Change in the Properties of Linear Low-Density Polyethylene (LLDPE

Article pubs.acs.org/IECR

Change in the Properties of Linear Low-Density Polyethylene (LLDPE)/Montmorillonite Clay Nanocomposites after a Fuel-Aging Process Daniel Komatsu,*,† Caio Márcio Paranhos,† Tiago Venâncio,† and Adhemar Ruvolo-Filho‡ †

Department of Chemistry and ‡Department of Materials Engineering, Federal University of São Carlos, Rodovia Washington Luís, km 235 - SP-310, São Carlos, São Paulo, Brazil ABSTRACT: Linear low-density polyethylene (LLDPE)/montmorillonite clay (MMT) nanocomposites were obtained by dilution of a master batch containing 20.0% clay content in a twin screw extruder to obtain samples with 1.5−10.0% clay content. Samples were aged in fuel (ethanol or gasoline) for 30 days at room temperature (25 °C). In the present study, wide-angle X-ray diffraction (WAXD) showed that ethanol interacts with clay more than gasoline does, rearranging the clay structure that was lost during the preparation of the nanocomposites. Solid-state 13C NMR spectroscopy showed that MMT modified the morphological structure of the LLDPE matrix, making it more heterogeneous. Moreover, the fuel-aging process generates structures that are more heterogeneous than nonaged nanocomposites. Finally, fuel sorption analysis was used to verify the structure of the LLDPE matrix after the addition MMT. It was observed that the gasoline diffusion coefficients were higher than the ethanol diffusion coefficients. thicknesses to be attained with smaller amounts of polymer.8 Whereas tortuosity is usually the primary mechanism by which nanofillers impact the barrier properties of polymer−clay nanocomposites, this is not always the case. The second way that nanoparticulate fillers influence the barrier properties is by causing changes to the polymer matrix itself in the interfacial regions. If the polymer−nanoparticle interactions are favorable, polymer strands located in close proximity to each nanoparticle can be partially immobilized. The result is that gas molecules traveling through these interfacial zones have attenuated hopping rates between free-volume holes or altered density and/or hole size, as has been observed directly by positron annihilation lifetime spectroscopy (PALS).9 In addition, the presence of surfactants or other additives used to efficiently incorporate the filler into the matrix can also affect the diffusivity or solubility of permeants. The effects of the interfacial regions have been found to be particularly important in polymer matrixes such as polyolefins that have very high native gas permeabilities. Attempts have been made to model the effect of the interfacial regions on the diffusivity properties of migrant gases through polymer films, but the relevant parameters are not always easy to measure. Molecular diffusion in general is a random-walk process and is proportional to a driving force (concentration gradient) and inversely proportional to the resistance (length). For example, Fick’s law can be described as JA = DAB dCA/dz, where JA is the flux per unit area of species A, DAB is the diffusion constant of species A through species B, and dCA/dz is the concentration gradient of species A. Fickian diffusion assumes that each molecule proceeds along a random path and only collides with

1. INTRODUCTION Over the past decade, interest in polymer layered silicate nanocomposites has increased at an unprecedented level, both in industry and in academia, because of their potential for enhanced physical, chemical, and mechanical properties compared to those of conventionally filled composites. These nanocomposites have the potential of being a low-cost alternative to high-performance composites for commercial applications in both the automotive and packaging industries. It is well-known that, when layered silicates are uniformly dispersed and exfoliated into a polymer matrix, the polymer properties can be dramatically improved. These improvements can include increased strength, higher modulus, better thermal stability and barrier properties, and decreased flammability. Hence, to capitalize on the potential offered by nanoparticles in areas such as reinforcement, higher levels of fully dispersed nanoparticles must be obtained.1,2 Polymer−clay nanocomposites are two-phase materials in which the polymers are reinforced by nanoscale fillers. The most heavily used filler material is based on the smectite class of aluminum silicate clays, of which the most common representative is montmorillonite, which has been employed in many polymer−clay nanocomposite systems because it has a potentially high aspect ratio and high surface area that could potentially lead to materials with significantly improved properties and lower cost.3−7 The dispersal of nanosized fillers into the polymer matrix affects the barrier properties of a homogeneous polymer film in two specific ways. The first way is by creating a tortuous path for gas diffusion. Because the filler materials are essentially impermeable inorganic crystals, gas molecules must diffuse around them rather than taking a (mean) direct path that lies perpendicular to the film surface. The result is a longer mean path for gas diffusion through the film in the presence of fillers. Essentially, the tortuous path allows larger effective film © 2013 American Chemical Society

Received: Revised: Accepted: Published: 7382

November 21, 2012 April 4, 2013 April 29, 2013 April 29, 2013 dx.doi.org/10.1021/ie303112w | Ind. Eng. Chem. Res. 2013, 52, 7382−7390

Industrial & Engineering Chemistry Research

Article

2. EXPERIMENTAL SECTION 2.1. Materials. The materials used in this work were as follows: LLDPE [melt flow index (MFI) = 0.5 g/10 min] purchased from Braskem, São Paulo, Brazil; montmorillonite clay (Cloisite 20A) was purchased from Southern Clay Products, Gonzales, TX; and linear low-density polyethylene grafted with maleic anhydride (LLDPE-g-MA, DuPont, Fusabond MX110D, MFI = 22.9 g/10 min) was supplied by Cromex Company, São Paulo, Brazil. 2.2. Preparation of Nanocomposites. The master batch was prepared by bmixing LLDPE-g-MA and montmorillonite clay at a 2:1 ratio in a mixer with a high shear rate (w = 3000 rpm); this melted the LLDPE-g-MA because of the friction generated inside the chamber of the mixer, to produce a concentrate with 20% montmorillonite clay. The master batch was then cryogenically ground in an IKA Werke M20 model mill with rotation of 20000 rpm. It was then diluted in the LLDPE matrix to obtain nanocomposites by melt intercalation using a B&P Process and Systems Equipment model MT19TC25 twin-screw extruder [length/diameter ratio (L/D) = 25] with a temperature profile of 150, 170, 170, 200, and 200 °C and a screw rotation speed of 120 rpm, to obtain nanocomposites with 1.5−10.0% clay content by weight in the polymer matrix. Samples were prepared in the form of films and plates by thermopressing at a temperature of 180 °C and subsequently cooled under pressure (9.65 MPa) to room temperature. 2.3. Analysis and Characterization Techniques. Structural analysis was performed by wide-angle X-ray diffraction (WAXD) using a Rigaku Geigerflex instrument with 2θ scanning in the range of 1.6−30.0°, with 1.54-Å Cu Kα radiation, operating at 40 kV and 25 mA. The XRD patterns obtained were used to calculate the crystallinities of aged and nonaged samples. For this purpose, a deconvolution procedure was applied to the XRD patterns of curves, calculating the areas relating to the crystalline portion and the total area (part crystalline and part amorphous) according to the Ruland method.16 Thus, the crystallinity of each sample was calculated by the equation

other molecules (alike or unalike) and that collisions with the pore walls do not contribute to the diffusion process. For Fickian diffusion to hold, the average distance that a molecule travels between collisions, defined as the mean free path λ, must be much shorter than the dimensions of the pore. Other types of molecular diffusion, such as Knudsen diffusion, occur when a molecule diffuses in a pore that is smaller than λ or when diffusion occurs at very low pressures. A collision with the pore walls makes a significant contribution to the overall diffusion under these conditions, and these collisions are taken into account in Knudsen diffusion.10 The transport of small molecules through polymeric membranes is due to the random motion of the individual molecules. The liquid transport process through the polymer occurs by a phenomenon whose initial mechanism is due to molecular sorption on the surface and subsequently by a molecular diffusion process. The equation that thermodynamically defines this process, described in terms of sorption and diffusion, is P = SD, where S is a term of thermodynamic nature that is determined by polymer/ penetrant liquid interactions and the excess free volume that exists in the polymer and D is a term of kinetic nature that is reflected in the mobility of the liquid through the polymer matrix, that is, it is related to the diffusion process.11 However, there are some factors that influence these processes. For example, solubility is affected by the presence of free volume12 and the relative polarity of the penetrant/polymer and matrix/ filler. The diffusion process is affected by the presence of free volume and the crystallinity degree of the polymer matrix, because the diffusion takes place exclusively in the amorphous phase of the polymer.13 Solid-state nuclear magnetic resonance (NMR) spectroscopy is a common technique for investigating the morphology of solid polymers. In particular, NMR spectroscopy offers the advantage that local order in noncrystalline phases can be studied, something that cannot be effectively achieved by other techniques such as small-angle X-ray or neutron scattering. The solid-state NMR method is based on the study of relaxation behavior, including determination of spin−lattice (T1) and spin−spin (T2) relaxation times of the nuclei of interest. The relaxation times vary as a function of Tc, the correlation time.14 Within this context, the determination of proton spin−lattice relaxation times through solid-state NMR spectroscopy was chosen in this work, because it is an important tool employed to study both molecular structure and dynamic behavior. It can generate new support and responses on nanocomposite intermolecular interactions and structural organization. The use of solid-state techniques will aid in the observation of changes in the structural/microstructural and dynamical behavior, focusing on the molecular motion of the polymer chains.15 In this study, linear low-density polyethylene (LLDPE), LLDPE grafted with maleic anhydride (LLDPE-g-MA), and montmorillonite clay (Cloisite 20A) were used. LLDPE/ montmorillonite clay nanocomposites were aged at room temperature (25 °C) for 30 days in ethanol or gasoline. This study aimed to investigate the influence of ethanol and gasoline aging processes on the properties of nanocomposites, simulating a real situation that happens with automotive fuel storage tanks during their lifetime. Wide-angle X-ray diffraction (WAXD), NMR spectroscopy, and gasoline and ethanol sorption analyses were performed.

degree of crystallinity (%) =

crystalline area × 100 total area

(1)

Solid-state 13C NMR spectroscopy was performed using a Bruker Avance III-400 spectrometer operating in a magnetic field of 9.4 T and with a probe for solid samples using 4-mmdiameter zirconia rotors at a rotation rate of 5 kHz. The longitudinal relaxation time (T1H) was determined indirectly using cross-polarization and magic angle spinning (CPMAS) with sample rotation around the magic angle. All experiments were performed at 25 °C. The ethanol and gasoline sorption analyses were performed at 30 °C. Samples were weighed to determine the initial mass and were then immersed in the fuel and weighed several times to complete an hour of measurements. After this first hour of measurements, samples were weighed hourly for 8 h, and after these 8 h, they were weighed twice daily until 192 h of testing had been completed. The equilibrium concentration values (Ceq) shown in Figure 6 and the diffusion coefficients shown in Figure 7 were obtained from plots of fuel sorption as illustrated in panels a and b, respectively, of Figure 1. In this case, the Ceq values were obtained in the balance region from the graph of mass of fuel sorbed versus time, where there is practically no 7383

dx.doi.org/10.1021/ie303112w | Ind. Eng. Chem. Res. 2013, 52, 7382−7390


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Furthermore, montmorillonite clay (Cloisite 20A) showed a main peak [d(001)] around 3.8° (2θ) and a weak second peak [d(001) with higher-order reflections] around 7.5° (2θ). This weak second peak [d(001)] with higher-order reflections around 7.5° (2θ) is related to a small amount of the original non-ion-exchanged MMT. It can shown that the ion-exchange process for sodium montmorilonite clay is not completely effective, because of sodium ion structures remaining in the interlamellar regions of montmorillonite clay (Cloisite 20A).18−20 The X-ray diffraction patterns obtained for organoclay and nanocomposites are shown in Figure 3. The influence of the fuel-aging process on clay (lower 2θ values) and the polymer matrix (higher 2θ values) can be seen, and it is possible to calculate the crystallinity of the LLDPE and nanocomposites. The fuel-aged montmorillonite clay (Cloisite 20A) exhibits an interesting effect. For example, in Figure 3a, one can see that the main peak [d(001)] around 3.8° (2θ) of nonaged montmorillonite clay (Cloisite 20A) practically disappears after gasoline aging, that is, gasoline aging could cause disruption of the coherent layer stacking of clay and result in a featureless diffraction pattern because of poor order in the lamellar structure. This is confirmed by the presence of weak shoulders near 2.75° and 4.75° (2θ) in the XRD diffractograms of gasoline-aged montmorillonite clay instead of a peak, due to an ordered lamellar structure, as seen for nonaged clay. On the other hand, ethanol aging causes a shift in the main peak [d(001)] of nonaged montmorillonite clay (Cloisite 20A) from 3.8° (2θ) to 3.5° (2θ). This shows that there was an increase in the distance between the lamellae relative to that in nonaged clay, specifically, from 2.3 to 2.5 nm, but without destroying the clay’s organization. This result could be related to the swelling of the clay due to contact with ethanol. Therefore, comparing the effect of gasoline and ethanol aging in the clay structure, it is possible to see that gasoline aging can swell the clay more than ethanol, disrupting the coherent layer stacking of montmorillonite clay, as seen at Figure 3a, where the gasoline-aged clay shows a featureless diffraction pattern because of its poorly ordered lamellar structure. It can be observed in Figure 3 that the main peaks [d(001)] related to montmorillonite clay in the nonaged nanocomposites are practically imperceptible whereas the fuel-aged samples show different behavior, that is, the peaks and shoulders reappear in the X-ray diffraction curves (Figure 3b−e). The difference in these X-ray diffraction curves could be an indication that the ethanol molecules interact with the montmorillonite clay more strongly than the gasoline molecules do, because the presence of the peaks is indicative of a reorganization of the ordered clay structure that was lost during the preparation of the nanocomposites. Brazilian gasoline contains 20 vol % ethanol. The presence of this amount of ethanol in gasoline would explain why the reorganization of the clay structure is less intense in gasoline than in pure ethanol. Moreover, unlike the gasoline-aged nanocomposites with 2.5% and 5.0% clay contents, which show small shoulders related to the main peak [d(001)] of montmorillonite clay, the nanocomposite with 7.5% clay content shows a well-defined peak. In this case, the reorganization of the clay structure could be due to the plasticization of the LLDPE matrix chains by gasoline molecules, which could organize these large structures formed by clay aggregates. In the nanocomposites with clay contents of 1.5%, 2.5%, and 5.0%, the clay structures are probably intercalated and exfoliated; however, this LLDPE

Figure 1. Graphs illustrating the sorption of fuel in LLDPE/ montmorillonite clay nanocomposites: (a) equilibrium concentration (Ceq) and (b) diffusion coefficient.

change in mass of fuel with the passage of time. The diffusion coefficient was obtained in the sloping region near the small sorption time values in a graph of the relative mass of fuel sorbed versus the square root of time (t1/2). The slope of the curve is related to the value of the diffusion coefficient by the equation according to Crank and Park.17 The diffusion coefficient (D) is given in centimeters squared per second (cm2·s−1) as shown in eq 2. D=

α 2L2π 57.600

(2)

where α is the slope of the sorption graph and L is the sample thickness.

3. RESULTS AND DISCUSSION The X-ray diffraction patterns obtained for organoclay (Cloisite 20A) and for sodium montmorillonite clay are shown in Figure 2. In Figure 2, one can see that the sodium montmorillonite clay showed a main peak [d(001)] around 6.1° (2θ).

Figure 2. XRD patterns of organoclay (Cloisite 20A) and sodium montmorillonite clay. 7384



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Figure 3. XRD patterns of fuel-aged and nonaged montmorillonite clay and LLDPE/montmorillonite clay nanocomposites: (a) montmorillonite clay, (b) nano 1.5% clay, (c) nano 2.5% clay, (d) nano 5.0% clay, (e) nano 7.5% clay.

matrix mobility is not sufficient for rearrangement of the clay structure to occur. Furthermore, it can be observed that the main peak [d(001)] of montmorillonite clay in the ethanolaged nanocomposite with 7.5% clay content appears at a higher angle than for other nanocomposites with lower clay contents. This could indicate the presence of large structures formed from clay aggregates that hinder the entry of ethanol molecules into the clay interior and make the disorganization of the clay structure difficult. Figure 4 shows curves for the nonaged (Figure 4a), gasolineaged (Figure 4b), and ethanol-aged (Figure 4c) samples at 2θ values between 0° and 10° to facilitate visualization of the clay region. The nonaged clay (Figure 4a) showed a diffraction main peak [d(001)] at around 3.83° (2θ) with an interlayer spacing of 2.30 nm. Furthermore, there was decrease in the intensity and a shift to lower 2θ values of the main peak [d(001)] of clay in the nanocomposites, which could suggest disorganization and an increase in interlayer spacing for clay the lamellae in the nanocomposite samples. This indicates the probable formation of nanocomposites with intercalated and exfoliated clay structures. Table 1 lists the interlayer spacing values for the nonaged and fuel-aged clay and nanocomposites. It can be seen that the processing of the nonaged nanocomposite was performed efficiently, as evidenced by the absence of main peaks [d(001)],

which indicates that the coherent layer stacking of montmorillonite clay was disrupted, that is, the lamellae were intercalated and exfoliated within the polymer matrix. In the case of gasoline-aged nanocomposites, an increase in the interlayer spacing value was observed as the clay concentration was increased, similar to that observed for ethanol aging. However, in the case of ethanol aging, the interlayer spacing values reached a maximum and then approached the value for ethanolaged clay (Cloisite 20A). Furthermore, the nonaged and fuelaged clays exhibited differences in the values of the interlamellar spacing [d(001)] with lower-order reflections. When these clays (nonaged and fuel-aged) were mixed with LLDPE, the values the basal clay interlamellar spacing [d(001)] in these nanocomposites were nearly identical, except for the nanocomposites with 7.5% clay aged in gasoline and ethanol. For the gasoline-aged nanocomposite with 7.5% clay content, the interlayer spacing of clay [d(001)] with lower-order reflections was greater than that in the ethanol-aged nanocomposite with the same clay content. This shows that the process of swelling of the clay by gasoline was more intense than the process of swelling of the clay by ethanol. Moreover, this swelling by gasoline of the nanocomposite with 7.5% clay content could favor the entrance of the polymer chains into this clay interlayer spacing, thereby generating intercalated clay structures with 7385



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Figure 4. XRD patterns of (a) nonaged, (b) gasoline-aged, and (c) ethanol-aged samples.

crystallinity values of the gasoline-aged nanocomposites tended to be greater than those of the nonaged nanocomposites, which can be explained by the plasticizing effect of gasoline on the polymer chains. This effect of plasticizing the polyethylene matrix can be seen in the article by Buck et al., who showed that effect of solvent diffusion through high-density polyethylene (HDPE) cause a distortion of the container’s morphology, which is caused by the plasticizing effect of the chains of the HDPE matrix generated by contact of the wall of the container with gasoline.21 This effect is more pronounced for nanocomposites with high clay contents (5.0% and 7.5%) than for those with low clay contents (1.5% and 2.5%). This is because, in the high-clay-content nanocomposites (5.0% and 7.5%), the clay is dispersed in the LLDPE matrix in aggregate form, causing an increase in the free volume ratio available in the LLDPE matrix for the crystallization of the LLDPE chains to occur, thereby generating high crystallinity values. In contrast, the low-clay-content nanocomposites (1.5% and 2.5%) are little affected by the plasticizing effect of gasoline because the clay is dispersed in the LLDPE matrix as small aggregates, generating intercalated and exfoliated clay structures that are well distributed in the LLDPE matrix. Therefore, the free volume ratio available in the LLDPE matrix is reduced for the crystallization process of the LLDPE chains to occur, and consequently, the crystallinity values are lower than in the highclay-content nanocomposites (5.0% and 7.5%). In the case of ethanol-aged LLDPE, it was observed that the crystallinity did not vary compared to that of the nonaged sample. This could be an indication that, because of good chemical affinity, the

Table 1. Interlayer Spacing Values of the Main Peak [d(001), nm] in Clay for Nonaged Samples and Fuel-Aged Samples sample

nonaged

gasoline-aged

ethanol-aged

Cloisite 20A nano 1.5% nano 2.5% nano 5.0% nano 7.5%

2.30 − − − −

− − 2.82 2.82 2.96

2.49 2.86 2.83 2.81 2.54

greater interlayer spacing than in the ethanol-aged nanocomposite with 7.5% clay content. At higher 2θ values, it is possible to verify the influence of clay and the effect of fuel aging on the polymer crystallinity. The results are reported in Table 2. In accordance with the crystallinity values reported in Table 2, it can be seen that, in the nonaged samples, the addition of clay reduced the crystallinity of the LLDPE matrix, that is, the clay did not act as a nucleating agent. Furthermore, the Table 2. Crystallinity Values (%) of Nonaged and Fuel-Aged Samples sample LLDPE nano 1.5% nano 2.5% nano 5.0% nano 7.5%

nonaged 59.93 54.71 53.31 52.03 51.58

± ± ± ± ±

0.85 0.81 0.75 0.73 0.18

gasoline-aged 53.61 54.90 55.38 57.70 58.02

± ± ± ± ±

0.76 0.77 0.78 0.81 0.82

ethanol-aged 59.80 58.56 55.54 52.16 51.47

± ± ± ± ±

0.84 0.83 0.78 0.74 0.73 7386



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of these low-clay-content nanocomposites (1.5% and 2.5%) are very similar, indicating that they have structures that are similar to intercalated and exfoliated structures. High-clay-content nanocomposites (7.5% and 10.0%) show greater heterogeneity because of the presence of clay particles dispersed not only as intercalated and exfoliated structures, but also with a significant presence of aggregate structures, thus resulting in more heterogeneous systems. The fuel-aging process decreases the spin-relaxation time (T1H) for nonaged samples. Gasoline-aged and ethanol-aged nanocomposites have lower T1H values than nonaged nanocomposites. This could be an indication that the fuel-aging process generates nanocomposites with more heterogeneous structures, resulting in faster energy transfer between the LLDPE polymer chains and their surroundings and leading to shorter relaxation times (T1H) than for the nonaged nanocomposites. IR spectra presented in another work by our group showed that the ethanol molecules interact with succinic groups present in the compatibilizer (LLDPE-g-MA), inactivating the interaction with the surfactant molecules present in the interlayer region of the clay.22 This can result in the reduction of structural organization, generating structures with different shapes and leading to an increasingly heterogeneous system, as can be seen in the X-ray diffractograms shown in Figure 4 for the fuel-aged nanocomposites. This confirms the results observed in the XRD spectra, where the presence of the main peak [d(001)] in the region near 3° and small structures due to the presence of weak diffraction peaks [d(001) with higher-order reflections] near 7° indicate intercalated structures. This agrees with what was discussed in another work by our group involving IR spectroscopy.22 In the nonaged nanocomposites, the main peak [d(001)] is not seen, and this could be an indication of the presence of exfoliated structures in the LLDPE matrix. In this case, the nonaged nanocomposites are more homogeneous than the fuel-aged nanocomposites. Table 3 reports the reduction in relaxation time (T1H) for the samples.

ethanol molecules interact with the quaternary ammonium salt present in the gallery of the clay and not with the LLDPE matrix, confirming what was discussed earlier in relation to the X-ray diffractogram (2θ = 0−10°). In ethanol-aged low-claycontent nanocomposites (1.5% and 2.5%), a slight increase in crystallinity was observed. This can be explained by the good distribution and dispersion of clay in the LLDPE matrix and also by the presence of the small quantity of ethanol sorbed by the LLDPE matrix that interacts with the quaternary ammonium salts present in the clay. This interaction results in weaker interactions between the ammonium salt and the maleic anhydride group present in the compatibilizer. Consequently, the LLDPE chains can be organized more easily, and the crystallinity degree of low-clay-content nanocomposites (1.5% and 2.5%) increases. However, because of the high concentration of clay in the high-clay-content nanocomposites (5.0% and 7.5%), this small amount of ethanol sorbed by the LLDPE matrix is not sufficient to break the interaction between the quaternary ammonium salt and the maleic anhydride group. Thus, polymer chain organization is not favored in this case, and the crystallinity values do not change compared to those of the nonaged nanocomposites. Figure 5 shows the spin-relaxation time (T1H) as a function of clay content.

Table 3. Reduction Rate in Relaxation Time (T1H) for Various Samples Figure 5. Relaxation time (T1H) as a function of clay content.

The longitudinal relaxation process is essentially an enthalpic process, in which it is necessary for energy transfer to take place between the spin-excited polymer chain and the surroundings. Thus, the more heterogeneous the system is, the more susceptible the surroundings are to dissipation energy, the more efficient the relaxation is, and, consequently, the lower the value of T1H is. Figure 5 shows that the spin-relaxation time (T1H) decreases with increasing amount of clay in the system, that is, the presence of the clay produces a polymer system that is more heterogeneous than the pure LLDPE matrix. The T1H values for the low-clay-content nanocomposites (1.5% and 2.5%) are greater than those for the high-clay-content nanocomposites (5.0% and 7.5%). This is because the former are more homogeneous systems, generated as a result of a better distribution and dispersion of the clay in the LLDPE matrix. Thus, the energy transfer between the excited spins of the polymer chains and the surroundings is impaired, as reflected in the higher T1H values for the low-clay-content nanocomposites (1.5% and 2.5%). Furthermore, the T1H values

sample

rate [ms (clay content)−1]

nonaged gasoline-aged ethanol-aged

42.48 42.09 29.74

Table 3 shows the reduction in relaxation time (T1H) as a function of clay content. It is observed that the reduction of the relaxation time (T1H) for ethanol-aged samples is lower than those for nonaged and gasoline-aged samples. This can be explained by the interaction between ethanol molecules, maleic anhydride groups, and the quaternary ammonium salt, which results in the reorganization of the clay in the LLDPE matrix so that the intercalated and aggregated structures are more homogeneous than for the nonaged and gasoline-aged samples. Regardless of the clay content, the X-ray diffractogram patterns for the ethanol-aged nanocomposites are similar to each other, that is, the difference between the relaxation times of the ethanol-aged samples is lower than that for the nonaged and gasoline-aged samples. This shows that there is no great variation in the heterogeneity of the ethanol-aged nanocomposites. 7387



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Figure 6 shows the equilibrium concentrations of ethanol and gasoline in LLDPE and the nanocomposites. It can be observed

Figure 7. Diffusion coefficient values for (a) gasoline-aged and (b) ethanol-aged nanocomposites. Figure 6. Equilibrium concentration as a function of clay content for (a) gasoline-aged and (b) ethanol-aged nanocomposites.

solvent, leading to a slight decrease in the value of the diffusion coefficient. This occurs because the crystalline phase is considered to be impermeable to gasoline. Although the diffusion coefficient values are very close to each other, the nanocomposite with 1.5% clay content has a higher diffusion coefficient than pure LLDPE. This can be explained by the greater amount of free volume in the matrix−clay interface, which favors an increase in the diffusion coefficient. In Figure 7b, it can be seen that the LLDPE has a higher diffusion coefficient than the ethanol-aged nanocomposites. This can be explained by the low interaction between ethanol and the LLDPE matrix, that is, there are no interactions between the hydroxyl groups in ethanol molecules and the LLDPE matrix. Thus, the ethanol diffusion coefficient for pure LLDPE is greater than those for the clay nanocomposites. This confirms what was discussed previously for Figure 6b. Figure 8 shows the diffusion coefficient and crystallinity as functions of clay content. Figure 8a shows that the diffusion coefficient of ethanol decreases with decreasing sample crystallinity. However, this reduction in crystallinity is not sufficient to explain the decrease in the diffusion coefficient, because one would expect the opposite behavior, namely, an increase in the value of the diffusion coefficient because the crystals are considered impermeable to the fuels. One way to try to explain this behavior is related to the balance between the diffusion coefficient and the solubility, where the permeability (P) is given by the product of the diffusion coefficient (D) and the solubility (S)

that the equilibrium concentrations of gasoline are higher than those of ethanol. This can be explained by the fact that the LLDPE matrix and gasoline present similar polarities, so that the interaction between LLDPE and gasoline is stronger than that between LLDPE and ethanol. In Figure 6a, it can be observed that the equilibrium concentrations of gasoline are larger than those of ethanol. This shows that the interaction between gasoline and the LLDPE matrix is stronger than the interaction between ethanol and the LLDPE matrix. Furthermore, the amount of gasoline sorbed by the system is less influenced by the clay−compatibilizer interface. This confirms that gasoline interacts preferentially with the LLDPE matrix. Figure 6b shows that the higher the clay content in a sample, the greater the amount of ethanol sorbed by the sample. This could be an indication that the ethanol molecules interact preferentially at the clay−compatibilizer interface, because both the ethanol molecules and the interface have the same polarity (polar), probably inactivating the interactions between them and thus leading to the formation of voids that increase the likelihood of ethanol molecule sorption. Furthermore, the interaction between the hydroxyl group in ethanol and the maleic anhydride group in the compatibilizer (LLDPE-g-MA) could facilitate the condensation of the ethanol at the clay− compatibilizer interface, increasing the solubility of the ethanol molecules in the nanocomposite matrix and consequently reducing diffusion through this system. No less important is the hypothesis that this interaction can occur simultaneously with the quaternary ammonium salt in the clay. This then could explain the significant increase in ethanol solubility with increased clay content. Figure 7 shows the ethanol and gasoline diffusion coefficient values for nanocomposites. It can be observed that the gasoline diffusion coefficients are higher than the ethanol diffusion coefficients. This can be explained again by the fact that the LLDPE matrix and gasoline present similar polarities, so that the interaction between gasoline and LLDPE is stronger than that between ethanol and LLDPE matrix. Figure 7a shows that the interaction between gasoline and the LLDPE matrix could induce their crystallization by the

P = DS

(3)

In other words, in Figure 6b, it can be seen that the ethanol concentration increases markedly with the amount of clay. In turn, the diffusion coefficient for ethanol in the nanocomposites decrease, clearly showing the balance between solubility and diffusion as previously mentioned. Therefore, in this case, we can say that the interaction between ethanol and the clay− compatibilizer interface exceeds the effect of crystallinity and decreases the diffusion process. Figure 8b shows different behavior than Figure 8a, because the gasoline interaction with the polymer matrix induces the crystallization of the polymer matrix, leading to a slight decrease in the value of the diffusion coefficient. This is because the crystalline phase is considered to 7388



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Figure 9. Permeability ratio (ratio of the value for the unfilled polymer to that for the nanocomposite) as a function of clay content.

barrier factor (Punfilled polymer/Pnanocomposite) and clay concentration. The barrier factor initially decreased for the nanocomposite with 1.5% clay content and then increased up to a clay content of 5.0%. This shows that the amount of gasoline sorbed by the system is less influenced by the clay− compatibilizer interface, and it confirms that gasoline interacts preferentially with the LLDPE matrix, as discussed in relation to Figure 6a, which shows that the interaction between gasoline and the LLDPE matrix is stronger than that between ethanol and the LLDPE matrix.

4. CONCLUSIONS The X-ray diffractograms show that ethanol interacts with clay more than gasoline does, rearranging the clay structure that was lost during the preparation of the nanocomposites. As Brazilian gasoline has 20.0 vol % ethanol in its composition, this could explain the reorganization of the clay structure, but it occurs in a less intense manner than for pure ethanol. Solid-state 13C NMR spectroscopy showed that the clay modified the morphological structure of the LLDPE matrix, making it more heterogeneous. This can be seen as a function of the variation of spin-relaxation time (T1H), which can indirectly provide clues about the clay structure in the LLDPE matrix (exfoliated, intercalated, and aggregate clay structures). Moreover, it was observed that the fuel-aging process decreased the spin-relaxation time (T1H) compared to that for nonaged samples. This is an indication that the fuel-aging process generates structures that are more heterogeneous in nanocomposites. Ethanol sorption tests showed that pure LLDPE has a higher diffusion coefficient than the nanocomposites. This can be explained by the low interaction between ethanol and the LLDPE matrix, that is, the lack of an interaction between the hydroxyl group in ethanol and the clay−compatibilizer interface. Thus, the diffusion of ethanol molecules in the pure LLDPE matrix is greater than that in the nanocomposites. In the case of gasoline, the interaction between the LLDPE matrix and gasoline can induce the crystallization of LLDPE, leading to a decrease in the diffusion coefficient because the crystalline phase is impermeable to any penetrant liquids.

Figure 8. Diffusion coefficient and crystallinity values as functions of clay content: (a) ethanol aging and (b) gasoline aging.

be impermeable to the fuel. In this case, for gasoline, the effect of crystallinity increase exceeds the effect of solubility, and thus, the diffusion coefficient decreases slightly. Furthermore, the interaction between gasoline and the polymer matrix could produce a positive synergistic effect with the effect of crystallinity, decreasing the diffusion process. Thus, there is a decrease in the value of the diffusion coefficient. Figure 9 shows a plot of the ratio of the permeability of unfilled polymer to that of the nanocomposite samples as a function of clay content. This ratio is the barrier factor for the diffusion of ethanol and gasoline through the LLDPE matrix. Figure 9 shows that the barrier factor (Punfilled polymer/ Pnanocomposite) decreased as the concentration of clay increased in ethanol-aged samples. This is an indication that the presence of clay reduced the barrier to ethanol molecules in the LLDPE matrix, confirming what was discussed with respect to Figure 6b, which shows that the amount of ethanol sorbed increased with increasing clay content in the nanocomposites. This could be an indication that the ethanol molecules interact preferentially at the clay−compatibilizer interface because both ethanol and the interface have the same polarity (polar), probably inactivating the interactions between them and thus leading to the formation of voids that increase the likelihood of ethanol molecule sorption. In contrast, for the gasoline-aged nanocomposites, no linear relation was observed between

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Notes

Nanocomposites and Polyethylene/Flame Retardants. Mater. Lett. 2007, 61, 2575. (19) Chrissopoulou, K.; Altintzi, I.; Anastasiadis, S. H.; Giannelis, E. P.; Pitsikalis, M.; N. Hadjichristidis, N.; Theophilou, N. Controlling the Miscibility of Polyethylene/Layered Silicate Nanocomposites by Altering the Polymer/Surface Interactions. Polymer 2005, 46, 12440. (20) Villaluenga, J. P. G.; Khayet, M.; López-Manchado, M. A.; Valentin, J. L.; Seoane, B.; Mengual, J. I. Gas Transport Properties of Polypropylene/Clay Composite Membranes. Eur. Polym. J. 2007, 43, 1132. (21) Buck, D. M.; Marsh, P. D.; Milcetich, F. A.; Kallish, K. J. Treated HDPE Containers Resist Solvent Permeation. Plast. Eng. 1986, 42, 33. (22) Paranhos, C. M.; Venâncio, T.; Ruvolo-Filho, A. Submitted to J. Macromol. Sci. B: Phys., 2012.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge CNPq for doctoral scholarship and research productivity grants and FAPESP for financial support through Project 2006/61008-5.

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REFERENCES

(1) Nguyen, Q. T.; Baird, D. G. An Improved Technique for Exfoliating and Dispersing Nanoclay Particles into Polymer Matrices Using Supercritical Carbon Dioxide. Polymer 2007, 48, 6923. (2) Ray, S. S.; Okamoto, M. Polymer/Layered Silicate Nanocomposites: A Review from Preparation to Processing. Prog. Polym. Sci. 2003, 28, 1539. (3) Lingaiah, S.; Sadler, R.; Ibeh, C.; Shivakumar, K. A Method of Visualization of Inorganic Nanoparticles Dispersion in Nanocomposites. Composites B 2008, 39, 196. (4) Pandey, J. K.; Reddy, K. R.; Kumar, A. P.; Singh, R. P. An Overview on the Degradability of Polymer Nanocomposites. Polym. Degrad. Stab. 2005, 88, 234. (5) Chrissopoulou, K.; Altintzi, I.; Anastasiadis, S. H.; Giannelis, E. P.; Pitsikalis, M.; Hadjichristidis, N.; Theophilou, N. Controlling the Miscibility of Polyethylene/Layered Silicate Nanocomposites by Altering the Polymer/Surface Interactions. Polymer 2005, 46, 12440. (6) Dennis, H. R.; Hunter, D. L.; Chang, D.; Kim, S.; White, J. L.; Cho, J. W.; Paul, D. R. Effect of Melt Processing Conditions on the Extent of Exfoliation in Organoclay-Based Nanocomposites. Polymer 2001, 42, 9513. (7) Zeng, Q.; Yu, A.; Lu, G. M. Interfacial Interactions and Structure of Polyurethane Intercalated Nanocomposite. Nanotechnology 2005, 16, 2757. (8) Duncan, T. V. Applications of Nanotechnology in Food Packaging and Food Safety: Barrier Materials, Antimicrobials and Sensors. J. Colloid Interface Sci. 2011, 363, 1. (9) Paranhos, C. M.; Soares, B. G.; Machado, J. C.; Windmöller, D.; Pessan, L. A. Microstructure and Free Volume Evaluation of Poly(vinyl ethanol) Nanocomposite Hydrogels. Eur. Polym. J. 2007, 43, 4882. (10) Hale, W. R.; Dohrer, K. K.; Tant, M. R.; Sand, I. D. A Diffusion Model for Water Vapor Transmission through Microporous Polyethylene/CaCo3 Films. Colloids Surf. A 2001, 187, 483. (11) Teixeira, E. M. Correlaçaõ entre morfologia e propriedades de transporte de diclorometano no Poli(eter imida) (PEI) e na benda PET/PEI. Ph.D. Dissertation, Federal University of São Carlos, São Paulo, Brazil, 2002. (12) Bandi, S.; Schiraldi, D. A. Glass Transition Behavior of Clay Aerogel/Poly(vinyl alcohol) Composites. Macromolecules 2006, 39, 6537. (13) Morales, A. R.; da Cruz, C. V. M; Peres, L.; Edson, N. I. Nanocompósitos de PEAD/PEBDLAvaliaçaõ da Esfoliaçaõ da ́ Argila Organofilica Pela Aplicaçaõ do Modelo de Nielsen e das ́ Propriedades Mecânicas, Ó pticas e Permeabilidade. Polim.: Ciênc. Tecnol. 2010, 20, 39. (14) Mingtao, W.; Guy, M. B.; Roderick, E. W.; Phillip, C. A SolidState 13C NMR Investigation of the Morphology of Single-Site and Ziegler−Natta Linear Low-Density Polyethylenes with Varying Branch Contents. Macromolecules 2007, 40, 6594. (15) Rodrigues, T.; Tavares, M. I. B.; Soares, I.; Moreira, A.; Ferreira, A. The Use of Solid-state NMR to Characterize High Density Polyethylene/Organoclay Nanocomposites. Chem. Chem. Technol. 2009, 3, 187. (16) Ruland, W. X-ray Determination of Crystallinity and Diffuse Disorder Scattering. Acta Crystallogr. 1961, 14, 1180. (17) Crank, J.; Park, G. S. Diffusion in Polymers; Academic Press: New York, 1968. (18) Barbosa, R.; Edcleide, M. A.; Tomas, J. A. M.; Edson, N. I. Comparison of Flammability Behavior of Polyethylene/Brazilian Clay 7390


Change in the Properties of Linear Low-Density Polyethylene (LLDPE

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