Primary Valence Networks

A specially good example showing how a large group of important compounds can develop all the above described types—primary valence lattices, primary valence networks and primary valence chains—is afforded by the silicates whose structure elucidation is due in the first place to the experiments of W. L. Bragg and his co-workers over many years. For these very compounds the old method borrowed from organic chemistry of description by the aid of constitutional formulas was shown to be inappropriate. 

A typical absorption curve for vitreous siUca containing metallic impurities after x-ray irradiation is shown in Eigure 12. As shown, the primary absorption centers are at 550, 300, and between 220 and 215 nm. The 550-nm band results from a center consisting of an interstitial alkah cation associated with a network substituent of lower valency than siUcon, eg, aluminum (205). Only alkaUes contribute to the coloration at 550 nm. Lithium is more effective than sodium, and sodium more effective than potassium. Pure siUca doped with aluminum alone shows virtually no coloration after irradiation. The intensity of the band is deterrnined by the component that is present in lower concentration. The presence of hydrogen does not appear to contribute to the 550-nm color-center production (209). 

Conventional rubbers are vulcanized, that is, cross-linked by primary valence bonding. For this reason vulcanized rubbers cannot dissolve or melt unless the network structure is irreversibly destroyed. These products cannot therefore be reprocessed like thermoplastics. Hence, if a polymer could be developed which showed rubbery properties at normal service temperatures but could be reprocessed like thermoplastics, it would be of great interest. 

The linking of plastics molecules by primary valencies into a mainly three-dimensional network. Suitable polymers can also be cross-linked chemically, by adding corresponding bridge-building molecules... 

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