Construction of polyporous polymer microspheres with a tailored mesoporous wall

Abstract

The fabrication of polymeric microspheres with a tailored hierarchically porous structure has great practical significance although it has been a longstanding challenge. Herein, we report a novel method to construct hierarchically porous microspheres with inner macropores and a mesoporous wall (Macro@Meso-SPS) through two consecutive phase separation processes, i.e., swelling-osmosis and polymerization-induced phase separation (PIPS) in solid sulfonated polystyrene (SPS) seed microspheres which were prepared by the sulfonation of polystyrene (PS) microspheres synthesized via γ-ray-radiation-initiated dispersion polymerization with polyvinylpyrrolidone (PVP) as a stabilizer. As PVP is less radiation-stable and more hydrophilic than PS, PVP macromolecular radicals produced under the radiation chemistry effect slightly crosslinked the surface of PS microspheres. A polyporous structure was fabricated through a swelling-osmosis process inside the seed SPS microspheres, and a mesoporous wall was constructed via the PIPS process in the crosslinked shell using methyl methacrylate (MMA) as a secondary monomer. The pore size of the Macro@Meso-SPS microspheres could be tuned from 3.1 to over 50 nm by adjusting the absorbed dose during the synthesis of PS microspheres, the amount and the conversion of MMA in PIPS. The Macro@Meso-SPS microspheres with surface mesopores around 3.1 nm showed better controlled-release performance of the loaded small molecules like methyl orange, whose cumulative release ratio was merely 45.3% after 24 h, compared to those with larger surface pores above 11.2 nm. This work not only provides a new way to construct hierarchically macro-@mesoporous polymer microspheres, but also proves that radiation-initiated polymerization may be a versatile synthetic platform for porous polymer materials with a bespoke structure for various applications like sustained release.