On the Ultimate Elongation of Natural Rubber Vulcanizates

Following the previous paper1) on the tensile properties of the noncrystallized amorphous rubber, the properties of natural rubber (NR) crystallized in a high elongated state are reported in the present studies. The gum vulcanizates of various crosslink densities and the filled vulcanizates of various carbon black contents were prepared. The tensile properties of these samples were studied, with particular reference to their ultimate elongation and their comparative studies were made with the molecular chain mobility and the polymer structure. The following conclusions were obtained.(1) The crystallized rubber showed “the large” ultimate elongation up to about 100°C, while noncrystallized rubber showed it no more than about -20°C. The abrupt increase of ultimate elongation in the transition region is consistent with the previous finding.(2) The large ultimate elongation at high temperature is interpreted as the result of increase in strength, based on the crystallization on stretching, and of increased mobility of molecular chains. The ultimate elongation and the temperature of maximum ultimate elongation were correlated with the network chain length. That is, while the former is reciprocally proportional to the square-root of the crosslink density, the latter is related to the crosslink density inverse.(3) The influence of the apparent network chain density on the tensile properties of the filled vulcanizates, is apparently opposite to the case of the gum vulcanizates. This tendency is considered to be due to the fact that the rubber molecules constitute the heterogeneous dense structure on the filler surface. In the region far from the filler particles where the molecules take part in the crystallization on stretching, the molecular chain length between the crosslinks is larger as the filler content increases. Thus the temperature of maximum ultimate elongation increases. On the other hand, the ultimate elongation is assumed to be determined by the apparent network chain density.