Why is a material such as plastic air-tight? I'm not too familiar with fluid mechanics, but i always wondered how let's say plastic containers would be able to contain a fluid, such as gas or water. How does plastic prevent the molecules of the fluid from crossing it? Is it because of a high density relation at the boundary? Does it have anything to do with energy levels?
 A: When a molecule from a fluid approaches molecules from a solid boundary, interaction repelling forces between them increase. As molecules from the solid are prevented from moving by their interactions with other molecules from the solid, it is the molecule from the fluid  that conserves all of the momentum and bounces back. In doing so, it exerts a force on the solid which averages as the fluid's pressure when you consider all the molecules from it hitting a patch of surface.
If the material is porous, the fluid molecule may enter a cavity and bounce within it. If it is porous enough, there are some odds that from cavity to cavity it reaches the other surface of the solid material. Of course, the size of the molecule from the gas is important for this. This is called permeation.
A: Much depends on the structure of the plastic. I learned from a talk by an engineer working in the field of production of plastic tubes that low density plastics such as ordinary PVC can be permeable for gases. There are many pockets not filled by the material. 
I noticed myself that cats are able to locate meat or fish in a closed PVC garbidge bag and rip it apart in the area where the former is located. 
A course for concern has been in agriculture where bromides are used to prepair the soil. The worry was about the diffusion of bromides through PVC drainage pipes from the outside into the pipes.
