Influence of Structure of Elastomers on Their Permeability to Gases

Abstract

Abstract It is a known fact that rubber membranes are permeable to gases and that the rate of permeation is a specific function of a given gas and rubber. We may picture this permeation of gas through rubber as a process in which gas molecules dissolve on one side of the membrane, then diffuse through the rubber to the other side, and there evaporate out of the membrane. Thus the process of permeation is divided into several independent processes, those of solution and evaporation being governed by Henry's law and that of diffusion by the diffusion equation (Fick's law). The problem to be considered is why a given rubber should possess a given permeability to a given gas. It will at once be evident from what has just been stated that, while studying the problem, the process of solution (or evaporation) and that of diffusion must be dealt with separately. It was shown in earlier investigations on a series of rubber varieties and gases how the chemical structure of the rubber molecules affects the rate at which the gas passes through, and, notably, that the introduction of methyl and polar groups in the polymer molecules and crystallization reduce this rate of diffusion. It was also found that a gas diffuses more slowly as its molecules are larger. The solubility of the gas in the rubber is determined by its tendency to condense and by the interaction between the rubber and the gas molecules. These views will now be put to the test with reference to another series of rubbers and gases. We shall also see that knowledge of the necessary conditions for low permeability may help to evolve new kinds of rubber of low permeability.