Aqueous-phase regulation in Pickering emulsions yields thermally expandable microspheres with high onset expansion temperature and high expansion ratio

Thermally expandable microspheres (TEMs) are 5–50 μm core–shell particles that expand near the shell’s glass transition temperature (Tg). They are widely used as blowing agents in polymer foaming. Typically, TEMs are prepared by Pickering emulsion–based suspension polymerization. In this process, the oil phase contains monomers, a volatile blowing agent, and an initiator, and it is dispersed as oil-in-water droplets. The aqueous phase contains inorganic particles and an organic dispersant to stabilize the interface. However, solely regulating the oil phase has failed to deliver TEMs that couple a high onset expansion temperature (Tstart) with a large expansion ratio. Therefore, these materials remain unsuitable as blowing agents for high-processing-temperature polymers. This study systematically investigates how regulating the aqueous-phase environment affects Tstart and the expansion ratio of TEMs. Specifically, we tune silica concentration, pH, and ionic strength. By enhancing emulsion stability and optimizing emulsion morphology, we obtain TEMs that combine a high Tstart (188 °C) and a 12× diameter expansion. These findings highlight the importance of aqueous-phase regulation in controlling the Tstart and expansion ratio of TEMs, providing a promising route to microspheres suited for high-temperature polymer foaming.