Mechanical strength chapter

The polymers which have stimulated the greatest interest are the polyacetylenes, poly-p-phenylene, poly(p-phenylene sulphide), polypyrrole and poly-1,6-heptadiyne. The mechanisms by which they function are not fully understood, and the materials available to date are still inferior, in terms of conductivity, to most metal conductors. If, however, the differences in density are taken into account, the polymers become comparable with some of the moderately conductive metals. Unfortunately, most of these polymers also have other disadvantages such as improcessability, poor mechanical strength, instability of the doped materials, sensitivity to oxygen, poor storage stability leading to a loss in conductivity, and poor stability in the presence of electrolytes. Whilst many industrial companies have been active in their development (including Allied, BSASF, IBM and Rohm and Haas,) they have to date remained as developmental products. For a further discussion see Chapter 31. 

Examples of using metal, polymer, and glass microreactors appear in other chapters of this volume. The present chapter focuses on microreactors created in silicon, a material that has high mechanical strength,... 

High-performance liquid chromatography (HPLC) and fast protein liquid chromatography (FPLC) rely on the same separation principles as the traditional chromatography columns, but tend to be much faster because of high flow rates that are possible due to the uniform bead size and the mechanical strength of the beads. See also Chapter 4, section 1.2.2. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

In principle, metallic monoliths have attractive features, e.g., fast warm-up, high mechanical strength, flexibility in shaping (see also Chapter 3 of this book). However, they are usually less porous and have, in general, small surface areas. Often, deposition of an oxidic layer is necessary to increase the surface area or improve the surface properties of the monoliths, but adhesion of an oxide layer on a metal surface can lead to weak structure. 

In this chapter, recent progress on the study and development of hydrogels with tough mechanical strength, low frictional coefficient, and cellular viability will be described. Furthermore, attempts to apply such gels as substitutes for biological tissues will be introduced. 

In the opposite case of adsorption from non-polar medium, e.g. adsorption of octadecylamine on glass in heptane, the adsorption layer reveals a certain finite strength, i.e. has properties of structural-mechanical barrier (Chapter VII, 5). 

Catalyst may be useful for either activity or selectivity, or both. Another important issue is the catalyst stability. A catalyst with good stability will change very slowly over the course of time under the conditions of use. Indeed, it is only in theory that the catalyst remains unaltered during the reaction. Actual practice is far from this ideal, as the progressive loss of activity could be associated with coke formation, attack of poisons, loss of volatile agents, changes of crystalline structure, which causes a loss of mechanical strength. Due to the extreme importance of catalyst deactivation, the kinetic aspects of this phenomenon will be treated in a separate chapter. 

Ceramic materials have an advantage in terms of mechanical strength and thermal and chemical stability. A number of investigations have been carried out on humidity sensors utilizing porous ceramic elements. There have been a number of reviews of ceramic humidity sensors [1, 5], and some examples of recent developments are introduced in this chapter. 

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