A polymer nanocomposite is a composite material comprising a polymer matrix and an inorganic dispersive phase that has least one dimension that is nanometric in scale. Due to the large aspect ratio of the inorganic layers, large polymer–clay surface interactions can occur, which enables the improvement in the properties.

In traditional polymer composites, the addition of large micrometric inorganic loads (between 10 and 40% weight concentration) can significantly increase the mechanical properties of the polymers. These properties can also be increased by using loads with high aspect ratios (such as glass and carbon fiber). The higher the aspect ratio of the inorganic load is, the greater the contact area with the polymer matrix is, which may increase the reinforcement by transferring a higher voltage from the matrix to the inorganic load. Therefore, the addition of a nanometric material with a high aspect ratio and a high stiffness to a polymer matrix may improve the performance of the polymer even more. Studies of this type of composite have been reported in the literature since 1950. However, since 1990, more attention has been given to nanocomposites because researchers at Toyota presented a study of the use of polyamide 6 (PA6) with organophilic clay in the construction of timing belts for motorized vehicles. The addition of clay at 5% by weight improved the characteristics of this material significantly when compared to unmodified PA6 resin [1].

Since then, different polymer matrices and inorganic loads have been combined to obtain new materials with different properties. Among the more frequently used inorganic loads, one can highlight laminar silicates [1–6], metal nanoparticles [7], carbon nanotubes [8,9], silica [10], calcium carbonate [11], and zinc oxide [12]. This chapter discusses the production, morphology, and properties of nanocomposites of polymer matrices with lamellar clays.

Naturally, clay minerals are hydrophilic and can be dispersed in water to form thixotropic gels. To make these clay minerals compatible with hydrophobic polymers such as polyolefins, modifications of the clay surface are performed by exchanging hydrated cations with organic cations, decreasing the surface energy of the clay and increasing the distance between the layers [13]. The vast majority of organophilic clays are obtained from smectite clays; montmorillonite is the most common due to intrinsic characteristics of this clay mineral, which include the small size of its crystals and its high cation exchange capacity (CEC), which is used to characterize the degree of isomorphous substitution [1,14]. The CEC of smectites varies from 80 to 150 meq/100 g, which is much higher than those of other clay minerals (which are smaller than 40 meq/100 g).

In addition to the salt used as the organic treatment agent in clay minerals, the form in which this substitution is performed affects the formation of nanocomposites. One of the methods most frequently used to introduce alkylammonium ions between the layers is the ion exchange reaction [14]. This reaction consists of forming, in solution, the desired ion by dissolving the amine with a strong acid or with a salt that has a long alkyl chain containing atoms bonded to counterions, such as chloride and bromide, in hot water at approximately 80°C. These solutions are poured into a montmorillonite dispersion in hot water. A mixer is used to precipitate the particles, which are collected, washed, and then dried. During the drying process, the particles pile up again [14].

The surface treatment is important because it not only makes the clay organophilic and improves the characteristics of its interactions with nonpolar polymers but also increases the distance between the layers. In fact, surface treatment is used even when the polymers are polar and polarity modification of the clay is not fundamental to the production of nanocomposites.

By chemically modifying the surface of the clay, the organic salts allow a favorable penetration of the polymer precursors into the interlamellar regions. The role of the organic salt in the process of clay delamination depends on its chemical nature as well as the length and polarity of its chains [16].

After the separation of the clay lamellae, it is expected that polymer chains bond physically or chemically to their surfaces to form interfaces that are strong enough to maintain their bonds under high strains and flexible enough to allow the transfer of these strains from the polymer matrix to the clay lamellae.

Depending on the nature of the components used (clay, an organic modifier of the clay, a polymer matrix, and a compatibilizing agent) and on the preparation method, it is possible to obtain composites with the three main types of structure shown in Fig. 7.2.

When the polymer is not able to intercalate between the silicate layers, the structure formed is similar to that of a conventional composite, shown in Fig. 7.2A, which provides little or no improvement of the properties. The second structure, which occurs in intercalated nanocomposites, is formed when one or more extended polymer chains are intercalated between the clay lamellae, which increases the distance between them while preserving the lamellar organization, as shown in Fig. 7.2B. In a third case, that of exfoliated nanocomposites, the clay lamellae are completely separated and dispersed, and the system does not present any order, as shown in Fig. 7.2C [1].

Structural characterization of polymer nanocomposites with lamellar clay is usually performed using two main complementary techniques: high-angle X-ray diffraction (XRD) and transmission electron microscopy (TEM). The intercalation and/or exfoliation process can be verified through XRD by observing the shift of the (0 0 1) diffraction peak of clay for values smaller than 2θ (for intercalation) or the absence of this peak (for exfoliation), which implies a loss of the structural regularity of the clay layers. The dispersion state and the distribution of the inorganic load in the polymer matrix are observed using TEM.

When a polymer is intercalated in solution, the organically modified clay and the polymer are dispersed in an organic polar solvent. The silicates in layers can be easily dispersed in an appropriate solvent. The polymer dissolves in the solvent and then adsorbs to the layers of expanded silicates. When the solvent evaporates, the layers regroup and form an intercalated structure [17,18]. The selection of an appropriate solvent is a primary criterion for obtaining the desired level of exfoliation of an organophilic clay.

In situ polymerization involves the insertion of an appropriate monomer into the clay galleries before the polymerization process. The silicate layers are “swollen” by the liquid monomer (or a solution of the monomer) to ensure that the polymer forms between the intercalated layers. Polymerization can be initiated by heat, radiation, or the diffusion of an appropriate initiator, such as an organic initiator or a fixed catalyzer, through cation exchange [1,15].

When nanocomposites are obtained by melted intercalation, the clay is directly dispersed in the melted polymer. During mixing in the melted state, the strain that the polymer exerts on the clay depends on its molecular weight. High levels of shear stress help reduce the size of the clay particles, which aids the intercalation and/or exfoliation process. The mechanism proposed for the action of the shear flow during the exfoliation of organically modified clay in melted intercalation is shown in Fig. 7.3. Initially, particles break down and form piles (tactoids) that disperse through the matrix, as shown in Fig. 7.3A. The transfer of strain from the polymer to these tactoids leads to stronger shearing, which breaks these tactoids into smaller piles (Fig. 7.3B). Finally, the individual layers are separated through a combination of shearing and diffusion of the polymer chains in the galleries; this step depends fundamentally on time and on the chemical affinity between the polymer and the clay (Fig. 7.3C) [19].

Thermodynamic models [20–22] have been used to explain the formation of nanocomposites by melted intercalation. Vaia and Giannelis [22] showed that this process occurs as a result of the concurrent action of entropic and enthalpic changes. The main factors contributing to changes in the free energy during the formation of the nanocomposite are the confinement of polymer chains between the clay layers, changes in the conformation of the surfactant molecules, and the appearance of new molecular interactions between the polymer, the surfactant, and the surface of the clay lamellae. The global decrease in the entropy of the system due to the confinement of polymer chains in silicate galleries can be compensated for by increasing the conformational freedom of the surfactant molecules and by separating the clay lamellae with a less confining medium. Therefore, these two opposing effects make the total entropic variation of the system small and negative, that is, the formation of a nanocomposite is entropically unfavorable. Therefore, the intercalation of the polymer must be managed, mostly by enthalpic variations.

The enthalpy of the mixture can be assumed to be favorable because the magnitude and number of interactions between the polymer chains and the surfactant molecules fixed on the surfaces of the clay lamellae are maximized when both contain polar groups. Therefore, intercalated nanocomposites can be obtained in systems that feature weak interactions between polymers and clays, and exfoliated nanocomposites can be produced by systems that feature strong interactions between polymers and clays, which exfoliates the layers. For nonpolar polymers, direct intercalation with clay lamellae is difficult and requires the use of compatibilizers.

Two main and joint strategies are currently used to enable chemical and physical interactions between the components of nanocomposites with nonpolar matrices. The first consists of making the surfaces of the inorganic particles organophilic, and the second consists of incorporating a comonomer with hydrophilic characteristics into the polymer chains.

Hydrophilic units can be directly inserted into the polymer chain in the matrix using copolymerization techniques in the presence of monomers and using reactive extrusion in the presence of polymers grafted with maleic anhydride, acrylic acid, or another functional group. An ideal compatibilizing agent between two noncompatible components must have parts that combine thermodynamically with both components. Surfactants provide these functions only partially because their ionic parts interact favorably with the surfaces of the clay particles. The long branches of alkyl groups, however, exhibit only limited compatibility with polymer chains. More efficient compatibilization can be obtained by using macrosurfactants, such as block or grafted copolymers, as shown in Fig. 7.4, to combine blocks that can react with the surface of a solid particle and with the polymer matrix and then improve the interaction between the surface of the clay and the polymer chains [20].

However, obtaining exfoliated nanocomposites from polyolefins and clay is still a significant challenge because polyolefins are hydrophobic and do not interact appropriately with the polar surface of the clay. The surfaces of clays are treated with quaternary ammonium salts to make them hydrophobic. For PE and PP, organophilization of the clay is not enough to guarantee the formation of exfoliated nanocomposites, and therefore, it is necessary to use compatibilizing agents, which are more commonly added during in situ polymerization and melted intercalation.

Different routes of compatibilization and/or types of compatibilizing agents used in the formation of nanocomposites with intercalated/exfoliated structures have been reported in the literature. Among the compatibilizing agents that are most common and most appropriate for nanocomposites are polyolefins grafted with maleic anhydride. They increase the chemical and structural affinity of the clay for the polymer matrix and promote significant improvement of mechanical and barrier properties in addition to being able to mix with polymer matrices, as shown in the following sections.

Two large groups of polymer matrices can be used to prepare these nanocomposites, polar matrices, and nonpolar matrices, and it is possible to combine materials from the two groups. The nanocomposite of PA6 and organophilic lamellar clay is an excellent example of how the polarity of the polymer matrix affects the interactions between the polymer and the clay, in addition to modifying the morphology, and the thermal and mechanical properties of the nanocomposite, which can be compared to those of pure PA6. For nonpolar matrices, such as polyolefins, obtaining exfoliated nanocomposites is more difficult because polyolefins are hydrophobic and there is a lack of appropriate interactions with the polar surface of aluminosilicate clay. The compatibility of inorganic loads with polymer matrices is a big challenge in the preparation of nanocomposites that can be improved by chemically modifying the surfaces of the particles of the components and by adding compatibilizing agents between the surfactant and the polymer matrix. The strengthening promoted by the clay is determined by the efficiency of the compatibilization and the restricted mobility of the polymer chains that are in contact with the clay lamellae.