Fine-tuning the architecture of microgels by varying the initiator addition time†

Poly-N-isopropylacrylamide (PNIPAM) microgels are versatile colloidal-scale polymer networks that exhibit unique responsiveness to external stimuli, such as temperature. While the synthesis of PNIPAM microgels is well-documented, there is limited exploration of how their structural properties can be modified by subtle changes in the polymerization process. In this work, we carefully investigate how varying the time of addition of a common initiator, such as potassium persulfate, during the polymerization process allows a precise control over microgel architecture. Our findings, based on a combination of numerical simulations, scattering, and rheology experiments, reveal that delayed initiator addition results in a more heterogeneous network, characterized by a less extended corona. In contrast, more homogeneous microgels are obtained by adding the initiator all at the start of the synthesis. In this way, the internal mass distribution of the particles can be tuned, highlighting the importance of synthesis timing for optimizing microgel conformation and functionality in tailored applications.