An inorganic polymer is defined as a giant molecule linked by covalent bonds but with an absence or near-absence of hydrocarbon units in the main molecular backbone; these may be included as pendant side chains. Carbon fibers, graphite, and so forth are considered inorganic polymers. Much of inorganic chemistry is the chemistry of high polymers.
For compounds that do not melt or dissolve without chemical change, both the absence of an equilibrium vapor pressure and the observation of a dissociation pressure resulting from de-polymerization bring them into the framework of the definition.
Some special characteristics of many inorganic polymers are a higher Young’s modulus and a lower failure strain compared with organic polymers. Relatively few inorganic polymers dissolve in the true sense, or alternatively, if they swell, few can revert. Crystallinity and high glass transition temperatures are also much more common than in organic polymers. In highly cross-linked inorganic polymers, stress relaxation frequently involves bond interchange.
The properties of inorganic polymers require a different technology from that of their organic counterparts. Such technology is either completely new (such as reconstructive processing — the spinning of an inorganic compound on an organic support or binder subsequently removed by oxidation/volatilization), or it has been adapted from other fields, for example, glass technology. Thus, reconstructed vermiculite can give flexible sheets. Yarn, paper, woven cloth, and even textiles can be made from alumina and zirconia fibers by the spinning/volatilization process. A mica-forming glass ceramic is resistant to thermal and mechanical shock and can be worked with conventional metalworking tools.
Inorganic polymers can be classified in a number of ways. Some are based on the composition of the backbone, such as the silicones (Si-O), the phosphazenes (P-N), and polymeric sulfur (S-S). Others are based on their connectivity, that is, the number of network bonds linking the repeating unit into the network. Thus, the silicones based on R2SiO, the phosphazenes based on NPX2, and polymeric sulfur each have a connection of two, while boric oxide based on B2O3 has a connectivity of three, and amorphous silica based on SiO2 has a connection of four.
The number of inorganic polymers is very large. Sulfur, selenium, and tellurium all form high polymers. Polymers of sulfur are usually elas-tomeric, and those of selenium and tellurium are generally crystalline. In the melt at 220°C, the molecular weight of the sulfur polymer is about 12,000,000 and that of selenium about 800,000.
Perhaps best known of all the synthetic polymers based on inorganic molecular structures are the silicones, which are derived from the basic units.
These are amorphous cross-linked polymers with a connectivity of three. Probably the best known is arsenic sulfide, (As2S3)n, which can be used for infrared transparent windows. Threshold and memory switching are also interesting properties of these glasses. Ultraphosphate glasses resemble glassy organic plastics and can be processed by the same methods, such as extrusion and injection molding. They are used for antifouling surfaces for marine applications and in the manufacture of nonmist-ing spectacle lenses.
This is a well-known two-dimensional polymer with lubricating and electrical properties. Intercalation compounds of graphite can have super-metallic anisotropic properties.
Structurally related to graphite is hexagonal boron nitride (BN). Like graphite, it has lubricating properties, reflecting the relationship between molecular structure and physical properties, but unlike graphite, it is an electrical insulator. Molybdenum disulfide, (MoS2)n,with a similar and related structure, is also a solid lubricant. Both graphite and hexagonal boron nitride can be readily machined. Outstanding properties of the latter include high thermal and chemical stability and good dielectric properties. Crucibles and such items as nuts and bolts can be made from this material.
Borate glasses with comparatively low softening points are used as solder and sealing glasses and can be prepared by fusing mixtures of metal oxides with boric oxide, (B2O3)n.
The silicates, both crystalline and amorphous, supply a very large number of inorganic polymers. Examples include the naturally occurring fiberlike asbestos and sheetlike mica. The industrially important water-soluble alkali metal silicates can give highly viscous polymeric solutions. Borosilicate glasses form another important group of silicate polymers. The Pyrex type is well known for its resistance to thermal shock; the leached Vycor type is porous and can be used for filtering bacteria and viruses. Asbestos occurs as ladder polymers, of which crocidolite is the most important, and as layer polymer, exemplified by chrysotile. The zeolites, many of which have been found naturally or have been synthesized, are three-dimensional network polymers. Their uses as molecular sieves are well known.
Silicon nitride, (Si3N4)n, is another macromol-ecule with interesting properties. Prepared by heating of silicon powder in an atmosphere of nitrogen (nitridation) at above 1200°C, the product is a material that can be machined readily and whose good thermal shock resistance and creep resistance at high temperatures, which is further improved by admixture of another inorganic macromolecule silicon carbide, make it useful for applications in gas turbine, diesel engines, thermocouple sheaths, and a variety of components.
Allotropic forms of carbon boron nitride are diamond and cubic boron nitride, both prepara-ble by high-temperature and high-pressure syntheses and characterized by extreme hardness, which make them useful industrially in cutting and grinding tools.
Inorganic polymeric materials are growing in importance as a result of a combination of two major factors: the depletion of the world’s fossil fuel reserves (the basis of the petrochemical industry) and the ever-increasing demands of modern technology, coupled with environmental and health regulations, such as flame retar-dancy and nonflammability.