Recent Advances in the Physics and Chemistry of Rubber. III. The Solubility and Swelling of Rubbers

Abstract

Abstract In Parts I and II we have built up a picture of the molecular structure of a piece of rubber, which we have envisaged as a mass of interpenetrating chains, each of considerable length, but randomly kinked and in constant thermal motion, so that its shape is continually changing. Vulcanization restricts the possible motions of the mass by introducing permanent connections between pairs of chains at a series of irregularly spaced points. The purpose of Part III is to show how the picture can be extended to account for the behavior of such a system in contact with a liquid. If we imagine a liquid as a mass of more or less spherical molecules in constant thermal motion, it is clear that, when a liquid is brought into contact with a rubber, there is a tendency for molecules of liquid to diffuse into the rubber structure. Considering first the case of an unvulcanized rubber, we should expect the liquid to enter in increasing amounts, thereby separating the rubber chains and giving them very much greater freedom of motion, until finally some of them begin to find their way out of the main body of rubber and into the liquid. The rubber would thus be observed first to swell and then slowly to disperse until a uniform mixture of rubber and liquid is produced. This is, of course, the behavior actually observed when raw rubber is brought into contact with a liquid in which it is soluble. It is important to emphasize that the driving force which causes the mixing is simply the thermal motion of the components, and is not essentially dependent on any molecular attraction between rubber and liquid. It can be given a quantitative measure in terms of the thermodynamic concept of the increase of entropy consequent on mixing, but this development will not be considered here, our purpose being only to discuss the essential physical basis of the phenomenon.