Effect of Highly Exfoliated and Oriented Organoclays on the Barrier

Article pubs.acs.org/JPCC

Effect of Highly Exfoliated and Oriented Organoclays on the Barrier Properties of Polyamide 6 Based Nanocomposites S. Alix,†,‡ N. Follain,§ N. Tenn,§ B. Alexandre,§ S. Bourbigot,†,#,⊥,∥ J. Soulestin,†,‡ and S. Marais*,§ †

Université Lille Nord de France, F-59000 Lille, France Ecole des Mines de Douai, Department of Polymers and Composites Technology & Mechanical Engineering, 941 Rue Charles Bourseul, BP 10838, F-59508, Douai, France § Université de Rouen, Laboratoire, Polymères, Biopolymères, Surfaces, UMR 6270 CNRS & FR 3038 F-76821 Mont-Saint-Aignan Cedex, France # ENSCL, ISP-UMET, F-59652 Villeneuve d’Ascq, France ⊥ USTL, ISP-UMET, F-59655 Villeneuve d’Ascq, France ∥ CNRS, UMR 8207, F-59652 Villeneuve d’Ascq, France ‡

ABSTRACT: The influence of the incorporation of nanoparticles (organo-modified montmorillonite Cloisite 30B) in polyamide 6 (PA6) on the transport of small molecules was investigated. Nanocomposites were prepared by melt blending followed by cast extrusion for obtaining film-forming materials. The nanoclay content of the materials was varied from 0 to 5 vol%. Differential scanning calorimetry measurements, electronic microscopy, X-ray diffraction, and rheology were used for characterizing the nanocomposite structure. Because of the presence of nanofillers, a high barrier effect to nitrogen and water was clearly evidenced and mainly attributed to the increase of tortuosity because of the increase of the diffusion pathway generated by impermeable nanofillers. In this study, the high barrier effect was attributed to the very good dispersion of montmorillonite platelets in the PA6 matrix resulting from a twostep melt processing. This peculiar processing has induced a good exfoliation and an orientation parallel to the film surface of lamellar montmorillonite platelets which is clearly demonstrated from transmission electron microscopy analysis. The permeation results were evaluated on the basis of geometrical models proposed by Nielsen and Bharadwaj, and by analyzing the plasticization phenomenon in the case of water permeation involving a concentration-dependence diffusivity correctly approached by an exponential law.

1. INTRODUCTION The transport properties of polymer nanocomposites have been attracting much attention in recent years1−7 resulting from the promising results concerning the high reinforcement of polymer composites even at very small filler content of layered silicates. In fact, in most studies the nanoparticles were estimated to be exfoliated to individual platelets within the polymer matrix inducing extremely large surfaces and interfaces8,9 which generated the considerable reinforcement. Their addition in polymer matrix improves the barrier effect when the dispersion of nanofillers leads to increase the tortuosity effect in the matrix.10,11 This effect is much more important with far less nanofiller content compared to conventional composite films. The quality of nanoparticle dispersion is a function of clay/polymer compatibility, clay amount and processing conditions. In that way, to enhance the barrier effect by improving the affinity between inorganic clays and the organic substrate, nanoparticles are often modified by incorporating an organic modified surfactant.12,13 Moreover, Fornes and Paul14 observed that the dispersion state decreases © 2012 American Chemical Society

when the organoclay content increases. It is well-known that the methods used to elaborate nanocomposites impact the degree of nanoparticle dispersion.15,16 Two main techniques are usually used to process nanocomposites and concern the intercalation of polymer from solution and the melt intercalation method.17 Intercalation of polymer from solution is obtained from a soluble polymer and swellable nanoclays, while the melt intercalation implies a blend of a polymer with nanoparticles under shear above the softening point of the polymer. This last method is convenient for current industrial process, such as injection molding and extrusion and is estimated to be more environmentally friendly than solution methods since organic solvents are not used. Independently of the above-mentioned methods, the in situ intercalative polymerization allows a good dispersion of clays in a polymer matrix.18,19 This technique, which involves the swelling of Received: June 3, 2011 Revised: January 23, 2012 Published: January 30, 2012 4937

dx.doi.org/10.1021/jp2052344 | J. Phys. Chem. C 2012, 116, 4937−4947

The Journal of Physical Chemistry C

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nanoparticle contents on transport properties of PA6/C30B nanocomposites associated with the evaluation of the exfoliation effect resulting from the two-step melt processing. To have a better insight into the quality of the nanoplatelet dispersion, X-ray diffraction measurements (XRD) and transmission electron microscopy (TEM) observations were performed on nanocomposites coupled with physical characterizations, such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic rheological measurements. The impact of the presence of nanofillers and their content on resulting matrix crystalline morphology was put forward. The resulting barrier properties were investigated by considering the interactions between the diffusing molecules differing in interaction capacity and the nanocomposite composition from a detailed evaluation of the permeation kinetics and thereafter correlated with the physical properties. N2 and H2O molecular probes were selected owing to very weak interactions and to polar-type interactions capacity with polymeric materials, respectively. To achieve this objective, the transport properties of these nanocomposites were simulated using a geometrical approach relative to the tortuosity concept and by taking into account interaction effects of diffusing molecules by polymer plasticization. The relative permeability deduced from the permeation flux analysis was discussed as a function of the nanoclay content, and of the selected diffusing molecules. The correlation between barrier properties of PA6/C30B nanocomposites and their physical properties will be discussed and especially on the basis of the nanocomposite structure characterized by a very good dispersion of clay layers with a high exfoliation and orientation.

nanoclays within the monomer solution followed by the polymer formation between the intercalated layers, is not so very adaptable to common industrial processes. The first commercial application of polyamide 6 (PA6)/ montmorillonite (MMT) nanocomposites was performed by Liu et al.20 in 1999 using the melt intercalation method with a twin screw extrusion process. A few years later, Fornes et al.21,22 reported the preparation of PA6/MMT using a twin screw extrusion too and studied the effect of molecular grade of PA6 and MMT structure on composite material structure, properties, and rheology. During the melt intercalation, the structural organization of PA6/nanoclay nanocomposites is also related to the elaboration protocol used. Preprocessing treatments for achieving the ideal dispersion state are proposed in the literature.23−26 Nevertheless, the formation of nanocomposite structure containing intercalated/exfoliated nanoparticles is claimed in most papers.27 If the extent of exfoliation is not considered sufficient in the literature, a higher exfoliation with an orientation parallel to the film surface of nanoparticles can be reached using a two-step melt processing.28,29 In our study, this two-step melt processing was applied on PA6 based nanocomposites in order to enhance the orientation and individualization of nanoparticles. It is also worth noting that nanocomposites based on semicrystalline matrix present a more complex barrier effect compared to those based on amorphous polymer. In the literature, PA6 was usually selected for elaborating polymer nanocomposites because of the interesting properties associated with the possibility to extend the exfoliation.9,30,31 Some references also mentioned that the crystalline part of PA6 matrix which contain two major phases, α-form phase and γform phase, can impact these resulting properties including barrier properties. These properties were influenced by the nanoparticle incorporation too and that can make changes in permeability of gas and water molecules at the same time. Some authors reported the modification of the crystalline part of the PA6 matrix by nanoparticles such as MMT. It is assumed that the γ-form phase grows with the introduction of layered silicates.32,33 Furthermore, barrier properties of polymer nanocomposites are mainly based on the tortuous pathway concept taking into account the impermeable character of nanofiller for small diffusing species.10 Generally, the models developed to describe the transport mechanisms in nanocomposite are purely geometrical models governed by tortuosity effects. However the limit of these models is in the fact that the interactions between diffusing molecules and polymer matrix as well as nanoclay/polymer interfaces are neglected. To our knowledge, in order to model the nanocomposite transport properties appropriately, a few studies showed a set of considerations: (1) the matrix crystalline phase variation, (2) the MMT structure polydispersity, (3) the surfactant presence at the clay surface, (4) the probable interactions with diffusing molecules, and (5) the clay/polymer compatibility.4,34,35 Alexandre et al.34 pointed out a concentration-dependent diffusion for PA12/MMT nanocomposites which should be included in the modeling for describing the transport of interacting diffusing molecules. They also suggested the presence of percolation paths of low diffusion resistance based on the increase in the sorbed water content for less exfoliated nanocomposites. Since the individualization of nanoclay platelets plays a major role in the transport properties of PA6 based nanocomposites, the aim of the present study was to analyze the influence of

2. MATERIALS AND METHODS 2.1. Materials. The polymer matrix used in this study was a commercial polyamide 6 (PA6) with melting temperature at 220 °C and with a molecular weight of 66 000 g/mol (determined by viscometric measurements), referenced as Akulon F132-E1 from DSM. The density of PA6 is 1.13 g/ cm3. The organo-modified montmorillonite Cloisite 30B (C30B), bearing a (tallow alkyl) methyl bis(2-hydroxyethyl) quaternary ammonium whose specific gravity is 1.98 g/cm3 was supplied by Southern Clay Products (Rockwood Additives, Ltd.). The density of the Cloisite Na+ (organic-free MMT) is 2.86 g/cm3. The composition of the tallow alkyl chains corresponds to ∼65% C18; ∼30% C16; ∼5% C14. The amount of ammonium cations of C30B is estimated to be ∼25 wt % from TGA analysis. 2.2. Preparation of Nanocomposite Films. The PA6/ C30B nanocomposites were obtained after melt blending using a two step extrusion masterbatch processing. The first step corresponded to the preparation of the masterbatch containing PA6 with 12 wt % of C30B nanoclay by melt compounding using a Clextral BC45 twin screw extruder at a rotation speed of 50 rpm and a flow rate of 9 kg/h. The barrel and die temperature settings ranged from 245° to 270 °C. The nanoclay content close to 12.2 wt % in the masterbatch was measured by TGA analysis. The second step corresponded to the dilution of the masterbatch with a single screw Haake Buchler Rheocord System 40 extruder for obtaining nanocomposites with different clay contents (3, 5, and 7 wt %). The extrusion temperature range was set up from barrel to die between 215 and 240 °C 4938



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The polymer crystalline structure and the crystallinity index of nanocomposites were studied by differential scanning calorimetry (DSC). DSC data were obtained by using the conventional differential scanning calorimeter Perkin-Elmer DSC-7. Before all DSC experiments, the baseline was calibrated using empty aluminum pans followed by the calibration using melting temperature and enthalpy of a high-purity indium standard (156.6 °C and 28.45 J/g). The given temperatures for the melting or the crystallization process were taken at the extremum of the peak. Samples with a mass of about 20 mg, in closed aluminum pans, were heated under nitrogen atmosphere (20 mL/min) to minimize the oxidative degradation. A first heating step was carried out from 25 to 250 °C, then followed by a cooling step from 250 to 25 °C, and a second heating step from 25 to 250 °C at heating and cooling rates of 10 °C/min. The crystallinity index (Xc) was determined by the following relation:

with a screw rotation speed at 100 rpm to minimize the feeding time. Before each processing step, the materials were dried in a vacuum oven at 80 °C overnight. Prior to processing into films by melt molding in order to get 200−250 μm thick plates, the nanocomposites pellets obtained after both processing steps were dried under a vacuum for 24 h at 80 °C to remove residual water moisture. A cast extrusion in a single screw Haake Buchler Rheocord System 40 extruder equipped with a calander was performed. The temperature settings from barrel to die were 215/220/230/225 °C with a screw rotation speed at 50 rpm. Finally nanocomposites films containing 3, 5, 7, and 12.2 wt % C30B in the PA6 matrix were obtained. The corresponding volume fractions of C30B and of impermeable fillers (organicfree MMT) are given in Table 1. Neat PA6 film was also Table 1. Comparison between Theoretical Nanoclay Mass Fraction and Nanoclay Mass Fraction Measured by Thermogravimetric Analysis of PA6/C30B Nanocomposites and Conversion into Volume Fraction of C30B (ϕ) (Density of C30B 1.98 g/cm3) and Volume Fraction of Impermeable Fillers (ϕi) (Density of Free Ammonium Cation Modifier Cloisite 2.86 g/cm3)

PA6/ C30B

theoretical clay mass fraction (%)

measured clay mass fraction (%)

ϕ volume fraction (%)

ϕ i volume fraction (%)

3

2.23

1.28

0.89

5 7 12.2

3.63 5.45 9.27

2.10 3.18 5.51

1.47 2.23 3.88

Xc =

(ΔH m − ΔHc) (1 − ϕ)ΔH0m

× 100 (1)

where ΔHm is the apparent melting enthalpy, ΔHc is the apparent crystallization enthalpy, ΔHm0 is the theoretical value of the enthalpy corresponding to the melting of 100% crystalline PA6 (ΔHm0 = 190 J/g36) and ϕ is the C30B mass fraction determined by TGA analysis. The dynamic rheological tests were carried out by using a rotational rheometer ARES (Rheometric Scientific) in dynamic frequency sweep mode starting from 0.1 to 100 rad/s at 230 °C under air atmosphere. The plate and plate configuration was used in this study. All the tests were performed at 1% fixed strain rate in the linear viscoelastic domain. 2.4. Permeation Experiments. Methods appropriate to the nature of the diffusing molecules were used for measuring the permeation properties of PA6/C30B nanocomposites at 25 °C. It is worth noting that the film-forming samples were dried in desiccators under pressure with P2O5 before measurements. Nitrogen permeation measurements were performed by using a barometric technique named the “time-lag” method.37 This variable pressure method was carried out as previously published with minor modifications. The method is based on the increase of pressure p of a gas through the polymer films as a function of time t. The difference in pressure between upstream compartment and downstream compartment surrounding the polymer sample was recorded with a datametric pressure sensor (Pfeiffer Vacuum 0−10 mbar). A preliminary high vacuum desorption was realized on both sides of the film lasting at least 72 h. The coefficient of permeability, P expressed in barrer units (1 barrer = 10−10 cm3(STP)·cm·cm−2·s‑1·cmHg), was calculated from the slope of the steady state line (dp/dt) resulting from the measurement by using

similarly prepared using the two step masterbatch process to overcome the effect of melt blending conditions. 2.3. Film Structure Characterization. The mass fraction of C30B particles included in nanocomposite films was determined by TGA. The experiments were performed on film-forming samples (approximately 20 mg) placed in open alumina pans from 30 to 600 °C at 10 °C/min under nitrogen atmosphere (20 mL/min) with a TGA device (TGA-7 PerkinElmer). The morphology and the structure of the nanocomposite films was observed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Concerning the TEM observations, all samples were ultra microtomed with a diamond knife on an ultracut microtome (Leica UCT, Switzerland), at cryo temperature (−120 °C) for obtaining sections with a nominal thickness of 70 nm. Sections were transferred to Cu grids of 400 mesh. Bright-field TEM images of nanocomposites were obtained at 300 kV under low dose conditions with an electronic microscope (Philips CM30), using a Gatan CCD camera (Gatan, USA). For XRD, the measurements in reflection mode were performed on a X-ray diffraction D8 Advance diffractometer from Bruker AXS equipped with a Cobalt source (λ = 1.79 Å corresponding to the α line of Co). Using a Bragg−Brentano configuration, the 2θ diffraction traces of samples were recorded at room temperature under similar experimental conditions in the range between 2° and 10° since the diffraction patterns of montmorillonite is located in this diffraction angle range. The diffraction peaks were thereafter fitted by using Lorentzian equations.

P=

dp L·V dt A ·R ·T ·Δp

(2)

where dp/dt is the slope of the steady state line, L is the thickness of the film, V is the downstream volume (98.13 cm3), A is the exposed area (11.341 cm2), R is the ideal gas constant, T is the experiment temperature (298 K), and Δp is the pressure difference measured between the two faces of the film. Water vapor permeation measurements were carried out from the pervaporation method previously described by Métayer et al.38,39 and regularly updated. The water flux passing through a polymer film was measured as a function of 4939



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local permeant concentration. From the simulated curves obtained from experimental flux data, Marais et al.45 have already described in more detail the method used to determine the different parameters of this dependence law: DM (the maximum value of D when C = Ceq), D0, (the mean integral diffusion coefficient), γ and Ceq (the permeant concentration in the polymer at the stationary state).

time from the dew point temperature of a sweeping gas. The water concentration measurement was monitored by using a hygrometric sensor (a chilled mirror hygrometer from General Eastern Instruments) and a data acquisition system. The flux J(L,t) is given by the following relation: J (L , t ) =

f (x out − x in) pt A RT

(3)

4. RESULTS AND DISCUSSION 4.1. Dispersion and Structure of PA6/Montmorillonite Systems. Figure 1 shows the thermogravimetric curves

where f is the flow rate of the carrier gas sweeping the downstream face (9.33 cm3/s), (xout − xin) the water concentration expressed in ppmV and determined at outlet and inlet of the permeation cell from the dew point temperature, A is the film surface area (6.86 cm2), R is the ideal gas constant, T is the experiment temperature (298 K), and pt is the total pressure (1 atm).

3. MODELING OF TRANSPORT MECHANISMS 3.1. Relative Permeability for Gas Permeant (Geometrical Approach). The relative permeability, determined from the ratio between Pn and Pm, the permeability of nanocomposite and the PA6 matrix, respectively, highlighted the effect of the presence of nanoclays on the barrier properties of nanocomposites. On the basis of the tortuosity concept, Nielsen and Bharadwaj10,11 have proposed the modeling of this relative value based on the following expression: 1 − ϕi Pn = 2 1 α Pm 1 + ϕi O+ 2

( 3 )(

2

)

Figure 1. Thermogravimetric curves of the PA6 matrix and PA6/C30B nanocomposites. (4)

obtained for PA6/C30B nanocomposites from which the mass fraction of montmorillonite in the nanocomposite was determined. The values were reported in Table 1. The measured values are relatively close to the theoretical ones; values correspond to the compositions defined during the preparation of the PA6/C30B nanocomposites. TEM analysis offers some support for evaluating the nanodispersion of C30B because of the nanometric size of the nanoclay layers. For evaluating the quality of the clay dispersion, the TEM micrographs taken from the PA6/C30B nanocomposites, at two magnifications (100 n.m. and 200 n.m.), is presented in Figure 2. TEM observation did not reveal the presence of aggregated structure inside the PA6 matrix. However, the presence of very small tactoids inside nanocomposites cannot be neglected because of the limited area scanned (even if representative images are shown taken from more than 50 different pictures). On the basis of the TEM analysis, one may conclude that the PA6 based nanocomposites contained highly exfoliated nanoclays, and this result is certainly related to the processing conditions. In addition to this good homogeneity in the nanoclay dispersion, a high degree of orientation can also be observed. An orientation of the clay nanoplatelets parallel to the surface of the film, in other words in the direction of extrusion, was obtained. This effect is directly linked to the thickness and the shearing forces induced during the cast extrusion which align the anisotropic individualized nanoplatelets in the flow direction.46 Concerning the extent of exfoliation, the PA6/C30B nanocomposites contained mainly individual platelets with more or less associated platelets. For the intermediate filler content of 5 wt % C30B, a mean aspect ratio close to 38 was calculated from a statistical analysis of the TEM micrographs (Table 2) obtained with the PA6/5 wt % C30B nanocomposite. This aspect ratio was determined by

where ϕi corresponds to the volume fraction of the impermeable fillers (volume fraction of montmorillonite without surfactant, Table 1), α is the aspect ratio of nanoclays and O is the orientation parameter of nanoclay platelets varying from 0 to 1. A random particle orientation is characterized by O = 0, while an orientation parallel to the film surface of lamellar nanoplatelets induces O = 1. These different parameters values are reported in Table 1. 3.2. Relative Permeability for Water Permeant. The relative permeability for gas permeant of nanocomposites can be considered to be well described by the Nielsen and Bharadwaj models, resulting from a geometrical approach, whereas these models became inadequate in the case of liquid or vapor permeants. Indeed, since the diffusing molecules can interact with the composite materials, the diffusion mechanisms became more complex in comparison with gas permeation where the diffusivity is assumed to be ordinary in the amorphous phase of the matrix (D is supposed constant in rubbery polymer according to Henry’s Law). In the case of polyamide aliphatic polymer, the water diffusivity increases strongly with the water concentration in the film sample according to exponential law during the permeation process.40,41 Usually, a plasticization phenomenon induced by organic and water vapors takes place in the polymer film leading to an increase of the free volume42 during the penetration of permeant vapor molecules. In that way and from the free volume theory, the concentration dependentdiffusion coefficient, relative to a Fickian type B mechanism, is expressed by the following equation:43,44 D = D0 exp(γC)

(5)

where D 0 represents the diffusion coefficient for nil concentration, γ is the plasticization coefficient, and C is the 4940



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Figure 3. XRD diffractograms of C30B nanoclays, PA6 matrix, and PA6/C30B nanocomposites.

d001 found to be 1.8 nm is deduced from the Bragg relation. Concerning the elaborated nanocomposites, it can be noted that no apparent characteristic diffraction peak attributed to C30B nanoclays are exhibited by PA6 based nanocomposites containing 3, 5, 7 wt % of nanoclays, whereas an inconspicuous shoulder centered on 2θ = 3.5° (corresponding to an interreticular distance of 2.9 nm) is observed for the nanocomposite containing the highest nanoclay content (Figure 3). Usually, in the literature, the authors claimed that the nanocomposites are characterized by an intercalated structure if the diffraction peak attributed to nanofillers is shifted to lower 2θ values, while its disappearance is interpreted as the evidence of an exfoliated structure.40,52 From these observations, the XRD results reveal that a fully exfoliation of nanoclays in PA6 matrix is reached for low and medium nanoclay contents. This is in agreement with the TEM micrographs displaying the individualization of nanoplatelets (Figure 2). Indeed, our results agreed well with several works focused on the elaboration of polyamide-silicate nanoclay nanocomposites as functions of nanoclay exfoliation and melt extrusion process conditions.14,40,54,59,61 The authors reported the exfoliation of inorganic nanofillers into PA matrix on the basis of the disappearance of diffraction peak attributed to nanofillers on XRD diffractograms. The presence of hydroxyl groups on the alkylammonium cation within the organomodified nanoclays galleries of C30B favored the PA6− nanoclays interactions which are necessary to achieve the exfoliation of the clays. The diffusion of PA6 polymer chains between the nanoplatelets is clearly promoted at low nanoclay contents. As expected, at high nanoclay content (12.2 wt % in this case), the shoulder centered on 2θ = 3.5° suggests that the dispersion is less accomplished and that an intercalated structure with few aggregations of nanoparticles can take place in the PA6 matrix. This is consistent with the TEM images showing some nanoplatelets more or less intercalated (Figure 2) and with the reported XRD results of polyamidesilicate nanoclay nanocomposites at high nanoclay content (>10 wt %) through the detection of resulting diffraction peak in XRD traces.40 Since the fully exfoliation of nanoclays is reached after melt blending using a two step extrusion masterbatch processing, one can estimate that the exfoliated structure containing some nanoclay aggregates for PA6 + 12.2 wt % C30B is mainly due to a lack of space in bulk PA6 volume for the nanoclay delamination at this high content (12.2 wt %). This result may not impact the resulting barrier properties because of some aggregates within nanocomposite structure still behave as barriers inducing additional tortuosity effect

Figure 2. TEM micrographs of PA6/C30B nanocomposites: (a, b) PA6 + 3 wt % C30B, (c, d) PA6 + 5 wt % C30B, (e, f) PA6 + 7 wt % C30B, (g, h) PA6 + 12.2 wt % C30B.

Table 2. Platelet Dimensions and the Aspect Ratio α for the PA6/5 wt % C30B Nanocomposite nanocomposites

platelet length (nm)

platelet thickness (nm)

α

PA6 + 5 wt % C30B

142 ± 58

4±2

38 ± 10

measuring the size of the clay nanoplatelets directly from TEM images. Nevertheless, it is worth noting that the determination was obtained from a few pictures. Consequently, XRD and rheological measurements were performed on nanocomposites for supporting the TEM observations in order to evaluate the degrees of exfoliation and of dispersion of C30B nanoparticles, respectively. XRD analysis is usually estimated as a standard method for characterizing polymer nanocomposite structure containing layered silicates from the presence, the shift, and/or the disappearance of the silicate diffraction peak in XRD diffractograms. For evaluating the extent of exfoliation in elaborated PA6/C30B nanocomposites, the XRD diffractograms are shown in Figure 3. For the sake of comparison, the XRD analysis of C30B powder is added. The diffraction peak located at 2θ = 5.7° is obviously observed and the silicate basal spacing 4941



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tion threshold (defined as the formation of long-range connectivity) is reached inducing a frequency-independent behavior at lower frequencies. The silicate network is formed under specific conditions, in other words, when sufficient exfoliation is achieved and/or when interconnections between nanoclay aggregates are established. However, the terminal behavior (liquid-like behavior) was still observed for the PA6 + 12.2 wt % C30B nanocomposite that showed the absence of a percolation threshold. In this case, one can assume that any interconnection between nanoclay aggregates took place even if some aggregates as well as the intercalation of nanofillers by polymer chains were put forward from TEM observations and XRD measurements at high nanoclay content. One of the most specific characterizations for proving the silicate network establishment is rheology, and on the basis of data, the Winter-Chambon criterion48,49 can be used for detecting the existence of the percolation threshold in polymer based nanocomposites. The presence of changes in frequency dependence of storage (G′) and loss (G″) moduli, complex viscosity (η*), and/or loss tangent tan δ indicates the formation of percolation of nanoparticles,. Typically, the dynamic moduli flatten at low frequencies reaching a plateau and the tan δ became frequency independent.50 Thus, the percolation threshold can be determined by representing tan δ versus nanoclay content at different frequencies. The percolation is defined as a function of the clay content when the tan δ value becomes constant at all frequencies. As reported in Figure 5 the

toward diffusing molecules. In fact, we can keep in mind that only the establishment of interconnections between aggregates would reduce the barrier properties of nanocomposites by creating preferential diffusion pathways. This point will be underlined through the rheological measurements. Besides, the solid-state NMR characterization of PA6/C30B nanocomposites discussed in a separate paper47 offers some support for evidencing the full exfoliation of nanoclays in PA6 matrix. The authors reported that the dispersion of the nanoplatelets was clearly achieved and that the PA6/C30B nanocomposites exhibited an exfoliated morphology, whatever the nanoclay content. They based their interpretation on the fitting of the experimental data obtained in solid-state NMR spectroscopy which gave information about the degree of nanoplatelets separation (f value) and the homogeneity of dispersion (ε value). For the tested nanocomposites,47 the homogeneity of nanoclay dispersion is found to be optimum as attested by the ε values (∼100%), while a clearly dispersed structure is evidenced by the f values obtained very close to 1. Furthermore, rheological measurements were carried out for all the elaborated nanocomposites for further confirming the observations concerning the degree of dispersion of PA6/C30B nanocomposites. Indeed, the rheological properties of the material at the molten state are sensitive to the structure, size, and shape of nanoparticles, and to the surface properties of the dispersed phase and by extension to the dispersion state. The rheological data provided an overview of the dispersion state of the clay nanoparticles at the macroscopic level that is complementary to TEM observations and XRD diffraction measurements. Figure 4 represents the frequency-dependence

Figure 5. Tan δ as a function of nanoclay content at different frequencies tested for PA6 matrix and PA6/C30B nanocomposites.

percolation is not observed as, even for a clay content of 12.2 wt %, tan δ remained frequency dependent. In spite of welldispersed nanoparticles, the absence of percolation even for high clay content may be attributed to the high orientation promoted by the two-step melt processing which is clearly highlighted by observing TEM micrographs. However, since the tan δ values tend to be very close to a constant value at all the frequencies, the percolation threshold is probably slightly higher than 12.2 wt % of nanoclay incorporated (Figure 5). Some references indicate that a percolation threshold for PA/ nanoclay nanocomposites can be defined at ∼2.5−3 vol % (∼6 wt %) of nanoclays.34 This is mainly governed by several factors, like the aspect ratio, polymer used, surfactant, or polymer/clay affinity, and significantly by the processing conditions and the extent of nanoclay exfoliation. This level is usually evidenced by enhanced macroscopic properties such as rheology, mechanical properties, barrier properties, for

Figure 4. Storage modulus/oscillatory frequency curves of PA6 matrix and PA6/C30B nanocomposites.

of the storage modulus (G′) of the PA6/C30B nanocomposites measured at 230 °C. It appears that G′ modulus of the nanocomposites increases with the increase of C30B content. In the case of the PA6 matrix, at low frequencies polymer chains are fully relaxed and exhibit typical terminal behavior (liquid-like behavior). In the case of PA6/C30B nanocomposites, the G′ value increases when increasing the nanoclay content which tends to become frequency-independent at low frequency (solid-like behavior). The existence of the solid-like rheological behavior for the polymer/nanoclay nanocomposites is attributed to the frictional interactions between the highly anisotropic silicate layers inducing a reduction in the mobility of PA6 polymer chains. These interactions become particularly significant when the percola4942



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instance. Compared to the literature, the percolation network for these studied PA6 based nanocomposites is obtained at a higher threshold. This significant difference can be related to the effects induced by the two-step melt processing and by the film casting that led to better dispersion and orientation, respectively. From a qualitative point of view, the results are in agreement with properties obtained for PA based nanocomposites and presented in the literature: a higher degree of exfoliation leads to improve properties and the performances are more marked with a more exfoliated structure. Finally, the rheological measurements and the TEM observations demonstrated a high degree of dispersion of nanoparticles in the PA6 based nanocomposites. Nevertheless, any percolation network is obtained because of the large dispersion and the high orientation of the C30B nanoparticles created during the film processing. 4.2. Crystalline Structure and Crystallinity of Polyamide 6 Matrix. Before discussing the water and gas permeation results, the thermal analysis was performed on nanocomposites for determining the change in crystallinity and ensuring that the processing conditions did not degrade the nanoclays during the preparation of cast films. The dynamic TGA measurements (in the temperature range of 30−600 at 10 °C/min) revealed that the nanostructure of nanocomposites was not affected by the processing conditions since the degradation temperature is measured after 270 °C (Figure 1). Some references presented similar results ensuring the integrity of C30B during the melt processing.51,52 Since the permeation properties could be affected by a modification in the crystalline state of the material due to the presence of nanoclays, it was necessary to determine their effect on the crystallinity index and the possible change in morphology. Indeed, the crystalline part of the nanocomposite material is usually considered as an impermeable zone to small molecules. It is recognized that the glass transition temperature (Tg) of PA6 polymer can vary from 30 to 60 °C.53,54 However, the determination of Tg value is difficult since its value depends on the plasticization rate due to the affinity of water for PA6. Figure 6 displays thermograms obtained from thermal analysis using differential scanning calorimetry (DSC). In the first heating step, all samples give an endothermic peak with a maximum value at ∼225 °C associated with the melting of the α-form crystal of the PA6 polymer. It can be also observed an endothermic shoulder located at 212 °C attributed to the γform crystal which appears with the presence of nanoclays in PA6 matrix. It is well-known that PA6 presents a polymorphic structure that exhibits two major crystal forms, namely, monoclinic α-form and pseudohexagonal γ-form depending on the thermal history, processing conditions, mechanical stress, and crystallization conditions.54−57 In the more thermodynamically stable α-form crystal, hydrogen bonds are formed between antiparallel polymer chains, while in the γform, hydrogen bonds are formed between parallel polymer chains. These results are consistent with the fact that the layered silicates can promote growth of the γ-form crystalline of PA6.54,58 According to Fornes et al.56 the formation of the γphase would be due to the reduction of the molecular mobility of macromolecular chains in the presence of clays. Moreover, the clay layers were shown to orient along the direction of shear, causing orientation of the polymer chains as well.59 From Figure 6, it can be observed an exothermic peak located at 195 °C, before the melting step. According to several

Figure 6. DSC curves of PA6 matrix and PA6/C30B nanocomposites.

authors,59−61 this exothermic phenomenon would be due to the crystallization of α-form crystal. DSC thermograms allow the crystallinity index (Xc) to be deduced from the melting and crystallization enthalpies. The Xc values are reported in Table 3. Table 3. Crystallinity Index of PA6 Matrix and PA6/C30B Nanocomposites Xc (%) PA6 PA6 PA6 PA6 PA6

+ + + +

3 wt % C30B 5 wt % C30B 7 wt % C30B 12.2 wt % C30B

33 28 30 33 30

By taking into account the measurement errors (due to the DSC and to the determination of enthalpies), the crystallinity index of PA6 seems to be quite constant and so would not be influenced by the nanoparticle content. To conclude, even if the morphology of PA6 is modified by the presence of nanoclays leading to the partial transformation of α-form in γ-form, the transport properties of PA6 based nanocomposites should not be strongly influenced by the crystallinity of the material since the crystallinity index is apparently not dependent on the nanoparticle content. However, it can be assumed that the possible orientation of crystals with the introduction of clays may have a slight impact on the diffusivity because of the change in tortuosity. At this stage of discussion, it is difficult to estimate this effect since no information is obtained on the orientation of the crystalline phases. 4.3.1. Transport Properties. Nitrogen Permeability. In a first step of analyzing the barrier properties of PA6/C30B nanocomposites, it was interesting to use inert gas molecules such as nitrogen that is recognized to not interact with the organic matrix. In that case, the nitrogen permeation measurements allow only the influence of the structure of the nanocomposite on the barrier properties to be shown. 4943



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This discrepancy between experimental and calculated aspect ratio values could be explained by the limitation of the geometrical model which neglects the possible physical modifications of the crystalline phase of the matrix and above all the role played by the clay/matrix interface on the mobility of the amorphous phase of the matrix near this interface. In our case, from DSC results, since the crystallinity index of PA6 is still the same in the presence of MMT, the only effect of the PA6 crystalline structure modified by the introduction of clay layers on permeation properties could be the change in orientation of the matrix crystals able to impact on the tortuosity. The change in morphology should not so much affect the permeation properties except if the density was really modified. But this is not the case when PA6 is used as a matrix (ρ = 1.23 g/cm3 for α-form and ρ = 1.19 for γ-form).56 Xu et al.63 have developed a chain-segment immobility factor and have shown that the chain confinement could enhance the barrier properties of nanocomposites. So as to explain our results and interpret the difference in experimental and calculated aspect ratios, the reduction of chain-segment mobility around the filler should be also taken into account in the geometrical model. The values for the diameter of montmorillonite C30B platelets reported by Southern Clay Products range between 70 and 100 nm. Considering the average value measured from TEM micrographs (α = 38 for 5 wt % of C30B) and the high orientation degree, it seems to be reasonable to consider that the value obtained from the model (α = 133 with the highest orientation) is too high. However, on one hand, the highest orientation is difficult to obtain experimentally for all nanoplatelets in the whole film. In our case, one can estimate that a good orientation can be observed but probably not the highest so that, on the basis of the model used, the α value should be found higher knowing that the value of the orientation parameter O is slightly lower than 1. On the other hand, as mentioned previously, the stiffness (that is, not taking into account in the geometrical model) is increased with the nanofiller content at the C30B/matrix interface, and the calculated value of α becomes overestimated. From these considerations with antagonist effects on the aspect ratio, the values of α deduced from the simple modeling should be relatively close to 100 which corresponds to the highest value of the aspect ratio given by Southern Clay Products. One can note that some works characterizing exfoliated or partially exfoliated systems showed an aspect ratio clearly below 100.40 4.3.2. Water Permeability. Contrary to nitrogen molecules, water molecules may interact with the PA6 matrix and with the clay/polymer interfaces. Like for nitrogen permeation, the water permeability systematically decreases when the C30B content increases (Table 4). This behavior was previously observed with the same type of nanocomposites, PA6/OMMT (organically modified montmorillonite), but a lower reduction of the relative water permeability was obtained.40,64 The decrease of water permeation is mostly due to the tortuosity effect which can be related to the presence of nanoclays inside the PA6 matrix (Figure 8). The plot J·L versus t/L2, shown in Figure 8, allows to compare all water permeation curves without a thickness effect. It can be seen that the reduction of the stationary flux observed with the presence of fillers comes systematically with an increase of the delay time of diffusivity. As shown in Figure 9, the decrease of the relative permeability is correctly predicted for the filled nanocomposites by using the geometrical Nielsen and Bharadwaj model (eq 4). In the case of

From that, the nitrogen molecule was used as a probe at a molecular scale to study the impact of the exfoliated structure of clay platelets on the transport properties of PA6/C30B films. Table 4 gives the coefficients of nitrogen permeability Table 4. Nitrogen and Water Permeabilities for PA6 and PA6/C30B Nanocomposites films PA6 PA6 PA6 PA6 PA6 a

+ + + +

3 wt % C30B 5 wt % C30B 7 wt % C30B 12.2 wt % C30B

P N2 (barrera) 1.32 0.75 0.62 0.56 0.43

× × × × ×

−2

10 10−2 10−2 10−2 10−2

P H2O (barrer) 5480 3695 3370 2510 1640

1 barrer = 10−10 cm3 (STP) cm cm−2 s−1 cmHg−1.

corresponding to the studied nanocomposites. It can be observed that when increasing the amount of C30B, the nitrogen permeability is decreased inducing an improvement of the barrier effect. This is in accordance with the tortuosity concept. The nitrogen relative permeability plotted versus C30B impermeable volume fraction ϕi (Figure 7) highlights the

Figure 7. Relative nitrogen permeability as a function of C30B volume fraction ϕi (impermeable volume fraction of fillers); experimental results and predictions according to the tortuous path model including the aspect ratio α and the orientation parameter of the clay platelets O.

high performance of these nanocomposites in terms of barrier properties. The results of nitrogen relative permeability are in the same scale of ones concerning nanocomposites based on isobutylene isopropene rubber (IIR) and MMT with complete dispersion of particles in the polymer matrix obtained by melt intercalation and press molding.62 As an example, in the case of the PA6/5 wt % C30B nanocomposite (intermediate filler amount in the range of 0− 12.2 wt %), the mean aspect ratio αmes is estimated to 38 (Table 2). By fitting the experimental data of the relative permeability according to eq 4, assuming oriented particles (O = 1) and on the basis of the root-sum-square (RSS) method, the calculated α value is estimated to be 133. If we consider the case of nonoriented particles, the α value will be found higher and so very different from the estimated one. According to the modeling approach, nanoclay platelets could be considered as high aspect-ratio particles. However, despite the difficulty of estimating an aspect ratio from TEM micrographs because of the orientation, the flexibility, the recovery, and the curvature of clay nanoplatelets, the aspect ratio value superior to 100 is certainly too high. 4944



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modeling the relative permeability against molecule vapors of nanocomposites. Indeed, in these models, the dependence of the diffusion coefficient with the water concentration is not taken into account, whereas the plasticization phenomenon by water often occurs in polymers and especially hydrophilic polymers. Generally, with increasing the sorbed water content, the free volume is increased during the permeation measurement which emphasizes the polymer chain mobility. In order to compare experimental and theoretical flux curves, the water permeation kinetics were analyzed by using the reduced time scale τ (= D·t/L2) and plotted in Figure 10. The

Figure 8. Experimental water flux curves obtained for PA6/C30B nanocomposites in the adimensioned scale J·L versus t/L2.

Figure 10. Normalized water transient fluxes through the PA6 + 5 wt % C30B nanocomposite.

differences observed in the permeation patterns represent the intrinsic behavior of the materials. Figure 10 shows clearly a deviation between the normalized water transient flux through PA6/5 wt % C30B nanocomposite and the theoretical curve calculated from Fick’s Law assuming D constant. From that and by considering a water concentration-dependent diffusion coefficient, an excellent agreement between the experimental and the new calculated fluxes (eq 5) was obtained for all the nanocomposites tested. An example of this good correlation is shown in Figure 10 with the case of PA6/5 wt % C30B nanocomposites. These results indicate that the content of water sorbed in the polymer phase is high enough to induce an increase in the polymer chain mobility leading to a plasticization phenomenon by water and hence an increase of the water diffusion rate during the water permeation process. The influence of the organically modified montmorillonite and the role played by the clay/polymer interface on the water plasticization effect of the PA6 matrix was then examined from the parameters of the concentration-dependence law relative to a Fickian type B mechanism by using eq 5.45,68 As far as nanocomposites are concerned, the decreases of the limit diffusion coefficient D0 and of the mean integral diffusion coefficient are in accordance with the tortuous path model (Table 5). Up to 5 wt% mass fraction of C30B, the plasticization factor γCeq increases slightly with the C30B amount and then decreases. The interpretation of this variation of γCeq can be done by analyzing separately the influence of the nanoclay mass fraction on the equilibrium water concentration Ceq and on the plasticization coefficient γ (Table 5). For PA6/C30B nanocomposites, γ value remains practically unchanged. The fact that γ values were found to be independent of the filler loading seems to indicate that the volume occupied by the water

Figure 9. Relative water vapor permeability as a function of C30B volume fraction ϕi (impermeable volume fraction of fillers); experimental results and predictions according to the tortuous path model including the aspect ratio α and the orientation parameter of the clay platelets O.

PA12 based nanocomposites with more than 4% volume fraction of C30B, Alexandre et al.52 have shown that the tortuosity effect could be counterbalanced by a percolation effect leading to a slight increase of water permeability. In this work, for the highest clay content (∼5 vol %), this percolation effect is not observed but on the contrary a high decrease of the relative permeability (Pn/Pm = 0.3) is obtained. This result validates the very good dispersion of exfoliated platelets in the PA6 matrix and as observed from TEM micrographs (Figure 2). On the basis of the assumption of oriented particles (O = 1), the fitting of experimental data from Nielsen’s formula leads to α = 100. With another model named the Cussler model,65 Messersmith & Giannelis66 obtained a particle aspect ratio of 70 for PCL/silicate nanocomposites elaborated by the solvent casting technique. These authors showed a reduction of water vapor transmission through the film due to an increase of the diffusion tortuous pathway induced by the plan orientation of silicate layers. In the case of poly(vinyl acetate)/montmorillonite nanocomposites, Chien & Lin67 have found an average aspect ratio of MMT particles calculated close to 327 with the generalized Nielsen’s permeability model, which effectively increased the tortuosity factor and decreased the permeability coefficient. Usually, the calculated α values from TEM analysis are found lower than those calculated from the geometrical models applied to permeation experiments. However, Nielsen and Bharadwaj models cannot be considered appropriate for 4945



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polymeric materials. The influence of the nanoclay/matrix interface on the transport properties of the nanocomposites was investigated by using water molecules interacting differently with the interfaces and the PA6 matrix. The high barrier effect (reduction in permeability ∼70%) was observed for low C30B contents (

Effect of Highly Exfoliated and Oriented Organoclays on the Barrier

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