Thermodynamics of the Swelling of Natural Rubber and Sodium-Butadiene Rubber in Binary Mixtures

Abstract

1. The swelling of vulcanizates of natural rubber and sodium-butadiene rubber in the saturated vapors of binary systems of benzene and low-molecular alcohols (from methyl to butyl) at 20° and 50° C was studied in the entire range of composition of binary solutions. 2. The general form of the curves of isothermic equilibrium between the swollen polymer and binary solution were found to conform to Konovalov's law. 3. The absorption of a binary solution by both polymers has a very pronounced selective character. The selectivity of absorption of the solution is more pronounced, the greater is the density of energy of cohesion of the alcohol. In natural rubber, the capacity of selective absorption is more pronounced than in sodium-butadiene rubber. This is probably due to the larger concentration of oxidized groups in natural rubber. 4. The changes of the free energy, the heat content, and entropy for the passage of one gram-mole of alcohol from an infinitely dilute solution into the polymer were calculated from the experimental data. These calculations indicate that the reaction of the alcohol molecules with the polar groups of the polymer is dipole-dipole. 5. The passage from pure benzene to a swollen polymer of any particular composition is characterized by a large increase of entropy in comparison with the passage to a binary solution of the same composition. The relation in alcohol is the reverse. The increase of the entropy of benzene can serve as a measure of the elasticity of the polymer chains. A decrease of the entropy of alcohols indicates the content of free polar groups in the polymer phase. 6. The values of the partial heat content of the low-molecular components are positive; consequently, the decrease of free energy during their sorption by the polymer is due exclusively to an increase of entropy.