Microcomposites, phase-separated

When the polymer is unable to intercalate between the lamella (for example, in silicate sheets) a phase-separated (aggregated) composite is obtained, whose properties are in the same range as for traditional microcomposites. The two types of lamellar PNCs are depicted in Fig. 1. 

Figure 6.5 Schematic representation of (a) phase-separated microcomposite, (b) intercalated nanocomposite, and (c) exfoliated nanocomposite [15].

Figure 8.9 Schematic representation of different types of polymeric composites, (a) microcomposite with separated phases, (b) nanocomposite presenting intercalated phases 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. 

The dispersion state of a typical phyllosilicate (except sepiolite and halloysite) in a matrix polymer depends on the preparation conditions and the matrix-nanolayer affinity. This effect determines the structure of the resulting composites, which can be either phase separated composites (microcomposites), intercalated nanocomposites, or exfoliated nanocomposites (Alexandre and Dubois 2000). 

Phase-separated microcomposites are formed when the polymer is unable to intercalate with the silicate layers. This can also be called an immiscible system, which normally would not be regarded as a nanocomposite. 

A layered particle-reinforced bionanocomposite, also known as a layered polymer nanocomposite (LPN), can be classified into three subcategories depending on how the particles are dispersed in the matrix. Intercalated nanocomposites are produced when polymer chains are intercalated between sheets of the layered nanoparticles, whereas exfoliated nanocomposites are obtained when there is separation of individual layers, and flocculated or phase-separated nanocomposites are produced when there is no separation between the layers due to particle-particle interactions. This last class of composites is often named microcomposites as the individual laminas do not separate, thus acting as microparticles dispersed in the polymeric matrix. Their mechanical and physical properties are poorer than exfoliated and intercalated nanocomposites [17, 20, 21, 36, 37]. Figure 11.1 shows a schematic drawing of the structure of layered nanocomposites. 

If the polymer is unable to intercalate into the galleries, a phase-separated composite is formed, whose properties are similar to that of traditional microcomposites the poor interaction between the organic and the inorganic... 

PC nanocomposites are an emerging class of organic-inorganic hybrids that contain a relatively low wt% of nanometer-sized clay. These were first developed in the late 1980s. The dispersion of the nanometer-sized clay in the polymer matrix significantly improves the mechanical, thermal, barrier properties and flame retardancy of the base polymer. Three main types of nanocomposites can be obtained when clay is dispersed in a polymer matrix. This depends on the nature of the components used, including polymer matrix, clay and organic cation. If the polymer cannot intercalate between the silicate sheets, a microcomposite is obtained. The phase-separated composite that is obtained has the same properties as traditional microcomposites. 

Depending on the types of clay and polymer, their physiochemical interactions, and the melt-mixing conditions, clays may not be fully dispersed, forming an agglomerated tactoid. In this case, clays are dispersed as a phase segregated and separated from the polymer matrix, and the properties of the composites are in the same range as in traditional microcomposites [1]. 

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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