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Enhanced thermal conductivity of rheologically percolated carbon nanofiber reinforced polypropylene composites

Three different types of carbon nanofibers (CNF) were incorporated in the same polypropylene (PP) matrix by twin-screw extrusion. The rheological and thermal properties were investigated. The rheological characterization of CNFs/PP composites as function of their volume fraction shows different microstructures: percolated and non-percolated behaviors of their CNF's networks. In this work, the laser flash technique is employed in the experimental determination of the thermal diffusivity and conductivity of composites at room temperature. The ultimate aim is to correlate microstructure described by rheological analysis with final thermal properties. The results show that thermal diffusivity and conductivity are clearly higher for rheologically percolated composites suggesting that above certain critical content of nanofibers thermal transport is mainly controlled by percolated structures caused by interconnected CNFs' networks. Finally, thermal conductivity results are described by means of percolation theory from which an intrinsic thermal conductivity for the CNFs' network of approximately 6.5 W/m K, i.e. close to three times lower than some values reported in literature for SWCNTs' networks, was calculated. Copyright © 2015 John Wiley & Sons, Ltd.