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Collections of molecules can exist in three possible physical states: solid, liquid and gas. In polymeric materials, things are not so straightforward. For example most polymers will decompose before they boil, and cross-linked polymers decompose before they melt. For many polymers the transition between the solid and liquid states is rather diffuse and difficult to pinpoint. Amorphous polymers are viscous liquids when they are held at temperatures above their glass transition temperature, Tg. Below Tg, the material is solid, yet has no long-range molecular order and so is non-crystalline. In other words, the material is an amorphous solid, or a glass.
The glass transition temperature is different for each polymer, but many polymers are above Tg at room temperature. In many cases the polymers are at least partially crystalline at room temperature and the temperature at which the crystals melt (Tm) is above room temperature. The graph below shows how some polymers are above Tg but below Tm at room temperature. Such polymers are rubbers (so long as they are largely amorphous) at room temperature. However, the polymer may flow like a liquid over long time periods as its amorphous component relaxes under the polymer's weight.
The glass transition of a polymer is related to the thermal energy required to allow changes in the conformation of the molecules at a microscopic level, and above Tg there is sufficient thermal energy for these changes to occur. However, the transition is not a sharp one, nor is it thermodynamically well defined. It is therefore different from melting a crystal, as will be explained later.
A distinct change from rubbery (above Tg) to glassy (below Tg) behaviour is readily observable in a wide range of polymers over a relatively narrow temperature range. In the following sections the behaviour of polymers around the glass transition temperature will be explored. The effects of strain rate, cooling or heating rate and other factors affecting the glass transition temperature will also be explained.