Network modifier

Fig. 16.5. Glass formation. A 3-co-ordinoted crystalline network is shown at (a). But the bonding requirements are still satisfied if o random (or glassy) network forms, as shown at (b). The network is broken up by adding network modifiers, like NojO, which interrupt the network as shown at ( ).

The network-modifying ions (commonly alkali and alkaline-earth ions) are ionically bound to the network although the field strength and diameter of the alkali ions allow them some mobility. 

The action of water and acids During attack the alkali and alkaline earth network-modifying ions are exchanged by H" or HjO" from acid solution. 

Addition of an alkali metal oxide as a "network modifier to the "network former causes pH sensitivity, i.e., small amounts of alkali metal induce superficial gel layer formation as a merely local chemical attack and so with limited alkali error larger amounts will result in more pronounced dissolving properties of the glass up to complete dissolution, e.g., water-glass with large amounts of sodium oxide. Simultaneous addition of an alkaline earth metal oxide, however, diminishes the dissolution rate. Substitution of lithium for sodium in pH-sensitive glass markedly reduces the alkali error. 

Inorganic networks, especially glasslike gnes can be characterized by the ratio of network forming (e.g. [siO ] ) to network modifying (e.g. =Si-0 Na ) units. Network formers (e.g. SiO, B 03, A 2°3 T 2 are (in opposition to the majority of organic units) three-dimensional crosslinking units. 

Organic groupings incorporated into this network (e.g. by =Si-R) act as network modifiers as shown in Figure la. 

Introduction of Organic Network Modifiers. The introduction of organic network modifiers into SiO glasses leads to drastic changes of properties. SiO glass, for example, has a thermal expansion coefficient of about 0.5 10 K, monomethyl-SiO glass ([ch SiO. A ) about 100 10 K (40). 

Figure 2. Adsorption isotherms of CC>2 on different network modified adsorbents 70, network former Si02 to network modifier (am) ratio (molar) 30 70 50, 50 50 10, 90 10 0, 100% Si02 ...

Fig. 12.7 Some examples of network formers and network modifiers used for the synthesis of ormosils.

The hydrolytic polycondensation of silicon alkoxides of general formula Si(OR)4 or R/ Si(OR)4 , where the non-reactive organofunc-tional group R acts as a network modifier, is carried out in the presence of dopant molecules resulting in the formation of highly porous, reactive organosilicates whose applications span many traditional domains of chemistry. 

Transition metal Modern raw material Colouring ion Colour in tetrahedral coordination (network former) Colour in octahedral coordination (network modifier)... 

Fig. 4.2 Typical variations in ionic conductivity with composition. In all cases, variations in alkali or silver content are very low compared to the observed variation in log a (a) influence of the network modifier (LijS) (b) influence of a doping salt (c) mixed alkali effect (d) mixed anion effect. References for data are indicated in Souquet and Perera (1990).

The glassy systems mentioned in Figs. 4.1(h) and 4.2 show that quite complex chemical compositions have been prepared in the glassy state. Up to three basic constituents are present in all ionically conducting glasses network formers, network modifiers and ionic salts, in different proportions. 

Qualitatively, the dipole-dipole interactions between the macro-molecular chains and the halide salt compensate for the lattice energy of the halide crystal and tend to decrease the interactions existing in the glass between the oxide macroanions. This decrease is probably the reason for the significant drop in the glass transition temperature resulting from the addition of a halide salt (Reggiani et al, 1978). Furthermore this type of reaction is consistent with the fact that dissolution of a halide salt in a vitreous solvent requires the existence of ionic bonds provided by a network modifier. 

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