Influence of pendent groups on the tensile properties of polyisoprene vulcanizates

Gum compounds of polyisoprene were vulcanized with a number of different cu ring systems to give networks with crosslink densities in two different ran ges. Stress-strain curves were obtained upon rapid (500 mm/min) and slow (0 .5 mm/min) extension. In tetramethylthiuram disulfide (TMTD)/sulfur and zin c dimethyldithiocarbamate/sulfur vulcanizates, which crystallize readily, f ailure occurred at higher tensile values upon rapid than upon slow extensio n and this is attributed to a greater contribution to tensile strength by a larger amount of stress-induced crystallites. X-ray diffraction showed tha t 2-benzothiazole-2,2 ' -disulfide (MBTS)/sulfur vulcanizates did not stres s-crystallize and failure occurred at lower tensile values. Furthermore, sa mples extended rapidly failed at lower tensile values than did slowly exten ded samples. These differences, compared to TMTD vulcanizates, are attribut ed to extensive main-chain modifications (pendent groups), causing delays i n the movement of sections of the chain, leading to the load being unequall y distributed between chains. The fewer load-bearing chains ensure earlier failure. The addition of zinc stearate to TMTD/sulfur and MBTS/sulfur formu lations increases the ability of vulcanizates of similar crosslink density to crystallize and enhances tensile properties of vulcanizates with similar crosslink densities, outcomes that are attributed to zinc stearate's promo ting crosslinking of pendent groups and reducing impediments to crystalliza tion and chain movement. Dicumyl peroxide-cured networks crystallize readil y and exhibit a very rapid upturn in the stress-strain curve. However, fail ure occurs at lower stress values than apply to accelerated sulfur networks and it is suggested that the distribution of subchain lengths between cros slinks may contribute to their inferior properties. (C) 2001. John Wiley & Sons, Inc.