Vulcanization. Part IV. The Effects of Compounding Variables on the Nature of Rubber Networks

It is generally accepted for sulfur vulcanizates of natural rubber that the crosslinks consist of monosulfidic and polysulfidic links between polymer chains. There is indication in the literature that the average number of sulfur atoms per crosslink decreases with an increasing ratio of accelerator concentration to sulfur concentration. Further work was needed and in Paper III of this series a rapid method for determining the relative proportions of monosulfidic and polysulfidic crosslinks was described. The method was based on solvent swelling measurements of thin film vulcanizates before and after a lithium aluminum hydride reduction. By using this method along with a newly developed method for measuring stress-relaxation in a Monsanto Oscillating Disk Rheometer, it is possible to determine the relative proportions of three types of crosslinks: those which are nonreducible, those which are reducible but not rapidly stress-relaxable and those which are reducible and rapidly stress-relaxable. It is assumed that the stress-relaxation is due to cleavage of some of the polysulfidic crosslinks, but not others (those of the type R—S—S—R). It was our purpose here to define network characteristics as a function of compounding variations. The networks were characterized in respect to the distributions of the types of crosslinks as well as the number of crosslinks. It is felt that the nature of the networks as measured on the microscale would be significantly reflected in gross properties; this should be borne out in future studies.