Temperature of vitreous transition
The temperature of vitreous transition from a material T g, is the Température below which the Molécule S have a low relative mobility or in other words, with the greatest modification of the capacity of deformation. The temperature T g is usually used for the phases completely or partially amorphous S such as the Verre S and the plastic S.
For glasses mineral (or inorganic), such as glass of Silica (SiO2), it is defined as being the medium of the interval of temperature in which they become gradually more viscous and pass from the state Liquide in a solid state .
The polymers Thermoplastique S (not réticulés) are more complex because, in addition to having a melting point, T m, above them structure Cristal line disappears, of such plastics has one second temperature, lower T g below which they become rigid and breakable, and can fissure or break in glares. The pure substances of weak Molecular weight such as the Eau have only one temperature of transition at the solid state: below they are crystalline solids (or amorphous Glace if cooling in lower part of T g is rather fast) and above they are liquids.
Above T g, the secondary connections not Covalente S between the chains of polymers become weak compared to thermal Agitation, and the polymer becomes flexible and able to become deformed or elastically plastically without rupture. This behavior is one of the properties which return the majority of the so useful plastics. Such a behavior does not exist for the plastics Thermodurcissable S réticulés which, once manufactured, are it in an irreversible way and will break in glares rather than to become deformed, without never becoming again plastic nor to melt while being heated.
The temperature of vitreous transition from a material can be measured, for example, by DSC or with a Viscoanalyseur.
Temperature of vitreous transition from some materials
They are only orders of magnitude, because the temperature of vitreous transition depends the speed of cooling and on the distribution of the molecular weights, and it can be influenced by the additives.
It should be also noted that for a semicrystalline material such as the polyethylene which is crystalline to 60 80% with room temperature, the temperature of vitreous transition indicated corresponds to that of the amorphous part of material when the temperature drops.
References
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