plasma treated by CF4/ C2H2 plasma treated by TMS plasma

46.5 40.5

17 ± 3 13 ± 3

186.5 186.0

82 ± 3 84 ± 3

65.0 ± 6 70.6 ± 6

40.5

25 ± 3

185.5

89 ± 3

64.1 ± 6

37.0

37 ± 3

185.5

92 ± 3

55.0 ± 6

38.5

28 ± 3

186.0

87 ± 3

58.9 ± 6

sample

In general, from TMDSC data and taking into account the error of measurements, one can conclude that the cold plasma treatment has practically no influence on the thermal properties of EVOH films. After the cold plasma treatments of EVOHAT44 film, no change can be observed except the slight 12608



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Table 5. Water Transport Properties through Untreated and Treated by Cold Plasma EVOHAT44 films

EVOHAT44

treated by CF4 plasma

treated by CF4/H2 plasma

treated by CF4/C2H2 plasma

treated by TMS plasma

a

1 barrer = 10

−10

sample N

P (barrera)

D0·1010 (cm2·s−1)

DI ·1010 (cm2·s−1)

DL ·1010 (cm2·s−1)

⟨D⟩ ·1010 (cm2·s−1)

γCeq

γ (cm3·mmol−1)

Ceq (mmol·cm−3)

1 2 average 1 2 average 1

404 414 409 314 271 293 429

0.48 0.50

1.41 1.40

1.96 1.94

3.80 3.75

3.30 3.24

0.29 0.28

11.2 11.6

0.36 0.42

1.05 1.31

1.52 1.83

2.83 3.64

3.30 3.4

0.28 0.43

11.8 7.9

1.80

2.32

2.62

3.62

1.25

0.1

12.5

2 average 1

414 421 478

1.62

2.12

2.35

3.40

1.34

0.1

12.9

2.55

3.74

4.31

6.73

1.71

0.23

7.5

2 average 1

480 479 357

1.82

1.59

1.15

3.42

0.43

8.0

2 average

313 335 −2 −1

BIF (%)

28.4

−2.9

16.0

24.4

30.3

45.5

−17.1 0.54

1.70

2.41

3.07

2.60

4.73

Improvement of Water Barrier Properties of Poly(ethylene-co-vinyl

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