Tactoid dispersion

From macroscopic observations, it appears that in the DMDBTDMA-dodecane system the nature of the third phase (liquid, gel, or solid) (140) depends to a large extent on the extracted species. In some cases, microphase separations can be obtained, that is, the coexistence of a more crystalline phase with domains of diluted phase that do not separate upon centrifugation. In classical colloidal literature (141), this situation is described as a dispersion of tactoids in the form of small amounts of liquid crystals, giving macroscopically a gel. 

In the de-stabilization of lyophilic colloids when coa-cervation occurs, the dispersed phase can initially separate into small, anisotropic droplets having shapes such as cylinders, called tactoids . With concentrated colloids, droplets of dilute colloid can separate out within the concentrated colloid these droplets are sometimes referred to as negative tactoids . 

Such a value appears to be intermediate between the two types of motion mentioned above. The explanation is that the above values apply to fully dispersed systems. We are monitoring here the "fast reorientation of a local director for clay platelets that have stacked-up into tactoids and that therefore are much slower in reorienting. 

Dispersion-flocculation processes are generally controlled by double layer swelling, adsorbed hydrolyzed Fe or Al, and chemical bridging (tactoid formation) (Stumm and O Melia, 1968). Once dispersed, clay colloids are kept dispersed by repulsive double layers (Van Olphen, 1971). The force of repulsion is related to the thickness of the double layer (see Chapter 4). This dimension is represented by the ions concentrated near the oppositely charged colloid surface. Any colloid that has a net negative or a net positive charge repulses a like-charged colloid. 

Figure 11. Some possible modes of clay particle association (a) dispersed, (b) coagulated into tactoids, (c and d) flocculated primary particles, and (e-g) coagulated and flocculated particles. Face-face mode is shown as b, edge-face mode is c, edge-edge mode is d, and others are mixtures of these. (Reproduced with permission from reference 51. Copyright 1980 Laurier L. Schramm.)...

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... 

Various factors such as size, shape, dispersity, charge, electrolyte, and external stimuli can affect the liquid crystallinity, pitch, domain size, ordering and other properties [9]. The sample suspension was submitted to ultrasonic treatment at 800 W for 8 min. After high power sonication, we could observe a trip-like texture [Fig. 13.6a]. And "tactoid could be observed in the following 24 h, which would aggregate with each other gradually [Figs. 13.6b-e]. Then the pitch characteristic of a chiral nematic phase is clearly observed and the domain size of the chiral nematic... 

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. 

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... 

TEM analyses showed that melt-compounded EAA/M2(HT)2 composites have homogeneous dispersion of thin intercalated tactoids with several individual silicate platelets... 

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