Unit 3- Class 9- High Polymers (Glass transition temperature (Tg))

Glass transition temperature (Tg): is the temperature, below which a polymer is hard and above which it is soft and flexible is called the glass transition temperature. The hard, brittle state is known as the glassy state and soft flexible state as the rubbery state.

Parameters affecting Tg:

1.    Crystallinity:  Higher the crystallinity, higher is the Tg value of a polymer. In crystalline polymer, the linear or stereo regular chains are lined up parallel to each other and are held by strong cohesive forces. This leads to a high Tg value of the polymer.
Egs: polyethylene has low Tg compared to that of nylon6,6.

2.    Molecular weight: The Tg of all polymers, in general, increases with molecular weight up to 20,000 and beyond which the effect is negligible.
3.    Effect of side group: Poly [a- methyl styrene] has higher Tg value while polystyrene has lower Tg value, is due to the presence of effective methyl side group which hinders the free rotation about carbon-carbon bond of chain back bone, and restricts the chain mobility, thereby increase Tg value.
4.    Intermolecular forces: Presence of large number of polar groups in the molecular chain lead to strong intermolecular cohesive forces which restrict the molecular mobility. This leads to an increase in Tg. Egs: polypropylene has lower Tg compared to nylon6,6 .
5.    Presence of plasticizers: Addition of plasticizers reduces the Tg value. Egs: Diisooctyl phthalate, which is added to PVC reduces its Tg.
6.    Srereoregularity of polymers increases Tg. Thus Tg of a isotactic polymer is greater than that of the syndiotactic polymer whose Tg is greater than atactic polymer.

We will discuss about the importance of glass transition temperature in our next class.