Dispersion-informed structure–property relationships in carbon nanotube–reinforced polypropylene thermoplastic composites

Carbon nanotube (CNT)-reinforced polypropylene (PP) thermoplastic nanocomposites were systematically investigated to examine how CNT dispersion and CNT–matrix interfacial interactions influence the coupled mechanical, rheological, and electrical behavior of multifunctional thermoplastic composites under industrially relevant processing conditions. PP/CNT nanocomposites containing 0.1–5 wt% multi-walled carbon nanotubes (MWCNTs) were fabricated through twin-screw melt compounding followed by injection molding. The resulting composites were characterized using tensile and impact testing, oscillatory rheology, electrical resistivity measurements, scanning electron microscopy (SEM), atomic force microscopy (AFM), and Fourier transform infrared spectroscopy (FTIR). Uniform CNT dispersion at low filler contents promoted efficient stress transfer and interconnected network formation, leading to simultaneous enhancements in stiffness and strength. An optimum CNT loading of 1 wt% was identified, at which tensile strength and Young’s modulus increased by approximately 27% and 33%, respectively, while maintaining acceptable ductility and processable melt rheology. Rheological measurements demonstrated enhanced melt elasticity, increased storage modulus, and partial stress recovery associated with CNT-induced viscoelastic networks. Electrical resistivity decreased progressively with CNT loading, indicating increasing development of CNT-mediated conductive pathways. However, excessive CNT loading resulted in pronounced agglomeration, reduced tensile performance, and increased brittleness. Combined SEM, AFM, and mechanical analyses showed that increasing CNT agglomeration at higher loadings coincided with reduced tensile reinforcement efficiency and greater mechanical heterogeneity. SEM and AFM observations further indicated that CNT dispersion state and CNT–matrix interfacial interactions play important roles in determining the multifunctional response of PP/CNT thermoplastic nanocomposites. Overall, the results provide a dispersion-informed understanding of the coupled mechanical, rheological, and electrical behavior of PP/CNT nanocomposites and identify a favorable formulation window within the investigated melt-processing conditions.