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Osmotic pressure: Spontaneous balloon popping

One common demonstration presented in science classes involves a set of balloons that are filled sulfur hexafluoride (SF₆) almost to the point of popping. SF₆ is a large, heavy molecule that forms a colorless, odorless gas at standard temperature and pressure. What makes these demonstrations interesting is the fact that, if the balloons are filled at the beginning of the class period, they will spontaneously pop sometime during the class.

Why would these balloons spontaneously pop? Take a look at the simulation on this page, which depicts a cross-sectional view of a balloon, and see if you can figure it out.

The concentric dotted lines represent the balloon’s rubber membrane. Also, the blue particles represent SF₆ molecules, while the green particles represent nitrogen molecules from the air outside the balloon. In this simulation, the balloon won't actually pop but by paying attention to what happens to the two different types of molecules, you can probably make a good guess about why the balloon might pop.