GLASS TRANSITION TEMPERATURE (Tg)

1)      When a plastic or rubber is cooled up to certain temperature, it becomes so hard and brittle that it breaks into pieces like glass, on application of stress.

2)      The temperature below which the polymer become hard, brittle and glassy and above which it is softer and flexible, is known as glass transition temperature.

SIGNIFICANCE OF GLASS TRANSITION

1)      A polymer  below its Tg, is hard, stiff and brittle and therefore it is not preferred material for use at temperature below its Tg. In general, a polymer materials should have               much lower Tg than the operating temperature. E.g  polymers to be used for refrigerators, air conditioners or used in cold countries, should have lower Tg, so that they will           not break during use.

FACTORS AFFECTING  Tg

1)      Crystallinity : Higher the crystallinity of polymer, higher is its Tg value . Amorphous polymers have lower its Tg.

2)      Molecular weight : Higher the molecular weight of polymer , greater is the Tg of polymer. However, Tg value depends on molecular weight up to 20,000, for vinyllic polymers and after that there negligible effects.

3)      Cross linking of polymer chain : The Tg of polymer increases with higher extent of cross linking of polymer chains.

4)      Coiled polymer chains : The polymer having coiled or spring like chain structure, shows lower Tg. E.g rubbers.

5)     Sided group on chain : Presence of bulky side groups on the polymer chain, increases the Tg value E.g polyethylene has no side groups on chain and has Tg of – 125 degree         Celsius while polystyrene has –C6H5 groups on the chain and has much higher Tg of 100 degree Celsius . Bigger sized side groups and greater polarity of the side groups             have larger effect on Tg value.

6)       Plasticizer effect : Addition a plasticizer of polymer decreases the Tg of polymer. Because, the polymer becomes amorphous and there is increase in chains mobility.

7)       Force of attraction between chain : The polymers having stronger force of intermolecular attraction, have greater Tg, as the chain mobility is lesser in such cases.