Anisotropic ultra-small-angle X-ray scattering in carbon black filled polymers

Carbon black (CB) filled polymers display mechanical and electrical propert ies that depend not only on the volume fraction phi of the filler particles but also on their structure and on their distribution. Recently it has bee n shown that ultra-small-angle X-ray scattering measurements (USAXS) yield information on the arrangement of aggregates in the fluffy or pelletized CB powder and in particular on their mutual interpenetration. In this paper, USAXS measurements using synchrotron radiation and a two-dimensional detect or are reported. They reveal anisotropic features in carbon black filled po lymers having a length scale around 100 nm. For CB concentrations below the percolation threshold, no anisotropy is observed, even in uniaxially stret ched samples. For concentrations above the threshold, however, the results are highly sensitive to the degree of interpenetration of the CB aggregates , The role of aggregate interpenetration (which can occur because the fract al dimension is smaller than 2) in the mechanical behavior of CB-polymer co mposites is shown. These features are fully consistent with recent models o f reinforcement of rubber by fractal aggregates.