Clay, in nanocomposites

X-rays were discovered in 1895 by W.C. Roentgen. When incident on a crystalline material. X-rays interfere with each other. This phenomenon is known as XRD. The WA-XRD is the most cormnonly used method to characterize the stmcture of bionanocomposites because of its ease of use and availability. The WA-XRD has been used to characterize dispersion of layered clays in nanocomposites based on protein (Chen and Zhang 2006 Shabeer et al., 2007 Yu et al., 2007) and starch (Dimonie et al., 2008 Tang et al., 2008). 

The XRD is a simple and convenient method to determine d-spacing for immiscible or intercalated arrangements of layered silicates in bio-nanocomposites. However, it may be insufficient to characterize exfoliated nanostmctures. Absence of peak in diffraction pattern is often misinterpreted as an indication of exfoliation. Other than exfoliation, dilution, and preferred orientation of clay in nanocomposites might result in a diffraction pattern with no peak. The XRD cannot be used to determine the spatial distribution and dispersion of layered silicates in bio-nanocomposites (Morgan and Gilman 2003). Therefore, XRD should always be used in conjunction with some other techniques such as TEM, SEM, or AFM. 

Different clays having different stmcmres and compositions give different types of nanocomposites. In this chapter, clay-based nanocomposites will be discussed in detail. 

Chitosan-clay bio-nanocomposites are very stable materials without significant desorption of the biopolymer when they are treated with aqueous salt solutions for long periods of time. In this way, they act as active phases of electrochemical sensors for detection of ions (Figure 1.8). The particular nanostructuration of the biopolymer in the interlayer region drives the selective uptake of monovalent versus polyvalent anions, which has been applied in electrode arrays of electronic tongues [132]. 

In rubber-plastic blends, clay reportedly disrupted the ordered crystallization of isotactic polypropylene (iPP) and had a key role in shaping the distribution of iPP and ethylene propylene rubber (EPR) phases larger filler contents brought about smaller, less coalesced and more homogeneous rubber domains [22]. Clays, by virtue of their selective residence in the continuous phase and not in the rubber domains, exhibited a significant effect on mechanical properties by controlling the size of rubber domains in the heterophasic matrix. This resulted in nanocomposites with increased stiffness, impact strength, and thermal stability. 

From the calculation in (7), the softer PEB region was shown to have maximum adhesive force in nature with the calculated modulus in the range of 15 1 MPa (Table 2). The harder PS domains found to have modulus in the range of 24 1 MPa in the SEBS/clay nanocomposite. The non attractive clay regions generally did not fit the JKR model. This was the reason for obtaining much less modulus than that of the literature values for clays in the GPa range. The discussion infers that the bulk modulus of the SEBS/clay nanocomposite (26 1 MPa as shown in Table 2) was dictated by the contribution from PS domains in the matrix. 

The X-ray diffraction peaks observed in the range of 3°-10° for the modified clays disappear in the rubber nanocomposites. photographs show predominantly exfoliation of the clays in the range of 12 4 nm in the BIMS. Consequently, excellent improvement in mechanical properties like tensile strength, elongation at break, and modulus is observed by the incorporation of the nanoclays in the BIMS. Maiti and Bhowmick have also studied the effect of solution concentration (5, 10, 15, 20, and 25 wt%) on the properties of fluorocarbon clay nanocomposites [64]. They noticed that optimum properties are achieved at 20 wt% solution. At the optimized solution concentration, they also prepared rubber/clay nanocomposites by a solution mixing process using fluoroelastomer and different nanoclays (namely NA, 10A, 20A, and 30B) and the effect of these nanoclays on the mechanical properties of the nanocomposites has been reported, as shown in Table 4 [93]. 

Maiti and Bhowmick [205] have established a good correlation between the aspect ratio of clay in fluoroelastomer/clay nanocomposites and the transport properties of solvent. For fluoroelastomer/clay nanocomposites, permeability decreases significantly with the addition of only 4 phr of the unmodified mont-morillonite clay (0.14 x 10 s cm2 s ) compared with that of neat polymer (2.29 x 10 8 cm2 s-1)-... 

Number of clay platelets per stack Effective surface area contribution of clay platelets (A) Extent of exfoliation in nanocomposite sample as obtained from image analysis ( ) ... 

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