Insulating groups

In this way, many green azo dyes have been made by combining separate conjugated systems in the same molecule, eg, one yellow and the other blue. The blocking or the insulating group prevents the electronic interaction of one chromophore system with the second. Chloramine Fast Scarlet 4BS... 

Chung [34] concluded that the semiconducting properties of a metal species influence discoloration. In contrast to metals belonging to the insulator group, metals belonging to the semiconductor group promote yellowing, perhaps due to catalysis of the polymerization of vinyl esters. The formation of chromophores is enhanced when the metal has a variable valency with a reduction potential near to zero. 

Intramolecular transfers between two ehromphores separated by insulating groups can lead to absorption by one chromophore and emission from the other. Complexes of rare earths specially of Eu+3, Sm+3. Gd+3 and Dy+3 emit line spectrum characteristic of the central metal ion when absorption takes place in the ligand moeity. 

A dye developer is a compound composed of a silver halide developer fragment linked via an insulating group (Z) to a dye moiety. The compounds are intrinsically soluble in alkali because of acid functions contained in the developer fragment, normally a hydroquin-one. On interaction with exposed silver halide the hydroquinone becomes oxidized to the corresponding quinone. This renders the compound insoluble in alkali (Scheme 7). The reaction can be enhanced by the use of a mobile auxiliary developer such as p-methylphenyl-hydroquinone. In the areas of the emulsion layer where there has been no exposure of silver halide, the dye developer cannot be oxidized. It therefore remains soluble and free to transfer to the mordant layer. Thus, the transfer of dye is inversely proportional to the amount of exposed silver halide and the overall result is a positive of an original scene on the receiver. 

Up to a certain degree the properties of fluorous solvents can be calctdated and thus tuned , especially when observing the strong electron withdrawing effects of insulating groups between the fluorous ponytails and the basic molecule [40] -... 

Appreciable interaction between chromophores does not occur unless they are linked directly to each other, or forced into close proximity as a result of molecular stereochemical configuration. Interposition of a single methylene group, or meta orientation about an aromatic ring, is sufficient to insulate chromophores almost completely from each other. Certain combinations of functional groups afford chromophoric systems which give rise to characteristic absorption bands. 

Polarization which can be induced in nonconducting materials by means of an externally appHed electric field is one of the most important parameters in the theory of insulators, which are called dielectrics when their polarizabiUty is under consideration (1). Experimental investigations have shown that these materials can be divided into linear and nonlinear dielectrics in accordance with their behavior in a realizable range of the electric field. The electric polarization PI of linear dielectrics depends linearly on the electric field E, whereas that of nonlinear dielectrics is a nonlinear function of the electric field (2). The polarization values which can be measured in linear (normal) dielectrics upon appHcation of experimentally attainable electric fields are usually small. However, a certain group of nonlinear dielectrics exhibit polarization values which are several orders of magnitude larger than those observed in normal dielectrics (3). Consequentiy, a number of useful physical properties related to the polarization of the materials, such as elastic, thermal, optical, electromechanical, etc, are observed in these groups of nonlinear dielectrics (4). 

Phosphazene polymers are inherently good electrical insulators unless side-group stmctures allow ionic conduction in the presence of salts. This insulating property forms the basis for appHcations as wire and cable jackets and coatings. Polyphosphazenes also exhibit excellent visible and uv radiation transparency when chromophoric substituents are absent. 

Electrical Properties. Due to the comparatively low content of polar groups, most commercial ionomers ate very good insulating resins. Typical electrical properties (6) for a zinc ionomer are as foUows ... 

Substituted heat-reactive resins are most widely used in contact-adhesive appHcations and, to a lesser extent, in coatings (77,78) -butylphenol, cresol, and nonylphenol are most frequendy used. The alkyl group increases compatibiHty with oleoresinous varnishes and alkyds. In combination with these resins, phenoHcs reduce water sensitivity. Common appHcations include baked-on and electrical insulation varnishes, and as modifiers for baking alkyds, rosin, and ester gum systems. Substituted heat-reactive resins are not used for air-dry coatings because of theh soft, tacky nature in the uncured state substituted nonheat-reactive phenoHcs are the modifying resin of choice in this case. 

Insulating Brick ASTM classifies insulating firebrick under C155 by group. The group number corresponds to the service temperature divided by 100 (Table 13). For example, group 16 corresponds to a test temperature of ca 1600°F (871°C). 

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