Morphology of polyethylene-carbon black composites

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.