Small-Angle X-ray Scattering Study of the Morphology of Carbon Black Mass Fractal Aggregates in Polymeric Composites

Carbon blacks (CB) demonstrate varied structural features on length scales from angstroms to micrometers. Widely used as fillers in polymers, carbon blacks improve the mechanical and electrical properties of the host material. In general, CB is a low-dimensional mass-fractal aggregate of carbonaceous primary particles. Here we investigate the effect of processing on the interpenetration of aggregates in CB/polymer composites by small-angle X-ray scattering (SAXS). We performed SAXS measurements on a series of N330/EPR and N330/HDPE composites containing different amounts of a commercially available N330 carbon black. N330/HDPE composites were prepared by two different methods. The first method is Brabender dispersion of depelletized N330 in molten polymer; the second method is high-shear mixing of depelletized N330 in HDPE dissolved in a good solvent and subsequent addition of this mixture to a poor solvent for polyethylene. SAXS experiments were performed at the University of New Mexico/Sandia National Laboratories SAXS Laboratory. Data were collected on the Bonse-Hart camera with a q-range of 0.003 < q < 1 nm-1. This wide q-range probes the structure of both the primary carbon black particles and aggregates of these particles. Depelletized N330 displays two power law regimes from which we deduce a surface-fractal dimension Ds = 2.3 for the primary particles and a mass fractal dimension D m = 1.8 for the aggregate. For pelletized N330, the mass fractal domain vanishes as a result of aggregate interpenetration and, therefore, loss of correlation between primary particles. For N330/HDPE composites prepared by the Brabender method, the SAXS curve is similar to that obtained for the depelletized samples, indicative of little to no interpenetration of the aggregates. For N330/HDPE composites prepared by the solvent method and N330/EPR composites the SAXS curve is similar to that obtained for pelletized N330, indicative of extensive interpenetration of the aggregates.