Balancing processability and percolation behavior of ultrahigh aspect ratio carbon nanotubes in polymers through ball milling

Abstract The effects of ball milling on the processability and percolation behavior of ultralong (~100 μm) carbon nanotubes (CNTs) in an amorphous thermoplastic polymer, polycarbonate, were examined. The Burgio–Rojac model was applied to establish the relationship between impact energy and milled CNT morphology. The minimum single impact energy for fracture of ultralong CNTs (125 mJ/impact) was found to be nearly an order of magnitude greater than that of short, commercial CNTs (~10 mJ/impact) due to bundle diameter size differences. The kinetic length reduction behavior of ultralong CNTs was found to align with previously studied short CNTs, allowing for accurate prediction of CNT length reduction based on cumulative impact energy. Ball milling CNTs to 50, 20, and 5 μm lengths resulted in composite melt viscosity reductions of 26%, 52%, and 58%, respectively. A moderate increase in composite degradation temperature with increased CNT milling duration (+7°C for length reduction to 5 μm) suggested that polymer molecular weight is preserved. However, ball milling resulted in significant increases in rheological and electrical percolation thresholds vs. the unmilled CNTs due to the formation of small, undispersed agglomerates, suggesting a balance must be struck when employing ball milling as a processing aid for polymer composites filled with high aspect ratio fillers. Highlights Length reduction kinetics of ultralong CNTs are similar to short CNTs. Ball milling CNTs improves composite processability by reducing agglomeration. Highly aligned CNT bundles do not disperse well during melt mixing. Ball milling CNTs results in an increased percolation threshold due to shortening.