Particles platelet-like

Figure 1.4 SEM images and EDX data from a Mo9V3W12Ox catalyst after activation during the oxidation of acrolein [35], The pictures indicate that needle-like (A), platelet-like (B), and spherical (not shown) particles are formed during exposure to the reaction mixture. EDX analysis at different spots, two of which are exemplified here, point to V, Mo, and W contents that vary from 19 to 29, 60 to 69, and 11 to 13 atom%, respectively. It was determined that the in situ formation of a (MoVW)5014-type phase accounts for the increase in acrolein conversion observed during the initial reaction stages. (Reproduced with permission from Elsevier.)...

The barrier effect can be achieved, for example, by using pigments with a platelet-like or lamellar particle shape. This allows them to form a wall of flat particles within a paint film and therefore water and electrolytes have to take an extended/less direct route through the paint film to the substrate. 

Prevention of the coalescence of the sticky, partially polymerized particles is a major problem in suspension polymerizations, and proper selection of stabilizing agents is important. Two kinds of additives are used to hinder coalescence of particles in suspension polymerizations. These are platelet-like mineral particles that concentrate at the organic-water interface, like Ca3(P04)2, and/or macromolecu-lar species that are soluble in water and insoluble in the particular organic phase. Poly(vinyl alcohol) and starch products are examples of the latter type. 

When the shapes of crystallites are isotropic, random distribution of their orientations is not a problem and deviations from an ideal sample are usually negligible. However, quite often the shapes are anisotropic, e.g. platelet-like or needle-like and this results in the introduction of distinctly non-random crystallite orientations due to natural preferences in packing of the anisotropic particles. The non-random particle orientation is called preferred orientation and it may cause considerable distortions of the scattered intensity. 

Consider two limiting anisotropie particle shapes platelet-like and needle-like. The platelets, when packed in a flat sample holder, will tend to align parallel to one another and to the sample surface. Then, the amount of plates that are parallel or nearly parallel to the surface will be much greater than the amount of platelets that are perpendicular or nearly perpendicular to the surface. In this case, a specific direction that is perpendicular to the flat... 

Both approaches work in a similar way. In the case of platelet-like particles, the function is stretched along T (T > Tx), while in case of needles it is stretched along Tx (Ty < Tx). Therefore, in both cases x < 1 describes preferred orientation of the platelets and x > 1 describes preferred orientation of the needles. Obviously x = 1 corresponds to a completely random distribution of reciprocal lattice vectors and the corresponding radial distribution functions become a circle with unit radius (both Eq. 2.78 and 2.79 result in Tuu = 1 for any... 

When powder particles have thin platelet-like shapes, they will tend to agglomerate, aligning their flat surfaces nearly parallel to one another Figure 3.20, left). As a result, the orientations of platelets are randomized via rotations about a common axis normal to their largest faces, and such samples are expected to have a uniaxial preferred orientation (or texture). 

Figure 3.20. The two critical cases of non-random particle orientation distributions due to distinctly anisotropic particle shapes platelet-like (left) and needle-like (right) particles. The arrows indicate the directions around which the particles may rotate freely.

Protein-vesicle systems have been studied also, such as the thylakoid membranes of the phototropic bacterium Rhodopseudomonas spheroides [270]. Thus the apoli-poprotein C-III/phosphatidylcholine complex has been studied with X-rays to show that a single particle contains about 9 apo C-III and 454 phospholipid molecules, is smaller than the average parent phosphoUpid vesicle, and has a platelet-like shape... 

Compared to cellulose or chitin, the morphology of constitutive nanocrystals obtained from starch is completely different. Figure 19.12 shows a TEM obtained from a dilute suspension of waxy maize starch nanocrystals. They consist of 5-7 nm thick platelet-like particles with a length ranging 20-40 nm and a width in the range of 15-30 nm. The detailed investigation on the stmcture of these platelet-like nanoparticles was reported [36]. 

This can be observed in Figure 12.2, which shows the surface area-to-volume ratio (A/V) as a function of the aspect ratio (a = l/d) of a cylindrical particle (disk/platelet-like or cylinder/fiber-like), for a given particle volume. Values of l/d < 1 correspond to platelet-like particles, while l/d > 1 correspond to rod-like particles. It can be seen that A/V increases faster for platelet-like particles than for rod-like particles with respect to their aspect ratio [6]. Hence, for an equivalent volume of particles and for the same aspect ratio, platelet-like particles have higher contact surfaces, which makes them more difficult to be dispersed. 

Following from the same considerations, it can be demonstrated that a fiber-like shape, instead of a platelet-like, can minimize the sharp increase of viscosity due to the addition of nanoclays to polymer melts, and therefore improve the processability of the nanocomposite. Different factors contribute to the viscosity of a nanocomposite (a) the polymer-polymer network, (b) the clay-clay network, and (c) the polymer-clay interaction. Supposing that the chemistry of the system is fixed, the first contribution is invariant and the second depends only on the particle aspect ratio, according to percolation theory [7] the higher the aspect ratio, the higher the viscosity of the composite. Comparing the effect of two nanoclay particles with the same aspect ratio, one with a rod-like shape and the other with... 

An additional advantage of using a fiber-like reinforcement over a platelet-like reinforcement is its higher reinforcing efficiency in case of unidirectionally aligned systems [8], as can be demonstrated by micromechanical models like Halpin-Tsai [9]. A particle is said to reinforce efficiently a polymeric matrix if the increase in Young s modulus is close to the theoretical limit given by the rule of mixtures [10]. 

NR/maize starch whiskers (platelet-like particles) 0.90 2.9 7.5 97... 

Transmission electron microscopy shows that allophane particles are nearly spherical with diameters in the range of 3.5-5 nm (by the way of comparison of the platelet-like... 

Asymmetrical flow field-flow fractionation was used for size determinations of SLN in comparison to an emulsion and oil-loaded SLN. The differences found in the sizes and elution profiles were attributed to differences in the particle shapes. Due to their anisometric, platelet-like shape it is likely that SLN are more retained by the cross flow applied compared to spherical emulsion droplets. This method appears very promising as additional size determination method particularly with regard to separation and detection of different colloidal structures. 

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