Silica platelets

Fig. 4.18 Formation ofhexagonal silica platelets with poly-L-lysine helical chains in the presence of monosilicic acid and phosphate. Reprinted with permission from [170], M.M.Tomczaketa/.J. Am. Chem. Soc. 2005, 727, 12577. 2005, American Chemical Society.

The silica Aerosil was from Degussa. Silica platelets for spectroscopic use were preheated in air at 800° C, as described elsewheer (16). The hydrogen and deuterium gases were from J. T. Baker. The purity was 99.95% and 99.50%, respectively. The gases were dried carefully before use. 

The development of nanoparticles with different sizes and shapes, including spherical particles such as silica, platelets such as layered silicates, and carbon nanotubes have been used as fillers in elastomeric matrices to improve their properties like mechanical and barrier. The interaction of the polymer chains with the filler particles is also cmcial in controlling the performance of nanocomposites. 

Figure 19.10 Novel nanocomposites extruded in film form with silica platelets both oriented and localized in numerous internal layers to give plastics with low permeability and other property enhancements. (Layers of matrix polymer separate both the platelets and platelet-rich layers. Internal layers and platelets are optimally oriented parallel to the film surface.)...

Figure 19.11 Examples of dispersions. (A) LDPE droplets in a blend of 20% LDPE and PR (B) nanodispersion of silica platelets in an extruded film where platelets are volumetrically oriented.

Figure 1.20 Proposed model of PLL-induced condensation of silica platelets. Reprinted with permission from Ref [83] 2005, American Chemical Society.

Crystalline fructose Crystalline nylons Crystalline platelets Crystalline polymers Crystalline polypropyL Crystalline Si Crystalline silica... 

As surface area and pore structure are properties of key importance for any catalyst or support material, we will first describe how these properties can be measured. First, it is useful to draw a clear borderline between roughness and porosity. If most features on a surface are deeper than they are wide, then we call the surface porous (Fig. 5.16). Although it is convenient to think about pores in terms of hollow cylinders, one should realize that pores may have all kinds of shapes. The pore system of zeolites consists of microporous channels and cages, whereas the pores of a silica gel support are formed by the interstices between spheres. Alumina and carbon black, on the other hand, have platelet structures, resulting in slit-shaped pores. All support materials may contain micro, meso and macropores (see text box for definitions). 

An excellent example of PLC applications in the indirect coupling version is provided by the works of Miwa et al. [12]. These researchers separated eight phospholipid standards and platelet phospholipids from the other lipids on a silica gel plate. The mobile phase was composed of methylacetate-propanol-chloro-form-methanol-0.2% (w/v) potassium chloride (25 30 20 10 10, v/v). After detection with iodine vapor (Figure 9.2), each phospholipid class was scraped off and extracted with 5 ml of methanol. The solvent was removed under a stream of nitrogen, and the fatty acids of each phospholipid class were analyzed (as their hydrazides) by HPLC. The aim of this study was to establish a standardized... 

The new lipid occurred only in the plasma hpids of newborns and was not present in membrane hpids of red cell membranes or platelets. Total lipids were extracted from plasma and from red blood cell membranes and platelets. A total lipid profile was obtained by a three-directional PLC using silica gel plates and was developed consecutively in the following solvent mixtures (1) chloroform-methanol-concen-trated ammonium hydroxide (65 25 5, v/v), (2) chloroform-acetone-methanol-ace-tic acid-water (50 20 10 15 5, v/v), and (3) hexane-diethyl ether-acetic acid (80 20 1, v/v). Each spot was scraped off the plate a known amount of methyl heptadecanoate was added, followed by methylation and analysis by GC/MS. The accmate characterization of the new lipid was realized using NMR technique. 

Silicate is another type of widely distributed substance that induces the release of histamine from platelets. It is a common constituent of plant tissues in the form of dissolved hydrous silica or silicic acid (24), which may contribute to the byssinotic syndrome. Cotton bracts contain 0.4-0.8% silica (24). For many of the byssinogenic substances, nanogram concentrations are sufficient to release histamine (48). 

As our experimental work proceeded, I realized that we were dealing with a silica-alumina system (silica-alumina gel and platelet crystalline silica-alumina Kaolin) that should be similar to the alumina gel/alumina trihydrate crystal system of RD-150. It seemed possible to achieve a similar pore diameter spectrum structure here by judicious combination of crystals and gel. 

The dominant class of lustre pigments is based upon oxide coatings of mica platelets, but newer materials have appeared e.g. silica and alumina flakes from Merck, with better performance characteristics than traditional mica." " There has also been developments in the technology for the coating of reflective metal platelets, e.g. aluminium, with oxides using chemical vapour deposition that has led to exciting new products. ... 

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. 

There has been some interest in generating silica-like particles using templates, as is done by Nature in biosilicification processes.113-118 Various particle shapes have been obtained, and platelet forms would be of particular interest with regard to their abilities to provide reinforcement and decrease permeability. 

Using the total platelet lipid extract as described in the section entitled Isolation of Human Platelets, single-dimensional thin-layer chromatography on a 250- pm-thick silica gel G-coated plate with a solvent system of chloroform-methanol-water (65 35 7, v/v) will yield the pattern shown in Figure 3-2. 

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