Carbon black is a common polymer additive that is used for reinforcement an
d for its ability to enhance physical properties, such as conductivity. Thi
s article pertains to an X-ray scattering (SAXS) study of a conductive grad
e of carbon black and carbon black-polymer composites. The scattering patte
rn for such blacks displays a surface-fractal-like power-law decay over man
y decades in scattering vector q. It is often assumed that small-angle scat
tering from carbon black aggregates can be described in terms of surface-fr
actal models, related to particles with fractally rough surfaces. Such self
-similar surface roughness is usually easy to identify by microscopy; howev
er, electron microscopy from these blacks fails to support this assumption.
It is proposed here that this apparent surface-fractal scattering actually
represents a more complicated morphology, including overlapping structural
features and a power-law scaling of polydispersity. One use of conductive
black-polyethylene composites is in circuit protection devices where resist
ive heating leads to a reversible association of carbon black aggregates th
at controls switching between a conductive and a nonconductive state. Scatt
ering 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 form
ation of linear agglomerates associated with the matrix polymer's semicryst
alline morphology. (C) 1999 John Wiley & Sons, Inc.