Platelet exfoliation

The dispersion of clay platelets (exfoliation and intercalation level of the silicate layers) and surface area of silicate platelets have the potential to alter the rheological behavior of the nanocomposites. In-situ polymerized nano composites exhibit more exfoliated structure than the composites prepared by the melt blending technique. Irrespective of the processing parameter, the nanocomposites show shear thinning behavior at high shear rate (Figure 9.14), whereas the pristine polyamide exhibits Newtonian behavior (i.e., the viscosity remains almost the same). It has also been reported that the polymer nanocomposite possesses higher steady shear viscosity than pristine polyamide at low shear rates. 

MMT has been established as a successful filler for polymers because it is cheap, plentiful in nature, and if the chemistry of the surface of the MMT and the polymer matrix is favorable, the micron-sized day tactoids can be dispersed into the nanometer-sized clay platelets (exfoliation), effectively forming a nanocomposite. 

Abstract In this chapter, we report the findings of experimental investigations conducted on durability of glass fiber-reinforced polymer (GFRP) composites with and without the addition of montmorillonite nanoclay. First, neat and nanoclay-added epoxy systems were characterized to evaluate the extent of clay platelet exfoliation and dispersion of nanoclay. GFRP composite panels were then fabricated with neat/modified epoxy resin and exposed to six different conditions, i.e. hot-dry/wet, cold-dry/wet, ultraviolet radiation and alternate ultraviolet radiation-condensation. Room temperature condition samples were also used for baseline consideration. 

The efficiency of a surfactant in platelet exfoliation and interfacial bonding should be considered as the main criterion for its selection. However, it is advisable to select a surfactant with respect to its thermal stability a well. The number and length of alkyl chains, type of counterion, and class of amine (or ammonium salt) - primary, secondary, or tertiary - were considered important parameters influencing the thermal stability of nanocomposites. 

Bright areas in the images show PLA matrix while dark areas indicate cross section of LDH platelets. Exfoliated LDH layers in PLA matrix can be seen in PLA/ LDH-CO3 5% (Figure 9(a)) while well distributed LDH platelets in PLA-LDH-C,j 5% (Figure 9(b)). 

Bio-nanocomposites can be prepared by several methods which include in situ polymerization, solution exfoliation, and melt inteicalatioa In the in situ polymerization method, monomers are migrated into the galleries of layered silicates and subsequently polymerized via heat, radiation, or catalyst. In solution exfoliation, layered clays are exfoliated into single platelets. Exfoliation is achieved by dispersing the layered clays in a solvent. The polymer is adsorbed onto the platelets by mixing in the clay suspension. The solvent is removed either by evaporation or by precipitation. In melt intercalation, layered clays are mixed with the polymer matrix in molten state (Zeng et al., 2005). 

These simple models have been widely used to describe the barrier properties of polymer layered silicate nanocomposites (Yano et al, 1997 Gusev and Lusti, 2001 Lu and Mai, 2007), however, some issues like the effect of clay platelets exfoliation / intercalation / aggregation ratio and multiphase polymer matrix system (polymer blend) remain a challenging problem for barrier property modeling. 

Fig. 1.9 (A) Exfoliation of clay platelets (white Cloisite25A and Cloisite30B after (B) two and a arrows) in a commercial polylactide matrix using half months hydrolysis and (C) after five and a a masterbatch process. (B, C) Visual aspect half months hydrolysis. (A) adapted from [144] of unfilled PLA, microcomposite based on reproduced by permission ofWiley-VCH, and CloisiteNa+, and nanocomposites based on (B, C) from [147] with permission from Elsevier.

Clays are usually cation-exchangeable aluminosilicates, and exfoliated clay particles have a platelet shape with nanoscopic size. Cast protein-clay films on electrodes have been used to immobilize proteins. The Clay/Mb electrode has good electrocatalytic properties for the reduction of oxygen and hydrogen peroxide [236] and the biosensors can also be made based on these properties. 

The pyridine intercalate is blue-black. Examination of the individual platelets under a microscope reveals a characteristic exfoliated appearance. Hexagonal symmetry is retained with a = 3.325 A and c/2 = 12.03 A. Comparison with the slab thickness for the parent 2H(a)-TaS2 shows a c-axis expansion, 8, of 5.99 A. The following d values have been obtained for low-angle X-ray diffraction lines 12.03, 6.015,4.010,3.008, 2.880, and 2.859 A. 2TaS2 C5HsN is also super conducting with a transition temperature of 3.5 0.3°K2,3... 

The effect of polymer-filler interaction on solvent swelling and dynamic mechanical properties of the sol-gel-derived acrylic rubber (ACM)/silica, epoxi-dized natural rubber (ENR)/silica, and polyvinyl alcohol (PVA)/silica hybrid nanocomposites was described by Bandyopadhyay et al. [27]. Theoretical delineation of the reinforcing mechanism of polymer-layered silicate nanocomposites has been attempted by some authors while studying the micromechanics of the intercalated or exfoliated PNCs [28-31]. Wu et al. [32] verified the modulus reinforcement of rubber/clay nanocomposites using composite theories based on Guth, Halpin-Tsai, and the modified Halpin-Tsai equations. On introduction of a modulus reduction factor (MRF) for the platelet-like fillers, the predicted moduli were found to be closer to the experimental measurements. 

Morphology evolution is thus found to be dependent on the processing technique applied to disperse the nanoparticles. The latex-blended and prevulcanized nanocomposites show predominant exfoliation with some intercalation, especially in uncured and prevulcanized samples. In conventionally cured but latex-blended nanocomposites, realignment of NA particles is visible, with a greater tendency of NA platelets towards agglomeration. In solid state mixing, the dispersion is still poorer. XRD studies also corroborate the above observations. 

The exact number of clay stacks having the above three categories of platelet stacking were measured by taking at least six different HRTEM images for each type of nanocomposite sample and the average distribution of the clay platelets were noted down (Fig. 38). This has been represented as the extent of exfoliation (B) in Table 9. It is apparent that as the level of exfoliation increases, the number of clay platelets per stack decreases and their effective surface area contribution increases. 

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 ( ) ... 

A good state of dispersion of the organoclay has been found in the CR matrix. The exfoliated structure can be directly observed from the of the OMMT-filled CR composite (left-hand image in Fig. 30). It is noticed from this micrograph that all silicate layers are exfoliated and distributed very nicely throughout the whole rubber matrix. It is also observed that some of the exfoliated clay platelets form a house of cards structure (right-hand image in Fig. 30). 

In this type of system, the polymer chains are constrained by a surface. They can lie between two hard surfaces such as in the galleries within two parallel clay platelets (as is illustrated in Figure 7), have one layer absorbed on to a hard surface as a coating, with the other free (as in Figure 8), they can be absorbed by the surfaces of exfoliated clay platelets (Figure 9), or by the surface of a solid reinforcing particle completely surrounded by an elastomeric phase (Figure 10). 

There have been a number of studies of polymers at surfaces, as illustrated in Figure 8, including some that have been tethered [10, 122-137]. Also in this category are the polymer-clay composites after the previously paired platelets have been sufficiently separated to be independent of one another. Such exfoliated arrangements are shown in Figure 9. 

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