Abstract
The tensile strength of lightly-filled sulfur vulcanizates of 1,4-cis polyisoprene is lower the more the specimen has undergone deformation after removal of a definite stress. With equal tensile strength, this deformation multiplied by the modulus is proportional to the square of the filler concentration. Along with this deformation the volume of the elongated specimen increases through the formation of internal stress cracks. Rupture originates even at lower stresses in the neighborhood of the filler particles. It is affected by the size of the filler particles as well as by the chemical nature of the filler and by the structure of the network. Encouraged by these results we checked the network structure. We examined the stresses at high elongations, the photoelaetic homogeneity and the turbidities and viscosities of solutions of the degraded vulcanizates, and found small local variations in crosslink density and differences in the number of elastically-ineffective chain ends. The number of these chain ends and the local variations of crosslink density may increase greatly during vulcanization, and substantially reduce the tensile strength.