3D Hierarchical orientation in polymer–clay nanocomposite films

Abstract

Organically modified clay was used as reinforcement for HDPE using maleated polyethylene (PEMA) as a compatibilizer. The effect of compatibilizer concentration on the orientation of various structural features in the polymer-layered silicate nanocomposite (PLSN) system was studied using two-dimensional (2D) small angle X-ray scattering (SAXS) and 2D wide-angle X-ray scattering (WAXS). The dispersion (repeat period) and three-dimensional (3D) orientations of six different structural features were easily identified:

• clay clusters/tactoids (0.12 μm),

• modified clay (002) (24–31 Å),

• unmodified clay (002) (13 Å),

• clay (110) and (020) planes normal to (b) and (c),

• polymer crystalline lamellae (001) (190–260 Å), and

• polymer unit cell (110) and (200) planes.

A 3D study of the relative orientation of this hierarchical morphology was carried out by measuring three scattering projections for each sample. Quantitative data on the orientation of these structural units in the nanocomposite film is determined through calculation of the major axis direction cosines and through a ternary, direction-cosine plot. Surprisingly, it is the unmodified clay which shows the most intimate relationship with the polymer crystalline lamellae in terms of orientation. Association between clay and polymer lamellae may be related to an observed increase in lamellar thickness in the composite films. Orientation relationships also reveal that the modified clay is associated with large-scale tactoid structures.

Introduction

Organically modified layered silicates have been widely studied for the past decade as property enhancers for polymeric materials. Various studies report improvement in mechanical [1], [2], [3], thermal [4], [5], flammability [4], [5], and barrier [6], [7] properties of thermoplastics by addition of organically modified layered silicates to polymer matrices. These modified thermoplastic systems are called polymer-layered silicate nanocomposites (PLSN). Due to this property enhancement at low filler content (2–6 wt%), PLSN systems have drawn tremendous attention. In general these PLSN systems possess several advantages including; (a) they are lighter in weight compared to conventionally filled polymers due to property enhancement even at small clay loadings; (b) they exhibit outstanding barrier properties without requiring a multi-layered fabrication, allowing for recycling.

PLSN systems are made of two components; the base resin, and a modified layered silicate (clay). A potential third component is a compatibilizer. Modified layered silicates are composed of silicate layers that can intercalate organic polymer chains if appropriate ionic or hydrogen bonding groups are present on the polymer. For example, montmorillonite is a 2:1 type layered silicate and is the most commonly used filler in PLSN systems [8]. 2:1 layered silicates are composed of an octahedral alumina or magnesia sheet sandwiched between two tetrahedral sheets of silica. The silica sheets have Na⁺, Ca²⁺, or K⁺ ions on their surfaces. The combined thickness of the two silica and one alumina or magnesia sheet is about 0.95 nm [8]. The presence of positive ions on the surface of the silica sheets increases the d-spacing in the normal (002) direction of the clay platelet which generally varies from 1.0 to 1.3 nm. The presence of positive ions on the surface also makes the clay platelet hydrophilic and thus incompatible with many polymers. The organophilicity of the clay platelets can be increased by exchange of these ions with organic cations (alkyl ammonium ions) [9], [10]. Ion exchange and surfactant treatment are not absolutely effective in commercially modified clay. Generally, two clay species might result: unmodified clay with small layer spacing on the order of 1 nm, and onium (alkyl ammonium) modified clay with a layer spacing on the order of 1–5 nm depending on the number of carbon atoms in the chain of the onium ion [8]. For the commercially modified montmorillonite studied here, about one-third of the clay remains with unmodified spacing as discussed below. The onium modified clay is thought to retain planar 0.95 nm thick aluminosilicate trilayers. The main evidence for this comes from the existence of a stacking period after intercalation and X-ray reflection from in-plane crystallographic structure. Two diffraction peaks are generally observed from these trilayer structures; (a) a ‘long-period’-like layer spacing, (002), oriented normal to the layer face, and (b) a weaker (110)/(020) combined orthogonal reflection at about 0.44 nm. The 0.44 nm reflections should always orient orthogonal to the layer spacing peak for planar aluminosilicate trilayers [11].

For the greatest property enhancement in PLSN systems it is generally believed that the clay layers should disperse as single trilayer (0.95 nm) platelets throughout the polymer matrix, exfoliation. To attain such dispersion of clay platelets the polymer should first penetrate between the clay platelets. This intercalation is possible if the polymer and the clay surfaces are compatible. Depending on the interaction between the clay and the polymer, and the clay loading [12], [13] different regimes of dispersion are expected. If the polymer just enters between the clay platelets the system is said to be intercalated. Some models predict a nematic phase for the platelets with polymer chains co-planar to the trilayers [12], [13]. Intercalation increases the d-spacing of the clay platelets by around 0.5–1.5 nm, a distance associated with a polymer monolayer [12], [13]. Exfoliated systems are formed when the polymer enters between the clay platelets and force them apart so they no longer interact with each other. This condition is modeled as a low concentration limit for relatively weakly-interacting systems [12], [13].

Recent studies [14], [15], [16] mention that along with exfoliation, orientation of the clay platelets plays a major role in tuning some property enhancements in PLSN systems. For example, the effect of shear on the orientation of the clay platelets and the polymer unit cells in PLSN systems has been studied [11], [17], [18], [19], [20], [21]. In some cases three-dimensional (3D) orientation of clay platelets and unit cells in polymer nanocomposites have been studied [15], [16], [20], [21].

PLSN's are generally composed of a collage of interacting structural features. On the nanoscale, polymer crystallites in the form of lamellar sheets of about 5–10 nm thickness coexist with the 1–2 nm thick clay platelets. Both the clay platelets and the polymer lamellae prefer to stack, especially at high volume fractions [22]. In studies on preferential orientation of clay platelets and polymer crystallites in nylon–clay nanocomposite films, Kojima et al. [20] observed that both clay platelets and polymer crystallites align parallel to the surface of the film and along the flow axis (machine direction). In injection molded nylon–clay nanocomposite samples, Kojima et al. [21] found that the polymer crystallites either align parallel (high shear region) or perpendicular (low shear region) to the clay platelets. Contrary to Kojima's observation, Vaia et al. [11] observed that polymer crystallites align perpendicular to the clay platelets in nylon–clay electrospun fibers where exceedingly high elongational strain rates are expected. Varlot et al. [15] observed that in intercalated nylon–clay nanocomposites, the clay platelets aligned with normals both parallel and perpendicular to the thickness of an injection molded sample consistent with Kojima et al. [21]. Although it is clear that the polymer lamellae align in different directions depending on the type of deformation, the cumulative strain and the strain rate, the relationship of clay platelet orientation to the orientation of other structural units, such as the polymer unit cells and polymer lamellae still remains unclear. Most of literature studies use nylon-6 as the base resin. Nylon, being a polar polymer, disperses clay platelets without addition of compatibilizer. On the other hand polymers like polyethylene (PE) and polypropylene (PP) are non-polar in nature, so are not compatible with modified clays. For these systems, the loading of compatibilizer becomes an important parameter. Orientation of clay platelets and polymer lamellae are expected to depend on clay loading, polymer degree of crystallinity, the enthalpic interaction between the clay surface and polymer, as well as polymer chain flexibility and molecular weight in addition to the accumulated shear strain and rate of strain and type of deformation.

PE is one of the most important commodity polymers with a worldwide consumption of about 44 million metric tons/year [23]. A large portion of the polyethylene produced is consumed in the film market. Due to its low cost, high-density polyethylene (HDPE) is increasingly finding acceptance as a wrapping material for food products. HDPE is known to have poor barrier properties for gases, organic solvents and hydrocarbons [24], [25]. Reports on enhancement of barrier, mechanical and thermal properties on addition of clay to a polymer have opened new fields of research in the polyolefin industry. As mentioned earlier, polyolefins being non-polar show poor compatibility with modified clays. Various authors [1], [26], [27], [28] reported on the dispersion of clay platelets in polyolefins by addition of a compatibilizer such as maleated polypropylene or maleated polyethylene. The compatibilizer is generally believed to first enter between the clay platelets, separate the clay trilayers and increase the gallery height facilitating intercalation of the non-polar polymer. This could be advantageous as strong clay/polymer interactions are believed to favor intercalation over exfoliation [12]. Although previous studies showed the effect of compatibilizer on property enhancement in polyolefin nanocomposites, the literature lacks a clear picture of the effect of compatibilizer on the orientation/dispersion of the clay platelets and the effect of this clay orientation on the orientation of other structural units such as polymer unit cells and polymer lamellae. Moreover, all the orientation studies mentioned above provide only qualitative data on orientation and little quantitative data are available in the literature concerning the relative orientation of structures in these systems.

In this study two HDPE–clay nanocomposite films cast from the melt (that is extruded from a coat-hanger die to form a film) were investigated. Organically modified clay was used as reinforcement while maleated polyethylene (PEMA) was used as compatibilizer. Various studies [1], [3], [4], [5], [6] report improvement in properties of polymers by addition of 2.5–5% organically modified layered silicates to polymer matrices. Oya et al. [3] observed that the intensity of the diffraction peak from clay was weak due to lower content of montmorillonite (3%) in the nanocomposite. In our earlier studies (not published) the properties of polyethylene nanocomposites were found to monotonically increase with increasing clay loading up to 6–8 wt%. Thus in order to enhance scattering from clay in both small angle X-ray scattering (SAXS) and WAXS, the concentration of clay was chosen to be 6% by weight in this study. The concentration of clay was kept constant for two films while the compatibilizer concentration was varied. The effect of compatibilizer concentration on the orientation of various structures in the PLSN system was studied using two-dimensional (2D) SAXS and 2D wide-angle X-ray scattering (WAXS) in three sample/camera orientations. Reflections and orientation of six different structural features were easily identified:

• clay clusters/tactoids (0.12 μm),

• modified/intercalated clay stacking period (002) (24–31 Å),

• stacking period of unmodified clay platelets (002) (13 Å),

• clay (110) and (020) planes, normal to (b) and (c),

• polymer crystalline lamellae (001) (190–260 Å), long period¹ and

• polymer unit cell (110) and (200) planes.

The corresponding reflections are identified in Fig. 2 as discussed below. A 3D study of the relative orientation of all the above mentioned structures was carried out by measuring three projections for each sample. Quantitative data on the orientation of these structural units in the nanocomposite film is determined through calculations of the major axis direction cosines and through a ternary, direction-cosine plot called a ‘Wilchinsky triangle’ [29], [30], [31], [32] previously proposed in lamellar orientation studies [30]. It allows a direct comparison of average preferred orientation for different structural features. In this way it is conceptually more useful than stereographic projections involving orientation density maps for a single X-ray reflection, pole figure.