Clays platelets

Other interesting Langmuir monolayer systems include spread thermotropic liquid crystals where a foam structure forms on expansion from a collapsed state [23]. Spread monolayers of clay dispersions form a layer of overlapping clay platelets that can be subsequently deposited onto solid substrates [24]. 

Therefore, the clay should be modified before it can be used to make a nano-composite. After all, these stacks of clay platelets are larger than one nanometer in every dimension. 

When nanoclays are added to the system, we may assume that the clay layers are randomly placed in the matrix. The diffusion of the solvent will detour around the impermeable clay layers. Diffusion will be diverted to pass a clay platelet in every layer and, hence, the solvent must have to travel a longer path d in the filled system compared to that d( for the neat polymer. 

FIG. 13 Reduction of the relative permeability coefficient is dependent on the clay platelet aspect ratio in the system of polyimide-clay hybrid with water vapor as the permeate. Each hybrid contains 2 wt% clay. The aspect ratios for hectorite, saponite, montmorillonite, and synthetic mica are 46, 165, 218, and 1230, respectively. (From Ref. 71.)... 

Stresses caused by chemical forces, such as hydration stress, can have a considerable influence on the stability of a wellbore [364]. When the total pressure and the chemical potential of water increase, water is absorbed into the clay platelets, which results either in the platelets moving farther apart (swelling) if they are free to move or in generation of hydrational stress if swelling is constrained [1715]. Hydrational stress results in an increase in pore pressure and a subsequent reduction in effective mud support, which leads to a less stable wellbore condition. 

Uniaxial deformations give prolate (needle-shaped) ellipsoids, and biaxial deformations give oblate (disc-shaped) ellipsoids [220,221], Prolate particles can be thought of as a conceptual bridge between the roughly spherical particles used to reinforce elastomers and the long fibers frequently used for this purpose in thermoplastics and thermosets. Similarly, oblate particles can be considered as analogues of the much-studied clay platelets used to reinforce a variety of materials [70-73], but with dimensions that are controllable. In the case of non-spherical particles, their orientations are also of considerable importance. One interest here is the anisotropic reinforcements such particles provide, and there have been simulations to better understand the mechanical properties of such composites [86,222],... 

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.

The long-term goal of this investigation is an understanding of the effect of a particular mode of adsorption on montmorillonite which ensures that under pressure the clay platelets align parallel to a solid surface even under saline conditions. This is an important phenomenon in petroleum recovery processes. 

The non-linear response of plastic materials is more challenging in many respects than pseudoplastic materials. While some yield phenomena, such as that seen in clay dispersions of montmorillonite, can be catastrophic in nature and recover very rapidly, others such as polymer particle blends can yield slowly. Not all clay structures catastrophically thin. Clay platelets forming an elastic structure can be deformed by a finite strain such that they align with the deforming field. When the strain... 

If we select the z axis to be normal to the plane of the clay film, the angle 0 can be imagined to fluctuate rapidly as the probe tumbles in the interlayer. This time-averaged system is symmetric about the z axis because of the random orientation of the a and b axes of the clay platelets in the x y plane. 

Use the snperposition principle to calcnlate the electrostatic swelling pressnre generated between parallel clay platelets with surface potentials of -110 mV, at a separation of 35 nm in a 1.5 mM aqneons solntion of NaCl at 25 °C. 

Values of Example were calculated for the constituting domains of SEBS (PS and PEB) and for the nanoclay regions in the SEBS/clay nanocomposite using (6) and are provided in Table 2. The modulus of the clay platelets was found to be 100 MPa, whereas the modulus for PS and PEB blocks was determined to be 22 and 12 MPa, respectively. These modulus values tallied with the slow strain-rate macromechanical tensile data of 26 MPa for the SEBS/clay nanocomposite (Table 2). The lower calculated modulus values of nanoclays compared to the literature might be due to adhering soft rubber on the nanoclays, which reduces the overall modulus of clay regions in the composite. 

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). 

The ratio (a/b), called the axial ratio of the ellipsoid, is frequently used as a measure of the deviation from sphericity of a particle. It plays an important role, for example, in our discussions of sedimentation and viscosity in Chapters 2 and 4, respectively. In the event that a > b, the prolate ellipsoid approximates a cylinder and, as such, is often used to describe rod-shaped particles such as the tobacco mosaic virus particles shown in Figure 1.12a. Likewise, if a < b, the oblate ellipsoid approaches the shape of a disk. Thus, even the irregular clay platelets of Figure 1.12b may be approximated as oblate ellipsoids. 

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). 

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