Delaminated exfoliation

Dispersion The degree of dispersion of the nanoplatelets is determined by the degree of delamination of the clay. The fully delaminated (exfoliated) nanocomposite presents much higher values for the tortuosity factor and the aspect ratio in comparison with the partially delaminated (intercalated) nanocomposite. This means that the clay particles that grow as aggregates or books of sheets must be broken up or exfoliated into individual sheets that have a thickness of the order of 1 nm, with lengths and widths of the order of 500 nm. 

FIGURE 5.90 Schematic presentation of two extremes of composite structures that can be obtained by polymer melt intercalation of layered silicates. The rectangular bars represent individual silicate layers (1 nm thick), (a) Single polymer layers intercalated in the silicate galleries, (b) Delamination (exfoliation) of layered silicates and dispersion in a continuous polymer matrix. (After Giannelis, E. R 1996. Adv. Mater., 8(1), 29. With permission.)... 

The introduction of a few percent of nanofUlers, such as layered aluminosilicate clays, has been extensively considered to enhance PLA properties. However, the greatest improvement is usually achieved when nanoparticles are fully and uniformly delaminated (exfoliated) in the polymer matrix, a challenge that cannot be fully met by direct melt blending of the clay as this usually leads to intercalated nanocomposites [43, 44]. The use of in-situ polymerization techniques appears fully justified since the monomer can penetrate and then polymerize inside the clay sheets, enhancing the delamination efficiency. 

It is well know that the zeolite materials synthesized in alkaline systems usually have a high number of silanol groups (=SiOH) named defect groups [10] which possess a moderated Bronsted acidity [11]. Oppositely, Silicalite-1 synthesized in fluorine media are relatively defect-free [12] and the fluorine ions remain in the small cages of the MFI structure even after the calcination process [12]. The 29Si-NMR analyses carried out on samples Na-Silicalite-1 and F-Silicalite-1 confirm the presence of silanol groups only on the SI support surface (results not showed). Delaminated zeolites (ITQ-6) are obtained by exfoliation of as-synthesized lamellar precursor zeolites [13]. After this process, the final structure of the delaminated zeolite results in a completely hydroxylated and well-ordered external surface [13]. 

Exfoliated or delaminated This is observed when the nanomaterial is completely dispersed in the polymer matrix, a highly desirable condition. 

LDH materials are not readily exfoliated in contrast to other layered materials such as smectite clays or MS2 type-chalcogenides (M0S2, NbSe2, etc...) [15-17] the delamination of LDH sheets requires elaborate syntheses as discussed above in Sect. 2.1. 

In exfoliated nanocomposites, delaminated structures are obtained if a large number of polymer chains are present between the layers and the layers stand >10 nm apart. Thus, the interlayer expansion is comparable to the radius of gyration of the polymer rather than that of an extended chain, as in the case of intercalated hybrids [35]. 

The exfoliation or delamination configuration is of particular interest because it maximizes the polymer-clay interactions, making the entire surface of layers available for a polymer, and thereby yields better properties than intercalated ones. 

Figure 7 shows the representative bright field HRTEM images of nanocomposites of NR and unmodified montmorillonite (NR/NA) prepared by different processing and curing techniques. It is apparent that the methodology followed to prepare the nanocomposites by latex blending facilitates the formation of exfoliated clay structure, even with unmodified nanoclays. It has been reported in the literature that hydration of montmorillonite clay leads to extensive delamination and breakdown of silicate layers [94, 95]. It has also been shown that NA disperses fully into the individual layers in its dilute aqueous dispersion (clay concentration <10%)... 

FIGURE 11.18 Representative TEM images (a) immiscible (b) intercalated with many clay layers stacking together (c) intercalated and uniformly dispersed with a few clay layers stacking together (d) exfoliated (delaminated) with a uniform dispersion. 

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