Monday, December 14, 2009

Thermal Hystersis

Pretty much as long as synthetic polymers have been known, it's also been known that the crystalline polymers don't melt and crystallize at the same temperature. This doesn't violate thermodynamics as thermodynamics requires equilibrium and what is observed here is a matter of kinetics - the polymer is cooled too quickly to crystallize at the melting temperature. Going the other direction - from crystal to melt, the system is able to take in energy, rearrange itself (annealing) and then melt at or just below the equilibrium melting temperature. That is why it is common to run two melting passes on the DSC of a polymer. The first heating pass is the result of the most recent thermal history of the sample, while the second (run after slow cooling) will better define the true melting temperature of the sample.

As Uncle Al has observed, "Thermodynamics propose, kinetics disposes".
"Man Propose, God Disposes" by Sir. Edwin Landseer, depicting the fate of some early artic explorers.

I've never had a nifty handle for this behavior, but now, courtesy of biologists, I do: thermal hysteresis. A good name: succinct but able to communicate what the item is. As I mentioned, biologist are all over this term: All the "anti-freeze" proteins exhibit this behavior, as well as the newly discovered protein-free anti-freeze glycogen.

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