Annealing-induced particle reorientation enhanced through-plane thermal conductivity in PPS/h-BN composites

: Thermally conductive polymer composites are promising lightweight materials for thermal management in high-power electronics; however, the effect of short-term thermal annealing on directional heat transport remains insufficiently understood. Herein, polyphenylene sulfide (PPS)/hexagonal boron nitride (h-BN) composites were prepared by twin-screw extrusion and melt pressing, then thermally annealed in air at 340 °C for 5, 30, or 60 min. In-plane and through-plane thermal conductivities were evaluated as a function of annealing time and correlated with rheological behavior, crystallization, chemical-state evolution, and filler-network morphology using parallel plate rheology, differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). At low filler loadings, annealing primarily altered the PPS matrix response, with limited changes in thermal conductivity. In contrast, at 50 vol. % h-BN, through-plane thermal conductivity increased from 1.41 ± 0.12 W/m·K after 5 min to 2.31 ± 0.22 W/m·K after 30 min and remained elevated at 2.11 ± 0.16 W/m·K after 60 min. In-plane thermal conductivity remained comparatively insensitive to annealing time across the compositions examined. XRD showed a decrease in the h-BN A (002) /A (100) peak-area ratio with increasing annealing time, consistent with reduced preferential in-plane orientation and improved through-plane network contribution. SEM observations and semi-empirical Lewis-Nielsen model fitting further supported annealing-dependent changes in the effective h-BN network structure. These results indicate that short-term thermal annealing can serve as a post-processing strategy to modify filler-network architecture and reduce thermal transport anisotropy in highly filled PPS/h-BN composites.