Influence of the Compatibilizer Polarity and Molar Mass on the Morphology and the Gas Barrier Properties of Polyethylene/Clay Nanocomposites

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1 Influence of the Compatibilizer Polarity and Molar Mass on the Morphology and the Gas Barrier Properties of Polyethylene/Clay Nanocomposites E. PICARD, 1 A. VERMOGEN, 2 J-F. GÉRARD, 3 E. ESPUCHE 1 1 Laboratoire des Matériaux Polymères et des Biomatériaux, Ingénierie des Matériaux Polymères, IMP, UMR CNRS5223, Université de Lyon, Université Lyon 1, Bât. ISTIL, 15 Bd A. Latarjet, Villeurbanne Cedex, France 2 MATEIS, UMR CNRS 5510, INSA de Lyon, Bât Blaise Pascal, 20 Av. A. Einstein, Villeurbanne Cedex, France 3 Laboratoire des Matériaux Macromoléculaires, Ingénierie des Matériaux Polymères, IMP, UMR CNRS5223, INSA de Lyon, Bât Jules Verne, 20 Av. A. Einstein, Villeurbanne Cedex, France Received 22 May 2008; revised 11 September 2008; accepted 14 September 2008 DOI: /polb Published online in Wiley InterScience ( ABSTRACT: In this study, different modified polyethylenes with different molar masses and different modification rates were examined as compatibilizers to prepare high density polyethylene/organoclay nanocomposites. Nanocomposites having 5 wt % organo-modified clay and 20 wt % interfacial agent were prepared by melt blending. The effect of compatibilizer molar mass and polarity was investigated on the clay dispersion and on the gas barrier properties. It was observed that the amount of large and dense fillers aggregates was considerably reduced by introduction of an interfacial agent. The nanocomposite final morphology was governed by a diffusion/shear mechanism. A high degree of clay delamination was obtained with the high molar mass compatibilizers, whereas highly swollen clay aggregates resulted from the incorporation of the low molar mass interfacial agents. In the investigated nanocomposites series, the barrier properties could not be directly related to the clay dispersion state but resulted also from the matrix/clay interfacial interactions. A gas transport mechanism based on these both parameters was proposed to explain the barrier properties evolution in these low polar nanocomposites series. VC 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: , 2008 Keywords: barrier properties; compatibilizer; gas permeation; morphology; nanocomposites; polyethylene/clay nanocomposite INTRODUCTION Correspondence to: E. Espuche ( eliane.espuche@ univ-lyon1.fr), Vol. 46, (2008) VC 2008 Wiley Periodicals, Inc. Organic/inorganic nanocomposites have been the subject of increased interest during the past decades because of their potential to combine the features of organic materials with those of inorganic materials. In particular, nanocomposites based on polymer and lamellar silicate fillers such as clays have been particularly studied for barrier applications Indeed, because of their high aspect ratio and their impermeable character, the clay platelets are expected to induce significant tortuosity effects when they are dispersed within a polymer matrix. Different models have been proposed in the literature to express the tortuosity factor as 2593

2 2594 PICARD ET AL. a function of the shape, orientation, dispersion state, and volume fraction of the dispersed impermeable particles. All these geometrical laws demonstrate that the barrier properties must be highly improved when the silicate platelets are individualized, uniformly dispersed within the polymer, and are lying in the plane of the film. Hence, much attention has been paid to obtain such morphologies, characterized by a high degree of clay exfoliation. If exfoliated structures can be quite easily obtained with polar matrices such as polyamide, it is not so obvious with apolar matrices such as polyolefins. Indeed, because of the low compatibility between the apolar matrix and the hydrophilic filler, high degree of clay delamination is rarely obtained by melt blending a polypropylene or polyethylene with a natural clay. The strategy to reinforce interfacial interactions between the clay and the nonpolar matrix is to use a organo-modified clay and to introduce also in the nanocomposite formulation modified polyolefins bearing polar groups, namely compatibilizers or interfacial agents. Maleic anhydride grafted polyolefins have been widely used as interfacial agents 14 16,18 20,40 45 and more recently, the compatibilizer family suitable for polyolefins has been enlarged. In particular, for polyethylene that is the subject of interest of this work, acrylic acid grafted polyethylenes, 19 surlyn V R, 21 different copolymers (ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymer), 19,22 or oxidized polyolefins 14,17 have been used as compatibilizers instead of the classical maleic anhydride grafted polyethylenes. In most cases, an enhanced clay dispersion state is observed with addition of these compatibilizers in comparison with the reference PE/clay nanocomposite. Nevertheless, the improvements observed in barrier properties seem to be limited in these systems in comparison with nylon-based nanocomposites. 5,6,14 22 Furthermore, the permeability decreases remain very similar (about 30%) despite the different kinds of morphologies that have been reported and the different kinds of compatibilizers that have been used. It is then of great interest to improve the understanding of the role of the compatibilizers on these properties. In this study, effects of the compatibilizer structure and polarity on the clay dispersion and on the gas barrier properties of nanocomposites prepared for the same amount of clay were studied. For all nanocomposite formulations, the compatibilizer to clay ratio was fixed to 4. Indeed, this ratio led, according to numerous studies, 41,44,46 48 to the optimized composition for clay exfoliation. The interfacial agents series was chosen to investigate the specific role of the compatibilizer molar mass and of the compatibilizer polarity on the clay dispersion state and on the interfacial interactions between the clay and the matrix. The nanocomposites barrier properties were discussed as a function of these two parameters, and they showed the importance of the control of the interfacial interactions for optimized properties. EXPERIMENTAL Materials In this study, a high density polyethylene (PE) from Polimeri Europa: Eraclene ML74 with an average molecular weight, M w, equal to g mol 1 was used as the reference polymer. The organoclay was a commercial organo-modified montmorillonite, Nanofil 15 from Süd Chemie. It was organically modified with a dimethyl ditallow quaternary ammonium. A previous detailed analysis of this OMMT allowed the determination of its organic surfactant weight content (35.7%). 49 Five modified polyethylenes were considered as interfacial agents to improve the interactions between the clays and the polymer: three of them, coded MA1, MA9, and lwma6, respectively, were maleic anhydride grafted polyethylenes with different molar masses and maleic anhydride contents. An oxidized paraffin named lwox16 and an acrylic acid modified high molecular weight polyethylene denoted AA63 were also studied. The compatibilizers characteristics, in particular, their melting temperature, T m,theirmolarmass,m w, and their KOH index, I KOH,thatisrepresentative of the compatibilizer modification rate are presented in Table 1. The abbreviations used to name the compatibilizers allowed to distinguish these different characteristics in addition to the type of polar groups bore by the chains. Matrix and Nanocomposites Preparation PE-based nanocomposites were prepared by melt compounding the reference polyethylene PE, the compatibilizer, and the clay using a Clextral twin screw corotating extruder (L/D ¼ 36). Compounding was carried out at a barrel temperature of 200 C, a screw speed of 100 rpm, and a feed rate of 1.3 kg/h. The compatibilizer and the OMMT

3 POLYETHYLENE/CLAY NANOCOMPOSITES 2595 Table 1. Characteristics of the Compatibilizers Used to Prepare the PE-Based Nanocomposites Code Compatibilizer Commercial Name M w (g/mol) I KOH (mg KOH /g) d (g/cm 3 ) T m ( C) MA9 MA1 lwma6 lwox16 AA63 High molar mass maleic anhydride grafted polyethylene High molar mass maleic anhydride grafted polyethylene Low molar mass maleic anhydride grafted polyethylene Low molar mass oxydized polyethylene Acrylic acid modified high molar mass polyethylene Polybond 3009 (Uniroyal) Priex (Solvay) (Sigma Aldrich) Licowax PED (Clariant) Polybond 1009 (Uniroyal) M w, Mean molar mass; I KOH, KOH index; d, density; and T m, melting temperature. weight amounts in the nanocomposites were fixed at 20 and 5 wt %, respectively. A reference PE/ clay blend was also melt compounded. The respective matrix of each nanocomposite was at last prepared according to the same processing conditions. Matrix and nanocomposites films with a thickness of lm were obtained by blow molding of the blends with a Clextral E20T equipment. The extrusion temperature was set between 190 and 220 C, and the screw speed was fixed at 80 rpm. Nanocomposites Characterization Clay dispersion was analyzed at two scales (i) by transmission electron microscopy (TEM) to get information up to a few micron square area and (ii) by wide angle X-ray diffraction (XRD) to get d- spacing, d 001, between the platelets. For transmission electron microscopy observations, the samples were embedded in an epoxy (Epofix) resin. As the montmorillonite particles tend to lie parallel to the film surface, the samples were cut (with an ultramicrotome Reichert S) to observe the particles on their edges. To obtain sections thin enough and to avoid distortions such as thickness ripples, the surface of the pyramid was roughly set to mm 2,anda35 diamond knife was used. The temperature was set at 130 C, the knife speed to 1 mm/s. Sections with a thickness of 60-nm were obtained, and they were finally mounted on a 400 mesh copper grid. The TEM bright field observations were performed on a JEOL-200CX (LaB 6 filament) microscope operating at 200 kv. The diffraction patterns were obtained at room temperature in the range of 2y between 1 and 10 by step of 0.02 using a Cu tube and a Siemens D500 diffractometer where the Kb line was removed with a nickel filter. The films were deposited on neutral monosubstrates with a thin transfer adhesive with low scattering response. The matrix crystallinity, X c, was determined by differential scanning calorimetry using a DSC2020 apparatus from TA Instruments. The DSC data were measured on 10 mg samples at a heating rate of 10 K/min under helium atmosphere. X c was calculated from the ratio between DH, the matrix melting enthalpy, and DHf 1. DHf 1 value was taken equal to 290 J/g for the reference PE and all the compatibilized matrices. 50 X c value was the average value calculated from the experiments that were performed on five different samples of each material. Permeation measurements were performed at 20 1 C on the nanocomposites and on the respective matrices for He, H 2,O 2,andCO 2. The permeation cell consisted of two compartments separated by the studied film (effective area 3 cm 2 ). A preliminary high vacuum desorption was realized within the permeation cell prior to the experiments to ensure that the static vacuum pressure changes in the downstream compartment were smaller than the pressure changes due to the gas diffusion. The gas was then introduced in the upstream compartment, and the upstream pressure was fixed at 3 bars. The pressure variations in the downstream compartment were recorded as a function of time with a Datametrics pressure sensor. A steady-state line was obtained after a transitory state. The permeability

4 2596 PICARD ET AL. Table 2. Characteristic Parameters of the Clay Dispersion State within the Different Matrices and of the Matrix Crystalline Phase in the Different Nanocomposite Films d-spacing Value, d 001, Determined by DRX (Å) Amount of Micron- Sized-Tactoids, f MST, Determined from TEM Analysis (%) T m ( C) X c (%) OMMT 28 / / / PE/OMMT PE/MA1/OMMT No diffraction peak PE/MA9/OMMT No diffraction peak PE/lwMA6/OMMT PE/AA63/OMMT PE/lwOX16/OMMT coefficient expressed in barrer units (1 barrer ¼ cm STP 3 cm cm 2 s 1 cm Hg 1 ) was calculated from the slope of the steady state line. For CO 2, the diffusion coefficient was calculated from the time lag value determined by the intercept of the steady state line with the time axis. All the penetrants used in this study had purity of at least 99%. The precision on the permeability and diffusion coefficients value was estimated to be better than 5%. RESULTS AND DISCUSSION Matrix Crystalline Morphology Study The crystallinity values of the composite films and of the respective matrices were determined from DSC analysis. Indeed, the polymer crystalline phaseisconsideredtobeanimpermeablephasefor gas transport. It was thus important to check that no significant modification of the matrix crystallinity occurred after addition of nanoclays to discuss the sole effects of the lamellar fillers. The temperature at the maximum of the melting peak, T m, determined from the DSC thermograms are reported in Table 2 for the nanocomposites. The crystallinity values, X c, of the different matrices and associated nanocomposites films have been reported in Figure 1. T m and X c values measured on the nanocomposites films do not differ from those of the respective matrices, allowing us to assign the gas permeability variation to a sole clay effect. representative of each nanocomposite is presented in Figure 2. In all cases, the platelets are lying in the plane of the film. A significant improvement of the filler dispersion state is observed when a compatibilizer is added in the blend. Indeed, the large and dense aggregates of fillers evidenced in the neat PE are no more observed in the compatibilized matrices. Even if some filler tactoids remain in the PE/compatibilizer blends, they are much smaller than those found in the noncompatibilized blends. However, and despite this general trend, very different types of filler dispersion state are obtained as a function of the compatibilizer nature and molar mass. The nanocomposite film prepared with MA9 clearly exhibits individualized inorganic sheets, whereas more numerous small stacks of platelets are observed with lwma6. A mixed structure composed of small and dense stacks of platelets in addition to intercalated/exfoliated platelets is observed for the nanocomposite films based on AA63. At last, large but highly Fillers Dispersion State The fillers dipersion state has been investigated by TEM and XRD analysis. A TEM micrograph Figure 1. Influence of the compatibilizer on the crystallinity values of the matrices and of the respective nanocomposite films.

5 POLYETHYLENE/CLAY NANOCOMPOSITES 2597 Figure 2. TEM micrographs of the nanocomposite films prepared with different compatibilizers. swollen filler aggregates are mainly evidenced in the nanocomposite films prepared with lwox16. This qualitative filler dispersion state analysis is in total agreement with the conclusions drawn from the XRD study performed on the nanocomposite films. Figure 3 shows the XRD curves relative to the organo-modified montmorillonite and the different nanocomposites. The XRD pattern of the clay exhibits a broad diffraction peak at an angular position of 3.2 corresponding to a d-spacing, d 001,of28Å. This peak is clearly observed for the PE/organoclay composite film, whereas it is shifted to lower 2y values for the nanocomposites based on AA63, lwma6, and lwox16 and it totally disappears for the nanocomposites based on MA1 and MA9. The values of the filler d-spacing were deduced for the different nanocomposites from the diffraction peak position. They are reported in Table 2. The d 001 value increases going from uncompatibilized PE to PE/AA63, PE/lwMA6, and PE/ lwox16 meaning that the polymer diffusion in the clay gallery is more efficient with lwox16.

6 2598 PICARD ET AL. Figure 3. XRD patterns of the organo-modified MMT-Tallow clay and of the nanocomposite films prepared with different compatibilizers. For nanocomposites based on MA1 and MA9, the absence of a well defined diffraction peak at low diffraction angle suggests, as confirmed by the TEM micrographs, that the fillers are rather well exfoliated. A quantitative and statistical analysis of the TEM pictures has been realized to determine, for each composite film, the amount of micron-sizedtactoids, f MST. Indeed, this parameter seems to be a discriminating parameter of the filler dispersion state according to the previously discussed morphological data. The details of the method that we used to perform this statistical TEM image analysis has already been presented in a previous article, 40 and the results derived from our study are given in Table 2 for each composite film. The micron-sized-tactoids amount values have also been reported in Figure 4(A,B) as a function of two parameters, the molar mass of the compatibilizer, and the compatibilizer I KOH value, respectively. From these figures, it can be confirmed that the introduction of a compatibilizer in the PE/clay film considerably reduces the amount of large fillers aggregates in the polymer matrix. Furthermore, the amount of micron-sized-tactoids is lower with the compatibilizers of high molar mass (MA1, MA9 et AA63) than with the compatibilizers of low molar mass (lwox16 et lwma6) independently on the nature and content of the polar groups bore by the compatibilizer chains. From all these results, it can then be concluded that the presence of polar species within the polymer matrix, which favors the clay/matrix interactions is necessary to obtain an efficient transfer of the shear strength from the matrix to the fillers. As soon as these polar entities are present in the systems by means of the compatibilizer, a combined diffusion/shear process, as proposed by Fornes et al. 51 governs the final morphology. For the low molar mass compatibilizers (lwma6 and lwox16), the diffusion contribution plays the major role in the mechanism. Indeed, these low molar mass compatibilizers are able to diffuse within the interlamellar galleries leading to a swelling effect. Nevertheless, their introduction in PE leads to an important reduction of the viscosity of the blend in melt state and consequently to a significant decrease of the shear applied to the filler agglomerates. A complete delamination of the fillers can then not be achieved and large but swollen clays aggregates are finally mainly observed in these compatibilized matrices. It can at last be observed that the interplatelet distance in these filler stacks increases as the polarity of the compatibilizer increases. Indeed, lwma6 and lwox16 have a similar molar mass but different Figure 4. Evolution of the amount of micron-sizedtactoids as a function of (A) the molar mass of the compatibilizer introduced in the nanocomposite formulation and (B) the KOH index of the compatibilizer.

7 POLYETHYLENE/CLAY NANOCOMPOSITES 2599 Table 3. Gas Transport Properties of the Matrices and Nanocomposites as a Function of the Compatibilizer Introduced in the Formulation Matrix Nanocomposite Compatibilizer P (H 2 ) P (He) P (O 2 ) P (CO 2 ) P (H 2 ) P (He) P (O 2 ) P (CO 2 ) D r (CO 2 ) MA MA lwma AA lwox The permeability coefficients (P) have been measured for different gases and D r is the relative diffusion determined by the ratio of the diffusion coefficient of the nanocomposite to that of the respective matrix for CO 2. I KOH index values and a higher d 001 value is obtained with lwox16. For the higher molar mass compatibilizers (MA1, MA9, and AA63), the shear process seems to play a major role as the micron sized tactoids rate significantly decreases. Nevertheless, the f MST values remain non-negligible (in the range between 33 and 40%). It is then important to keep in mind that the optimal conditions for a complete exfoliation of the organoclays have never been achieved in these PE-based nanocomposites series. Gas Transport Properties The permeability coefficients have been measured for different gases (He, H 2,O 2, and CO 2 )onthe nanocomposites films and on the respective matrices. The permeability values have been reported in Table 3. The relative permeability, the ratio between the permeability of the nanocomposite to that of the respective matrix, has been calculated and reported in Figure 5 as a function of the compatibilizer nature. This factor should directly reflect the impact of clay introduction on the gas barrier reinforcement because the introduction of fillers does not modify the matrix crystallinity. From data of Figure 5, it can be observed that for each nanocomposite film, the relative permeability values do not depend on the gas nature. Furthermore, these values are in the same range than those of the relative CO 2 diffusion reported in Table 3. These both observations allow then to analyze at first the permeation data according to the classical geometrical approaches based on a tortuosity effect. According to the different models proposed in the literature, the relative permeability should decrease as the fraction of micron-sized tactoids f MST decreases. The data of Figure 6(A) that represent the relative permeability as a function of f MST clearly show that if the fillers do not lead to any significant improvement of the barrier properties in neat polyethylene, in agreement with the formation of a high content of dense and large aggregates, there is either no significant improvement of the barrier properties for the nanocomposites films based on MA1 and MA9 compatibilizers which are representative of the films that present the smallest amounts of micron sized tactoids. Indeed, the relative permeability coefficients remain close to one for these two nanocomposites films. On the other hand, higher permeability reductions are observed with lwma6 and lwox16, thus, with the nanocomposite films Figure 5. Relative permeability values measured for different gases on the nanocomposite films prepared with different compatibilizers.

8 2600 PICARD ET AL. Figure 6. Evolution of the relative permeability as a function of (A) the amount of micron-sized-tactoids dispersed within the matrices, (B) the compatibilizer molar mass, and (C) the KOH index of the compatibilizer. that present the highest rate of micron-sized-tactoids. This result clearly underlines that the classical geometrical approach can not succeed in describing the experimental permeability data and that the clay dispersion state is not the factor that governs the barrier properties for this PE nanocomposite series. The role of some physicochemical parameters that are characteristic of the compatibilizers have then been investigated. Figure 6(B,C) represent the evolution of the relative permeability as a function of the compatibilizer molar mass and I KOH index, respectively. No direct relation can be established between the molar mass of the compatibilizer and the relative permeability of the nanocomposites [Fig. 6(B)]. This result is not surprising as our previous morphological analysis clearly showed that the molar mass was the main factor governing the amount of micron-sized tactoids in the films and as a consequence the filler dispersion state. Considering now the modification rate of the compatibilizer by means of I KOH value, we can observe from the data reported in Figure 6(C), that the relative permeability decreases with the increase of I KOH whatever the type of polar functions bore by the compatibilizer. Furthermore, similar relative permeability values are obtained with lwox16 and AA63. The decrease of the relative permeability as a function of I KOH is not related to an increase in the global medium polarity. Indeed, as already mentioned, the relative permeability is defined as the ratio between the permeability of the nanocomposite to that of the respective matrix. The polarity is then similar in the considered matrix and in its associated composite. However, according to our morphological analysis, a higher compatibilizer polarity has been shown to promote higher interactions toward the clay as it favors the chain diffusion in the clay gallery. Thus, if it is considered that I KOH is representative of the interactions at the interface between the fillers and the matrix, it appears that this parameter can explain the evolution of the relative permeability in the nanocomposites series, in combination with the filler dispersion state that is representative of the interfacial area developed in each composite film. Indeed, the permeability decrease observed in a polymer matrix filled with impermeable fillers results from (i) a tortuosity effect that lengthens the diffusive gas path way and (ii) from the diffusion rate at the polymer/filler interface. It is generally assumed that the diffusion rate at the interface is equal to that in the bulk polymer, and, that is why the tortuosity effect becomes predominant and higher permeability reductions are expected when the clay dispersion state is improved. However, some authors 19,23,52 have already proposed to take into account the matrix/clay interface to improve the understanding of nanocomposite permeability properties. Specific interfacial properties can indeed result from the strength of the interactions developed between the polymer and the clay

9 POLYETHYLENE/CLAY NANOCOMPOSITES 2601 Figure 7. Comparison between the barrier effect values collected from literature data and those calculated for the nanocomposites based on the oxidized paraffin and different clay contents. The dotted curve is representative of Nielsen law with a form factor equal to 200. The two straight lines delimits the results obtained in this work. or from an effect of polymer chain confinement in the filler vicinity. Waché 19 has in particular shown that a high diffusive interface could be formed in PE-based nanocomposites, even when compatibilizers were introduced in the formulation. If we assume that the viscosity of the melt compatibilized matrix can play an important role on the clay dispersion state, whereas the compatibilizer polarity can play a role on the matrix/clay interactions, we can envisage the possibility to obtain a relatively good dispersion of the nanoclays with rather weak interfacial interactions such as in the case of MA1 or on the contrary to keep large and swollen aggregates of fillers with stronger interfacial interactions such as in the case of lwox16. In both cases, the permeability decrease results from the combination of a tortuosity effect and of the diffusion rate at the filler/ matrix interface. With MA1, the tortuosity is quite important because of the high shear rate induced by the high molar mass compatibilizer, but the diffusion rate at the interface is rather high because of the rather low polarity of the compatibilizer. No significant effect of the clay introduction is then observed on the nanocomposite permeability in comparison with the reference matrix. With lwox16, the tortuosity is not so important but because of the high polarity of the compatibilizer and of its low molar mass, swollen aggregates with strong interfaces are obtained. The diffusion rate at the interface is then expected to be decreased and the final result is a higher reduction of the relative permeability. According to our approach, the contribution of AA63 compatibilizer to interfacial interactions toward the clays seems to be higher than maleic anhydride grafted polyethylenes but lower than oxidized polyethylene. Indeed, similar relative permeability values are observed with lwox16 and AA63 despite a lower content of micron-sized tactoids with AA63. In conclusion, a gas transport mechanism closely related to the clay dispersion state and to the clay/matrix interfacial interactions has been

10 2602 PICARD ET AL. proposed to explain the evolution of the barrier properties in nanocomposites based on a nonpolar matrix and oxidized polyethylenes have demonstrated all their interest in this context. To optimize the gas barrier properties, nanocomposites have been prepared for increasing amounts of organo-modified clays from neat PE and lwox16. The barrier effects, that means the ratio between the nanocomposites permeability and the neat PE permeability, obtained with these systems were compared in Figure 7 with those calculated from literature data concerning different PE-based nanocomposites. The theoretical relative permeability values calculated from Nielsen law 23 considering a platelet aspect ratio equal to 200 was added in the graph. Different remarks can be drawn from this figure. First, whatever the system, the permeability decrease is much lower than that predicted by Nielsen law that would be representative of a total exfoliation of the clays. Second, all the experimental results are very similar despite the different nature and amount of compatibilizers introduced in the PE matrix and the different morphologies that were described in these different works. 8,14,18,20,22 Last, and as already underlined by one of our previous work, 14 quite interesting results are obtained with lwox16, meaning that in all the systems based on apolar matrices, the contribution of the compatibilizer and the optimization of the filler matrix interface can play a key role on the final barrier properties. CONCLUSIONS In this study, polyethylene/organoclay nanocomposites were prepared by melt processing using different interfacial agents varying by their molar mass, polarity, and type of polar groups. The organoclay and compatibilizer contents were fixed to 5 and 20 wt %, respectively. Nanocomposite morphology was investigated by different characterization techniques. It was found that the matrix crystallinity was not modified by the introduction of the organo-modified clays neither in the neat nor in the compatibilized polyethylene matrix. Furthermore, the large and dense filler aggregates initially observed in the neat PE were no more evidenced after addition of a compatibilizer in the nanocomposite formulation. Nevertheless, very different clay dispersion states were obtained as a function of the compatibilizer nature and molar mass. High clay delamination states were achieved with the high molar mass compatibilizers, whereas highly swollen clay aggregates were mainly obtained with polar and low molar mass interfacial agents. We proposed that a combined diffusion/shear process defined the final morphology as soon as polar entities were present in the systems by means of the compatibilizer. For the low molar mass compatibilizers, the diffusion contribution governed the final morphology, whereas the shear process was, on the other hand, predominant for the high molar mass compatibilizers. The gas transport properties were analyzed for different diffusing molecules with two main goals. The first goal was to propose a gas transport mechanism for these nanocomposites based on a nonpolar matrix and the second goal was to optimize a nanocomposite formulation for barrier properties. The relative permeability directly reflected the effect of the nanoclay introduction within the different matrices. It did not depend on the gas nature and it was very similar to the relative diffusion. However, it could not be directly related to the clay dispersion state in our nanocomposites series. Indeed, the lowest relative permeability was obtained for the nanocomposite based on the low molar mass oxidized polyethylene, which evidenced a large amount of highly swollen clay aggregates. In fact, the relative permeability values were the result of a tortuosity effect and of the matrix/clay interfacial interactions. With the high molar mass maleic anhydride grafted polyethylenes (MA1, MA9), the tortuosity was quite important due to the high shear rate developed in the system but the interfacial interactions were not very strong due to the rather low polarity of the compatibilizers. No significant effect of the clay introduction was then observed on the nanocomposite permeability in comparison with the reference matrix. With the low molar mass oxidized polyethylene (lwox16), the tortuosity was not so important, but the strong interfaces developed in the swollen clay aggregates led to a decrease of the diffusion rate at the interface and the final result was a higher reduction of the relative permeability. Very interesting properties in comparison with the literature data were achieved for this system by varying the clay content. Thus, the stiffness of the interfacial clay/matrix interactions was a key parameter in the optimization of the barrier properties of nanocomposites based on nonpolar matrices, and the interest

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