Last week I posted about flow-induced crystallization, an eminently useful concept. PET is a useful material because of it. Without it, the polymer would crystallize too slowly to be useful. Another closely related field which is far more fascinating from an intellectual stand is flow-induced phase separation.
This is actually a much broader field, as the term "phase" can include both polymer-polymer phase separation and polymer-solvent phase separation. Regardless of the specifics, think of it as de-mixing caused by the flow field. This is the opposite of normal experience. If you are trying to dissolve sugar in your lemonade, you use a flow field to help the dissolution, don't you?
As was mentioned earlier, the flow field changes the shape of the polymer and so the thermodynamics of the system are changed. In many cases, the change is too small to have an impact, but in some cases it can lead to tremendous changes.
In my studies, I was working with a dilute solution (0.1 wt%) of ultrahigh molecular weight polyethylene dissolved in hot xylene. When working with polymers in solvents, "dilute" is not just a qualitative description, but is actually a semi-quantitative term. For those working in the field, "dilute" means that the polymer concentration was so low that each polymer chain was physically isolated from the other chains. There was no overlap of the coils.
When this solution was put in an extensional flow field however, the coils begin to overlap and came crashing out of solution. That may seem obvious, but look at it from a different perspective: the flow field caused the polymer chains to move UP a concentration gradient from 0.1% to 100%. Powerful stuff.
Of course, the industrial application were very limited. Does anybody really want to work with a 0.1 wt% solution? Even with a top-notch solvent recovery system, you still will have very low output.
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