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Structural characterization of vulcanizates. Part I. Crosslinking efficiency of sulfur in unaccelerated natural rubber–sulfur systems
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1961
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Materials ScienceChemical EngineeringMacromolecular ScienceStructural CharacterizationEngineeringDepolymerizationMacromolecular ChemistryCross-linkPolymer ScienceChain ScissionOrganic ChemistrySulfur Contents ScChemistryDesulfurizationChemical CrosslinkChemical KineticsPolymer ChemistryPolymers
The sulfur contents Sc and degrees of chemical crosslinking 1/2M*c,chem have been determined for a series of vulcanizate networks obtained by curing unaccelerated natural rubber–sulfur mixes for various times at 140°C. The independent estimates of 1/2M*c,chem were obtained for each vulcanizate from stress–strain measurements (C1 and λ*) on the unswollen vulcanizate and use of the methods described by Mullins. The disparity in the values of 1/2M*c,chem obtained by the two methods reflects the scission of natural rubber main chains during the vulcanization process, the extent of such scission being found to increase with increasing cure time. When allowance is made for chain scission the Sc and 1/2M*c,chem data yield values of the efficiency of sulfur as a crosslinking agent. It is found that between about 40 and 55 sulfur atoms must be combined in the network to produce one chemical crosslink, values which reflect the marked structural complexity of the network as is also inferred from the structures of the products derived from sulfur–olefin interaction at 140°C.