Question: How do you create polymer molecules that are that thin. And are they mainly addition, condensation or another type?

Dave Farmer answered on 18 Jun 2013:

Hello kerenhb,

First and foremost, the polymers I use are made in a variety of methods, but that’s not something I worry about, I buy them already made. I’m a polymer physicist not a chemist! When I make very thin polymer films, however, it’s important to understand a bit about the size of the polymers.

In my last publication, we made polystyrene films ~100 nm (nanometre is 10^-9 m) thick. The polystyrene molecules we used to make these were made up of roughly 1800 repeated monomers. If you know how big a monomer is, you can then calculate the contour length (or how long the polymer is stretched out). In this case, that’s ~370 nm.

So how did make them into a thin film? Well what you need to remember is that polymers don’t lie all flat and stretched out like they appear in your textbook. That’s a very unfavourable state for them to be in. Actually they are all coiled and curled up on themselves. We can estimate how big these coiled ‘blobs’ of polymer are as well. For the case above, we get ~10 nm. So even quite large polymers can be squeezed into small dimensions.

The way we create the films is very simple. We dissolve the polymer chains in a solvent (normally something quite volatile so it evaporates quickly). We then place a few drops of this solution on to the substrate that we want (I mostly use silicon) and spin it very fast (a few thousand rpm). This spreads the drop out, and as the solvent evaporates, we are left with a very thin, very flat polymer film.

I hope this answers your question, let me know if you’d like to know anything else.

Dave