Monday, August 12, 2013

Combating Molecular Weight Reduction from Shear Forces

The dirty little secret of polymer processing - one that everyone in industry knows but is seldom mentioned in any textbook - is that pumping a polymer causes the molecular weight to breakdown. Since most polymers are "solid" at room temperature, pumping them usually requires heating them too which only acerbates the breakdown [*]. Whether the "pumping" is from an extruder, an injection molder, a piston, or any other process, the result is the same - the molecular weight drops, and there is little you can do about it.

But that might be changing. Nature Chemistry has a new report on an approached to offset the reductions in molecular weight. As is shown in the illustration, the researchers incorporated dibromocyclopropane in the backbone of the polymer. The shear stresses then force open the ring, creating allylic bromides which can then crosslink with other chemicals mixed into the polymer.
The researchers were able to show that it is shear forces and not thermal energy opening the rings by creating polymers where the dibromocyclopropane were moieties hanging off the backbone rather than in the backbone. The sidearms experience very little of the shear forces and the results showed that - very few of the rings opened up.

You can criticize this research as just being the latest addition to the arena of "self-healing polymers", one that is already well-crowded and filled with academic creations that are without real-world merit, but this result is far more than just a self-healing polymer. This repair occurs while the polymer is molten and being processed, not later after it has been made into a final product. It appears to be the first such example of that concept. Which means that it has the usual bumps and warts and will likely never leave the lab, but it will provide the inspiration for others to do better.

It's about time our dirty little secret is confronted head on.

[*] It is this breakdown that leads many people to erroneously conclude that recycling plastics is downcycling them, an issue I addressed back in May.

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