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Polymer materials can have a change in physical properties at certain temperatures not attributed to melting. The polymer softens or changes from a relatively hard, brittle state to a rubber like state. The temperature at which this occurs is referred to as the glass transition temperature, Tg. The following table lists the Tg (oC) of some common polymers.

Polymer Tg
polyethylene (PE) -125
polypropylene (PP) -20
polyvinyl chloride (PVC) 80
polytetrafluoro-
ethylene (PTFE)
127
polystyrene (PS) 100
nylon 6,6 57
polyethylene
terephthalate (PET)
70

References
Carlos J. Hilado, "Flammability Handbook for Plastics", Technomic Publishing
Malcolm P. Stevens, "Polymer Chemistry, An Introduction", Oxford Press, 3rd Edit.