Tactoids exfoliation

Figure 6.21 Exfoliated tactoids of montmorillonite. Note the irregular shapes of exfoliated platelets compared to the larger tactoids in Figures 6.18-6.19.

T. Lan, T.J. Pinnavaia, Mechanism of clay tactoid exfoliation in epoxy-clay nanocomposites, Chem. Mater., vol. 7, pp. 2144-2150,1995. 

However, in addition to this, there is evidence of improvements found with partial exfoliation [102, 103] of tactoids formed by 10-20 lamellas, due to required flexibility of the dispersed particles. Interfacial interactions are of fundamental importance in the structure and properties of nanocomposites. Lee et al. [104] proposed that the interactions of the polar maleic groups grafted to PP with hydroxyl groups of... 

Lu and Xiao [72] studied the structure and properties of nanocomposites made from CMKGM and MMT. TEM showed that MMT dispersion in CMKGM was mainly in the intercalated state and depended on the CMKGM/MMT weight ratio. At 5 wt% MMT, most MMT layers existed as tactoids however, some intercalation was seen. At 15 wt% MMT, there were still some tactoids, exfoliation was observed a little in some parts, MMT layers were separated by ultrathin CMKGM, and intercalation was seen clearly. Pure MMT had a sharp XRD peak at 29 = 7.1°, corresponding to a basal spacing of 1.25 nm. All diffraction peaks of... 

Pluta [49] studied the structure and properties of PLA/MMT nanocomposites and showed an effective enhancement of MMT dispersion with prolongation of the blending time (from 6 to 30 min). They stressed that this was possible due to strong interaction between PLA-MMT and shearing forces during melt compounding. The nanostructure was induced by the intercalation followed by tactoids formation and exfoliation of MMT, as confirmed by TEM analysis and XRD. The studies performed also clearly revealed the influence of MMT s dispersion in the PLA matrix on the physical properties of the nanocomposites formed as the improved MMT s dispersion (at their constant concentration) had increased the thermal stability of the nanocomposites under oxidative and nonoxidative conditions was improved with MMT s dispersion. Besides that, the crystallization ability of PLA also improved with incorporation of MMT. 

Fig. 5. Schematic of the hierarchy of clay structures in polypropylene nanocomposites of mixed morphology. Clay tactoids and exfoliated platelets comprise the mesoscale morphology. The internal intercalation structure of clay tactoids is determined by the compatibilizer and compounding conditions. (View this art in color at www.dekker.co...

Fracture toughness, always important in composites and adhesives, remains an important issue, as does a better understanding of the relative importance of intercalation/exfoliation and the influence of their size-scale (micron vs. nanometer). Indeed, the effect of the size-scale of materials from individual layers to tactoids to clusters to larger clay clumps, many of which seem to exist simultaneously in a sample, is still not fully understood or able to be independently manipulated. The ability to make full use of the anisotropy of the clays in thermoset components Ijy appropriate processing techniques also is of interest. likewise, the ability to usefully add greater concentrations of clay in certain applications for particular property balances (as opposed to widely-used 3-5 wt %) may also allow a greater range of properties. 

Figure 1.15 Schematic representation of formation of polymer-based CMP systems viz. (a) conventional bulk CMP (tactoid), (b) intercalated NCs, and(c) exfoliated NCs, via physical mixing (dry/solution phase mixing or melt compounding) and in-situ polymerization routes.

Here, on the contrary, the two schemes depict profoundly different situations with respect to solvent-clay interactions. In Figure 3.1, a clay dispersion comprising solvent-intercalated tactoids is considered, whereas complete exfoliation of the organoclay is assumed in... 

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