Question

What is glass transition temperature ?Write its significance .

Answer 1

1) There is a temperature boundary for almost all amorphous polymers and many crystalline polymers,only above which the substance remains soft,flexible,rubbery and below which it becomes hard,brittle and glassy.
Eg. An ordinary rubber ball if cooled below -70*C becomes so hard and brittle that it will break into pieces like a glass ball falling on a hard surface. 

2) The temperature below which a polymer is hard,brittle and glassy and above which itis soft and flexible is called as ‘Glass Transition Temperature’(Tg).
3) A polymer is not preferred to be used at temperatures below its glass transitiontemperature since it becomes hard,stiff and brittle.
4) A polymer material should have much lower Tg than the operating temperature.
Eg polymers to be used for refrigerators, air conditioners or used in cold countries should have lower Tg,so that they donot break during use.
5) At glass transition temperature,the internal energy of the polymer increases to thatextent where chain segments of a polymer molecule just start leaving their lattice sites.