Tuesday, November 09, 2010

Flow-Induced Crystallization #3

So the big question about flow-induced crystallization is "WHY?". Why does it occur. And this is where what I use to know and what is now known begin to diverge.

Any flow field takes the random coil that a polymer is in and begins to orient it to some degree. Extensional flow fields (a zone that necks down) is better than a shear flow, but both will do fine. Given this, the old explanation was that the partially oriented polymer had a lower entropy, and so the entropy change upon crystallization would be less than before. At a phase change (such as crystallization), T = DH/DS, where DH is the heat of crystallization and DS is the entropy change upon crystallization. Since DS is now less and DH is constant, the temperature at which crystallization occurs is increased.

Nice idea, but this now appears to be totally wrong - note that the orientation does occur, and the formula given is also correct - it's just that it's now been shown that the entropy change induced by the flow is too small to give a meaningful change in the crystallization temperature.

And so an old idea goes down in flames. But we're just getting started. A bigger blow will come tomorrow in the next post of this series.

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