material behavior at subambient temperature

[tmvo]

I am perfoming subambient Tg test on a thermosetting prepreg. However, I am not clearly understand what happen to the prepreg at extremely cold temperature ? and what happen to the prepreg when heating up from cold temperature to room temperature.
This is what I know:

At extremely cold temperature, themosetting molecules are freezed and they have very minimal mobility, which the material state is like solid. When heating it up, the mobility of molecules are increasing, which resulting relaxation of polymers. As such, the material state is changing from solid to rubbery state.

However, I feel like this explaination have some problem, perhap someone could help me with clear understanding of this phenomenon.

 

[Demon3]

You understand it right. Heating materials makes them softer and cooling makes them harder like ice turning to water and back.

Of course if you heat the epoxy enough it will react and become permanently hard.

Chris DeArmitt PhD FRSC CChem

 

[Compositepro]

Subambient Tg (by DSC)is run on uncured prepreg to detect advancement (aging) that will affect tack and drape. There is a very slight change in heat capacity of resin at Tg and this is what is detected. I've seen very few labs that have the instruments and skill to be able to get reliable and consistent results with this test.

 

[tmvo]

Thank you Demon3 and Compositepro for the explaination.
In addition to that, do you know who is moisture effect on subTg determination? I'd been told by someone that moisture has no effect on DSC's results, I wonder if it is right or not?

 

[Compositepro]

Absorbed moisture will act as a plasticizer or solvent and will reduce the sub-ambient Tg. It will also reduce the cured Tg but this effect is reversible and the sample will dry before getting to the Tg.