The effect of conductive network on positive temperature coefficient behaviour in conductive polymer composites

Flexible and controllable self-regulating heating devices with positive temperature coefficient (PTC) behaviour are potentially excellent candidates in applications like healthcare, soft robotics, artificial skin and wearable electronics. Although extensive studies have been carried out in this field to understand the mechanism of PTC effect, rather limited conclusions have been reached. Many controversies remain on the dominating factors that influence the PTC performance of composites, hence limiting their design and broader applications. Herein, we propose a systematic study to explore the PTC phenomenon and the underlying mechanism, from a conductive network viewpoint, taking account of both conductive fillers and polymer matrices. Three representative conductive fillers with distinct dimensions and shapes (0D silver coated glass spheres, 1D carbon nanotubes and 2D graphene nanoplatelets), in combination with three different polymer matrices (high density polyethylene, thermoplastic polyurethane and polycarbonate) were selected to elucidate the effect of the robustness of different conductive networks on PTC behaviour in conductive polymer composites (CPCs). The desired conductive network can be obtained by selecting preferentially larger filler size, lower filler aspect ratio and/or selective distribution of filler (e.g. in the amorphous region of semi-crystalline polymers). The highest PTC intensity was observed around the critical percolation threshold, in correspondence of networks with the lowest number of inter-particle contacts. This study can serve as a guideline in the selection of the most appropriate conductive filler and polymer matrix for various self-regulating heating requirements and final applications.