Nano-galleries

Since the possibility of direct melt intercalation was first demonstrated [11], melt intercalation has become a method of preparation of the intercalated polymer/ layered silicate nanocomposites (PLSNCs). This process involves annealing, statically or under shear, a mixture of the polymer and organically modified layered fillers (OMLFs) above the softening point of the polymer. During annealing, the polymer chains diffused from the bulk polymer melt into the nano-galleries between the layered fillers. 

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. 

The soluble polymers can be intercalated into the galleries using this method. Polymer is dissolved in the solvent containing the desired weight fraction of nano-clay. The polymer chains intercalates into the interlayer spacing of the clay platelets. The nanocomposite is formed by the evaporation of the solvent. This method is good for the intercalation of polymers... 

Fig. 17.24 TEM micrographs of nylon 6/organoclay/EOR-g-MA (76/4/20) ternary nanocomposite showing (a) submicron and nano-voids which are associated with intra-gallery delamination of some organoclay layers (note that the section is not selectively stained in order to clearly reveal delaminations of clay layers), (b) cavitation of EOR-g-MA particles which preferentially starts from the larger particles as indicated by arrows, and (c) extensive matrix shear yielding at the arrested crack tip which in turn causes the EOR-g-MA particles and delaminated clay layers to collapse within the matrix. A schematic of the arrested crack tip illustrating different locations from where TEM micrographs (a-c) were taken is also shown. Note that the schematic is not to scale (Lim et al. 2010)...

The same situation exists for nano-clays, but is complicated by the presence of surface-active species used in their preparation. These organic treatments comprise a significant part of most nano-clays, and fulfil a number of very important functions. Thus, they increase the gallery spacing, making it easier for other species, such as monomers or polymers, to intercalate. The increased spacing also makes it easier to disperse, or delaminate the structure. In the simplest cases, these intercalants would also provide the compatibility and interaction with the polymer matrix described previously. It is thus obvious that the nature and amount of intercalant have to be chosen carefully and controlled. For ease of treatment, it is also helpful for the intercalant to be water soluble. 

In general, the dispersion of clay particles in a polymer matrix can result in the formation of three general types of composite materials (Figure 1). Conventional composites contain clay tactoids with the layers aggregated in unintercalated face - face form. The clay tactoids are simply dispersed as a segregated phase. Intercalated clay composites are intercalation compounds of definite structure formed by the insertion of one or more molecular layers of polymer into the clay host galleries and the properties usually resemble those of the ceramic host. In contrast, exfoliated polymer-clay nanocomposites have a low clay content, a monolithic structure, a separation between layers that depends on the polymer content of the composite, and properties that reflect those of the nano-confmed polymer. 

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