Surface Modification by Low-Pressure Plasma of Polyamide 12 (PA12

Langmuir 2002, 18, 10411-10420

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Surface Modification by Low-Pressure Plasma of Polyamide 12 (PA12). Improvement of the Water Barrier Properties F. Dreux,† S. Marais,*,† F. Poncin-Epaillard,‡ M. Me´tayer,† and M. Labbe´† UMR 6522, CNRS/Universite´ de Rouen, “Polyme` res, Biopolyme` res et Membranes”, 76831 Mont-Saint-Aignan Cedex, France, and UMR 6120, CNRS/Universite´ du Maine, “Polyme` res, Colloı¨des et Interfaces”, 72017 Le MANS Cedex, France Received June 24, 2002. In Final Form: October 1, 2002 Polyamide 12 (PA12) films have been modified by CF4 and CF4+H2 (50/50 v/v) microwave plasma with different treatment times. The surface modifications have been followed versus plasma exposure duration by water contact angle measurements and atomic force microscopy (AFM) Pervaporation measurements were carried out to characterize the effects of these plasma treatments on water transport through PA12 films. From these measurements, water permeability was determined for each duration time of treatment. The efficiency of these plasma treatments in reduced permeability is compared. For both plasma treatments (CF4 and CF4+H2), the analysis of the experimental data shows an increase and then a decrease of the permeability coefficient P with treatment durations. These observations are related with the evolution of the surface versus treatment time. From all these experimental results, it is clearly shown that the barrier effect to water in plasma-treated layers of PA12 is improved significantly, especially with CF4.

1. Introduction Owing to their favorable performances as efficient barrier materials, polymers are used in many applications and are therefore widely used in packaging and protective coating. Moreover, it is more and more frequently recommended that the barrier effect is improved against molecular species such as hydrocarbons in fuel containers that are responsible for air pollution. The modification of the permeability of polymeric materials is now, therefore, a much explored field. Among all the new technologies involved in improving the permeability of polymer films, one in particular is often used: the creation of multilayers systems, mostly through the plasma technique.1 The plasma treatment of membranes could also be applied as a simple chemical technique to modify the permselectivity properties. For example, a composite polybutadiene/polycarbonate (PB/PC) 2 membrane treated in CHCl3 plasma shows higher oxygen permeation with transport properties depending on the plasma parameters such as duration and power. These transport properties alterations are associated with a physical modification of the extreme surface, i.e., densification and/or cross-linking, rather than a chemical modification (chlorination) as membrane pores tend to disappear and the PB fraction to cross-link. However, if the polymer matrix cannot be cross-linked, like the poly(trimethylpropyne) membrane,3 the CF4 plasma treatment induces an increase of its permselectivity toward oxygen and nitrogen compared to the virgin membrane. But aside from this increase of permselectivity, the aging of the treated surface can degrade the perm-properties.2 The surface modification induced * Author to whom correspondence should be addressed. † UMR 6522, CNRS/Universite ´ de Rouen. ‡ UMR 6120 CNRS/Universite ´ du Maine. (1) D’Agostino R. Plasma deposition and modifcation of polymers; Academic Press: Boston, 1990. (2) Chen, S. H.; Chuang, W. H.; Wang, A. A.; Ruaan, R. C.; Lai, J. Y. J. Membr. Sci. 1997, 124 (2), 273-281. (3) Lin, X. A.; Jun, X. A.; Yu, Y. L.; Jie, C.; Zheng, G. D.; Xu, J. P. J. Appl. Polym. Sci. 1993, 48 (2), 231-236.

by the plasma treatment is proved to limit the exudation of bigger molecules included in the original poly(vinyl chloride) (PVC) film, such as the di(2-ethylhexyl) phthalate or adipate.4,5 In an opposite manner, a faster diffusion of biomolecules such as insulin through a copolymer acrylonitrile and methallylsodium sulfonate for an artificial pancreas organ is observed.6 The barrier effect is also obtained with different plasma polymerizations or plasma-enhanced chemical vapor deposition processes. Various hybrids in organic SiO2 or SI3N4 layers can grow onto the membrane through the plasma polymerization of precursors such as hexamethyl disiloxane (HMDSO, C6H18SiO2) and its mixtures with SiH4 and N2. Their thickness, L, ranges from 8 to 200 nm.7-9 The oxygen and water vapor permeations are approximatively 0.5 scc m-2 day-1 and 0.3 g m-2 day-1 for a threshold thickness value of 15 nm, respectively. The major fact is that the transport properties are more dependent on the layer topography (cracks, smoothness, etc.) than on its thickness above this minimum. Pervaporation membranes have been prepared either with the plasma deposition of fluorinated monomers (tetrafluoroethylene, perfluoropropane, or perfluoropriopylene)10 or with organosilicium monomers (hexamethyldisiloxane, octmethyltrisiloxane, and decamethyl tetrasiloxane)11,12 (4) Ishikawa, Y.; Honda, K.; Sasakawa, S.; Hatada, K.; Kobayashi H. Vox Sang. 1983, 45, 68-73. (5) Audic, J. L; Poncin-Epaillard, F.; Reyx, D.; Brosse, J. C. J. Appl. Polym. Sci. 2001, 79 (8), 1384-1393. (6) Kessler, L.; Legeay, G.; West, R.; Belcourt, A.; Ponget, M. J. Biomed. Mater. Res. 1997, 34, 235-245. (7) Sobrinho, A. S. D.; Latreche, M.; Czeremuszkin, G.; KlembergSapieha, J. E.; Wertheimer, M. R. J. Vac. Sci. Technol. A 1998, 16 (6), 3190-3198. (8) Vallon, S.; Hofrichter, A.; Guyot, L.; Drevillon, B.; KlembergSapieha, J. E.; Martinu, L.; Poncin-Epaillard, F. J. Adhes. Sci. Technol. 1996, 10, 1287-1311. (9) Vallon, S.; Brenot, R.; Hofrichter, A.; Drevillon, B.; Gheorghia, A.; Senemand, C.; Klemberg-Sapieha, J. E.; Martinu, L.; PoncinEpaillard, F. J. Adhes. Sci. Technol. 1996, 10, 1313-1332. (10) Masuoka, T.; Iwatsubo, T.; Mizoguchi, K. J. Appl. Polym. Sci. 1992, 46 (2), 311-317. (11) Matsuyama, H.; Kariya, A.; Teramoto, M. J. Appl. Polym. Sci. 1994, 51 (4), 689-693.

10.1021/la020584d CCC: $22.00 © 2002 American Chemical Society Published on Web 12/02/2002

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onto polypropylene films. These new membranes are described as perm-selective toward ethanol, and their efficiency depends on the length of grafted siloxane chains. The combination of the Ar plasma modification followed by a chemical grafting of acrylic monomers13-16 onto porous polypropylene or PVC increase its perm-selectivity toward water for water-ethanol mixtures, a phenomenon emphasized by the ionization of grafted chains. The grafting yield and the value of the pH medium changes also the perm-selectivity. On this basis, our first purpose is to reduce the water permeability of polymer films such as polyamide 12 (PA12) by using a surface treatment, the low-pressure plasma treatment. This technique appears suitable because it does not affect the bulk phase properties of the material, solvents are not used, and short times of treatment are enough to introduce an important effect. Some applications of this kind of process are already in development i.e., for dialysis membranes.17 PA12, belonging to the class of dense polymers, is characterized by very low water permeabilities and used in many industrial fields. Its success arises, in part, from its performance as a good barrier material with good mechanical properties, and from its low cost. Despite its good performances and taking into account the new requirements in terms of permeability for fuel containers in the future (new European standards against pollution) the barrier properties of PA12 toward organic vapors are not sufficient and need to be improved. The effect of fluorinated plasma has been studied in a preliminary work performed on an unsaturated polyester resin.18 From these first results, it appears that the CF4 plasma treatment used for these polymers enables water permeability to be reduced. In this work, plasma treatment conditions were studied: time of treatment and gas quality, pure gas CF4 and then the (50/50 v/v) mixture CF4+H2, given that it is well-known that the introduction of di-hydrogen in CF4 plasma reacts with atomic fluorine, which is responsible for degradation.1 Both sides of a PA12 film were modified by using these two types of tetrafluoromethane low-pressure plasma with different treatment times. The modifications of the water diffusivity and permeability values resulting from these plasma treatments were analyzed. The pervaporation measurements were performed with apparatus developed in our laboratory.19 To characterize the effects of the plasma treatment on the surface morphology, contact angles were measured and the topography of the treated surfaces were analyzed by means of atomic force microscopy (AFM). 2. Materials and Methods 2.1. Materials. Polyamide 12 (PA12) films are provided by ATO FINA (27 Serguigny, France). The thickness of these films, L, is measured using a Roch-O-25 micrometer (error: ( 1µm) on several evenly distributed points. In our case, LPA12 ) 100 µm. (12) Inagaki, N. Plasma Surface Modification and Plasma Polymerization; Technomic Publishing: Lancaster, U.K., 1996. (13) Hirotsu, T. J. Appl. Polym. Sci. 1987, 34, 1159-1172. (14) Hirotsu, T.; Arita, A. J. Appl. Polym. Sci. 1988, 36, 177-183. (15) Hirotsu, T.; Arita, A. J. Appl. Polym. Sci. 1991, 42, 3255-3261. (16) Vigo, F.; Uliana, C.; Traverso, M. Eur. Polym. J. 1991, 27 (8), 779-783. (17) Lecacheux, H. Elaboration et caracte´ risation de mate´ riaux membranaires fluore´ s par PECVD. Application a` l’extraction d’ions bromure en solution aqueuse. Thesis of the University of Montpellier II, France, 1999. (18) Marais, S.; Me´tayer, M.; Labbe´, M.; Valleton, J. M.; Alexandre, S.; Saiter, J. M.; Poncin-Epaillard, F. Surf. Coat. Technol. 1999, 122, 247-259. (19) Me´tayer, M.; Labbe´, M.; Marais, S.; Langevin, D.; Brainville, M.; Chappey, C.; Dreux, F.; Belliard, P. Polym. Test. 1999, 18, 533549.

Dreux et al. Before the plasma treatment, the PA12 films are precleaned with acetone on both faces and then stored under vacuum until required. 2.2. Methods. (i) Low-Pressure Plasma (CF4) Treatment. The equipment used in this work is a microwave plasma20 composed of three parts. 1. The excitation is provided by a microwave generator (433 MHz) with variable power (0-250 W), which is coupled to a surfatron. The incident power Pi and the reflected power Pr are measured with a power meter (Hewlett-Packard, 435B). The impedance is adjusted until the reflected power is very low (Pr < 10-2 W). The glow is generated at the top of the reactor. 2. The reactor is a quartz cylinder of 500 mm length and 76 mm diameter. The reactor is set up in a chamber used for the sample introduction. The sample substrate can be moved in or out of the plasma volume. 3. The pumping system is composed of a primary pump (CIT Alcatel 2012) and an oil diffusion pump (CIT Alcatel Crystal). The pressure P is measured with Penning and Pirani gauges at the base of the plasma device. The gas flow is controlled by a MKS mass flowmeter type 1259B. Before each run, the system is kept to 10-4 Pa for at least 2 h, with incident power Pi ) 70 W, reflected power Pr < 10-2 W, gas flow f ) 10 cm3 min-1 (STP, sccm), pressure P ) 30 Pa. The gases used for this study are tetrafluoromethane (CF4, Messers France, purity >99,99%) and tetrafluoromethane associated with hydrogen (CF4+H2, 50/50 f/f). The plasma treatment procedure is as follows: the film is introduced into the reactor, then the system is pumped up to an ultimate pressure of 10-4 Pa. The gases are introduced at a controlled flow. The samples are simultaneously treated on both sides of the polymer film with different times of treatment duration. For both treatments, CF4 and CF4+H2 (50/50 f/f), the plasma conditions are the same (power ) 50 W and total gas flow ) 10 sccm). (ii) Atomic Force Microscopy (AFM). Atomic force microscopy experiments are performed with a Nanoscope II model from Digital Instruments (Santa Barbara, CA), in the contact mode, with a 150 and a 15 µm scanner for molecular resolution studies. The cantilevers used are characterized by a low spring constant of about 0.06 N/m. All the measurements are performed with the feedback loop in operation (constant force ) 10-9-10-8 N) in ambient air. Treated samples are investigated by AFM before and after plasma treatment. All the AFM images are presented in the height mode (palette of color for height: dark colors for low zones, light colors for high zones) and are top-view images. (iii) X-ray Photoelectron Spectroscopy (XPS). The XPS spectra are obtained with CF4-treated samples using a LHS 12 LEYBOLD spectrometer equipped with an Mg source. After being referenced with respect to the 284.6 eV carbon 1s level (observed for hydrocarbon), the C1s spectra are deconvoluted using computing curve fitting. These measurements are carried out at the “Institut des Mate´riaux de Nantes” (BP 32229, 44322 Nantes Cedex 3, France). From these XPS measurements, two kinds of information are searched in this work. The elementary composition is determined from the relative intensities of F1s, O1s, N1s, and C1s core level spectra. (iv) Contact Angle Measurements. Water contact angle (θ) and surface energy measurements are performed at room temperature (23 °C). A few seconds after the treatment, water contact angles are measured with a goniometer. The θw values obtained are summed up in Table 1 (θw1 values); they correspond to an average of three measurements made with the same volume of the drop (2 µL)21 and at different locations on the sample. After plasma treatment (3 days later and before permeation tests), the contact angle measurements are performed again but with three different liquids: ultrapure water (milliQ Water system, resistivity 18Ω/cm, Millipore), diiodomethane (Aldrich, (20) Poncin-Epaillard, F.; Wang, W.; Ausserre, D.; Scharzenbach, W.; Derouard , J.; Sadeghi, N. Eur. Phys. J. Appl. Phys. 1998, 4, 181191. (21) Morra, M.; Occhiello, E.; Garbassi, F. Adv. Colloid Interface Sci. 1990, 32, 79-116.

Surface Modification of PA12 by Low-Pressure Plasma

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Table 1. Contact Angles and Surface Energy of PA12 Film First Untreated and Then Treated by CF4 or CF4+H2 Plasmaa

Eastern Instruments, Massachussetts) was used for its high accuracy ((0.07 part per million (volume) of water vapor in a gas). The previously dried film was mounted in the cell and dry nitrogen was flushed in both compartments over many hours until a dew point lower than -70 °C was obtained. Next, a stream of liquid water was pumped through the upstream compartment, then the water concentration in the initially dry sweeping gas was monitored in the downstream compartment via the hygrometers and a data acquisition system. The flux J(L,t) at the dry interface is obtained from

treatment time (min) CF4

CF4+H2

θw1 (°)

θw2 (°)

γd (mJ/m2)

γp (mJ/m2)

γ (mJ/m2)

0 1 2 3 5 8 10 12 15

64 99 109 112 115 118 118 117 125

64 111 111 111 115 113 113 113 113

24 8 9 8 7 8 5 6 6

17 18 14 14 13 6 14 12 11

41 26 23 22 20 14 19 18 17

0.5 1 3 6 8 10 12 15

59 70 99 106 109 112 108 100

60 73 91 95 93 101 94 90

20 23 15 9 24 24 23 24

27 15 12 13 2 2 2 2

47 39 28 22 26 26 25 26

(xout - xin) f pt J(L,t) ) 10-6 A RTr

(5)

with A the film surface area (30 cm2), R the ideal gas constant, and Tr the temperature (in K) of the experiment. The water concentration x (ppmV) is calculated from the water vapor pressure p, which is directly related to the sweeping gas dew points Tdp at the inlet and the outlet of the cell (x ppmv ) 106 p/pt, pt being the total pressure, usually 1 atm.).19

3. Theoretical Background of Diffusion

Plasma conditions: P ) 50 W and f ) 10 sccm. b θw1: contact angle of water measured immediately after the plasma treatment. c θ : contact angle of water measured 3 days after the plasma w2 treatment. a

99%), and glycerol (Aldrich, 99%). Samples are stored under vacuum. In this case, the values for each liquid are obtained by measuring θa, the advancing contact angle, and θr, the receding contact angle. A drop of a liquid of 2 µL was deposited with a microsyringe, the support was tilted, and the drop was photographed with a black-and-white CCD camera (500 × 500) just before falling. The contact angle was determined from a computerized contact anglemeter (NFT Communications Company, Tours, France). The θw values presented in Table 1 (θw2 values) are the means of θa and θr values obtained from at least three different drops. The surface energy is then calculated according to the WendtOwens method.22 This method uses the Young-Dupre23 and Fowkes 24 equations:

γsv ) γlv cos θ + γsl

(1)

Wsl ) γs + γl - γsl

(2)

γl(1 + cos θ) ) 2xγds γdl + 2xγps γpl

(3)

γ ) γp + γd

(4)

where γ is the surface energy and θ is the contact angle. The indications s and l correspond respectively to the solid and the liquid phases, and the exponents d and p correspond, respectively, to the dispersion force and to the polar components.25 Knowing the surface energies from the three liquids used (water: p p γdl ) 21.8 mJ/m2, γl ) 51.0 mJ/m2; glycerol: γdl ) 37.0 mJ/m2, γl p d 2 2 ) 26.4 mJ/m ; and diiodomethane: γl ) 49.5 mJ/m , γl ) 1.3 mJ/m2), the surface energy of the polymer can be calculated. (v) Water-Specific Permeameter. The permeameter consists of a measurement cell, a dry nitrogen supply, and a hygrometric unit consisting of two sensors. The first sensor, a capacitancetype hygrometer (gold-plated alumina device, from Shaw Gruter & Marchand, F92000 Nanterre Ltd., Bradford, England), was selected because of its fast response (response time shorter than 3 s), and the second one (chilled mirror hygrometer, General (22) Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969, 13, 17411747. (23) Dupre, A. Theorie de me´ canique de la chaleur; Gauthier-Villars: Paris, 1969. (24) Fowkes, F. M. J. Phys. Chem. 1963, 67, 2538-2541. (25) Good, R. J. Contact angle, wetting, and adhesion; K. L. Mittal: Washington, 1993; pp 3-36.

The mathematical treatment of diffusion transport is based on the following assumptions:26,27 (1) the polymer film is homogeneous, (2) the process is Fickian, (3) the interfacial sorption (penetrant/polymer) equilibrium is instantaneous and steady, and (4) the mass transfer occurs in a direction perpendicular to the plane sheet. Concentration and flux profiles, C(x,t) and J(x,t), are described by Fick’s laws, and the boundary conditions used are

for t ) 0, C(x,0) ) 0 ∀ x ∈ ]0,L[ at x ) 0, C(0,t) ) Ceq ∀t

(6)

at x ) L, C(L,t) ) 0 ∀t The measurement principle, based on the differential permeation, and its procedure were described in a previous paper.19 When the upstream face of an initially dry film is suddenly put into contact with an atmosphere at fixed water concentration, while the downstream face is swept with a dry gas at the flow rate f, a water permeation flux J occurs through the film. The initially nil flux increases progressively with time up to a limit Jst typical of the steady state. The variation of the reduced water flux J/Jst with time is obtained by integrating Fick’s laws in our specific boundary conditions. Permeability. The permeability coefficient is the product of gas flux and film thickness divided by the pressure (or concentration) difference across the film. From the steady state (with stationary flux Jst for water or gas), it is possible to determine the permeability coefficient P:

P)

JstL ∆a

(7)

where L is the thickness of the polymer film, and ∆a is the difference in activities between the two faces of the film. In pervaporation at the upstream interface with pure water, ah ) 1. In permeation, ah ) ph/pst, where ph and pst are vapor and saturated vapor pressures, respectively. Usually P is expressed in Barrer (10-10 cm3 STP.cm/cm2 s cmHg). (26) Crank, J.; Park, G. S. Diffusion in polymers; Academic Press: London and New York, 1968. (27) Carslaw, H. S.; Jaeger, J. C. Conduction of heat in solids, 2nd ed.; Oxford University Press: London, 1959; Chapters 3 and 12.

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Diffusivity. One of the main problems in transport phenomena is the determination of the ad-hoc value (or expression) of the diffusion coefficient D. If we assume that D is constant, its value can be calculated in at least two different ways: (i) from the time-lag tL:28

DL )

L2 6tL

(8)

The calculated value of DL is determined on the transient permeation curve at the J point corresponding to a value of J/Jst ) 0.6167, for which t ) tL. (ii) from the time t0.24 corresponding to a value of J/Jst ) 0.24, i.e., at the inflection point I of the transient permeation curve:29

DI )

0.091L2 t0.24

(9)

If DL is found to be practically equal to DI, D can generally be assumed to be constant. In permeation, this particular case of D constant is characterized by the reduced curve j ) f(τ) (j ) J/Jst) with τ ) Dt/L2 ∞

j)1+2

(-1)ne(-n ∑ n)1

2 2 ) π τ

(10)

This reduced curve j ) f(τ) shows an inflection point I(τI ) 0.091, jI ) 0.24), (cf Figure 9). At this inflection point I of the plot of the dimensionless flux J/Jst versus the reduced time τ ) Dt/L2, the slope R ()∆j/∆τ) depends on the D variation law. When D is constant, increases, or decreases with concentration, R ) R0 ) 5.82, R > R0 or R < R0, respectively. The slope R could be used as a significant parameter of the concentration dependence. If DL is different from DI, a model which takes into account a possible variation of D with the concentration C of sorbed molecules must be tested. The interpretation of the experimental curves is thus more complex and requires the numerical integration of the experimental data. When D is not constant, the diffusion coefficient is generally considered to increase exponentially with the local permeant concentration in the film during the course of water penetration and is generally attributed to a plasticization effect of the materials by the permeant (water):30,31

D ) D0eβC

(11)

where D0 is the limit diffusion coefficient, β is the plasticization coefficient, and C is the local permeant concentration; this includes the cases of linear or quadratic change of the diffusivity with the permeant concentration. To determine the two parameters of this diffusion law, we use a new method which is described in more detail in a separate paper.32 During the fitting procedure of the experimental transient flux data, the values of DM ()D0eβCeq), D0, β, Ceq and D h are computed. Ceq is the (28) Frisch, H. L. J. Polym. Sci. 1957, 61, 93-95. (29) Marais, S.; Me´tayer, M.; Labbe´, M. J. Appl. Polym. Sci. 1999, 74, 3380-3395. (30) Prager, S.; Long, F. A. J. Am. Chem. Soc. 1951, 73, 4072-4075. (31) Stern, S. A.; Trohalaki, S. In Barrier Polymers and Barrier Structures; Korros W. J., ACS Symposium Series, American Chemical Society: Washington, D.C., 1990; pp 22-59. (32) Marais, S.; Me´tayer, M.; Nguyen, Q. T. Macromol. Theory Simul. 2000, 9, 207-214.

penetrant concentration in the polymer at sorption equilibrium and D h is an integral mean diffusion coefficient which can be used to characterize the average diffusion coefficient of water in the materials.21,28 4. Results Atomic Force Microscopy. From AFM images obtained, at a scale of 50 × 50 µm, Figure 1 shows the aspect of the surface topography of different PA12 samples, first untreated (a) and then treated by CF4 for 15 min (b) and CF4+H2 for 12 min (c) (these treatment times are the durations corresponding to the best permeability reduction for each treatment). It can be observed that before the plasma treatment, the surface morphology of the PA12 sample at scale 50 × 50 µm appears as a flat surface with no characteristic information except two parallel scratches which are the result of the extrusion process (cf Figure 1a). As shown in Figure 1b,c, drastic changes appear on the surface after treatment, but with different structures depending on the type of the plasma used, CF4 (b) and CF4+H2 (c). However, in both cases, the scratches have been almost filled in by a coating formation. The images in Figure 1b,c show that the CF4 plasma treatment leads to a soft layer, exhibiting nodular structures,18 which covers the surfaces in a more homogeneous way, whereas for CF4+H2 plasma treatment (Figure 1 c), a layer can also be observed but appears less dense. The new reduced scale of 5 × 5 µm allows a better view of the secondary structure of these treated surfaces (cf Figure 2). Images obtained at this new reduced scale highlight the presence of two kinds of structures, depending on the type of plasma treatment used. For the CF4, the previous observations remain valid, with the presence of nodular structures which appear as heaps (cf Figure 2a), whereas for the CF4+H2 plasma treatment, structures in the form of short sticks appear on the polymeric surface (Figure 2b). X-ray Photoelectron Spectroscopy. The large spectra obtained for the nontreated and CF4-treated PA12 films with different times of treatment duration are presented in Figure 3. The PA12 sample modified by CF4 plasma treatment is characterized by a high peak corresponding to fluorine. The increase of the hydrophobic character is directly due to the introduction of fluorine at the polymer surface. As shown in Figure 3, from the first seconds of treatment (the sample treated by CF4 for 0.5 min), a peak of high intensity appears and then increases with treatment durations. At the same time, peaks corresponding to oxygen and nitrogen decrease and tend to vanish for the long durations of treatment. The comparison of the areas obtained for the 1 s peaks give us a relative elementary composition of the surface. These compositions are gathered in Table 3. For the untreated sample, the elements only present, except carbon, are oxygen and nitrogen but in small quantities. They are probably due to the nature of the polymer itself but can also come from adsorption of these gases (N2, O2 in the atmosphere) and particularly from dioxygen (the percentage of oxygen seems to be quite high). It is wellknown that oxygen is easily adsorbed on polymeric surfaces compared to nitrogen. After CF4 plasma treatment, a large quantity of fluorine is introduced, and this element represents the higher proportion on the surface.


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Figure 1. Atomic force microscopy images (50 × 50 µm) for (a) non treated sample, (b) CF4-treated sample, and (c) CF4+H2-treated sample. (Plasma conditions: P ) 50 W and f ) 10 sccm.)

The deconvolution spectrum of the C1s peaks allows the environment of the C atoms to be determined. Figure 4 shows this deconvolution for untreated (Figure 4a) and

treated PA12 surfaces (for 30 min) (Figure 4b). For the untreated sample, only C-H, C-O, and C-N contributions are observed. For the CF4 plasma-treated sample,

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Figure 2. Atomic force microscopy images (5 × 5 µm) for (a) CF4-treated sample, and (b) CF4+H2-treated sample. (Plasma conditions: P ) 50 W and f ) 10 sccm.)

the environment of C is essentially CF2, knowing that other fluorinated groups exist such as CF3, CF, etc. (see Table 4). Contact Angles. For both plasma treatments, the values of water contact angles, θw, determined for different times of exposure, are reported in Table 1. θw1 and θw2 are water contact angles measured a few seconds after the treatment and 3 days later, respectively. In a general point of view, from these results, and as shown in Figure 5, the same behavior can be observed for CF4 and CF4+H2 plasma treatments, i.e., a rapid increase of θw up to about 5 min before reaching a plateau. However, after 10 min of plasma treatment, the maximum value of θw seems to be obtained with CF4 (θw ≈ 120°) compared to the value obtained for CF4+H2 (110°). Surface energy follows the same pattern, decreasing rapidly up to about 5 min, then reaching a plateau (cf. Table 1 and Figure 6). From experimental results of contact angles (cf. Table 1 and Figure 5), it can also be observed that the decrease of the surface free energy γ with treatment time is mainly due to the decrease of γp (polar component) for CF4+H2, whereas in the case of the CF4 plasma treatment, the variation is more significant with γd (dispersive component). The decrease of the surface

Figure 3. Large spectra obtained by XPS for the untreated and CF4 plasma-treated PA12 surfaces (50 W, 10 sccm, 0, 0.5, 3, 5, 10, 15, 30 min).


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Table 2. Experimental Results of a PA12 Sample (L ) 100 µM) Tested in Pervaporation, before and after the Plasma Treatment (CF4 or CF4 + H2)a treatment duration (min)

P (Barrer)b

DI × 109 (cm2 s-1)

DL × 10 (cm2 s-1)

tL (s)

D h × 109 (cm2 s-1)

Ceq (mmol × cm-3)

β (cm3 mmol-1)

CF4

0 1 3 5 8 10 12 15

318 350 287 279 265 246 233 220

1.63 1.83 1.77 1.61 1.50 2.00 1.74 1.63

1.91 2.18 2.48 1.83 2.45 2.58 2.26 2.21

8710 7647 6714 9100 6787 6449 7370 7541

3.46 3.71 3.75 3.05 2.77 4.23 3.18 3.34

0.98 1.00 0.81 0.97 1.01 0.78 0.87 0.91

2.33 2.09 2.77 1.91 1.75 2.89 2.56 2.36

CF4+H2

0 0.5 1 3 6 8 10 12 15

361.2 364.1 356.5 404.6 379.1 370.7 357.4 330.1 350.8

1.99 1.79 1.97 2.07 1.88 1.99 1.92 1.75 1.74

2.53 2.36 2.56 2.65 2.44 2.58 2.45 2.32 2.24

6586 7058 6518 6285 6835 6450 6794 7197 7428

4.03 3.85 4.15 4.11 3.84 4.09 3.80 3.67 3.51

0.95 1.00 0.91 1.04 1.05 0.96 1.00 0.95 1.06

2.19 2.30 2.44 1.94 2.02 2.22 2.03 2.31 1.97

aP

) 50 W and f ) 10 sccm. b 1 Barrer ) 10-10 cm3STP.cm/cmHg.cm2.s.

Table 3. Evolution of Surface Relative Composition Determined by XPS Measurement for the Untreated and CF4 Plasma-Treated PA12 Surfaces (50 W, 10 sccm) duration time of treatment (min)

%C

%O

%N

%F

0 0.5 3 5 10 15 30

84 38 38 37 39 36 35

12 4 4 2 3 2 2

4 2 2 2 2 2 2

0 56 56 59 55 60 61

energy highlights the fluorination of the surface and its growth with the duration of treatment. Overall, the hydrophobic character of the polymer surfaces is clearly enhanced, since the initial value for the PA 12 is close to 60° and the final value above 100° for a contact angle measured with pure water. Permeation. From an experimental point of view, it was first necessary to test the reproducibility of our pervaporation measurements. Preliminary tests were performed on untreated PA12 films and the results have shown a very good reproducibility of water flux and have allowed the efficiency of the plasma treatment to be tested.33 From the six tested films, the mean value of the permeability coefficient P was (3.2 ( 0.1) Barrer. Pervaporation measurements were then carried out with PA12 films before and after CF4 and CF4+H2 (50/50 f/f) plasma treatments with various treatment durations. These measurements allow, in part, the characterization of the role played by the surface treatments with the effect of durations on the permeametric parameters such as the permeability coefficient P (eq 7), and the diffusion coefficient D (eqs 8, 9, 11). In determining these parameters, it will be assumed that all samples exhibit, before and after plasma treatments, the same surface exposed to water. In this case, the apparent values of P, tL, DI, and DL can be calculated under the same conditions, as reported in Table 2. In all cases, the values of DI and DL (eqs 8 and 9) corresponding to the transient permeation extents of 0.24 and 0.62, respectively, are different. Table 1 shows that, (33) Dreux, F. Modification des proprie´ te´ s barrie` re a` l’eau et au tolue` ne d’un polyamide 12 par traitement plasma froid. Thesis, University of Rouen, 2001.

Figure 4. Deconvolution spectra for the untreated and CF4 plasma-treated PA12 surfaces (50 W, 10 sccm, 10 min). (a) Untreated PA12, (b) CF4 plasma-treated PA12.

before and after both plasma treatments and whatever the time of treatment duration used, the DI value is smaller than the DL value. As the first value obtained corresponds to an earlier period of the transient regime compared with the second one obtained, the smaller DI means that the

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Figure 5. Contact angle variations versus time of treatment. (Plasma conditions: P ) 50 W and f ) 10 sccm.) Figure 7. Experimental water flux curves (corresponding to the best reduction of permeability in each case) obtained with the nontreated PA12 film and with the PA 12 films treated by CF4 and CF4+H2 plasma. (Plasma conditions: P ) 50 W and f ) 10 sccm.)

Figure 6. Surface energy versus treatment duration for CF4 and CF4+H2 plasma. (Plasma conditions: P ) 50 W and f ) 10 sccm.) Table 4. XPS Measurement for the Untreated and CF4 Plasma-Treated PA12 Surfaces (50 W, 10 sccm). Evolution of Contribution of Each Component for Peak C1s contribution of component for peak C1s (in %) type of bonds 0 min 0.5 min 3 min 5 min 10 min 15 min 30 min C-H 83 C-N 10 CH-CF CdO CF-CFx CF CF2 CF3

5

12

15

12

7

2

35

30

20

36

22

27

9 18 28 5

9 16 27 6

5 19 35 6

9 16 22 5

9 21 33 8

9 21 33 8

water diffusivity increases as the permeation proceeds from the starting point, where the film is dry. Figure 7 shows the typical flux curves J ) f(t) obtained with a PA12 film before and after CF4 and CF4+H2 plasma treatments with a treatment duration of 15 and 12 min, respectively. These treatment durations correspond to the best reduction of water permeability (lowest value of the water flux Jst calculated at the steady state) obtained for each plasma treatment (Table 2). In terms of permeability, to see the efficiency of the plasma treatments on PA12 films with exposure treatment durations, the relative variation of the permeability coefficient P′ ) (P - Pt)0)/Pt)0 versus time of treatment is plotted in Figure 8. For short times of treatment duration, about 1 min for CF4 and 3min for CF4+H2, the water permeability increases up to 10 and 12%, respectively. Then, for longer times, water permeability decreases, but at 5 min of treatment duration for CF4+H2, water permeability stops decreasing and tends to increase

Figure 8. Variation of the relative permeability coefficient versus treatment duration for CF4 and CF4+H2 plasma treatments. (Plasma conditions: P ) 50 W and f ) 10 sccm.)

again, while for CF4 water permeability continues to decrease. 5. Discussion First of all, concerning the water contact angles, a slight difference between θw1 and θw2 appears. This difference can be explained, in part, by the experimental conditions knowing that the measurements for θw1 and θw2 are obtained by using two differents apparatus. Nevertheless, the tendency in the decrease of θw with time, θw1 > θw2 (see Table 1), seems to be the result of an aging of the treated surfaces from a post-oxidation due to the presence of free radicals created during the plasma treatement. However, the surfaces treated by fluorinated plasma are relatively stable under ambiant atmosphere because of their low energy compared to oxidizing treatements. The contact angle results clearly show that CF4 plasma treatments lead to an increase in the hydrophobic character of the treated PA12 surfaces, the water contact angle varying from 64° for the untreated sample to at least 120° for samples treated during 15 min. For both plasma treatments, the evolution of θw versus time of treatment occurs in two steps (Figure 5). The first part of the curves presents a fast increase of θw before


reaching a plateau. Some authors attribute this behavior to the increase in the amount of fluorine on the surfaces34 and assume that during this period, the increase of θw is nearly proportional to the quantity of fluorine introduced. Moreover, it can be mentioned that the rate of this increase depends on the type of plasma treatment used. The introduction of di-hydrogen in CF4 plasma seems to be less efficient, the increase of θw being slower and the maximum value lower. This is confirmed by the surface energy data and which variations are wholly reversed to those of contact angles (Figure 6). The polar contribution to the total surface energy decreases with the plasma treatment time. However, the polar component of the surface energy of the CF4-treated sample is less decreased than for CF4+H2 and that can be explained by a more degradative effect since the fluorine atom, the degrading agent, is quenched when hydrogen is added to the plasma phase. In a general point of view, these water contact angle results are consistent with XPS measurements which highlight a rapid increase of the quantity of fluorine on the CF4-treated PA12 surfaces for the first seconds of treatment durations. A slight fluctuation of this amount can be noted but probably due to ablation phenomena as already observed in fluoration mechanisms. The second part of the curves exhibits a plateau rather than a variation. This can be explained by the establishment of an equilibrium between the different effects produced by the plasma treatment and, in particular, the ablation and the grafting reactions which are usually in competition.17 As given in Table 1, the maximum value obtained for θw is lower for CF4+H2 than for CF4 plasma treatment. Furthermore, for long treatment times (when the plateau of the curve is reached, t > 10 min), it should be noticed that the surface energy of these fluorinated surfaces (in particular for CF4) is much lower than that of commercial polymers comprised between 15 and 20 mJ m-2. It must be noted that for the first seconds of CF4+H2 plasma treatment, a slight decrease of the water contact angle exists. This result can be interpreted by the degree of surface fluorination. The θw values are measured just a few minutes after the plasma treatment. At the polymer surface, some sites are activated by the plasma and then can react with fluorine. But for very short treatment durations, these activated sites have not enough time to react with the plasma and react with di-oxygen during their return in the ambient atmosphere rather than with fluorine, and so lead to a more hydrophilic surface compared to the initial state.35,36 For both plasma treatments, it is now interesting to compare the variations of θw and P′ with treatment durations. In the same way as for contact angles, the evolution of P′ with the time of treatment can be characterized in two parts. For short times of treatment, the low increase of the permeation parameter can be explained by the same phenomena observed for the decrease of the water contact angle: some activated sites may not yet have reacted with the fluorine in the reactor, but rather with the di-oxygen present in the ambient air. Moreover, in the first seconds of the treatment, the cleaning step, which may favor the water sorption into the polymer, must also be considered. The treatment time (34) Bretagne, J.; Epaillard, F.; Ricard, A. J. Polym. Sci. 1992, 30, 323-328. (35) Anand, M.; Cohen, R. E.; Baddour, R. F. Polymer 1980, 21, 139140. (36) Mournet, S.; Arefi, F.; Montazer-Rahmati, P.; Goldman, M.; Amouroux, J. Rev. Int. Hautes Temp. Re´ fract. 1989, 25, 219-225.

Langmuir, Vol. 18, No. 26, 2002 10419

Figure 9. Comparison between experimental and calculated fluxes on a reduced scale for a constant diffusion coefficient and for a concentration-dependent diffusion coefficient following D ) D0eγC. (Plasma conditions: P ) 50 W and f ) 10 sccm.)

for which the water permeability begins to decrease (after increasing) corresponds approximately to the slowing down of the contact angle increase (t > 1 min for CF4, and t > 3min for CF4+H2). It appears that, for this kind of plasma treatment, a minimum exposure time is necessary to obtain a sufficient effect leading to barrier property enhancement. As the water permeability decreases with the duration of treatment, the water contact angle remains practically constant. This period is mainly due to the fluorination followed usually by the establishment of a fluorination-degradation equilibrium. However, it can be noticed that after 12 min of treatment, the water permeability stops decreasing for CF4+H2 plasma treatment, but not for CF4. It is interesting to see that at 15 min of treatment, θw seems to decrease slightly for CF4+H2 and the opposite occurs for CF4 plasma treatment. These observations tend to show a good correlation between the water contact angle and the water permeability of treated PA12 films. If we consider the permeation and the contact angles, CF4+H2 treatment appears once again, less effective than the CF4 plasma treatment. The more hydrophobic the polymeric surface, the greater the reduction in water permeation. The best barrier effect (reduced rate of 35%) is obtained by using the CF4 plasma treatment rather than the CF4+H2 plasma treatment and for some durations (15 min). The permeation factor increases for short durations of treatment and then decreases to reach a minimum value (Figure 8). This value is lower for CF4 treatment than for CF4+H2 treatment which is compatible with a better increase of the hydrophobic character of the surface for a CF4 plasma treatment. A simple explanation for the better reactivity of the CF4 plasma treatment can be given by the topography of the treated surfaces and particularly the nature of the structures created at the polymer surface which modify the access to water molecules. According to AFM images, the CF4+H2 plasma treatment leads to small stick-shaped structures which should be less efficient in terms of barrier layers than the soft structure obtained by CF4 plasma treatment. The water molecules can thus pass through paths between the sticks, allowing better mobility of small molecules compared to the soft layer. The comparison of the DI and DL values has highlighted the concentration-dependent diffusion coefficient. As shown in Figure 9, using dimensionless scales J/Jst and

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Figure 10. Variation of the relative time-lag versus treatment duration for CF4 and CF4+H2 plasma. (Plasma conditions: P ) 50 W and f ) 10 sccm.)

τ ) DIt/L2, the experimental transient water fluxes are lower than the calculated flux, assuming D to be constant (eq 10) in the early part of the permeation, for times less than tI, when the PA12 films are mostly dry. When the transient fluxes are computed according to eq 11, an excellent agreement between the calculated and the experimental fluxes is obtained with a positive plasticization factor (Figure 9). Such an increase in the diffusivity is generally attributed to a plasticization of the materials by the permeant (water), though this phenomenon cannot be determined by the transient permeation experiments. Table 1 gives the values of the diffusion law parameters obtained with PA12 films, before and after plasma treatments. The value of the integral mean diffusion coefficient D h represents the overall diffusivity of water in the polymer considered; it takes into account the plasticization effect, i.e., the enhancement of water mobility in the polymer due to the presence of sorbed water under given experimental conditions. This means that there is an increase in the free volume, thus an increase in the diffusivity, due to the added free volume of water. In Table 2, it can be noted that the water plasticization factor, β, and the equilibrium water concentration in the materials, Ceq, are not really affected by either plasma treatment. In other words, at this scale of characterization, the plasma treatments do not seem to modify the plasticization phenomena. From that, the bulk properties of the material do not seem to be modified. Finally, one way to better correlate the effect of the duration of plasma treatment on water permeability, could be to study the relative variations of the time-lag results (Figure 10). In this case, the comparison of the relative time-lag values between both plasma treatments with treatment duration shows pratcically the same behavior: a decrease until 3 min, a peak at 5-6 min, then a slight increase before reaching a plateau. These similar variations are stronger for CF4 plasma treatment compared to CF4+H2, which is not surprising seeing that CF4 plasma treatment is more efficient and thus lead to more important changes. However, a significant difference appears in the first period of plasma treatments, in particular, for the first seconds of treatment, the presence of a slight increase of the relative time-lag values can be observed for CF4+H2. Nevertheless, it is interesting to

Dreux et al.

note that during the first period of plasma treatments (CF4 and CF4+H2), the variations of the relative time-lag data are apparently in good agreement with those of the relative permeability (the permeability being proportional to the diffusivity and consequently inversely to the timelag). At this stage of discussion, the interpretation of the whole variations is more difficult knowing that after few minutes the ablation and the grafting reactions are usually in competition, changing the affinity and the topography of the treated surface. In this way, by thinking that surface modifications due to plasma treatment can alternate at the same time the topography (roughness, bumps, pinholes, etc.) and the affinity (polarity...) of the material surfaces and thus by integration of fluorine groups, giving a more difficult access to water molecules, it might have been interesting to compare these relatives variations of time-lag with the variations of the water contact angles. However the analysis of these complex variations shows behaviors that are difficult to interpret. Anyway, keeping in mind that time-lag values correspond to the first period after molecules have passed through the polymer film, it would be rather judicious to compare these surface results (contact angle, surface free energy) to the first kinetics of water sorption. Taking into account the fact that both plasma treatments do not give the same relative variations of time-lag and permeability and which are not constant for longer time treatment durations, it would not be surprising that plasma treatment not only modifies the surface but also can change the bulk properties. 6. Conclusion Fluorination, obtained by CF4 or CF4+H2 plasma treatment, allows reduction of water permeability of Polyamide 12 films. However, the surface modifications obtained by these surface plasma treatments are different. The implantation of fluorinated groups seems to be less efficient in the case of CF4+H2 plasma treatment. The effect of surface modification on water permeability depends not only on the affinity of the treated surface with the permeant molecules, but also on the form that these modifications take. These first experimental data show that in order to obtain a better water permeability reduction, it would be necessary to delay the establishment of the equilibrium between the different effects of the plasma, i.e., the step where θw remains constant. With our experimental results, it is rather difficult to discuss the variation of the water permeability with duration of treatment because the diffusion coefficients are calculated after a latent period (time needed for molecules to pass through the polymer film) and the solubility is not determined directly. For this reason and to observe the possible changes in the bulk properties of PA12 due to the plasma treatments (able to modify cristallinity and density), it will be necessary to measure furthermore the water sorption kinetics and analyze the general behavior from sorption isotherms. Acknowledgment. Acknowledgements to RMPP, “Re´seaux Normand Mate´riaux Polyme`res Plasturgie”. LA020584D

Surface Modification by Low-Pressure Plasma of Polyamide 12 (PA12

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