Microcomposite, layered silicates

As illustrated in Fig. 1, layered silicate composite structures fall into three different classes (a) microcomposites with no interaction between the clay galleries and the polymer, (b) intercalated nanocomposites, where the silicate is well-dispersed in a polymer matrix with polymer chains inserted into the galleries between the parallel, sihcate platelets, and (c) exfohated nano composites with fully separated silicate platelets individually dispersed or delaminated within the polymer matrix [12]. However, these terms describe only ideal cases and most observed morphologies fall between the extremes. A more detailed nomenclature will be presented later in this review. 

Fig.1 Schematic illustration of different possible structures of layered silicate polymer composite (a) microcomposite (b) intercalated nanocomposite (c) exfoliated nanocomposite [12]...

Clays are strongly hydrophilic in nature, making their dispersion in organic matrices difficult [26]. There are usually three possible arrangements of these layered silicate clays, which can be obtained when they are dispersed in a polymer matrix. If the polymer cannot intercalate between the silicate sheets, a non-intercalated microcomposite is obtained. Beyond this traditional class of polymer-filler composites, two other types of composites can be obtained. An intercalated structure is one in which the separation of clay layers occurs to some extent by increasing the interlayer spacing. 

Figure 27.8 Different types of composites arising from the interaction of layered silicates, pol5miers (a) phase-separated microcomposite (b) intercalated nanocomposite and (c) exfoliated nanocomposite.

Depending on the nature of the components used (layered silicate, organic cation, and polymer matrix) and the preparation method, three types of hybrid PCNs can be obtained [17]. Phase-separated microcomposites (conventional composites) are obtained when the polymer chains are unable to intercalate within the inorganic sheets clay lamellae remain stacked in structures marked as tactoids as in the pristine mineral. Otherwise, when the polymer chains penetrate in between the clay galleries, an intercalative system is obtained. In this case, the nanocomposite shows, at least in principle, a well-ordered multilayer morphology built up with alternating polymeric and clay layers. When clay platelets are randomly dispersed in the polymer matrix and the lamellae are far apart from each other, so that the periodicity of this platelet arrangement is totally lost, an exfoliated structure is achieved. 

Fig. 3 Composite stmctures obtained using layered silicate (a) conventional composite or microcomposite,...

Based on the interaction between the polymer matrix and layered silicate polymer layered silicate nanocomposites are classified into intercalated and exfoliated nanocomposites [56-58]. Figure 7 represents the intercalated and exfoliated nanocomposites along with the conventionally filled microcomposite. 

Fig. 7 (a) Conventionally filled polymer or microcomposite, (b) polymer chains intercalated into the clay layers, (c) exfoliation of the layered silicate... 

The XRD patterns for various possible arrangements of layered silicates in biopolymers are shown in Figure 1. For an itmniscible arrangement (microcomposite) of layered clays in a biopolymer matrix, the structure of the layered silicate in the composite is not affected. Thus, XRD pattern for the microcomposite should remain same as that obtained for the pure layered silicate. Intercalation of the polymer... 

Toughness reduction was also noticed when layered silicate was incorporated in PS. In the related study PS/layered silicate micro- and nanocomposites were produced. Microcomposite was generated by direct melt blending of PS with the silicate. Intercalated nanocomposites were, however, achieved when the pristine layered silicate was swollen in water before adding the related slurry in... 

In other studies [307, 308], this same group produced bionanocomposites by melt intercalation of PCL and MMT modified by various alkylammonium cations. Depending on whether the ammonium cations contain nonfunctional alkyl chains or chains terminated by carboxylic acid or hydroxyl functions, microcomposites or nanocomposites were produced. The layered silicate PCL nanocomposites exhibited some improvement in mechanical properties and increased thermal stabihty as well as enhanced flame retardancy. The authors concluded that formation of PCL-based nanocomposites, not only depended on the nature of the ammonium cation and its functionaHty, but also on the selected synthetic route, that is, melt intercalation versus in situ intercalative polymerization. 

In contrast with the tactoid structure predominating in microcomposites (conventional composites), in which the polymer and the clay tactoids remain immiscible, resulting in agglomeration of the clay in the matrix and poor macroscopic properties of the material (Alexandre et al., 2009 Luduena, Alvarez, and Vasquez, 2007), the interaction between layered silicates and polymer chains may produce two types of ideal nanoscale composites. The properties of the resulting material are dependent on the state of the nanoclay in the nanocomposite, that is, if it is exfoliate or intercalate. Intercalation is the state in which polymer chains are present between the clay layers, resulting in a multilayered structure with alternating polymer/inorganic... 

The main limitations of these biodegradable polymers towards their wider application are their relatively low thermal and mechanical resistance and limited gas barrier properties, which limit their access to certain industrial sectors, such as food packaging, in which their use would be justified when biodegradability is required. Nevertheless, the above drawbacks could be overcome by enhancing their properties through the use of filler and/or additives. In the last two decades, the addition of nanofillers to polymers has attracted great attention for the potentiality of these materials to improve a high number of polymer properties for example, polymer layered silicate nanocomposites, because of the nanometer size of the silicate sheets, exhibit, even at low filler content (1-5 wt%), markedly improved mechanical, thermal, barrier and flame retardant properties, in comparison to the unfilled matrix and to the more conventional microcomposites. ... 

The preparation of nanocomposites by melt intercalation is a very attractive environmentally friendly process since no solvent is required. In this method, the polymer matrix is blended in the molten state with a known amount of layered silicate. Under these conditions, if the layered surfaces are sufficiently compatible with the chosen matrix, the polymer can crawl into the interlayer space and forms an intercalated nanocomposite. In the case of poor compatibility between the silicate host and the polymer matrix, polymer intercalation is not allowed and micro-size clay particles are randomly dispersed in the matrix, forming a microcomposite. 

Similarly to PCL layered silicate nanocomposites, melt intercalation of plasticized PLA with a constant amount of nanoclays (3 wt%) leads to an intercalated nanostructure , even for the unmodified natural montmorillonite (Cloisite Na. This particularity can be explained by the sole intercalation of the plasticizer (PEG chains) into the interlayer spacing of the filler, leading to an increase of the interlayer distance from 1.21 to 1.77 nm, as already observed for simple blends of natural montmorillonite with PEG alone. Selective PEG intercalation was further confirmed by the impossibility to form a nanocomposite by melt blending (non plasticized) PLA with Cloisite Na, only microcomposites could be recovered. XRD analysis performed on organoclay based blends (Figure 8) does not allow... 

Polymers filled with layered silicates, in comparison with unfilled polymers or common microcomposites, have shown improvements in several properties. The incorporation of nanolayers of high aspect ratio at very low volume fractions can improve the tensile modulus, strength, gas barrier properties, heat resistance, and decrease flammability of polymer nanocomposites. Thus, one should expect a reduction in thickness of commercial packaging containers and improvement in barrier properties by incorporation of nanoplatelets [1-3]. 

Filled polymer systems of industrial importance, e.g., filled rubber compounds, filled thermoplastics are thus meso or microcomposites, possibly with a structuration (of the dispersed phase) at the nano or meso scale. Whilst no sizeable commercial application yet exist for nanocomposites rubbers or thermoplastics (to the author s knowledge), considerable research has been made since 1984 with so-called ex-foliated layered silicate "nano-clays." Exfoliation means that individual clay sheets, of around 1 nm thickness, have been separated and adequately dispersed in the matrix. Some reinforcement has indeed been demonstrated with such exfoliated nanoparticles but, generally with very specific rubber systems and/or at a cost of preparation that is hardly compatible with reasonable chances of commercialization. 

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