For which types of polymers, the crystallization peak in heating cycle of DSC test can be observed and for which types cant? In another word, when this exothermic crystallization can be observed in the heating cycle (also in its cooling cycle) and when just in its cooling cycle?
Dear Majid Sohrabian ,
Crystallization peaks in polymers are observed when the polymers are semicrystalline. You won't observe any peaks in amorphous polymers.
A polymer being semicrystalline or amorphous will depend on the structure of the polymer, and the capacity of the chains to become compact and form cristallyne domains or not.
Hope this helps!
Mostly, the crystallization peak appears during cooling. However, you can see the peak both during heating and cooling for polylactide (Polylactic acid).
Crystallization during heating requires the amorphous (or semicrystalline) polymer to be significantly above its glass transition temperature (Tg), but of course below its melting point. The requirement concerning Tg is due to the fact that the polymer chains need enough mobility to be able to order themselves into crystalline domains. It follows that the crystallization temperature depends on heating (or cooling) rate, and the phenomenon can be missed entirely if the heating (or cooling) rate is too fast to allow this ordering process.
Tg of a polymer is not always visible by DSC (high heating rate can make the glass transition more obvious), but if you can find it you'll have an idea of where to look for a crystallization peak.
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