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Gas Barriers: An Introduction Basic Reference 4

Activated Diffusion Mechanism¹⁾

In Introduction Basic Reference 3, we explained that the transmission of air inside a rubber balloon is caused by gas permeation in the activated diffusion mechanism. In this article, the activated diffusion mechanism will be explained in detail.

In order to explain the activated diffusion mechanism, take as an example of two rooms that are separated by a film barrier, as seen in the top part of figure 4-1. Now, suppose that the pressure in the room on the left is higher than that in the room on the right. In this case, gas flows from the room with the high pressure to that with the low pressure.

The lower part of figure 4-1 shows an enlargement of the film. Gas transmission goes through the following process. First, gas molecules in the room with the higher pressure condense on the surface of the film. Next, the molecules dissolve into the film. The concentration of gas dissolved into the film is proportional to the gas pressure.

Driven by a concentration gradient, the dissolved gas molecules diffuse into the room with the lower concentration of gas molecules. The molecules that reach the other side of the film are desorbed from the surface of the film. Gas transmission by the activated diffusion mechanism undergoes the three processes of dissolution, diffusion, and desorption. The reason why there are gas permeable or impermeable plastics is because the amount of dissolution or diffusion that can take place varies depending on the type of gas and plastic.

Next, let us look at the diffusion process in more detail. To do this, we first need to know how plastic is structured.

A water molecule has one oxygen atom and two hydrogen atoms while an oxygen molecule has two oxygen atoms. In this way, most substances are made up of atom-bound molecules. Ethylene gas is a molecule made up of two carbon atoms and four hydrogen atoms. It is possible to join several hundred ethylene molecules through a reaction. By doing so, a long, large chain-like molecule called polyethylene is produced. In this case, the base substance, i.e., ethylene, is called a monomer. The large molecule formed is called a polymer, and plastic is made up of polymers.

Figure 4-3 (a) shows a pattern diagram of the inside of a piece of plastic film enlarged. When long molecules are represented by lines, they are seen to be intertwining or aligned in an orderly fashion in the plastic. Cuboids in the figure represent the crystallization of the orderly aligned molecules, and the remaining part is called amorphous. With most polymers, gas does not permeate through the crystallized parts, but only through the amorphous part.