Impact of hydrophobic plasma treatments on the barrier properties of poly(lactic acid) films

Abstract

Different hydrophobic plasma treatments (CF₄, CF₄/H₂, CF₄/C₂H₂, tetramethyl silane (TMS)) were applied to the poly(lactic acid) (PLA) film in order to improve its water and oxygen barrier properties. The plasma parameters, such as power, gas flow and treatment time, were optimized according to the water contact angle measurements. X-ray photoelectron spectroscopy measurements revealed the presence of either fluorine (CF, CF₂, CF₃) or silicon (SiO_xC_y) functional groups on the film surface after the fluorinated or TMS plasma treatments, respectively. The thermal properties of the treated PLA films were studied by means of the differential scanning calorimetry (DSC) measurements and were found not to be influenced by the plasma treatment. The water permeability measurements showed an improvement of the PLA barrier properties as a result of all plasma treatments used and, particularly, after CF₄/C₂H₂ plasma. The water vapour sorption measurements confirmed well the improvement of the water barrier properties by the reduction of the water solubility. No impact of the plasma treatment on the oxygen barrier properties of the PLA film was observed, even at high relative humidity (up to 90%).

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Introduction

The amount of polymer films being used in various sectors, particularly in food packaging applications, has been significantly increased recently.¹ The polymers have enormous advantages, such as the flexibility of thermal and mechanical properties, the low weight and the low price, in comparison with other classical materials (glass, metals, etc.). However, the polymers possess some application limitations. For instance, their inherent permeability to the transport of low molecular weight components like oxygen can lead to the food oxidation.² Besides, the use of polymers gives rise to a number of environmental problems, i.e. the pollution and accumulation of the solid waste products and the petroleum residua. Therefore, the development of polymer films based on biopolymers instead of petroleum-based materials has attracted great attention and renewed interest due to their environmental friendly nature and their potential use in the packaging industry.²

The biobased materials are divided into three families: (i) the polymers extracted directly from the biomass, such as the starch and the cellulose; (ii) the polymers prepared by a classical synthesis reaction from monomers based on the biomass, such as poly(lactic acid) (PLA); (iii) the polymers produced from natural or genetically modified micro-organisms, such as poly(hydroxyalkanoates) (PHA).

PLA is a linear aliphatic polyester (Fig. 1), synthesized from the lactic acid, which is derived from renewable resources by the bacterial fermentation of carbohydrate,³ that means it is a biodegradable and biocompostable polymer. Historically, the PLA application was limited to biomedical and pharmaceutical fields.^4,5 Recently, PLA was approved by Food and Drug Administration (FDA) which authorizes its direct contact with the biological fluid and with the food. So, PLA was used for packaging.^6–8 PLA becomes increasingly used as a biodegradable polymer due to its high mechanical strength and easy processability compared to other biopolymers.⁹ It is important to note that oxygen, water, carbon dioxide and nitrogen barrier performances of PLA are lower than those of poly(styrene) (PS), but higher than those of poly(ethylene terephthalate) (PET).^4–6

Fig. 1 Chemical structure of PLA.

However, the main drawback of PLA is its easy degradation, mainly caused by the hydrolysis process of the ester bonds under the humid conditions. This disadvantage limits the PLA application in the packaging industry. In order to overcome this problem and to improve the mechanical, barrier and adhesion properties of PLA, different solutions have been already proposed. One of them is the melt blending with different types of plasticizer (lactide, oligomer, glycerol),^4,7 natural fibers,¹⁰ with stabilizing agents (peroxides),⁶ or with other polymers, for example poly(caprolactone) (PCL), poly(3-hydroxy butyrate) (PHB)⁷ or polyphosphazene.¹¹ The other widely used methods are the incorporation of nanoparticles⁴ and the surface treatments, for example, the fluorination with perfluoropolyether (PFPE).¹² Each of these methods has its own advantages and limitations depending on the type of PLA and its further application.

The plasma treatment appears to be a suitable solution, since it is an environmental friendly technique (sterile dry chemistry). The treatment time is short that favours its industrial application. Besides, the plasma treatment induces a surface modification without affecting the bulk properties.¹³ However, in plasma chemistry several different elementary reactions (degradation, polymerization, crosslinking, etc.) can occur simultaneously; therefore the choice of the plasma parameters is a key feature of the plasma treatment. The PLA films treated by the plasma are, in most cases, intended for biological applications in order to enhance the adhesion between the cells and the PLA film and to improve the cell growth.^14–19

It is known that the packaging material requires certain mechanical strength and special physical, chemical and biological properties, such as barrier protection, transparency, safe food contact, etc. Barrier properties depend essentially on the biochemical reactions taking place in food, mainly on its respiratory activity, i.e. on the reaction between sugar and oxygen resulting in forming carbon dioxide and water. In order to increase the shelf life of food, the respiratory intensity should be decreased, that can be obtained by moderating the water permeability and/or by increasing the selectivity between carbon dioxyde and oxygen.

The main objective of the present research was to improve the water and oxygen barrier properties of the PLA film. The water barrier properties were improved in order to overcome the high sensibility of the PLA film to the moisture, and the oxygen barrier properties were improved in order to enhance the PLA use in packaging application. For this purpose, hydrophobic plasma treatments such as CF₄, CF₄/H₂ and CF₄/C₂H₂ mixtures and TMS were used. The presence of fluorine on the surface of food packaging films may have a negative part, because of its known negative effect on our health. But the matter is in the value of fluorine. That's why it is important to clarify this point. According to FDA regulations, for the content less that 0.05 mg kg⁻¹ day⁻¹, fluorine is considered to be beneficial for teeth: it prevents dental caries.²⁰ But with the content more than 2 mg kg⁻¹ day⁻¹, the fluorine presence leads to undesirable effects (for example, dental fluorosis). The use of the fluorine based cold plasma treatment for the polymer film modification induces the presence of very small quantity of fluorine (a few ppm) on the film surface as the thickness of the plasma modified layer is only a few nanometers. Besides, the fluorine liberation is limited due to the covalent bonds formed between the fluorine compounds formed during the plasma treatment and the polymer surface, as the fluorine is attached by covalent bonds to the film surface.^21,22 Therefore, the fluorine plasma treatment can be used for the polymer films designated for the food package application. To obtain the maximum hydrophobic effect after the plasma treatment, the optimization of the plasma conditions (power, gas flow and treatment time) as a function of the water contact angle value was investigated. The influence of different plasma treatments on the PLA surface composition was analyzed by XPS measurements. The water and oxygen permeability measurements were performed in order to determine the effect of plasma treatments on the barrier properties of the PLA films. In addition, the water vapour sorption measurements were carried out for correlating the sorption and permeation properties of the plasma treated PLA films. Finally, the thermal properties of the PLA films after the different plasma treatments were studied by means of DSC measurements to ensure the stability of the bulk properties of the PLA treated films, since the thermal properties may strongly affect the barrier properties.

Experimental

Materials

PLA (PLE005) (L/D isomer ratio = 96 : 4, T_g = 64 °C, T_m = 150 °C, M_n = 6 × 10⁴ g mol⁻¹) was supplied by NaturePlast (France). CF₄ (99.99% purity, Messer), C₂H₂ (99.95% purity, Air Liquid), Ar (99.99% purity, Air Liquid), H₂ (99.99% purity, Air Liquid), N₂ (99.99% purity, Air Liquid), O₂ (99.99% purity, Air Liquid) and N₂/H₂ (95%/5%, Air Liquid) gases, and TMS (>99.0% purity, Sigma-Aldrich) were used as received.

Elaboration of the PLA films

The films were prepared by the compression molding technique using the molding press (CAMEX 20T type). The PLA grains were placed between two sheets of aluminium. These sheets were placed for 10 min between the platens of the press, which was previously heated to 185 °C, without any applied pressure. Then, a pressure of 100 bar was applied for 10 min followed by cooling to the room temperature. The thickness of obtained films was 110 ± 20 μm. All films were conditioned in a dessicator over P₂O₅ under vacuum at room temperature (23 ± 2 °C) before testing.

Experimental techniques

Cold plasma treatment. For plasma treatment, an aluminium vacuum chamber with 310 mm of length and 255 mm of width was used.²³ Plasma was excited with a capacitively coupled 13.56 MHz radio frequency (RF) generator, capable of delivering continuously varying power output from 0 to 600 W. The base pressure of the plasma chamber was down to 1 · 10⁻⁷ mbar. The injected amount of gas was controlled by mass flowmeters and the flow value is presented in standard cm³ min⁻¹ (sccm). The influence of the nature of plasma treatment gas (CF₄, CF₄/H₂, CF₄/C₂H₂ and TMS), the power (15–60 W), the gas flow (10–25 sccm) and the treatment time (2–10 min) on the hydrophobicity of the PLA surface was investigated by means of water contact angle measurements.

Water contact angle measurements. A goniometer Rame-Hart N 100-00 was used to determine the water contact angle values at room temperature (23 ± 2 °C). The measurements consist in dropping a 5 μL of water (MilliQ Water System, resistivity 18 MΩ cm⁻¹) on the PLA film surface before and after plasma treatment. Then, the angle between the water drop and the PLA surface was measured by a sessile drop method. To provide a statistical mean value, the average of six drops at different positions on the film surface was calculated.

X-ray photoelectron spectroscopy (XPS). The surface chemical composition of the plasma treated PLA films was examined using a VG 220i-XL ESCALAB system. The Mg X-ray source (Kα = 1253.6 eV) was activated. The acquisition of high-resolution spectra was done at 20 eV. A flood gun was activated for the charge compensation. The binding energies were reliable to ±0.2 eV for such insulating samples. The elemental composition was calculated from high resolution spectra using Scofield sensitivity factors. The Avantage software provided by ThermoFisher Scientific was used to fit the XPS spectra.

Differential scanning calorimetry (DSC). A TA Instruments apparatus (DSC 2920) equipped with a low-temperature cell (minimal temperature = −50 °C) was used for DSC measurements. The calibration was performed with indium standard (T_m = 156.6 °C, ΔH_m = 28.66 J g⁻¹).²⁴ The calorimetric analysis was performed under dry nitrogen (70 mL min⁻¹) using the standard aluminium pan containing ∼2 mg of sample. The study of crystallization and melting processes was carried out in the heat-only mode. The temperature was varied from 30 to 200 °C at the rate of 2 °C min⁻¹. The degree of crystallinity of the PLA films was calculated from the equationwhere ΔH_c is the crystallization enthalpy, ΔH_m and ΔH⁰_m are the enthalpy for melting and for a 100% crystalline PLA polymer (93 J g⁻¹),²⁵ respectively.

Water permeation measurements. Water permeation through the PLA film before and after the plasma treatments was measured at 25.0 ± 0.2 °C using the permeadiffusiometer previously described.^23,26 After a drying step with a dry nitrogen, the upstream compartment of the permeation cell containing the film was full with the liquid water (MilliQ water system, resistivity 18 MΩ cm⁻¹). The water transfer through the PLA film was measured as a function of the time by controlling the variation of the humidity in the dry nitrogen gas in the downstream compartment using a chilled mirror hygrometer. The permeation measurement was considered as completed, when the humidity value was stable, which corresponds to the steady state. The measurements were repeated twice for each PLA film.

Water vapour sorption measurements. For water vapour sorption kinetic measurements, an automatic gravimetric dynamic vapour sorption system (DVS1 Advantage, Surface Measurement Systems, Ltd., London, UK) equipped with a Cahn microbalance was used. The measurements were carried out at the constant temperature (25.0 ± 0.1 °C) and for different relative humidity (RH) values ranging from 0% to 95%. Approximately 10 mg of polymer film was loaded onto the stainless steel mesh pan that then was placed in a closed chamber. First, the film was dehydrated (RH = 0%) until a constant dry weight was reached. Then, the PLA film was submitted to a hydration cycle. For this purpose, humidified nitrogen of known RH was passed through the chamber constantly. The change of the film weight was recorded for each water activity tested and the mass at each equilibrium state was used to construct the sorption isotherm. Mass at steady state was determined at each humidity stage by measuring the percentage change in mass with respect to time, i.e. the curve slope. Once the mass slope appeared to be below a predetermined threshold value and the equilibrium was achieved, the experiment proceeded to the next programmed humidity stage.

The experimental sorption isotherms were fitted to Park's sorption isotherm model that supposes the association of three mechanisms: the specific sorption on special sites (Langmuir's mode) at low water activity a_w (a_w < 0.2); the non-specific sorption (Henry's law) for water activity range up to 0.5 and the water molecule aggregation at high water activities (a_w > 0.5).^27,28 Thus, Park's model comprises three terms with five parameters to be defined:

where A_L is Langmuir's capacity constant, b_L is an affinity constant, K_H is Henry's law coefficient, K_agr is an aggregation equilibrium constant and n is an average size of the aggregate.

The parameters of Park's model were calculated using the non-linear regression analysis by Table Curve 2D® software. The mean relative deviation modulus E was defined by the equation:

where m_i and m_ci are the experimental and predicted moisture contents at observation i, respectively, N is the number of observations. The modulus value below 10% indicates a correct fitting.²⁹

The sorption data were used to calculate the diffusion coefficient D, which is a measure of the ability of the water molecules to move through the polymer film. In order to determine the diffusion coefficient, Fick's second law must be solved, which depends on geometry and concentration conditions.³⁰ For a thin film geometry, in which diffusion from the edges of the film can be neglected and assuming a constant diffusion coefficient, the total mass sorption of the water can be described by:³⁰

where M_t and M_∞ are the quantity of sorbed water at time t and infinite time, respectively, L is the film thickness and D is the diffusion coefficient. This equation can be simplified to the so-called short-half sorption (up to 50% of M_∞):where is the slope of the initial linear portion of the curve versus .

Oxygen permeability measurements. The oxygen transmission rate measurements were performed at 23 °C and atmospheric pressure using a OX-TRAN model 2/21 equipped with a Coulox^® sensor and supplied by Lippke (Mocon company, Germany). This equipment consists of two measurement cells that allow performing two different tests simultaneously. Each measurement cell is composed of two compartments separated by the polymer film – the inside chamber and the outside chamber. The PLA film was twice masked with tight aluminium foil leaving a circular exposure area of 5 cm². The system was initially purged with an oxygen-free carrier gas (N₂/H₂ (95%/5%) mixture) to remove sorbed oxygen from the film. The carrier gas was conducted to the coulometric sensor and the drying step was continued till the steady state was reached. Then, the pure oxygen gas (99.99%) was introduced into the outside chamber of the cell, while the inside chamber was being exposed to the carrier gas. This carrier gas, containing the oxygen transferred through the film, was conducted to the coulometric sensor. The flow rate through the PLA film at steady state was measured. The RH of both gases was controlled by the humidifier and varied from 0 to 90%. The oxygen permeability (P_O2) was calculated by normalizing the flow rate at steady state with respect to the oxygen pressure gradient over the film and the film thickness.

Results and discussion

Plasma treatment optimization

Plasma treatment is a technique that can be used to modify the surface properties of the polymer films without altering their bulk properties that may affect their function. For this reason, to improve the barrier properties, the PLA films were subjected to RF cold plasma treatment. This type of treatment implies low energy processes and the created species have slight penetration energy.³¹ Therefore, the modification is limited to a few molecular layers from the surface, providing specific functionality. The various excitation, ionization and dissociation reactions occurring during the plasma process generate high-energy and reactive species (electrons, ions, radicals, photons) that can interact with the film surface, changing its chemical and physical characteristics. Besides, one should remember that the plasma treatment is accompanied by two antagonist processes – functionalization and degradation^31–33 – and the compromise between them should be found. The type of the surface modification induced by the plasma treatment is strongly dependent on the choice of reactive gas. To improve the water and oxygen barrier properties the hydrophobic plasma treatment should be preferentially used.^23,31–34 The effect of the plasma treatment on the surface properties can be first evaluated by the water contact angle measurements since the variation of the water contact angle as a function of plasma parameters (power, gas flow and treatment time) allows determining the optimum plasma conditions. As the water and oxygen barrier properties are expected to be improved, the highest water contact angle values should be obtained.

The change of the water contact angle values for the PLA film after CF₄ plasma treatment as a function of the plasma parameters is shown in Fig. 2. The water contact angle for the untreated PLA film is about 76 ± 2°. The increase of the RF power leads to an increase of the water contact angle (Fig. 2a). This result can be explained by the fact that the initial RF power increase to 10 W provokes the dissociation of the plasma species in the reactor that, in its turn, enhances the functionalization effect. The further increase of the RF power does not lead to significant changes in the water contact angle value and almost constant value is observed starting from 15 W (110 ± 2°) due to a more pronounced effect of degradation. Under such working conditions (CF₄ flow = 25 sccm and 10 min of the treatment time), the compromise between the functionalization and degradation effects was reached at 15 W.

Fig. 2 Water contact angle of the PLA surface treated by CF₄ plasma as a function of (a) RF power, (b) gas flow and (c) treatment time.

The variation of the water contact angle value as a function of CF₄ flow for the PLA film shows an inverted parabola dependence with the maximum value of about 119 ± 2° for 20 sccm (Fig. 2b). The decrease of the water contact value to 100 ± 2° was observed for the further CF₄ flow increasing (over 20 sccm) due to the degradation process on the PLA surface. Consequently, CF₄ flow less than 20 sccm favours the functionalization character of the plasma treatment. It is known that the degradation effect of CF₄ plasma process is caused by the presence of the atomic fluorine in the plasma phase.^23,32,33 Therefore, one may suppose that the contribution of the degradation process during the plasma treatment increases with the gas flow increase as the concentration of the atomic fluorine grows.

The increase of the plasma duration to 10 min revealed an increase of the water contact angle value up to 118 ± 2° (Fig. 2c). The longer treatment had only small effect on the water contact angle since a practically constant value of the water contact angle was obtained. The slight decrease of the water contact angle value observed at the beginning of the treatment (Fig. 2c) may be associated with the so-called cleaning of the PLA surface, i.e. the removal of the surface impurities.³¹ The optimum conditions for CF₄ plasma treatment that correspond to the highest water contact angle value are summarized in Table 1.

Table 1 Optimized plasma conditions for the PLA film

Treatment	RF power (W)	Gas flow (sccm)	Treatment time (min)	Content of H2 or C2H2 (%)
CF4 plasma	15	20	10	—
CF4/H2 plasma	15	18/2	10	10
CF4/C2H2 plasma	15	19/1	10	5
TMS plasma	50	10	2	—

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To limit the degradation effect during CF₄ plasma process, different solutions may be found in the literature.^31–33 D'Agostino proposed to add hydrogen to the fluorine plasma in order to combine the atomic fluorine into stable HF molecules.³¹ Also, the influence of the acetylene addition to the CF₄ plasma was studied by Jacobsohn et al.,³³ as it is known that the presence of acetylene in plasma phase allows the creation of a compact tridimensionnel network which plays an important role in the barrier properties.^35–37 The addition of H₂ and C₂H₂ gases to CF₄ plasma was studied in the case of poly(ethylene-co-vinyl alcohol) (EVOH) films with different content of the ethylene groups (29 (EVOHDT29) and 44 (EVOHAT44) mol%) and quite satisfactory results were obtained.²³ It was found that the best barrier improvement factor for EVOHAT44 film was obtained after CF₄ plasma treatment (about 28%) and in the case of EVOHDT29 film – after CF₄/C₂H₂ plasma treatment (about 25%). The established difference between the effects of the plasma treatment on EVOH films was related to the polymer film composition, i.e. to the content of ethylene and hydroxyl groups, and the crosslinking reactions produced during the plasma treatment.

To reduce the degradation effect and to improve the hydrophobicity of the PLA films, H₂ and C₂H₂ gases were added to CF₄ plasma phase (Fig. 3). The water contact angle value decreases after the introduction of H₂ (Fig. 3a) or C₂H₂ (Fig. 3b) gases to CF₄ plasma. This fact can be explained by the formation of the stable HF molecules and, thus, by the decrease of the fluorine species in the plasma phase. The similar results were found by Montazer Rahmati et al. in the case of polyethylene films treated by the mixture of CF₄ with H₂.³² Table 1 presents the optimized conditions for CF₄/H₂ and CF₄/C₂H₂ plasma treatments.

Fig. 3 Water contact angle of the PLA surface as a function of H₂ (a) and C₂H₂ (b) content in CF₄ plasma: RF power = 15 W, treatment time = 10 min.

Transparent materials such as silicon oxide (SiO_x) and silicon nitride (SiN_x) deposited onto the polymer substrate were widely used as gas barrier films.^21,38–40 In the case of such organic/inorganic barrier coating, the inorganic oxide layer acts as a diffusion barrier to water vapour and/or oxygen permeation. The defects of this barrier inorganic layer are covered by the polymer, thus improving the barrier performance of the composite film. Different organosilicon reactants were used for this purpose, for example, hexamethyldisiloxane (HMDSO),^39,41 the mixtures of HMDSO and ammonia³⁸ or of tetramethoxysilane and oxygen,⁴² etc. In the case of using TMS as a silicon source, the presence of hydrophobic groups, such as SiMe₄⁺, SiMe₃⁺, SiMe₂⁺, SiMe₃˙,^34,43 generated by the dissociation of TMS molecules, induces a hydrophobic character of the surface after the plasma treatment.

The activation of the substrate surface is usually required in order to increase the effect of the liquid precursor use (TMS).⁴¹ Such pretreatment activation may be performed by an inert gas, like Ar or He. A pure inert gas does not contain any chemically reactive species and normally leads to radical formation, crosslinking or double bond formation on the polymer surface. In our case, Ar plasma pretreatment during 2 min was used in order to activate the PLA surface before TMS plasma as it is known that Ar plasma exposure always leads to a fresh surface for the PLA film.⁴⁴ The water contact angle value as a function of the RF power is shown in Fig. 4. As one can see, practically no variation of the water contact angle is observed up to 30 W. Thereafter, the significant improvement of the film hydrophobicity was observed – the water contact angle value increased from 76 ± 2° to 96 ± 2° at 50 W. The observed water contact angle variation may be explained by the changes of the surface chemical composition.^40,42 Further increase in the RF power value did not change water contact angle (Fig. 4), meaning that 50 W might be taken as the optimum value of the RF power (Table 1).

Fig. 4 Water contact angle of the PLA surface treated by TMS plasma as a function of RF power: flow = 10 sccm, treatment time = 2 min.

For further investigations, the PLA films treated under the optimum plasma conditions (Table 1) were taken.

XPS analysis

XPS analysis was performed to examine and to quantify the chemical composition of the PLA film surface after the cold plasma treatment. Table 2 shows the atomic composition of the PLA films before and after plasma treatments. Contamination of silicon detected on some films may arise from the film preparation technique.

Table 2 Experimental atomic composition (at.%) and atomic ratio obtained by XPS analysis for the PLA films treated under the optimized plasma conditions (Table 1)

	Atomic percentage					Atomic ratio
	C	O	F	Si	Other	O/C	F/C	Si/C
PLA	59.6	40.4	—	—	—	0.67	—	—
Treated by CF4 plasma	46.0	21.0	33.0	—	—	0.46	0.72	—
Treated by CF4/H2 plasma	52.1	28.7	13.9	2.2	1.3	0.55	0.27	0.04
Treated by CF4/C2H2 plasma	46.0	14.5	36.9	2.6	—	0.31	0.80	0.06
Treated by TMS plasma	59.5	30.6	—	9.9	—	0.51	—	0.17

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After all plasma treatments, the O/C atomic ratio decreases, indicating that the incorporation (grafting) of the fluorinated or silicon functional groups occurs on oxygen atoms of PLA (Fig. 1). Besides, the fluorine plasma treatments (CF₄, CF₄/H₂, CF₄/C₂H₂) introduce a significant amount of fluorine on the PLA surface (Table 2). To obtain a better insight into the chemical bonds on the PLA surface, the fitting of the high resolution spectra was performed (Fig. 5). The high resolution C_1s spectrum for the untreated PLA film (Fig. 5a) may be decomposed into seven components according to the PLA composition (Fig. 1): the hydrocarbonated structure with C–H bonds (284.6 eV), C neighbouring to C–OH (285.3 eV), –C–OH (286.0 eV), –C=O (287.1 eV), –COO⁻ (288.3 eV) and O–C=O (289.9 eV) groups.^13,19,44,45

Fig. 5 High resolution C_1s XPS spectra of the PLA films before (a) and after plasma treatments: (b) CF₄ plasma, (c) CF₄/H₂ plasma, (d) CF₄/C₂H₂ plasma, (e) TMS plasma.

During the plasma treatment, active plasma species attack the polymer surface that leads to the incorporation of additional functional groups. As one can see from Table 2, the fluorine treatment of PLA results in a substantial amount of fluorine at the surface. In this case, the spectra become more complex and the appearance of the fluorinated group peaks may be observed at 290.9, 291.9 and 292.9 eV for CF-, CF₂- and CF₃-groups, respectively (Fig. 5b–d).^22,23,46,47 The decomposition of the high resolution F_1s spectra obtained for the PLA surface after the fluorine plasma treatment (Fig. 6a–c) also reveals the presence of the fluorinated groups at 688.3 eV, 686.9 eV and at 685.4 eV.^22,47 Besides, it should be noted that the peak attributed to the presence of CF₂-groups on the PLA surface (around 686.8 eV) is the highest one (Fig. 6a–c). These results correlate well with the water contact angle measurements, as it is known that CF₂ groups make the main contribution to the hydrophobic character of the film.^32,33 The presence of the peak at 285.4 eV was observed on the high resolution C_1s spectrum after TMS plasma treatment (Fig. 5e). At the same time, the binding energies of silicon chemical bonds locate at 101.0 eV for C₃–SiO, at 102.4 eV for C₂–SiO₂ and at 104.0 eV for SiO₄ (Fig. 6d). The deposition of the inorganic SiO_xC_y layer on the PLA surface results in the decreasing of the oxygen content and in the presence of ∼10 at.% of silicon (Table 2).

Fig. 6 High resolution F_1s and Si_2p XPS spectra of the PLA films after plasma treatments: (a) CF₄ plasma, (b) CF₄/H₂ plasma, (c) CF₄/C₂H₂ plasma, (d) TMS plasma.

DSC measurements

The plasma, corona discharge, laser and electron-beam irradiation are considered as the extreme surface treatments not affecting the material bulk properties.^13,31 However, an impact of the laser irradiation on the crystallinity degree of the PLA films has been shown.⁴⁸ The crystallinity changes have been attributed to the generated thermal effects during the treatment process. And so, DSC measurements were performed in order to investigate the influence of the different plasma treatments on the bulk properties of the PLA films. The degree of crystallinity X_c, the glass transition temperature T_g, and the melting temperature T_m of the plasma treated PLA films are summarized in Table 3. As one can see, no significant changes in the values of X_c, T_g, and T_m were observed after the cold plasma treatment. A slight decrease of the melting temperature after CF₄ plasma treatment (144 °C in comparison with 150 °C for the untreated PLA) may be related to the modification of the crystalline structure size. These results are in good agreement with the study of Chaiwong et al. concerning the influence of SF₆ plasma on the hydrophobicity properties of PLA.⁴⁹ Therefore, one can conclude that the bulk structure of PLA is unchanged after the hydrophobic plasma treatments used.

Table 3 Thermal properties of the PLA films before and after plasma treatments

	Tg (°C)	ΔHC (J g^−1)	Tm (°C)	ΔHm (J g^−1)	XC (%)
PLA	56.7 ± 2.0	33 ± 3	150 ± 1	34 ± 3	1 ± 3
Treated by CF4 plasma	56.9 ± 2.0	28 ± 3	144 ± 1	33 ± 3	6 ± 3
Treated by CF4/H2 plasma	57.2 ± 2.0	29 ± 3	151 ± 1	35 ± 3	6 ± 3
Treated by CF4/C2H2 plasma	56.6 ± 2.0	29 ± 3	151 ± 1	32 ± 3	4 ± 3
Treated by TMS plasma	58.6 ± 2.0	32 ± 3	151 ± 1	33 ± 3	1 ± 3

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Ageing behaviour

The ageing effect of the plasma treated PLA films was studied using water contact angle measurements. After the plasma treatment, the PLA films were stored in the dessicator at room temperature (23 ± 2 °C). Fig. 7 shows the evolution of the contact angle of the plasma treated films as a function of storage time. As shown in Fig. 7, the ageing process is characterized by a decrease in water contact angle values during the storage time and, thus, may be referred to as hydrophilic recovery.^19,49 The mechanism of ageing and methods to delay the recovery are the subject of active research. The change in the contact angle can be explained by the phenomenon of orientation of mobile groups into the bulk of the material.^31,34,44 At the beginning, just after the plasma treatment, hydrophobic groups are incorporated on the polymer surface and, as a result, the improvement of hydrophobic properties is observed for freshly treated films. However, for the polymer films stored for some months, the functional groups start getting reoriented toward each other and also toward the interior to reach a more energetically favorable position. Besides, as the plasma treatment is accompanied by two antagonistic processes (functionalization and degradation), some small molecules from the plasma phase as well as degradation products may attach to the PLA surface by the physisorption process. So, another effect which could cause the ageing behavior is the release of the less stable functional groups from the plasma treated PLA surface.^32,34 It is because of this fact that the contact angle changes during the storage resulting in a partial loss of the treatment effect (Fig. 7). Faster degradation of hydrophobic properties in the case of TMS plasma treated PLA film may be explained by the slow hydrolysis of Si–O–C moiety³⁴ (Fig. 6d) occurred during the storage. However, it should be highlighted that the contact angle value reached after the storage during 2 years differs from the water contact angle value for the untreated PLA film (76°), which means that the plasma surface modification is being kept even after 24 months of ageing.

Fig. 7 The time evolution of the water contact angle of the PLA surface after plasma treatments: (□) CF₄ plasma, (◊) CF₄/H₂ plasma, (○) CF₄/C₂H₂ plasma, (Δ) TMS plasma.

Water barrier properties

Water barrier properties of the plasma treated PLA films were evaluated by measuring water permeation and water vapour sorption kinetics in the film. The water permeability measurements allow determining the quantity of water molecules diffused through the polymer film by the surface and time unities and the water vapour sorption measurements enable determining the quantity of water molecules sorbed by the polymer film in time. Both types of measurements depend on the solubility and the diffusion coefficients of water in the polymer film. In its turn, the solubility coefficient (thermodynamic parameter) is related to the affinity between water molecules and the polymer, and the diffusion coefficient (kinetic parameter) is related to the size of the permeant, the nature of the polymer and also its structure and, particularly, to the crystallinity degree and the stiffness of the polymer film. The correlation of water permeation and water vapour sorption measurements with the physical and structural characteristics allows better understanding the transport properties of the plasma treated PLA films.

Water permeation measurements. The thickness-corrected water flux JL through the PLA films as a function of the thickness-corrected time tL⁻² is presented in Fig. 8a. All permeation curves can be divided into three parts.⁵⁰ At the beginning, the water molecules have not passed through the PLA film yet, so no water flux was determined, i.e. JL = 0. Then, in some space of time, water molecules started to cross the PLA film and the strong increase of the water flux was observed. Such increase corresponds to the transient step, from which the diffusion coefficient (D) can be calculated. After this step, the steady state is reached, i.e. the water flux remains practically constant (J_st), and the permeability coefficient (P) can be calculated as the product of water flux (J) and film thickness (L) divided by the pressure difference between the two faces of the film. The diffusion coefficient (D) is evaluated from the permeation curve with normalized flux (J/J_st) as a function of time. On the basis of Fick's law, taking D as a constant,³⁰ the values of D_I and D_L, corresponding to the transient permeation curve at J/J_st = 0.24 and J/J_st = 0.62, respectively, can be calculated (Table 4).

Fig. 8 Experimental reduced (a) and normalized (b) water permeation flux curves for the PLA films before () and after plasma treatments: () CF₄ plasma, ()CF₄/H₂ plasma, ()CF₄/C₂H₂ plasma, () TMS plasma.

Table 4 Water transport properties through the untreated and plasma treated PLA films

	P (Barrer^a)	Diffusion coefficient at J/Jst = 0.24 DI × 10^10 (cm^2 s^−1)	Diffusion coefficient at J/Jst = 0.62 DL × 10^10 (cm^2 s^−1)	Mean integral diffusion coefficient 〈D〉 × 10^10 (cm^2 s^−1)	Plasticization factor γCeq	Barrier improvement BI (%)
a 1 Barrer = 10^−10 cm^3(STP) cm cm^−2 s^−1 cmHg^−1.
PLA	2030 ± 32	94 ± 15	117 ± 18	187 ± 37	2.0 ± 0.1
Treated by CF4 plasma	1688 ± 47	162 ± 32	227 ± 42	428 ± 103	3.2 ± 0.1	17
Treated by CF4/H2 plasma	1670 ± 36	107 ± 24	140 ± 25	246 ± 43	2.6 ± 0.1	18
Treated by CF4/C2H2 plasma	1608 ± 78	103 ± 4	136 ± 9	242 ± 34	2.7 ± 0.7	21
Treated by TMS plasma	1910 ± 126	112 ± 18	144 ± 30	255 ± 73	2.5 ± 0.6	6

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As one can see from Fig. 8a, all plasma treatments result in the decreasing of the water permeability (the steady state of water permeation flux decreases), especially after CF₄/C₂H₂ plasma. These results are in good agreement with the water contact angle (Fig. 2–4) and XPS (Table 2) measurements. Indeed, CF₄ plasma induces an important hydrophobic effect (the increasing of the water contact angle from 76° to 120° and the presence of 33 at.% of fluorine on the PLA surface). However, the presence of the atomic fluorine favours the degradation (etching) effect, which negatively impact the barrier properties.^23,49 The addition of H₂ to CF₄ plasma will promote the decrease of the degradation effect of the atomic fluorine, but, in its turn, will give a slightly hydrophilic character to the treated surface.³² In the case of CF₄/C₂H₂ plasma, the hydrophobic character of CF₄ plasma is strengthened by the crosslinking effect of acetylene.^33,35–37 Really, in this case the higher fluorine content was obtained on the PLA surface (∼37 at.%, Table 2). Therefore, the treatment by the mixture of CF₄/C₂H₂ seems to be the most efficient, as in this case the barrier improvement (BI) is about 21% (Table 4). For comparison, Chaiwong et al. have obtained the BI that equals to 14% for the PLA film treated by SF₆ plasma.⁴⁹

The comparison of D_L and D_I coefficients (Table 4) shows that D_L value is higher than D_I value for all PLA films. As the former value corresponds to a latter period of the transient state (J/J_st = 0.62), the smaller D_I value means that the water diffusion increases during the permeation process. Generally, such diffusion increase is attributed to the plasticization of the material by water which leads to an increase of the material free volume. All permeation curves were fitted to the well known exponential law of diffusion^23,30

D = D₀e^γC, (6)

where D₀ is the limit diffusion coefficient, γ is the plasticization coefficient and C is the local permeant concentration. From the obtained curves it was possible to determine the mean integral diffusion coefficient 〈D〉 and the plasticization factor γC_eq, with C_eq as the limit condition of concentration. One can see that the plasticization factor (γC_eq) slightly increases as a result of the plasma treatments (Table 4). The obtained 〈D〉 values demonstrate that the reduction of the water permeability for the plasma treated films cannot be attributed to the decrease of the water diffusivity. Indeed, the increase of the diffusion coefficients is observed after all plasma treatments (Table 4). This result is unexpected since the hydrophobic coating should lead to an increase of the tortuosity by increasing the diffusion pathway for water molecules through the fluorinated layer. During the plasma treatment, the degradation occurs at the same time with the functionalization process. It is known that CF₄ plasma treatment can generate pinholes and chain scissions on the film surface.³¹ This degradation effect could favour the water diffusivity by reducing the molecular mass of the treated layer as it can be observed from the normalized permeation curves of flux (Fig. 8b) showing that the transient regime is more easily reached for the treated PLA films than for the untreated film.

Therefore, one can deduce that the decrease of the permeability can be explained by the reduction of the affinity between the water molecules and the polymer surface due to the presence of hydrophobic groups on the treated surface, i.e. by decreasing the solubility coefficient. To valid this hypothesis the water vapour sorption measurements were performed.

Water vapour sorption measurements. It is well known that the water sorption isotherms give additional information about the film structure. Fig. 9 shows the water sorption isotherm curves for the plasma treated PLA films. The isotherm for the untreated PLA film is given for comparison. In general, the curves display the familiar sigmoidal shape: with increasing environmental water activities a_w to about 0.8 the equilibrium water content increases slowly but then sharply rises. As one can see, untreated PLA do not absorb a lot of water molecules – ∼1.1% at a_w = 0.95. Such behaviour may be explained by the difference between the solubility parameter of water (40 MPa^1/2) and PLA (19–20.5 MPa^1/2).⁶

Fig. 9 Water vapour sorption isotherms for the PLA films before (●) and after plasma treatments: (□) CF₄ plasma, (◊) CF₄/H₂ plasma, (○) CF₄/C₂H₂ plasma, (Δ) TMS plasma. Solid lines are the best fit to Park's model (eqn (2)).

The water sorption measurements performed for the plasma treated PLA films reveal the same sorption behaviour as the untreated PLA film (Fig. 9). The obtained isotherms clearly show that all plasma treatments reduce the water sorption capacity of PLA in the whole water activity range. However, even if the moisture resistance is enhanced, the plasma treatment of PLA has not strongly reduced the ability of PLA to absorb and accept water. The low sensitivity of PLA to the water (∼1%) and the small thickness of the plasma treated layer may explain not very significant barrier improvement.

As the increase in a_w is accompanied by a nonlinear increase in water content, the experimental water sorption isotherms were fitted according to Park's model (eqn (2)). The results presented in Table 5 confirm that Park's model is a good representation of the experimental data – the mean deviation modulus E values (eqn (3)) are ranging from 2.5 to 9.6% that validate the performed fit. Consequently, the water sorption behaviour can be understood by analyzing the constants of Park's model. Mathematical fitting of the sorption data to eqn (2) has shown the decrease of A_L value for all plasma treated PLA films (Table 5). This value represents the Langmuir's capacity constant related to the concentration of specific sites on which water molecules can be sorbed. The decrease of A_L value indicates the reduction of the surface solubility of plasma treated surfaces for water molecules (Fig. 9). The reduction of this adsorption (Langmuir's mode) testifies to the presence of the plasma grafted hydrophobic groups on the binding sites. In the case of PLA, the specific sites of sorption are mainly the –OH groups (Fig. 1). This result is in good agreement with the XPS analysis, i.e. with the decrease of the oxygen atomic concentration after the plasma treatments (Table 2).

Table 5 Parameters of Park's model (eqn (2)) for the untreated and plasma treated PLA films

	AL	KH	Kagr	n	E (%)
PLA	0.0830	0.42	1403	13	2.5
Treated by CF4 plasma	0.0003	0.42	2054	13	7.9
Treated by CF4/H2 plasma	0.0090	0.40	2259	12	9.6
Treated by CF4/C2H2 plasma	0.0020	0.41	2086	13	5.3
Treated by TMS plasma	0.0035	0.45	1027	13	4.8

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As one can see from the results in Table 5, the found K_H and n values for the untreated and plasma treated PLA films are quite close that means that Henry’s law (i.e. non-specific absorption) and water clustering components were practically not influenced by the plasma treatments. The change of the water sorption behaviour of the plasma treated PLA films is in accordance with the result obtained by Chaiwong et al. for the PLA film treated by SF₆ plasma.⁴⁹ The authors ascertained that the water absorption time increased twice after the plasma treatment due to the presence of the fluorinated functional groups on the PLA surface.

In conclusion one can say that the hydrophobic plasma treatment by CF₄, CF₄/H₂, CF₄/C₂H₂ and TMS makes the entrance of the first water molecules into the polymer structure difficult, thus affecting Langmuir's mode (the decrease of A_L value (Table 5)). Therefore, the reduction of the water permeability can be caused by the decrease of the water solubility. In other words, the decrease of the water permeation flux occurs after the plasma treatment as if the total surface of the film, which is in contact with water, is reduced because of the presence of hydrophobic groups. From the moment when the first water molecules penetrate into the polymer and as a_w value increases, the PLA film starts to swell, opening up new (deeper in the film thickness) sites for water to adsorb. It should be also noted that the acceleration of the sorption process was observed at the highest water activity (a_w = 0.95) for the PLA films treated by CF₄/H₂ and TMS plasma (Fig. 8). It means that the access of the water molecules into the polymer becomes easier and the water aggregation increases. This phenomenon cannot be explained clearly for the moment.

The diffusion coefficient (eqn (5)) can more directly reflect the effect of the performed plasma treatment on the transport mechanism of the water molecules in the PLA films. Fig. 10 shows the variation of the water diffusion coefficient as a function of water vapour activity for untreated PLA film as well as for all plasma treated PLA films. Three domains may be distinguished from the experimental data represented in the semi-logarithmic scale either for untreated PLA or for the plasma treated films. At low water activity (a_w ≤ 0.3) the increase of the diffusion coefficient is observed due to the plasticization effect.³⁰ Indeed, the absorption of water molecules by the specific sites leads to the free volume increase in PLA and, thus, improves the mobility of the water molecules. In the case of the plasma treated PLA films the increase of the diffusion coefficient is less pronounced because of the plasma grafted hydrophobic groups' presence on the specific sites (Langmuir's contribution). At medium water activity (0.3 < a_w < 0.6) the diffusion coefficient remains practically constant (Fig. 10). For more hydrophilic polymers, e.g. derivatives of cellulose, the plasticization effect induced by water molecules is usually observed for a larger water activity (0 < a_w < 0.5).³⁰

Fig. 10 Influence of the water activity on the diffusion coefficient for the PLA films before (●) and after plasma treatments: (□) CF₄ plasma, (◊) CF₄/H₂ plasma, (○) CF₄/C₂H₂ plasma, (Δ) TMS plasma.

A significant decrease of the diffusion coefficient is observed for a_w > 0.5 (Fig. 10). Such variation of the diffusion coefficient with the water activity may be explained by the presence of water aggregates which are formed during the water vapour sorption process.^23,30 Thus, the reduction of the mobility of the water molecules is observed. Besides, the variation of the diffusion coefficient with the water activity increase is the same for both untreated and plasma treated PLA films.

The plasticization phenomenon has been revealed by both sorption and permeation measurements (Table 4 and Fig. 10). Moreover, it is interesting to note that the sorption kinetic measurements have also revealed the aggregation phenomenon. It is important to remind that the boundary conditions are different for sorption and permeation processes. It means that the PLA film is not under the same experimental conditions, i.e. both faces of the polymer film are in contact with the water vapours during the permeation measurements whereas only one film face is in contact during the sorption measurements, so that the water concentration gradient is different. Therefore, as the aggregation of water molecules is considered as rather slow process, so mostly it may be observed by the sorption measurements.

Oxygen barrier properties

The good oxygen barrier properties of polymer films used for food packaging are critical for a long shelf life of the packaged food products. The oxygen permeation through the food package may cause the loss of the food quality because of the oxidative degradation reactions.⁵¹ It was found that the maximum oxygen ingress of 1–5 ppm is enough to limit the shelf life of some foods at 25 °C.⁵² Therefore, the oxygen barrier properties of the plasma treated PLA film with the highest water barrier effect, i.e. after CF₄/C₂H₂ plasma treatment, were studied as a function of the RH level (Fig. 11).

Fig. 11 Relative humidity effect on the oxygen permeability of the PLA films before (●) and after (○) CF₄/C₂H₂ plasma treatment.

The oxygen permeability coefficient value measured for the untreated PLA film (0.2–0.3 Barrer) is close to that reported in the literature – 0.31–0.61 Barrer depending on the polymer structure.⁵ The oxygen permeability of the PLA film is not really affected by the RH (Fig. 11). Only a slight decrease of the oxygen permeability is observed at RH = 50% following by the oxygen permeability increase with the further RH increase. Two explanations of this phenomenon can be found in the literature. The first one is that such decrease of the oxygen permeability at RH = 50% may be attributed to the facility of the water molecules to faster occupy the free volume in the polymer matrix in comparison with the oxygen molecules.⁵³ This will decrease the diffusion pathway of the oxygen molecules and, consequently, the oxygen permeability will be reduced. The second one explains the observed decreasing of the oxygen permeability by the antiplasticization effect.⁵⁴ In this case the water molecules fill the available free volume in the amorphous region of the polymer, thus strengthening the intermolecular hydrogen bonding across the free volume. Thus, the water molecules hinder the polymer chain mobility at this RH level and reduce the diffusion of the oxygen molecules. At higher RH levels (>50%), the water molecules form agglomerates inside the PLA film by increasing the water concentration in the polymer that leads to the increase of the oxygen permeability in the PLA film.

The oxygen permeability values for the untreated and plasma treated PLA films are found to be of the same order of magnitude even at high RH (90%) (Fig. 11). Besides, the PLA film treated by CF₄/C₂H₂ plasma exhibits similar oxygen barrier performance compared to the untreated PLA film, i.e. the decrease of the oxygen permeability up to 50% RH with its further increase. Such a result is not very surprising as the water plasticization effect is not vanished even after the plasma treatments (Fig. 8 and 10). Therefore, in order to reduce the oxygen permeability, a decrease of the plasticization should be obtained. For this purpose, the successive plasma treatment will be the subject of further investigation.

Conclusion

In order to improve the barrier properties and especially the water resistance of the PLA film, different hydrophobic plasma treatments were studied (CF₄, CF₄/H₂, CF₄/C₂H₂, TMS). XPS analysis revealed the presence of fluorinated (CF_x) or silicon (SiO_xC_y) functional groups on the film surface after the corresponding plasma treatment under the optimum conditions used. The invariability of the bulk PLA properties was confirmed by DSC measurements, i.e. no variation of the thermal properties was observed after the plasma treatments.

The increase of the water barrier properties of PLA after all plasma treatments was shown by the water permeability measurements. The highest improvement was obtained after CF₄/C₂H₂ plasma treatment – up to 21%. The barrier effect was explained by the hydrophobic character of the grafted fluorinated groups and by the crosslinking reaction caused by the presence of C₂H₂ in the plasma phase. The analysis of the water permeation and sorption measurements confirmed that the decrease of the water permeability was mainly due to the decrease of the solubility inside the plasma treated PLA films for the water molecules. However, no significant effect of the plasma treatment on the oxygen barrier properties of the PLA films was established, even at high RH levels. This result may be explained by the water plasticization and the formation of water clusters after the plasma treatment.

Thus, one can conclude that the hydrophobic plasma treatment is the effective means to improve the water barrier properties of the PLA films.

Acknowledgements

The authors would like to thank the French Ministry for Research and Technology for the financial support and N. Delpouve from LECAP laboratory, University of Rouen (France) for DSC measurements.

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