On the role of the interphase in the AC electrical conductivity of low-conducting polypropylene/carbon nanofiber composites

This work presents a comparative investigation of the AC electrical conductivity of melt-mixed polypropylene (PP)/carbon nanofiber (CNF) composites prepared using two different CNF grades supplied by Applied Sciences Inc.: non-commercial as-grown PR 25 AG and commercial Pyrograf® III PR 25 PS XT. The study aims to clarify how the CNF surface state and post-processing influence filler dispersion, conductive-network formation, charge-transport mechanisms, and polymer–filler interphase contribution. PP/CNF films were produced by melt mixing and electrically characterized as a function of frequency and CNF content. For both composite systems, the conductivity response shows a transition from dielectric to conductive behavior near 0.9 vol.% CNF, indicating the onset of percolative pathways within the PP matrix. This transition is associated with frequency-dependent hopping and tunnelling charge transport between neighboring CNFs and CNF-rich regions. The main novelty of this work lies in the comparative application of modified generalized effective medium and mixing rule models that explicitly include the interphase as an active electrical region. These models allow estimation of the interphase conductivity and interphase volume fraction, providing physical insight beyond classical binary descriptions of polymer composites. The results reveal that the interphase contribution is dominant in low-frequency conduction, where polarization and interfacial charge accumulation are more significant, whereas its influence progressively decreases at higher frequencies. Overall, this study establishes a physics-based modelling framework for correlating CNF grade, interphase properties, and AC conductivity, supporting the design of low-conducting PP/CNF composites for antistatic and electrostatic dissipative applications.