Learning how to create a polystyrene film

Yesterday, we made solutions of approximately 1.4% polystyrene dissolved in toluene using an analytical balance (5 significant figures). In experiments that require exact concentrations of solute, solvent volume is measured by mass (using specific gravity to convert volume to mass) as it is more precise. We had to abandon the lab after noon however, as there was a chemical spill and the site needed to be cleared of toxins by reverse air flow.

Today, we used a spin coating device in the lab, which is a bit like a centrifuge with a flat plate that spins at 128,000 rpm. A glass slide is placed exactly in the center of the plate (if it is off center, it flies off and shatters), then the 1.4% polystyrene is pipetted onto half of the glass slide and spun for one minute. This creates a coating of polystyrene on the glass slide that is about 100 nm thick. After the coated slide is removed from the spin coating device, a template and diamond tipped pen is used to cut the thin coat of polystyrene into a precise circle about 2cm wide. Of course, one cannot see the polystyrene coat... it is too thin to be visible; only the difference in light reflection and refraction allow us to tell which half of the glass slide has the coating on it.



Once we have scored the circle on the polystyrene coat, we remove the circle and float it on water. We do this by slowly immersing the slide into the water at a 45 degree angle. This liberates the circle of polystyrene from the slide, and the 100 nm thick circle of polystyrene then floats on the water. Again, we can only see this by differences in reflection and refraction.

Next, we used a microsyringe to put a 0.2 microliter drop of water ON the thin floating film of polystyrene. A distinct (but so tiny!) pattern of wrinkles appears around the drop that is characteristic of polystyrene in number and length of the wrinkles. This is where Young's modulus, Hooke's law, Poisson's ratio and surface tension come into play. There are actually three interfaces here: air and polystyrene, air and water, and water and polystyrene. Each has a characteristic "pull" in the interactions at the interfaces.

If you are interested by all of this, you can do a crude mock up of it at home using plastic food wrap cut into a small circle, floating it on a water bath (water in a bowl), then putting a small drop of water in the middle. Look at the wrinkling pattern..then change the size of the drop of water..or what you are floating it on..or the size of the film...or whatever you think of. See what happens!

Click here to learn more about:

Hooke's Law
Poisson's Ratio
Surface Tension