Interfacial interaction exploration and oxygen barrier potential of polyethylene/poly(ethylene-co-vinyl alcohol)/clay hybrid nanocomposites

2.1 Materials

HDPE grade HF4760 (Jam Petrochemical Co., Iran) with the density of 0.956 g/cm³ was used as continuous matrix phase of the nanocomposite samples. Maleic anhydride-grafted polyethylene (PE-g-MA) with the trade name of NF911E and the density of 0.9 g/cm³, a product of ADMER Mitsui Chemicals, was used as the compatibilizing agent. Montmorillonite grades of nanoclay, respectively with the trade name of Cloisite 15A (with density of 1.66 g/cm³ and the interlayer distance of 31.5 Å and Cloisite 20A (with the density of 1.77 g/cm³ and the interlayer distance of 24.2 Å) were both supplied by the Southern Clay, USA. The EVOH copolymer was the grade E151B manufactured by EVAL Europe Kuraray, Japan, with the ethylene molar ratio of 44% and the density of 1.14 g/cm³.

2.2 Preparation of HDPE/Clay nanocomposite samples

Prior to compounding process, nanoclay and PE-g-MA compatibilizer were placed in a vacuum oven at 80°C for 24 h to remove any trapped moisture. The ingredients were then melt blended in a Brabender Plasticorder static mixer with the capacity of 50 ml at 180°C and rotor speed of 60 rpm for about 10 min. Different nanocomposite samples were prepared and tested in which the amount of filler (1:1 weight ratio of 15A and 20A) was changed from 3 and 5 wt.%. The name and composition of prepared samples are given in Table 1. The required sheets for the tests were prepared by a hot press machine. For this purpose and after preheating the samples at 180°C for 5 min, the samples were held under pressure at the same temperature for further 2 min and then experienced indirect water-cooling process.

2.3 Sample characterization

In order to analyze the morphology of samples and evaluation of the dispersion and distribution state of nanoclay throughout the polymer matrix, thin cuts of the samples were prepared via ultra-microtome using a Reichert omu₃ and then the images were captured by a transmission electron microscope (TEM) (Philips, CM30 model) working at 150 kV. Wide angle X-ray scattering (WAXS), MICRO-S3 model manufactured by Austrian company of Hecus with the resolution strength of 0.01 degree, emission with the wavelength of λ=1.542 Å and the acceleration voltage of 50 kV from the angle 2θ=1–8, was employed to determine the interlayer distance of nanoclay sheets. To measure the oxygen gas permeability, a Munchen instrument (Brugger Co.) has been employed. This test was performed according to ASTM D1434 standard in the ambient temperature. The crystallinity of compounds and the neat polymer was measured by a differential scanning calorimeter (DSC) instrument model DSC1 manufactured by Mettler Toledo. Samples 5–6 milligrams in weight prepared by hot press machine were heated from 25°C up to 180°C with the heating rate of 10 K/min and then held at the assigned temperature for 2 min to eliminate their thermal history. Then, the samples were cooled from 180°C down to 25°C and reheated up to the 180°C at the same heating rate. The extent of crystallinity was measured from the second calorimetric scanning. The thermo-mechanical properties were measured by the dynamic mechanical thermal analysis (DMTA) instrument, DMTA-Triton model Tritec 2000, UK. The test was performed according to the ASTM D1640 at the frequency of 1 Hz altering the temperature within the interval −150 to 115°C with the heating rate of 5 K/min.

3.1 Morphological assessment

The effect of polyethylene and the compatibilizing agent insertion into the galleries between the 15A and 20A nanoclay platelets has been depicted in Figure 1.

Figure 1:

WAXS analysis; (A) Comparison between nanoclay and all other samples, (B) the effect of nanoclay and compatibilizer agent, and (C) the effect of compatibilizer and nanoclay combination.

In Figure 1A, an overall view of WAXS results is provided for the selected samples. It can be realized from this figure that the scattering peak related to the pure nanoclay has been eliminated in all samples in the vicinity of 2θ=7 due to the lower nanoclay content in comparison with the peak assigned to OMMT. It can be also observed that the area beneath the OMMT sample diagram is wider compared to the other samples due to the lower amount of nanoclay in the nanocomposites. Moreover, the peak pertinent to the interlayer distance of nanoclay has been decreased in the nanocomposite samples followed by an ascending trend in the direction that polyethylene chains penetrate into the nanoclay platelets. Correspondingly, the scattering peak appeared at around 2θ=2.65 assigned to (P₈₈/C₉/O15A₃) nanocomposite has been shifted to the angle of 2θ=2.37, which signifies a rise in the interlayer gap from 33.34 to 37.28 Å. Nonetheless, such interlayer distance is still higher than that for pure nanoclay.

Figure 1A also suggests that increase of nanoclay content to 5 wt.% in (P₈₈/C₉/O15A₃) sample has slightly shifted the angle at which scattering peak has been recorded. The characteristic which can be detected for all samples is broadening of scattering peaks reflecting suitable dispersion of nanoclay throughout the polymer matrix. In the case of (P₈₈/C₉/O20A₃) sample, the scattering peak is slightly higher than that of (P₈₈/C₉/O15A₃), however in the case of (P₈₈/C₉/O20A₃) it has been nearly disappeared. Figure 1B shows the effect of compatibilizer on the nanocomposites containing 15A and 20A nanoclay. In case of (P₈₈/C₉/O20A₃) sample, the PE-g-MA and nanoparticles should be appropriately interacted in view of enlargement of clay galleries. In Figure 1C, the influence of nanoclay type and interaction between them, polyethylene and PE-g-MA has been illustrated. It can be seen that (P₈₈/C₉/O20A₃) sample appears better than (P₈₈/C₉/O15A₃), while combination of nanoclays in (P₈₈/C₉/O15A_1.5/O20A_1.5) led to no obvious compatibility.

Figure 2 illustrates TEM micrographs for the non-compatibilized polyethylene nanocomposites having 15A and 20A clays and the ones containing PE-g-MA compatibilizer.

Figure 2:

TEM images, (A) and (B) (P₉₇/O15A_1.5/O20A_1.5), and (C) and (D) (P₈₈/C₉/O15A_1.5/O20A_1.5).

It goes without saying that state of dispersion/distribution of nanoclay platelets in the polymer matrix depends on the extent to which clay layers are detached from each other, providing sufficient space for polymer chains to go through. According to Figure 2, an intercalated microstructure can be observed for samples free of compatibilizer (micrographs A and B) suggesting inadequate penetration of HDPE chains into the clay galleries. By contrast, however, images (C) and (D) provided for compatibilized samples prove that addition of PE-g-MA compatibilizer facilitates penetration of macromolecules into the interlayer gap of nanoclay platelets leading to creation of exfoliated structures together with some intercalated morphology. It can be concluded that partial exfoliation in the (P₈₈/C₉/O20A₃) sample takes its origin in improved interaction due to the presence of compatibilizer molecules.

3.2 Thermal properties

Figure 3 provides detailed information on melting and crystallinity behavior of samples, where the melting point follows an inverse trend with respect to crystallinity temperature (Figure 3A). Among studied samples, (P₉₇/O15A₃) has the lowest melting point, while that of (P₈₈/C₉/O15A_1.5/o20A_1.5) takes the highest value. In the (P₉₇/O15A₃) sample, a chain can easily form as spherulitic structures, for O15 sheets pose no serious limitation on movement of polyethylene chains towards formation of lamellar structures. By the contrary, the combined effect of two types of clays in (P₈₈/C₉/O15A_1.5/O20A_1.5) sample can suppress crystal formation, especially at higher loadings. As a result, the required energy for the formation of polymer crystals ceases to increase in the assigned nanocomposite. It can be concluded from the XRD patterns that the P₈₈/C₉/O20A₃ sample has more exfoliated structure in comparison to (P₉₇/O15A₃) sample, which is responsible for increased melting point of this sample. The presence of PE-g-MA compatibilizer in (P₈₈/C₉/O15A_1.5/O20A_1.5) sample caused an abrupt fall in the melting point due to stronger interfaces between components. It can be seen that the samples containing EVOH copolymer represent a relatively low melting point proposing the formation of thinner crystals and lower extent of crystallinity. During the crystallization process, (P₈₈/C₉/O15A_1.5/O20A_1.5) shows a lower crystallinity temperature in comparison to the other samples, a consequence of higher number of crystallites (Figure 3B). In addition, EVOH copolymer involvement has entailed an increase in the crystallinity temperature arising from an entropy rise. It can be concluded that the best area for crystallization in the mentioned samples would be the zone specified in Figure 3A.

Figure 3:

Evaluation of DSC results, (A) the comparison between the melting point and crystallinity temperature, (B) the comparison between the crystallinity (%) and crystallinity temperature, (C) the comparison between the melting point and crystallinity (%) and (D) the comparison between the melting point and crystallinity with the alteration in the crystallinity rate in the selected sample.

The trend of crystallization rate in terms of crystallinity temperature and melting point has been demonstrated in Figure 3C. The crystallization rate shows a direct dependence on the melting point, but an inverse trend against crystallization temperature. Though the (P₈₈/C₉/O15A_1.5/O20A_1.5) sample adheres to this trend, the (P₉₇/O15A_1.5/O20A_1.5) sample shows a different behavior. All in all, samples which have contained EVOH had obviously higher crystallinity temperature and low melting point indicating that EVOH copolymer makes complex detection of crystallinity in this nanocomposite. In Figure 3D, changes in the crystallization rate of composites are compared with that of neat polyethylene (P₁₀₀ sample) as a function of melting point and crystallization temperature. In this figure, (P₉₇/O15A₃), (P₉₇/O20A₃) and (P₈₀/C₁₅/O15A_2.5/O20A_2.5) samples all exhibited higher crystallization rate in comparison to P₁₀₀, whereas the samples which contained EVOH and PE-g-MA compatibilizer possess a lower rate of crystallinity. From the results of thermal behavior we came to the conclusion that addition of compatibilizer enhances interaction between polymer chains and clay, which has been appeared as confined crystallization.

3.3 Dynamic mechanical analysis

Since dynamic mechanical analysis reflects mechanical behavior under dynamic conditions, it provides useful information about the state of interaction between gradients in the composite. Figure 4 shows storage moduli of some selected samples as a function of temperature. According to Figure 4A, addition of nanoclay increases storage modulus with respect to neat polyethylene sample, which was expected on account of higher modulus of mineral filler compared to polymer. Of note, nanoclay 15A has shown a significantly less effect on the modulus compared to 20A. However, the combination of clays in (P₉₇/O15A_1.5/O20A_1.5) sample resulted in a higher storage modulus. It should be admitted that such an effect cannot be understood easily in this work and necessitates detailed characterization, possibly the subject of a future study. Figure 4B makes evident that the presence of PE-g-MA compatibilizer results in a reduction in the storage modulus, at the same time loading up to 5 wt.% has a negative effect on this criterion. The effect of EVOH involvement on the storage modulus is shown in Figure 4C. Accordingly, the more the nanoclay content in the samples having EVOH, the higher the storage modulus. Nonetheless, such quantities are still much lower than those for (P₉₇/O15A₃) and (P₉₇/O20A₃) samples. To explain such a behavior we came on this conclusion that PE-g-MA plays the role one expects from a plasticizing agent, unless EVOH is apt to hold polyethylene chains in contact because of having ethylenic segments. In addition, EVOH has potential to interact with organically modified clay. Such a plasticizing effect has been reinforced at higher contents of clay. We may need further investigations to make this interpretation concrete.

Figure 4:

Storage modulus against temperature; (A) effect of nanoclay type, (B) effect of nanoclay content and compatibilizer, and (C) effect of EVOH addition.

3.4 Rheological behavior assessment

Rheological test results including storage modulus and complex viscosity measured at frequency of 1 Hz are summarized in Table 2. Alteration of storage modulus, complex viscosity, and Tan (δ) have also been monitored in terms of frequency for polyethylene and its nanocomposites filled with 3, 5, and 7 wt.% of nanoclay, which are shown and compared in Figure 5.

Figure 5:

Rheological properties; (A) Storage modulus, (B) Complex Modules and (C) Tan (δ).

Figure 5A and B reveal that storage modulus and complex viscosity data corresponding to nanocomposites are obviously above those of neat polyethylene over the whole frequency range. A comparison between (P₉₇/O15A_1.5/O20A_1.5) and (P₈₈/C₉/O15A_1.5/O20A_1.5) samples suggests that nanocomposite having PE-g-MA compatibilizer have higher storage modulus and complex viscosity values (Table 2), which can be ascribed to better interaction between polyethylene and nanoclay in the compatibilized sample. Moreover, good dispersion of nanoclay in the compatibilized (P₈₈/C₉/O15A_1.5/O20A_1.5) nanocomposite makes this conclusion well-grounded (Figure 2). The increase of nanoclay from 3 to 5 and then 7 wt.% in the compatibilized samples causes a rise in storage moduli and complex viscosities from 48.9 to 114.4, and 199.3, and from 17 to 37.8, and 62.4, respectively. The constrained mobility of polymer chains in the vicinity of clay platelets is believed to be intensified at higher clay contents via formation of a three-dimensional network that resists against chain movement. Among samples (P₈₈/C₉/O15A₃), (P₈₈/C₉/O20A₃), and (P₈₈/C₉/O15A_1.5/O20A_1.5), the second one featured this effect obviously, possibly due to the combined effects of 15A and 20A clays, where the latter possesses more mineral fraction. Table 2 suggests that addition of Cloisite 15A, especially at 3 and 5 wt.% loading levels, has more effects on rheological response of nanocomposites. In case of EVOH incorporated samples, however, such quantities remain almost unaffected by the introduction of clay. This can be interpreted in this manner that the compatibility between HDPE and EVOH macromolecules has not been governed by the clay introduction. A negative slope in Tan (δ) plot illustrated in Figure 5C is representative of viscous liquid-like behavior, while the positive slope is a characteristic of elastic response of the viscoelastic material. Accordingly, addition of clay in the nanocomposite samples changed the behavior of material from a liquid-like to a solid-like, which is consistent with explanations provided earlier in this work on other test results.

3.5 Permeability analysis

Figure 6 shows the results obtained from permeability testing as well as illustrating the performance improvement in the barrierity of the prepared nanocomposites.

Figure 6:

The results of permeability test (A) permeation rate, and (B) performance effectiveness.

To uncover the correlation between permeability and other features of nanocomposites one may need a close view of molecular-scale developments. A comparison between the permeability coefficients of different samples has been represented in Figure 6A. It is evident, except in case of (P₉₁/C₉) and (P₈₈/C₉/O15A_1.5/O20A_1.5) samples, that nanocomposite samples reveal good barrierity against oxygen. The extent to which barrierity has been improved springs from several motives which are not certainly known. It is crystal clear that introduction of compatibilizer increases the polarity of system. This phenomenon fades the interactions between polyethylene and nonpolar oxygen molecules which, in turn, facilitates permeation of oxygen by enlargement of free-volume holes between polyethylene segments. Additionally, spacious anhydride groups which exist in the compatibilizer give rise to increase of the gap between the polymer chains and consequently larger channels for gas transmission are to be acquired. By the contrary, compatibilizer molecules delay the rate of crystallinity of sample. These opposed tendencies, though crystallinity makes permeation of gas molecules difficult, leads to facilitation of oxygen permeation. Thus, the compatibilizer effect overwhelms over the crystallinity effect.

Overall, nanoclay addition enhances barrier properties via increasing the tortuous pathway for the permeating molecules. Certain ambiguities are still in recognition of the contribution of nanoclay and crystalline areas to barrierity enhancement. Although the oxygen permeation rate was expected to go into decline following the introduction of nanoclay platelets, the results show that permeability coefficients are still higher than the value measured for neat polyethylene. It is apparent that addition of 3 wt.% of nanoclays with 1:1 weight ratio was not sufficient to overcome the adverse effect due to compatibilizer. The permeability coefficient of the aforementioned sample is 15.5% lower than that of neat polyethylene. This fall reaches to 19.62% when 5 wt.% nanoclay is used. As it can be concluded from Figure 6B, a decreasing trend of permeation rate takes higher rate at lower clay contents. This indicates that by increasing the nanoclay content in the absence of compatibilizer the interaction between polyethylene and nanoclay should be hindered compared to the case with lower content of nanoclay, hence, the dispersion and distribution of nanoclay should significantly be improved at lower loadings. The sample with the nanoclays combination shows a lower permeability compared to (P₉₇/O15A₃) sample in spite of its lower crystallinity. On the other hand, the comparison between (P₉₁/C₉), (P₈₈/C₉/O15A_1.5/O20A_1.5), and (P₉₇/O15A_1.5/O20A_1.5) samples illustrates that the presence of compatibilizer prevails over nanoclay effect on permeability reduction. The oxygen permeability of (P₉₃/C₂/E₅) sample which contains 5 wt.% of EVOH has decreased by the rate of 17%. EVOH copolymer has a very low permeability against oxygen and acts as a permeability barrier in the polyethylene bulk leading to decrease in oxygen transmission across the specimen (19). Addition of 3 wt.% nanoclays combination to (P₉₃/C₂/E₅) sample, namely the (P₉₀/C₂/E₅/O15A_1.5/O20A_1.5), resulted in decreasing the oxygen permeability from 17 to 20.4 percent. The extent of such a decrease in the oxygen permeability rate by addition of 3 wt.% of nanoclay to the EVOH-contained sample is less than that of EVOH-free sample (20), (21). Moreover, increasing nanoclay content up to 5 wt.% in the (P₈₈/C₂/E₅/O15A_2.5/O20A_2.5) sample is liable for reduction of oxygen permeability down to 26.54% compared to the neat polyethylene. The rate of oxygen permeability reduction in EVOH-contained samples is more severe at higher content of nanoclay, which shows in inverse trend in the case of EVOH-free samples.

Considering the situation of interaction between ingredients in the system was not easy and detailed quantitative analysis of permeability was not possible to do, we attempted to illustrate such a complex system schematically (Figure 7). Morphological assessments provided support for the fact that introduction of PE-g-MA is responsible for formation of exfoliated structures together with some intercalated zones in which clay platelets are not efficiently detached from each other in the absence of compatibilizer molecules (Figure 2). According to the illustration in Figure 7, direction and detachment of clay platelets from each other has been governed by compatibilizer addition to the system. From a molecular perspective, the introduction of such a compatibilizer with a dual character (with tendency towards both polyethylene and clay phases) makes permeation rate of oxygen through HDPE/EVOH/clay nanocomposites dependent on interaction. In other words, nonpolar oxygen molecules are habitually allowed to pass through the prepared nanocomposite sheets in the absence of compatibilizer. Therefore, it is the higher fraction of free volume in the (P₉₁/C₉) sample (Figure 7A) that facilitates permeation of oxygen molecules, whilst strengthening of interface in (P₈₈/C₂/E₅/O15A_2.5/O20A_2.5) sample takes origin in lengthening of tortuous path provided for oxygen permeate, or enhanced interaction between clay, EVOH and HDPE chains (Figure 7B). As a result of this molecular packing, the permeability performance takes higher values in case of compatibilized nanocomposites (Figure 6B). Thus, the extent to which the barrierity of the studied nanocomposites will be enhanced has been outlined by the microscale intermolecular nature of the system.

Figure 7:

Schematic illustration of oxygen permeation through (A) (P₉₁/C₉) and (B) (P₈₈/C₂/E₅/O15A_2.5/O20A_2.5).