Tg - Glass transition temperature - (May/12/2005 )

hello
how to define"Tg"- glass transition temperature.What will happen to a substance below/above Tg?What does it mean "glass transition"?

hi

Glass transition temperature is the temperature at which a material's characteristics change from that of a glass to that of rubber. This temperature varies widely for thermoplastics.

Amorphous polymers at temperatures well below the Tg value are hard, stiff, glassy materials. On the other hand, at temperatures well above Tg polymers are rubbery. The temperature region around Tg is often called the leathery region. The glass transition is not a sharp transition but a gradual transition. A plot of modulus against temperature shows that the elastic modulus drops by a factor of about 103.

For semi-crystalline polymers the changes in properties around Tg are not so dramatic, but above Tg semi-crystalline polymers are not normally brittle.


good link here

but a little googling would help surely better....