Spectroscopic Characterization of the Molecular Structure of Elastomeric Networks

Abstract

Abstract In vulcanization, chemical crosslinks are formed across elastomeric polymer chains improving both the strength and elastic properties of the rubber. An understanding of the formation, structure, and stability of vulcanizates is therefore very important. Solid-state NMR and NMR imaging have been effective methods to study many different aspects of vulcanization. In solid-state NMR, several peaks appear in the C-13 spectrum of vulcanized rubber. Through model studies, NMR analysis, and chemical shift additivity calculations, these peaks were assigned to their respective vulcanizate structures. Once this assignment was made, the concentration of each vulcanizate structure formed could be followed with time under a variety of different conditions. In unaccelerated sulfur vulcanization of natural rubber (NR) and polybutadiene rubber (BR), many inefficient (cyclic or intramolecular) structures were formed as compared to intermolecular crosslinks. In accelerated NR and BR sulfur vulcanization, NMR was used to study vulcanizate concentration dependence on (a) type of formulation (efficient, semi-efficient, or conventional), (b) type of accelerator, (c) extent of cure, and (d) different concentration of ingredients (sulfur, activator, etc.). Solid-state NMR was also used to study different parameters in butyl rubber and to identify elastomers in binary blends of chloroprene rubber (CR) and NR, CR and chlorosulfonated polyethylene (CSM), NR and CSM, and styrene—butadiene rubber (SBR) and acrylonitrile—butadiene rubber (NBR) as well as the tertiary blend of NR/SBR/BR. In several studies, the effect of filler (carbon black or silica) on vulcanization was studied. Additionally, the thermo-oxidative degradation of sulfur vulcanizates in NR with heating time and temperature was observed using NMR. NMR imaging has been useful in the determination of internal inhomogeneities arising from inadequate mixing, gradients in crosslinking chemistry, filler distribution, blends, and coagents.