Multi-Dimensional Filler Design for Enhanced Thermal Conductivity and Tunable Dielectric Properties in Natural Rubber Composites.

Modern electronics demand materials that simultaneously manage heat and provide electromagnetic responses due to high integration and multifunctionality. Therefore, polymer composites with high thermal conductivity and tunable dielectric properties are critical for next-generation electronic devices. Here, natural rubber (NR) was engineered with multi-dimensional fillers-hexagonal boron nitride (h-BN), halloysite nanotubes (HNTs), poly(3-hydroxybutyrate-co-4-hydroxyvalerate) (P34HB), and multi-walled carbon nanotubes (MWCNTs)-to systematically tailor thermal, dielectric, and mechanical properties. Synergistic combinations of h-BN and MWCNTs form an effective three-dimensional thermal network, while HNTs and MWCNTs generate highly effective phonon pathways, achieving a peak thermal conductivity of 0.287 W/(m·K). Dielectric tunability is enabled via percolating h-BN/MWCNT networks, where interfacial polarization allows broad-frequency modulation of the dielectric constant. MWCNTs also regulate curing behavior and provide mechanical reinforcement. In contrast, phase separation between P34HB and NR disrupts the filler network, enabling good electrical insulation while retaining partial thermal pathways, whereas weak interfacial bonding in HNT/MWCNT composites constrains mechanical enhancement. This study demonstrates a systematic multi-dimensional filler strategy enabling tunable thermal and dielectric properties in NR composites and provides a versatile platform for multifunctional polymer materials in flexible and wearable devices.