Tailor-made microgel particles: Synthesis and characterization

• PNIPAM-based microgels with controlled crosslink distributions were synthesized.
• PNIPAM-co-PMAA microgels with controlled charge distributions were synthesized.
• Crosslink distribution influences temperature-dependent deswelling of microgels.
• Charge distribution influences microgel swelling in response to pH.

Microgels are soft particles that consist of chemically cross-linked three-dimensional polymer networks. They are usually synthesized by precipitation polymerization. However, this method often leads to an inhomogeneous spatial distribution of crosslinks and functional groups within the microgel particles caused by differences in the reactivity ratios of the applied monomers, co-monomers, and crosslinkers. This lack of control of the structure has a profound effect on the properties of microgels, which in turn significantly limits their applications. In this work, poly(N-isopropylacrylamide) (PNIPAM)-based microgels with controlled distribution of crosslinks and charges were synthesized by batch polymerization or starve-feeding semi-batch polymerization. The structures of microgels with different crosslink distributions were characterized by laser light scattering (LLS). Our results show that the distributions of crosslinks influence the deswelling behaviors of microgels in response to temperature. When organic acid like methacrylic acid (MAA) was incorporated into the microgel, the resulting microgel particles added responsiveness to pH. The charge distributions, namely the spatial distribution of the functional MMA groups in the microgels were verified by potentiometric titration and electrophoresis. These microgels exhibit different swelling properties in response to pH. The developed approach for improving control of functional group distribution in microgels is essential for the design of more advanced microgel structures for specific applications.

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