Carbon black-filled immiscible blends of poly(vinylidene fluoride) and high density polyethylene: The relationship between morphology and positive and negative temperature coefficient effects

Conductive polymer composites were prepared by melt-mixing of an immiscible blend of poly(vinylidene fluoride) (PVDF), high density polyethylene (HDPE ), and carbon black (CB). Three major factors-the carbon black content, the carbon black type, and the composite morphology-were shown to have remarka ble effects on the positive temperature coefficient (PTC) and negative temp erature coefficient (NTC) effect of the composites. The relationship betwee n the morphology and the PTC and NTC effects of the composites was investig ated using optical microscopy (OM) and scanning electron microscopy (SEM). The OM micrographs indicated that CB was selectively located in the HDPE ph ase and the SEM micrographs showed that there were some gaps between the tw o phases. The PTC effect of the composites is caused by the thermal expansi on as a result of the melting of the HDPE crystallites. The morphology of t he composites greatly affects the PTC and NTC behaviors of the composites. When the CB-filled HDPE formed a continuous phase and the PVDF formed a dis persed phase, the PTC and NTC behaviors of the composites were similar to t hose of CB-filled neat HDPE composite without crosslinking. When the compos ite exhibited an interlocking structure, a normal PTC effect could also be observed, but the NTC effect was delayed to higher temperatures. A mechanis m was proposed to explain this new physical phenomenon, and the mechanism w as verified by another CB-filled polymer blend comprising an alternating co polymer of tetrafluoroethylene-ethylene and HDPE.