Glass Transition Temperature (Tg)
Glass transition temperature (Tg) is defined as the temperature at which the internal energy of polymer molecules increases to such an extent that the chain segments of the polymer molecules are just ready to leave their lattice position. At this temperature, the chain segments start moving past one another even if an infinitesimally small strain is applied to the polymer.
In simple way, the temperature at which a polymer changes from a hard, glassy state to a soft, leathery state, or vice versa is called glass transition temperature. It is directly related to a strength of material and mechanical properties. The glass transition temperature is a thermal property of amorphous as well as semi-crystalline region while crystalline region exhibit a Tm (melt temperature). The value of Tg depends on the mobility of the polymer chain, and for most synthetic polymers its value lies between 170K to 500K.
Below the glass transition temperature, the chain segments of the polymer are frozen on the lattice sites. The polymers in this conditions is hard, invariably brittle and hence, breakable like glass.
Above the glass transition temperature, the segments of the polymer begin to exhibit difussional motion as do the molecules of a liquid.
At temperature sufficiently higher than Tg, the polymer is present in the molten state. In this state there is exchange of bonds between the atoms.
The crossed linked inorganic polymers show a wide range in their glass transition temperatures. In this temperature range, they change from rigid solids to leathery solids, from leathery solids to rubbery solids and finally to highly viscous liquids.
The glass transition temperature of a linear polymer is fairly sharp. This is because the movement of chain segments from one site to another does not involve the exchange of bonds as happened in the case of crossed linked inorganic polymers. The chain in a linear polymers are held together by weak Van der Waal forces which are overcome by the supply of even a small amount of energy. Therefore, the energy required for the movement of chains in such polymers is small. Consequently, the glass transition temperature of linear polymer is lower than of a cross linked polymer in which the movement of chain segments requires the breaking of some bonds.
Glass transition temperature of a polymer can be affected by its chemical structure, molecular weight, degree of crystallinity, and the presence of plasticizers or additives. Amorphous polymers generally have lower Tg values compared to semi-crystalline polymers, which exhibit both Tg and a melting temperature (Tm) due to their crystalline regions
Glass transition temperature is measured by using Differential Scanning Calorimetry (DSC) or Dynamic Mechanical Analysis (DMA), where changes in heat capacity, thermal expansion, or mechanical properties are observed as the temperature changes.
