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When different elastomers are being described, a fundamental property which is often quoted is the 'glass transition temperature, Tg', which differs from one elastomer to another. For example, for natural rubber Tg is -70°C (-95°F). Broadly this means that above -70°C the material behaves as a rubber, but below -70°C the material behaves more like a glass. When glassy, natural rubber is about one thousand times as stiff as it is when rubbery. When glassy a hammer blow on natural rubber will cause it to shatter like a glass; when rubbery the hammer is likely just to bounce off.

Of course in practice the dividing line between the glassy and rubbery behaviour just described is not as sharp as this. In fact the transition is spread over some tens of degrees - but it is centred around -70°C. Thus, although a Tg can be accurately defined (although varying somewhat with the precise test conditions), for practical purposes we have to consider a glass transition region within which the properties are slowly changing from rubbery to glassy or vice versa (the processes are completely reversible). The broadness of this transition region varies from elastomer to elastomer.

Strictly speaking we should only use the term 'elastomer' to describe a material when it is above its glass transition temperature, but such a distinction would generally be regarded as pedantic. Definitions of this type (such as 'elastomer' or 'glassy polymer') are accepted as applying to the state of the polymer at ambient temperature.

The reason for the existence of a glass transition temperature can be understood in terms of the molecular model of rubber which has been described above. It has been seen that, at normal temperatures, the molecular chains are in a constant state of thermal motion, that they are constantly changing their configuration, and that their flexibility makes them reasonably easy to stretch. It is not difficult to appreciate that as the temperature is lowered the chains become less flexible and the amount of thermal motion decreases. Eventually, a low temperature, the glass transition temperature, is reached, where all major motion of the chains essentially ceases. The material no longer has the properties which make it an elastomer, and it behaves as a glass.

For all practical engineering uses of elastomers we require good flexibility - so it is essential that we use them only at temperatures which are comfortably above the glass transition. This is generally no problem for natural rubber with a Tg of -70°C, or cis-butadiene rubber with a Tg of -108°C (-160°F). But many elastomers, especially those which have been designed to be highly heat or oil resistant, have much higher Tgs, and this must be borne in mind when selecting them for service applications. For example some fluoroelastomers, which have excellent oil and heat resistance, have a Tg not far below normal room temperature. This can result in problems if a component required to work at high temperature also has to serve the same function on cooling down, and must be considered in design.

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