Highly Thermally Conductive PDMS/h-BN Composites Enabled by Aspect-Ratio-Driven Alignment

Shear-induced alignment of hexagonal boron nitride (h-BN) platelets offers a scalable route to high-performance, electrically insulating thermal management materials, yet the role of filler geometry under practical shear processing remains unclear. Here, we examine how platelet aspect ratio governs alignment and heat transport in PDMS/h-BN composites processed by sequential roll-gap controlled two-roll milling. Using a geometric moment-arm perspective, we relate the platelet effective radius to the shear-driven rotational driving moment. High-aspect-ratio platelets (L-BN) exhibit more stable flow-parallel alignment than small platelets (S-BN), forming a better-connected conductive network. At 175 wt% loading, the aligned L-BN composite achieves 10.3 W m^-1 K^-1 (94% higher than its random counterpart) and outperforms the S-BN system while also improving stiffness and device-relevant heat dissipation. These results identify aspect ratio as an alignment-enabling design criterion for scalable thermal management.