Equilibrium between Rubber and Alcohol-Benzene Solutions

1. The swelling in the gaseous phase at 20° C of a digested vulcanizate of natural rubber was studied through the entire range of compositions of the following binary systems: methyl alcohol-benzene, ethyl alcohol-benzene, n-propyl alcohol-benzene, n-butyl alcohol-benzene, and n-hexanol-benzene, and in a system of octadecanol-benzene at 60° V in the liquid phase. The swelling-composition curves show a maximum in the first three systems enumerated and a minimum in the latter system. 2. In the first five systems, the concentrations of the components of the equilibrium solution phase and the rubber phase were measured by means of an interferometer. 3. The coefficient of distribution of alcohol between the polymer and solution phases decreased from 1.6 in the methyl alcohol-benzene system to 0.3 in the n-hexanol-benzene system. The decrease of the coefficient of distribution of alcohol through the series of homologs is attributable to the dipolar character of the alcohol molecules. The coefficient of distribution of benzene between the two phases decreased from 16.6 in the methyl alcohol-benzene system to 6.3 in the n-hexanol-benzene system. This decrease of the coefficient of distribution of benzene is caused by the weakening of the quasi-crystalline structure in passing from methyl alcohol to the higher alcohols. 4. From an analysis of the distribution of alcohol between the solution and polymer, and also from a comparison of these curves with the curves of distribution of alcohol between the binary solution and its saturated vapor, it follows that rubber absorbs individual alcohol molecules, but not associates of them. 5. From a comparison of data on the distribution of the components of the binary solution between the polymer and equilibrium solution, and also from an analysis of the curves of partial free energy of the components of both phases, it follows that the general nature of the curves of isothermic equilibrium in the systems studied accords with the hypothesis of Konovalov; at the maximum points on the swelling curves, the composition of both phases is equal. The degree of swelling increases with increase of the concentration of the component in highest concentration in the polymer. 6. The existence of points of constant composition on the swelling curves is confirmed by measurements of the swelling by the volumetric method. Only for the compositions of solutions which correspond to the maximum points on the swelling curves, the volume in the condenser in whose vapors the specimen is suspended does not change with time. In more dilute solutions, the volume decreases ; in more concentrated solutions it increases, with time. 7. There is a close relation between the degree of swelling and the structure of a solution. The more pronounced the quasi-crystalline structure of the solution, the higher is the swelling-composition curve on the graph. The values of the degree of swelling and the Kirkwood-Anselm correlation parameter change in parallel for a given composition of the solution, in passing from one alcohol to another.