Dynamic Mechanical Properties of Cross-Linked Rubbers. I. Effects of Cross-Link Spacing in Natural Rubber1

Abstract Complex shear compliances of five samples of natural rubber crosslinked by cumyl peroxide and three samples crosslinked by sulfur, covering a broad range of average crosslink spacing, have been measured over a frequency range of 0.1 to 1000 cps and a temperature range from −18 to 55° C. The data were all reduced to 25° C by shift factors calculated from an equation of the WLF type. In the transition zone of frequencies, the viscoelastic functions were closely similar; the loss tangents of the cumyl peroxide vulcanizates were identical within experimental error, and those of the sulfur vulcanizates were shifted somewhat to the left on the logarithmic frequency scale with increasing degree of vulcanization. In the rubbery zone, the losses persisted to much lower frequencies than could be expected from configurational rearrangements within individual strands on the basis of current molecular theories, reflecting slow relaxation mechanisms whose presence has also been deduced from other measurements....