EFFECTS OF THE TYPE OF CROSS-LINK ON VISCOELASTIC PROPERTIES OF NATURAL-RUBBER

The viscoelastic properties of various crosslinked natural rubbers, NR , were investigated by mechanical spectroscopy. The glass transition t emperature, T-g, was found to be dependent on both the crosslink densi ty and the crosslink type. Higher values of T-g were obtained for sulf ur-crosslinked NR than for peroxide-crosslinked NR at the same crossli nk density. The greater influence of the sulfur content on T-g may be attributed to polysulfidic crosslinks and cyclic sulfide structures fa vored at high sulfur contents. Sulfur-vulcanized NRs with monosulfidic crosslinks, favored at relatively high accelerator/sulfur ratios, hav e properties more similar to the peroxide-cured NR with simple carbon- carbon crosslinks covalent bonds, resulting in only small shifts in T- g. A qualitative analysis of monosulfidic crosslinks and polysulfidic structures was performed with C-13 solid-state NMR spectroscopy. The s torage modulus, E', in the rubbery plateau region increased with incre asing crosslink density. However, the crosslink type did not influence the moduli values as much as it influenced the T-g values. Different methods of detecting the crosslink density were also discussed. (C) 19 96 John Wiley & Sons, Inc.