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ORMOCER®s - A new class of polymeric material.

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Introduction

ORMOCER(r)s - A new class of polymeric material 1. Introduction The field of polymer science is one of the most important areas of modern materials research. Much of the technological progress of the last century has been dependent on the development of new materials with specialized properties, and the majority of these new substances are polymers. A polymer is defined as a substance consisting of long chains of much smaller repeating units known as monomers. There are many naturally occuring polymers, including the molecule fundamental to life itself, DNA, which consists of long chains of bases known as nucleotides. However the great majority of polymers used commercially are man-made, being synthesised by the direct polymerisation of the starting monomers. One of the simplest examples of a synthetic polymer is familiar by name to everyone: polythene, or polyethylene. This is formed by the linking of a large number of discrete molecules of ethylene, C2H4, into long hydrocarbon chains. Fig 1. Scheme showing the polymerization of n molecules of ethylene into a polyethylene chain of n repeat units. As can be seen, polythene is formed from a single type of monomer, ethylene, and it is termed a homopolymer. Polymers formed from the polymerization of more than one type of monomer are called copolymers. Materials of this second class are particularly important because their physical properties are often superior to those of the respective homopolymers. Among the best-known examples of commercially useful copolymers are the nylons, formed by the copolymerisation of organic compounds known as diamines and dicarboxylic acids. 2. Organic and inorganic polymers Like most polymers, both natural and synthetic, the above examples are organic: they are based upon a long chain of carbon atoms formed by the polymerisation of organic starting molecules. The properties of synthetic organic polymers vary enormously, depending for the most part (though not exclusively) on the chemical identity of the repeat units. ...read more.

Middle

followed by curing to promote organic crosslinking of the Si-O-Si networks. R* is a reactive organic group capable of polymerizing to the species R. A significant advantage of this method of ORMOCER(r) synthesis over the first two is the dramatic reduction in shrinkage of the gel during evaporation of the liquid. Shrinkage is a serious problem in many sol-gel syntheses and often restricts the extent to which the gel can be used as a bulk material; organically crosslinked ORMOCER(r)s show much less shrinkage than normal due to the preformation of the inorganic network. In special cases it is possible to specifically design non-shrink crosslinking ORMOCER(r) syntheses which do not require an evaporation step at all because the alcohol used to contain the sol/gel, and the alcohol expelled by the hydrolysis reaction, are both polymerizable and crosslink during the curing step. A wide range of polymerizable organic entities can be used for this third class of ORMOCER(r) synthesis, giving rise to a wide variation in the properties of the products. Among the most important classes of polymerizable group are epoxides, acrylates and thiol/enes. Epoxides contain an oxygen atom bound to two carbon atoms which are also bound to each other, forming a three-membered ring. On polymerization the ring is broken, or "opened", and a polymer is formed containing C-O-C, or polyether, linkages. Acrylates contain carbon-carbon double bonds (C=C) adjacent to carbonyl groups (C=O) and polymerize, by a variety of mechanisms, to acrylics. Thiols are organic entities terminating in the group -SH. Alkenes contain C=C double bonds. The two can copolymerize in the thiol/ene addition to form thioether linkages, C-S-C. a) b) c) Fig 5. General polymerization schemes for a) epoxides, b) acrylates and c) alkenes and thiols (thiol/ene addition). 5. Properties In general, ORMOCER(r)s are transparent, homogeneous, electrically insulating materials with a density slightly above that of typical organic polymers. They are also strongly resistant to abrasion, and can be used as abrasion-resistant coatings. ...read more.

Conclusion

species to the polymer matrix. Electrically conducting polymers can be used as electrolytes in, for example, batteries and capacitors. Research on applying the above technique to ORMOCER(r)s has yielded promising results. One research group has found that, by synthesising type III ORMOCER(r)s in the presence of sulfur trioxide, the species SO3H can be incorporated into the network and the ORMOCER(r) can conduct electricity by the flow of protons, H+. By varying the proportion of the SO3H species from 0.1 to 0.6, the degree of electrical conductivity could be varied by a factor of 100,000. These and other researchers have also developed type III ORMOCER(r)s capable of conducting electricity by the flow of lithium ions, Li+. This is done by dissolving lithium salts in the sol before the curing step. These ORMOCER(r)s may have applications in lithium batteries. Conducting ORMOCER(r)s can be prepared as surface coatings or as molded gels and offer the same advantageous physical properties (combination of strength and elasticity) as ORMOCER(r)s in general. In addition, they have two advantages over purely organic conducting polymers due to their inorganic backbone: they are stable at higher temperatures, and they do not show the tendency to adopt crystalline structures and hence show the decrease in electrical conductivity which typically accompanies this. 7. Conclusion In the last decade much research has been devoted to inorganic-organic nanocomposites, or ORMOCER(r)s, substances in which organic and inorganic networks are mixed at a molecular level. Synthesised via the sol-gel process, ORMOCER(r)s include molecular-level polymer dispersions (type I), organically functionalized silica networks (type II), and organically crosslinked silica networks (type III). These hybrid polymers combine some of the advantageous physical properties of organic polymers (such as flexibility) with those of inorganic materials (such as mechanical strength), and can be tailored further by the choice of organic components. Due to these useful and variable properties, ORMOCER(r)s have found a wide range of applications including specialised coatings, bulk material for dental fillings, and battery electrolytes. Research into their potential applications continues, and promises to yield interesting results. 8. ...read more.

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