Interfacial design and property regulation of modified poly(phenylene oxide)/silica dielectric composites

Poly(phenylene oxide) (PPO) is a promising resin matrix for high-frequency packaging, owing to its outstanding dielectric properties and high glass transition temperature (~210 °C). However, the abundant rigid phenyl ring structures restrict chain segment mobility, resulting in intrinsic brittleness and inferior processability. Furthermore, PPO exhibits a relatively high coefficient of thermal expansion (CTE, 76 ppm/°C), which restricts its application in advanced dielectric composites demanding high thermal dimensional stability. Although hydrogenated styrene-butadiene-styrene block copolymer (SEBS) and inorganic fillers have been incorporated into PPO to improve toughness and reduce CTE, respectively, weak interfacial bonding in such ternary systems often causes increased dielectric loss and degraded mechanical performance. Herein, we proposed an interfacial regulation strategy based on carbon-carbon double-bond reactions. PPO was first end-capped with styryl groups to obtain crosslinkable styryl-terminated poly(phenylene oxide) (SPPO), and vinyl groups were grafted onto the silica surface to produce functionalized silica (f–SiO₂). During curing, free-radical polymerization of the double bonds generated covalent linkages between the SPPO/SEBS matrix and f–SiO₂. Consequently, a stable chemically bonded interface and a three-dimensional crosslinked network were constructed between the resin phase and the filler phase, thus effectively resolving the interfacial mismatch issue in the ternary composite system. The as-fabricated dielectric composites exhibited a low dielectric constant (Dk = 2.65) and low dielectric loss (Df = 2.58 × 10^–3) at 10 GHz, as well as a low z-direction coefficient of thermal expansion (CTE = 27 ppm/℃) and excellent thermal stability.