What is inhibiting polystyrene's ability to dissolve in traditional solvents like ethanol or acetone? My friend and I have been struggling with this problem for a couple days. Because polystyrene has a similar structure to many plastics, we assumed that it would dissolve in acetone. However, after running several tests, we found that polystyrene would not dissolve in a measurable quantity even if we let it sit in acetone for a significant amount of time.

Here is the plastic we are attempting to dissolve:

https://imgur.com/rdawCIP

And here is a plastic more easily dissolved:

https://imgur.com/hAVMUsM

We understand that most molecular-bonded plastics dissolve via van der Waals dispersion forces, so why isn't polystyrene dissolving? My theory is that the benzene ring is interfering with the dissolving process -- that only half the compound of polystyrene is interacting with the acetone and thus it does not dissolve like a polyethylene (which obviously has twice as much surface area to dissolve).
Polystyrene is soluble in acetone, MEK and other ketones, but solubility and the speed with which it dissolves depends, in part, on the molecular mass of the polymer.

If the solvent diffuses very slowly into the polystyrene, only the surface will appear to be affected. Applying a bit of energy with heat or mechanical crushing of the plastic greatly speeds the action of the solvent.

For more technical information, see Solubility Parameters: Theory and Application.