What is Glass-Transition Temperature?

In this lecture, we have discussed about a few important aspects of polymeric materials which are strongly related to their mechanical properties. A very unique characteristics of polymer is the presence of Glass-transition temperature.

There is a certain characteristic temperature(different for each polymer) called the glass transition temperature, or Tg for short. When a polymer is cooled below this temperature, it becomes hard and brittle, like glass.

Some polymers are used above their glass transition temperatures, and some are used below. Hard plastics like Polystyrene and PMMA are used below their glass transition temperatures. The Tg of PMMA is about 105^oC and the same for Polystyrene is about 95⁰ C.

On the other hand, some elastomers like Polyisoperene and Polyisobutylene, are used above their Tg's, that is, in their rubbery state, where they are soft and flexible.