Unified modeling and experimental realization of electrical and thermal percolation in polymer composites

Correlations between electrical and thermal conduction in polymer composites are blurred due to the complex contribution of charge and heat carriers at the nanoscale junctions of filler particles. Conflicting reports on the lack or existence of thermal percolation in polymer composites have made it the subject of great controversy for decades. Here, we develop a generalized percolation framework that describes both electrical and thermal conductivity within a remarkably wide range of filler-to-matrix conductivity ratios ( Y f / Y m), covering 20 orders of magnitude. Our unified theory provides a genuine classification of electrical conductivity with typical Y f / Y m & GE; 10 10 as insulator-conductor percolation with the standard power-law behavior and of thermal conductivity with 10 2 & LE; Y f / Y m & LE; 10 4 as poor-good conductor percolation characterized by two universal critical exponents. Experimental verification of the universal and unified features of our theoretical framework is conducted by constructing a 3D segregated and well-extended network of multiwalled carbon nanotubes in polypropylene as a model polymer matrix under a carefully designed fabrication method. We study the evolution of the electrical and thermal conductivity in our fabricated composites at different loading levels up to 5 vol. %. Significantly, we find an ultralow electrical percolation threshold at 0.02 vol. % and a record-low thermal percolation threshold at 1.5 vol. %. We also apply our theoretical model to a number of 23 independent experimental and numerical datasets reported in the literature, including more than 350 data points, for systems with different microscopic details, and show that all collapse onto our proposed universal scaling function, which depends only on dimensionality. Published under an exclusive license by AIP Publishing.

Automated summary

The relationship between electrical and thermal conduction in polymer composites is complex due to the contributions of charge and heat carriers at the nanoscale junctions of filler particles. In this study, a generalized percolation framework was developed to describe the conductive properties in a wide range of filler-to-matrix conductivity ratios. Experimental verification of the framework was conducted using carbon nanotubes in polypropylene composites, showing ultralow percolation thresholds for electrical and thermal conductivity.