Structural characterization of vulcanizates. Part IV. Use of triphenylphosphine and sodium di‐n‐butyl phosphite to determine the structures of sulfur linkages in natural rubber, cis‐1,4‐polyisoprene, and ethylene–propylene rubber vulcanizate networks

Abstract

AbstractDeterminations of the degree of chemical crosslinking and crosslinking efficiency values E (i.e., the number of sulfur atoms combined in the network per chemical crosslink present) have been used in conjunction with the chemical probes triphenylphosphine and sodium di‐n‐butyl phosphite to establish the structural features of sulfur links in vulcanizate networks derived from a variety of accelerated natural rubber–sulfur systems, from a cis‐1,4‐polyisoprene–TMTD–zinc oxide system, and from natural rubber and ethylene–propylene rubber crosslinked with a dicumyl peroxide–sulfur system. The triphenylphosphine converts polysulfide links into monosulfide and, to a lesser extent, disulfide links. The values of E′, i.e., the number of sulfur atoms combined in the network per chemical crosslink present after triphenylphosphine treatment are, therefore, a measure of the extent of main‐chain modification in the network by cyclic monosulfide groups and/or pendant groups of the type: SxAccel. (where x ≥ 1 and Accel. is an accelerator fragment). Sodium di‐n‐butyl phosphite cleaves di‐ and polysulfide crosslinks but leaves monosulfide and carbon–carbon crosslinks intact, and thus determination of the degree of chemical crosslinking before and after treatment with this reagent yields estimates of these two different classes of crosslinks. The combined results indicate that the efficiency of utilizing sulfur for crosslinking and, therefore, the structural complexity of the derived networks are very sensitive to the nature of the vulcanizing system (type and relative concentrations of crosslinking agent, accelerator, and activator) and vulcanizing conditions (time and temperature of cure). In general, the proportion of crosslinks which are di‐ and/or polysulfidic decreases with increasing cure time, and for the accelerated sulfur systems the structural complexity of the network increases with cure time, especially at higher vulcanizing temperatures and with low concentrations of fatty acid activator.