What is the glass transition temperature (Tg)?
The glass transition temperature (Tg) - or more precisely: the glass transition range - is the temperature range in which an amorphous or semi-crystalline plastic or. Elastomer from a rubbery-elastic to a glassy-hard state. Above the glass transition temperature, the molecular chains are flexible enough to allow elastic deformation. Below this temperature, the material solidifies and largely loses its flexibility. In practice, the glass transition temperature is a decisive parameter in the selection of cold-resistant materials. For example, the Tg of silicone rubber (VMQ) well below -70 °C, while FKM depending on the type, changes to the glassy state at around -40 °C.
Factors influencing the glass transition temperature (Tg)
- Polymer structure: flexible main chains and side groups reduce the Tg, rigid structures increase it.
- Plasticisers: become embedded between the polymer chains and increase their flexibility, which lowers the Tg.
- Fillers: can increase or decrease the Tg depending on the type and interaction with the polymer matrix.
- Degree of cross-linking: higher cross-linking density restricts chain mobility and usually increases the Tg.
Measuring methods of the glass transition temperature (Tg)
Two methods have become established for determining the glass transition temperature, which are most frequently used in both research and industrial quality control:
- Differential scanning calorimetry (DSC): In DSC measurement, a small material sample is heated or cooled in a controlled manner. The device records the heat flow in comparison to a reference sample (usually an empty sample capsule). The heat capacity of the sample material can be determined from the difference in heat flow. The heat capacity of the material changes during the glass transition and is shown as a characteristic step in the heat flow diagram. The method is fast, reproducible and particularly well suited to clearly visualising thermal transitions.
- Dynamic mechanical analysis (DMA): In DMA, the sample is subjected to a small, periodic mechanical load (bending or tensile impulse) while the temperature is gradually changed. The test device measures the deformation of the material sample and uses this to calculate the complex shear modulus, consisting of the storage modulus (real part) for the elastic component and the loss modulus (imaginary part) for the viscous component. In the glass transition area, the complex shear modulus drops sharply, while the loss modulus reaches a maximum. The DMA tests the real mechanical behaviour of an elastomer particularly precisely.