Monday, September 2, 2013

RIPE: Polymers and Entanglements

Let's start with three everyday examples of entanglements.

(i) I am love my iPod and listen to it quite a bit. Usually, after I am done listening, I just stick it, along with headphones, into my pocket. The next time I pull it out, the headphones are all tangled up, and I often wonder what the heck happened to them in my pocket.

(ii) A similar thing happens every time I use my leaf-blower, and don’t stow away the cables carefully. It is a mess!

(iii) As a father of daughters, I am a big fan of short hair. Why? Long hair gets tangled up more easily, and needs more effort combing (de-entangling).

Headphones, cables, and hair – they all have a natural tendency to get all tangled up.

The “longer” these things get, the more trouble they are.

What has this got to do with polymers?

Polymers are long chain-like molecules formed by stringing together units called monomers. As the number of units increases, their “molecular weight” increases, as does their propensity to get tangled up.

Indeed, there is a threshold molecular weight – different for different polymers – called the entanglement molecular weight, beyond which their rheology is strongly influenced by these entanglements. (This opens up a fascinating cascade of research questions that I have spent countless hours pondering!)

Below the entanglement molecular weight, the viscosity (the resistance to flow) of polymer melts increases gently. Double the molecular weight, and you double the viscosity.

Beyond the entanglement molecular weight, this linear increase in viscosity is abruptly disturbed (see a picture here). In this highly entangled regime, a doubling of molecular weight results in nearly a 10-fold increase in viscosity.

Take that!

Footnote: Polyethylene in your milk jars, and polystyrene in Styrofoam cups have entanglement molecular weights of about 1000 and 15,000 daltons respectively.

Footnote2: RIPE = Research In Plain English

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