The Vulcanization of Rubber. XI. Influence of the Degree of Vulcanization on the Internal Friction of Unloaded Butadiene-Styrene Vulcanizates

Abstract

Abstract 1. The relation between the internal friction, which is responsible for heat formation during the dynamic deformation of unloaded vulcanizates of butadiene-styrene rubber, and the content of combined sulfur, which was changed within the limits of 0.5 to 26 per cent in the rubber, was studied. 2. It is shown that internal friction and the related relaxation properties do not depend on the percentage of combined sulfur up to 9.0 per cent. Further increase of the combined sulfur content causes a sharp increase of the internal friction of the rubber. 3. For the same vulcanizates of butadiene-styrene rubber, the equilibrium stress, as well as its energy and entropy components, were measured. It is shown that in vulcanizates which contain combined sulfur in proportions not greater than 8.0 per cent, the nature of the stress is predominantly entropic. Further increase of the combined sulfur content causes an increase of the energy component of the stress. 4. Proceeding from the values of the entropy component of the equilibrium stress, on the basis of the approximate theory of Wall, the values of the molecular weights of a chain segment between the cross-links of the spatial network M* were calculated for vulcanizates containing varying percentages of combined sulfur. 5. It is shown that the value of M* is 7000 for the polymer studied and is critical in the sense that further densening of the network causes a sharp increase of both internal friction and the energy component of the equilibrium stress. The hypothesis is advanced that the value of M*crit. can be regarded as a criterion of the flexibility of the molecular chains of the polymer studied.