Exfoliated silicate layer

Figure 31a and b show the TEM images. Exfoliated silicate layers are dispersed more widely and uniformly in the UlOOl-type PPCN (PP-I-U1001/C18-Mt) than in the UlOlO-type PPCN (PP-i-U1010/C18-Mt). Many silicate layers, whose interlayer distances were expanded to more than 5 nm, were also... 

Further, incorporation of nanoclays tends to increase the T of PLA to 275 and 260"C respectively for PLA/C20A and PLA/C93A nanocomposites. On the other hand, nanocomposite prepared using C93A and C30B nanoclays exhibited T. of 306 and 307 C respectively. The nanoclays act as a heat barrier over matrix macromolecules, which enhances the thermal stability in the layered silicate nanocomposites as well as assists in the formation of char after thermal decomposition. PLA/OMMT nanocomposite showed the optimimi T. of 320 C, which confirms the improved capacity of exfoliated silicate layers as a superior heat insulator and mass transport barrier to the volatile products generated during the thermal decomposition. 

Layered silicate nanoparticles have also been used to prepare PEN-based nanocomposites through the direct intercalation of PEN polymer chains from the melt into the surface-treated clay. An internal mixer was used and exfoliated silicate layers within a PEN matrix were obtained. Mechanical and barrier properties measnred by dynamic mechanical and permeability analysis showed significant improvanents in the storage modulus and water permeabihty when compared to neat PEN (Wu and Liu, 2005). 

Initiation of e-CL polymerization from the clay surface accounts for the tethering of the in-situ grown chains onto the clay surface and for clay delamination. As reported elsewheregrafted and/or tethered polymer chains have a key role in preventing the exfoliated silicate layers from aggregating during post-thermal and processing treatments of the composites. 

WAXD difiractograms of intercalated or otherwise stacked tactoid structures may show second-order and even third-order reflections, which are assodated to very ordered stacking of the intercalated layers [24,26,28,45-48]. On the other hand, the absence of high order (001) peaks in the WAXD patterns of nanocomposites, especially with low amounts of MMT fillers, can be due to different reasons, such as a decrease in the regularity of the stacking layers or a diminution of the size and number of tactoids with highly parallel stacking, and a consequent increase in the proportion of exfoliated silicate layers dispersed in the polymer matrix [27,49]. 

XRD and TEM results, a mixed morphology including tactoids, and intercalated and exfoliated silicate layer structures... 

In a 1975 Japanese patent application, Fujiwara and Sakamoto [71 ] of Unitika adsorbed various lactams onto montmorillonite and found swelling of the clay silicate layers and spontaneous polymerization. Similar but more extensive studies were subsequently published and patented byKojima,Okada,Usuki, and their coworkers [72 to 77] from Toyota from the late 1980s. They then examined the structure and properties of these compoimds. In these compounds, the clay is dispersed as individual exfoliated silicate layers. Various other silicate minerals were studied [76], but montmorillonite compounds were found to be superior in mechanical properties. Similar polymerizations were carried out for a polyamic acid to produce a polyimide [77]. Mechanical property enhancements of exfoliated montmoriUonite/polyamide-6 [78,79] are shown in Fig. 7.3. 

The cold-fractured surface morphologies of the various fibres, associated with the matrix phase homogeneity are shown by the SEM photomicrographs in Fig. 19.5. The relatively smooth morphology exhibited by PPEX, close to that of the neat PP, is an indication of the presence of significantly exfoliated silicate layers that preserved a homogeneous PP phase. In contrast, PPEZ... 

Pluta et al. also observed an increase in thermal stability of PLA nanocomposites with the clay content by TGA, with a maximum clay loading of 5 wt.%. When further increasing the filler content, a decrease in thermal stability was observed. Such behaviour was explained by the relative extent of exfoliation/delamination as a function of the amount of OMLS. Indeed at low filler content, exfoliation dominates but the amount of exfoliated silicate layers is not sufficient to promote any significant improvement of the thermal stability. Increasing the filler content leads to relatively more exfoliated particles, and increases the thermal stability of the nanocomposites. However, when the silicate content is more than some critical value, complete exfoliation of such high aspect ratio silicate layers becomes increasingly hindered because of the geometrical constraints within the limited space available in the polymer matrix and no more increase in thermal stability or even some decrease is detected. 

Transparent exfoliated-intercalated PEA/bentonite nanocomposites are prepared by in situ emulsion polymerization in aqueous dispersions containing bentonite, following directly casting the resulting emulsion into film. XRD and TEM reveal that disorderedly exfoliated silicate layers and intercalated silicate layers coexist in the PEA matrix. Thermal stability, mechanical properties and barrier properties of the obtained materials were greatly improved. Because relatively small amounts (<10%) of nanometer-size clay particles can provide large... 

Exfoliated polymer/silicate systems have been found to exhibit mechanical, thermal and solubility properties, as well as water vapor transmission rates, which are superior to conventionally filled systems. Furthermore due to their nanoscale dispersion of filler, they retain optical clarity. In our PVA/MMT hybrids, and especially at the application relevant low i mmt (below lOwt.%), TEM and XRD reveal a coexistence of intercalated and exfoliated silicate layers. For these systems we will briefly describe some of their materials properties. 

The intercalation of organic cations enhances the interlayer distance resulting in delamination and exfoliation of the silicate layers. However, a homogeneous dispersion and distribution of exfoliated silicate layers in polymer composites is easy to realize with polar polymers but more difficult to achieve in unpolar polymers such as olefins. After proper surface modification and exfoliation of the silicates the general nanocomposite manufarturing techniques later discussed in this chapter can be applied. However, depending on the type... 

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