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 (T g ).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 (T start ) 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 T start 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 T start (188 °C) and a 12× diameter expansion.These findings highlight the importance of aqueous-phase regulation in controlling the T start and expansion ratio of TEMs, providing a promising route to microspheres suited for high-temperature polymer foaming.