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13 Polymeric Nanoclay Composites Hamid Dalir, Rouhollah D. Farahani, Martin Lévesque and Daniel Therriault École Polytechnique de Montréal, Canada 1. Introduction Traditionally, polymeric materials have been filled with synthetic or natural inorganic compounds in order to improve their properties, or simply to reduce cost. Conventional fillers are materials in the form of particles (e.g. calcium carbonate), fibers (e.g. glass fibers) or plate-shaped particles (e.g. mica). However, although conventionally filled or reinforced polymeric materials are widely used in various fields, it is often reported that the addition of these fillers imparts drawbacks to the resulting materials, such as weight increase, brittleness and opacity (Alexandre & Dubois, 2000; Fischer, 2003; Lagaly, 1999; Giannelis, 1996; Varlot et al., 2001). Nanocomposites, on the other hand, are a new class of composites, for which at least one dimension of the dispersed particles is in the nanometer range. Depending on how many dimensions are in the nanometer range, one can distinguish isodimensional nanoparticles when the three dimensions are on the order of nanometers, nanotubes or whiskers when two dimensions are on the nanometer scale and the third is larger, thus forming an elongated structure, and, finally, layered crystals or clays, present in the form of sheets of one to a few nanometers thick and hundreds to thousands nanometers in extent (Alexandre & Dubois, 2000; Fischer, 2003; Lagaly, 1999; Giannelis, 1996). Among all the potential nanocomposite precursors, those based on clay and layered silicates have been most widely investigated, probably because the starting clay materials are easily available and because their intercalation chemistry has been studied for a long time (Gorrasi et al., 2002). Polymer-layered silicate nanocomposites, which are the subject of the present contribution, are prepared by incorporating finely dispersed layered silicate materials in a polymer matrix (Fischer, 2003). However, the nanolayers are not easily dispersed in most polymers due to their preferred face to face stacking in agglomerated tactoids. Dispersion of the tactoids into discrete monolayers is further hindered by the intrinsic incompatibility of hydrophilic layered silicates and hydrophobic engineering plastics. Therefore, layered silicates first need to be organically modified to produce polymer-compatible clay (organoclay). In fact, it has been well-demonstrated that the replacement of the inorganic exchange cations in the cavities or “galleries” of the native clay silicate structure by alkylammonium surfactants can compatibilize the surface chemistry of the clay and a hydrophobic polymer matrix (LeBaron et al., 1999). Thereafter, different approaches can be applied to incorporate the ion-exchanged layered silicates in polymer hosts by in situ polymerization, solution intercalation or simple melt mixing. In any case, nanoparticles are added to the matrix or matrix precursors as 1-100 µm 290 Advances in Diverse Industrial Applications of Nanocomposites powders, containing associated nanoparticles. Engineering the correct interfacial chemistry between nanoparticles and the polymer host, as described previously, is critical but not sufficient to transform the micron-scale compositional heterogeneity of the initial powder into nanoscale homogenization of nanoparticles within a polymeric nanocomposite (Vaia & Wagner, 2004). Therefore, appropriate conditions have to be established during the nanocomposite preparation stage. The resulting polymer-layered silicates hybrids possess unique properties - typically not shared by their more conventional microscopic counterparts - which are attributed to their nanometer size features and the extraordinarily high surface area of the dispersed clay (Alexandre & Dubois, 2000; Fischer, 2003; Lagaly, 1999; Giannelis, 1996). In fact, it is well established that dramatic improvements in physical properties, such as tensile strength and modulus, heat distortion temperature (HDT) and gas permeability, can be achieved by adding just a small fraction of clay to a polymer matrix, without impairing the optical homogeneity of the material. Most notable are the unexpected properties obtained from the addition of stiff filler to a polymer matrix, e.g. the often reported retention (or even improvement) of the impact strength. Since the weight fraction of the inorganic additive is typically below 10%, the materials are also lighter than most conventional composites (Fischer, 2003; Ginzburg et al., 2000; Osman et al., 2004; Balazs et al., 1999; Lincoln et al., 2001). These unique properties make the nanocomposites ideal materials for products ranging from high-barrier packaging for food and electronics to strong, heat-resistant automotive components (Balazs et al., 1999). Additionally, polymer-layered silicate nanocomposites have been proposed as model systems to examine polymer structure and dynamics in confined environments (Lincoln et al., 2001; Vaia & Giannelis, 2001). However, despite the recent progress in polymer nanocomposite technology, there are many fundamental questions that have not been answered. For example, how do changes in polymer crystalline structure induced by the clay affect overall composite properties? How does one tailor organoclay chemistry to achieve high degrees of exfoliation reproducibility for a given polymer system? How do process parameters and fabrication affect composite properties? Further research is needed that addresses such issues (Fornes et al., 2001). The objective of this work is to review recent scientific and technological advances in the field of polymer-layered silicate nanocomposite materials and to develop a better understanding of how superior nanocomposites are formed. 2. Nanoclay 2.1 Geometry and structure Layered silicates used in the synthesis of nanocomposites are natural or synthetic minerals, consisting of very thin layers that are usually bound together with counter-ions. Their basic building blocks are tetrahedral sheets in which silicon is surrounded by four oxygen atoms, and octahedral sheets in which a metal like aluminum is surrounded by eight oxygen atoms. Therefore, in 1:1 layered structures (e.g. in kaolinite) a tetrahedral sheet is fused with an octahedral sheet, whereby the oxygen atoms are shared (Miranda & Coles, 2003). On the other hand, the crystal lattice of 2:1 layered silicates (or 2:1 phyllosilicates), consists of two-dimensional layers where a central octahedral sheet of alumina is fused to two external silica tetrahedra by the tip, so that the oxygen ions of the octahedral sheet also belong to the tetrahedral sheets, as shown in Fig. 1. The layer thickness is around 1 nm and the lateral dimensions may vary from 300 Å to several microns, and even larger, depending Polymeric Nanoclay Composites 291 Fig. 1. The structure of a 2:1 layered silicate (Beyer et al., 2002). Reproduced from Beyer by permission of Elsevier Science Ltd., UK. on the particulate silicate, the source of the clay and the method of preparation (e.g. clays prepared by milling typically have lateral platelet dimensions of approximately 0.1-1.0 µm). Therefore, the aspect ratio of these layers (ratio length/thickness) is particularly high, with values greater than 1000 (Beyer et al., 2002; McNally et al., 2003; Solomon et al., 2001). Analysis of layered silicates has shown that there are several levels of organization within the clay minerals. The smallest particles, primary particles, are on the order of 10 nm and are composed of stacks of parallel lamellae. Micro-aggregates are formed by lateral joining of several primary particles, and aggregates are composed of several primary particles and micro-aggregates (Ishida et al., 2000). 2.2 Surface modification as a compatibilizer Since, in their pristine state layered silicates are only miscible with hydrophilic polymers, such as poly(ethylene oxide) and poly(vinyl alcohol), in order to render them miscible with other polymers, one must exchange the alkali counter-ions with a cationic-organic surfactant. Alkylammonium ions are mostly used, although other “onium” salts can be used, such as sulfonium and phosphonium (Manias et al., 2001; Zanetti et al., 2000). This can be readily achieved through ion-exchange reactions that render the clay organophilic (Kornmann et al., 2001). In order to obtain the exchange of the onium ions with the cations in the galleries, water swelling of the silicate is needed. For this reason alkalications are preferred in the galleries because 2-valent and higher valent cations prevent swelling by 292 Advances in Diverse Industrial Applications of Nanocomposites water. Indeed, the hydrate formation of monovalent intergallery cations is the driving force for water swelling. Natural clays may contain divalent cations such as calcium and require exchange procedures with sodium prior to further treatment with onium salts (Zanetti et al., 2000). The alkali cations, as they are not structural, can be easily replaced by other positively charged atoms or molecules, and thus are called exchangeable cations (Xie et al., 2001). The organic cations lower the surface energy of the silicate surface and improve wetting with the polymer matrix (Giannelis, 1996; Kornmann et al., 2001). Moreover, the long organic chains of such surfactants, with positively charged ends, are tethered to the surface of the negatively charged silicate layers, resulting in an increase of the gallery height (Kim et al., 2001). It then becomes possible for organic species (i.e. polymers or prepolymers) to diffuse between the layers and eventually separate them (Kornmann et al., 2001; Zerda et al., 2001). Sometimes, the alkylammonium cations may even provide functional groups that can react with the polymer or initiate polymerization of monomers. The microchemical environment in the galleries is, therefore, appropriate to the intercalation of polymer molecules (Huang et al., 2001). Conclusively, the surface modification both increases the basal spacing of clays and serves as a compatibilizer between the hydrophilic clay and the hydrophobic polymer (Zerda et al., 2001). There are two particular characteristics of layered silicates that are exploited in polymer-layered silicate nanocomposites. The first is the ability of the silicate particles to disperse into individual layers. Since dispersing a layered silicate can be pictured like opening a book, an aspect ratio as high as 1000 for fully dispersed individual layers can be obtained (contrast that to an aspect ratio of about 10 for undispersed or poorly dispersed particles). The second characteristic is the ability to fine-tune their surface chemistry through ion exchange reactions with organic and inorganic cations. These two characteristics are, of course, interrelated since the degree of dispersion in a given matrix that, in turn, determines aspect ratio, depends on the interlayer cation (Giannelis, 1996; Ishida et al., 2000). 3. Nanocomposite 3.1 Structural phases Any physical mixture of a polymer and silicate (or inorganic material in general) does not necessarily form a nanocomposite. The situation is analogous to polymer blends. In most cases, separation into discrete phases normally takes place. In immiscible systems, the poor physical attraction between the organic and the inorganic components leads to relatively poor mechanical properties. Furthermore, particle agglomeration tends to reduce strength and produce weaker materials (Giannelis, 1996). Thus, when the polymer is unable to intercalate between the silicate sheets, a phase-separated composite is obtained, whose properties are in the same range as for traditional microcomposites (Alexandre & Dubois, 2000; Beyer et al., 2002). Beyond this traditional class of polymer-filler composites, two types of nanocomposites can be obtained, depending on the preparation method and the nature of the components used, including polymer matrix, layered silicate and organic cation (Alexandre & Dubois, 2000; Beyer et al., 2002). These two types of polymer-layered silicate nanocomposites are depicted in Fig. 2 (McGlashan et al., 2003). Intercalated structures are formed when a single (or sometimes more) extended polymer chain is intercalated between the silicate layers. The result is a well ordered multilayer structure of alternating polymeric and inorganic layers, with a repeat distance between Polymeric Nanoclay Composites 293 Fig. 2. Types of composite structure of polymer-layered silicate clay materials (McGlashan et al., 2003). Reproduced from McGlashan et al., by permission of John Wiley & Sons, Inc., US. them. Intercalation causes less than 20-30 Å separation between the platelets (Beyer et al., 2002; Kim et al., 2001; Dennis et al., 2001). On the other hand, exfoliated or delaminated structures are obtained when the clay layers are well separated from one another and individually dispersed in the continuous polymer matrix (Kim et al., 2001). In this case, the polymer separates the clay platelets by 80-100 Å or more (Dennis et al., 2001). It is not easy to achieve complete exfoliation of clays and, indeed with few exceptions, the majority of the polymer nanocomposites reported in the literature were found to have intercalated or mixed intercalated-exfoliated nanostructures. This is because the silicate layers are highly anisotropic, with lateral dimensions ranging from 100 to 1000 nm, and even when separated by large distances (i.e. when delaminated) cannot be placed completely randomly in the sea of polymer. Furthermore, the majority of the polymer chains in the hybrids are tethered to the surface of the silicate layers. Thus, it can be expected that there are domains in these materials, even above the melting temperature of the constituent polymers, wherein some long-range order is preserved and the silicate layers are oriented in some preferred direction. 3.2 Morphological characterization Two complementary techniques are generally used to characterize the structures of nanocomposites: X-ray differaction (XRD) and transmission electron microscopy (TEM) (Alexandre & Dubois, 2000; Huang et al., 2001; Porter et al., 2003; Ma et al., 2003). ... - tailieumienphi.vn
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