Upon cooling an amorphous material from liquid state, there is no sudden change in volume such as occurs in the case of cooling of a crystalline material through its freezing point. The glass transition temperature, Tg, occurs over a range of temperatures.

The determination of Tg ,for amorphous polymers by various methods, is found to be rate- dependent. For an amorphous polymer, the higher Tg value is obtained with a substantially higher cooling rate than the lower Tg value.

This rate dependence can be understood in terms of intermolecular relaxation processes. Since a glass is not an equilibrium phase, its properties will exhibit a time- dependence. The main part of the relaxation behavior governing the glass transition in polymers can be related to their tangled chain structure where cooperative molecular motion is required for internal readjustments. At temperatures well above Tg, few repeat units of the polymer backbone are relatively free to move in cooperative thermal motion to provide conformational rearrangement of the backbone. Below Tg, the motion of these individual chain segments becomes frozen with only small scale molecular motion remaining, involving individual or small groups of atoms. Thus a rapid cooling rate or "quenching" takes rubbery material into glassy behavior at higher temperature (higher Tg).

The main structural relaxation in polymers is the segmental, so-called alpha-relaxation. It controls diffusion, viscosity, and rotation of monomeric segments. The alpha-process is usually attributed to micro-Brownian motion of chain segments. Most scientists agree that this process is related to conformational changes (like gauche- trans transition).

Thank you very much Mr Matar for the great explanation.