The structure of carbon black-elastomer composites by small-angle neutron scattering and the method of contrast variation

We have been exploring the use of small-angle neutron scattering and the method of contrast variation to give a new look at very old problem-reinforcement of elastomers by carbon black in durable rubber products. Carbon black has a hierarchy of structures consisting of particles covalently bound into aggregates, which in turn associate by weak interactions into agglomerates. We found that in one carbon black, HSA, the aggregates are rod-like, containing an average of 4-6 particles. The aggregates have an outer graphitic shell and an inner core of lower density carbon. The core is continuous throughout the carbon black aggregate. Contrast variation of swollen HSA-polyisoprene gels shows that the HSA is completely embedded in polyisoprene and that the agglomerates are formed predominantly by end on associations of the rodlike aggregates. The surface structure of the carbon black appears smooth over length scales above about 10 Angstrom. Further studies using production carbon blacks suggest that these structural characteristics are generally present in commercial rubber composites.