SAXS/USAXS Studies of Polyethylene/Carbon Black Composites. Greg Beaucage, Dept. Materials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221-0012

Carbon black is a common polymer additive for reinforcement and enhanced physical properties such as conductivity. This paper pertains to a study of a conductive grade of carbon black and carbon black/polymer composites using x-ray scattering, SAXS and USAXS. The scattering pattern for such blacks displays a surface-fractal-like power-law decay over many decades in q. It is often assumed that small-angle scattering from carbon black aggregates can be described in terms of surface-fractal models, related to particles with fractally rough surfaces. Such self-similar surface roughness is usually easy to qualitatively identify by microscopic techniques, however, scanning electron microscopy from these blacks fails to clearly support this assumption. It is proposed here that an apparent surface-fractal regime in scattering actually reflects a more complicated morphology including overlapping structural features and a power-law scaling of polydispersity. One use of conductive black composites with polyethylene is in circuit protection devices where resistive heating leads to a reversible association of carbon black aggregates that controls switching between a conductive and a non-conductive state. Scattering can be used as an in situ tool to observe the morphological signature of this reversible structural change. Scattering patterns support a model for this switching based on local enhancement of concentration and the formation of linear agglomerates associated with the matrix polymer's semi-crystalline morphology. This work was done in association with S. Rane and D. W. Schaefer of the University of Cincinnati, G. Long and D. Fischer of NIST and K. Schwartz and M. Wartenberg of Raychem Corporation and was partially funded by Sandia National Laboratories.