Stability - Overview

J.P. Galbo, Light Stabilizers (Overview), in J.C. Salamone (ed.). Concise Polymeric Materials Encyclopedia, CRC Press,... 

Microbiol Stability. Microbial growth is hindered by reducing water activity and adding preservatives. An overview is available (30). Reduction in water activity is typically obtained by including approximately 50% of a polyalcohol such as sorbitol or glycerol. Furthermore, 20% of a salt like NaCl has a pronounced growth inhibiting effect. 

Poly(p-pheny lene)s, PPPs, constitute the prototype of rigid-rod polymers and are currently being intensively investigated [1]. The key role of PPPs follows from their conceptually simple and appealing molecular structure, from their chemical stability, and from their superior physical properties [2], In turn, this is the result of important advances made in aromatic chemistry over the last few years. The following section gives an overview of the most common methods to generate poly(p-phenylene)s via different synthetic approaches. 

Support materials are commonly u.sed in heterogeneous catalysis. Their major function is to maximize the dispersion of the active phase by providing a large surface area over which the active phase can be distributed. In this way the cataly.st material is shaped into a form suitable for use in technical reactors. Supports are not always chemically inert they can also show certain catalytic activity and often they act as a stabilizer for the actual active phase. A number of materials are u.sed as catalyst supports. Table 3.2 gives an overview. 

Important steps during RM production are the control of properties that determine the quality of the final product and its long-term stability (see the overview of techniques in Table 2.4). As some of the methods discussed below, for example determination of homogeneity, stability and shelf life (expiry date) are also mentioned in other Chapters and in the interest of brevity the reader is referred to them for further details. 

All the above-mentioned methods will be discussed in this chapter. A number of filtration and reactor units that have been used in published work will be described and an overview of the different membrane materials will be given. As generally catalytic reactions deal with organic substrates and are usually carried out in an organic solvent, the membranes have to be solvent resistant. Thermal stability can also be an issue depending on the process conditions. 

In recent years, the amount of research time devoted to materials chemistry has risen almost exponentially and sulfur-based radicals, such as the charge-transfer salts based upon TTF (tetrathiafulvalene), have played an important role in these developments. These TTF derivatives will not be discussed here but are dealt with elsewhere in this book. Instead we focus on recent developments in the area of group 15/16 free radicals. Up until the latter end of the last century, these radicals posed fundamental questions regarding the structure and bonding in main group chemistry. Now, in many cases, their thermodynamic and kinetic stability allows them to be used in the construction of molecular magnets and conductors. In this overview we will focus on the synthesis and characterisation of these radicals with a particular emphasis on their physical properties. 

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