IntermediateTopic 44 of 100Materials & Chemical
Glass transition temperature
Polymers change from glassy to rubbery at Tg.
In depth
Tg is where amorphous polymers change from hard glassy to soft rubbery behaviour as chain segments become mobile. Below Tg brittle (PS Tg≈100 °C); above, flexible (PE Tg≈-120 °C). Controls service temperature; plasticisers lower Tg. Measured by DSC or DMA.
Key points
- Tg vs Tm
- Cooling rate dependence
- Crystallinity effects
ExamplePVC is rigid at room temperature but plasticised PVC is flexible because Tg is lowered.
What is Glass transition temperature?
Polymers change from glassy to rubbery at Tg.
Why is it important to understand glass transition temperature?
The glass transition temperature marks when a polymer changes from stiff to soft, so it sets the maximum service temperature for many plastics.
What is Tg?
The temperature at which an amorphous polymer changes from a glassy, rigid state to a rubbery state, and it is a key design limit.
Why do some plastics have both Tg and a melting point?
Semi-crystalline polymers have a glassy amorphous region and crystalline regions, so they show a Tg and a melting temperature.
How is Glass transition temperature used in practice?
Typical applications include selecting a polymer for a component that must stay stiff at 80 degrees, explaining why a cold plastic shatters when struck and choosing an amorphous plastic for optical clarity with a known Tg.
What should I learn before Glass transition temperature?
Study "Viscoelasticity" first (topic 43 of 100 in Materials & Chemical). It covers the groundwork this topic builds on.
What are the key points of Glass transition temperature?
Tg vs Tm. Cooling rate dependence. Crystallinity effects.
Can you give an example of Glass transition temperature?
PVC is rigid at room temperature but plasticised PVC is flexible because Tg is lowered.
What comes after Glass transition temperature?
The next topic is "Polymer processing", which builds directly on this one.
How difficult is Glass transition temperature?
It is classed as Intermediate in Materials & Chemical (topic 44 of 100). It is an intermediate topic. Work through the beginner topics in Materials & Chemical first.